Comparison Study of the SCR Performance over Mn–TiO₂ and Ce–TiO₂ Catalysts: An Experimental and DFT Study

MnOx- and CeOx-based catalysts are the most popular low-temperature selective catalytic reduction (SCR) catalysts for the substitution of the VO5–WO3/TiO2 catalyst. The comparison study was conducted to reveal the inner mechanism for the difference of the SCR performance over Mn–TiO2 and Ce–TiO2 catalysts via the experimental analysis and density-functional theory (DFT) calculations. The results suggested that the inhibited low-temperature NO conversion rate over the Ce–TiO2 catalyst is attributed to the highly inhibited Langmuir–Hinshelwood mechanism (L–H mechanism pathway) due to the insufficient redox properties, restricting the activation of adsorbed NO and the formation of the intermediate NH4NO2 species. As for the middle and high temperatures, the NO conversion rate is the synergistic effect of the inhibited SCR reaction following Eley–Rideal mechanism (E–R pathway) and the C–O reaction rate (catalytic oxidation reaction). The strong interaction between NH3 and the Ce–TiO2 catalyst is the main reason for the inhibition of the kE–R value over the Ce–TiO2 catalyst, inhibiting the reactivity of the adsorbed NH2 with gaseous NO. Meanwhile, the C–O reaction over the Ce–TiO2 catalyst was inhibited by the weakened oxidation properties and lower concentrations of Ce3+/Ce. Therefore, the Ce–TiO2 catalyst presented a slightly lower middle-temperature NO conversion rate compared to the Mn–TiO2 catalyst.