Catalytic NO activation and NO-H2 reaction pathways

* NO-H2 turnover rates depend weakly on Pt cluster size (1.5-10nm). * HD formation rates in NO-H2-D2 reactions show quasi-equilibrated H2 dissociation. * Isotopic effects are consistent with kinetically-relevant * HNOH * formation. * Theory indicates that N-O cleaves only after H * -additions to form NOH * and * HNOH * . 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.