Publications (3)3.57 Total impact
-
Article: Catalytic ammonia oxidation on platinum: mechanism and catalyst restructuring at high and low pressure.
[show abstract] [hide abstract]
ABSTRACT: Catalytic ammonia oxidation over platinum has been studied experimentally from UHV up to atmospheric pressure with polycrystalline Pt and with the Pt single crystal orientations (533), (443), (865), and (100). Density functional theory (DFT) calculations explored the reaction pathways on Pt(111) and Pt(211). It was shown, both in theory and experimentally, that ammonia is activated by adsorbed oxygen, i.e. by O(ad) or by OH(ad). In situ XPS up to 1 mbar showed the existence of NH(x)(x= 0,1,2,3) intermediates on Pt(533). Based on a mechanism of ammonia activation via the interaction with O(ad)/OH(ad) a detailed and a simplified mathematical model were formulated which reproduced the experimental data semiquantitatively. From transient experiments in vacuum performed in a transient analysis of products (TAP) reactor it was concluded that N(2)O is formed by recombination of two NO(ad) species and by a reaction between NO(ad) and NH(x,ad)(x= 0,1,2) fragments. Reaction-induced morphological changes were studied with polycrystalline Pt in the mbar range and with stepped Pt single crystals as model systems in the range 10(-5)-10(-1) mbar.Physical Chemistry Chemical Physics 08/2007; 9(27):3522-40. · 3.57 Impact Factor -
Article: Catalytic ammonia oxidation on platinum: mechanism and catalyst restructuring at high and low pressure
[show abstract] [hide abstract]
ABSTRACT: Catalytic ammonia oxidation over platinum has been studied experimentally from UHV up to atmospheric pressure with polycrystalline Pt and with the Pt single crystal orientations (533), (443), (865), and (100). Density functional theory (DFT) calculations explored the reaction pathways on Pt(111) and Pt(211). It was shown, both in theory and experimentally, that ammonia is activated by adsorbed oxygen, i.e. by Oad or by OHad. In situ XPS up to 1 mbar showed the existence of NHx(x= 0,1,2,3) intermediates on Pt(533). Based on a mechanism of ammonia activation via the interaction with Oad/OHad a detailed and a simplified mathematical model were formulated which reproduced the experimental data semiquantitatively. From transient experiments in vacuum performed in a transient analysis of products (TAP) reactor it was concluded that N2O is formed by recombination of two NOad species and by a reaction between NOad and NHx,ad(x= 0,1,2) fragments. Reaction-induced morphological changes were studied with polycrystalline Pt in the mbar range and with stepped Pt single crystals as model systems in the range 10–5–10–1 mbar.Physical Chemistry Chemical Physics, v.9, 3522-3540 (2007). -
Article: Bridging the pressure and material gap in the catalytic ammonia oxidation: structural and catalytic properties of different platinum catalysts
[show abstract] [hide abstract]
ABSTRACT: To understand the “pressure and material gap” in the platinum-catalyzed ammonia oxidation over Pt, the reaction was studied over a wide range of pressures (10−3–105 Pa) and temperatures (293–1073 K) with different Pt catalysts: stepped Pt(533) single crystal, knitted Pt gauze, and Pt foil. Experiments were supplemented by theory applying DFT calculations. It was concluded that the primary reaction step of NH3 oxidation, stripping of hydrogen from the NH3 molecule, is favored by the presence of surface O and OH species. The latter species are more active for dehydrogenation of NH2 and NH fragments. NO and N2 are the only nitrogen-containing products detected under UHV conditions (p<10−1 Pa). N2O was observed, however, at about 6 Pa (peak pressure) in the temporal analysis of products (TAP) reactor and under ambient pressure conditions of 100 kPa in a microstructured reactor. The pressure dependence of N2O formation is suggested to be related to a minimum surface coverage by reaction intermediates required for N2O formation, which is a real “pressure gap” phenomenon. Independently of the pressure range (10−3–105 Pa) and the type of Pt specimen, N2 formation prevails at low temperatures (<700 K), whereas NO production increases with temperature and becomes the dominant reaction channel at high temperature. Catalyst characterization by SEM revealed a reconstruction of the Pt surface after ammonia oxidation at ambient pressure. The degree of surface restructuring is related to the total exposure of the catalyst to the reactants. Surface roughening contributes to activation of the catalyst and changes in its selectivity.Journal of Catalysis.
