Mechanistic Study Of NO Reduction By Acetylene On ZSM-5 And Selective Catalytic Reduction Of NO By Acetylene Over Y/HZSM-5

NO_x is the main air pollutant, the elimination of which has received much attention allover the world. Selective catalytic reduction of NO by hydrocarbons (HC-SCR) is the mosteffective way to remove nitric oxide from the exhaust gas of lean-burn engines. Up to now, ithas been found in many researches that proton form zeolitic catalysts exhibit much higheractivity than the sodium form ones in HC-SCR. However, the reason for this great distinctionhas not been mentioned.In this work, a mechanism of C₂H₂-SCR is first proposed on the basis of studying theaimed reaction over ZSM-5. Accordingly, the function of protons in C₂H₂-SCR over HZSM-5catalyst is well explained. The mechanism is as follows:NO is oxidized to NO₂ in the first step over HZSM-5, which is catalyzed by protons, andthen the resulting NO₂ adsorbs on the catalyst surface to form bidentate and bridging nitratesbeing underwent reduction by adsorbed reductants. On the other hand, acetylene adsorbs onthe HZSM-5 catalyst to form vinyl alcohol bounded to the Br(o|¨)nsted acid sites throughhydrogen bond, which then converts to acetate species active towards the nitrate species in thepresence of O₂. The active nitrous species and the carbonous species then react with eachother to form cyanides that further react to form N₂. Over NaZSM-5, not only that NO cannotbe catalyzed to form NO₂ and that nitrates (which may be formed via non-catalytic oxidationof NO) associated with Na⁺ are inert for the aimed reaction, but also acetylene is hardlyactivated on NaZSM-5, due to the weak adsorption of acetylene on the catalyst (forming vinylalcohol bounded to the Na⁺ ions). Thus, both the active nitrous and the active carbonousintermediates for the aimed reaction are hardly generated, leading to NaZSM-5 inactive forthe C₂H₂-SCR of NO.A new catalyst with much higher activity for the C₂H₂-SCR of NO is also investigated.Over 0.5%Y/HZSM-5, at 325℃, under 800 ppm C₂H₂, 1600 ppm NO, 9.95 % O₂ in He, aNO conversion to N₂ up to 84.8% is achieved, which is 13% higher than that of HZSM-5under the same reaction condition. It is proposed that highly dispersed yttrium speciespromote the catalytic activity to the aimed reaction through increasing the amount of activenitrates (bidentate nitrates and nitrates associated with yttrium).