Catalytic NO activation and NO-H-2 reaction pathways

Hibbitts, DD; Jimenez, R; Yoshimura, M; Weiss, B.; Iglesia, E

Keywords: platinum, density functional theory, Nitric oxide reduction, Cluster size effects, Kinetic isotope effects


Kinetic and isotopic data on Pt clusters and activation free energy barriers from density functional theory (DFT) on Pt(1 1 1) are used to assess the elementary steps involved in NO-H-2 reactions. Pt clusters 1-10 nm in diameter gave similar turnover rates, indicating that these elementary steps are insensitive to surface-atom coordination. N-O cleavage occurs after sequential addition of two chemisorbed H-atoms (H*) to NO* which are quasi-equilibrated with H-2 and NO co-reactants. The first step is equilibrated and forms HNO*, while the second addition is irreversible and forms *HNOH*; this latter step limits NO-H-2 rates and forms OH* and NH* intermediates that undergo fast reactions to give H2O, N2O, NH3, and N-2. These conclusions are consistent with (i) measured normal H/D kinetic isotope effects; (ii) rates proportional to H-2 pressure, but reaching constant values at higher pressures; (iii) fast H-2-D-2 equilibration during catalysis; and (iv) DFT-derived activation barriers. These data and calculations, taken together, rule out N-O cleavage via N-O* reactions with another NO* (forming O* and N2O) or with vicinal vacancies (forming N* and O*), which have much higher barriers than H*-assisted routes. The cleavage of N-O bonds via *HNOH* intermediates is reminiscent of C-O cleavage in CO-H-2 reactions (via (HCOH)-H-**) and of O-O cleavage in O-2-H-2 reactions (via OOH* or *HOOH*). H*-addition weakens the multiple bonds in NO, CO, and O-2 and allows coordination of each atom to metal surfaces; as a result, dissociation occurs via such assisted routes at all surface coverages relevant in the practice of catalysis. (C) 2014 Published by Elsevier Inc.

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Título según WOS: Catalytic NO activation and NO-H-2 reaction pathways
Título según SCOPUS: Catalytic NO activation and NO-H2 reaction pathways
Título de la Revista: JOURNAL OF CATALYSIS
Volumen: 319
Fecha de publicación: 2014
Página de inicio: 95
Página final: 109
Idioma: English