Different Carbonate Isomers Formed by the Addition of CO2 to M3O6- for M = Ti, Zr, and Hf.

The chemisorption addition of CO2 to M3O6 and M3O6- for M = Ti, Zr, and Hf was examined using couple cluster CCSD(T) theory using density functional theory B3LYP geometries. For neutral M3O6CO2, a bridge chemisorbed tridentate carbonate cluster is the lowest energy for Ti and Zr, and a terminal chemisorbed bidentate carbonate is the lowest energy for Hf. For anionic M3O6CO2-, the lowest energy structure is a terminal chemisorbed bidentate carbonate for all three metals. The use of correlation-consistent weighted core basis sets for the CCSD(T) calculations is shown to be necessary to obtain the correct energy ordering for the isomers. Only for Ti3O6CO2- is a center tridentate carbonate cluster a low energy isomer. The electron affinities of M3O6CO2 are ∼0.2 eV larger than for M3O6. The CO2 chemisorption binding energies increase slightly for M3O6- as compared to those for M3O6.