2009. "Characterization of Dispersed Heteropoly Acid on Mesoporous Zeolite Using Solid-State P-31 NMR Spin-Lattice Relaxation." Journal of the American Chemical Society 131(28):9715-9721. doi:10.1021/ja901317r Abstract Dispersion and quantitative characterization of supported catalysts is a grand challenge in catalytic science. In this paper, heteropoly acid H3PW12O40 (HPA) is dispersed on mesoporous zeolite silicalite-1 derived from hydrothermal synthesis using carbon black nanoparticle templates, and the catalytic activity is studied for 1-butene isomerization. The HPAs supported on conventional zeolite and on mesoporous zeolite exhibit very different activities and thus provide good model systems to investigate the structure dependence of the catalytic properties. The HPA on mesoporous silicalite-1 shows enhanced catalytic activity for 1-butene isomerization, while HPA on conventional silicalite-1 exhibits low activity. To elucidate the structural difference, supported HPA catalysts are characterized using a variety of techniques, including 31P magic angle spinning nuclear magnetic resonance, and are shown to contain a range of species on both mesoporous and conventional zeolites. However, contrary to studies reported in the literature, conventional NMR techniques and chemical shifts alone do not provide sufficient information to distinguish the dispersed and aggregated surface species. The dispersed phase and the nondispersed phase can only be unambiguously and quantitatively characterized using spin-lattice relaxation NMR techniques. The HPA supported on mesoporous zeolite contains a fast relaxation component related to the dispersed catalyst, giving a much higher activity, while the HPA supported on conventional zeolite has essentially only the slow relaxation component with very low activity. The results obtained from this work demonstrate that the combination of spinning sideband fitting and spin-lattice relaxation techniques can provide detailed structural information on not only the Keggin structure for HPA but also the degree of dispersion on the support.
2009. "Simultaneous MS-IR Studies of Surface Formate Reactivity Under Methanol Synthesis Conditions on Cu/SiO2." Topics in Catalysis 52(10):1440-1447. doi:10.1007/s11244-009-9320-3 Abstract The coverages and surface lifetimes of copper-bound formates on Cu/SiO2 catalysts, and the steady-state rates of reverse water-gas shift and methanol synthesis have been measured simultaneously by mass (MS) and infrared (IR) spectroscopies under a variety of elevated pressure conditions at temperatures between 140 and 160°C. DCOO lifetimes under steady state catalytic conditions in CO2:D2 atmospheres were measured by 12C – 13C isotope transients (SSITKA). The values range from 220s at 160°C to 660s at 140°C. The catalytic rates of both reverse water gas shift (RWGS) and methanol synthesis are ~100-fold slower than this formate removal rate back to CO2+1/2 H2, and thus they do not significantly influence the formate lifetime or coverage at steady state. The formate coverage is instead determined by formate’s rapid production / decomposition equilibrium with gas phase CO2+H2. The results are consistent with formate being an intermediate in methanol synthesis, but with the rate-controlling step being after formate production (for example, its further hydrogenation to methoxy). A 2-3 fold shorter life time (faster decomposition rate) was observed for formate under reactions conditions when both D2 and CO2 are present than in pure Ar or D2+Ar alone, attributed to effects of coadsorbates (produced in D2 and CO2) on adsorbed formate reaction pathways. The carbon which appears in the methanol product spends a longer time on the surface than the formate species, 1.8 times as long at 140°C. The additional delay on the surface is attributed in part to readsorption of methanol on the catalyst, thus obscuring the mechanistic link between formate and methanol.
2009. "SO2-induced stability of Ag-alumina catalysts in the SCR of NO with methane." Applied Catalysis. B, Environmental 88(1-2):98-105. Abstract We report on a stabilization effect on the structure and activity of Ag/Al2O3 for the selective catalytic reduction (SCR) of NOx with CH4 imparted by the presence of SO2 in the exhaust gasmixture. The reaction is carried out at temperature above 600 8C to keep the surface partially free of sulfates. In SO2-free gases, catalyst deactivation is fast and measurable at these temperatures. Time-resolved TEM analyses of used samples have determined that deactivation is due to sintering of silver from well-dispersed clusters to nanoparticles to micrometer-size particles with time-on-stream at 625 8C. However, sintering of silver was dramatically suppressed by the presence of SO2 in the reaction gas mixture. The structural stabilization by SO2 was accompanied by stable catalyst activity for the NO reduction to N2. The direct oxidation of methane was suppressed, thus the methane selectivity was improved in SO2-laden gas mixtures. In tests with high-content silver alumina with some of the silver present in metallic form, an increase in the SCR activity was found in SO2-containing gas mixtures. This is attributed to redispersion of the silver particles by SO2, an unexpected finding. The catalyst performance was reversible over many cycles of operation at 625 8C with the SO2 switched on and off in the gas mixture.
2009. "First-principles Analysis of NOx Adsorption on Anhydrous γ-Al2O3 Surfaces." Journal of Physical Chemistry C 113(18):7779-7789. doi:10.1021/jp8103563 Abstract The interaction of nitrogen oxides NOx (x=1-3) with gamma Al2O3 has been investigated using first-principles density functional theory calculations. NO and NO2 weakly physisorb on the clean, dehydrated (100) and (110) surfaces of gamma Al2O3, whereas the adsorption of the NO3 radical is rather strong. Only the basic-like O-down adsorption configurations were found to be stable. The interaction between NOx and gamma Al2O3 can be described as a surface mediated electron transfer process. For single NOx adsorption, greater electron transfer from the surface to the adsorbate (negatively charged) yields stronger interactions between NOx and the surface. The adsorption of four combinations of NOx+NOy (x=1-3, y=2, 3) pairs on the (100) and the (110) facets of gamma Al2O3 were investigated. Except for the NO2+NO2 pair, a strong cooperative effect that substantially enhances the stability of NOx on both gamma Al2O3 surfaces was found. This cooperative effect consists of surface-mediated electron transfer processes resulting in a favorable electrostatic interaction between two adsorbed NOx species. The pair was found to be the thermodynamically most stable state among the co-adsorbed NOx+NOy pairs on both gamma Al2O3 surfaces. The results are used to analyze the experimentally observed NOx evolution during temperature programmed desorption from NO2-saturated gamma Al2O3 substrates. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2009. "Characterization of Surface and Bulk Nitrates of γ-Al2O3-Supported Alkaline Earth Oxides using Density Functional Theory." Physical Chemistry Chemical Physics. PCCP 11(18):3380-3389. doi:10.1039/b819347a Abstract “Surface" and "bulk" nitrates formed on a series of alkaline earth oxides (AEOs), AE(NO3)2, were investigated using first-principles density functional theory calculations. The formation of these surface and bulk nitrates was modeled by the adsorption of NO2+NO3 pairs on gamma-Al2O3-supported monomeric AEOs (MgO, CaO, SrO, and BaO) and on the extended AEO(001) surfaces, respectively. The calculated vibrational frequencies of the surface and bulk nitrates based on our proposed models are in good agreement with experimental measurements of AEO/gamma-Al2O3 materials after prolonged NO2 exposure. This indicates that experimentally observed "surface" nitrates are most likely formed with isolated two dimensional (including monomeric) AEO clusters on the gamma-Al2O3 substrate, while the "bulk" nitrates are formed on exposed (including (001)) surfaces (and likely in the bulk as well) of large three dimensional AEO particles supported on the gamma-Al2O3 substrate. Also in line with the experiments, our calculations show that the low and high frequency components of the vibrations for both surface and bulk nitrates are systematically red shifted with the increasing basicity and cationic size of the AEOs. The adsorption strengths of NO2+NO3 pairs are nearly the same for the series of alumina-supported monomeric AEOs, while the adsorption strengths of NO2+NO3 pairs on the AEO surfaces increase in the order of MgO < CaO < SrO ~ BaO. Compared to the NO2+NO3 pair that only interacts with monomeric AEOs, the stability of NO2+NO3 pairs that interact with both the monomeric AEO and the gamma-Al2O3 substrate is enhanced by about 0.5 eV. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2009. "Understanding the nature of surface nitrates in BaO/gamma-Al2O3 NOx storage materials: A combined experimental and theoretical study ." Journal of Catalysis 261(1):17-22. Abstract The special role of the interface between the active catalytic phase (metal or metal oxide) and the oxide support in determining the properties of practical catalysts has long been recognized; however, it is still very poorly understood in most systems
2009. "Coordinatively unsaturated Al3+ centers as binding sites for active catalyst phases on γ-Al2O3." Science 325(5948):1670-1673. doi:10.1126/science.1176745 Abstract A combination of ultrahigh resolution spectroscopy and microscopy techniques (ultrahigh magnetic field solid state magic angle spinning nuclear magnetic resonance (MAS-NMR) and high-resolution scanning transmission electron microscopy (HR-STEM)) coupled with first principles DFT calculations reveal the nature of anchoring sites of a catalytically active phase onto the surface of γ-Al2O3. The results obtained unambiguously prove that coordinatively unsaturated penta-coordinate Al3+ (Al3+penta) centers present on the (100) facets of the γ-Al2O3 surface are the sites where the anchoring of Pt occurs. At low loadings, the active catalytic phase is atomically dispersed on the support surface (Pt/ Al3+penta=1), while two dimensional Pt rafts form at higher coverages.
2009. "Effects of sulfation level on the desulfation behavior of pre-sulfated Pt BaO/Al2O3 lean NOx trap catalysts: a combined H2 Temperature-Programmed Reaction, in-situ sulfur K-edge X-ray Absorption Near-Edge Spectroscopy, X-ray Photoelectron Spectroscopy, and Time-Resolved X-ray Diffraction Study." Journal of Physical Chemistry C 113(17):7336-7341. doi:10.1021/jp900304h Abstract Desulfation by hydrogen of pre-sulfated Pt(2wt%) BaO(20wt%)/Al2O3 with various sulfur loading (S/Ba = 0.12, 0.31 and 0.62) were investigated by combining H2 temperature programmed reaction (TPRX), x-ray photoelectron spectroscopy (XPS), in-situ sulfur K-edge x-ray absorption near-edge spectroscopy (XANES), and synchrotron time-resolved x-ray diffraction (TR-XRD) techniques. We find that the amount of H2S desorbed during the desulfation in the H2 TPRX experiments is not proportional to the amount of initial sulfur loading. The results of both in-situ sulfur K-edge XANES and TR-XRD show that at low sulfur loadings, sulfates were transformed to a BaS phase and remained in the catalyst, rather than being removed as H2S. On the other hand, when the deposited sulfur level exceeded a certain threshold (at least S/Ba = 0.31) sulfates were reduced to form H2S, and the relative amount of the residual sulfide species in the catalyst was much less than at low sulfur loading. Unlike samples with high sulfur loading (e.g., S/Ba = 0.62), H2O did not promote the desulfation for the sample with S/Ba of 0.12, implying that the formed BaS species originating from the reduction of sulfates at low sulfur loading are more stable to hydrolysis. The results of this combined spectroscopy investigation provide clear evidence to show that sulfates at low sulfur loadings are less likely to be removed as H2S and have a greater tendency to be transformed to BaS on the material, leading to the conclusion that desulfation behavior of Pt BaO/Al2O3 lean NOx trap catalysts is markedly dependent on the sulfation levels.
2009. "Studies of the Active Sites for Methane Dehydroaromatization Using Ultrahigh-Field Solid-State Mo95 NMR Spectroscopy." Journal of Physical Chemistry C 113(7):2936-2942. doi:10.1021/jp8107914 Abstract Abstract It is found that the spin-lattice relaxation time, T1, corresponding to the surface exchanged molybdenum species in Mo/HZSM-5 catalysts is short, i.e., less than about 100ms at 21.1 T while the value of T1 for the crystallite MoO3 molecules is longer, i.e., about 30 s. Such a difference, more than two orders in magnitude, is utilized to differentiate the exchanged Mo species from the agglomerate MoO3 in Mo/HZSM-5 catalyst. An approximately linear correlation between the amount of exchanged species and the aromatics formation rate is obtained. This result significantly strengthens our prior conclusion that the exchanged Mo species are the active centers for the methane dehydroaromatization reaction on Mo/HZSM-5 catalysts (J. Am. Chem. Soc. 2008, 130, 3722-3723). Our results also suggest that one exchanged Mo atom anchors on two ion exchange sites and the exchanged Mo species on catalysts are possibly monomeric. Analyzing the linshapes obtained from both the 95Mo MAS and the static spectra indicates that the exchanged sites are heterogeneous, resulting in a significantly broadened MAS spectrum and essentially a featureless but nearly symmetric static lineshape for the exchanged Mo species. Furthermore, for crystallite MoO3 powder sample, the parameters related to the electric-field-gradient (EFG) tensor, the chemical shift anisotropy (CSA) and the three Euler angles required to align the CSA principal axis system with the quadrupolar principal axis system are determined by analyzing both the 95Mo MAS and the static spectra obtained at ultra-high field of 21.1 T. The new results obtained from this study on crystallite MoO3 powders should help to clarify some of the contradictions in prior literature reports from other groups. Key words: 95Mo NMR, MAS, relaxation, surface exchanged species, HZSM-5, electric-field-gradient (EFG), chemical shift anisotropy (CSA), active centers.
2009. "An Isotropic Chemical Shift-Chemical Shift Anisotropic Correlation Experiment Using Discrete Magic Angle Turning." Journal of Magnetic Resonance 198(1):105-110. Abstract An isotropic-anisotropic shift 2D correlation spectroscopy is introduced that combines the advantages of both magic angle turning (MAT) and magic angle hopping (MAH) technologies. In this new approach, denoted DMAT for "discrete magic angle turning", the sample rotates clockwise followed by an anticlockwise rotation of exactly the same amount with each rotation less or equal than 360 degrees but greater than 240 degrees, with the rotation speed being constant only for times related to the evolution dimension. This back and forth rotation is repeated and synchronized with a special radio frequency (RF) pulse sequence to produce an isotropic-anisotropic shift 2D correlation spectrum. For any spin-interaction of rank-2 such as chemical shift anisotropy, isotropic magnetic susceptibility interaction, and residual homonuclear dipolar interaction in biological fluid samples, the projection along the isotropic dimension is a high resolution spectrum. Since a less than 360 degrees sample rotation is involved, the design potentially allows for in situ control over physical parameters such as pressure, flow conditions, feed compositions, and temperature so that true in-situ NMR investigations can be carried out.
2008. "Direct Observation of the Active Center for Methane Dehydroaromatization Using an Ultrahigh Field 95Mo NMR Spectroscopy." Journal of the American Chemical Society 130(12):3722-3723. doi:10.1021/ja7110916 Abstract Direct conversion of methane to value-added chemicals remains a challenge from both scientific and industrial points of view. In 1993, Wang et al. reported that methane can be transformed into aromatics on Mo/HZSM-5 catalysts under non-oxidative condition.1 Although remarkable progress has been made in the studies of the methane dehydroaromatization (MDA) reaction since that time, the reaction mechanism is still being debated,2 mainly due to the lack of understanding of the active center on Mo/HZSM-5 catalysts.3 It has been hypothesized that molybdenum may migrate into zeolitic channels and anchor on Brönsted acid sites during the synthesis. However, it is unclear whether the active molybdenum species are small crystallites or isolated exchanged species.
2008. "Isotope effects in methanol synthesis and the reactivity of copper formates on a Cu/SiO2 catalyst." Catalysis Letters 125(3-4):201-208. doi:10.1007/s10562-008-9592-4 Abstract Here we investigate isotope effects on the catalytic methanol synthesis reaction and the reactivity of copper-bound formate species in CO2-H2 atmospheres on Cu/SiO2 catalysts by simultaneous IR and MS measurements, both steady-state and transient. Studies of isotopic variants (H/D, 12C/13C) reveal that bidentate formate dominates the copper surface at steady state. The steady-state formate coverages of HCOO (in 6 bar 3:1 H2:CO2) and DCOO (in D2:CO2) are similar and the steady-state formate coverages in both systems decrease by ~80% from 350 K to 550 K. Over the temperature range 413K – 553K, the steady-state methanol synthesis rate shows a weak H/D isotope effect (1.05 ± 0.05) with somewhat higher activation energies in H2:CO2 (79 kJ/mole) than D2:CO2 (71 kJ/mole) over the range 473K-553K. The reverse water gas shift (RWGS) rates are higher than methanol synthesis and also shows a weak positive H/D isotope effect with higher activation energy for H2/CO2 than D2/CO2 (108 vs. and 102 kJ/mole). The reactivity of the resulting formate species in 6 bar H2, 6 bar D2 and 6 bar Ar is strongly dominated by decomposition back to CO2 and H2. H2 and D2 exposure compared to Ar do not enhance the formate decomposition rate. The decomposition profiles on the supported catalyst deviate from first order decay, indicating distributed surface reactivity. The average decomposition rates are similar to values previously reported on single crystals. The average activation energies for formate decomposition are 90 ± 17 kJ/mole for HCOO and 119 ± 11 kJ/mole for DCOO. By contrast to the catalytic reaction rates, the formate decomposition rate shows a strong H/D kinetic isotope effect (H/D ~ 8 at 413K), similar to previously observed values on Cu(110).
2008. "NOx uptake on alkaline earth oxides (BaO, MgO, CaO and SrO) supported on γ-Al2O3." Catalysis Today 136(1-2):121-127. doi:doi:10.1016/j.cattod.2007.12.138 Abstract NOx uptake experiments were performed on a series of alkaline earth oxide (AEO) (MgO, CaO, SrO, BaO) on γ-alumina materials. Temperature Programmed Desorption (TPD) conducted on He flow revealed the presence of two kinds of nitrate species: i.e. bulk and surface nitrates. The ratio of these two types of nitrate species strongly depends on the nature of the alkaline earth oxide. The amount of bulk nitrate species increases with the basicity of the alkaline earth oxide. This conclusion was supported by the results of infrared and 15N solid state NMR studies of NO2 adsorption. Due to the low melting point of the precursor used for the preparation of MgO/Al2O3 material (Mg(NO3)2), a significant amount of Mg was lost during sample activation (high temperature annealing) resulting in a material with properties were very similar to that of the γ-Al2O3 support. The effect of water on the NOx species formed in the exposure of the AEO-s to NO2 was also investigated. In agreement with our previous findings for the BaO/γ-Al2O3 system, an increase of the bulk nitrate species and the simultaneous decrease of the surface nitrate phase were observed for all of these materials.
