Preparation Of Catalysts With Highly Dispersed Palladium-Copper Nanoparticles By Supercritical Fluid Deposition Technique

Nano-metal materials have the advantages of relatively small volume,large specific surface area,large surface energy and surface binding energy,and have momentous applications in catalysis,environmental remediation,information storage,medical materials and other fields.The supported nano-metal materials avoid the inherent defects of nano-metal materials such as agglomeration inactivation,difficulty in separation and recovery,therefore it has been a hot research issue for the past few years.Compared with the supported single nanometal materials,bimetallic or polymetallic nanomaterials can not only form intermetallic synergy effect,but also obtain higher chemical stability and activity during the loading process.Compared with the traditional preparation methods of supported nano-metal materials,such as impregnation reduction method,deposition precipitation method and sol-gel method,supercritical fluid deposition(SCFD)has the advantages of no pollution,no toxicity,short process time and easy adjustment of process parameters,etc.It will not only protect the structure of the carrier not to be damaged,but also make the nano-metal particles disperse fully on the surface of the carrier.Combining SCFD method and heat treatment under normal pressure,palladium-copper bimetallic nanoparticles were supported on activated alumina balls as the matrix,and the influence of the related process parameters in the SCFD process and the heat treatment process under normal pressure on the supported metal nanoparticles were investigated.First,the effects of the supercritical process parameters such as deposition time,deposition temperature and deposition pressure on the preparation of palladium-copper nanoparticles were investigated.The experimental results showed that with the lengthening of deposition time,palladium acetylacetonate and copper acetylacetonate would gradually achieve the equilibrium of dissolution and the equilibrium of adsorption on alumina carrier in supercritical system.The deposition temperature and deposition pressure would affect the density,viscosity and surface tension of the supercritical system,then affect the solubility and diffusion ability of the supercritical system to palladium acetylacetonate and copper acetylacetonate,and also affect the adsorption properties of the alumina carrier for palladium acetylacetonate and copper acetylacetonate.When the deposition temperature was 65 ℃,the deposition pressure was 15 MPa,and the deposition time was 3 h,the average particle size of palladium-copper nanoparticles could reach 2.37 nm.Then,the effects of the feeding parameters such as the type and dosage of entrainer,the palladium theoretical loading and the copper/palladium theoretical molar ratio on the preparation of palladium-copper nanoparticles were investigated.The experimental results showed that using dichloromethane as entrainer,the increase of the metal precursors’ feeding amount would lead to the increase of the metal precursors’ total loading mass but the decrease of the metal precursors’ total loading efficiency.When the deposition temperature was 65 ℃,the deposition pressure was 15 MPa,the deposition time was 3 h,8 m L of dichloromethane was used as the entrainer,the theoretical loading of palladium was 0.50 %,and the theoretical molar ratio of copper/palladium was 1/1,the average particle size of palladium-copper nanoparticles could reach 1.81 nm.Finally,the effects of the atmosphere and the temperature of heat treatment under normal pressure on the preparation of palladium-copper nanoparticles were investigated.The experimental results showed that the heat treatment temperature of the metal precursor was an effective means to control the particle size of the supported metal nanoparticles.Under normal pressure,the supported metal nanoparticles obtained by the conversion of metal precursors in inert gas were much finer than those obtained in reducing atmosphere.The decomposition temperatures of palladium acetylacetonate and copper acetylacetonate both showed an increasing trend when supported on alumina carrier.