Study On Size,Geometry And Electronic Structure Of Supported Palladium Catalysts And Their Structure-catalytic Property Relationships

Supported Pd-based catalysts have been widely used in reactions of environmental engineering and chemical industry such as petroleum crackings,carbon-carbon couplings,alkane oxidations and so on.Additionally,because supported Pd catalysts possess strong ability to activate hydrogen,they are highly active in catalytic hydrogenation of organic pollutions in gas phase and a variety of drinking watercontaminants.There are two main goals for catalysis research,one is to to understand a catalytic reaction at moleculer-level,the other is to design and prepare catalysts with ideal catalytic performances.Because the size,geometry and electronic structure of supported Pd catalysts are three vital factors that determine their catalytic activity and selectivity,it is necessary to prepare supported Pd catalysts with desired structural properties(size,geometry and electronic structure)for better catalytic performances.The aim of this work was to explore delicate catalyst preparation methods and prepare supported Pd catalysts with desired structures to improve the catalytic performances of supported Pd catalysts in different catalytic systems.The catalysts were systematically characterized by X-ray diffraction,transmission electron microscope,X-ray photoelectron spectroscopy,infrared spectroscopy of CO adsorption,energy dispersive spectroscopy,H2 chemisorption and so on to investigate their structure-catalytic property relationships.The main results are as follows:1.Bimetallic Au@Pd/TiO2 catalysts with an Au loading amount of 0.94 wt.%and Pd loading amounts varying from 0.017 to 0.13 wt.%were prepared using a two-step photocatalytic deposition method,and the solvent-free aerobic oxidation of benzyl alcohol was investigated on the catalysts.The catalysts were systematically characterized.Characterization results suggested that in the bimetallic catalysts Pd atoms were preferentially deposited on exposed Au surface and core-shell structured Au@Pd particles with atomically dispersed Pd as the shell were formed.For the solvent-free aerobic oxidation of benzyl alcohol over Au@Pd/TiO2,the conversion of benzyl alcohol increased with the increase of Pd deposition amount from 0.017 to 0.049 wt.%,whereas kept constant when Pd deposition amount was further increased to 0.13 wt.%.The relationship between catalytic activity and Pd deposition amount could be explained in terms of monolayer dispersion model of metallic Pd.Accordingly,Au@Pd(0.049)/TiO2 had a highest turnover frequency value of 21961 h-1 for the oxidation of benzyl alcohol among the test bimetallic catalysts.The present findings suggest that atomical dispersion of Pd was achieved in Au@Pd/TiO2 prepared using two-step photodeposition method.2.Ag catalysts decorated by trace Pd supported on y-Al2O3 with different structure and chemical properties were prepared using a combined impregnation and galvanic replacement method.For comparison,monometallic Ag/y-Al2O3 and Pd/y-Al2O3 catalysts were prepared using the impregnation method.Gas-phase catalytic hydrodechlorination of 1,2-dichloroethane to ethylene was investigated on those catalysts.The structures and chemical compositions of bimetallic Pd-Ag particles in the catalysts were controlled by adjusting Pd replacement amount.The as-prepared catalysts were systematically characterized.The results demonstrated that contiguous Pd sites dominated in the monometallic Pd/y-Al2O3 catalyst,while Pd atoms were separately decorated on the surface of Ag particles in the bimetallic Pd-Ag/y-Al2O3 catalysts when Pd replacement amount was below 0.30 wt.%.At Pd replacement amount of 0.30 wt.%,Pd ensembles with contiguous Pd sites developed in the bimetallic catalyst.Thus,monometallic Pd/y-Al2O3 catalyst displayed negligible ethylene selectivity toward the catalytic hydrodechlorination of 1,2-dichloroethane,while bimetallic Pd-Ag/y-Al2O3 catalyst with a Pd replacement amount of 0.13 wt.%exhibited 94.6%of ethylene selectivity.Findings in this work provide a promising bimetallic catalyst prepared by galvanic replacement for the selective catalytic hydrodechlorination of 1,2-dichloroethane.3.Supported Pd catalysts on CeO2 modified SBA-15(Ce-SBA-15)were prepared using the strong electrostatic adsorption(SEA)method.For comparison,supported Pd catalysts on SBA-15 and Ce-SBA-15 were prepared using the impregnation method.Liquid-phase catalytic hydrogenation of three typical disinfection byproducts(i.e.,bromate,2,4-dichlorophenol and monochloro-acetic acid)was investigated on those catalysts.The catalysts were systematically characterized.Characterization results showed that for Pd/Ce-SBA-15 catalysts prepared using the SEA method,Pd particles were site-specifically deposited on CeO2 moieties.As a result,much higher Pd dispersion and stronger metal-support interaction(higher amount of cationic Pd species)were observed as compared with the catalysts prepared using the impregnation method.For the catalytic reduction of bromate,Pd/SBA-15 prepared using the impregnation method exhibited negligible catalytic activity.In contrast,markedly enhanced catalytic activities were observed on Pd catalysts supported on Ce-SBA-15.Additionally,Pd/Ce-SBA-15 prepared using the SEA method displayed much higher activity than that prepared using the impregnation method.In parallel,Pd/Ce-SBA-15 prepared using the SEA method displayed markedly higher activities for the liquid-phase catalytic hydrodechlorination of 2,4-dichlorophenol and monochloroacetic acid than those prepared using the impregnation method.The present findings clearly indicate that Pd/Ce-SBA-15 prepared by the SEA method can be used as a promising catalyst in liquid phase catalytic hydrogenation of disinfection byproducts.4.A series of supported Au@Pd/TiO2 catalysts with varied Au particle sizes andPd contents were prepared using a two-step photocatalytic deposition method and the selective hydrogenation of acetylene in excess ethylene were studied on those catalysts.The catalysts were systematically characterized.Characterization results suggested that in the bimetallic catalysts core-shell structured bimetallic Au@Pd particles with separate Pd atoms decoration were formed.For the selective hydrogenation of acetylene,bimetallic Au@Pd/TiO2 catalysts exhibited higher hydrogenation activity and selectivity to ethylene in comparion with the monometallic Pd catalyst with similar Pd content.For bimetallic Au@Pd/TiO2 catalysts,decrease Au particle size and Pd content resulted in increased ethylene selectivity.The present findings indicate that Au@Pd/TiO2 prepared using the two-step photocatalytic deposition method was a promising candidate for the selective catalytic hydrogenation of acetylene in excess ethylene.