Reaction of CO₂ with Groups 4 and 6 Transition Metal Oxide Clusters

Density functional theory (DFT) and coupled cluster theory (CCSD(T)) were used to study the addition of CO₂ to group 4 (MO₂)_n and group 6 (MO₃)_n (n = 1, 2, 3) nanoclusters. The structures and energetics arising from Lewis acid-base addition (physisorption) and formation of CO₃^2- (chemisorption) of CO₂ to these clusters were predicted. Physisorption and chemisorption of CO₂ are predicted to be thermodynamically allowed for group 4 (MO₂)_n clusters, with chemisorption being more favored energetically. Correlations of the ligand binding energies (LBEs) for the group 4 clusters are made with the fluoride affinities and M-O and M═O bond strengths of the clusters. The LBEs for chemisorption on the Zr and Ti clusters are consistent with published experimental and computational studies of bulk solids. Physisorption LBEs for the Ti and Zr clusters are more exothermic than the bulk values, as the cluster models allow for better relaxation at the metal sites. Chemisorption is not predicted to occur with group 6 (MO₃)_n clusters, as the larger chemisorbed structures were all found to be metastable. CO₂ is predicted to weakly physisorb to (WO₃)_n with physisorption correlating with the Lewis acidity of the metal site.