Selective Catalytic Reduction Of NO_x By Acetylene Over The Ce-HY And MoO₃/HZSM-5 Catalysts

The low selectivity becomes a serious problem of global significance in the NO_x selective catalytic reduction by hydrocarbon (SCR) in the temperature range of 300-400℃ at the exess oxygen conditions. The aim of this work is to investigate high selective catalyst for the SCR using acetylene as reductant (C2H2-SCR) under oxygen-rich conditions for providing the basis for the application of elimination of NOX from lean-burn and diesel engines exhaust.Catalytic activity of various metal-ion exchanged HY (Si/Al=2.5) catalysts in C2H2-SCR was studied extensively in the paper. It was found that Ce3+ ions drastically promoted the activity of HY zeolite. The maximum conversion of NO to N2 was increased from 16.8% to 75.4% at the temperature of 300-400 ℃ by the modification of HY (Si/Al=2.5) with Ce cations. Compared with the literatures in which the same level of NO removal was achieved using C3H6 as reductant, only ca. 1/3 of the reductant in carbon moles was used in the case of C2H2-SCR, which means that the selectivity of C2H2-SCR we studied is about 3 times as high as that in the literatures.The role of Ce3+ ions in the C2H2-SCR over Ce-HY catalyst was discussed by means of the NH3-TPD, C2H2 adsorption characterization technique as well as the reaction of NO oxidation with O2. The results indicate that proton in the zeolite plays a crucial role in the C2H2-SCR and the adsorption capacity for C2H2 of catalysts is independent of their performance in the C2H2-SCR. NO oxidation to NO2 was the rate-determining step of the C2H2-SCR of NO on the HY based catalysts. It was proposed tha the role of cerium species in the Ce-HY catalyst in the title reaction is to accelerate the oxidation of NO to NO2.MOO3/HZSM-5 (SiO2/Al2O3=25) catalysts prepared by an impregnation method were investigated for the NO reduction by acetylene. Over the optimized 2wt.%MoO3/HZSM-5(25) catalyst, 83.7% of NO conversion to N2 was achieved under the conditions of 1600 ppm NO, 800 ppm C2H2, and 9.95 % O2 in He. The nature and function of active molybdenum species in the MoO3/HZSM-5(25) catalysts were clarified by the combined characterization of XRD, UV-Vis DRS and NOX-TPD. It was demonstrated that the highly dispersed Mo(Td) species in the catalyst is responsible for catalyzing NO selective reduction, and it has a cooperation with the acid sites to active the NO oxidation to NO2 and facilitate NOX adsorption.CH4 and C3H6 (in the same concentration of carbon as that of C2H2) as reductant were tested individually over the 2wt.%MoO3/HZSM-5(25) catalyst for SCR of NO. No NOconversion to N2 and only 23.6% was observed at 350 °C, respectively. Even at 500°C, only 25.4% and 18.1% NO conversion to N2 were obtained individually over the 2wt.%MoO3/HZSM-5(25) catalyst. These results further indicate the promising advantage of C2H2 compared with CH4 and C3H6 as reductant for SCR of NOX in the exsess of O2.