ARTICLE pubs.acs.org/JACS Reactivity of Chemisorbed Oxygen Atoms and Their Catalytic Consequences during CH4O2 Catalysis on Supported Pt Clusters

ABSTRACT: Kinetic and isotopic data and density functional theory treatments provide evidence for the elementary steps and the active site requirements involved in the four distinct kinetic regimes observed during CH4 oxidation reactions using O2, H2O, or CO2 as oxidants on Pt clusters. These four regimes exhibit distinct rate equations because of the involvement of different kinetically relevant steps, predominant adsorbed species, and rate and equilib-rium constants for different elementary steps. Transitions among regimes occur as chemisorbed oxygen (O*) coverages change on Pt clusters. O * coverages are given, in turn, by a virtual O2 pressure, which represents the pressure that would give the prevalent steady-state O * coverages if their adsorptiondesorption equilibrium was maintained. The virtual O2 pressure acts as a surrogate for oxygen chemical potentials at catalytic surfaces and reflects the kinetic coupling betweenCHandOdOactivation steps. O * coverages and virtual pressures depend onO2 pressure whenO2 activation is equilibrated and on O2/CH4 ratios when this step becomes irreversible as a result of fast scavenging of O * by CH4-derived intermediates. In three of these kinetic regimes, CH bond activation is the sole kinetically relevant step, but occurs on different active sites, which evolve fro