Structure Adjustment And Catalytic Hydrogenation Properties Of Supported Bimetallic Catalysts

Chlorinated Volatile Organic Compounds(CVOCs)are released from a wide variety of chemical industrial processes.Due to its properties of difficult to degrade under natural conditions it is considered as one of the main air pollutants.Among the disposal ways for those pollutants,catalytic hydrodechlorination(HDC)has recently attracted more attention due to its strong capability to transformation of industrial waste products into commercial useful and environment friendly products.Additionally,the catalysts used in polyethylene production are usually poised by the presence of 0.1-1%acetylene as impurity,which degrade the quality of the products.The commonly used way to remove the acetylene is to convert acetylene to ethylene through the selective hydrogenation.1,2-dichloroethane(1,2-DCE)is one of the typical CVOCs with completely dechlorinated products of ethane and ethylene.Because ethylene posses high commercial value and can be used as raw material in the polyethylene production,improving ethylene selectivity in the HDC of 1,2-dichloroethane has recently gained wide research interest.Metals from group Ⅷ serves as active sites broadly used in the process due to their high catalytic hydrogenation activity for the conversion of 1,2-DCE into fully hydrogenated compounds.On the other hand,several researchers proposed that the bimetallic catalyst containing the metals from both Ⅷ and IB showed enhanced selectivity toward commercial valualuble product ethylene.Such enhanced selectivity can be ascribed to effective modification of noble metal by the metals from IB.The investigation revealed that the superficial component of the catalyst directly influences the selectivity during the HDC of 1,2-dichloroethane,and the different method of catalyst preparation can effectively tune structure of surfical metal.In the thesis,three methods were applied to prepare supported bimetallic catalysts and the activity and selectivity during the HDC was investigated.Besides the HDC reaction,the bimetallic catalysts prepared by the photodeposition method were also studied for the selectivie hydrogenation of acetylene in ethylene rich stream.For semiconductor supports,the preferential growth of metallic Ag on the surface of Pt could achieve by the sequential photodeposition method,leading to the formation of core-shell like Pt@Ag catalysts.For comparison,supported bimetallic catalyst was also prepared by impregnation method.The XPS characterization results showed that the catalyst synthesized via the photodeposition displayed higher Ag enrichment than that by the impregnation method under the similar Ag loading content.The Ag domain on the surface of the catalysts prepared by the photodeposition method was also identified by the UV-vis analysis.Additionally,surface Ag enrichment in bimetallic catalysts was accompanied with the decrease of exposed Pt atoms,which could be identified by CO chemical adsorption.Catalyst prepared by either the photodeposition or impregnation method showed an enhanced ethylene selectivity with the increasing of Ag loading in the process of catalytic HDC of 1,2-dichloroethane.Furthermore,given similar Ag amount,the catalysts prepared by the photodeposition mehtod displayed more stable catalytic activity and higher ethylene selectivity than that by the impregnation method.The sequential photodepositon method is equally applicable to adjust the surface metal structure of Pd-based catalyst used in selective acetylene hydrogenation in ethylene rich stream.Characterization results showed that Ag was preferentially deposited on high coordination Pd site which was favored the further hydrogenation of intermediate product ethylene to formation ethane.Decreasing the content of high coordination Pd site and low coordination site lead to the enhanced selectivity toward ethylene.The ethylene selectivity also improved by suppressdding of the formation ofβ-PdH.Therefore,the Pd-based bimetallic catalyst prepared by sequential photodeposition exhibit a higher selectivity toward ethylene than by the impregnation method.The suface reduction method is a way suitable to prepare surface modified catalysts with any supports.In order to investigate the influence of different preparation methods and different modified metals on the catalytic behavior,Pd-Ag/Al2O3 and Pd-Cu/Al2O3 catalyst were prepared by the impregnation and surface reduction method.The selectivie deposition of promoters on Pd-based catalysts through the reaction occurs between the active H provided by Pd and the promoter,and promoter only modifies the surface of Pd to obtain higher ethylene selectivity than that from the impregnation.Given the similar loading amount,the Ag modified catalyst shown more stable catalytic activity and a higher ethylene selectivity than Cu modified catalyst,attributed to following three factors.First,copper is a bivalent cation,whose deposition requires two hydrogen atoms provided by adjacent Pd active sites whereas the deposition of silver does not pose any constraint for Pd sites because Ag is a monovalent cation.Second,compared with the hydrated ions of Cu and Ag,Ag is smaller than Cu,and is easier to get access to the surface of Pd.Finally,Ag posess a higher reduction potential than Cu,and thus is easier to be reduced on the surface of Pd active sites.At identical Ag content,Pd catalysts were prepared by the impregnation method in the presence or absence of Ag.Different catalytic activity and ethylene selectivity was observed during the process of HDC of 1,2-dechloethane,ascribed to the different active sites on the two catalysts.For the Pd/ZrO2 catalysts,Pd is the only active site for the dechlorination and provides active hydrogen.On the contrary,Ag is active site for the dechlorination and Pd is responsible for providing H to regenerate the deactivated Ag.Accordingly,given the same Pd amount,the dispersion of Pd with Ag displayed higher ethylene selectivity.Furthermore,increasing the loading amount of Pd the ethylene selectivity decreases.