Influence of CeO₂ and WO₃ Addition to Impregnated V₂O₅/TiO₂ Catalysts on the Selective Catalytic Reduction of NOₓ with NH₃

Abstract

We investigate the effect of cerium and tungsten addition to optimize the deNOₓ activity of V₂O₅/TiO₂ catalysts over a broad temperature range in the catalytic reduction of NOₓ with NH₃ (NH₃-SCR) with and without the presence of water. The catalysts were synthesized following co-impregnation of TiO₂ with different loadings and varying content of V₂O₅, CeO₂ and WO₃ oxides as promoters. Based on surface and bulk characterization, we show that all catalysts undergo different structural changes depending on the chemical nature of the promoters. X-ray photoelectron spectra indicate a tendency for surface reduction after addition of CeO₂, surface oxidation after addition of WO₃, and after catalytic NH₃-SCR. Promotion of V₂O₅/TiO₂ catalysts with CeO₂ and/or WO₃ broadens the operation temperature window of the catalytic NH₃-SCR reaction under both dry and wet conditions and improves the N₂ selectivity at high temperatures. The thermal deactivation resistance of CeO₂- and WO₃-promoted catalysts improves with increasing amount of WO₃. We tentatively relate this to suppression of the sintering of the active VOₓ component and increasing the amount of CeVO₄. The latter, as a consequence of Ce-V interaction, detrimentally changes the surface composition of the catalysts and hides active V in the bulk structure inaccessible for reaction. Water slightly decreases the overall catalytic activity of SCR at low temperatures, while preventing the formation of N₂O at elevated temperatures. Addition of CeO₂ leads to a slight decrease in overall reducibility of the catalysts, while W causes an enhancement in quantitative H₂ uptake. On the contrary, the sole addition of CeO₂ leads to an enhancement of ammonia adsorption and the appearance of new acidic surface sites, which beneficially combine the reduced surface of the catalysts with an enhanced deNOₓ activity at low temperature.