Catalytic NO activation and NO–H2 reaction pathways

Abstract

Kinetic and isotopic data on Pt clusters and activation free energy barriers from density functional theory (DFT) on Pt(111) are used to assess the elementary steps involved in NO–H2 reactions. Pt clusters 1–10nm 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 H2 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–H2 rates and forms OH* and NH* intermediates that undergo fast reactions to give H2O, N2O, NH3, and N2. These conclusions are consistent with (i) measured normal H/D kinetic isotope effects; (ii) rates proportional to H2 pressure, but reaching constant values at higher pressures; (iii) fast H2–D2 equilibration during catalysis; and (iv) DFT-derived activation barriers. These data and calculations, taken together, rule out N–O cleavage via NO* 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–H2 reactions (via *HCOH*) and of O–O cleavage in O2–H2 reactions (via OOH* or *HOOH*). H*-addition weakens the multiple bonds in NO, CO, and O2 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.