Abstract:

Based on the classical nucleation theory and the related models of crystallization kinetics in the supercritical field, the main factors and laws affecting the nucleation rate are analyzed. Inorganic salt precursor was dissolved in supercritical carbon dioxide with the assistance of co-solvents, and it was deposited in the silicon-based SBA-15. When depressurized rate was increased gradually for the initial stage of depressurization (0.05—18 MPa/min, 20—14 MPa), the average particle size of supported nanoparticles gradually decreased to 1.5 nm from TEM observation, which is close to the typical crystal nucleus size of 1 nm. However, an increasing trend of metal loading had emerged. The critical nucleation radius of the crystal was taken as 0.5 nm, and nucleation rate in the nozzle of the rapid expansion of supercritical solution (RESS) process was calculated by Türk model and Debenedetti model, compared with the initial nucleation rate of population balance model (PBM) of supercritical anti-solvent (SAS), which was considered as the boundary condition of PBM. The nucleation rates calculated based on these three models were close to the estimated nucleation rate by Cu loading of the as-deposited sample under the rapid depressurization method. In the depressurization stage subsequent to the supercritical deposition stage, the desorption of CO₂ under supercritical conditions may be an inductive factor to nucleation of the metal precursor adsorbed on the surface of the supports, and the nucleation rate of the nanoparticles and the metal loading supported on SBA-15 could be controlled by the amount of instantaneous desorption of CO₂. The experimental and theoretical foundation were formed to investigate the performance of SAS process in micro-scale material.

Key words: supercritical CO₂ deposition, mesoporous material, rapid depressurization, crystallization kinetics, nanoparticles

摘要：

依据经典成核理论和超临界领域中的结晶动力学相关模型，分析影响成核速率的主要因素及其规律。研究共溶剂辅助超临界CO₂溶解无机盐，在SBA-15介孔材料表面沉积实验的结果，发现伴随初始阶段的泄压速率逐步提升（0.05~18 MPa/min，20~14 MPa），载体所负载的纳米颗粒的粒径逐渐减小至1.5 nm左右，与典型的晶核尺寸1 nm相接近，而担载量却出现逐渐增加的趋势。晶体的临界成核半径取为0.5 nm，通过Türk模型和Debenedetti模型计算超临界流体快速膨胀（RESS）工艺其喷嘴内的成核速率，与超临界反溶剂（SAS）群体平衡模型（PBM）的边界条件即SAS过程的初始成核速率相比较，三者的成核速率相接近，且利用快速泄压方法的沉积实验结果与按Cu担载量所估算的成核速率相接近。分析在沉积反应后的泄压阶段，超临界条件下的CO₂的脱附作用，可能成为吸附于载体表面的前驱物离子结晶的诱导因素。并且CO₂瞬时脱附量能够调控负载型纳米颗粒的成核速率，同时控制复合材料的金属担载量。为研究微观尺度下SAS过程的实现提供了实验与理论基础。

关键词: 超临界CO₂脱附, 介孔材料, 快速泄压, 结晶动力学, 纳米颗粒