Mechanistic investigation of the enhanced NH₃-SCR on cobalt-decorated Ce-Ti mixed oxide: In situ FTIR analysis for structure-activity correlation

Abstract

A series of transition metals (Co, Cu and Fe) were selected to decorate Ce-Ti mixed oxide to elevate the low-temperature activity of selective catalytic reduction of NOx by NH3 (NH3-SCR) reaction, by adjusting the ratio of surface Ce3+ species and oxygen vacancies. Among them, Co-Ce-Ti sample exhibited the excellent low-temperature activity and broadened temperature window, which could be attributed to the improvement of the physico-chemical properties and the acceleration of the reactions in the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. Owing to the different ionic sizes of Co2+ and Ce4+, the lattice distortion of Ce-Ti mixed oxide was greatly aggravated and subsequently increased the ratio of Ce3+ and the surface adsorbed oxygen, which benefited the generation of adsorbed NOx species and improved the reaction in the L-H mechanism. Meanwhile, the coordinatively unsaturated cationic sites over the Co-Ce-Ti sample induced more Lewis acid sites and enhanced the formation of the adsorbed NH3 species bounded with Lewis acid sites, which were considered as the crucial intermediates in E-R mechanism, and therefore facilitating the reaction between the adsorbed NH3 species and NO molecules. The enhancements in both the reactions from L-H and E-R mechanisms appeared to directly correlated with the improved deNOx performance on the Co-Ce-Ti sample, and the L-H mechanism could be the dominate one at low temperatures due to its rapid reaction rate.