Mixed VOx/WOx Sites for NH3‐SCR Activity in W‐Promoted V2O5/TiO2 Catalysts

Tungsten is often used as a promoter in VOx/TiO2 catalysts for the selective catalytic reduction of NOx by ammonia (NH3-SCR). To better understand the role of tungsten, we have investigated the dispersion of V and W and the activity of catalysts with variable V and W contents. Infrared spectroscopy of CO probe molecules shows that VOx spreads more easily on the TiO2surface, while WOx tends to form WO3 agglomerates before complete surface coverage. The estimated monolayer coverage is 5.7 (V + W) atoms/nm2. At molar W/V ratios below 1, the activity is proportional to the square of (V + W) surface density, while above 1, the activity remains constant upon increasing W content. WOx/TiO2 catalysts show minimal activity, showing that at least one V atom is required in the active site. The turnover frequencies per V atom in VOx/WOx/TiO2 catalysts are 1.5–2 times higher than in unpromoted VOx/TiO2. EPR reveals that the amount of V4+ is approximately the same in promoted and unpromoted catalysts at monolayer coverage. These results are consistent with randomly mixed layers of vanadium and tungsten oxide, with an approximately equal contribution of V–O–V and V–O–W sites to the activity, in which the vanadium atom provides the redox function.