Synthesis of novel Co(3-x)MnxO4@TiO2 core-shell catalyst for low-temperature NH3-SCR of NOx with enhanced SO2 tolerance

Abstract

In this study, the Co(3-x)MnxO4@TiO2 core-shell catalysts prepared by sol-gel and external kinetic coating method were investigated for DeNOx by response surface methodology (RSM) using 3-factor with 5-level experiments. Compared with CoMn2O4 sample, the CoMn2O4@TiO2 catalyst was optimized to obtain almost 100% NOx conversion at 125°C to 225°C that also maintained above 80% SCR activity with 100 ppm SO2 and 10 vol.% H2O in the testing time within 24 h. The XRD and Raman show a clear distinction of that CoMn2O4 spinel phase was well-dispersed on anatase TiO2 over CoMn2O4@TiO2 catalyst, which exhibited a core-shell structure with obvious distribution boundary of CoMn2O4 core coated by TiO2 shell confirmed by TEM images. This intact catalyst presented complete TiO2 coating structure due to the detected bonds Mn-O, Co-O and particular Ti-O via FT-IR analysis. NH3-TPD and H2-TPR profiles indicated that the CoMn2O4@TiO2 catalyst owned more acid sites, stronger acidity and enhanced redox capacities benefiting from its core-shell structure after TiO2 coating. According to XPS analysis, higher content of (Mn3++Mn4+)/(Mn2++Mn3++Mn4+) (77.7%), Co3+/ (Co2+ + Co3+) (53.5%) and surface oxygen (26.1%) were in favour of valence electron interaction (Mn4++Co2+↔Mn3++ Co3+, Mn3++Ti4+↔Mn4++ Ti3+). The NH3-SCR reaction pathways over CoMn2O4 and CoMn2O4@TiO2 catalysts were compared and proposed through DRIFTS experiments, which mainly follows the typical Eley-Rideal (E-R) mechanism, while the Langmuir-Hinshelwood (L-H) mechanism is weak at 225 °C. This study opens up a new avenue for designing efficient and environment-friendly NH3-SCR catalysts and looks promising for practical application.