DRIFT Study of the Adsorption of NH3 and NOx over Rare Earth Concentrate Enriched from Bayan Obo Tailings

Abstract

ABSTRACTIn this study, the reaction mechanism for the selective catalytic reduction (SCR) of NOx by NH3 over rare earth concentrate (RE-concentrate) was investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The catalyst sample was investigated by X-ray diffraction, Brunauer–Emmett–Teller and scanning electron microscope methods to characterize its crystal structure and surface topography. Some pore structures and the diffraction peaks of CeO2 appeared after the catalyst calcined at 500°C. Fe, Nd, La, and other metal elements well-dispersed inside the catalyst, which exist in the form of coexistence. Further, the presence of oxygen promoted the adsorption of NO. Moreover, the adsorption intensity of NOx on the RE-concentrate was higher than the pure metal oxidation (Fe2O3, CeO2, and La2O3). The result of the in-situ DRIFT shows that there are Lewis and Brønsted acidic sites on the surface of the catalyst. The presence of oxygen promotes the dehydrogenation of NH3. It is found that the selective catalytic reduction (SCR) reaction over RE-concentrate catalysts mainly followed the Langmuir–Hinshelwood mechanism. The decomposition of oxygen into active monomolecular oxygen (O(ad)) by the surface active sites and the adsorption of NO on the catalyst surface are the important steps in the NH3-SCR reaction. However, it is of great importance to reduce the oxidability of RE-concentrate and control further dehydrogenation of NH2 species, which could improve the N2 selectivity.