Effects of Hydrothermal\nAging on Standard and Fast\nSCR over CeWO<sub><i>x</i></sub>: A Quantitative Study on\nAdsorption Sites and Reaction Mechanism

CeWO<sub><i>x</i></sub> after hydrothermal\naging at 700\n°C for 10 h exhibited >90% nitrogen oxide (NO<sub><i>x</i></sub>) conversion at 200–450 °C either in\nthe presence\nor absence of H<sub>2</sub>O during fast selective catalytic reduction\nof NO<sub><i>x</i></sub> (fast SCR), which was higher than\nor comparable to other oxide catalysts without hydrothermal aging\nreported in the literature. Even after hydrothermal aging at 800 °C\nfor 10 h, CeWO<sub><i>x</i></sub> could achieve >80%\nNO<sub><i>x</i></sub> conversion at 200–400 °C,\neither\nin the presence or absence of H<sub>2</sub>O. After hydrothermal aging,\nthe adsorption amounts of NO and NH<sub>3</sub> on aged CeWO<sub><i>x</i></sub> decreased dramatically, and NO activation, nitrate\nproduction, and the “nitrite path” (Langmuir–Hinshelwood\nmechanism) were inhibited, causing a drastic decline in NO<sub><i>x</i></sub> conversion during standard SCR. During fast SCR,\nalthough the adsorption amount of NO was low, the interaction between\ngaseous NO<sub>2</sub> and NH<sub>3</sub> to form NH<sub>4</sub>NO<sub>3</sub> increased the adsorption amounts of NO<sub>2</sub> and NH<sub>3</sub>, compensating for the loss of adsorption sites. NH<sub>4</sub>NO<sub>3</sub> took part in the “NH<sub>4</sub>NO<sub>3</sub> fast path” and “NH<sub>4</sub>NO<sub>3</sub> path”\n(Eley–Rideal mechanism) and was an important intermediate for\nobtaining a high activity. These are the reasons why fast SCR maintains\nan excellent activity. The presence of water inhibited the “nitrite\npath” and “NH<sub>4</sub>NO<sub>3</sub> path”,\nleading to decreased NO<sub><i>x</i></sub> conversion.