Highly efficient Ce0.8WyMn0.2-yOx oxides for low-temperature reduction of nitrogen oxides: Balance of acidity and reducibility

This study aims to enhance the low-temperature NH 3 -SCR activity of CeWO x catalysts by optimizing the Mn/W ratio. CeWO x oxides have shown outstanding medium-temperature activity for ammonia selective catalytic reduction (NH 3 -SCR) of NO x emitted from stationary sources . However, the inferior low-temperature performances (<150℃) have significantly impeded their widely practical application. Herein, a series of Ce 0.8 W 0.2-y Mn y O x catalysts possessing distinct Mn/W molar ratios were successfully synthesized via a straightforward homogeneous precipitation method, and their catalytic performances were evaluated for NH 3 -SCR of NO x . The experimental results show that the surface acidity and redox properties of Ce 0.8 W 0.2-y Mn y O x catalysts are considerably affected by the Mn/W ratio. Therein, Ce 0.8 W 0.05 Mn 0.15 O x catalyst exhibits the optimal low-temperature efficiency and the widest temperature window with NO x conversion above 90 % in the temperature range of 130-315℃, which is notably wider than that of Ce 0.8 W 0.15 Mn 0.05 O x (220-300℃) and Ce 0.8 W 0.1 Mn 0.1 O x (180-310℃) counterparts. This should be due to its suitable acidity and excellent redox properties. In-situ DRIFTS results confirmed that appropriate Mn/W not only effectively promoted the reaction of NO with NH 4 NO 3 to generate active NO 2 and subsequently the “fast-SCR” reaction (NO + NO 2 + 2NH 3 → 2 N 2 + 3H 2 O), avoiding the accumulation of NH 4 NO 3 hindering the catalytic active sites, but also facilitated the generation of the active HONO and thereafter NH 4 NO 2 intermediate through the Langmuir-Hinshelwood (L-H) mechanism, thereby enhancing the low-temperature NH 3 -SCR performances of Ce 0.8 W 0.2-y Mn y O x (y < 0.2) catalysts. Nevertheless, excessive Mn/W ratio not only weakened the acidity and suppressed the adsorption of ammonia molecules at the high temperature but also boosted the generation of inert nitrates covering the catalytic active sites, thereby seriously suppressing the NH 3 -SCR performance of Ce 0.8 Mn 0.2 O x catalyst in the whole temperature range. This work provides a deep understanding of the structure-performance relationship in CeWMnO x catalysts, which may be beneficial for their further practical applications in the flue gas denitrification from fixed sources such as thermal power plants in the future.