Structure-Performance Relationships for Redox Promoted (CeOx and CuOx) and Surface Sulfated V2O5-WO3/TiO2 Catalysts for Selective Catalytic Reduction (SCR) of NO with NH3

Current use of the supported V₂O₅-WO₃/TiO₂ catalyst for the selective catalytic reduction (SCR) of NO_x with NH₃ is extensive. Improved low temperature NO conversion is critical to mitigate N₂O generation at currently used high temperatures (T>300°C) and to improve performance under “cold start” conditions. These problems can seemingly be overcome by the integration of redox promoters such as Ce and Cu. Furthermore, the effect of SO₂ typically found in industrial flue gas on the structure-performance relations of the catalysts is still poorly understood. In the present work, in situ Raman and IR spectroscopies are used to investigate the anchoring, interactions, surface structures and acidity of the metal oxide surface sites. The effects of CeO_x, CuO_x and SO_x surface sites on the SCR performance of the catalysts is measured using temperature programmed surface reaction (TPSR) spectroscopy. The results indicate that the redox promoters are fully dispersed on the TiO₂ support and create unique surface hydroxyls that serve as the primary anchoring sites for the surface VO_x sites. Both the redox promoters and the surface sulfates affect the surface acidity, with the basic redox and acidic sulfate surface sites strengthening the Lewis and Brønsted acid characters of the catalysts, respectively. Both redox promoters enhanced the low temperature NO conversion activity of the catalyst, but only the CeO_x promoted catalyst exhibited lower N₂O generation. While the unpromoted catalyst remained unaffected by sulfation, the NO conversion activity of the redox promoted catalysts was inhibited. Finally, it was found that the SCR reaction can efficiently proceed via both Lewis and Brønsted acid sites, resolving the long-lived debate regarding the superior role of one acid site over another. These results serve to establish currently missing fundamental structure-performance relations for the interactions between CeO_x, CuO_x and SO_x surface sites and theV₂O₅-WO₃/TiO₂ catalytic system.