Catalytic Consequences of Chemisorbed Oxygen during Methanol Oxidative Dehydrogenation on Pd Clusters

Abstract

Surface oxygen contents and thermodynamic activities of nanosized Pd clusters and their connection to the reactivities for methanol oxidative dehydrogenation are established from rate measurements in the kinetically controlled regime and oxygen uptake studies at chemical equilibrium. First-order rate coefficients for methanol oxidative dehydrogenation (turnover rates divided by CH3OH pressure) are single-valued functions of the oxygen-to-methanol ratio in the contacting gas phase because this ratio determines the oxygen coverages, the relative abundance of chemisorbed oxygen and unoccupied Pd sites, and in turn, the identities of the kinetically relevant steps at Pd cluster surfaces. As the oxygen-to-methanol ratio increases, the most abundant surface intermediates on Pd clusters vary from uncovered to saturated with chemisorbed oxygen; in response to this shift in coverages, the kinetically relevant step concomitantly varies from oxygen dissociation, to CH3OH activation on oxygen adatom and oxygen vacancy pairs (O*–*), and then to CH3OH activation on oxygen adatom pairs (O*–O*), during which the first-order rate coefficients initially increase and then reach a maximum value before decreasing to a constant value that does not vary with the oxygen-to-methanol ratio. These dependencies reflect the dual catalytic functionality of chemisorbed oxygen, first promoting the methanol conversion as the oxygen coverages increase and then inhibiting the reaction at near O* saturation, as oxygen displaces the unoccupied Pd sites, thus removing the O*–* centers while replacing them with less reactive O*–O* centers for CH3OH activation. Methanol acts as a surface oxygen scavenger that reduces the oxygen chemical potential at Pd cluster surfaces during its catalytic turnovers. The oxygen-scavenging step by methanol increases with temperature to a larger extent than the O2 activation step. As a result of more effective oxygen removal (by reactions) at higher temperatures, the critical oxygen coverage required for the transition of regimes from *–* to O*–* as the predominant surface sites occurs at higher oxygen-to-methanol ratios. Larger Pd clusters are more reactive for methanol oxidative dehydrogenation than smaller clusters in all regimes because cluster dimension influences the relative abundance of chemisorbed oxygen and unoccupied Pd sites, the oxygen binding strengths, and their reactivities. The direct connection of first-order rate coefficient and oxidant-to-reductant ratio appears to be general for methanol oxidative dehydrogenation on other transition metals and for oxidation catalysis, for which the reductant scavenges the chemisorbed oxygen effectively, thus dictating their coverages and thermodynamic activities.