Mechanism Study On Selective Catalytic Reduction Of NO_x By C₂H₂ Over ZSM-5 And Ferrierite Based Catalysts

The elimination of nitrogen oxides has received much attention all over the world due to the more and more serious pollution caused by the pollutant gases being contained in automobile exhaust. Selective catalytic reduction of NO by hydrocarbons (HC-SCR) has been believed to be the most effective way to remove nitrogen oxides from the exhaust gas of lean-burn engines. The thesis aimed to research the active catalyst for selective catalytic reduction of NO_x by C₂H₂ (C₂H₂-SCR) at low temperature and investigate the reaction mechanism. The feature of zeolites for adsorbing acetylene and nitrogen oxides was studied firstly, which are the main reactants for the aimed reaction.It was found that the NO_x conversion over HFER is as 3 times high as that of HZSM-5 under the reaction conditions of 1600 ppm NO + 800 ppm C₂H₂ + 10 % O₂ in N₂ at 250℃.The nitrate species formation over the zeolites by the co-adsorption of NO+O₂ was investigated by in situ FT-IR and the following results were obtained. (1) The activity of the zeolites for C₂H₂-SCR can be well correlated with the relative rate of the nitrate species formation. (2) The bands at 1629 and 1598 cm^-1 due to nitrate species, which were sharply appeared when the catalyst was exposed to NO+O₂, could not be observed in the in situ FT-IR spectra when the catalysts were exposed to C₂H₂+NO+O₂. Based on the results above, it was proposed that the rate-determining step of C₂H₂-SCR was nitrate species formation over the catalysts. It is the main reason for the high activity of HFER at low temperature in C₂H₂-SCR that the formation rate of nitrate species on HFER was higher than that on HZSM-5.Based on the in situ FT-IR investigation, acetamide, which was supposed to be an intermediate of HC-SCR in series of reaction systems, was excluded in C₂H₂-SCR over the zeolites based catalysts. It was proposed for the first time that formamide species was a crucial intermediate for C₂H₂-SCR. The formamide species produced by the reaction of nitrate species with C₂H₂ are quite high reactive towards NO+O₂ (and nitrate species). Accordingly, the mechanism of C₂H₂-SCR over HFER and HZSM-5 was proposed.The nitrate species formation on the catalysts was dramatically improved by loading 2 % Zr on HZSM-5 and HFER. Accordingly, the activity of the catalysts for C₂H₂-SCR were significantly enhanced, leading to the sharp increase of NO_x conversion from 20 % to 56 % (HZSM-5) and 70 % to 78 % (HFER) at 250℃, respectivly. The activity of Zr/HFER towards the aimed reaction influenced by present state of zirconium was also investigated. It was found that Zr isomorphous substituted Al in the framework of HFER is unfavorable for C₂H₂-SCR although the loading of zirconium on the zeolite could significantly enhance the aimed reaction. Correspondingly, no band at 1691 cm^-1 due to formamide species appeared in the in situ FT-IR spectra in steady state of C₂H₂-SCR over the catalyst in which part of Al in the framework of HFER was isomorphous substituted by Zr, which is in line with the much low activity of the catalyst for C₂H₂-SCR.