Coadsorption of CO and O2 on selected gold clusters:: Evidence for efficient room-temperature CO2 generation

Spurred by the recent demonstrations of the size- and support-dependent reactivity of supported gold clusters, here we present results on the coadsorption of CO and O-2 on selected anionic gold clusters, Au-N(-), in the gas phase. O-2 adsorbs in a binary (0, 1) fashion as a one-electron acceptor on the Au-N(-) clusters, with even-N clusters showing varying reactivity toward O-2 adsorption, while odd-N clusters show no evidence of reactivity. CO shows a highly size-dependent reactivity for Au-N(-) sizes from N = 4 to 19, but no adsorption on the gold dimer or trimer. When the gold clusters are exposed to both reactants, either simultaneously or sequentially, interesting effects have been observed. While the same rules pertaining to individual O-2 or CO adsorption continue to apply, the preadsorption of one reactant on a cluster may lead to the increased reactivity of the cluster to the other reactant. Thus, the adsorbates are not competing for bonding sites (competitive coadsorption), but, instead, aid in the adsorption of one another (cooperative coadsorption). New peaks also arise in the mass spectrum of Au-6(-) under CO and O-2 coadsorption conditions, which can be attributed to the loss of a CO2 molecule (or molecules). By studying the relative amount of reaction, and relating it to the reaction time, it is found that the gas-phase Au-6 anion is capable of oxidizing CO at a rate 100 times that reported for commercial or model gold catalysts.