Adsorption of carbon monoxide on smaller gold-cluster anions in an atmospheric-pressure flow-reactor: Temperature and humidity dependence

In the absence of moisture and at room temperature, the activity and saturation of CO on gold cluster anions, Au_N^-, are known to be highly dependent on the size of the cluster. Small Au_N ^- clusters (N = 2,3) showed no adsorption activity, and the saturation CO adsorption values did not increase proportionately to cluster size or area. Here, we report on the effects of water vapor and temperature on the ability of Au_N^- clusters to adsorb CO in a high-pressure, fast-flow reactor. In contrast to all earlier reports, our results using this method show that smaller gold-cluster anions bind single and multiple CO groups at ambient temperature and above. In particular, species previously unseen at room temperature, corresponding to Au₂(CO)^-, Au ₃(CO)^- and Au₄(CO)₂^-, have been observed. Apparently, the presence of water vapor facilitates the adsorption of CO on the smaller clusters, possibly by aiding in the release of adsorption energy. As the number of studies concerning gold catalysis has continually increased over the past decade, these results provide important new information on the possible role of moisture in gold catalysis.