Preparation And Catalytic Properties Of Supported Core-shell Structured Bimetallic Catalysts For Selective Hydrogenation

Catalysts can play a key role in achieving the green,sustainable recycling economy.For the catalytic science study and application,design and construct catalysts with high catalytic activities,selectivities and stabilities is one of the main challenge to overcomePrecious metal Pd has a strong ability to activate hydrogen.It is one of the most active and most studied catalysts for catalytic hydrogenation.However,the high activity of Pd catalyst also brings some problems such as low catalytic selectivity.Additionally,the high price of Pd strongly restricts the application of Pd catalysts.The above problems can be effectively solved by introducing a second metal into monometallic Pd catalysts,and the structural properties of the bimetallic catalysts will markedly influence the catalytic hydrogenation performances.Therefore,it is important to prepare supported Pd based bimetallic catalysts with uniform and designed structures using controllable preparation method for enhanced selective catalytic hydrogenation.The aim of this work was to explore delicate preparation methods to prepare catalysts with desired structures and high Pd atomic utility and thus improve the catalytic hydrogenation selectivity of supported Pd catalysts.The structural properties of the catalysts were analyzed by transmission electron microscope(TEM),X-ray photoelectron spectroscopy(XPS),X-ray diffraction(XRD),in situ infrared spectroscopy of CO adsorption(In-situ CO-IR),high angle annular dark-field-scanning transmission electron microscopy(HAADF-STEM),ultraviolet-visible spectroscopy(UV-Vis)and other characterization methods to investigate the relationship between catalyst structures and catalytic properties.The main findings are as follows:(1)Under acidic conditions,Pd was deposited on Au/TiO2 catalyst by sodium borohydride reduction method to prepare Au@Pd/TiO2 for selective catalytic hydrogenation of acetylene in a large amount of ethylene.The characterization results show that Pd is selectively deposited on the surface of Au particles to form a structure with Au as the core and Pd as the shell.Compared with the single metal Pd catalyst,Pd has higher dispersion on the Au particles,mainly in the form of isolated active sites.In the selective hydrogenation of acetylene,the catalytic selectivity decreases gradually with the increase of Pd loading from 0.129 wt.%to 0.061 wt.%in the bimetallic catalyst.17 When the loading is 0.089 wt.%,the catalytic selectivity is the highest.The Au@Pd(0.089)/TiO2 catalyst has higher activity and selectivity than a similar loading of single metal catalyst.(2)The surface reduction method was used to deposit Ag on the Pd/TiO2 catalyst,and the synthesized Pd@Ag/TiO2 was used for the selective hydrogenation of phenylacetylene.Combined with the results of various characterizations,it was found that metal Ag was deposited on the surface of Pd.When the loading of Ag reached 0.54 wt.%,the catalyst did not detect H2 adsorption,indicating that Pd was completely encapsulated and formed the structure of Pd core Ag shell.Compared to the single metal Pd catalyst,the Ag shell hinders the contact of the reactants with the catalyst,weakens the adsorption of styrene on catalyst surface,and promotes the formation of more separated Pd sites.For the selective hydrogenation of phenylacetylene,as the loading of Ag in the bimetallic catalyst increases from 0.24 wt.%to 0.54 wt.%,the catalytic selectivity continuously increased.When the loading of Ag is 0.54 wt.%,the Pd@Ag(0.54)/TiO2 catalyst exhibits excellent selectivity and maintains suitable catalytic activity.