Effect of Mn On The Performance and Mechanism of Catalysts For The Synthesis of (Ce,La)CO3F

Abstract

\n In accordance with the cerium-lanthanum ratio of fluorocerium ores in the mineralogy of the Baiyun Ebo process, pure substances such as Ce(NO3)3·6H2O, La(NO3)3·6H2O were used to synthesize (Ce,La)CO3F grains to simulate bastnaesite minerals by hydrothermal method, and used as NH3-SCR denitrification catalysts. After being roasted at a series of different temperatures, the catalyst surface produced a well-crystallised Ce7O12 species as the active component for denitrification. The activity results showed that the synthetic (Ce,La)CO3F was roasted at 500°C, and the NOx conversion was 27% at 200°C. The NH3-SCR catalytic activity of the synthesised (Ce,La)CO3F was improved by loaded transition metal Mn. The best catalyst was found to be produced by impregnating (Ce,La)CO3F with 1 mol/L manganese nitrate solution, with a NOx conversion of 80% at 250°C. The physicochemical properties were analysed using XRD, BET, H2-TPR, NH3-TPD and XPS. The loading of Mn resulted in the appearance of numerous well-dispersed MnOx species on the catalyst surface, the dispersion of Ce7O12 species was also greatly enhanced, and the reduction in grain size indicated that Mnn+ entered into the (Ce,La)CO3F lattice causing lattice shrinkage. The number of acidic sites on the catalyst surface and the redox capacity were enhanced. The amount of Ce3+ in the catalyst was also enhanced by the introduction of Mnn+, but the proportion of adsorbed oxygen decreased, which indicated that the introduction of Mnn+ was detrimental to the increase in the proportion of adsorbed oxygen. The reaction mechanisms of the (Ce,La)CO3F and Mn/(Ce,La)CO3F catalysts were investigated by in-situ Fourier transform infrared spectroscopy (FTIR), to provide theoretical guidance for the specific reaction pathways of bastnaesite in the NH3-SCR reaction. The results showed that catalysts followed both the E-R and L-H mechanisms throughout the reaction process. When loaded with Mn, the main reactive species in the L-H mechanism were the NH4+(ad) species on the Brønsted acidic site and the O-Ce3+-O-NO, O-Mn3+-O-NO species. The main reactive species for the E-R mechanism were NH3/NH4+(ad) species on the Brønsted/Lewis acidic sites and NO. The NH4+ (ad) species on the Brønsted acidic sites act as the main reactive NH3(g) adsorbing species, bonded to the Ce4+ in the carrier (Ce,La)CO3F to participate in the acid cycle reaction. The introduction of Mnn+ increases the number of Brønsted acidic sites on the catalyst surface, and acts as an adsorption site for NO, to react with NO to generate more monodentate nitrate species, to participate in the redox cycle reactions. The above results indicated that Mnn+ and (Ce,La)CO3F have a good mutual promotion effect, which makes the loaded catalyst have excellent performance, which provides a theoretical basis for the high value utilization of bastnaesite.