Activation of CO by supported Cu clusters.

Catalytic reduction of carbon dioxide to useful chemicals is a potent way to mitigate this greenhouse gas, but the challenge lies in finding active reduction catalysts. Using density functional theory we studied CO activation over TiO-supported Cu clusters of size 1-4 atoms. The linear to bent transformation of CO is necessary for activation, and we found that all the clusters stabilized bent CO, along with a significant gain of electrons on the CO (indicative of activation). On all the TiO supported Cu clusters, the interfacial sites were found to stabilize the bent CO adsorption, where the active site of adsorption on Cu dimer, trimer and tetramer was on the Cu atom farthest away from the TiO surface. Particularly, the Cu dimer stabilized bent CO very strongly, although this species was found to be unstable on the surface. A synthesis technique that could stabilize the Cu dimer could therefore lead to a very active catalyst. Furthermore we found (using vibrational and charge analysis) that the active sites for the CO activation predominantly had 0 and +1 oxidation states; the oxidation state of Cu is known to directly affect CO reduction activity. Our study shows TiO-supported small Cu clusters can be active catalysts for CO reduction and also provides further motivation for theoretical and experimental studies of metal clusters.