Tungsten oxide-promotion mechanism for supported V₂O₅/TiO₂ catalysts during NOx abatement: structural effects revealed by ⁵¹V MAS NMR.

The selective catalytic reduction (SCR) of NOx with NH3 to N-2 with supported V2O5(-WO3)/TiO2 catalysts is an industrial technology used to mitigate toxic emissions. Long-standing uncertainties in the molecular structures of surface vanadia are clarified, whereby progressive addition of vanadia to TiO2 forms oligomeric vanadia structures and reveals a proportional relationship of SCR reaction rate to [surface VOx concentration](2), implying a 2-site mechanism. Unreactive surface tungsta (WO3) also promote the formation of oligomeric vanadia (V2O5) sites, showing that promoter incorporation enhances the SCR reaction by a structural effect generating adjacent surface sites and not from electronic effects as previously proposed. The findings outline a method to assess structural effects of promoter incorporation on catalysts and reveal both the dual-site requirement for the SCR reaction and the important structural promotional effect that tungsten oxide offers for the SCR reaction by V2O5/TiO2 catalysts.