2008. "Adsorption and Formation of BaO Overlayers on Gamma-Al2O3 Surfaces ." Journal of Physical Chemistry C 112(46):18050–18060. doi:10.1021/jp806212z Abstract First-principles density functional theory slab calculations were used to investigate adsorption, clustering and overlayer formation of BaO on the gamma-Al2O3 surfaces. Multiple stable adsorption configurations were identified for the adsorbed BaO molecule and (BaO)2 on both (100) and (110) surfaces of gamma-Al2O3. Adsorption of BaO and (BaO)2 induces significant relaxation of the gamma-Al2O3 surfaces. At high BaO coverage, up to the ratio of BaO units to surface Al atoms being unity, the adsorbed BaO molecules were organized to form a buckled monolayer-like overlayer on the surface. Aggregation energy was used to characterize the organization of adsorbed BaO on the surface. Our results showed that the initial BaO adsorption configuration had a strong effect on clustering and overlayer formation. A weakly adsorbed BaO molecule will thermodynamically favor clustering over being isolated. On the fully dehydrated gamma-Al2O3(100) surface, the formation of BaO overlayer was thermodynamically unfavorable until 4.26 BaO/nm2 if the additional BaO was from the most stable site, corresponding to a low BaO loading, whereas aggregation became favorable if the additional BaO was from less stable sites, corresponding to a high BaO loading. On the fully dehydrated gamma-Al2O3(110) surface, the formation of a BaO dimer was found to have the highest energy cost. On the other hand, the presence of hydroxyls on the surface enhances the stability of the adsorbed BaO molecules. As such, isolated BaO islands, rather than a complete BaO overlayer, were expected on the hydroxylated gamma-Al2O3 surfaces, consistent with recent experimental observations. Pacific Northwest National Laboratory operated by Battelle for the U. S. Department of Energy.
2008. "The Role of PentaCoordinated Al3+ Ions in the High Temperature Phase Transformation of γ-Al2O3." Journal of Physical Chemistry C 112(25):9486–9492. doi:10.1021/jp802631u Abstract In this work, the structural stability of gamma-alumina (γ-Al2O3) was investigated by a combination of XRD and high resolution solid state 27Al MAS NMR at an ultra-high magnetic field of 21.1 tesla. XRD measurements show that γ-Al2O3 undergoes a phase transition to θ-Al2O3 during calcination at 1000oC for 10hr. The formation of the θ-Al2O3 phase is further confirmed by 27Al MAS NMR; additional 27Al peaks centered at 10.5 and ~78 ppm were observed in samples calcined at this high temperature. Both the XRD and NMR results indicate that, after calcination at 1000°C for 10 hrs, the ratio of the θ-Al2O3 phase to the total alumina in samples modified by either BaO or La2O3 is significantly reduced in comparison with γ-Al2O3. 27Al MAS NMR spectra revealed that the reduction in the extent of θ-Al2O3 formation was highly correlated with the reduction in the amount of penta-coordinated aluminum ions, measured after 500°C calcination, in both BaO- and La2O3-modified γ-Al2O3 samples. These results strongly suggest that the penta-coordinated aluminum ions, present exclusively on the surface of γ-Al2O3, play a critical role in the phase transformation of γ-Al2O3 to θ-Al2O3. The role of the modifiers, in our case BaO or La2O3, is to convert the penta-coordinated aluminum ions into octahedral ones, thereby improving the thermal stabilities of the samples. Oxide additives, on the other hand, had no beneficial effect on preventing the specific surface area reduction that occurred during high temperature (≤1000°C) calcination.
2008. "Excellent Sulfur Resistance of Pt/BaO/CeO2 Lean NOx Trap Catalysts." Applied Catalysis. B, Environmental 84(3-4):545-551. doi:10.1016/j.apcatb.2008.05.009 Abstract In this work, we investigated the NOx storage behavior of Pt-BaO/CeO2 catalysts, especially in the presence of SO2. High surface area CeO2 (~ 110 m2/g) with a rod like morphology was synthesized and used as a support. The Pt-BaO/CeO2 sample demonstrated slightly higher NOx conversion in the entire temperature range studied compared with Pt-BaO/γ-Al2O3. More importantly, this ceria-based catalyst showed higher sulfur tolerance than the alumina-based one. The time of complete NOx uptake was maintained even after exposing the sample to ~3 g/L of SO2. The same sulfur exposure, on the other hand, eliminated the complete NOx uptake time on the alumina-based NOx storage catalysts. TEM images show no evidence of either Pt sintering or BaS phase formation during reductive de-sulfation up to 600°C on the ceria based catalyst, while the same process over the alumina-based catalyst resulted in both a significant increase in the average Pt cluster size and the agglomeration of a newly-formed BaS phase into large crystallites. XPS results revealed the presence of about 5 times more residual sulfur after reductive de-sulfation at 600°C on the alumina based catalysts in comparison with the ceria-based ones. All of these results strongly support that, besides their superior intrinsic NOx uptake properties, ceria based catalysts have a) much higher sulfur tolerance and b) excellent resistance against Pt sintering when they are compared to the widely used alumina based catalysts.
2008. "Sequential high temperature reduction, low temperature hydrolysis for the regeneration of sulfated NOx trap catalysts." Catalysis Today 136(1-2):183-187. doi:doi:10.1016/j.cattod.2007.12.134 Abstract We describe a new method that minimizes irreversible Pt sintering during the desulfation of sulfated Pt/BaO/Al2O3 lean NOx trap (LNT) catalysts. While it is known that the addition of H2O to H2 promotes desulfation, we find that the significant and irreversible Pt sintering arising from the presence of water is unavoidable. Control of precious metal sintering is considered to be one of the critical issues in the development of durable LNT catalysts. The new method described here is a sequential desulfation process: the first step is to reduce the sulfates with hydrogen only at higher temperatures to form BaS, followed by a treatment of the thus reduced sample with water at low to moderate temperatures to convert BaS to BaO and H2S. The data showed that Pt sintering was significantly inhibited due to the absence of H2O during the desulfation at high temperatures, and also demonstrates the similar NOx uptake with the desulfated sample cooperatively with H2 and H2O. Therefore, the sequential desulfation process may find applications in realistic systems to inhibit the irreversible sintering of the Pt in the lean NOx trap catalyst, leading to a longer catalyst life.
2008. "Roles of Pt and BaO in the Sulfation of Pt/BaO/Al2O3 Lean NOx Trap Materials: Sulfur K-edge XANES and Pt LIII XAFS Studies." Journal of Physical Chemistry C 112(8):2981-2987. doi:10.1021/jp077563i Abstract The roles of barium oxide and platinum during the sulfation of Pt-BaO/Al2O3 lean NOx trap catalysts were investigated by S K edge XANES (X-ray absorption near-edge spectroscopy) and Pt LIII XAFS (X-ray absorption fine structure). All of the samples studied (Al2O3, BaO/Al2O3, Pt/Al2O3 and Pt-BaO/Al2O3) were pre-sulfated prior to the X-ray absorption measurements. It was found that barium oxide itself has the ability to directly form barium sulfate even in the absence of Pt and gas phase oxygen. In the platinum-containing samples, the presence of Pt-O species plays an important role in the formation of sulfate species. Even if barium and aluminum sites are available for SO2 to form sulfate, for the case of the BaO(8)/Al2O3 sample, where the barium coverage is about 0.26 ML, S XANES spectroscopy results show that barium sulfates are preferentially produced over aluminum sulfates . When oxygen is absent from the gas phase, the sulfation route that involves Pt-O is eliminated after the initially present Pt-O species are completely consumed. In this case, formation of sulfates is suppressed unless barium oxide is also present. Pt LIII XAFS results show that the first coordination sphere around the Pt atoms in the Pt particles is dependent upon the redox nature of the gas mixture used during the sulfation process. Sulfation under reducing environments (e.g. SO2+H2) leads to formation of Pt-S bonds, while oxidizing conditions (e.g. SO2+O2) continue to show the presence of Pt-O bonds. In addition, the former condition was found to give rise to a higher degree of Pt sintering than the latter one. This result explains why samples sulfated under reducing conditions had lower NOx uptakes than those sulfated under oxidizing conditions. Therefore, our results provide needed information for the development of optimum practical operation conditions (e.g. sulfation or desulfation) for lean NOx trap catalysts that minimize deactivation by sulfur.
2008. "Promotional Effects of H2O Treatment on NOx Storage over Fresh and Thermally Aged Pt-BaO/Al2O3 Lean NOx Trap Catalysts ." Catalysis Letters 124(1-2):39-45. doi:10.1007/s10562-008-9505-6 Abstract A simple liquid water treatment applied to fresh and thermally aged Pt(2wt%)-BaO(20wt%)/Al2O3 lean NOx trap catalysts at room temperature induces morphological and structural changes in the barium species as followed by XRD and TEM analysis. During the water treatment, liquid water sufficient to fill the catalyst pore volume is brought into contact with the samples. It was found that irrespective of the original barium chemical state (highly dispersed BaO or crystalline BaAl2O4), exposing the sample to this liquid water treatment promotes the formation of BaCO3 crystallites (about 15 – 25 nm of its size) without changing the Pt particle size. Such transformations of the barium species are found to significantly promote NOx uptake from 250 °C to 450 °C. The increase in the NOx uptake for the water-treated samples can be attributed to an enhanced Pt-Ba interaction through the redistribution of barium species. These results provide useful information for the regeneration of aged lean NOx trap catalysts since water is plentiful in the exhaust of diesel or lean-burn engines.
2008. "Effects of novel supports on the physical and catalytic properties of tungstophosphoric acid for alcohol dehydration reactions." Topics in Catalysis 49(3-4):259-267. doi:10.1007/s11244-008-9081-4 Abstract The catalytic behavior of tungstophosphoric acid supported on modified mesoporous silica materials for the dehydration of 2-butanol and methanol was studied. Specifically, the supports evaluated here consisted of unmodified MCM-41 and SBA-15 mesoporous silicas, and these materials coated with sub-monolayer quantities of alumina, titania, and zirconia. UV-Vis DRS and 31P-NMR spectroscopy showed that the tungstophosphoric acid species retained their chemical identity in the synthesized supported form, although the spectra were influenced by the specific support material used. In addition, their acidic properties were evaluated using temperature programmed oxidation of isopropyl amine. The differences in reaction rates between the samples reflect both the diversity in the amount of Brønsted acidic sites available for catalysis and dissimilarities in coking resistance. These two characteristics depend, in turn, on the type of support modifier used to prepare the catalyst.
2008. "Preface." Catalysis Today 136(1-2):1-2. doi:10.1016/j.cattod.2008.03.008 Abstract This manuscript is a preface to a special issue of the journal, Catalysis Today, on catalytic diesel emission control edited by the preface authors.
2008. "Carbonate Formation and Stability on a Pt/BaO/γ-Al2O3 NOx Storage/Reduction Catalyst." Journal of Physical Chemistry C 112(29):10952-10959. doi:10.1021/jp712180q Abstract There has been recent debate regarding the role or influence of BaCO3 species on the performance or operation of Pt/BaO/Al2O3 model NOx storage/reduction (NSR) catalysts. This influence is primarily regarded as negative, but the extent of its impact is not clear. For this reason, the formation and stability of barium carbonate species on a Pt/BaO/Al2O3 model NSR catalyst were characterized using Fourier transform infra-red (FTIR) spectroscopy. The catalyst sample was exposed to CO2, CO and CO + O2 at various temperatures, from 300K to >500K. Bidentate carbonate species readily form under all conditions while, at higher temperatures, unidentate species were also observed and likely formed from bidentate species as a result of a change in their coordination to the oxide surface. Reaction of COx species with residual hydroxide species on the catalyst led to the formation of bicarbonates, and when the sample was exposed to CO at low temperature, formate species were also formed. These formate species decomposed at elevated temperatures and contributed to the formation of carbonates. H2O exposure resulted in the agglomeration of various COx-containing phases to larger particles.
2008. "Propanal synthesis from aqueous propylene glycol/hydrogen peroxide on a Ru/alumina catalyst." Inorganic Chemistry Communications 11(5):561-563. doi:doi:10.1016/j.inoche.2008.01.024 Abstract The conversion of polyol materials, including 1,2-diols, into higher commodity chemicals is actively being pursued by many researchers. Here we report the production of propanal from propylene glycol and hydrogen peroxide using a Ru/alumina catalyst. Experiments were conducted by adding up to four peroxide equivalents under steady-state reflux conditions at 371 K. The product propanal and its subsequent reaction product with substrate, 1,3-dioxolane-2-ethyl-4-methyl, was observed to be an intermediate achieving a maximum concentration of 3% of substrate. Buffering using Mg(OH)2 at pH~10 resulted in propanal formation, whereas buffering at similar pH using Na2HSO4 did not, from which we propose that magnesium acts as a promoter in the reaction. The mechanism appears to be a dehydration to enol, followed by rearrangement to product. Experiments utilizing Ru/carbon did not yield any propanol suggesting that the acidic sites of alumina aid the dehydration reaction. To our knowledge, this represents the first time hydrogen peroxide has been used in an alcohol dehydration reaction.
2008. "NMR Studies of Cu/zeolite SCR Catalysts Hydrothermally Aged with Urea." Catalysis Today 136(1-2):34-39. doi:10.1016/j.cattod.2008.01.019 Abstract The effects of hydrothermal aging of Cu/zeolite urea-SCR catalysts on their reactivity and material properties was assessed by performance tests and multiple characterization techniques that included 27Al NMR and XRD. Three aging protocols were used that consisted of varying temperature during hydrothermal aging with or without exposure to aqueous urea solution. Differences in behavior were even found for samples hydrothermally aged immediately following exposure to the urea solution or if the sample was dried overnight before hydrothermal aging. The combination of urea and high temperature exposure increased the deactivation of Cu/zeolite SCR catalysts beyond that observed by hydrothermal aging alone, with an immediate high temperature exposure following wetting of the catalyst core with aqueous urea causing the most significant deterioration in performance. The impact of urea on SCR catalyst durability was also found to increase with the aging temperature. NMR analysis suggested that aging with urea resulted in relatively more dealumination of the zeolite for the SCR catalysts in this study.
2007. "Understanding Practical Catalysts Using a Surface Science Approach: The Importance of Strong Interaction between BaO and Al2O3 in NOx Storage Materials." Journal of Physical Chemistry C 111(41):14942-14944. doi:10.1021/jp0763376 Abstract Modern surface science techniques have been commonly applied to understand issues arising from practical catalytic systems.[1-4] However, the applicability of most of the results obtained from model systems has been limited, due, primarily, to the vastly different conditions studies on model and practical systems are carried out (catalyst composition, reaction conditions etc.).[5, 6] Therefore, the need to conduct experiments on compositionally similar systems (model and practical) is necessary to obtain valuable information on the workings of real catalysts. In this communication we demonstrate the utility of surface science studies on model catalysts in understanding the properties of high surface area, BaO-based NOx storage-reduction (NSR) catalysts.[7] We present evidence for the facile formation of surface barium aluminate-like species even at very low coverages of BaO. This Ba-aluminate layer, however, can react with NO2 resulting in the formation of a bulk-like Ba(NO3)2 phase. In order to construct model catalysts that are representative of the practical NOx storage systems, we first needed to estimate the BaO covareges on the high surface area catalysts. Since the publication of the work by Fanson et al.[8], BaO loadings of 8 – 10 wt.% on a γ-alumina support (200 m2/g) have been regarded as corresponding to one monolayer (ML) coverage, based on the unit cell size of bulk BaO. The coverage equivalent of one ML, however, was significantly underestimated. Assuming complete spreading of the BaO layer and using a Ba–O distance of ~ 2.77 Å (one unit of BaO occupies 1.53 × 10-19 m2), 10 wt.% loading of BaO would cover only about 1/3 of the alumina surface. Table 1 shows our calculated estimates of two-dimensional BaO coverages as a function of loading on a -Al2O3 surface (200 m2/g) based on the lattice parameters of bulk BaO[9] (5.54 Å). Based on these values, for our model system studies we prepared BaO/Al2O3/NiAl(110) materials in which the BaO coverages were very close to those of 4, 8, and 20 wt.% BaO/γ-Al2O3 high surface area catalysts used in prior studies.
2007. "Grafting Sulfated Zirconia on Mesoporous Silica." Green chemistry: an international journal and green chemistry resource:GC 9(6):540-544. doi:10.1039/b614928f Abstract Sulfated zirconia has received considerable attention as a potential solid acid catalyst in recent years. In this paper, the preparation and properties of acid catalysts obtained by grafting ziconia with atomic precision on MCM-41 mesoporous silica were studied. TEM and potential titration characterizations revealed that ZrO2/MCM-41 with monolayer coverage can be obtained using this grafting technique. Sulfated ZrO2/MCM-41 exhibits improved thermal stability than that of bulk sulfated zirconia, as evidenced by temperature programmed characterizations and XRD analysis. Temperature programmed reaction of isopropanol was used to evaluate the acidity of sulfated ZrO2/MCM-41. It was found that the acid strength of sulfated ZrO2/MCM-41 with monolayer coverage is weaker than bulk sulfated zirconia but stronger than SiO2-Al2O3, a common strong acid catalyst.
2007. "Water-induced morphology changes in BaO/γ-Al2O3 NOx storage materials: an FTIR, TPD, and time-resolved synchrotron XRD study." Journal of Physical Chemistry C 111(12):4678-4687. doi:10.1021/jp067932v Abstract The effect of water on the morphology of BaO/Al2O3-based NOx storage materials was investigated using Fourier transform infrared spectroscopy, temperature programmed desorption, and time-resolved synchrotron X-ray diffraction techniques. The results of this multi-spectroscopy study reveal that, in the presence of water, surface Ba-nitrates convert to bulk nitrates, and water facilitates the formation of large Ba(NO3)2 particles. This process is completely reversible, i.e. after the removal of water from the storage material a significant fraction of the bulk nitrates re-convert to surface nitrates. NO2 exposure of a H2O-containing (wet) BaO/Al2O3 sample results in the formation of nitrites and bulk nitrates exclusively, i.e. no surface nitrates form. After further exposure to NO2, the nitrites completely convert to bulk nitrates. The amount of NOx taken up by the storage material is, however, essentially unaffected by the presence of water, regardless of whether the water was dosed prior to or after NO2 exposure. Based on the results of this study we are now able to explain most of the observations reported in the literature on the effect of water on NOx uptake on similar storage materials.
2007. "Water-induced morphology changes in BaO/γ-Al2O3 NOx storage materials." Chemical Communications 2007(9):984-986. doi:10.1039/b613674e Abstract Exposure of NO2-saturated BaO/γ-Al2O3 NOx storage materials to H2O vapour results in the conversion of surface nitrates to Ba(NO3)2 crystallites, causing dramatic morphological changes in the Ba-containing phase, demonstrating a role for water in affecting the NOx storage/reduction properties of these materials.
