Improvement of activity, selectivity and H2O&SO2-tolerance of micro-mesoporous CrMn2O4 spinel catalyst for low-temperature NH3-SCR of NOx

Abstract

Cr-Mn mixed-oxide catalysts were prepared by citric acid method for low-temperature NH3-SCR of NOx. Mn(3)Cr(2)Ox (molar ratio) catalyst has an excellent NOx conversion of nearly 100% at 100–225 °C and good N2 selectivity above 70% at 100–200 °C. Results of XRD, BET, and XPS suggested that the micro-mesoporous CrMn2O4 spinel with high specific surface area, more active sites (Mn3+ and Mn4+) and effective electron transfer (Cr5+ + 2Mn3+ ↔ Cr3+ + 2Mn4+) were the important promotion factors for outstanding SCR performance. In-situ DRIFTS experiments indicated the SCR reaction pathway over CrMn2O4 spinel mainly followed the typical ER mechanism at the temperature around 200 °C. The higher Mn–O binding energy and lower dehydrogenation ability were the main reasons for CrMn2O4 spinel with low N2O by-product than Mn3O4. The CrMn2O4 spinel has satisfying tolerance to SO2 and H2O (about 72% NOx conversion at 200 °C for 20 h). In-situ DRIFTS showed SO2 could completely inhibit the adsorption of NO and weaken the adsorption of coordinated NH3 to Lewis acid sites but promote the adsorption of NH4+ to Brønsted acid sites. Analysis of TGA and FTIR results indicated that the sulfation of metal might be the main reason for the decline of SCR activity with SO2. The formation of Cr(III) sulfate could play an important role in protecting Mn active sites away from sulfating. Besides, the transform of HSO3 and SO42− to (H⋯SO42−) can provide new Brønsted acid sites for ionic NH4+, enhancing the SCR activity via fast-SCR (NO2 + NH4+ → NH4NO2 → N2 + H2O).