Chemical Synthesis and Hydrogenation Performances of Transition Metal Silicides
Author:Yang Kai Xuan
Supervisor:liang chang hai
Transition metal silicides,typical intermetallic compounds formed by strong interaction between silicon and metal,have unique physical and chemical properties.They have potential applications in hydrogenation,dehydrogenation,methanation,photocatalysis and electrocatalysis.However,the traditional preparation methods are unable to achieve the controllable preparation of metal silicide in particle size and phase state,which restrict their applications in catalysis.Therefore,polymer pyrolysis and chemical vapor deposition methods have been developed for the preparation of transition metal silicides.The selective hydrogenation,hydrodeoxygenation(HDO)and hydrodesulfurization(HDS)properties of transition metal silicide catalysts were investigated.The intrinsic relationship between the catalytic performance and structure of metal silicides were explained by combining the characterization and theoretical calculation results.The main research contents and results are listed as follows:(1)The pyrolysis precursor was successfully synthesized using polydimethylsilane as silicon source,and single-phase Ni2Si catalytic material was obtained by temperature programmed pyrolysis method.H2 atmosphere can promote the formation of SiH4 in precursor and reduce the preparation temperature of Ni2Si.Nickel silicide formation involves the following sequence as a function of increasing temperature:Ni→Ni2Si@Ni→Ni2Si→Ni@Ni2Si.The prepared Ni2Si was used for selective hydrogenation of phenylacetylene(PA)and showed good hydrogenation activity and high selectivity to styrene(ca.90%).In addition,alkali treatment can significantly improve the hydrogenation activity of Ni2Si catalyst by removing the SiOx passivation layer.(2)A chemical vapor deposition method using Si(CH3)2Cl2 as silicon source was developed.Single-phase Ni2Si,CoSi,Cu6.69Si,and mixed-phases FexSi supported on SBA-15 were prepared successfully.Ni2Si/SBA-15 showed the superior catalytic performances for selective hydrogenation of phenylacetylene using selective hydrogenation of phenylacetylene as probe reaction,and its selectivity to styrene(ST)was over 90%.The electron transfer effect between metal and Si atoms in metal silicides was proved by XPS and other characterization methods,which changed the adsorption and activation of Ni to reactants and intermediate products in Ni2Si,thus making the selectivity of Ni2Si catalyst to ST much higher than that of Ni catalyst.In addition,Ni2Si/SBA-15 catalyst maintained high activity and 90%high selectivity in five cycles.(3)A bifunctional Ni2Si/Al2O3-SiO2 catalysts were prepared by CVD with Ni-Al layered double hydroxides as metal precursor and Si(CH3)2Cl2 as silicon precursor.The single-phase Ni2Si was prepared successfully by CVD method.Furthermore,the amorphous silicon-aluminium structure was formed by the reaction of Al2O3 and SiOx during the CVD treatment,which improved the acidity of the support.The Ni2Si/Al2O3-SiO2 catalyst showed excellent catalytic performance in the HDO of diphenyl ether(DPE).The formation of Ni2Si and the increase of acidity change the reaction pathway of DPE over Ni2Si/Al2O3-SiO2,which improves the selectivity of BEN and reduces the consumption of H2 during HDO process.Because of the combination of[AI(V)]and silicide on the surface of the support,the sintering resistance of the catalyst was improved and no obvious deactivation was observed in the 80 h stability test.(4)Noble silicides supported by carbon nanotubes(CNTs)were prepared by CVD successfully.The modification of geometric structures and electronic structures of Pd2Si by Si were proved by characterization and DFT calculation It was found that the reaction pathway of dibenzothiophene(DBT)over Pd2Si catalyst changed because of the electron transfer between Si and Pd and the isolation of active sites.Up to 50%of DBT was converted into biphenyl(BP)through direct desulfurization path,which reduces the consumption of H2.Meanwhile,the sulfur resistance of Pd2Si was improved because of the weakness adsorption to S over active sites.Therefore,the Pd2Si showed high stability during the 80 h stability test.Furthermore,SiOx-Pd/CNTs and Pd2Si/CNTs-B catalysts were prepared by low temperature chemical vapor deposition and alkali treatment.By comparing the HDS performances of those two catalysts with that of Pd2Si/CNTs,it was proved that the high selectivity and stability of metal silicides originate from their special structures,and the SiOx oxide layer only has certain benefits on anti-sintering.