Mixed VO _x /WO _x Sites for NH ₃ ‐SCR Activity in W‐Promoted V ₂ O ₅ /TiO ₂ Catalysts

Abstract

Abstract Tungsten is often used as a promoter in VO x /TiO 2 catalysts for the selective catalytic reduction of NO x by ammonia (NH 3 ‐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 VO x spreads more easily on the TiO 2 surface, while WO x tends to form WO 3 agglomerates before complete surface coverage. The estimated monolayer coverage is 5.7 (V + W) atoms/nm 2 . 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. WO x /TiO 2 catalysts show minimal activity, showing that at least one V atom is required in the active site. The turnover frequencies per V atom in VO x /WO x /TiO 2 catalysts are 1.5–2 times higher than in unpromoted VO x /TiO 2 . EPR reveals that the amount of V 4+ 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.