Controllable Synthesis Of Supported Nanometal By Supercritical Adsorption And Desorption And Investigation Of The Underlying Mechanism

The quantum scale effect,surface effect and dielectric confinement effect of nanocomposites are closely related to the morphology,size and dispersion of the supported nanometal.The controllable synthesis of nanocomposites is the emphasis and difficulty in the field of nanometer materials.Supercritical fluid deposition(SCFD)is an effective process for preparing nanocomposites,it is often used as supercritical fluids(SCFs)to dissolve metal precursors and diffuse them into porous materials.In this paper,inorganic salt was used as the precursor,and the co-solvent assisted it to dissolve in scCO2.With mesoporous silica as support,the control strategies of preparing nanocomposites by SCFD method was investigated from the perspectives of adsorption and desorption,discussing regulation mechanism.The control strategy was applied to the preparation system of different metal precursors,co-solvents and supports to realize the controllable preparation of the morphology,size and metal load of the loaded nano metal.The main research contents can be summarized as follows:1.From the perspective of supercritical fluid adsorption,the adsorption and diffusion effect of copper nitrate with different co-solvents on the SBA-15 channels were investigated.Under the slow depressurization strategy(0.008MPa/min),the adsorption conditions such as the dosage of co-solvent,the proportion of double co-solvents and the deposition time were investigated respectively.The morphology of CuO nanophase was changed from nanowires to nanorods,and then the highly dispersed CuO nanoparticles were obtained,whose particle size distribution showed a single peak,with an average particle size around 5 nm and a Cu loading of 19.2 wt%.Combined with the adsorption kinetics experiment of the deposition system,the regulation mechanism of CuO nanophase morphology and dispersion under the double cosolvents system was studied in SCFD.2.From the perspective of supercritical fluid desorption,for the deposition system of copper nitrate+EG+water,the controllability of scCO2 desorption strategy on the nanometal on supports was investigated under the depressurization stage.According to the PR equation of state,the CO2 density-pressure data was calculated,and the grading depressurization and the rapid depressurization were designed accordingly.The depressurization stage was divided into high pressure region,medium pressure region and low pressure region,and the depressurized rate was adjusted respectively to obtain 1-6nm highly dispersed CuO nanoparticles with narrow single-peak particle size distribution.Combining the experimental results of scCO2 desorption strategy with the modified classical nucleation theory in supercritical fluid,the surface energy and nucleation rate of the nanophase were calculated,and the mechanism of scCO2 desorption strategy regulating the size and distribution of CuO nanoparticle was proposed.3.The control capability of scCO2 desorption strategy to control the morphology and size of nano-Ag was investigated.With silver nitrate as precursor and water as co-solvent,grading depressurization was adopted in SCFD.The size of Ag nanoparticles decreased significantly compared with slow depressurization,which was 2～3 nm,and the Ag load could reach 25.3wt%.The SBA-15 was impregnated with silver nitrate aqueous solution and placed in the reactor,after filling scCO2,and the grading depressurization was carried out directly,investigating the flow effect of bulk phase scCO2 and the heterogeneous nucleation and mechanical perturbation caused by scCO2 desorption in SCFD method.The mechanism of supercritical fluid desorption strategy regulating the size and distribution of Ag nanometer phase was described.4.Cu-Ag bimetallic nanocomposites were synthesized by the adsorption and desorption of scCO2.On the basis of obtaining highly dispersed Ag nanoparticles by scCO2 desorption strategy,the second supercritical fluid deposition with copper nitrate as precursor was carried out on Ag@SBA-15 by sequential supercritical deposition method,and the scCO2 desorption strategy was used to regulate Cu species again.Cu-Ag bimetallic nanocomposites were prepared.The two metal precursors of Ag and Cu were added into the co-solvent in simultaneous supercritical deposition method.The concentration of the two precursors was adjusted to control morphology,distribution and molar ratio of Cu-Ag bimetallic nanophase.5.To investigate the activity and stability of Cu catalyst prepared by scCO2 desorption strategy,the catalytic hydrogenation of dimethyl oxalate(DMO)to ethylene glycol(EG)in C1 reaction route was selected as probe reaction.The fixed bed reaction device was used,the reaction solution was 10 wt%DMO methanol solution,the pressure was 3.0 MPa,the molar ratio of hydrogen ester was 120,and the mass airspeed(WHSV)was 1.2 h-1.The change of DMO conversion rate and EG selectivity with the reaction temperature was investigated,and the stability of the catalyst was investigated at an optimized reaction temperature for 120 h.