2007. "The effect of H2O on the adsorption of NO2 on γ-Al2O3: an in situ FTIR/MS study." Journal of Physical Chemistry C 111(6):2661-2669. doi:10.1021/jp066326x Abstract The effect of water on the adsorption of NO2 onto a γ-Al2O3 catalyst support surface was investigated using Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS). Upon room temperature exposure of the alumina surface to small amounts of NO2, nitrites and nitrates are formed, and at higher NO2 doses only nitrates are observed. The surface nitrates formed were of bridging monodentate, bridging bidentate, and monodentate configuration. At elevated NO2 pressures, the surface hydroxyls were consumed in their reaction with NO2 giving primarily bridge-bound nitrates. A significant amount of weakly adsorbed N2O3 was seen as well. Exposure of the NO2-saturated γ-Al2O3 surface to H2O resulted in the desorption of some NO2 + NO as H2O interacted with the weakly-held N2O3, while the bridging monodentate surface nitrates converted into monodentate nitrates. The conversion of these oxide-bound nitrates to water-solvated nitrates was observed at high water doses when the presence of liquid-like water is expected on the surface. The addition of H2O to the NO2-saturated γ-Al2O3 did not affect the amount of NOx strongly adsorbed on the support surface. In particular, no NOx desorption was observed when the NO2-saturated sample was heated to 573K prior to room temperature H2O exposure. The effect of water is completely reversible; i.e., during TPD experiments following NO2 and H2O coadsorption, the same IR spectra were observed at temperatures above that required for H2O desorption as seen for NO2 adsorption only experiments.
2007. "Penta-coordinated Al3+ ions as preferential nucleation sites for BaO on γ-Al2O3: an ultra-high magnetic field 27Al MAS NMR study." Journal of Catalysis 251(2):189-194. doi:10.1016/j.jcat.2007.06.029 Abstract In this paper, we report the first observation of preferential anchoring of an impregnated catalytic phase onto penta-coordinated Al3+ sites on the surface of γ Al2O3. The interaction of barium oxide with a γ alumina support was investigated by high resolution solid state 27Al magic angle spinning NMR at an ultra-high magnetic field of 21.1T and at sample spinning rates of up to 23 kHz. Under these experimental conditions, a peak in the NMR spectrum at ~ 23 ppm with relatively low intensity, assigned to 5-coordinated Al3+ ions, is clearly distinguished from the two other peaks representing Al3+ ions in tetra-, and octahedral coordination sites. Spin-lattice 27Al relaxation time measurements clearly show that these penta-coordinated Al3+ sites are located on the surface of the γ alumina support. BaO deposition onto this γ alumina sample resulted in the loss of intensity of the 23 ppm peak. The intensity loss observed was linearly proportional to the amount of BaO deposited. The results of this study strongly suggest that, at least for BaO, these penta-coordinated Al3+ ions are the nucleation sites.
2007. "Water-induced Bulk Ba(NO3)2 Formation From NO2 Exposed Thermally Aged BaO/Al2O3." Applied Catalysis. B, Environmental 72(3-4):233-239. Abstract Phase changes in high temperature treated (> 900 °C) 8 or 20 wt% BaO supported on Al2O3 model lean NOx trap (LNT) catalysts, induced by NO2 and/or H2O adsorption, were investigated with powder X-ray Diffraction (XRD), solid state 27Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy, and NO2 Temperature Programmed Desorption (TPD) experiments. After calcination in dry air at 1000 °C, the XRD and solid state 27Al MAS NMR results confirm that stable surface BaO and bulk BaAl2O4 phases are formed for 8 and 20 wt% BaO/Al2O3, respectively. Following NO2 adsorption over these thermally treated samples, no additional phase changes are observed based on XRD results. However, when water was added to the thermally aged samples after NO2 exposure, the formation of crystalline Ba(NO3)2 particles was observed in both samples. Solid state 27Al MAS NMR is shown to be a good technique for identifying the various Al species present in the materials during the processes studied here. NO2 TPD results demonstrate a significant loss of uptake for the 20 wt% model catalysts upon thermal treatment. However, the described phase transformations upon subsequent water treatment gave rise to the partial recovery of NOx uptake, demonstrating that such a water treatment of thermally aged catalysts can provide a potential method to regenerate LNT materials.
2007. "Design of a reaction protocol for decoupling sulfur removal and thermal aging effects during desulfation of Pt-BaO/Al2O3 lean NOx trap catalysts." Industrial and Engineering Chemistry Research 46(9):2735-2740. doi:10.1021/ie061542d Abstract A novel reaction protocol was designed to decouple the effects of thermal deactivation from those due to, for example, incomplete de-sulfation during regeneration steps of Ba-based lean NOx trap catalysts. The protocol was applied to two samples: a Pt-BaO/Al2O3 model catalyst, and an enhanced model sample doped with promoter species. The results obtained from the reaction protocol demonstrate that regeneration (desulfation) temperatures need to be maintained below those that lead to significant Pt sintering in order to prevent permanent deactivation. In addition, the modified reaction protocol allows us to compare the regeneration behavior of samples with varying degrees of sulfation, while other approaches have difficulty differentiating the effects of thermal aging from those of sulfation. We believe that this approach provides a convenient way both to assess the relative sensitivities of various catalysts to regeneration conditions, and to develop regeneration strategies that minimize the separate but often linked deactivation effects of sulfation and high temperatures.
2007. "Reply to comment on "CO oxidation on ruthenium: the nature of the active catalytic surface" by H. Over, M. Muhler, and A.P. Seitsone." Surface Science 601(23):5663–5665. doi:10.1016/j.susc.2007.09.042 Abstract We first state that the premise of our Letter [1] was to state emphatically that the early studies of Peden and Goodman (PG) [2] can be entirely explained by and are only consistent with the active surface being a monolayer oxygen-covered Ru(0001) surface. Contrary to the authors’ contention in their Comment, the reaction conditions addressed in the experiments of PG [2] spanned the temperature range 375 to 600 K, a pressure range of O2 and CO pressures from 0.5 to 500 Torr, and CO/O2 ratios from 60 to .03. The authors state in their introduction that “the oxidation of Ru metal takes place to RuO2 only for temperatures higher than 500 K. Here we do not wish to address the oxidation of Ru in all morphologies, but rather the specific reaction conditions required for RuO2 to form on Ru(0001), the catalyst used in the experiments of PG and the subject of our previous Letter [1]. Indeed, as pointed out in our Letter [1], Over and coworkers in a very definitive piece of work [3] showed that the critical conditions required for the formation of RuO2 on Ru(0001) were far outside those used in the experiments of PG with respect to temperature and oxygen chemical potential. Based on our earlier studies [2] and the work of Over and coworkers [3] we concluded in our Letter that RuO2 could not have formed to any appreciable extent in the PG experiments.
2007. "CO oxidation on ruthenium: The nature of the active catalytic surface." Surface Science 601:L124 - L126. doi:10.1016/j.susc.2007.08.003 Abstract CO oxidation over ruthenium has been extensively studied over the last several decades from ultrahigh (UHV) vacuum to ambient pressures. While Ru is an inferior catalyst for this reaction compared to Pt, Rh, and Pd under ultrahigh vacuum conditions, it is comparable to these metals at or near atmospheric reaction conditions. Recent studies have suggested that the transformation from an inactive to an active catalyst can be attributed to a structural transformation of Ru to RuO2 and that an epitaxial film of RuO2(110) on Ru(0001) is the active catalytic surface under stoichiometric CO/O2 reaction condition at or approaching one atmosphere. Furthermore recent experimental and theoretical studies have suggested that under such elevated reactant pressures, a strongly bound CO (120 kJ/mol) on RuO2 reacts with O atoms from the oxide surface to form the CO2 product. However, that the active surface on Ru(0001) is a multilayer RuO2(110) oxide and that the CO reactant is strongly bound is inconsistent with early elevated pressure studies of CO oxidation on Ru showing: (i) the formation of a single surface oxide layer under elevated pressure reactant conditions; (ii) a weakly bound CO (<40 kJ/mol) is the reactant under elevated pressure reactant conditions; and (iii) the formation of RuO2 deactivates Ru as a catalyst. This article reviews CO oxidation over Ru for the last several decades emphasizing those aspects that rationalize the connection between the vacuum and high pressure results.
2007. "Oxidation of Ethanol to Acetaldehyde over Na-promoted vanadium oxide catalysts." Applied Catalysis. A, General 332(2):263-272. doi:10.1016/j.apcata.2007.08.024 Abstract Sodium-promoted vanadium oxide catalysts supported on MCM-41 and TiO2 (anatase) were investigated for the partial oxidation of ethanol to acetaldehyde. The catalysts were prepared by incipient wetness impregnation with a vanadium oxide content of 6 wt. %. The experimental characterization was performed by X-ray diffraction (XRD), N2 adsorption, temperature programmed reduction (TPR), and diffuse reflectance UV-Vis. Temperature programmed oxidation (TPO) was also used to identify carbon deposits on the spent catalysts. The presence of sodium plays a strong role in the dispersion and reducibility of the vanadium species as detected by TPR analysis and optical absorption spectroscopy. While sodium addition increases the dispersion of the VOx species, its presence also decreases their reducibility. Additionally, TPO of the spent catalysts revealed that an increase in the Na loading decreases the carbon deposition during reaction. In the case of the catalysts supported on MCM-41, these modifications were mirrored by a change in the activity and selectivity to acetaldehyde. Additionally, on the VOx/TiO2 catalysts the catalytic activity decreased with increasing sodium content in the catalyst. A model in which sodium affects dispersion, reducibility and also acidity of the supported-vanadia species is proposed to explain all these observations.
2006. "Fractional factorial study of HCN removal over a 0.5% Pt/Al₂O₃ catalyst: effects of temperature, gas flow rate, and reactant partial pressure ." Industrial and Engineering Chemistry Research 45(3):934-939. doi:10.21/ie048777e Abstract Fractional factorial design was used to determine which factors have significant effects on the HCN (hydrogen cyanide) oxidation reaction over 0.5% Pt/Al₂O₃ under lean conditions. We conclude that the reaction temperature and gas-hourly space velocity (GHSV) have significant effects on the HCN conversion, while no significant effects are caused by the presence of either NO (nitric oxide) or C₃H₆ (propene). A central composite design was used to study the effects of temperature and GHSV on HCN conversion, C₃H₆ conversion and NOx selectivity. Based on a second polynomial equation model, regression analysis was used to study the significance of each variable term and derive equations for each response. Our results show that HCN conversion was significantly affected by temperature (X3), GHSV (X4), a temperature polynomial term (X32), and a temperature and GHSV interaction term (X3X4). HCN conversion decreased with increasing values of GHSV and increased with increasing temperature, up to a transition temperature that depends on the GHSV value. The variables of temperature (X3), GHSV (X4), and the temperature polynomial term (X32) have significant effects on both C₃H₆ conversion and NOx selectivity, but in these two cases the interaction of temperature and GHSV was not significant. Contour plots of HCN conversion, C₃H₆ conversion, and NOx selectivity versus temperature and GHSV were constructed from an analysis of the measured data, and these plots can be utilized to estimate HCN conversion, C₃H₆ conversion, and NOx selectivity over the range of temperatures and GHSV investigated. Optimum catalyst operation is described by high HCN conversion and low NOx selectivity. These results show C and o that the highest HCN conversion was achieved at temperatures above 250 relatively low GHSV values, while low NOx selectivity was best achieved at a C.o temperature of 215
2006. "Catalytic oxidation of HCN over a 0.5% Pt/Al2O3 catalyst." Applied Catalysis. B, Environmental 65(2006):282-290. doi:10.1016/j.apcatb.2006.02.009 Abstract The adsorption of HCN on, its catalytic oxidation with 6% O2 over 0.5% Pt/Al2O3, and the subsequent oxidation of strongly bound chemisorbed species upon heating were investigated. The observed N-containing products were N2O, NO and NO2, and some residual adsorbed N-containing species were oxidized to NO and NO2 during subsequent temperature programmed oxidation. Because N-atom balance could not be obtained after accounting for the quantities of each of these product species, we propose that N2 and was formed. Both the HCN conversion and the selectivity towards different N-containing products depend strongly on the reaction temperature and the composition of the reactant gas mixture. In particular, total HCN conversion reaches 95% above 250 C. Furthermore, the temperature of maximum HCN conversion to N2O is located between 200 and 250 C, while raising the reaction temperature increases the proportion of NOx in the products. The co-feeding of H2O and C3H6 had little, if any effect on the total HCN conversion, but C3H6 addition did increase the conversion to NO and decrease the conversion to NO2, perhaps due to the competing presence of adsorbed fragments of reductive C3H6. Evidence is also presented that introduction of NO and NO2 into the reactant gas mixture resulted in additional reaction pathways between these NOx species and HCN that provide for lean-NOx reduction coincident with HCN oxidation.
2006. "Design and operating characteristics of a transient kinetic analysis catalysis reactor system employing in situ transmission Fourier transform infrared." Review of Scientific Instruments 77(9):Art. No. 094104. Abstract A novel apparatus for gas-phase heterogeneous catalysis kinetics is described. The apparatus enables fast isotopic transient kinetic analysis (ITKA) to be performed in which both the gaseous and adsorbed species inside the catalytic reactor are monitored simultaneously with rapid-scan transmission FTIR, and its gaseous effluent can be monitored by mass spectroscopy during rapidly switching of reagent gas streams. This enables a more powerful version of the well-known steady-state isotopic kinetic analysis (SSITKA) technique in which the vibrational spectra of the gas phase and adsorbed species are also probed: FTIR-SSITKA. Unique reactor characteristics include tungsten construction, liquid nitrogen cooling or heating (~200-700 K), fast reactor disassembly and reassembly, and catalyst loading in a common volume. The FTIR data acquisition rate of this apparatus (3 Hz) is 10-fold faster than previously reported instruments. A 95% signal decay time of ~3 seconds for gas switching was measured. Very good temperature reproducibility and uniformity (< ±3K) was observed by in-situ rotational temperature analysis, which allows accurate calibration of the reactor thermocouple to the reactor gas temperature. Finally, FTIR-SSITKA capabilities are demonstrated for CO2 isotope switching over a -alumina sample at 75 C, which reveal an adsorbed carbonate species with an average surface residence time of =148±5 seconds and a coverage of ~2.5x1015 molecules cm-2.
2006. "Morphological Evolution of Ba(NO3)2 Supported on -Al2O3(0001): An In-Situ TEM Study." Journal of Physical Chemistry B 110(24):11878-11883. doi:10.1021/jp060235i Abstract One of the key questions for the BaO-based NOx catalyst system is the morphological evolution of Ba(NO3)2 to BaO upon heating for releasing of NOx or vice versa from BaO to Ba(NO3)2 upon uptaking of NOx. However, associated with the small crystallite size of high-surface area Al2O3, it can be difficult to extract structural and morphological features of Ba(NO3)2 supported on -Al2O3 by any direct imaging method including transmission electron microscopy. In this work, by choosing a model system of Ba(NO3)2 particles supported on single crystal -Al2O3, we have investigated the structural and morphological features of Ba(NO3)2 as well as the formation of BaO from Ba(NO3)2 during the release of NOx using ex-situ and in-situ TEM imaging, electron diffraction, energy dispersive spectroscopy (EDS), and Wulff shape construction. We find that Ba(NO3)2 supported on -Al2O3 possesses a platelet morphology, with the interface and facets being invariably the 8 {111} planes. Formation of the platelet structure leads to an enlarged interface area between Ba(NO3)2 and -Al2O3, indicating that the interfacial energy is lower than the Ba(NO3)2 surface free energy. In fact, Wulff shape constructions indicate that the interfacial energy is ~1/4 of the {111} surface free energy of Ba(NO3)2. The orientation relationship between Ba(NO3)2 and the -Al2O3 is: -Al2O3[0001]//Ba(NO3)2[111] and -Al2O3(1-2 10)//Ba(NO3)2(110).
2006. "Characterization of NOx Species in Dehydrated and Hydrated Na- and Ba-Y, FAU Zeolites Formed in NO₂ Adsorption." Journal of Electron Spectroscopy and Related Phenomena 150(2-3):164-170. doi:10.1016/j.elspec.2005.05.007 Abstract Adsorbed ionic NOx species formed upon the interaction of NO₂ with dehydrated or hydrated Na-, and Ba-Y, FAU zeolites were characterized using FTIR/TPD, solid state NMR, and XANES techniques. NO₂ disproportionates on both dehydrated catalyst materials forming NO⁺ and NO₃⁻ species. These ionic species are stabilized by their interactions with the negatively charged zeolite framework and the charge compensating cations (Na⁺ and Ba²⁺), respectively. Although the nature of the adsorbed NOx species formed on the two catalysts is similar, their thermal stabilities are strongly dependent on the charge compensating cations. In the presence of water in the channels of these zeolite materials new paths open for reactions between NO⁺ and H₂O, and NO₂ and H₂O, resulting in significant changes in the adsorbed ionic species observed. These combined spectroscopic investigations afforded the understanding of the interactions between water and the NO₂ on these zeolite catalysts.
2006. "Reduction of Stored NOx on Pt/Al₂O₃ and Pt/BaO/Al₂O₃ Catalysts with H₂ and CO." Journal of Catalysis 239(1):51-64. doi:10.1016/j.jcat.2006.01.014 Abstract In situ FTIR spectroscopy coupled with mass spectrometry, and time resolved X-ray diffraction were used to study the efficiency of nitrate reduction with CO and H₂ on Pt/Al₂O₃ and Pt/BaO/Al₂O₃ NOx storage-reduction (NSR) catalysts. Surface nitrates were generated by NO₂ adsorption and their reduction efficiencies were examined on the catalysts together with the analysis of the gas phase composition in the presence of the two different reductants. H₂ was found to be a more effective reducing agent than CO. In particular, the reduction of surface nitrates proceeds very efficiently with H₂ even at low temperatures (~420 K). During reduction with CO, isocyanates were observed to form on every catalyst component. These surface isocyanates, however, readily react with water to form CO₂ and ammonia. The thus formed NH₃, in turn, reacts with stored NOx at higher temperatures (>473K) to produce N₂. In the absence of H₂O, the NCO species are stable to high temperatures, and removed only from the catalyst when they react with NOx thermal decomposition products to form N₂ and CO₂. The results of this study point to a complex reaction mechanism that involves the removal of surface oxygen atoms from the Pt particles by either H₂ or CO, the direct reduction of stored NOx with H₂ (low temperature NOx reduction), the formation and the subsequent hydrolysis of NCO species, as well as the direct reaction of NCO with decomposing NOx (high temperature NOx reduction).
