Promotion of Different Active Phases in MnOX-CeO2 Catalysts for Simultaneous NO Reduction and o-DCB Oxidation

Abstract

MnO_X-CeO₂ catalysts with different Mn and Ce content were prepared to evaluate the effect of metal content on catalytic properties and activity in the simultaneous NO reduction and o-DCB oxidation, in order to elucidate the most active species for the process. Catalytic properties were evaluated by ICP-AES, XRD, skeletal FTIR, STEM-HAADF, XPS, N₂-physisorption, H₂-TPR, NH₃-TPD and pyridine-FTIR. Catalysts with 85%Mn and 15%Ce molar content have been found to be the most active. Their excellent catalytic performance is related to the coexistence of Mn in different phases, i.e., Mn species strongly interacting with Ce and segregated Mn species. The effect of the preparation methods has also been deeply investigated: Co-precipitation method (CP) leads to Mn segregation as Mn₂O₃, whereas sol-gel preparation method (SG) promotes the formation of an amorphous powder. The synergy between segregated Mn₂O₃ species and Mn species in high interaction with Ce (resulting in a mixed oxide phase) leads to the presence of Mn with different oxidation states. This effect, together with the high oxygen mobility caused by structural defects, enhances redox, acidic and oxidative properties. The improvement of catalytic properties with Mn content also favors NO reduction side-reactions, with N₂O and NO₂ being the most important by-products, whereas it limits the production of chlorinated organic by-products in o-DCB oxidation.