2009. "Reaction of NO2 with a pure, thick BaO film: the effect of temperature on the nature of NOx species formed." Journal of Physical Chemistry C 113(6):2134-2140. Abstract The adsorption and reaction of NO2 on a thick (>30 ML), pure BaO film deposited onto an Al2O3/NiAl(110) substrate were investigated in the temperature range of 300 – 660 K using temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and x-ray photoelectron spectroscopy (XPS) techniques. The adsorption of NO2 on BaO at room temperature results in the formation of nitrite-nitrate ion pairs. During thermal desorption the nitrite species decompose first, releasing an NO molecule and leaving an O on the surface, while nitrate species decompose in two steps at higher temperatures: at lower temperature as NO2 only, then, at higher temperature, as NO + O2. In cyclic experiments when the BaO film was exposed to NO2 at 300 K, followed by annealing to 575 K, a large amount of NOx was stored as nitrates, and no saturation was achieved even after the 10th adsorption/anneal cycle. This suggests the gradual conversion of the BaO film into barium nitrate clusters at elevated temperatures. The rate of nitrate formation increases as the sample temperature during NO2 exposure increases up to 610 K, while at even higher temperatures the amount of nitrates formed decreases. NO2 adsorption on the thick BaO film at 610 K results in the formation of strongly bound nitrates as the major NOx species.
2009. "Interaction of D2O with a Thick BaO Film: Formation of and Phase Transitions in Barium Hydroxides." Journal of Physical Chemistry C 113(35):15692-15697. Abstract The interaction of D2O with a thick BaO film (≥ 20 mono-layer equivalent (MLE)) on ultra-thin Al2O3/NiAl(110) was investigated with temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Upon D2O exposure of a thick BaO film amorphous barium hydroxide formed at room temperature that readily converted to crystalline Ba(OD)2 phases during annealing in ultra-high vacuum (UHV). The formation of crystalline hydroxide phases depends on the initial D2O exposure at 300 K. Following low D2O exposure at room temperature that results in the formation of amorphous barium hydroxide with no hydrating water, only the a-Ba(OD)2 phase was observed after 400 K annealing. The sample that was exposed to D2O extensively (i.e. hydrated amorphous barium hydroxide formed) showed a series of phase transformations as the sample was annealed to increasingly higher temperatures: amorphous- Ba(OD)2•xD2O (x>1)à b-Ba(OD)2.D2Oà b-Ba(OD)2àa-Ba(OD)2. The results of TPD experiments completely agreed with this phase transformation scheme: hydrating water molecules desorbed first at 425 K, allowing the formation of the b-Ba(OD)2.D2O phase. Desorption of water from b-Ba(OD)2.D2O at around 475 K leads to the formation of b-Ba(OD)2 and its subsequent conversion to a-Ba(OH)2. All the barium hydroxides thermally decomposed at T < 550 K. When the BaO film was exposed to D2O at 425 K crystalline b-Ba(OD)2 formed initially, which lead to the formation of a small amount of a-Ba(OD)2 as well at low D2O exposures. At high D2O exposures the dominant phase was b-Ba(OD)2.xD2O, and no a-phase was seen.
2009. "BaO/Al2O3/NiAl(110) Model NOx Storage Materials: the effect of BaO film thickness on the amorphous-to-crystalline Ba(NO3)2 phase transition." Journal of Physical Chemistry C 113(2):716-723. Abstract The reaction of NO2 with BaO (0.15 – 2 ML and > 30 ML)/Al2O3(12 ML)/NiAl(110) model NOx storage materials was studied. A thick (~12 ML), ordered Al2O3 film was prepared as the support oxide on a NiAl(110) substrate in order to minimize the effect of the intermixing between the two oxide phases (BaO and Al2O3) on the NOx chemistry of BaO. The growth of a thick alumina film, prepared by atomic oxygen deposition onto NiAl(110), follows a layer-by-layer growth mode and the resulting film is much more stable when exposed to NO2 than the ultra-thin alumina films studied before. The interaction of NO2 with the model NOx storage systems at low coverages of BaO show fundamentally different behaviors from a thick BaO film, as nitrite species form at low exposures of NO2, followed by nitrate formation at high NO2 exposures. In contrast, on the thick BaO layer nitrite-nitrate ion pairs form at 300 K under UHV conditions (PNO2 ~ 1 10-9 Torr). However, at elevated NO2 pressures (≥ 1 10-5 Torr) the thick BaO film is gradually converted into amorphous Ba(NO3)2 at 300 K. Raising the temperature of the samples with ΘBaO > 1 ML after NO2 exposure (in the absence of gas phase NO2) leads to the phase transformation of the amorphous Ba(NO3)2 layer into crystalline Ba(NO3)2 particles in the temperature range of 500 – 600 K. No phase transformation is observed in samples with ΘBaO < 1 ML.
