Selectivity of chemisorbed oxygen in C–H bond activation and CO oxidation and kinetic consequences for CH₄–O₂ catalysis on Pt and Rh clusters

Rate measurements, density functional theory (DFT) within the framework of transition state theory, and ensemble-averaging methods are used to probe oxygen selectivities, defined as the reaction probability ratios for O* reactions with CO and CH₄, during CH₄–O₂ catalysis on Pt and Rh clusters. CO₂ and H₂O are the predominant products, but small amounts of CO form as chemisorbed oxygen atoms (O*) are depleted from cluster surfaces. Oxygen selectivities, measured using ¹²CO–¹³CH₄–O₂ reactants, increase with O₂/CO ratio and O* coverage and are much larger than unity at all conditions on Pt clusters. These results suggest that O* reacts much faster with CO than with CH₄, causing any CO that forms and desorbs from metal cluster surfaces to react along the reactor bed with other O* to produce CO₂ at any residence time required for detectable extents of CH₄ conversion. O* selectivities were also calculated by averaging DFT-derived activation barriers for CO and CH₄ oxidation reactions over all distinct surface sites on cubo-octahedral Pt clusters (1.8nm diameter, 201 Pt atoms) at low O* coverages, which are prevalent at low O₂ pressures during catalysis. CO oxidation involves non-activated molecular CO adsorption as the kinetically relevant step on exposed Pt atoms vicinal of chemisorbed O* atoms (on *–O* site pairs). CH₄ oxidation occurs via kinetically relevant C–H bond activation on *–* site pairs involving oxidative insertion of a Pt atom into one of the C–H bonds in CH₄, forming a three-centered HC₃–Pt–H transition state. C–H bond activation barriers reflect the strength of Pt–CH₃ and Pt–H interactions at the transition state, which correlates, in turn, with the Pt coordination and with CH₃ * binding energies. Ensemble-averaged O* selectivities increase linearly with O₂/CO ratios, which define the O* coverages, via a proportionality constant. The proportionality constant is given by the ratio of rate constants for O₂ dissociation and C–H bond activation elementary steps; the values for this constant are much larger than unity and are higher on larger Pt clusters (1.8–33nm) at all temperatures (573–1273K) relevant for CH₄–O₂ reactions. The barriers for the kinetically relevant C–H bond dissociation step increase, while those for CO oxidation remain unchanged as the Pt coordination number and cluster size increase, and lead, in turn, to higher O* selectivities on larger Pt clusters. Oxygen selectivities were much larger on Rh than Pt, because the limiting reac