2006. "Effects of Ba loading and calcination temperature on BaAl2O4 formation for BaO/Al2O3 NOx Storage and Reduction Catalysts." Catalysis Today 114(1):86-93. doi:10.1016/j.cattod.2006.02.016 Abstract The effect of thermal treatment on the structure and chemical properties of Ba-oxide-based NOx storage/reduction catalysts with different Ba loadings was investigated using BET, TEM, EDS, TPD and FTIR techniques. On the basis of the present and previously reported results, we propose that moderate (< ~873 K) temperature calcinations result in a single monolayer (ML) ‘coating’ of BaO on the alumina surface. At high Ba loading in excess of that required for a full monolayer ‘coating’ (> 8 wt.% BaO), small (~5 nm) particles of ‘bulk’ BaO are present on top of the 1 ML BaO/Al2O3 surface. We did not observe any detectable morphological changes upon higher temperature thermal treatment of 2 and 8 wt% BaO/Al2O3 samples, while dramatic changes occurred for the 20 wt% sample. In this latter case, the transformations included BaAl2O4 formation at the expense of the bulk BaO phase. In particular, we conclude that the surface (ML) BaO phase is quite stable against thermal treatment, while the bulk phase provides the source of Ba for BaAl2O4 formation.
2006. "Model NOx storage systems: Storage capacity and thermal aging of BaO/theta- Al2O3/NiAl(100)." Journal of Catalysis 243(1):149-157. doi:10.1016/j.jcat.2006.06.028 Abstract The NO;( storage properties of a BaO/θ-Al[2]O[3]/NiAl(100) model system, with a BaO coverage of ∼2 monolayer equivalent (MLE), was studied. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) techniques were used to investigate NO[2] adsorption and reaction on the BaO/θ-Al[2]O[3]/NiAl(100) surface. These results were compared with those of the θ-Al[2]O[3]/NiAl(100) support material, a thermally aged BaO/θ-Al[2]O[3]/NiAl(100) model system, and a realistic BaO (20 wt%)/γ-Al[2]O[3] high-surface area counterpart. At T > 300 K, adsorbed NO[2] is converted to nitrates on all of the surfaces studied. Nitrates residing on the alumina sites of the model catalyst surfaces are relatively weakly bound and typically desorb within 300-600 K, leading to NO(g) evolution; while nitrates associated with the baria sites are significantly more stable and desorb within 600-850 K, resulting in NO(g) or NO(g) + O[2](g) evolution. NO[x] uptake by the baria sites of the BaO/θ-Al[2]O[3]/NiAl(100) model system was found to be as much as five-fold greater than that of the θ-Al[2]O[3]/NiAl(100) support material. Thermal aging of a BaO/0-Al[2]O[3]/NiAl(100) surface at 1100 K before NO[x] uptake experiments brings about a significant (>70%) reduction in the NO[x] storage capacity of the model catalyst surface.
2006. "Low Temperature H2O and NO2 Coadsorption on θ-Al2O3/NiAl(100) ultrathin films ." Journal of Physical Chemistry B 110(15):8025-8034. doi:10.1021/jp057534c Abstract The co-adsorption of H2O and NO2 molecules on a well-ordered, ultrathin θ-Al2O3/NiAl(100) film surface was studied using temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS). For H2O and NO2 monolayers adsorbed separately on the θ-Al2O3/NiAl(100) surface, adsorption energies were estimated to be 44.8 kJ/mol and 36.6 kJ/mol, respectively. Coadsorption systems prepared by sequential deposition of NO2 and H2O revealed the existence of coverage and temperature dependent adsorption regimes where H2O molecules and the surface NOx species (NO2/N2O4/NO2-,NO3-) form segregated and/or mixed domains. Influence of the changes in the crystallinity of solid water (amorphous vs. crystalline) on the coadsorption properties of the NO2/H2O/θ-Al2O3/NiAl(100) system is also discussed.
2006. "Ba Deposition and Oxidation on θ-Al2O3/NiAl(100) ultrathin films. Part II: O2(g) assisted Ba oxidation." Journal of Physical Chemistry B 110(34):17009-17014. doi:10.1021/jp060669d Abstract Ba deposition on a θ-Al2O3/NiAl(100) substrate and its oxidation with gas phase O2 at various surface temperatures are investigated using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) techniques. Oxidation of metallic Ba in gas phase O2 at 800 K results in the growth of 2D and 3D BaO surface domains. Saturation of a metallic Ba layer deposited on θ-Al2O3/NiAl(100) with O2(g) at 300 K reveals the formation of BaO2-like surface states. These metastable peroxide (O22-) states are converted to regular oxide (O2-) states at higher temperatures (800 K). In terms of thermal stability, BaO surface layers that are formed by O2(g) assisted oxidation on the θ-Al2O3/NiAl(100) substrate are significantly more stable (with a desorption/decomposition temperature of c.a. 1050 K) than the metallic/partially oxidized Ba layers prepared in the absence of gas phase O2, which desorb at temperatures as low as 700 K.
2006. "Ba Deposition and Oxidation on θ-Al2O3/NiAl(100) Ultrathin Films. Part I: Anaerobic Deposition Conditions." Journal of Physical Chemistry 110:17001-17008. doi:10.1021/jp060668l Abstract Room temperature Ba deposition on an oxygen terminated θ-Al2O3/NiAl(100) ultrathin film substrate under ultra high vacuum (UHV) conditions is studied using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) techniques. In addition, Ba oxidation by the alumina substrate at 300 K < T < 1200 K in the absence of a gas phase oxidizing agent is investigated. Our results indicate that at room temperature Ba grows in a layer by layer fashion for the first two layers and Ba is partially oxidized. Annealing at T < 700 K results in further oxidation of the Ba species whereas annealing at higher temperatures leads to loss of Ba from the surface via desorption.
2006. "NOx Uptake Mechanism on Pt/BaO/Al2O3 Catalysts." Catalysis Letters 111(3-4):119-126. Abstract The NOx adsorption mechanism on Pt/BaO/Al2O3 catalysts was investigated by performing NOx storage/reduction cycles, NO2 adsorption and NO + O2 adsorption on 2%Pt/(x)BaO/Al2O3 (x = 2, 8 and 20 wt%) catalysts. NOx uptake profiles on 2%Pt/20%BaO/Al2O3 at 523 K show complete uptake behavior for almost 5 min, and then the NOx level starts gradually increasing with time and it reaches 75% of the inlet NOx concentration after 30 min time-on-stream. Although this catalyst shows fairly high NOx conversion at 523 K, only ~ 2.4 wt% out of 20 wt% BaO is converted to Ba(NO3)2. Adsorption studies by using NO2 and NO + O2 suggest two different NOx adsorption mechanisms. The NO2 uptake profile on 2%Pt/20%BaO/Al2O3 shows the absence of a complete NOx uptake period at the beginning of adsorption and the overall NOx uptake is controlled by the gas-solid equilibrium between NO2 and BaO/Ba(NO3)2 phase. When we use NO + O2, complete initial NOx uptake occurs and the time it takes to convert ~ 4 % of BaO to Ba(NO3)2 is independent of the NO concentration. These NOx uptake characteristics suggest that the NO + O2 reaction on the surface of Pt particles produces NO2 that is subsequently transferred to the neighboring BaO phase by spill over. At the beginning of the NOx uptake, this spill-over process is very fast and so it is able to provide complete NOx storage. However, the NOx uptake by this mechanism slows down as BaO in the vicinity of Pt particles are converted to Ba(NO3)2. The formation of Ba(NO3)2 around the Pt particles results in the development of a diffusion barrier for NO2, and increases the probability of NO2 desorption and consequently, the beginning of NOx slip. As NOx uptake by NO2 spill-over mechanism slows down due to the diffusion barrier formation, the rate and extent of NO2 uptake are determined by the diffusion rate of nitrate ions into the BaO bulk, which, in turn, is determined by the gas phase NO2 concentration.
2006. "Non-thermal plasma-assisted NOx reduction over Na-Y zeolites: The promotional effect of acid sites." Catalysis Letters 109(1-2):1-6. doi:10.1007/s10562-006-0049-3 Abstract The effect of acid sites on the catalytic activities of a series of H+-modified Na-Y zeolites was investigated in the non-thermal plasma assisted NOx reduction reaction using a simulated diesel engine exhaust gas mixture. The acid sites were formed by NH4+ ion exchange and subsequent heat treatment of a NaY zeolite. The catalytic activities of these H+-modified NaY zeolites significantly increased with the number of acid sites. This NOx conversion increase was correlated with the decrease in the amount of unreacted NO2. The increase in the number of acid sites did not change the NO level, it stayed constant. Temperature programmed desorption following NO2 adsorption showed the appearance of a high temperature desorption peak at 453 K in addition to the main desorption feature of 343 K observed for the base Na-Y. The results of both the IR and TPD experiments revealed the formation of crotonaldehyde, resulting from condensation reaction of adsorbed acetaldehyde. Strong adsorptions of both NOx and hydrocarbon species are proposed to be responsible for the higher catalytic activity of H+-modified Na-Y zeolites in comparison to the base NaY material
2006. "A New Class of Highly Dispersed VOx Catalysts on Mesoporous Silica: Synthesis, Characterization, and Catalytic Activity in the Partial Oxidation of Ethanol." Applied Catalysis. A, General 300(2):109-119. Abstract The morphology of vanadium oxide supported on a titania-modified mesoporous silica (MCM-41), obtained by means of a careful grafting process through atomic layer deposition, was studied using a variety of characterization techniques. The XRD together with TEM, 51V-NMR, Raman, FTIR and DRS-UV-Vis results showed that the vanadia species are extremely well dispersed onto the surface of the mesoporous support; the dispersion being stable upon thermal treatments up to 400 °C. Studies of the catalytic activity of these materials were performed using the partial oxidation of ethanol as a probe reaction. The results indicate an intrinsic relationship between dispersion, the presence of a TiO2-VOx phase, and catalytic activity for oxidation and dehydration.
2006. "Relationship of Pt Particle Size with the NOx Storage Performance of Thermally Aged Pt/BaO/Al2O3 Lean NOx Trap Catalysts." Industrial and Engineering Chemistry Research 45(26):8815-8821. doi:10.1021/ie060736q Abstract Relationship between Pt particle size and NOx storage performance was investigated over a model Pt/BaO/Al2O3 and an enhanced lean NOx trap catalyst (LNT). These catalysts were treated at elevated temperature to mimic the effect of thermal aging encountered during the desulfation step in NOx trap catalyst regeneration. Combination of in situ time-resolved X-ray Diffraction (TR-XRD), DRIFT measurement after CO adsorption and TEM observation over the thermally aged samples clearly shows the sintering behavior of Pt crystalline as a function of time and temperature. Under elevated temperatures, the crystalline growth of Pt occurs within a short time (e.g. 1.5 hr) followed by the gradual increase with time. NOx storage reaction performed after successive thermal treatments of catalyst under oxygen and in situ XRD experiment have allowed us to correlate the NOx storage performance with phase change. Comparing the simple Pt-BaO/Al2O3 sample with the enhanced one which shows much less Pt sintering, it was confirmed that the Pt crystalline size plays a critical role in determining the NOx storage activity, in other words, the retention of the small Pt particles after thermal aging is crucial to maintain a higher activity. In conclusion, the prevention of Pt sintering when operated at elevated temperature must be a key factor to design the more durable LNT catalyst, thus potentially implying the practical importance for the improvement of the LNT technology.
2006. "Effect of Barium Loading on the Desulfation of Pt-BaO/Al2O3 Studied by H2 TPRX, TEM, Sulfur K-edge XANES, and in Situ TR-XRD." Journal of Physical Chemistry B 110(21):10441-10448. doi:10.1021/jp060119f Abstract Desulfation processes were investigated over sulfated Pt BaO/Al2O3 with different barium loading (8 wt% and 20 wt%) by using H2 temperature programmed reaction (TPRX), transmission electron microscope (TEM) with energy dispersive spectroscopy (EDS), sulfur K-edge X-ray absorption near-edge spectroscopy (XANES), and in situ time-resolved X-ray diffraction (TR-XRD) techniques. Both sulfated samples (8 wt% and 20 wt%) form sulfate species (primarily BaSO4) as evidenced by S K-edge XANES and in situ TR-XRD. However, the desulfation behavior is strongly dependant on the barium loading. Sulfated Pt BaO(8)/Al2O3, consisting predominantly of surface BaO/BaCO3 species, displays more facile desulfation by H2 at lower temperatures than sulfated Pt BaO(20)/Al2O3, a material containing primarily bulk BaO/BaCO3 species. Therefore, after desulfation with H2 up to 1073 K, the amount of the remaining sulfur species on the former, mostly as BaS, is much less than on the latter. This suggests that the initial morphology differences between the two samples play a crucial role in determining the extent of desulfation and the temperature at which it occurs. It is concluded that the removal of sulfur is significantly easier at lower barium loading. This finding can potentially be important in developing more sulfur resistant LNT catalyst systems.
2006. "Synthesis, Characterization, and Catalytic Function of Novel Highly Dispersed Tungsten Oxide Catalysts on Mesoporous Silica ." Journal of Catalysis 239(1):200-211. Abstract The physical and chemical properties of tungsten oxide supported on SBA-15 mesoporous silica prepared by a controlled grafting process through atomic layer deposition (ALD) were studied using complementary characterization methods. X-ray diffraction, optical absorption and transmission electron microscopy showed that tungsten oxide species are highly dispersed on SBA-15 surfaces, even at 30 wt.% WOx contents (surface density: 1.33 WOx/nm2). ALD methods led to samples with much better thermal stability than those prepared via impregnation. Dehydration reactions of 2-butanol and methanol dehydration were used as probe reactions. Differences in reaction rates between the samples prepared by ALD and conventional impregnation reflect the sintering resistance of catalysts prepared by ALD. Notably, temperature programmed oxidation of spent catalysts showed that carbon formation was not responsible for the different dehydration rates in samples prepared by ALD and impregnation..
2006. "Synthesis of Nanodispersed Oxides of Vanadium, Titanium, Molybdenum, and Tungsten on Mesoporous Silica using Atomic Layer Deposition." Topics in Catalysis 39(3-4):245-255. doi:10.1007/s11244-006-0063-0 Abstract The advantages of the atomic layer deposition (ALD) method for preparation of tungsten, vanadium, titanium and molybdenum oxide catalyst supported on mesoporous silica are discussed, with emphasis on the importance of synthesis conditions on dispersion, structure and activity of the resulting materials. A suite of complementary techniques such as DRS-UV/Vis, BET, 1H-NMR, XRD, and TEM were used to study the structural properties of the supported metal oxides, and probe reactions such as 2-butanol dehydration and ethanol partial oxidation were used to demonstrate the potential advantages of the ALD prepared catalysts. Specifically, highly dispersed oxides of titanium, molybdenum, and tungsten oxide on mesoporous silica were synthesized using the ALD method. It is also demonstrated that attainment of high dispersions of vanadium oxide on mesoporous silica requires the presence of at least a single layer of titanium oxide due to the well-known poor interaction between vanadia and silica. The highly dispersed catalysts prepared here by ALD methods exhibited superior catalytic performance relative to those prepared using conventional incipient wetness impregnation.
2006. "Non-equilibrium Effects in the Hydrogenation-mediated Isomerization Mechanism of Olefins during Cavitating Ultrasound Processing." Catalysis Communications 7(6):348–350. doi:10.1016/j.catcom.2005.10.021 Abstract The process whereby cis-olefins isomerize to their trans form is generally understood as occurring through C-H activation of surface bound alkyl radical species. Here we present aqueous phase deuteration results of cis-2-buten-1-ol on Raney Nickel. In the context of the accepted olefin isomerization mechanism, our results illustrate that transition-state theory can accurately model the competition between C-H and C-D activation for olefin exchange (isomerization) for the case of conventional catalytic processing. This is the case also for a catalytic process that includes cavitating ultrasound, although the model then requires a much higher vibrational temperature (at least ~800 K) in order to simulate the selectivity of the deuterium exchange process. Thus, cavitating ultrasound likely incorporates a high level of molecular vibrational excitation, suggesting that the vibrational temperature is not in equilibrium with the thermal (e.g., translational) temperature as the chemistry proceeds along a traditional reaction path.
2006. "Developing Multiple-Site Kinetic Models in Catalysis Simulation: A Case Study of 02+2N0 ↔ 2 NO2 Oxidation-Reduction Chemistry on Pt(100) Catalyst Crystal Facets." Journal of Catalysis 238(1):1-5. doi:10.1016/j.jcat.2005.11.031 Abstract It is generally recognized that developing a kinetic model for a supported catalyst is difficult since multiple site types exist. These sites can arise from a distribution of crystal facets (e.g., (100), (110), etc.) each with their unique intrinsic site types (e.g., atop, bridge, hollow, etc.). Additional complexities arise from non-basel plane site types (defect, edge, corner, etc.), all whose differing lateral interaction energies may be coverage dependent for each site pairwise interaction. To demonstrate the complexities that develop even for a greatly simplified system, we examine a multiple site kinetic model of the reaction 2NO + O2 - 2NO2 on an ideal Pt(100) catalyst. A model of the Pt(100) surface is adopted where atop, bridge, and 4-fold hollow sites are responsible for O2, NO, and NO2 chemisorption to form Pt-O, Pt-NO, and Pt-NO2 species. In our kinetic scheme, equilibrium is assumed for O2, NO, and NO2 chemisorption due to their high sticking coefficients (all > 0.1). A single rate determining step of the Langmuir-Hinshelwood type was chosen to describe the oxidation of NO on platinum via the reaction PtH,A,B-O + PtH,A,B-NO - PtH,A,B + PtH,A,B-NO2, where H, A, and B represent 4-fold hollow, atop, and bridge sites. Equal kinetic parameters for all site combinations were assumed to exist and, in part, taken from the literature to be AH+=83 kJ/mol and AS+=20 J/K mol. The exercise here is largely hypothetical but offers insight into how more detailed kinetic models may be developed, such as through the use of reaction velocity matrices, a concept introduced here. Specifically for this system, the model yielded insight into NOx chemistry on Pt(100) in that it predicted that the greatest reaction velocities (forward and reverse) occurred via the reaction Pt-O(atop) + Pt-NO(bridge) A Pt(atop) + Pt-NO2(bridge). We believe the framework of a site-specific modeling scheme presented here is an important starting point for future site-specific microkinetic modeling. In particular, a definition and description of use of surface coverages, reaction rate coefficients, and computed reaction velocity matrices are presented.
