Cation synergies affect ammonia adsorption over VO_X and (V,W)O_X dispersed on α-Al₂O₃ (0001) and α-Fe₂O₃ (0001)

The catalytic behavior of oxide-supported metal oxide species depends on the nature of the support and the presence of co-catalysts. We use density functional theory (DFT) to explore the relationship between the structure and chemical behavior of vanadium oxide in light of its industrial use for the selective catalytic reduction of nitric oxide with ammonia (NO-SCR). The relative stabilities of dispersed VO_X monomers, dimers, and long-chain oligomers on two model oxide support surfaces with similar structure but drastically different chemical behavior, α-Al₂O₃ (0001) and α-Fe₂O₃ (0001), are determined. The effect of added tungsten, known to promote NO-SCR, is also investigated on the relatively inert α-Al₂O₃ (0001) support. We find that the adsorption behavior of NH₃, representing the first step of the NO-SCR reaction, depends strongly on the VO_X local structure. Protonation of NH₃ to NH₄⁺ over surface hydroxyls is energetically favorable over VO_X-WO_X dimers and VO_X oligomers, which are stabilized by the reducible α-Fe₂O₃ (0001) support.