Selectivity of chemisorbed oxygen in C–H bond activation and CO oxidation and kinetic consequences for CH 4–O 2 catalysis on Pt and Rh clusters

Abstract

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 CH 4, during CH 4–O 2 catalysis on Pt and Rh clusters. CO 2 and H 2O 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– 13CH 4–O 2 reactants, increase with O 2/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 CH 4, causing any CO that forms and desorbs from metal cluster surfaces to react along the reactor bed with other O * to produce CO 2 at any residence time required for detectable extents of CH 4 conversion. O * selectivities were also calculated by averaging DFT-derived activation barriers for CO and CH 4 oxidation reactions over all distinct surface sites on cubo-octahedral Pt clusters (1.8 nm diameter, 201 Pt atoms) at low O * coverages, which are prevalent at low O 2 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). CH 4 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 CH 4, forming a three-centered HC 3–Pt–H transition state. C–H bond activation barriers reflect the strength of Pt–CH 3 and Pt–H interactions at the transition state, which correlates, in turn, with the Pt coordination and with CH 3 * binding energies. Ensemble-averaged O * selectivities increase linearly with O 2/CO ratios, which define the O * coverages, via a proportionality constant. The proportionality constant is given by the ratio of rate constants for O 2 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 (1.8–33 nm) at all temperatures (573–1273 K) relevant for CH 4–O 2 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 reactants for CO oxidation were completely consumed in 12CO– 13CH 4–O 2 mixtures, consistent with lower CO/CO 2 ratios measured by varying the residence time and O 2/CH 4 ratio independently in CH 4–O 2 reactions. These mechanistic assessments and theoretical treatments for O * selectivity provide rigorous evidence of low intrinsic limits of the maximum CO yields, thus confirming that direct catalytic partial oxidation of CH 4 to CO (and H 2) does not occur at the molecular scale on Pt and Rh clusters. CO (and H 2) are predominantly formed upon complete O 2 depletion from the sequential reforming steps.