Binding of SO₃ 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 SO₃, 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. SO₃ is predicted to strongly chemisorb to these clusters, as do NO₂ and CO₂. For SO₃, these chemisorption processes favor binding to TMO clusters as SO₄^2- sulfate in both the terminal and bridging configurations. It is predicted that SO₃ fully extracts the bridging oxygen from the TMO lattice to form bridging SO₄^2-. This is favorable because of the lower S-O bond dissociation energy of SO₃, whereas other acid gases add across the bridging oxygen because of their higher A-O bond dissociation energy. SO₃ 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 SO₃ will prefer the SO₄^2- configuration. An approximately linear relationship exists between the physisorption energy and proton affinity for rows 2 and 3 elements.