Binding of SO 3 to Group 4 Transition Metal Oxide Nanoclusters.

Transition metal oxide (TMO) clusters are being studied for their ability to absorb acid gases generated by energy production processes. The interaction of SO 3 , a byproduct of common industrial processes, with group 4 metal (Ti, Zr, and Hf) oxide nanoclusters, has been predicted using electronic structure methods. The calculations were done at the density functional theory (DFT) and correlated molecular orbital coupled cluster singles and doubles CCSD(T) theory levels. There is a reasonable agreement between the DFT/ωB97x-D energies with the CCSD(T) results. SO 3 is predicted to strongly chemisorb to these clusters, as do NO 2 and CO 2 . For SO 3 , these chemisorption processes favor binding to TMO clusters as SO 4 2- sulfate in both the terminal and bridging configurations. It is predicted that SO 3 fully extracts the bridging oxygen from the TMO lattice to form bridging SO 4 2- . This is favorable because of the lower S-O bond dissociation energy of SO 3 , whereas other acid gases add across the bridging oxygen because of their higher A-O bond dissociation energy. SO 3 is capable of physisorption as long as an exposed metal center is present in the lattice. If a metal center has a terminal oxo-group, then SO 3 will prefer the SO 4 2- configuration. An approximately linear relationship exists between the physisorption energy and proton affinity for rows 2 and 3 elements.