Effects of Hydrothermal Aging on Standard and Fast SCR over CeWOx: A Quantitative Study on Adsorption Sites and Reaction Mechanism

CeWO_x after hydrothermal aging at 700 °C for 10 h exhibited >90% nitrogen oxide (NO_x) conversion at 200–450 °C either in the presence or absence of H₂O during fast selective catalytic reduction of NO_x (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, CeWO_x could achieve >80% NO_x 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 CeWO_x decreased dramatically, and NO activation, nitrate production, and the “nitrite path” (Langmuir–Hinshelwood mechanism) were inhibited, causing a drastic decline in NO_x 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 NO_x conversion.