Study On Adsorption And Catalytic Oxidation Of Ethanol Over TWCs Modified By MCM-41

Ethanol fuels can alleviate energy problems while reducing emissions from vehicle exhaust to relieve environmental stress.Although the addition of ethanol in gasohol vehicles can reduce the emission of HCs,NOx and CO in the three major pollutants of tail gas,the addition of ethanol will produce unconventional pollutants,mainly ethanol and its incompletely burned aldehydes.The problem of organic pollution caused by the tail gas of gasohol is particularly serious in the cold start of automobiles.In the cold start of the gasohol vehicle,since the exhaust gas temperature is difficult to reach the light-off temperature of the three way catalyst,the catalyst has no effect at this time,and the organic and other pollutants brought by the gasohol vehicle are equivalent to direct discharge into the atmosphere.It has generated severe environmental pollution.The main work of this paper is to deal with the pollutants such as ethanol and aldehydes discharged during the cold start of gasohol vehicles.The mesoporous MCM-41 zeolite is used to modify the traditional three-way catalyst to improve the catalytic oxidation and adsorption ability of ethanol at low temperature.The texture and physicochemical properties of the catalysts are characterized by using various characterization techniques.The mechanism of ethanol oxidation on the surface of the catalyst by analyzing the by-product distribution and in situ Fourier transform infrared diffuse reflectance technique for the catalytic oxidation of ethanol on the catalyst surface.The following conclusions were obtained:（1）When Pd/γ-Al₂O₃ is used for catalytic combustion of ethanol,T50 and T90are 199 ^oC and 247 ^oC,respectively.The adsorption capacity of ethanol at 40 ^oC is128.02 mg·g^-1.The addition of MCM-41 zeolite can effectively improve the catalytic combustion performance and adsorption performance of the TWCs on ethanol.The adsorption of ethanol on the T50 and T90 of Pd/MCM-41 for ethanol combustion at107°C and 136°C at 40°C,respectively.The adsorption capacity of ethanol at 40 ^oC is 160.14 mg·g^-1.After the addition of MCM-41 zeolite,the flakyγ-Al₂O₃ deposited on the surface of MCM-41 during the synthesis ofγ-Al₂O₃ did not change the original structure of the catalyst,and could effectively improve the BET specific surface area of the conventional three-way catalyst.Reduce the average pore volume and pore size.The BET specific surface areas of Pd/γ-Al₂O₃ and Pd/MCM-41 were 385 m²·g^-1 and910 m²·g^-1,respectively.The increase of specific surface area increases the dispersion of PdO species in the active center,decreases the crystal size of PdO,and increases the surface metal area.After the addition of MCM-41 zeolite,the valence of the PdO species in the surface of the catalyst shifts to the low valence.This shift enhances the reducing ability of PdO and the catalytic oxidation ability to ethanol.（2）The catalysts after high temperature aging at 1000°C have an increase in T90and T50 and a decrease in the adsorption capacity of ethanol during the catalytic oxidation of ethanol.Compared with the unaged catalyst,the T50 and T90 of the Pd/γ-Al₂O₃ catalyst increased by 16 ^oC and 1 ^oC,respectively,while the T50 and T90of the Pd/MCM-41 catalyst increased by 119 ^oC and 101 ^oC,respectively;The adsorption capacity of ethanol also decreased after aging,but the catalyst of the mixed carrier decreased less.The adsorption capacity of Pd/γ-Al₂O₃ catalyst and Pd/MCM-41 catalyst decreased after aging at 40 ^oC are 26.9%and 50.6%,respectively.The adsorption capacity of the catalyst with 50%zeolite added to ethanol decreased by 13.9%.The texture properties of the catalyst changed after aging.The BET specific surface areas of Pd/γ-Al₂O₃ and Pd/MCM-41 catalysts decreased by 40%and 66.3%,respectively.The BET specific surface area of the catalyst decreased by21.2%when the MCM-41 zeolite was added in an amount of 50%in the mixed carrier.SEM and XRD data showed that theγ-Al₂O₃ changed fromγphase toδphase after aging,while the structure ofγ-Al₂O₃ did not change after MCM-41 zeolite was added.（3）After the addition of MCM-41 zeolite,the amount of ethyl acetate formed increased,and the temperature of ethyl acetate decreased.At the same time,the addition of MCM-41 zeolite inhibits the formation of methane during the oxidation of ethanol,which is beneficial to improve the low-temperature catalytic oxidation activity of the catalyst and the selectivity of CO₂.Ethanol forms an ethoxy group on the surface of the catalyst.In the presence of oxygen in the reaction system,the ethoxy group is oxidized to form an acetate,and the acetate is adsorbed in two different forms on the surface of the catalyst.Firstly,Acetate is adsorbed as a monoatomic ligand;the second way is adsorbed as a diatomic chelate ligand.The acetate species adsorbed on the surface of the catalyst in the form of a diatomic ligand chelating ligand are more susceptible to further oxidation while the product is more selective for CO₂.The addition of MCM-41 zeolite can effectively increase the adsorption of acetate in the form of diatomic chelating ligand,thereby promoting the further oxidation of the catalyst to ethanol and improving the selectivity of the catalyst for CO₂.