2006. "An examination of the H/D isotope substitution effect on selectivity and activity in the cavitating ultrasound hydrogenation of aqueous 3-buten-2-ol and 1,4-pentadien-3-ol on Pd-black." Industrial and Engineering Chemistry Research 45(11):4015-4018. doi:10.1021/ie051347j Abstract An H/D isotope effect study of the (H2 versus D2) hydrogenation of the aqueous substrates 3-buten-2-ol (3B2OL) and 1,4-pentadien-3-ol (14PD3OL) was performed using Pd-black catalyst. Either H2O or D2O solvents were employed (for alcohol H/D isotope substitution). Two experimental processing conditions of cavitating ultrasound (CUS) and stirred/silent (SS) methods were used. Products formed include 2-butanol and 2-butanone for the former, and 3-pentanol and 3-pentanone for the latter. The observed selectivity and pseudo-first order reaction rate coefficients (e.g., activity) to these products enabled a mechanistic interpretation of the various reaction conditions to be proposed. Experiments utilized a 50 mL batch reactor maintained at 298 K, employed 5.4 atm of H2 or D2 gas, while seven aliquots were collected during the course of the reaction. We have utilized 1-propanol as an inert dopant in all experiments to enable the rapid onset of cavitation in the CUS systems as described earlier [R.S. Disselkamp et al., J. Catal., 227, 552 (2004)]. The following conclusions were noted. First, the activity of the CUS compared to SS processing were ~100-fold larger. Second, variable catalyst loading experiments for stirred/silent D2 hydrogenation processing indicated that mass transfer of hydrogen gas to the Pd-surface played a role such that higher catalyst loading reduced surface D-atom concentrations and reduced saturated alcohol formation (e.g., via reduced H-addition to surface alkyl radicals). Third, for CUS processing the ketone selectivities for experiments employing water compared to D2O indicated that 3B2OL were twice as large, whereas for 14PD3OL they were comparable. This suggests, somewhat surprisingly, that for 3B2OL enol tautomerization to ketone is a slow, and possibly rate-controlling, process. Finally, again for CUS processing, the similarity in ketone selectivities (all ~17%) for H2 compared to D2 hydrogenation for both 3B2OL and 14PD3OL suggest that both H/D isotopes have rapid surface diffusion and hence give rise to nearly equal selectivies. Restated, the thermal or cavitating ultrasound activation is much greater than the surface H/D diffusion barrier.
2005. "Differential Kinetic Analysis of Diesel Particulate Matter (Soot) Oxidation by Oxygen Using a Step-Response Technique." Applied Catalysis. B, Environmental 61(1-2):120-129. Abstract A novel step-response technique was developed for the kinetic study of diesel soot oxidation. Using this technique, various aspects of the oxidation process can be probed while consuming only differential amounts of carbon, and the impact of the reaction heat on the measured rates can be minimized. Due to its high throughput, the technique allows broad parametric studies to be performed rapidly and in a kinetically rigorous manner. The technique was applied to soot oxidation by O2, one of the major regeneration mechanisms for the catalytic soot filter systems. It was found that, after decoupling effects due to the sample history, carbon oxidation by O2 in the absence of H2O can be well described by an unmodified Arhenius equation, with similar activation energy values for diesel and model soot samples (137±8.7 and 132±5.1 kJ/mol, respectively). The reaction order in O2 for these samples was found to be 0.61±0.03 and 0.71±0.03, respectively, and was remarkably independent of the temperature, suggesting that the fractional order is not due to mixed kinetic control. The reaction mechanism was also found to be independent of carbon conversion. The density of the reaction sites, however, appeared to increase with oxidation. This increase could not be accounted for by the changes in the specific surface area, either directly measured, or derived from such simplified models as the shrinking core formalism. The entire set of obtained experimental results can be described using a kinetically uncomplicated model in a broad range of temperatures, partial pressures of oxygen and degrees of soot oxidation.
2005. "Differential kinetic analysis of diesel particulate matter (soot) oxidation by oxygen using a step–response technique." Applied Catalysis. B, Environmental 61(1-2):120-129. Abstract The effects of a catalytic coating on the oxidation of captured soot over diesel particulate filters (DPF) is debated in the literature, since a catalyzed filter wall appears to lack sufficiently tight contact with soot deposits to exercise direct catalytic action. The topology of soot– catalyst contact may change with progressive oxidation of the soot layer; hence, a technique capable of probing catalytic action via detailed kinetic analysis at different stages of oxidation is required to conclusively resolve this problem. A novel step–response technique was developed in this work as a methodological foundation for such study. Using this technique, various aspects of the oxidation process can be probed while consuming only differential amounts of carbon, and the impact of the reaction heat on the measured rates can be minimized. This technique was applied to soot oxidation by O2 and showed that, after decoupling effects due to the sample history, carbon oxidation by O2 in the absence of H2O can be well-described by an unmodified Arrhenius equation, with similar activation energy values for diesel and model soot samples (137 _ 8.7 and 132 _ 5.1 kJ/mol, respectively). The reaction order in O2 for these samples was found to be 0.61 _ 0.03 and 0.71 _ 0.03, respectively, and was remarkably independent of the temperature, suggesting that the fractional order is not due to mixed kinetic control. The reaction mechanism was also found to be independent of carbon conversion. The density of the reaction sites, however, appeared to increase with oxidation. This increase could not be accounted for by the changes in the specific surface area, either directly measured orderived from such simplified models as the shrinking-core formalism. The entire set of obtained experimental results can be described using a kinetically uncomplicated model in a broad range of temperatures, partial pressures of oxygen and degrees of soot oxidation.
2005. "Microstructure of ZrO2-CeO2 Hetero-Multi-Layer Films Grown on YSZ Substrate." Acta materialia 53(7):1921-1929. Abstract Multi-layer films of pure ZrO2 and CeO2 were grown using oxygen plasma assisted molecular beam epitaxy on yttria stabilized zirconia (YSZ) substrates. The microstructure of the film was analyzed using x-ray diffraction (XRD), conventional and high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), energy dispersive x-ray (EDX) elemental mapping, selected area electron diffraction, and dynamical electron diffraction calculations. The deposited pure CeO2 layers exist in the cubic fluorite structure, and the ZrO2 layers show a good epitaxial orientation with respect to the CeO2 layers. However, distinctive forbidden diffraction spots of (odd, odd, even) type were observed on the selected area electron diffraction patterns of the film. Dark-field imaging clearly reveals that these forbidden diffraction spots were contributed solely by the ZrO2 layers. Dynamical electron diffraction calculation based on the tetragonal phase of unity tetragonality (space group P42/nmc) with oxygen displaced along the c-axis does not match with the experimental pattern. Instead, a diffraction pattern calculated based on a cubic structure (space group P43m) for which the oxygen sub-lattice was displaced along the <111> matches with the experimental data. It is further suggested that the displacement of the oxygen from the ideal (¼,¼,¼) position was introduced during the film growth process.
2005. "The Catalytic Chemistry of HCN+NO₂ over Na- and Ba-Y, FAU: An In Situ FTIR and TPD/TPR Study." Journal of Physical Chemistry B 109(4):1481-1490. doi:10.1021/jp045671o Abstract The adsorption of HCN and the reaction of HCN with NO₂ over Na-, and Ba-Y,FAU zeolite catalysts were investigated using in situ FTIR and TPD/TPR spectroscopies. Both catalysts adsorb HCN molecularly at room temperature, and the strength of adsorption is higher over Ba-Y than Na-Y. Over Na-Y the reaction between HCN and NO₂ is slow at 473K. On Ba-Y HCN reacts readily with NO₂ at 473K, forming N₂, CO, CO₂, HNCO, NO, N₂O and C₂N₂. The results of this investigation suggest that initial step in the HCN+NO₂ reaction over these catalysts is the hydrogen abstraction from HCN, and the formation of ionic CN⁻ and NC⁻ species. The formation of N2 can proceed directly from these ionic species upon their interaction with NO⁺. Alternatively, these cyanide species can be oxidized to isocyanates which then can be further transformed to N₂, N₂O and COx in their subsequent reaction with NOx.
2005. "NO2 Adsorption on BaO/Al2O3: The Nature of Nitrate Species." Journal of Physical Chemistry B 109(1):27-29. doi:10.1021/jp045082i Abstract The nature of nitrate species formed in the Al₂O₃, 8wt%, and 20wt% BaO/Al₂O₃ catalysts was investigated in a combined TPD, FTIR and 15N solid state NMR study. The results strongly suggest the formation of a monolayer bidentate nitrate on the alumina support that forms upon NO₂ exposure. This monolayer nitrate decomposes at lower temperature than bulk Ba(NO₃)₂ and its only decomposition product is NO₂. A bulk-like Ba(NO₃)₂ phase also forms with its characteristic set of TPD, IR and NMR features. The amount of NOx stored in the monolayer nitrate is proportional to the surface area of the catalyst, while that in the bulk nitrate increases with BaO coverage.
2005. "Changing Morphology of BaO/AI₂O₃ during NO₂ Uptake and Release." Journal of Physical Chemistry B 109(15):7339-7344. Abstract The changes in the morphology of Ba-oxide-based NOx storage/reduction catalysts were investigated using time resolved x-ray diffraction, transmission electron microscopy and energy dispersed spectroscopy. Large Ba(NO₃)₂ crystallites form on the alumina support when the catalyst is prepared by the incipient wetness method using an aqueous Ba(NO₃)₂ solution. Heating the sample to 873K in a He flow results in the decomposition of the Ba(NO₃)₂ phase and the formation of both a monolayer BaO film strongly interacting with the alumina support, and nano crystalline BaO particles. Upon NO₂ exposure of these BaO phases at room temperature, small (nano-sized) Ba(NO₃)₂ crystals and a monolayer of surface nitrate form. Heating this sample in NO₂ results in the coalescence of the nano crystalline Ba(NO₃)₂ particles into large crystals. The average crystal size in the re-formed Ba(NO₃)₂ layer is significantly smaller than that measured after the catalyst preparation. Evidence is also presented for the existence of a monolayer Ba(NO₃)₂ phase after thermal treatment in NO₂, in addition to these large crystals. These results clearly demonstrate the dynamic nature of the Ba-containing phases that are active in the NOx storage/reduction process. The proposed morphology cycle may contribute to the understanding of the changes observed in the performances of these catalysts during actual operating conditions.
2005. "Partial encapsulation of Pd particles by reduced ceria-zirconia." Applied Physics Letters 87(20):Art. No. 201915. doi:10.1063/1.2132067 Abstract The interaction between metal particles and their oxide support can be strong so as to affect the reactivity of a catalyst system by, for example, encapsulation of the particles by the oxide. Direct observation of metal-oxide interfaces with atomic resolution is a challenge and can only be achieved by cross sectional high-resolution transmission electron microscopy (HRTEM). With this approach, we found partial encapsulation of Pd particles by reduced ceria-zirconia in a model, single-crystal thin film auto catalyst, indicating a strong interaction between Pd and the oxide. Besides obtaining HRTEM images, the valence of cerium was determined by electron energy loss spectroscopy (EELS). The effect of reduction and oxidation conditions on this interaction provides a qualitative explanation for a previously observed reversible reactivation of oxygen storage in model powder auto catalysts. The technique of cross sectional HRTEM can be applied to the study of other metal-particle-on-oxide systems.
2005. "Physical and Chemical Properties of Ce₁-xZrxO₂ Nanoparticles and Ce₁-xZrxO₂ (111) Surfaces: Synchrotron-based Studies." Journal of Molecular Catalysis. A, Chemical. 228(1-2):11-19. Abstract In this article, we review a series of studies that use synchrotron-based techniques (high-resolution photoemission, time-resolved x-ray diffraction, and x-ray absorption near-edge spectroscopy) to investigate the physical and chemical properties of Ce₁-xZrxO₂ nanoparticles and Ce₁-xZrxO₂ (111) surfaces (x ≤ 0.5). Ce O₂ and Ce₁-xZrxO₂ particles in sizes between 3 and 7 nm were synthesized using a novel microemulsion method. The results of XANES (O K-edge, Ce and Zr LIII-edges) indicate that the Ce₁-xZrxO₂ nanoparticles and Ce₁-xZrxO₂ (111) surfaces have very similar electronic properties. For these systems, the lattice constant decreased with increasing Zr content, varying from 5.40 Å in CeO₂ to 5.27 Å in Ce₀․₅Zr₀․₅O₂. Within the fluorite structure, the Zr atoms exhibited structural perturbations that led to different types of Zr-O distances and non-equivalent O atoms in the Ce₁-xZrxO₂ compounds. The Ce₁-xZrxO₂ nanoparticles were more reactive towards H₂ and SO₂ than the Ce₁-xZrxO₂ (111) surfaces. The Ce₁-xZrxO₂ (111) surfaces did not reduce in hydrogen at 300 °C. At temperatures above 250 °C, the Ce₁-xZrxO₂ nanoparticles reacted with H₂ and water evolved into gas phase. XANES showed the generation of Ce³⁺cations without reduction of Zr⁴⁺. There was an expansion in the unit cell of the reduced nanoparticles probably as a consequence of a partial Ce⁴⁺→ Ce³⁺ transformation and the sorption of hydrogen into the bulk of the material. S K-edge XANES spectra pointed to SO₄ as the main product of the adsorption of SO₂ on the Ce₁-xZrxO₂ nanoparticles and Ce₁-xZrxO₂ (111) surfaces. Full dissociation of SO₂ was seen on the nanoparticles but not on the Ce₁-xZrxO₂ (111) surfaces. The metal cations at corner and edge sites of the Ce₁-xZrxO₂ nanoparticles probably play a very important role in interactions with the H₂ and SO₂ molecules.
2005. "NO₂ Adsorption on Ultrathin Θ-Al₂O₃ Films: Formation of Nitrite and Nitrate Species." Journal of Physical Chemistry 109(33):15977-15984. Abstract Interaction of NO₂ with an ordered θ-Al₂O₃/NiAl(100) model catalyst surface was investigated using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The origin of the NOx uptake of the catalytic support (i.e. Al₂O₃) in a NOx storage catalyst is identified. Adsorbed NO₂ is converted to strongly bound nitrites and nitrates that are stable on the model catalyst surface at temperatures as high as 300 and 650 K, respectively. The results show that alumina is not completely inert and may stabilize some form of NOx under certain catalytic conditions. The stability of the NOx formed by exposing the θ-Al₂O₃ model catalyst to NO₂ adsorption increases in the order: NO₂ (physisorbed or N₂O₄) < NO₂(chemisorbed) < NO₂- < NO₃-.
2005. "Interaction of Water with Ordered Theta-Al₂0₃ Ultrathin Films grown on NiAl(100)." Journal of Physical Chemistry B 109(8):3431-3436. doi:10.1021/jp0449206 Abstract The structure of an ordered, ultra thin ⊖-Al₂0₃ film grown on a NiAl(100) single crystal surface was studied by Auger electron spectroscopy (AES), X-Ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED), and its interaction with water was investigated with temperature programmed desorption (TPD), and XPS. Our results indicate that H₂O adsorption on the ⊖-Al₂0₃/NiAl(100) surface is predominantly molecular rather than dissociative. For ⊖H₂O< 1ML (ML=monolayer), H₂O molecules were found to populate Al³⁺ cation sites to form isolated H₂O species aligned in a row along the cation sites on the oxide surface with a repulsive interaction between them. For ⊖H₂O> 1ML, H₂O overlayers were observed to form three dimensional ice multilayers where water molecules start occupying both cationic and anionic adsorption sites on the oxide surface allowing the formation of hydrogen boding. A small extent of H₂O dissociation was observed to occur on the ⊖-Al₂0₃/NiAl(100) surface which was attributed to the presence of a low concentration of surface defects. Titration of these defect sites with absorbed H₂O molecules revealed an estimated defect density of ~0.-5 ML for the ⊖-Al₂0₃/NiAl(100) system consistent with the ordered nature of the synthesized oxide film.
2005. "Structural and Catalytic Properties of the Alkali Metal Ion-Exchanged Y-Zeolites by 29Si and 27Al Solid-State NMR and FT-IR Spectroscopy." Key Engineering Materials 277-279:708-719. Abstract A series of cation exchanged Y-zeolites were prepared by exchanging cations with various alkali (M+, M=Li, Na, K, Cs) metals. The structural and catalytic properties of the alkali metal exchanged Y-zeolites have been investigated by a number of analytical techniques. Comparative elemental analyses were determined by an Energy Dispersive Spectroscopy X-ray (EDS), X-ray Photoelectron Spectroscopy (XPS), Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and X-ray Fluorescence (XRF) before and after cation substitution. The framework and non-framework Al coordination and the Si/Al ratios of the Y-zeolites were investigated by MAS Solid-State Nuclear Magnetic Resonance (NMR) spectroscopy. The Al NMR spectra were characterized by two 27Al resonance signals at 12 and 59 ppm, indicating the presence of the non-framework and framework Al respectively. The intensities of these resonances were used to monitor the amount of the framework and non-framework of Al species in the series of exchanged zeolites.
2005. "Physical and Catalytic Properties of the Na-exchanged Y-Zeolites with Different Si/Al Ratios." Key Engineering Materials 277-279:720-731. Abstract A series of sodium-exchanged Y-zeolites with different Si/Al ratios have been prepared from the proton and ammonium ion forms of Y-zeolite(faujasite) by means of a solution ion exchange. The prepared catalysts have been characterized by the various analytical techniques and the NOx conversion rates of the catalysts were measured with non-thermal plasma assisted catalytic system. Specific surface areas measured by the BET increased in the order of the NaY series 708, 712, 100, 720, 760. The results of the elemental analyses showed that the Si/Al ratio of the catalysts increased gradually in the order of NaY 100, NaY 712, NaY 720, NaY 760, NaY 780.
2005. "Changes in Ba phases in BaO/Al₂O₃ upon thermal aging and H₂O treatment." Catalysis Letters 105(3-4):259-268. doi:10.1007/s10562-005-8700-y Abstract The effects of thermal aging and H₂O treatment on the physicochemical properties of a BaO/Al₂O₃ model catalyst were investigated by means of XRD, BET, TEM/EDX and NO₂ TPD. Thermal aging at 1000 °C for 10 hrs resulted in conversion of dispersed BaCO₃ into low surface area crystalline BaAl₂O₄. It was found that H₂O treatment on a BaO/Al₂O₃ sample at room temperature transformed not only the BaAl₂O₄, but also the dispersed BaCO₃ into highly crystalline BaCO₃ segregated from the Al₂O₃ support, as evidenced in TEM/EDX and XRD analysis. The sample containing dispersed BaCO3 in the initial phase segregated more severely than the BaAl₂O₄ containing one, with the Ba in the BaAl₂O₄ matrix exhibiting higher resistance towards segregation. Contacting the BaO/Al₂O₃ sample with liquid water over a prolong period of time leads to an increase in crystallinity of the segregated BaCO₃. These phenomena imply that special care must be taken during catalyst synthesis and during realistic operation of Pt/BaO/Al₂O₃ NOx trap catalysts since both processes involve potential exposure of the material with liquid H₂O. Based on the results, a model to explain the behavior of Ba containing species upon thermal aging and H₂O treatment is proposed.
