RESUMO
A composite catalyst for the selective catalytic reduction (SCR) of NOx 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 (MnOx/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 MnOx/ASC. The catalyst with a Mn:Ce molar ratio of 10:1 yields an over 90% NOx 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 NOx adsorption and then accelerates the SCR reaction due to the formation of more chemisorbed oxygen (Oß), 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 MnOx/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.