Selective Catalytic Reduction Of NO_x By Acetylene Over Cerium Based Zeolites

NO_x is the main atmospheric pollutant, causing photochemical reaction, acid rain and so on which leads to the destroying of environment and human beings' respiration disease. High selectivity of de-NO_x at lower temperature in lean-burn conditions, which mainly comes from automobile exhaust, has become one of the greatest challenges in catalytic field of environmental protection. SCR of NO_x by hydrocarbons was considered the most promising method in practical application. Using acetylene as a reductant in the selective catalytic reduction (C₂H₂-SCR) of NO with the presence of excess oxygen was investigated for the first time, which was expected to obtain higher selectivity under low temperature (³00℃) compared with the former researches in SCR.Various Ce-exchanged and impregnated zeolites were studied in C₂H₂-SCR under the conditions of 1600 ppm NO, 800 ppm C₂H₂, and 9.95 % O₂ in He in a fixed bed flow reactor. The Ce-H-ZSM-5 (Si/Al=25) catalyst showed higher activity of about 83% NO conversion to N₂ and significantly high selectivity at the temperature of 300⁴00℃. Compared with the achievements of C₃H₆-SCR studies reported in literatures, only ca. 1/3 of the reductant in carbon moles was used in the case of C₂H₂-SCR, which means the selectivity of C₂H₂-SCR is almost 3 times as that of C3H6-SCR. The selectivity of C₂H₂-SCR is on the same level as that of CH4-SCR, however the reaction temperature for the maximum activity of NO to N₂ conversion decreases nearly 200 degree C.The important rules of the research were as follows: The activity order of the different Ce-H-zeoltes catalysts in C₂H₂-SCR of NO is Ce-H-ZSM-5 (Si/Al=25) > Ce-H-Y (Si/Al=2.5) > Ce-H-p (Si/Al=20³0) > Ce-H-SAPO (Si/Al=34) > Ce-H-5A (Si/Al=12), which is in good consistent with that of H-zeolites order, indicating that the type of the zeoltes plays an essential part on the C₂H₂-SCR. The activity of the Ce-H-zeolites for the C₂H₂-SCR increases dramatically when the Si/Al ratio was decreased. Almost no detectable conversion of NO to N₂ could be observed over the Ce³⁺ incorporated to the sodium form zeolite, but for proton form zeolite, specially the H-ZSM-5 (Si/Al=25), the activity of the catalysts improved apparently after impregnated or exchanged Ce. This result indicates that proton in the zeolites plays a crucial role in the C₂H₂-SCR, and only with the proton assistance, Ce³⁺ could shows its full effect in C₂H₂-SCR reaction. In other words, there is a synergetic effect for C₂H₂-SCR between Ce³⁺ and proton.The research found that O2 competed with NO for reductant in C2H2-SCR, but the C2H2-SCR reaction could not progress without O2. On the other hand, C2H2 could not be oxidized in the absence of NO. Therefore, we suggested that the oxidization of NO to NO2 in C2H2-SCR is the rate-determined step. This reaction mechanism is in consistent with that of using other reductants over zeolites catalysts in the literatures.The performance of Ce-H-ZSM-5 (Si/Al=25) in application was investigated. Ce-H-ZSM-5 (Si/Al=25) was treated at the high temperature of 650 °C for lh in the reaction feed, and the behavior of the catalyst did not change at 325 °C. Therefore, besides the high selectivity, Ce-H-ZSM-5 (Si/Al=25) exhibited good stability and resistance of thermal impact. The activity loss of catalyst caused by vapor (6.8%) in de-NOx reaction is reversible. The activity increasing is benefit from the presence of SO2 (50 ppm) under the dry condition, while decreasing seriously under the wet condition.Using of CH4 and C3H6 as reducing agent containing the same amount of C as C2H2 over Ce-H-ZSM-5 (Si/Al=25) in SCR of NO, the conversions of NO to N2 were 0 and 17% respectively at 300 °C, 27% and 29% respectively at high temperature of 500 °C. The results farther prove the promising advantage of C2H2 in application contrast with CH4 and C3H6 as reductant in SCR of NOX.