2005. "Line narrowing in H-1 MAS spectrum of mesoporous silica by removing adsorbed H2O using N-2 ." Solid State Nuclear Magnetic Resonance 27(3):200-205. Abstract The peaks for silanol protons in the high resolution 1H NMR spectrum obtained on mesoporous silica materials may be broadened and shifted downfield by hydrogen bonding with absorbed water molecules. The spectrum may be further complicated by overlapping of the resonance for hydrogen-bonded silanol with the corresponding broad peak due to hydrogen-bonded water. These complications hamper a quantitative analysis of the spectra for these and similar materials. It is demonstrated in this paper that absorbed water can be removed by exposing the sample to dry N2 during magic angle spinning. This results in significantly enhanced spectral resolution for the silanol protons in the 1H MAS spectrum. The enhanced spectral resolution makes it possible to quantify the various hydroxyl groups in a complex metal-oxide catalyst. Results obtained on tungsten oxide supported on SBA-15 mesoporous silica materials are reported.
2005. "Ultrasound-Assisted Hydrogenation of Cinnamaldehyde." Ultrasonics Sonochemistry 12(4):319-324. Abstract They hydrogenation, employing hydrogen gas, of cinnamaldehyde was performed using Pd-black and Raney Ni catalysis at 298± K in a water-cooled (jacketed) reaction vessel. Sampling at pre-determined time intervals and GC/MS analysis yielded time-dependent product state distribution information. A kinetic modeling of the data revealed that cinnamaldehyde was both hydrogenated to the final product benzene propanal, as well as a fraction being converted to the intermediate benzene propanal, where the latter was subsequently hydrogenated to benzene propanal. Comparing the ultrasound-assisted and blank (stirred) experiments revealed that a higher maximum relative concentration of intermediate benzene propanal was formed in the ultrasound experiments compared to the stirred experiment. The activities of the ultrasound experiments compared to blank were 9-fold and 20-fold greater for the Pd-black and Raney Ni catalysts, respectively. Finally, an application of Polayni’s principle to yield an estimate of the ratio of rate coefficients for benzene propanal and benzene propanal formation was performed by considering chemical group energy differences and surface adsorption energy differences in the first mechanistic step of hydrogenation.
2005. "The Effect of Cavitating Ultrasound on the Aqueous Phase Hydrogenation of Cis-2-buten-1-ol and Cis-2-penten-1-ol on Pd-black." Applied Catalysis. A, General 288(1-2):62-66. Abstract We have studied the effect of cavitating ultrasound on the heterogeneous aqueous hydrogenation of cis-2-buten-1-ol (C4 olefin) and cis-2-penten-1-ol (C5 olefin) on Pd-black to form the trans-olefins (trans-2-buten-1-ol and trans-2-penten-1-ol) and saturated alcohols (1-butanol and 1-pentanol, respectively). Silent (and magnetically stirred) experiments served as control experiments. As described in an earlier publication by our group, we have added an inert dopant, 1-propanol, in the reaction mixture to ensure the rapid onset of cavitation in the ultrasound-assisted reactions that can lead to altered selectivity compared to silent reaction systems [Disselkamp et al., J. Catal., 227 (2004) 552]. The motivation for this study is to examine whether cavitating ultrasound can reduce the [trans-olefin/saturated alcohol] molar ratio during the course of the reaction. This could have practical application in that it may offer an alternative processing methodology of synthesizing healthier edible seed oils by reducing trans-fat content. We have observed that cavitating ultrasound results in a [(trans-olefin/saturated alcohol)ultrasound/(trans-olefin/saturated alcohol)silent] ratio quantity less than 0.5 at the reaction mid-point for both the C4 and C5 olefin systems. This indicates that ultrasound reduces trans-olefin production compared to the silent control experiment. Furthermore, there is an added 30% reduction for the C5 versus C4 olefin compounds again at reaction mid-point. We attribute differences in the ratio quantity as a moment of inertia effect. In principle, the C4 versus C5 olefins has a ~52% increase in moment of inertia about C2=C3 double bond slowing isomerization. Since seed oils are C18 multiple cis olefins and have an moment of inertia even greater than our C5 olefin here, our study suggests that even a greater reduction in trans-olefin content may occur for partial hydrogenation of C18 seed oils.
2005. "The effect of cavitating ultrasound on the aqueous phase hydrogenation of cis-2-buten-1-ol and cis-2-penten-1-ol on Pd-black." Applied Catalysis. A, General 288(1-2):62-66. doi:10.1016/j.apcata.2005.04.040 Abstract We have studied the effect of cavitating ultrasound on the heterogeneous aqueous hydrogenation of cis-2-buten-1-ol (C4 olefin) and cis-2-penten-1-ol (C5 olefin) on Pd-black to form the trans-olefins (trans-2-buten-1-ol and trans-2-penten-1-ol) and saturated alcohols (1-butanol and 1-pentanol, respectively). Silent (and magnetically stirred) experiments served as control experiments. As described in an earlier publication by our group, we have added an inert dopant, 1-propanol, in the reaction mixture to ensure the rapid onset of cavitation in the ultrasound-assisted reactions that can lead to altered selectivity compared to silent reaction systems [R.S. Disselkamp, Ya-Huei Chin, C.H.F. Peden, J. Catal. 227 (2004) 552]. The motivation for this study is to examine whether cavitating ultrasound can reduce the [trans-olefin/saturated alcohol] molar ratio during the course of the reaction. This could have practical application in that it may offer an alternative processing methodology of synthesizing healthier edible seed oils by reducing trans-fat content.We have observed that cavitating ultrasound results in a [(trans-olefin/saturated alcohol)ultrasound/(trans-olefin/saturated alcohol)silent] ratio quantity less than 0.5 at the reaction mid-point for both the C4 and C5 olefin systems. This indicates that ultrasound reduces trans-olefin production compared to the silent control experiment. Furthermore, there is an added 30% reduction for the C5 versus C4 olefin compounds again at reaction mid-point. We attribute differences in the ratio quantity as a moment of inertia effect. In principle, the C4 versus C5 olefins has a ~52% increase in moment of inertia about C2 C3 double bond slowing isomerization. Since seed oils are C18 multiple cis-olefins and have a moment of inertia even greater than our C5 olefin here, our study suggests that even a greater reduction in trans-olefin content may occur for partial hydrogenation of C18 seed oils.
2005. "Chemistry - Oxygen Vacancies and Catalysis on Ceria Surfaces." Science 309(5735):713-714. Abstract Chemistry occurring at the surface of metal oxides is critical in a variety of industrial applications including catalysis and photocatalysis, optical display technology, solar energy devices and corrosion prevention. Defects have long been recognized to be the most reactive sites on the surfaces of many oxide materials. The most common types of defects present on the surfaces of metal oxides are oxygen vacancies and step edges. The nature of surface oxygen vacancies, and their number, distribution and diffusion across the surface of oxides, are thus issues of major scientific importance. One of the most interesting oxides in this respect is CeO2, since oxygen vacancies play the key role in giving this material it’s industrially important “oxygen-storage capacity.” This capacity makes modern automotive exhaust treatment catalysts containing CeO2 much more effective than their predecessors without CeO2. Ceria is also well known as a support which enhances the performance of transition metal catalysts, relative to other oxide supports, in a variety of other reactions including water-gas shift, steam reforming of oxygenates and PROX 1-7, all of which hold promise for enabling a hydrogen economy 1. Related to ceria’s facile redox capacity (ability to rapidly form and eliminate oxygen vacancy defects) is the poorly understood observation that some less reducible oxides, such as zirconia (ZrO2), are used as additives that actually enhance this “oxygen storage” property of CeO2. In this issue, Esch and coworkers in Trieste, Italy report an exciting study that for the first time clearly elucidates the structure, distribution and formation of oxygen vacancies on a cerium oxide surface 8. They have elegantly combined beautiful, atomic-resolution imaging using scanning-tunneling microscopy (STM) on a ceria surface with state-of-the-art quantum mechanical calculations using Density Functional Theory (DFT) to raise our understanding of CeO2 surfaces to a much higher level. They show that surface oxygen vacancies on CeO2(111) are immobile at room temperature, but at higher temperatures linear clusters of these vacancies are formed which expose exclusively Ce3+ ions to gas-phase reactants. The resulting exposed Ce3+ ions are thus grouped into rather large ensembles, with the sites immediately adjacent to these vacancy clusters remaining as pure Ce4+ ions. The authors further show that one subsurface oxygen vacancy is required to nucleate each such vacancy cluster. Guided by this knowledge, they performed DFT calculations that suggest an exciting new explanation for the role of Zr promoters in ceria-based catalysts: to enable growth of these linear vacancy chains without the need for a subsurface vacancy, which is energetically more costly. It should be noted that Namai et al. 9,10 also recently reported such linear vacancy clusters on CeO2(111), for which Esch et al. now provide much needed atomic-level structural detail.
2005. "Nanoscale Effects on Ion Conductance of Layer-by-Layer Structures of Gadolinia-doped Ceria and Zirconia." Applied Physics Letters 86(13):131906-131909. Abstract Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.
2004. "Mechanisms of Sulfur Poisoning of NOx Adsorber Materials." Chapter III.I in Advanced Combustion Engine R&D: 2003 Annual Progress Report, ed. G Singh, pp. 141-145. US Department of Energy, Washington, DC. Abstract This annual report will review progress of the initial 4 months of a three-year effort between Cummins Engine Company and Pacific Northwest National Laboratory to understand and improve the performance and sulfur tolerance of the materials used in the NOx adsorber after-treatment technology in order to meet both performance and reliability standards required for diesel engines. The goal of this project is to enable NOx after-treatment technologies that will meet both EPA 2007 emission standards and customer cost, reliability and durability requirements. The project will consist of three phases. First, the efforts will focus on understanding the current limitation of capture, regeneration and durability of existing NOx adsorber materials, especially with respect to their sulfur tolerance. With this developing understanding, efforts will also be focused on the optimization of the NOx absorber chemical and material properties to increase performance and durability over many regeneration cycles. We anticipate that improved materials will be tested and evaluated, in partnership with Cummins, on diesel vehicle engines over expected operating conditions.
2004. "Plasma Catalysis for NOx Reduction from Light-Duty Diesel Vehicles." In Advanced Combustion Engine Research & Development, 2004 Annual Progress Report, pp. 180-183. US Department of Energy, Energy Efficiency and Renewable Energy, Washington DC. Abstract The control of NOx (NO and NO2) emissions from so-called ‘lean-burn’ vehicle engines remains a challenge. In this program, we have been developing a novel plasma/catalyst technology for the remediation of NOx under lean (excess oxygen) conditions, specifically for compression ignition direct injection (CIDI) diesel engines that have significant fuel economy benefits over conventional stoichiometric gasoline engines. Program efforts included: (1) improving the catalyst and plasma reactor efficiencies for NOx reduction; (2) studies to reveal important details of the reaction mechanism(s) that can then guide our catalyst and reactor development efforts; (3) evaluating the performance of prototype systems on real engine exhaust; and (4) studies of the effects of the plasma on particulate matter (PM) in real diesel engine exhaust. Figure 1 is a conceptual schematic of a plasma/catalyst device, which also shows our current best understanding of the role of the various components of the overall device for reducing NOx from the exhaust of a CIDI engine. When this program was initiated, it was not at all clear what the plasma was doing and, as such, what class of catalyst materials might be expected to produce good results. With the understanding of the role of the plasma (as depicted in Figure 1) obtained in this program, faujasite zeolite-based catalysts were developed and shown to produce high activity for NOx reduction of plasma-treated exhaust in a temperature range expected for light-duty diesel engines. These materials are the subject of a pending patent application, and were recognized with a prestigious R&D100 Award in 2002. In addition, PNNL staff were awarded a Federal Laboratory Consortium (FLC) Award in 2003 “For Excellence in Technology Transfer”. The program also received the DOE’s 2001 CIDI Combustion and Emission Control Program Special Recognition Award and 2004 Advanced Combustion Engine R&D Special Recognition Award.
2004. "Plasma Catalysis for NOx Reduction from Light-Duty Diesel Vehicles." Chapter III.G. in Advanced Combustion Engine R&D: 2003 Annual Progress Report, ed. G Singh, pp. 129-136. Department of Energy, Washington, DC. Abstract This annual report reviews FY 2003 progress of a program aimed at the development of a novel plasma/catalyst technology for the remediation of NOx under lean (excess oxygen) conditions, specifically for compression ignition direct injection (CIDI) diesel engines that have significant fuel economy benefits over conventional stoichiometric gasoline engines. Our previous work has shown that a non-thermal plasma in combination with an appropriate catalyst can provide NOx emission reduction efficiency of 60-80% using a simulated diesel exhaust. Based on these levels of NOx reduction obtained in the lab, a simple model was developed in this program that allows for the estimation of the fuel economy penalty that would be incurred by operating a plasma/catalyst system. Results obtained from this model suggest that a 5% fuel economy penalty is achievable with the then current (FY2000) state-of-the-art catalyst materials and plasma reactor designs. In this last year, we have continued to focus on (1) improving the catalyst and plasma reactor efficiencies for NOx reduction, (2) studies to reveal important details of the reaction mechanism(s) that can then guide our catalyst and reactor development efforts (focus 1), and (3) evaluating the performance of prototype systems on real engine exhaust. While studies of the effects of the plasma on PM in real diesel engine exhaust is meant to be part of the program, this year we did not conduct any experiments along these lines due to the major effort required to carry out the engine testing (focus 3).
2004. "Distortion of the Oxygen Sublattice in Pure Cubic-ZrO₂ ." Journal of Materials Research 19(5):1315-1319. Abstract Multi-layer films of pure ZrO₂ and CeO₂ were grown using molecular beam epitaxy on a yttria-stabilized zirconia (YSZ) substrate. Distinctive forbidden diffraction spots of (odd, odd, even) type were observed on the selected area electron diffraction patterns of the film. Dark-field imaging clearly revealed that these forbidden diffraction spots were solely due to the ZrO₂ layers. Comparison of the electron diffraction pattern with one simulated by dynamical calculations suggest that the pure ZrO₂ layers possess a cubic structure of space group with the oxygen sub-lattice being displaced diagonally, rather than along the c-axis as suggested for YSZ. Our results further suggest that the displacement of the oxygen from the ideal (¼,¼,¼) position might have been introduced during the film growth process.
2004. "Adsorption and Reaction of Acetaldehyde on Stoichiometric and Defective SrTiO₃(100) Surfaces." Journal of Physical Chemistry B 108(5):1646-1652. Abstract The adsorption and reaction of acetaldehyde (CH₃CHO), on stoichiometric (TiO₂-terminated) and reduced SrTiO₃(100) surfaces, have been investigated using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Acetaldehyde adsorbs molecularly on the stoichiometric SrTiO₃(100) surface that contains predominately Ti₄⁺ cations. The Ti₄⁺ sites on the stoichiometric SrTiO₃(100) surface are not sufficiently active for surface reactions such as aldol condensation, as opposed to the Ti₄⁺ ions on the TiO₂ (001) surface. However, decomposition and redox reactions of acetaldehyde occur in the presence of surface defects created by Ar⁺ sputtering. The decomposition products following reactions of acetaldehyde on the defective surface include H₂, C2H₄, CO, C₄H6 and C₄H₈. Reductive coupling, to produce C₂H₄ and C₄H₈, is the main reaction pathway for decomposition of acetaldehyde on the sputter reduced SrTiO₃ (100) surface.
2004. "Adsorption and Reaction of Acetaldehyde on Stoichiometric and Defective SrTiO₃ (100) Surfaces." Journal of Physical Chemistry B 108(5):1646 - 1652. Abstract The adsorption and reaction of acetaldehyde (CH₃CHO), on stoichiometric (TiO₂-terminated) and reduced SrTiO₃(100) surfaces, have been investigated using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Acetaldehyde adsorbs molecularly on the stoichiometric SrTiO₃(100) surface that contains predominately Ti₄⁺ cations. The Ti₄⁺ sites on the stoichiometric SrTiO₃(100) surface are not sufficiently active for surface reactions such as aldol condensation, as opposed to the Ti₄⁺ ions on the TiO₂(001) surface. However, decomposition and redox reactions of acetaldehyde occur in the presence of surface defects created by Ar⁺ sputtering. The decomposition products following reactions of acetaldehyde on the defective surface include H₂, C₂H₄, CO, C₄H₆ and C₄H₈. Reductive coupling, to produce C₂H₄ and C₄H₈, is the main reaction pathway for decomposition of acetaldehyde on the sputter reduced SrTiO₃(100) surface.
2004. "The Effect of Water on the Adsorption of NO₂ in Na- and Ba-Y,FAU Zeolites: A Combined FTIR and TPD Investigation." Journal of Physical Chemistry B 108(12):3746-3753. Abstract The adsorption of NO₂ was investigated and compared on Na- and Ba-Y,FAU zeolites both in the absence and presence of adsorbed water using FTIR and TPD techniques. The same ionic NOx species (NO⁺, NO⁺:NO₂, NO₃-), formed by the disproportionation of NO₂, were observed to form on both materials under dry conditions at room temperature. The thermal stabilities of these species, however, were vastly different on the two materials. Room temperature evacuation was sufficient to decompose the NO⁺NO₂ adduct in Na-Y, while this species was stable up to 350K over Ba-Y. The adsorbed NO⁺ was also much more stable over Ba-Y than on Na-Y. Water significantly affected the adsorbed NOx species on both materials. In the presence of water the IR signatures of adsorbed NO+ were eliminated from both catalysts, however it did not affect the IR feature of the NO⁺NO₂ species on Ba-Y. In the TPD spectra the NO₂ desorption peak shifted from 350K to 520K on Na-Y pre-exposed to water. In Ba-Y the high temperature NO₂ desorption feature of ~470K shifted to ~620K as a result of adsorption on the water containing sample, while the low temperature peak remained unchanged.
2004. "Adsorption, Coadsorption and Reaction of Acetaldehyde and NO₂ on Na-Y,FAU: an in situ FTIR Investigation." Journal of Physical Chemistry B 108(44):17050-17058 . Abstract The adsorption of acetaldehyde and its co-adsorption and reaction with NO₂ were investigated on a Na-Y, FAU zeolite using in situ FTIR spectroscopy. Acetaldehyde adsorbs strongly over Na-Y and desorbs molecularly at around 400K with very limited extent of condensation or polymerization. Reaction between CH₃CHO and NO₂ takes place in co-adsorption experiments even at 300K. In the initial step, acetaldehyde is oxidized to acetic acid accompanied by the formation of NO, which can be observed as N2O₃ formed via a further reaction between NO and NO₂. The key intermediates in the overall NOx reduction in this process are nitro- and nitrosomethane, which form in the next step. Their decomposition and further reaction with adsorbed NOx species lead to the formation of HCN, HNCO, N₂O, CO₂ and organic nitrile species identified by their characteristic IR vibrational signatures. At 473K, the reaction between adsorbed CH₃CHO and NO₂ is very fast. The results seem to suggest a mechanism in which N-N bond formation takes place among ionic nitrogen containing species (NO⁺ and CN⁻ or NCO⁻). No evidence has been found to suggest the participation of NHx⁺NOy⁻ type species in the N⁻N bond formation under the experimental conditions of this study.
