Abstract:

A series of MnSAPO-34 molecular sieves were synthesized by a hydrothermal method for selective catalytic reduction (SCR) of NO with NH₃ and characterized using X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and temperature-programmed desorption (TPD). Three factors were studied, including Mn-loading, calcination temperature, and synthesis time. The MnSAPO-34, which was synthesized in 6 h and calcined at 550 ℃ with the Mn-loading (n_(MnO)/n_(P2O5)= 0.1), exhibits the highest activity among all the samples, with NO_x conversion of almost 100% and N₂ selectivity higher than 80%. The results show that the porous and crystalline structures of MnSAPO-34 are greatly affected by addition of manganese, and excessive Mn-loading could result in lower crystallinity and the generation of nonframework manganese oxides. Meanwhile, a decrease in specific surface area and pore volume are observed in MnSAPO-34 with higher Mn-loading; however, the opposite characteristics are observed with a decreasing calcination temperature and shorter synthesis time. Manganese species of high oxidation state, mostly Mn⁴⁺, are shown to be on the catalysts surface after high temperature calcination, and the increase ratio of Mn³⁺ could help to improve the catalytic activity. Under proper synthesis conditions, the incorporation of manganese could improve the adsorption of nitric oxide and ammonia, and the interaction between the strongly adsorbed NO and strongly adsorbed NH₃ might be the reason for the enhancement in their catalytic efficiency.

Key words: Selective catalytic reduction, Nitrogen oxide, Manganese, SAPO-34 molecular sieve, X-ray photoelectron spectroscopy, Temperature-programmed reduction, Temperature-programmed desorption