Influence Of Tungsten Dispersion And Tungsten Species Distribution On The Performance Of W/HZSM-5 In C2H2-SCR

The elimination of NO_x with high selectivity in the temperature range of 300-400℃from lean-burn and diesel engines exhaust is a very important subject in the world. The aim of this work is to investigate the catalyst with high activity for the selective catalytic reduction of NO using acetylene as reductant (C₂H₂-SCR) under oxygen-rich conditions, in order to provide the basis of the practical elimination of NO_x.To improve the selectivity at low temperature in the selective catalytic reduction (SCR) of NO_x, the performance of the catalysts prepared by incorporating tungsten into HZSM-5 zeolite was investigated for the SCR of NO_x in the lean-burn conditions by using acetylene as reducing agent. By means of the XRD, UV-vis DRS, and FTIR, it was proved that monomeric tungsten species dispersed on HZSM-5 are the active tungsten species contributing to the selective catalytic reduction of NO, whereas WO₃ crystallites have negative effects on the C₂H₂-SCR.By investigating the preparation conditons of W/HZSM-5 catalysts and the catalytic performance of the catalysts influenced by the SiO₂/Al₂O₃ ratio of the zeolites, the 8%W/HZSM-5 catalyst with a better tungsten dispersion on HZSM-5 and relatively more monomeric tungsten species were obtained by impregnation of the zeolite with lower SiO₂/Al₂O₃ ratio (25) in ammonium tungstate ammonia solution and calcination of the resulting material at higher temperature (550℃). As high as 86.3% of NO conversion to N₂ was obtained at 350℃over the 8%W(A)/HZSM-5 catalyst (0.2 g) under the reaction conditions of 1600 ppm NO, 800 ppm C₂H₂, and 10 % O₂ in He with a total flow rate of 50 ml·min^-1.By in-situ FTIR, it was found that active nitrous species on the catalyst contributing to the NO removal were bidentate and bridging nitrate species. The nitrate species formation is the rate determining step of the C₂H₂-SCR over HZSM-5. Thus, the monomeric tungsten species improving the C₂H₂-SCR can be attributed to accelerate the formation of active nitrate species over the W/HZSM-5 catalyst.