2004. "Combination of Low and High Temperature Catalytic Materials to Obtain Broad Temperature Coverage for Plasma-Facilitated NOx Reduction." Catalysis Today 89(1-2):143-150. Abstract Two catalysts, Ba/zeolite Y and Ag/⁻alumina, were combined to reduce NOx in(Two catalysts, Ba/zeolite Y and Ag/ simulated lean exhaust using plasma-facilitated catalysis. Steady-state experiments conducted at 473, 623, and 773K show that ordering of the catalysts impacts NOx conversion, and maximum efficiency is obtained when the zeolite material precedes the alumina. Optimal ordering results in greater than 80% conversion at 473K, and efficiencies greater than 95% were obtained at 623 and 773K under steady operation when propene was added as a reductant at C₁:NOx = 12. Temperature cycling experiments covering the temperature range 373 to 773K were used to determine a ‘transient’ cycle efficiency of 70% for the optimal catalyst configuration. Results of these experiments suggest that preferred ordering likely results in better management of stored NOx when transients to higher temperature occur. Studies also show that improved hydrocarbon utilization is evident in the optimal configuration due to the nature of the partially oxidized hydrocarbons that are formed at various stages of during the plasma-facilitated catalytic NOx reduction process. Keywords: NOx Reduction, Non-Thermal Plasma, Hydrocarbons, Silver-Alumina, Barium-Zeolite Y
2004. "Adsorption and Reaction of SO₂ on Model Ce₁-xZrxO₂(111) Catalysts." Journal of Physical Chemistry B 108(9):2931-2938. Abstract The interaction of SO₂ with ceria-zirconia model catalysts was studied using high-resolution synchrotron-based X-ray photoelectron spectroscopy (XPS). Epitaxial Ce₁ xZrxO₂(111) (x = 0.1 and 0.3) thin films (500-700 Å in thickness) were grown by oxygen-plasma-assisted molecular beam epitaxy on single crystal yttria-stabilized ZrO₂(111). Slightly defective surfaces were achieved by vacuum annealing at 900 K and highly defective surfaces with O vacancies were obtained by 1.5 keV Ne⁺ sputtering. On the slightly defective Ce₀․₉Zr₀․₁O₁․₉₅ (111) and Ce₀․₇Zr₀․₃O₁.₉₅ (111) surfaces, the only product upon SO₂ adsorption at 300 K is sulfate, in the form of Ce(SO₄) ₂, which gradually desorbs from the surface between 300 and 900 K. SO₂ adsorption on the heavily reduced surfaces results in a different behavior. A complex set of compounds is observed during adsorption and thermal conversion processes. The Ce₃⁺ states play a dominant role in the adsorption of SO₂ and cleavage of S-O bonds. The relative amount of sulfur-derived adsorbates depends on the defect concentration: the higher the Ce₃⁺ concentration, the larger the amount of chemisorbed species. On Ce₀․₉Zr₀․₁O₁.₅₀ (111) and Ce₀․₇Zr₀․₃O₁.₅₀ (111) surfaces, SO₄ (in the form of Ce₂(SO₄)₃), SO₃, and atomic sulfur species coexist at 300 K. Thermal annealing (for Ce₀․₉Zr₀․₁O₁.₅₀ (111), > 400 K; for Ce₀․₇Zr₀․₃O₁.₅₀ (111), > 700 K) leads to the formation of an oxy-sulfide (Ce₂O₂S), which is converted from either SO₄ or SO₃.
2004. "Non-Thermal Plasma-assisted NOx Reduction over Alkali and Alkaline Earth Ion Exchanged Y, FAU Zeolites." Catalysis Today 89(1-2):135-141. Abstract The catalytic activities of a series of alkali and alkaline earth cation exchanged Y,FAU zeolites were investigated in the non-thermal plasma-assisted NOx reduction reaction using a simulated diesel engine exhaust gas mixture. The catalytic activity of the Y,FAU zeolite showed significant variations with both the nature of the charge compensating cation, and the method of catalyst preparation. Our results show that conventional multiple solution ion exchange is insufficient to prepare the most active catalyst for the given cationic form. The highest NOx conversion level was achieved over a Ba-Y,FAU which was prepared by a multiple ion exchange method, in which each solution ion exchange step was followed by a high temperature calcination. A systematic change in the catalytic activity was observed as a function of the charge density around the charge compensating cation. For both catalyst series (alkali and alkaline earth ion exchanged Y,FAU), the specific activity decreased with increasing electrostatic field around the charge compensating cation. The large difference in the NOx reduction activity at a given e/r ratio, however, may suggest different reaction mechanisms for the two sets of catalysts. Indeed, there is a noticeable difference in the product distribution (selectivity) for the alkali and alkaline earth series of catalysts. Our results also reveal that extreme care must be taken when catalytic activities are compared for seemingly similar materials. We found that two base zeolite materials with identical Si/Al ratios, obtained from the same manufacturer but from different synthesis batches show significantly different catalytic behavior.
2004. "X-ray Photoelectron Spectroscopy Studies of Oxidized and Reduced CeO₂(111) Surfaces." Surface Science Spectra 11(1-4):73-81. Abstract We have studied the electronic structure of oxidized and reduced CeO₂ (111) surfaces using X-ray photoelectron spectroscopy (XPS). The 50 nm thick Co₂(111) film was grown on a YSZ(111) substrate using oxygen plasma assisted molecular beam epitaxy (OPA-MBE). This film has been characterized using in-situ RHEED (reflection high energy electron diffraction) and ex-situ XRD (X-ray diffraction), HRTEM (high resolution transmission electron microscopy) and RBS (Rutherford backscattering spectroscopy). The lattice mismatch between CeO₂(111) and YSZ(111) is less than 5% and yields a flat surface that is comprised of an equivalent number of Ce⁴⁺ and O₂₋ ions. Oxidation with O₂ at 773 K under UHV conditions was sufficient to fully oxidize the CeO₂(111). Surface reduction was carried out by annealing in UHV at 973 K.
2004. "The Effect of Cavitating Ultrasound on the Heterogeneous Aqueous Hydrogenation of 3-buten-2-ol on Pd-black." Journal of Catalysis 227(2):552-555. Abstract The effect of ultrasound at 20 kHz on the heterogeneous aqueous hydrogenation of 3-buten-2-ol employing a Pd-black catalyst has been studied isothermally at 295 K, forming 2-butanone and 2-butanol products. Our work here shows that adding 1-pentanol as an inert dopant had the effect of inducing cavitation in the ultrasound-treated reaction where it otherwise would not occur. The selectivity showed a 700% increase toward 2-butanol formation and the activity enhanced a factor of 10.8 compared to the noncavitating high-power ultrasound experiment. This study demonstrates that “inert dopants” may have use as synthetic tools in sonocatalysis.
2004. "A Comparison Between Conventional and Ultrasound-Mediated Heterogeneous Catalysis: Hydrogenation of 3-buten-1-ol Aqueous Solutions." Journal of Catalysis 221(2):347-353. Abstract A power flow scheme applicable to probe-type ultrasound reactors is presented, that has been deduced from both experimental measurements employing an unjacketed vessel and theoretical predictions. Under typical conditions for water, 77% of the electrical power is converted into mechanical motion of the probe, that in turn is dissipated to both acoustic power (~12%) and cavitational heating (~88%). Approximately 92% of the mechanical power of the probe was converted into heat, with the remaining power presumably converted into audible acoustic and/or mechanical motion. Heterogeneous catalysis experiments have been performed at 298 K in an isothermal (i.e., jacketed) reaction vessel comparing chemistry in conventional (e.g., thermal) versus ultrasound-assisted systems. Both product state distribution and reaction rate measurements have been performed for the hydrogenation (using hydrogen gas) of aqueous 3-buten-1-ol solutions employing Pd-black powder. Products from the heterogeneous catalysis include isomerization to cis and trans 2-buten-1-ol, as well as hydrogenation to 1-butanol. Based on the observed differences in cis- to trans- 2-buten-1-ol ratios in conventional experiments, employing untreated and pre-reduced catalysts, it has been determined that a kinetic effect controls the observed product state distribution. In addition, differences in the ratio between cis- plus trans- 2-buten-1-ol to 1-butanol, comparing ultrasound-assisted to conventional catalysis, reveal a ~5-fold enhancement in isomerization relative to the more energetically favored hydrogenation due to application of ultrasound. Finally, the product formation rates for 1-butanol, as well as isomerization plus hydrogenation, revealed that conventional and ultrasound experiments showed both a non-linear dependence with applied ultrasound power and no differences between untreated and pre-reduced catalysts. The observed reaction rate enhancements were 1:36:183 for the conventional, 90 W ultrasound, and 190 W ultrasound experiments, respectively.
2004. "X-ray Photoelectron Spectroscopy Studies of Oxidized and Reduced Ce₀․₈Zr₀․₂O₂(111)." Surface Science Spectra 11(1-4):82-90. doi:10.1116/11.20050202 Abstract We have studied the electronic structure of oxidized and reduced Ce₀․₈Zr₀․₂O₂(111) using x-ray photoelectron spectroscopy (XPS). The 50 nm thick Ce₀․₈Zr₀․₂O₂(111) film was grown on a YSZ(111) substrate using oxygen assisted molecular beam epitaxy (OPA-MBE). This film has been characterized using in-situ RHEED (reflection high energy electron diffraction) and ex-situ XRD (x-ray diffraction), HRTEM (high energy resolution transmission electron spectroscopy) and RBS (Rutherford backscattering spectroscopy). Surfaces of the Ce₀․₈Zr₀․₂O₂(111) film used in this study is found to be unreconstructed and exhibits the structure of bulk CeO₂(111) where Zr atoms occupy the lattice sites of Ce in the fluorite structure of ceria. The extent of surface reduction as a result of vacuum annealing has been reported here in addition to the electronic structure of defect-free Ce₀․₈Zr₀․₂O₂(111) surface.
2003. "Interface Structure of an Epitaxial Cubic Ceria Film on Cubic Zirconia." Journal of the American Ceramic Society 86(2):363-365. Abstract A cubic CeO₂ (001) film of thickness equal to ~58 nm was epitaxially grown on Y₂O₃-stablized cubic ZrO₂ by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interface was characterized using high resolution transmission electron microscopy (HRTEM). The interface exhibited coherent regions separated by equally-spaced misfit dislocations. When imaged from the [100] direction, the dislocation spacing is 3.3 ? 0.5 nm, which is slightly shorter than the expected value of 4.9 nm calculated from the differences in lattice constants given in the literature, but is fairly consistent with the 3.9 nm lattice mismatch measured by electron diffraction. Thus, the results presented here indicate that the lattice mismatch between the film and the substrate is accommodated mainly by interface misfit dislocations above some critical thickness.
2003. "Interface Characteristics of Iso-Structural Thin Film and Substrate Pairs." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):1-9. Abstract Cubic-CeO₂ and -Fe₂O₃ thin films have been epitaxially grown on yttria-stabilized ZrO₂ and -Al₂O₃ substrates, respectively, by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interface structural features between the films and the substrates were characterized by Rutherford backscattering spectrometry (RBS), high resolution transmission electron microscopy (HRTEM), and x-ray diffraction (XRD). RBS channeling spectra for both CeO₂/ZrO₂ and Fe₂O₃/Al₂O₃ show interface disorder-related scattering peaks. It is believed that the observed interface disorder-related scattering peaks on RBS spectra are attributed to the interface misfit dislocations. Cross sectional HRTEM reveals that interfaces of both systems are similarly characterized by coherent regions that are separated by misfit dislocations periodically distributed along the interface. The experimentally observed dislocation spacings are approximately consistent with those calculated from the lattice mismatch, implying that the lattice mismatch is accommodated mainly by interface misfit dislocations above the critical thickness.
2003. "The Adsorption of NO₂ and the NO+Oֿ² Reaction on Na-Y,FAU: an in situ FTIR Investigation ." Physical Chemistry Chemical Physics. PCCP 5(18):4045-4051. Abstract The adsorption of NO and NO₂ and the reaction between NO and O₂ were investigated on a Na-Y,FAU zeolite. The interaction between NO and Na-Y is weak and no IR absorption feature is seen upon room temperature adsorption. On the other hand, several NOx species were identified in the adsorption of NO₂ bonded to Lewis acidic (NO₃-, NO₂-) and basic sites (NO⁺ and [NO⁺][NO₂] and [NO⁺][N₂O₄]). In the NO⁺O₂ reaction, N₂O₃ was formed and adsorbed N₂O₃ was observed in addition to the species detected upon NO₂ adsorption. A series of experiments were conducted to unambiguously assign the IR features in the 2000-2120cm-1 spectral range. Through reaction and isotopic substitution (N15O and O182) experiments, these bands were assigned to NO⁺ adsorbed onto framework O- sites as charge compensating cations. Key words: NOx reduction; Na-Y,FAU; IR spectroscopy of adsorbed NOx, isotopic substitution.
2003. "Non-thermal Plasma-Assisted Catalytic NOx Reduction over Ba-Y,FAU: The Effect of Catalyst Preparation." Journal of Catalysis 220(2):291-298. Abstract The effects of catalyst preparation on the NOx reduction activity of a series of Ba-Y,FAU zeolites were investigated using a simulated exhaust gas mixture. The introduction of Ba²⁺ions into Na-Y,FAU results in a large increase in their non-thermal plasma-assisted NOx reduction activity. The NOx reduction activities of Ba-Y,FAU catalysts were found to increase with increasing Ba²⁺ concentration in the aqueous ion exchange solutions, which translated into increased Ba²⁺/Na² ratios in the resulting materials. Consecutive ion exchange procedures at a given Ba²⁺concentration in the aqueous solution, however, did not improve the NOx reduction activities of Ba-Y,FAU catalysts, i.e. the activity of the four times ion exchanged material was the same as that of the one that was ion exchanged only once. The reaction profiles for all of these Ba-Y,FAU catalysts were the same. In contrast, a significant increase in NOx reduction activity was observed when a 773K calcination step was implemented after each solution ion exchange. The reaction profile was also altered as a result of the ion exchange/calcination cycles. Calcination that followed each ion exchange step seems to further increase the Ba²⁺/Na⁺ ratio in the zeolite, and in turn increases the NOx reduction activities of the catalysts prepared this way. Key differences in Na-, and Ba-Y catalysts were found in NO₂ adsorption and TPD experiments. The amount of chemisorbed NO₂ is about twice as high in Ba-Y than in Na-Y, and Ba-Y holds NOx much stronger than Na-Y.
2003. "Redox Properties of Water on the Oxidized and Reduced Surfaces of CeO₂ (111)." Surface Science 526(1-2):1-18. Abstract We present x-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) results probing the surface chemistry of water on the oxidized and reduced surfaces of a 500 Å epitaxial CeO₂ (111) film grown on yttria-stabilized ZrO₂ (111). Oxidation with O₂ at 773 K under UHV conditions was sufficient to generate XPS spectra reflective of fully oxidized CeO₂ (111). Surface reduction was carried out by annealing in UHV between 773 and 973 K, and the level of reduction was quantified using changes in the Ce₃d₃/₂ 4f0 photoemission peak at 917 eV which results primarily from Ce₄⁺ sites. As expected, the level of surface reduction (generation of Ce₃⁺ sites) increased with increasing temperature. These Ce₃⁺ sites were primarily in the first layer based on the fact that exposure of the film to O₂ at RT resulted in nearly complete conversion of Ce₃⁺ to Ce₄⁺. Annealing at 773 K led to a surface in which approximately 40% of the surface Ce₄⁺sites were reduced to Ce₃⁺, whereas annealing at higher temperatures led to more substantial reduction of the first layer along with some subsurface reduction that was not reoxidized by RT exposure to O₂.
2003. "Insights into Photoexcited Electron Scavenging Processes on TiO₂ Obtained from Studies of the Reaction of O₂ with OH Groups Adsorbed at Electronic Defects on TiO₂ (110) Defects on TiO2(110)." Journal of Physical Chemistry B 107(2):534-545. Abstract In this study we show that molecular oxygen reacts with bridging OH (OHbr) groups that are formed as a result of water dissociation at oxygen vacancy defects on the surface of rutile TiO₂ (110). The electronic structure of an oxygen vacancy defect on TiO₂ (110) is essentially the same as that of electron trap states detected on photoexcited or sensitized TiO₂ photocatalysts, being Ti₃⁺ in nature. Electron energy loss spectroscopy (EELS) measurements, in agreement with valence band photoemission results in the literature, indicate that water dissociation at oxygen vacancy sites has little or no impact on the electronic structure of these sites. Temperature programmed desorption (TPD) measurements show that O₂ adsorbed at 120 K reacts with near unity reaction probability with OHbr groups on TiO₂ (110) to form an unidentified intermediate that decomposes to generate terminal OH groups at non-defect sites. Commensurate with this process, the electronic defect associated with the original oxygen vacancy defect (Ti₃⁺) is oxidized. Both vibrational and electronic EELS results indicate that the reaction between O₂and OHbr occurs at about 230 K. Detailed TPD experiments in which the precoverage of water was varied indicate that O₂ need not chemisorb to cation sites on the TiO₂ (110) surface in order for the reaction between O₂ and OHbr to occur, which implies a direct interaction between weakly bound O₂ and the OHbr groups. In agreement with this conclusion, we find that second layer water, which selectively hydrogen-bonds to bridging O and OH groups, blocks the reaction of O₂with OHbr groups and prevents oxidation of the vacancy-related Ti₃⁺ electronic state.
2003. "Conversion of N₂O to Nֿ² on TiO₂ (110)." Catalysis Today 85 (2-4):251-266. Abstract In this study we examine the interaction of TiO₂ with TiO₂ (110) in an effort to better understand the conversion of NOx species to N₂ over TiO₂-based catalysts. The TiO₂ (110) surface was used as a model system because this material is commonly used as a support and because oxygen vacancies on this surface are perhaps the best available models for the role of electronic defects in catalysis. Annealing TiO₂ (110) in vacuum at high temperature (above 800 K) generates oxygen vacancy sites that are associated with reduced surface cations (Ti₃⁺sites) and that are easily quantified using temperature programmed desorption (TPD) of water. Using TPD, x-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), we found that the majority of N₂O molecules adsorbed at 90 K on TiO₂ (110) are weakly held, and desorb from the surface at 130 K. However, a small fraction of the N₂O molecules exposed to TiO₂ (110) at 90 K decompose to N₂ via one of two channels, both of which are vacancy-mediated. One channel occurs at 90 K, and results in N₂ ejection from the surface and vacancy oxidation. We propose that this channel involves N₂O molecules bound at vacancies with the O-end of the molecule. The second channel results from an adsorbed state of N₂O that decomposes at 170 K to liberate N₂ in the gas phase and deposit oxygen adatoms at non-defect Ti₄⁺ sites. The presence of these O adatoms is clearly evident in subsequent water TPD measurements. We propose that this channel involves N₂O molecules that are bound at vacancies with the N-end of the molecule, which permits the O-end of the molecule to interact with an adjacent Ti₄⁺ site. The partitioning between these two channels is roughly 1:1 for adsorption at 90 K, but neither is observed to occur for moderate N₂O exposures at temperatures above 200 K. EELS data indicate that vacancies readily transfer charge to N₂O at 90 K, and this charge transfer facilitates N₂O decomposition. Based on this result, it appears that the decomposition of N₂O to N₂ requires trapping of the Molecule at vacancies and that the lifetime of the N₂O-vacancy interaction may be key to the conversion of N₂O to N₂.
