The Synthesis And Catalytic Performance Of Palladium-based Catalysts In Selective Hydrogenation Of Acetylene

The selective hydrogenation of acetylene to ethylene is an efficient way to remove trace amounts of acetylene from ethylene feed,which will otherwise cause the deactivation of polymerization catalysts.Compared with other catalysts,the supported Pd catalyst has been widely applied in such hydrogenation reaction due to its superior catalytic performance,but its activity and selectivity remains to be improved.Generally,as an important part of supported catalyst,support and additive has significant effect on the catalyst performance.Recentlly,much attention has been paid to the Pd catalysts supported on carbon materials because of their specific physical and chemical properties.The main purpose of the present work is to investigate how the properties of carbon supports and the amount of Ga2O3 additives affect the activity and selectivity of Pd/C catalyst in partial hydrogenation of acetylene to ethylene.In this study,a series of Pd/C catalysts were firstly prepared by deposition of Pd NPs on various carbon-based materials using sol-immobilization method.Furthermore,Ga2O3-promoted Pd/AC catalysts with different content of Ga2O3 were also prepared and tested in the selective hydrogenation of acetylene.The role of supports and the effect of Ga2O3 promoters were investigated and discussed on the basis of the characterization results and catalytic performance of supported Pd catalysts.The main results are as follows:(1)Three different carbon materials including actvated carbon(AC),graphite oxide(GO)and reduced graphite oxide(RGO)were used as the supports to prepare Pd/C catalysts while the PdNPs has a similar average size and size distribution.The catalytic performance of various Pd/C catalysts was tested in the selective hydrogenation of acetylene.The results indicate that the nature of carbon support plays an important role in selective hydrogenation reaction.Pd/AC exhibited a higher activity and selectivity compared with Pd/GO and Pd/RGO.On the basis of the TEM,XRD and XPS results,the possible factors of the carbon supports affecting the catalytic performance were discussed.It was observed that the chemical state of Pd nanoparticles is different for three Pd/C catalysts,as evidenced by the shift of the binding energy of Pd 3ds/2.Pd/AC has the highest BE of Pd 3d5/2 compared to Pd/GO200 and Pd/RGO.So the above results may suggest that the difference in the activity and selectivity of various Pd/C catalysts was related to the changes of chemical state of Pd nanoparticles induced by the metal-support interaction.(2)The effect of surface modification of AC supports on the catalytic properties of Pd/AC catalysts were examined,which were prepared by Pd deposition on thermally and chemically-treated AC supports.The results showed that thermal and chemical treatment of AC supports indeed has some influences on the catalytic performance of Pd/AC catalysts in selective hydrogenation of acetylene.Pd/AC-heat has a slightly higher catalytic performance than untreated Pd/AC catalyst while chemical treatment of AC supports by NaOH suppresses the activity and selectivity of Pd/AC to some extent.On the basis of the analysis of XPS results,it was suggested that a relatively weak Pd-support interaction may account for the decrease in the selectivity of ethylene on Pd/AC-12h.(3)Ga2O3-promoted Pd/AC catalysts were synthesized by addition of Ga2O3 to the above prepared Pd/AC catalysts using the deposition-precipitation method.By comparing the activity and selectivity of Ga2O3/Pd/AC catalysts,it has been observed that the modification of Pd/AC with Ga2O3 had an essential impact on the performance of the Pd/AC catalysts.With an increase of Ga2O3 content,the catalytic activity increases at first and decreases afterwards,and the maximum catalytic activity was achieved when the Ga2O3 content reached?1 wt%.By combining the characterization results of HRTEM,XPS and H2-TPR,it was suggested that the concentration of PdGa alloy is crucial for the catalytic performance of Ga2O3/Pd/AC catalysts in selective hydrogenation of acetylene.