Preparation Of Supported Au-Pd Catalyst And Investigation Its Performance In Solvent-free Oxidation Of Benzyl Alcohol

The benzaldehyde,as an important fine chemical intermediate,has been widely used in the fields of medicine,perfume and food.The traditional production methods of benzaldehyde,such as benzyl chloride hydrolysis method and toluene catalytic oxidation method,not only have the problems of environmental pollution and more by-products,but also the generated benzaldehyde often contains chloride,which limits its application in medicine and food industry.At present,the main production process of chlorobenzene formaldehyde is the selective oxidation of benzyl alcohol,but this process usually uses permanganate or chromate as the oxidant,which will not only produce a large number of heavy metal containing three wastes but also difficult to avoid the occurrence of excessive oxidation reaction.The catalytic oxidation of benzyl alcohol in liquid phase under solvent free condition has become a research hotspot in recent years because of its advantages such as cheap raw materials,green environmental protection,chlorine-free products and high efficiency.However,the oxidation process of benzyl alcohol is prone to disproportionation reaction,and the selectivity of the product is difficult to regulate,resulting in a series of by-products such as benzene,toluene and methyl benzoate.Therefore,the preparation of the catalyst with excellent performance,so that it can obtain a higher conversion of benzyl alcohol and benzaldehyde selectivity,which is the key of benzyl alcohol catalytic oxidation reaction.In this paper,in view of the technical defects existing in the current catalyst,The CeO₂/TiO₂ with different morphologies and C₃N₄ support were synthesized,and the active components of Au and Pd were supported.The structure characterization and catalytic oxidation performance of benzyl alcohol were investigated.The physical properties(particle size,Pd²⁺,O_α)of catalysts with different morphologies and supported metal ratios were investigated by N₂-BET,XRD,XPS and TEM,and the following conclusions were drawn:（1）Firstly,CeO₂ carriers with different morphologies（nanorods,polyhedrons and cubes）were synthesized by hydrothermal method.Then,the bimetallic Au-Pd was loaded on the carriers with three different morphologies respectively by deposition method.Combined with the catalytic oxidation reaction of benzyl alcohol,it was found that the morphology of CeO₂carriers had a great influence on the catalytic activity of benzyl alcohol.The nano-rod-shaped of Au-Pd/CeO₂ catalyst has a higher benzyl alcohol conversion rate than the polyhedral and cubic catalysts,and the TOF value of the nano-rod-shaped catalyst is also the highest.The XRD results showed that the XRD patterns of Au-Pd nanoparticles were not significantly changed,indicating that Au-Pd nanoparticles had no effect on the structure of CeO₂ carriers.The Raman test results showed that after loading Au-Pd nanoparticles on CeO₂,no ownership bands of Au nanoparticles or Pd nanoparticles are found,indicating that the dispersion of metal particles is good.The TEM results show that the metal particles on the surface of the catalyst are uniformly distributed.The particle size distribution of more than 100 Au-Pd nanoparticles on each catalyst was calculated by HAADF-STEM.The results showed that the size of Au-Pd nanoparticles remained about 3 nm,that is,the distribution of Au-Pd nanoparticles on all CeO₂ support was not affected by the morphology and exposed crystal plane of the support.The XPS results showed that in Au-Pd/CeO₂ catalysts,100%Pd²⁺concentration can be detected in ACR catalyst,and the highest Ce³⁺on the ACR surface,which can provide oxygen for Pd particles,has the highest proportion of oxygen vacancy,thus improving the REDOX performance of the catalyst.（2）Secondly,different forms of TiO₂ carriers were synthesized by hydrothermal method in this paper.Then,bimetallic Au-Pd was respectively supported on three different forms of TiO₂ carriers through deposition and precipitation method.Combined with benzyl alcohol catalytic oxidation reaction,the effects of different forms of TiO₂ carriers on benzyl alcohol conversion,the benzaldehyde selectivity and catalyst stability were obtained.According to the reaction test,the rutile Au-Pd/TiO₂ catalyst has the highest conversion of benzyl alcohol,but its thermal stability is the worst.The XRD results show that there is no diffraction peak of Au or Pd in the Au-Pd/TiO₂ catalyst after the Au-Pd nanoparticles are supported on the TiO₂support,which indicates that the Au and Pd are highly dispersed on the surface of the support,and the crystal structure of the TiO₂ support is not affected by Au and Pd.The XPS characterization results showed that Au³⁺ions were not detected on Au-Pd/TiO₂ catalysts,and the surface Pd²⁺content of rutile and platytitanite TiO₂ catalysts was the highest,that is,TiO₂also promoted the formation of Pd²⁺,the proportion of oxygen（O_α）adsorption on rutile TiO₂was the highest（43.8%）.The oxygen vacancy ratio on its surface is the highest,which is very important to improve the catalytic activity.TEM results show that for Au-Pd/TiO₂（rutile type）catalyst,the dispersion of Au-Pd nanoparticles is the most uniform and the average particle size（4.1 nm）is the smallest,that is,the particle size has a strong correlation with the shape of the catalyst support.Therefore,the higher ratio of Pd²⁺on the surface of the catalyst,the higher ratio of adsorbed oxygen（O_α）and the smaller metal particle size all contribute to the improvement of the REDOX performance of the catalyst.（3）Finally,the g-C₃N₄ carrier was synthesized by calcining melamine at high TE M perature.The 2%Au/g-C₃N₄,2%Pd/g-C₃N₄,1%Au-1%Pd/g-C₃N₄,0.5%Au-1.5%Pd/g-C₃N₄,1.5%Au-0.5%Pd/g-C₃N₄ were prepared by photodeposition method,respectively.Th e effects of g-C₃N₄ supported with single metal,bimetal and different proportions of Au-Pd on the catalytic oxidation of benzyl alcohol were investigated.When loading dif ferent proportion of bimetallic,2%Pd/g-C₃N₄,0.5%Au-1.5%Pd/g-C₃N₄,1%Au-1%Pd/C₃N₄ activity is higher,1.5%Au-0.5%Pd/C₃N₄ activity is lower.The XRD results showed that the dispersion of Au and Pd on the surface of g-C₃N₄ carrier was better than tha t on the surface of g-C₃N₄ carrier.The XPS characterization results showed that the P d²⁺concentration on the surface of 2%Pd/g-C₃N₄,1%Au-1%Pd/g-C₃N₄ and 0.5%Au-1.5%Pd/g-C₃N₄ catalysts were higher than 11.23%.Moreover,1%Au-1%Pd/g-C₃N₄ and 0.5%Au-1.5%Pd/g-C₃N₄ have higher concentrations ofO_α.TEM characterization results sh ow that the single metal 2%Pd/g-C₃N₄ and all the bimetallic catalysts indicate that the metal particle size is small.These results indicate that higher concentration of Pd²⁺,higher concentration ofO_αand smaller size of metal nanoparticles contribute to obtain ing high catalytic activity.