Selective Catalytic Reduction Of NO_x By Acetylene Over Tungsten Based Zeolites Catalyst In Lean-burn Condition

NO_x is the main atmospheric pollutant, causing photochemical smog, acid rain and so onwhich leads to the destroying of environment and human beings' respiration disease. For theabatement of NO_x from the exhaust gases of diesel and lean-bum gasoline engines, theselective catalytic reduction of NO_x by hydrocarbons (HC-SCR) is believed to be a promisingmethod. 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 toHZSM-5 zeolite was investigated for the SCR of NO_x in the lean-burn conditions by usingacetylene as reducing agent, which is compared with methane and propene.Various W-impregnated zeolites werestudied in C₂H₂-SCR under the conditions of 1600ppm NO, 800 ppm C₂H₂, and 10 % O₂ in He in a fixed bed flow reactor. On the6%W/HZSM-5 catalyst that has the optimal tungsten loading, 89% of NO conversion to N₂was obtained at 350℃using acetylene as reducing agent. However, when C₃H₆ or CH₄ wasused as reducing agent in the condition of containing the same amount of "C" as C₂H₂ overthe same catalyst, only 33% or 28% of the maximum NO conversion to N₂ was observed,respectively. The results indicate that C₂H₂ is much more effective than C₃H₆ and CH₄ as areductant on the W/HZSM-5 catalyst.The monomeric and polymeric tungsten species dispersed on HZSM-5 were the activetungsten species contributing to the selective reduction of NO by means of the XRD, UV-visDRS, and NH₃-TPD. Tungsten had significant doping effect to HZSM-5 zeolite on C₂H₂-SCRof NO. It displayed two primary functions: (1) catalyzing the NO oxidation to NO₂ bycooperating with the protons in HZSM-5 zeolite; (2) increasing the strongly adsorbed nitrousspecies on the catalyst.The active nitrous species on the catalyst contributing to the NO removal were the partsof bidentate and bridging nitrate species that could remain on the catalyst surface at thereaction temperature. The active carbonous species towards the NO_x reduction were thespecies containing the carbonyl group that were easily formed under the reaction conditionsof C₂H₂-SCR of NO over the 6%W/HZSM-5 catalyst. Adsorption and desorption of acetylene and propylene on the zeolites ZSM-5, Na13X, Y,HMOR, H5A, HSAPO-34 as well as on some catalyst supports were investigated at 80℃. Itreveals that the adsorbates almost do not adsorb on SiO₂ andγ-Al₂O₃, while they havestronger interaction with the zeolites. The saturated adsorption amount decreases with theSi/Al ratio of the zeolites. The adsorption of propylene is always larger than that of acetyleneon each zeolite in the same conditions. Over Hβzeolite, as high as 0.11 mmol/g and 4.89mmol/g of acetylene and propylene were obtained in saturated adsorption, respectively, whichare greatly larger than that reported in literatures.