A theoretical study on Brønsted acidity of WO3 clusters supported on metal oxide supports by “paired interacting orbitals” (PIO) analysis

Ammonia adsorption on Brønsted acid sites of WO3 cluster supported on metal oxide supports: SnO2, ZrO2, and TiO2, is analyzed by PIO analysis. We employed (HO)(WO3)4(H) and (HO)(WO3)9(H) on (SnO2)12, (ZrO2)12, and (TiO2)12, respectively, as supported Brønsted acid models and examined two types of Brønsted acid sites, an edge type and a face type. We estimated ammonia adsorption strength by total overlap population (∑OP) of all PIOs between the Brønsted acid site and NH3. The order of acidity (∑OP) of each model is as follows: edge type: SnO2, 0.0096 > ZrO2, 0.0048 > TiO2, −0.0001 ≫ face type: ZrO2, −0.0759 > TiO2, −0.0761 > SnO2, −0.0867. The edge type adsorption is far stronger than the face type one. This order in the edge type coincides with the experimental results. The reason of these results is explained by the difference of the influence of oxygen atoms sitting near the N atom of NH3.