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

The addition of NO₂ to Group IV (MO₂) and Group VI (MO₃) (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) 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) clusters in the form of a terminal nitrate containing species whereas the larger chemisorbed nitrate structures for (MoO₃) and (WO₃) 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) and (WO₃) at lower temperatures and therefore unlikely to bind NO₂ at temperatures ≥298 K. Correlations between the (MO₃)NO₂ ligand bond energies and the chemical properties of the parent (MO₃) clusters (Lewis acidity, ionization potentials, excitation energies, and M = O/M-O bond strengths) are described.