PNNL

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 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 DFTderived activation barriers for CO and CH4 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 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 (1.8–33 nm) at all temperatures (573–1273 K) 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 reactants for CO oxidation were completely consumed in ¹²CO–¹³CH4–O2 mixtures, consistent with lower CO/O2 ratios measured by varying the residence time and O2/CH4 ratio independently in CH4–O2 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 CH4 to CO (and H2) does not occur at the molecular scale on Pt and Rh clusters. CO (and H2) are predominantly formed upon complete O2 depletion from the sequential reforming steps.