Role of Vanadium in Thermal and Hydrothermal Aging of a Commercial V₂O₅-WO₃/TiO₂ Monolith for Selective Catalytic Reduction of NOx: A Case Study

In recent years, increased attention to air pollutants such as NOx has led the scientific community to focus meaningfully on developing strategies for NOx reduction. Selective catalytic reduction of NOx by ammonia (NO SCR by NH3) is currently the main method to remove NOx from diesel engine exhaust emissions. The catalysts with typical V2O5-WO3/TiO2 (VWTi) composition are widely used in NH3-SCR for their high NOx conversion activity, low cost, and robustness, especially concerning sulfur poisoning. However, in real diesel engine working conditions, the thermal and hydrothermal aging of catalysts can occur after several hours of operation at high temperature, affecting the catalytic performance. In this study, the stability of a commercial VWTi monolith, self-supported and containing glass fibers and bentonite in its matrix, was investigated as a case study. In laboratory conditions, NO SCR tests were performed for 50 h in the range of 150 to 350 °C. Subsequently, the VWTi monolith was thermally and hydrothermally aged at 600 °C for 6 h. The thermal aging increased the NOx conversion, especially at low temperature (<250 °C), while the hydrothermal aging did not affect the SCR. The differences in NOx conversion before and after aging were associated with the change in vanadium and tungsten oxide surface coverage and with the reduction in the surface area of catalysts. In order to correlate the change in SCR activity with the modifications occurring after aging processes, the monolithic samples were characterized by several techniques, namely XRD, SSA and pore analysis, TPR, XPS, Raman, TGA and SEM/EDX.