Structure sensitivity via decoration of low-coordination exposed metal atoms: CO oxidation catalysis on Pt clusters

Abstract

The effects of CO and O2 concentrations on turnover rates and 18O2–16O2 exchange rates during catalysis are used to assess the relevant elementary steps and the consequences of Pt coordination for CO oxidation catalysis at moderate temperatures (700–800K) on supported Pt clusters 1.8–25nm in diameter. Turnover rates, measured under conditions of strict kinetic control, are proportional to O2 pressure and inhibited by CO; these data are consistent with kinetically-relevant O2 dissociation steps on cluster surfaces covered partially by chemisorbed CO (CO*). O2 dissociation also limits CO oxidation rates at higher temperatures, which lead to bare Pt surfaces, and at lower temperatures, where saturation CO* coverages require O2 dissociation to be assisted by CO* because of a dearth of vacant sites. At the intermediate temperatures used here, kinetic coupling between irreversible O2 activation and CO* reactions with O* causes edge and corner atoms to become decorated by unreactive O* species; consequently, turnovers occur predominantly on exposed low-index planes, which account for a decreasing fraction of exposed atoms with increasing metal dispersion. These decoration effects confer the appearance of structure sensitivity to the prototypical structure insensitive reaction by rendering only a fraction of exposed metal atoms able to turnover. These active sites, residing at exposed low-index planes, show similar CO* binding energies on large and small Pt clusters, but their relative abundance decreases as clusters become smaller, leading to a sharp decrease in turnover rates with increasing Pt dispersion. These trends stand in marked contrast with the absence of cluster size effects on CO oxidation rates at low temperatures, where high CO* coverages dampen the intrinsic site non-uniformity of metal clusters, and at high temperatures, where all Pt atoms remain accessible irrespective of coordination and active for catalytic turnovers.