EMSL: Janos Szanyi's Publications

Kim DH, J Szanyi, JH Kwak, X Wang, JC Hanson, MH Engelhard, and CHF Peden. 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.

Kwak JH, JZ Hu, D Mei, CWW Yi, DH Kim, CHF Peden, L Allard, and J Szanyi. 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.

Kwak JH, D Mei, CWW Yi, DH Kim, CHF Peden, L Allard, and J Szanyi. 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

Mei D, Q Ge, JH Kwak, DH Kim, CM Verrier, J Szanyi, and CHF Peden. 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.

Mei D, Q Ge, J Szanyi, and CHF Peden. 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.

Mudiyanselage K, CW Yi, and J Szanyi. 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.

Mudiyanselage K, CWW Yi, and J Szanyi. 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.

Nachimuthu P, YJ Kim, SVNT Kuchibhatla, Z Yu, W Jiang, MH Engelhard, V Shutthanandan, J Szanyi, and S Thevuthasan. 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.

Yi CWW, and J Szanyi. 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.

Yi CW, and J Szanyi. 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.