2009. "Growth and characterization of barium oxide nanoclusters on YSZ(111)." Journal of Physical Chemistry C 113(32):14324-14328. doi:10.1021/jp9020068 Abstract Barium oxide (BaO) was grown on YSZ(111) substrate by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE). In-situ reflection high-energy electron diffraction, ex-situ x-ray diffraction, atomic force microscopy and x-ray photoelectron spectroscopy have confirmed that the BaO grows as clusters on YSZ(111). During and following the growth under UHV conditions, BaO remains in single phase. When exposed to ambient conditions, the clusters transformed to BaCO3 and/or Ba(OH)2 H2O. However, in a few attempts of BaO growth, XRD results show a fairly single phase cubic BaO with a lattice constant of 0.5418(1) nm. XPS results show that exposing BaO clusters to ambient conditions results in the formation BaCO3 on the surface and partly Ba(OH)2 throughout in the bulk. Based on the observations, it is concluded that the BaO nanoclusters grown on YSZ(111) are highly reactive in ambient conditions. The variation in the reactivity of BaO between different attempts of the growth is attributed to the cluster size.
2009. "Reactivity of a Thick BaO Film Supported on Pt(111): Adsorption and Reaction of NO2, H2O and CO2." Langmuir 25(18):10820-10828. doi:10.1021/la901371g Abstract Reactions of NO2, H2O, and CO2 with a thick (> 20 MLE) BaO film supported on Pt(111) were studied with temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). NO2 reacts with a thick BaO to form surface nitrite-nitrate ion pairs at 300 K, while only nitrates form at 600 K. In the thermal decomposition process of nitrite–nitrate ion pairs, first nitrites decompose and desorb as NO. Then nitrates decompose in two steps : at lower temperature with the release of NO2 and at higher temperature, nitrates dissociate to NO + O2. The thick BaO layer converts completely to Ba(OH)2 following the adsorption of H2O at 300 K. Dehydration/dehydroxylation of this hydroxide layer can be fully achieved by annealing to 550 K. CO2 also reacts with BaO to form BaCO3 that completely decomposes to regenerate BaO upon annealing to 825 K. However, the thick BaO film cannot be converted completely to Ba(NOx)2 or BaCO3 under the experimental conditions employed in this study.
2009. "Oxygen Coverage Dependence of NO Oxidation on Pt(111)." Journal of Physical Chemistry C 113(14):5766-5776. Abstract The interaction of NO with adsorbed atomic oxygen on Pt(111) was studied with temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS) and low energy electron diffraction (LEED). Atomic oxygen adlayers with 0.25 and 0.75 ML coverages were prepared on a Pt(111) single crystal by dissociative chemisorption of O2 at 300 K and NO2 at 400 K, respectively. These two oxygen pre-covered surfaces were used to study the oxygen coverage dependence of NO oxidation at different sample temperatures. The well ordered p(2x2)-O layer, corresponding to ΘO = 0.25 ML, does not react with NO to form NO2 in the temperature range of 350 - 500 K, in contrast to CO oxidation which takes place readily at a sample temperature as low as 300 K. At ΘO = 0.75 ML, however, the NO oxidation reaction is facile, and the formation of NO2 is observed even at 150 K. However, the NO oxidation reaction completely stops as the atomic oxygen coverage drops below 0.28 ML, because all the weakly bound oxygen atoms available only at higher O coverages have been consumed. The remaining oxygen atoms are bound too strongly to the Pt(111) surface and, therefore, unable to participate in NO oxidation in the 150 – 500 K temperature range.
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.
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. "NOx Reduction on a Transition Metal-free γ-Al2O3 Catalyst Using Dimethylether (DME)." Catalysis Today 136(1-2):46-54. Abstract NO2 and dimethylether (DME) adsorption as well as DME and NO2 coadsorption on a transition metal-free γ-alumina catalyst were investigated via in-situ transmission Fourier transform infrared spectroscopy (in-situ FTIR), residual gas analysis (RGA) and temperature programmed desorption (TPD) techniques. NO2 adsorption at room temperature leads to the formation of surface nitrates and nitrites. DME adsorption on the alumina surface at 300 K leads to molecularly adsorbed DME, molecularly adsorbed methanol and surface methoxides. Upon heating the DME-exposed alumina to 500-600 K the surface is dominated by methoxide groups. At higher temperatures methoxide groups are converted into formates. At T > 510 K formate decomposition takes place to form H2O(g) and CO(g). DME and NO2 coadsorption at 423 K do not indicate a significant reaction between DME and NO2. However, in similar experiments at 573 K, fast reaction occurs and the methoxides present at 573 K before the NO2 adsorption are converted into formates, simultaneously with the formation of isocyanates. Under these conditions, NCO can further be hydrolyzed into isocyanic acid or ammonia with the help of water which is generated during the formate formation, decomposition and/or NCO formation steps.
