Effects of Hydrothermal Aging on Standard and Fast SCR over CeWO_<i>x</i>: A Quantitative Study on Adsorption Sites and Reaction Mechanism

CeWOx after hydrothermal aging at 700 °C for 10 h exhibited >90% nitrogen oxide (NOx) conversion at 200–450 °C either in the presence or absence of H2O during fast selective catalytic reduction of NOx (fast SCR), which was higher than or comparable to other oxide catalysts without hydrothermal aging reported in the literature. Even after hydrothermal aging at 800 °C for 10 h, CeWOx could achieve >80% NOx conversion at 200–400 °C, either in the presence or absence of H2O. After hydrothermal aging, the adsorption amounts of NO and NH3 on aged CeWOx decreased dramatically, and NO activation, nitrate production, and the “nitrite path” (Langmuir–Hinshelwood mechanism) were inhibited, causing a drastic decline in NOx conversion during standard SCR. During fast SCR, although the adsorption amount of NO was low, the interaction between gaseous NO2 and NH3 to form NH4NO3 increased the adsorption amounts of NO2 and NH3, compensating for the loss of adsorption sites. NH4NO3 took part in the “NH4NO3 fast path” and “NH4NO3 path” (Eley–Rideal mechanism) and was an important intermediate for obtaining a high activity. These are the reasons why fast SCR maintains an excellent activity. The presence of water inhibited the “nitrite path” and “NH4NO3 path”, leading to decreased NOx conversion.