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

本研究的目的是开发一种简单的技术来测量热液金刚石砧细胞（HDAC）的压力，并利用该技术与HDAC一起直观地研究超临界水中氧化物的形成。在温度高达421℃，压力高达850 MPa的条件下，用金刚石砧槽体系研究了Al(NO_3)_3在水中的水热反应。观察到许多凝胶相转变似乎在薄水铝石（AlOOH）的形成中起关键作用。在高Al(NO_3)_3浓度（0.32 M）和高密度（1060 kg/ M ^3）下，高压诱导凝胶状相的形成，形成的相似乎比低密度（400 ~ 670 kg/ M ^3）下的溶液反应性要低得多。当密度接近水的临界密度时，随着加热的发生，凝胶状物质沉淀并再溶解成溶液，在表观临界点附近发生强烈沉淀。建立了溶液中铝氢氧配合物的模型，表明在高密度条件下，阳离子种类是主要的种类。硝酸离子似乎在凝胶状相的形成和薄水铝石颗粒的产生中起关键作用。我们在此基础上进行了额外的批量实验，发现在超临界情况下，硝酸盐不会在结晶薄水铝石相中形成。我们发现在超临界水热条件下可以制备出不含硝酸盐杂质的薄水铝石纳米晶，这表明以硝酸盐为原料的水热法制得的薄水铝石受到难以去除的硝酸盐的污染。

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.