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. "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.
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. "The interaction of NO2 with BaO: from cooperative adsorption to Ba(NO3)2 formation." Journal of Physical Chemistry C 111(42):15299-15305. doi:10.1021/jp074179c 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. The conversion of surface to bulk Ba-nitrates 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. "D2O Adsorption on an Ultrathin Alumina Film on NiAl(110)." Journal of Physical Chemistry C 111(47):17597-17602. doi:10.1021/jp074459s Abstract The structure of an ordered, ultra-thin Al2O3 film grown on a NiAl(110) single-crystal surface and its interaction with D2O were studied by low energy ion scattering spectroscopy (LEISS), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). LEISS demonstrated that the surface was terminated by an oxygen layer, and TPD data of adsorbed D2O revealed that most of the water molecularly adsorbed onto Al2O3/NiAl(110). However, we observed that a small amount of water molecules dissociated during adsorption and/or TPD measurements, and as a consequence the alumina film thickness increased after water adsorption/desorption. These results suggest that atomic oxygen and/or hydroxyl species, which are formed by dissociation of water, interact with sub-surface aluminum atoms through defects sites and cause the increase in the alumina film thickness. In addition, due to the weak interaction between adsorbed water molecules and the alumina film, a few monolayer of water can be transformed from amorphous solid water (ASW) to crystalline ice (CI) phase, as it was seen by IRAS.
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. "Physisorption of CO2 on non-ice materials relevant to icy satellites." Icarus 191(1):371-380. doi:10.1016/j.icarus.2007.04.012 Abstract CO2 is known to adsorb onto clay and other minerals when a significant atmospheric pressure is present. We have found that CO2 can also adsorb onto some clays when the CO2 partial pressure is effectively zero under ultra-high vacuum (UHV) if cooled to the surface temperatures of the icy satellites of Jupiter and Saturn. The strength of adsorption and the spectral characteristics of the adsorbed CO2 infrared (IR) ν3 absorption band near 4.25 μm depend on the composition and temperature of the adsorbent. CO2 remains adsorbed onto the clay mineral montmorillonite for >10 s of min when exposed to a vacuum of ∼1×10−8 Torr at ∼125 K. CO2 does not adsorb onto serpentine, goethite, or palagonite under these conditions. A small amount may adsorb onto kaolinite. When heated above 150 K under vacuum, the CO2 desorbs from the montmorillonite within a few minutes. The ν3 absorption band of CO2 adsorbed onto montmorillonite at 125 K is similar to that of the CO2 detected on the saturnian and Galilean satellites and is markedly different from CO2 adsorbed onto montmorillonite at room temperature. We infer the adsorption process is physisorption and postulate that this mechanism may explain the presence and spectral characteristics of the CO2 detected in the surfaces of these outer satellites.
2007. "Effects of surface coordination on the temperature-programmed desorption of oxalate from goethite." Journal of Physical Chemistry C 111(45):17072-17081. doi:10.1021/jp075576q Abstract The temperature-programmed desorption (TPD) of weakly-bound, hydrogen-bonded and metal-bonded oxalate complexes at the goethite surface was investigated in the 300-900 K range with concerted Fourier Transform Infrared (FTIR) measurements (TPD-FTIR). These reactions took place with the concomitant dehydroxylation reaction of goethite to hematite and decarbonation of bulk-occluded carbonate. The measurements revealed three important stages of desorption. Stage I (300-440 K) corresponds to the desorption of weakly-and/or un-bound oxalate molecules in the goethite powder with a thermal decomposition reaction pathway characteristic of oxalic acid. Stage II (440-520 K) corresponds to a thermally-driven dehydration of hydrogen-bonded surface complexes, leading to a partial desorption via oxalic acid thermal decomposition pathways and to a partial conversion to metal-bonded surface complexes. This latter mechanism led to the increase in FTIR bands characteristic of these complexes. Finally, Stage III (520-660 K) corresponds to the thermal decomposition of the metal-bonded oxalate complex, proceeding through a 2 electron reduction pathway.
