Promotional roles of ZrO2 and WO3 in V2O5-WO3/TiO2-ZrO2 catalysts for NOx reduction by NH3: Catalytic performance, morphology, and reaction mechanism

Abstract

V2O5/TiO2-ZrO2 catalysts containing various amounts of WO3 were synthesized. The catalyst morphologies, catalytic performances, and reaction mechanisms in the selective catalytic reduction of NOx by NH3 were investigated using in situ diffuse-reflectance infrared Fourier-transform spectroscopy, temperature-programmed reduction (TPR), X-ray diffraction, and the Brunauer-Emmett-Teller (BET) method. The BET surface area of the triple oxides increased with increasing ZrO2 doping but gradually decreased with increasing WO3 loading. Addition of sufficient WO3 helped to stabilize the pore structure and the combination of WO3 and ZrO2 improved dispersion of all the metal oxides. The mechanisms of reactions using V2O5-9%WO3/TiO2-ZrO2 and V2O5-9%WO3/TiO2 were compared by using either a single or mixed gas feed and various pretreatments. The results suggest that both reactions followed the Eley-Ridel mechanism; however, the dominant acid sites, which depended on the addition of WO3 or ZrO2, determined the pathways for NOx reduction, and involved [NH4+–NO–Brönsted acid site]* and [NH2–NO–Lewis acid site]* intermediates, respectively. NH3-TPR and H2-TPR showed that the metal oxides in the catalysts were not reduced by NH3 and O2 did not reoxidize the catalyst surfaces but participated in the formation of H2O and NO2.