Preparation Of Ag (Au)/SiO₂ Nanocomposites By Supercritical Fluid Deposition And Process Simulation

Currently, nanocomposite has attracted a great deal of attention in the field of materials and has a wide range of applications in catalytic, mechanics, optics, electrochemistry etc. Supercritical Fluid Deposition (SCFD) technology has been proved to be an effective way for confining metal nanoparticles and nanowires in the nanoscale channels of mesoporous substrate. The influencing factors, mechanism, thermodynamics and kinetics behaviors of preparing nanocomposites using SCFD method were investigated in this work. The main work and experimental results are as follows:(1) Using KIT-6 as substrate, AgNO3 as precursor, ethanol and ethylene glycol as co-solvent, the Ag/KIT-6 nanocomposite was prepared at 50℃and 16-24MPa. The experimental results indicated that the deposition process can be completed in only 3h using 3mL (3.59mol%) ethanol and 0.5mL (0.6mol%) ethylene glycol. Highly dispersed Ag nanoparticles with the average size of 5.3nm were found in the channels of KIT-6.(2) The metal loading increased with the deposition time and it didn't change until the adsorption approached equilibrium. Moreover, the metal loading increased with the decrease of the mass ratio of substrate to precursor and it reached the maximum value of 26.3wt% when the ratio was 1:1. The experiments were repeated at the same conditions and the results showed that SCFD method has good repeatability.(3) Using SBA-15 as substrate, HAuCl4·4H2O as precursor, ethanol, ethylene glycol and water were chose as co-solvent, the Au/SBA-15 nanocomposite was prepared at 50℃and 20-24MPa. Using ethanol and water as co-solvent, Au nanoparticles or nanowires were found in the nanocomposite. A high loading of Au can be obtained even at a low concentration of the precursor. Au nanowires with diameter of 5.8 nm and highly dispersed nanoparticles with the average size of 5.0-6.Onm were obtained in the Au/SBA-15 nanocomposite when ethanol plus ethylene glycol were used as co-solvent and the deposition time was only 0.5h.(4) The metal loading in nanocomposite were measured at 35℃,50℃,60℃at different concentrations of precursors while keeping the density of carbon dioxide constant. At a fixed precursor concentration, the loading was higher at low temperature; while at the same temperature, the loading increased with the concentration of the precursor. Langmuir model, Modified Langmuir model and Freundlich model were used as adsorption model to fit the equilibrium data. (5) The adsorption model was established to simulate the kinetics in SCFD process. The system of Cu(NO3)2 (adsorbate) and SBA-15 (adsorbent) as well as AgNO3 (adsorbate) and SBA-15 (adsorbent) were studied and the simulation showed that the equilibrium time for Cu(NO3)2 was 2000-3000s and AgNO3 was about 10000s. The results were consistent with the experiments. There is little difference between the equilibrium concentration simulated using Langmuir and Modified Langmuir model. The Modified Langmuir model can simulate the experimental data better.