Preparation And Application Of Zeolite-based Catalysts For Removing Low-concentration CO And NO_x In Air

With the rapid development of the world economy,air pollution has become increasingly prominent.The emissions of the harmful gases including CO and NO_x are increasing drastically,which pose a significant threat to the environment and human health.CO can be eliminated by adsorption,catalytic oxidation and solvent absorption.In particular,the catalytic oxidation is wildly used because of the advantages such as easy operation,energy saving and high efficiency.However,for most conventional fixed-bed catalytic reactors in which granular or powder cata-lysts are used,they suffer from poor contacting efficiency,relatively higher mass/heat transfer as well as bed pressure dorp.Thereby,it is promising to design and fabricate novel structured catalysts based on zeolite film materials which have high voidage.The mass/heat transfer effi-ciency,contacting efficiency and reaction rates can therefore be improved.Meanwhile,the emissions of NO_x can be lessened by the selective catalytic reduction?SCR?and the selective non-catalytic reduction?SNCR?.And the SCR is widely used due to the high efficiency and low energy consumption.The conventional vanadium-based catalysts have several drawbacks such as narrow operating temperature window,low hydrothermal aging stability and toxicity.It is promising to synthesize environment-friendly catalysts which exhibit wide operating temper-ature window and high hydrothermal aging stability.First of all,we investigated the synthesis of LTA and FAU zeolite film on the paper-like sintered stainless-steel fibers?PSSF?support.We studied the various synthesis parameters such as molar ratio,crystallization temperature,hydrothermal duration,synthesis times on the growth of zeolite film.As for LTA zeolite film,the crystallization temperature,hydrothermal duration and synthesis times had effects on the morphology.In particular,crystallization tem-perature,synthesis times affected the thickness and the appearance of impurities while the syn-thesis times had effects on the thickness.Besides,the pretreatment with APTMS led to the uniform distribution of zeolite crystals.As for the FAU zeolite film,the morphology was vul-nerable to the slight change of all the synthesis parameters.The continuous and dense LTA and FAU zeolite film were successfully fabricated on the surface of PSSF via the in-situ hydrother-mal synthesis under the typical condition.The thickness of the LTA and FAU zeolite film was about 2?m.In addition,the FAU zeolite film,which was the NaX type,had the surface area of121 m²/g.Secondly,the preparation process of granular zeolite catalysts was investigated over LTA?4A,5A?and FAU?NaX,NaY?zeolites.The modified zeolite catalysts were prepared via the excess-solution impregnation and the incipient wetness impregnation using copper nitrate and cerium nitrate as precursor salts.We explored the effects of impregnation methods,Cu/Ce ratio,total metal loading,impregnation duration and calcination temperature on the catalytic activity of CO oxidation.Results showed the high dispersion of active speices favored the excess-solu-tion impregnation.The NaX zeolite-supported catalyst prepared by incipient wetness impreg-nation which was loaded with 12.4%Cu and 18.6%Ce,impregnated for 12 h and calcined at500°C for 4h exhibited the best catalytic activity for CO oxidation,showing T_90%at 145°C.Thirdly,the preparation process of 4A and NaX zeolite film-supported catalysts was stud-ied.The modified 4A and NaX zeolite film catalysts were synthesized via incipient wetness impregnation using copper nitrate,cerium nitrate,cobalt nitrate and ferric nitrate as precursor salts.We explored the effects of metal ratio,total metal loading,impregnation duration and calcination temperature on the morphology of structured catalysts and catalytic activity of CO oxidation.Results showed that the Cu-Ce/NaX zeolite film catalyst of the best activity(T_90%=100°C)were loaded with 6.2%Cu and 24.8%Ce,impregnated for 12 h and further cal-cined at 500°C for 2h.The Cu-Ce-Fe/NaX zeolite film catalyst of the best activity(T_90%=130°C)were loaded with 3%Cu,12%Ce and 4%Fe,impregnated for 2 h and further calcined at 400°C for 2 h.The Co-Ce/NaA zeolite film catalyst of the best activity(T_90%=107°C)were loaded with24%Co and 6%Ce,impregnated for 2 h and further calcined at 500°C for 2 h.The outstanding performance of structured catalysts based on zeolite film was due to the high contacting effi-ciency,excellent catalytic reaction rate and reaction stability.Fourthly,the catalytic reaction kinetics of CO oxidation over the granular Cu-Ce/NaX ze-olite catalyst and the Cu-Ce-Fe/NaX zeolite film catalyst were investigated.The external and inter mass transfer effects were evalted over the granular Cu-Ce/NaX zeolite catalyst.The intristic kinetics of CO oxidation were studied over these reactors using Power-rate Law kinetic model.The reaction order?9?)and the activation energy(_?6?can be obtained based on the Arrhenius Law.Results showed that9)=1.245 and_{?6?=57.17 kJ·mol^-1 for the granular Cu-Ce/NaX zeolite catalyst.It can be regared as first-order reaction.Besides,for the Cu-Ce-Fe/NaX zeolite film catalyst,9}=1.261 and₍?6?=69.59kJ·mol^-1.It can also be regared as first-order reaction.Finally,the CHA?SAPO-34,SSZ-13?and ZSM-5 zeolite supported Cu,Fe catalysts were investigated for NH₃-SCR.We explored the effects of preparation parameters of Cu-SAPO-34on SCR activity.The SO₂ tolerance and hydrothermal aging stability were also evaluated.Results showed the 1%Fe/1%MnCe/3.86%Cu-SAPO-34 was highly active for SCR,showing NO conversion of 85%at 350°C.As for the Cu-catalysts including Cu-SAPO-34,Cu-SSZ-13 and Cu-ZSM-5,N₂O emissions were mainly from NSCR,NH₃ oxidation and Eley-Rideal mecha-nism.As for Fe-catalysts including Fe-SAPO-34,Fe-SSZ-13 and Fe-ZSM-5,N₂O emissions were low due to the N₂O decomposition and N₂O-SCR.