Modulating the acidity and reducibility of FeMn/TiO2 to boost the low-temperature selective catalytic reduction of NOx

The design and fabrication of selective catalytic reduction (SCR) catalyst with ultra-low-temperature activity is diligent, however, remains challenging hitherto. Herein, a variety of TiO2-based catalysts with various Fe/Mn ratios were fabricated using a coprecipitation technique and tested for SCR of NOx with ammonia. The experimental results demonstrate that the Fe/Mn ratio considerably affects the acidity, redox property and thereafter the NH3-SCR behavior of FeMn/TiO2 catalysts. Therein, Fe0.3Mn0.7/TiO2 exhibits the broadest activity temperature window from 75 to 210 °C because of its more reactive oxygen species and higher Mn4+ ratio on the catalyst surface, leading to improved oxidation performance and surface acidity, thereby promoting the generation of active NO2 and HONO intermediates, and thereafter enabling the catalyst to adsorb more NH3 to generate active nitrate species and increasing the NH3-SCR performance. Further increasing or decreasing the Fe/Mn ratio would downgrade the SCR performances of corresponding catalysts due to the mismatch between acidity and redox ability, which inhibits the formation of active NO2 and HONO intermediates. In-situ DRIFTS results further illustrate that the SCR of NOx over FeMn/TiO2 catalysts follow both the L-H and E-R mechanisms at the low temperature, however, the types of active nitrates flourish at high temperatures, primarily via the E-R mechanism.