2003. "Adsorption and Reaction of NO on Oxidized and Reduced SrTiO₃ (100) Surfaces." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 21(4):1307-1311. Abstract Adsorption and reaction of NO on oxidized and reduced SrTiO₃ (100) surfaces have been studied using temperature programmed desorption (TPD). Major desorption peaks for NO from the fully oxidized surface as found at 140 and 260 K, along with a long tail that continues up to 500 K. The desorption features at 140 and 260 K correspond to activation energies of 36 and 66 kJ/mol, respectively, using a simple Redhead analysis. NO reacts non-dissociatively on the fully oxidized surface. Reactivity of reduced SrTiO₃ (100) is relatively higher than that of the fully oxidized surface and is influenced by the adsorption temperature of the NO molecules on the surface. NO and N₂O are the major desorption products following adsorption of NO on the reduced surface at 110 K. Desorption of N₂O from significantly reduced SrTiO₃ (100) indicates that the oxygen atoms of the adsorbed NO molecules are preferentially extracted by the surface oxygen vacancy sites whereas the surface TI₃⁺ sites are oxidized as a result of the de-oxygenation of the adsorbates. Adsorption of NO on the reduced surface at 297 K is followed by breakage of the N-O bond producing adsorbed N and O atoms and recombination of these ad-species results in desorption of NO and N₂ from this surface. Adsorption of NO on the significantly reduced surface at 200 K is followed by desorption of NO, N₂ and N₂O as TPD products and the reactivity of this surface at 200 K presumable is a composite of the behavior observed for NO adsorption at 110 and 297 K.
2002. "Electronic and Chemical Properties of Ce0.8Zr0.2O₂ (111) Surfaces: Photoemission, XANES, Density- Functional, and NO₂ Adsorption Studies." In National Synchrotron Light Source Activity Report 2001, BNL--52653, p. Abstract No. liu189 . Brookhaven National Laboratory, Upton, NY. Abstract Zirconia-doped ceria (Ce₁-xZrxO₂) is a complex material and an important component of catalysts used in automotive exhaust gas converters. The exact role of pure and Zr-doped ceria to reduce the emission of toxic pollutants such as nitrogen oxides NOx (NO₂, NO, N₂O) in automobile catalytic converters is not clear. Understanding the details of NOx chemistry on Ce₁-xZrxO₂ surfaces has both practical and academic interests.
2002. "HRTEM Characterization of Interface Between Iso-Structural Thin Solid Film and Substrate." Microscopy & Microanalysis 8(Suppl S02):1160-1161. doi:10.1017/S1431927602107975 Abstract Alpha-Fe₂O₃ and c-CeO₂ thin films have been epitaxially grown on alpha-Al₂O₃ and yttria- stabilized c-ZrO₂ substrates, respectively, by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interface structural features between the films and the substrates were characterized by high resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), Rutherford backscattering spectrometry (RBS), and x-ray diffraction (XRD). For the two systems studied, the interfaces are similarly characterized by coherent regions that are separated by misfit dislocations periodically distributed along the interface. These results will be presented along with the results from molecular dynamics (MD) simulations of these interfaces.
2002. "Growth and Structure of Epitaxial Ce₁-xZrxO₂ Thin Films on Yttria-Stabilized Zirconia (111)." Journal of Electron Spectroscopy and Related Phenomena 126(1-3):177-190. Abstract We describe here studies aimed at the identification of optimum parameters for the epitaxial growth of the mixed-oxides films, Ce₁ xZrxO₂ with x = 0.1, 0.2 and 0.3, by oxygen-plasma-assisted MBE on single crystal Y-stabilized ZrO₂ (YSZ) substrates. The resulting films were characterized by RHEED, LEED, XPS/XPD, XRD, and RBS/C in order to determine their bulk and surface structures and compositions. Pure-phase, epitaxial Ce₁ xZrxO₂ films readily grew on YSZ(111) without showing any contamination of yttria from the substrate. The resulting epitaxial film surfaces are unreconstructed and exhibit the structure of bulk CeO2(111). XPS data indicate that both Ce and Zr cations are formally in the ⁺⁴ oxidation state for all films prepared here. Small differences in the photoemission results for Zr-doped ceria films as compared to those obtained for pure ZrO₂ may be explained by changes in electronic structure when Zr is added to ceria that, in turn, results from longer Zr O bond distances in the mixed oxides. The minimum yields obtained from the random and channeling spectra of these films also provide evidence that high quality single crystal CeO₂ and Ce₀․₇Zr₀․₃O₂ materials were grown. For the Zr-doped films, Zr atoms are shown to occupy the lattice sites of Ce in the bulk structure of CeO₂ (111). Indeed, based on minimum yield values, the fraction of Zr substitution for Ce cations in the film was estimated to be 88%.
2001. "E. Plasma Catalysis for NOx Reduction from Light-Duty Diesel Vehicles." In FY 2001 Progress Report for Combustion and Emission Control for Advancd CIDI Engines, pp. 65-72. US Department of Energy. Office of Transportation Technologies, Washington, D.C.. Abstract In this program, we have been developing a novel plasma/catalyst technology for the remediation of NOx under lean (excess oxygen) conditions, specifically for compression ignition direct injection (CIDI) diesel engines that have significant fuel economy benefits over conventional stoichiometric gasoline engines. Our previous work has shown that a non-thermal plasma in combination with an appropriate catalyst can provide NOx emission reduction efficiency of 60-80% using a simulated diesel exhaust. Based on these levels of NOx reduction obtained in the lab, a simple model was developed in this program last year that allows for the estimation of the fuel economy penalty that would be incurred by operating a plasma/catalyst system. Results obtained from this model suggest that a 5% fuel economy penalty is achievable with the then current state-of-the-art catalyst materials and plasma reactor designs.
2001. "Preparation of Highly Dispersed Cs-Tungstophosphoric Acid Salt on MCM-41 Silica." Catalysis Letters 75(3-4):169-173. Abstract This paper describes a grafting technique that we developed for preparing highly dispersed Cs-tungstophosphoric acid (Cs-TPA) salt on a mesoporous silica carrier (MCM-41) using alcohols as solvents during impregnation. It is shown that this procedure leads to a novel class of catalysts with high dispersion of Cs-TPA salts which exhibit significantly improved catalytic performances for the alkylation of 1,3,5-trimethylbenzene with cyclohexene than those prepared using water as the solvent. Catalyst acitivity demonstrated to have a clear correlation with the type of alcohol used during impregnation which decreased in the following order: 1-butanol > 1-propanol = ethanol > water. Adsorption data using molecules of various sizes suggest that our novel catalysts maintained open pore structures after the impregnation with tungstophosphoric acid (TPA). For comparison purposes, we also used a prior published method to prepare Cs-TPA on MCM-41, which seemed to have suffered unfavorable structural changes such as partical pore clogging leading to significant transport limitations for the reactants examined here.
2001. "Self-Diffusion in Ceria." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 19(4 PT 2):1942-1946. Abstract Ceria (CeO₂) is an oxygen storage material vital to the proper functioning of automobile three-way catalysts and is typically viewed as an anion conductor. Prior experimental work using temperature programmed static secondary ion mass spectrometry (TPSSIMS) has indicated that for rutile TiO₂, a prototypical oxide, the mobile species are Ti cations rather than O anions. To further expand on the mobile species in CeO₂ we have investigated the diffusion of both cerium and oxygen ions by TPSSIMS. The CeO₂(111) film was heteroepitaxially grown by molecular beam epitaxy on a yttria stabilized zirconia substrate. Although high quality LEED patterns and AES spectra free of impurity signals were obtained after just a few sputtering and annealing cycles, further cleaning was necessary to remove intense alkali and alkaline earth signals observed in SSIMS. The CeO₂(111) surface was slightly enriched in 18O by first annealing the film in UHV at 830 K and then exposing the 130 K crystal to 18O₂. TPSSIMS data in conjunction with temperature programmed desorption data demonstrate that surface oxygen atoms begin to diffuse into the bulk around 550K. Physical deposition of sub-monolayer amounts of isotopically enriched cerium (136Ce) in an 18O₂ background allowed the simultaneous study of the diffusion of both cerium and oxygen ions. Surface cerium cations were found to be immobile with no diffusion into the bulk for temperatures up to 900K, the highest temperature studied.
2001. "Electronic and Chemical Properties of Ce0.8Zr0.2O2(111) Surfaces: Photoemission, XANES, Density Functional, and NO2 Adsorption Studies." Journal of Physical Chemistry B 105(32):7762-7770. Abstract Synchrotron-based high-resolution photoemission, conventional X-ray (Mg K?) photoemission (XPS), X-ray absorption near-edge spectroscopy (XANES), and first-principles density functional calculations have been used to study the electronic properties of a Ce0.8Zr0.2O2 mixed-metal oxide. The results of density-functional calculations show that the band gap in bulk Ce0.8Zr0.2O2 is ~ 0.6 eV smaller than in bulk CeO2, with the Zr atoms in the mixed-metal oxide showing smaller positive charges than the cations in ZrO2 or CeO2. When present in a lattice of CeO2, the Zr atoms are forced to adopt larger metal-O distances than in ZrO2, leading to a reduction in the oxidation state of this element. Due to non-equivalent Zr-O distances, at least three different types of oxygen atoms are found in the Ce0.8Zr0.2O2 system. O K-edge XANES spectra for a series of Ce1-xZrxO2 (x=0, 0.1, 0.2, 0.3, 1) compounds show a distinctive line shape for the mixed-metal oxides that cannot be attributed to a sum of CeO2 and ZrO2 features, supporting the idea that the O atoms in Ce1-xZrxO2 are in a special chemical environment. XPS Ce 3d core level spectra show the presence of Ce3+ cations even after prolonged oxidation with oxygen gas, which may be related to the relative stability of oxygen vacancy defects upon incorporation of zirconia into ceria. The interaction of NO2 gas with Ce0.8Zr0.2O2-x(111), CeO2-x(111), and Zr(Y)O2-x(111) reduced surfaces was addressed. Ne+ ion sputtering was used to generate substantial concentrations of Ce3+, Zr2+ and Zr0 centers on the oxide surfaces. On CeO2-x(111), NO3, NO2 and N were seen upon adsorption of NO2. In contrast, only NO2 and N were detected after adsorption of NO2 on Ce0.8Zr0.2O2-x(111) and Zr(Y)O2-x(111). Adsorption of NO2 induced an increase in the oxidation state of the metal cations (Ce3+-->Ce4+; Zr0-->Zr2+).
2001. "Effect of Platinum Nanocluster Size and Titania Surface Structure upon CO Surface Chemistry on Platinum-Supported TiO2 (110)." Journal of Physical Chemistry B 105(12):2412-2416. Abstract The adsorption chemistry of CO on clean and Pt-supported TiO2 (110) was investigated. It was found that surface structure of TiO2 plays an important role in the chemistry that takes place at the surface. On the reduced (1 x 2)-reconstructed surface, CO desorbed at 140 and 170 K, while only desorption at 140 K was observed on the stoichiometric (1 x 1) surface. Additionally, CO dissociation, possibly due to the reduction by Ti 3+ , was observed on the Pt-supported (1 x 2) surface. On the Pt-covered surfaces, the chemistry of CO adsorption and desorption strongly depends on the size of Pt nanoclusters. With a decrease in cluster size, CO was found to desorb at higher temperatures. This unusual desorption chemistry is likely related to quantum size effects of Pt nanoclusters. Scanning tunneling spectra revealed that clusters below 20 ? in diameter exhibited nonmetallic behavior, while those above 40 ? were metallic. This transition of the properties of Pt nanoclusters from metallic to nonmetallic as the cluster size decreases correlates with stronger interaction of CO with Pt observed in temperature-programmed desorption spectra.
2000. "Cs-Substituted Tungstophosphoric Acid Salt Supported on Mesoporous Silica." Catalysis Today 55(1-2):117-124. Abstract Describes the characterization and catalytic properties of mesoporous silica supported Cs-substituted tungstophosphoric acid salt (CS-TPS/MS) with improved dispersion of the active clusters compared to materials described previously in the literature. In particular, transmission electron micrographs and the activity results for a model alkylation reaction are presented as evidence for the enhanced dispersion and performance. In addition, demonstrates improvements in the physical and thermal stability of these materials with Cs substitution using various characterization techniques.
1999. "The Ion Beam Materials Analysis Laboratory at the Environmental Molecular Sciences Laboratory." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 420(1-2):81-89. Abstract Describes the equipment capabilities at EMSL with special emphasis on the accelerator, ion beam analysis and ion beam modification.
1999. " Model Catalyst Studies with Single Crystals and Epitaxial Thin Oxide Films." Catalysis Today 51(3-4):513-519. Abstract The paper describes recent results from our relatively new program to perform detailed studies of the catalytic properites of metal-oxide materials; in particular, to effect a determination of the active catalytic site(s) and the mechanism for reactions over this especially important class of heterogeneous catalysts. Issues of structure-sensitivity, poisioning and promotion, and competing reaction mechanisms are critical questions that need to be addressed in a detailed manner for catalysis by oxides. As just one important example, both surface (Langmui-Hinhelwood) and direct (Eley-Rideal) reaction mechanism have been proposed for the selective catalytic reduction (SCR) reaction of nitrogen oxides (NOx) over vanadia/titania catalysts. For this program, we are using a number of unique, state-of-the-art capabilities available in the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory; for example, the first molecular beam epitaxy (MBE) system dedicated to the growth of model metal-oxide films, and a unique moderate-pressure catalytic reactor/surface science apparatus. We describe the growth, characterization, and water adsorption properties of a thin Fe3O4(001) film grown on a lattice-matched MgO(001) substrate. Because our moderate pressure catalysis studies are preliminary at this point, we instead describe our previous results on the CO oxidation reaction over a Ru(0001) model catalyst to demonstrate the utility of the experimental approach. We specifically discuss the possibility that this reaction occurs by an Eley-Rideal mechanism.
1999. "Segregation of K at the TiO2(100) Surface." Colloids and Surfaces. A, Physicochemical and Engineering Aspects 154(*-2):187-192. Abstract We have studied the segregation of potassium to the surface of TiO2(100) using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), direct recoil spectroscopy (DRS), and mass-spectroscopy of recoiled ions (MSRI). It is found that the concentration of potassium at the surface is strongly influenced by hte sample temperature. MSRI results indicate the presence of potassium starting at annealing temperatures of 675 K and the subsequent disappearance above 875 K. Conversely, AES only indicates the presence of potassium after annealing the sample to 975 K. Together these results indicate that below 875 K, potassium exists in the very topmost surface layer, while above 875 K, potassium is located below this layer.
1999. "Interaction of D2O with CeO2(001) Investigated by Temperature Programmed Desorption and X-ray Photoelectron Spectroscopy." Langmuir 15(11):3993-3997. Abstract Studies the interaction of D2O with the CeO2(001). Finds that D2O desorption occurs in three states with temperature of 152, 200, and 275 K and defines them as multilayer D2O, weakly bound surface D2O, and hydroxyl recombination.
1999. "A Comparison of the NO-CO Reaction over Rh(100), Rh(110) and Rh(111)." Catalysis Letters 62(2-4):131-138. Abstract Reaction rates and product selectivities were measured over the Rh(100) surface as a function of temperature, and CO and NO partial pressures. These results are compared with our prior studies of the NO-CO reaction on the Rh(111) and Rh(110) surfaces. The only products detected for all three surfaces were CO2, N2O and N2. Furthermore, for the Rh(100) surface we have found a significant change in the apparent activation energy (Ea) with reaction temperature. For the Rh(100) surfaces it was found that the Ea can change by a factor of 2.3 in the temperature range investigated here, 528 to 700 K, with the lower values obtained at higher temperatures. In contrast, Ea's were found to remain constant over the same temperature range for the Rh(110) and Rh(111) surfaces. The results oberved for the Rh(100) surface suggest that reaction kinetics are dominated by variations in NO coverages. At low temperatures, the surfaces is fully saturated with NO, and dissociation is limited by the availability of vacancy sites through NO desorption. At high temperatures, the surface is still primarily covered with NO, however the number of vacancy sites has increased substantially. In this case, we propose that the apparent activation energy is now reflecting NO dissociation kinetics rather than those for NO desorption.
1999. "Interaction of Molecular Oxygen with the Vacuum Annealed TiO2 (110) Surface: Molecular and Dissociative Channels." Journal of Physical Chemistry B 103(25):5328-5337. Abstract We have examined the interaction of molecular oxygen with the TiO2 (110) surface using temperature programmed desorption (TPD), isotopic labeling studies, sticking probability measurements and electron energy loss spectroscopy (ELS). Based on the variety of oxygen adsorption states observed in this study, further work is needed in order to determine which oxygen-related species play important roles in chemical photochemical oxidtion processes on TiO2 surfaces.
1999. "Epitaxial Growth and Characterization of Ce1-xZrxO2 Thin Films." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 17(3):961-969. Abstract Epitaxial films have been grown on SrTiO3(001) by oxygen-plasma-assisted molecular beam epitaxy. The film growth at 600 degrees C is predominantly nucleation and growth of 3-D islands. The films become much smoother after rapid thermal annealing at 700 degrees C for 30 seconds in the oxygen plasma. High-energy ion channeling reavelas that Zr atoms substitutionally incorporate at cation sites in the CeO2 lattice for all doping levels, leading to Ce1-xZrxO2 solid solutions. Analysis of Zr 3d and Ce 3d core-level binding energies shows that the oxidation state of both Zr and Ce is +4. Lattice distortion induced by incorporation of Zr in the CeO2 lattice beomes prevalent for high doping levels, and surfaces roughen accordingly.
1998. "Chromyl Chloride Chemistry on the TiO2(110) Surface." Journal of Physical Chemistry B 102:111-122. Abstract Examines the surface chemistry of chromyl chloride on TiO2(110). Results suggest that the reduction and potential immobilization of Cr(VI) species on TiO2 materials may occur thermally if the appropriate surface defect sites are present.