Size-dependent catalytic activity for CO oxidation over sub-nano Au clusters

Abstract

Gold (Au) nanocatalysts present outstanding activity for many reactions and have long attracted much attention, but the size effect of sub-nano clusters on catalytic activity lacks systematic research. Using CO oxidation as a probe reaction, the size-dependent catalytic capability of sub-nano Au clusters was explored. The global-minimum structures of AuN (N=2-300, <2.5nm) were obtained utilizing revised particle swarm optimization (RPSO) combined with density functional theory (DFT) calculation and Gupta empirical potential. Geometric structural descriptors built a bridge among geometric features, adsorption energy, and CO oxidation rate of each site of any given sub-nano Au clusters, making it possible for high-throughput evaluation on adsorption energy and catalytic activity of the whole sub-nano Au cluster. The activity per unit mass of sub-nano Au clusters shows a volcano-shape relationship with cluster size, where the sub-nano Au clusters with 0.75 nm diameter possess the highest CO2 formation rate per unit mass, since the Edge and Kink site with higher turnover frequency (approximately 106) than Face site (approximately 102), contribute the most to CO2 formation. The increasing size after 0.75nm diameter of sub-nano Au clusters brings about higher percentage of Face site. The weak adsorption of CO and O2 was found to be a crucial factor determining the inferior activity of Face site to Kink and Edge site. The adsorption process rather than the surface reaction step becoming the rate-determining step on Face site, attributing to the decreasing activity per unit mass of sub-nano Au clusters. This work provides an in-depth mechanistic understanding of size-dependent catalytic activity for the Au clusters at the sub-nano level.