The Study Of Functinal Defects Of Y Zeolite As Catalyst For The Selective Catalytic Reduction Of NO_x By Hydrocarbon

Pentasil zeolites (HZSM-5, HFER and HMOR) based catalysts are believed to be promising catalysts for the selective catalytic reduction of NO by hydrocarbons (HC-SCR), due to their high selectivity towards the aimed reaction at low temperature. However, although the reported catalytic systems gave high N₂ conversion, the gas hourly space velocity (GHSV) employed in the reactions was far lower than that required by practical application. Thus, improving the activity of catalysts and further searching for feasible catalytic systems for practical NO_x elimination has been a challenge for the people all over the world.In this thesis, the behaviors of HY, which are quite different from those of the pentasil zeolites, in the selective catalytic reduction of NO by acetylene (C2H2-SCR) and in some related processes were studied to explore the functional defects of HY in catalyzing the reaction of HC-SCR.Under the reaction condition of 800 ppm C₂H₂, 1600 ppm NO, 9.95 % O₂ in He, the four zeolites gave the following order in NO conversion at 300℃(represented in the parenthesises): HFER (85.5 %) > HMOR (71.9 %) > HZSM-5 (58.9 %) > HY (16.9 %). Obviously, although HY zeolite possesses comparatively larger pore opening (0.74 nm×0.74 nm), its catalytic activity for C₂H₂-SCR is much lower than those of pentasil zeolites with narrower pore opening (corresponding pore opening: HZSM-5 (0.53nm×0.56 nm), HFER (0.42 nm×0.54 nm), HMOR (0.65 nm×0.70 nm)). It indicated that some functions must be deficient for HY zeolite compared to the pentasil zeolites for catalyzing the reaction of HC-SCR, otherwise, HY would be more active for the reaction, as it has the advantage for the transportation of the reactants and products due to its larger pore opening.The different behaviors between HY and the pentasil zeolites in the aspects of catalyzing NO oxidation to NO₂, adsorbing NO_x to form nitrosonium ions (NO⁺) and nitrate species, and activating nitrate species to react with hydrocarbons were compared in this thesis. The following results were obtained: (1) The ability of HY in catalyzing NO oxidation was much weaker than those of the pentasil zeolites; (2) Although HY wasn't inferior to the pentasil zeolites in adsorbing NO_x to form nitrate species, it could not produce active NO⁺ species, as the pentasil zeolites do over zeolite surface; (3) The most different performance of HY, compared to those of pentasil zeolites, was that HY could not activate the nitrate species over its surface to react with hydrocarbons (C₂H₂ and C₃H₆) at reaction conditions.Therefore, it can be concluded that the poor behavior of HY in HC-SCR arises from the above three functional defects of the zeolite, which can be reasonably interpreted by the deficiency of strong Bronsted acid sites on the surface of HY (characterized by the FTIR spectra of pyridine adsorption on the zeolite at 500℃).