Promotional effect of rare earth-doped manganese oxides supported on activated semi-coke for selective catalytic reduction of NO with NH₃

A composite catalyst for the selective catalytic reduction (SCR) of NO_x with NH₃ is investigated, in which the rare earth (RE, including La, Ce, Pr, and Nd) is doped into manganese oxides supported on activated semi-coke (MnO_x/ASC) via hydrothermal method at the molar ratio of Mn:RE = 1:5. It is evidenced that the addition of RE at a rather low molar ratio can enhance the catalytic activity of MnO_x/ASC. The catalyst with a Mn:Ce molar ratio of 10:1 yields an over 90% NO_x removal efficiency in the temperature range of 150–250 °C. An approximate 100% NO conversion and 95% N₂ selectivity are achieved at about 200 °C. The catalysts are characterized by N₂ physisorption, X-ray powder diffraction (XRD), scanning electron microscope (SEM), hydrogen temperature-programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS). The results indicated that the Ce additive is conducive to the NO_x adsorption and then accelerates the SCR reaction due to the formation of more chemisorbed oxygen (O_β), which is favored during the oxidation of NH₃ and NO. Moreover, the in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) results confirm that the Ce additive on MnO_x/ASC catalyst could provide more active Brønsted acid sites, which eventually contributes to the SCR reaction. The generation of ad-NH₄⁺ and nitrite species is proved to play the crucial role in the promotional effect of RE addition.