1,2-Ethanediol and 1,3-Propanediol Conversions over (MO3)3 (M = Mo, W) Nanoclusters: A Computational Study

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 SO42- 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 SO42-. 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 SO42- configuration. An approximately linear relationship exists between the physisorption energy and proton affinity for rows 2 and 3 elements.

Reaction of NO2 with Groups IV and VI Transition Metal Oxide Clusters

The addition of NO₂ to Group IV (MO₂)_n and Group VI (MO₃)_n (n = 1-3) nanoclusters was studied using both density functional theory (DFT) and coupled cluster theory (CCSD(T)). The structures and overall binding energetics were predicted for Lewis acid-base addition without transfer of spin (a physisorption-type process) and the formation of either cluster-ONO (HONO-like or bidentate bonding) or NO₃^- formation where for both the spin is transferred to the metal oxide clusters (a chemisorption-type process). Only chemisorption of NO₂ is predicted to be thermodynamically allowed at temperatures ≥298 K for Group IV (MO₂)_n clusters with the formation of surface chemisorbed NO₂ being by far the most energetically favorable. The ligand binding energies (LBEs) for physisorption and chemisorption on the TiO₂ nanoclusters are consistent with computational studies of the bulk solids. Chemisorption is only predicted to occur for (CrO₃)_n clusters in the form of a terminal nitrate containing species whereas the larger chemisorbed nitrate structures for (MoO₃)_n and (WO₃)_n were found to be metastable and unlikely to form in any appreciable amount at temperatures of 298 K and higher. NO₂ is predicted to only be capable of physisorbing to (MoO₃)_n and (WO₃)_n at lower temperatures and therefore unlikely to bind NO₂ at temperatures ≥298 K. Correlations between the (MO₃)_nNO₂ ligand bond energies and the chemical properties of the parent (MO₃)_n clusters (Lewis acidity, ionization potentials, excitation energies, and M = O/M-O bond strengths) are described.