| Because of their extensive use as a chemical building blocks,light olefins,such as propylene and ethylene,are among the most important types of compounds in the petrochemical and fine chemical industry,which production is a significant symbol of the development level of national petrochemical industry.Steam cracking technology is the most common method for obtaining these light olefins.Typically,light olefins obtained from this method contains trace alkynes,which could poison the catalysts employed in polymerization process as well as decrease the quality of the product.Therefore,selective hydrogenation of alkynes is a necessary purification process to remove trace alkynes in the steam.In recent years,hydraulic fracturing technologies gave improved to the point where large volumes of shale gas can be extracted,and increasing supply of natural gas created opportunities for on-purpose catalytic dehydrogenation of light paraffins into the corresponding olefins.Pd and Pt,due to high intrinsic activity,are commonly selected as the active metals in the selective hydrogenation/dehydrogenation reactions.However,noble metal catalysts suffer poor stability and low olefins selectivity.As a result,the design and synthesis of highly performed supported catalysts for selective hydrogenation and dehydrogenation of light hydrocarbons have attracted much attention.Formation of a bi-component catalyst exhibits the feature of continual turning through changes in composition of the active metals and flexible modification of the electronic and geometric structure of bi-component catalysts.In view of low olefins selectivity and poor stability of noble Pd and Pt catalysts,in this paper,the design and preparation of supported bi-component catalysts were carried out to enhance the catalytic performance.First,surface segregation was taken to turning surface composition and structure of Pd-Co bimetallic nanoparticles,and the influence of surface structure of active metal on the catalytic performance was discussed.Subsequently,a new strategy was developed to synthesis core-shell nanoparticle with alloy shell,and the electronic and geometric effect over Ag@PdAg/LDHs catalyst induced by core-shell heterostructure on the ethylene selectivity were investigated.In order to further improve the catalytic performance,Cu vacancy defects were constructed on the surface of Au-Cu alloy,and the role of vacancy defects over Au-based catalysts in selective hydrogenation reaction was also revealed.In this work,four novel bi-component catalysts were obtained,which greatly improve the olefins selectivity in selective hydrogenation/dehydrogenation reactions.Surface composition and structure of as-prepared Pd-Co bimetallic nanoparticles with different Pd/Co ratios was tuned by H2-induced surface segregation.1.5 nm Pd enriched shell is formed on Pd75Co25/Al2O3-700 sample while isolated Pd sites with enriched electron is constructed on Pd15CO85/Al2O3-700 sample.The enrichment and redistribution of Pd atoms on the nanoparticle surface exhibits significantly increased activity in acetylene selective hydrogenation,which can be attributed to the increased Pd dispersion and the fabricated surface site.The relatively lower Ea of the Pd15Co85/Al2O3-700 catalyst(34.1 kJ/mol),with comparison to the corresponding as-prepared Pd15Co85/Al2O3-400(47.9 kJ/mol),suggests that the post-treated catalyst could surely effectively decrease the activation barrier.Moreover,Pd15Co85/Al2O3-700 catalyst also exhibits excellent carbon deposition resistance and stability during 100 h reaction.SiO2 supported Ru2P and RuP phases were prepared by composition turning of P content with H3PO4 as the phosphorus source.In situ XPS,synchrotron in situ XAS,and in situ CO-IR are employed to identify the surface structure of nanoparticles with different P/Ru atomic ratios,changing from Ru to Ru2P and to RuP with increasing P loading.For the series of Ru-P catalysts,the propylene selectivity increased almost linearly with increasing P level in the nanoparticles,and the RuP catalyst showed 91%propylene selectivity at 25%propane conversion.The fully isolated Ru sites on RuP structure instead of Ru ensembles significantly improved the C-H activation selectivity during dehydrogenation by suppressing cracking and hydrogenolysis reactions.We have demonstrated an effective strategy to construct Ag@PdAg core-shell nanostructure with controllable PdAg shell thickness under the mild wet conditions.Both the relative reduction potentials and atomic ratios of metal precursors are the key factors for the formation of the metal@alloy core-shell architecture.In partial hydrogenation of acetylene,the Ag@PdAg showed 97%ethylene yield,which is 2.0 times and 8.1 times higher than PdAg alloy and Pd monometallic catalysts respectively.The enhancement of ethylene selectivity can be attributed to the electronic-rich Pd atoms on the surface of PdAg alloy,and this electronic modification can be enhanced by the construction of core-shell heterostructure,which favors the desorption of ethylene.Moreover,this core-shell structure exhibits preferable stability and few coke were detected after 100 h reaction,with comparison to PdAg alloy catalyst.Two Au-Cu alloys with Au:Cu atomic ratios of 3:1(Au3Cu/TiO2)and 1:1(AuCu/TiO2)were prepared via deposition precipitation method.Then Cu vacant defect and low coordination of Au atoms were fabricated on the surface of Au-Cu nanoparticles via dealloying nanoparticles.In situ EXAFS was carried out to analysis the number of defects on Au-Cu nanoparticles with different Au:Cu atomic ratios.In partial hydrogenation of acetylene,the E-Au3Cu/TiO2 showed 78%ethylene yield under mild reaction conditions,which is 14 times and 9 times higher than Au3Cu/TiO2 alloy and Au/TiO2 monometallic catalysts respectively.The the Cu vacancy defects on the surface of dealloyed Au3Cu nanoparticles is beneficial for H2 activation,and the enhancement of ethylene selectivity can be attributed to the low coordination Au atoms. |
