TY - JOUR

T1 - Bridging the pressure and material gap in the catalytic ammonia oxidation

T2 - Structural and catalytic properties of different platinum catalysts

AU - Baerns, M.

AU - Imbihl, R.

AU - Kondratenko, V. A.

AU - Kraehnert, R.

AU - Offermans, W. K.

AU - Van Santen, R. A.

AU - Scheibe, A.

N1 - Funding Information: The authors thank Deutsche Forschungsgemeinschaft (DFG) for financial support in the frame of the competence network SPP1091, “Bridging the gap between real and ideal systems in heterogeneous catalysis.” V. Kondratenko thanks the Institute for Applied Chemistry Berlin-Adlershof e.V. for a Ph.D. fellowship. The provision of SEM results by Gisela Weinberg and Robert Schlögl was highly appreciated.

PY - 2005/4/12

Y1 - 2005/4/12

N2 - 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.

AB - 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.

KW - Ammonia oxidation

KW - Microstructured reactor

KW - NO

KW - NH

KW - Platinum

KW - SEM

KW - Surface restructuring of Pt catalysts

KW - TAP reactor

KW - UHV reactor

UR - http://www.scopus.com/inward/record.url?scp=17644400675&partnerID=8YFLogxK

U2 - 10.1016/j.jcat.2005.03.002

DO - 10.1016/j.jcat.2005.03.002

M3 - Article

AN - SCOPUS:17644400675

VL - 232

SP - 226

EP - 238

JO - Journal of catalysis

JF - Journal of catalysis

SN - 0021-9517

IS - 1

ER -