Observation of Hydrothermal Synthesis of Metal Oxide Fine Particle in Supercritical Water by Diamond Anvil Cell

为了发展一种简单的水热金刚石对顶砧装置（HDAC）压力测量技术，并将其用于超临界水中氧化物生成的可视化研究，利用该装置研究了Al（NO_3）_3在水中的水热反应，温度高达421 ℃，压力高达850 MPa。一些凝胶状相变观察到，似乎发挥了关键作用，在勃姆石（AlOOH）的形成。在高Al（NO_3）_3浓度（0.32 M）和高密度（1060 kg/m^3）下，高压诱导形成凝胶状相，并且所形成的相似乎比低密度（400至670 kg/m^3）下的溶液反应性低得多。当密度接近水的临界密度时，随着加热的发生，凝胶状材料在表观临界点附近强烈沉淀之前沉淀并重新溶解到溶液中。铝的羟基络合物在溶液中的模型的开发表明，阳离子物种的主要物种存在于高密度条件下。硝酸根离子似乎在凝胶状相的形成和勃姆石颗粒的产生中起着关键作用。我们随后对此进行了额外的批次实验，发现对于超临界的情况下，硝酸盐不会形成在结晶的勃姆石phase.We发现，勃姆石纳米晶体可以在超临界水热条件下制备没有硝酸盐杂质，与其他结果相反，这表明水热法生产的勃姆石从硝酸盐的起始材料被污染，难以去除硝酸盐。

The objective of this research was to develop a simple technique for measuring pressure for a hydrothermal diamond anvil cell (HDAC) and to use the technique with a HDAC to visually study formation of oxides in supercritical water.Hydrothermal reaction of Al(NO_3)_3 in water was investigated at temperatures up to 421 ℃ and pressures up to 850 MPa with the diamond anvil cell system. A number of gelatinous phase transitions were observed that seemed to play a key role in boehmite (AlOOH) formation. At high Al(NO_3)_3 concentrations (0.32 M) and high densities (1060 kg/m^3), high pressure induced formation of a gel-like phase and the phase formed seemed to be much less reactive than solutions at lower densities (400 to 670 kg/m^3). At densities closer to the critical density of water, as heating occurred, the gelatinous material precipitated and redissolved into solution prior to intense precipitation around the apparent critical point. A model for the aluminum hydroxocomplexes in solution was developed that showed that cationic species were the main species present at high density conditions. Nitrate ion seemed to play a key role in the gel-like phase formation and boehmite particle production. We followed up on this with additional batch experiments and found that for the supercritical case, that nitrate does not form in the crystallized boehmite phase.We found that boehmite nanocrystals can be prepared without nitrate impurities under supercritical hydrothermal conditions in contrast with the other results, which suggested that hydrothermally produced boehmites produced from nitrate starting materials were contaminated with difficult to remove nitrates.