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. "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. "A Combined FTIR and TPD Study on the Bulk and Surface Dehydroxylation and Decarbonation of Synthetic Goethite." Geochimica et Cosmochimica Acta 70(14):3613-3624. Abstract The thermal dehydroxylation of dried goethite was studied with combined Fourier Transform Infrared (FTIR)-Temperature Programmed Desorption (TPD) experiments in the presence and absence of adsorbed carbonate and oxalate. The TPD data revealed a dehydroxylation peaks involving the intrinsic dehydroxylation of goethite at 575 K but also a low temperature peak at 475 K which was shown to be associated to the release of non-stoichiometric water in the goethite bulk and surface. The FTIR and the TPD data of goethite in the absence of adsorbed species revealed the presence of adventitious carbonate mostly sequestered in the goethite bulk. The release of carbonate was however not only related to the dehydration of goethite but also to the compaction of the hematite phase at temperatures exceeding 700 K. The relative abundance of surface hydroxyls was shown to change systematically upon goethite dehydroxylation with a preferential stripping of -OH sites followed by a dramatic change in the dominance of the different surface hydroxyls upon the crystallization of hematite. The presence of surface-bound carbonate revealed similar FTIR bands as those of bulk-carbonate, suggesting similar bonding structures. The release of surface-bound carbonate was moreover shown to be associated to the low temperature dehydration peak and concomitantly with the preferential stripping of –OH sites more than 100 K below the dehydroxylation edge of goethite. The presence of chemisorbed oxalate species produced an important release of CO at 425 K, followed by the presence of a transient carbonyl-bearing surface species (e.g. oxalate of different coordination geometry or formate) which were subsequently decomposed to formic acid and CO2 at higher temperatures. The thermal decomposition of oxalate/formate did not, however, reveal additional adsorbed CO2(g) at the goethite surface.
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. "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. "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.
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. "Thermal Chemistry of Trimethyl Acetic Acid on TiO₂(110)." Journal of Physical Chemistry B 108(11):3592-3602. Abstract Based on temperature programmed desorption and isothermal reaction mass spectrometry, the thermal surface chemistry of trimethyl acetic acid, (CH₃)₃CCOOH, dosed onto a well characterized single crystal TiO₂(110) surface is described. Deprotonation occurs at or below 300 K to form trimethyl acetate, (CH₃)₃CCOO-, and hydroxide, OH-. (CH₃)₃CCOO- is bound to exposed Ti₄⁺ cations and OH- involves a bridging oxygen atom of the substrate. Based on temperature programmed desorption and isothermal reaction mass spectrometry, the desorbing products include (CH₃)₃CCOOH, isobutene (i-C₄H₈), carbon monoxide and water accompanied by smaller amounts of other products including methyl isopropenyl ketone (CH₂=C(CH₃)C(=O)CH₃), isobutane (i-C4H10), and di-t-butyl ketone, (CH₃)₃CC(=O)C(CH₃)₃. Much of the (CH₃)₃CCOO- is relatively stable and decomposes to release mainly carbon monoxide and isobutene above 550 K with a maximum rate at 660 K. Thermal desorption to 750 K leaves a carbon-free surface that is indistinguishable from the initially clean surface. During dosing at 550 K, a steady-state reaction condition is realized with about half the adsorption sites being occupied at any instant.
2004. "Interaction of H₂O and NO₂ with BaY Faujasite: Complex Contraction/Expansion Behavior of the Zeolite Unit Cell ." Journal of Physical Chemistry B 16613-16616 . Abstract In situ time-resolved X-ray diffraction in combination with the Rietveld method was used to study the contraction/expansion behavior of a BaY faujasite zeolite during dehydration and NO₂ adsorption/desorption reactions. The adsorption of water and NO₂ molecules induced a significant contraction of hte zeolite unit cell. The unexpected contraction of the unit cell was closely related to the amount of water and NOx species (NO⁺, NO₂, NO₃⁻) inside the framework. The reaction H₂O + NO₂ --> NO₃⁻ + H⁺ had a substantial effect on the structural properties of the BaY system. The NO₃⁻ groups produced a shift in the position of the Ba cations and a reduction in the zeolite unit cell.
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. "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.
2003. "The Photon-Driven Hydrophilicity of Titania: A Model Study Using TiO₂(110) and Adsorbed Trimethyl acetate." Journal of Physical Chemistry B 107(34):9029-9033. Abstract The behavior of H₂O on clean and trimethylacetate (TMA)-covered TiO₂(110)-(1x1), prepared with or without oxygen vacancies and associated Ti₃⁺, reveals the hydrophilic nature of clean surfaces and the hydrophobic nature of TMA-covered surfaces. UV irradiation of a hydrophobic surface in the presence of 10-6 Torr of O₂ removes TMA with a cross section of at least 10-17 cm₂ photon-1 and rapidly restores hydrophilicity. The presence of oxygen vacancies does not detectably increase the hydrophilicity of either clean or TMA-covered TiO₂(110).
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. "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.