Multiple Spectroscopic and Microscopic Study on Hydrolysis of Tetravalent Cerium: In-situ Characterization of Ce(IV) Hydrolytic Species in an Aqueous Solution

Because of its chemical similarity and stable tetravalent oxidation state in solution, cerium (Ce) is often employed as an analogue of tetravalent actinides (An(IV)) in various research fields related with actinides. Besides, its dioxide compound, CeIVO₂, has recently attracted considerable attention owing to its wide variety of industrial applications, such as catalysis, oxygen storage, fuel cells, or luminescent materials.
Hydrolysis is one of the most fundamental reactions of metal ions in an aqueous solution. In fact, the hydrolysis behaviour of tetravalent cerium (Ce(IV)) has been studied and often compared with those of An(IV) in order to evaluate its suitability as an analogue of An(IV). Furthermore, the hydrolysis–precipitation process of cerium ions is also of great importance in its industrial applications, since it is one of the most straightforward and simple methods to produce CeO₂. However, despite these importance of hydrolysis reaction in the basic- and applied research fields for Ce, most of the precedent studies have focused on its thermodynamical aspects, and still little has been known about the complex structure of Ce(IV) hydrolytic species formed in solution. In this study, multi-spectroscopic (i.e., X-ray absorption spectroscopy (XAS), high energy X-ray scattering (HEXS), dynamic light scattering (DLS)) and microscopic (TEM) techniques have been employed to structurally characterize the hydrolytic Ce(IV) species in an aqueous solution. The results indicate that nanocrystalline CeO₂ forms and grows up in an aqueous solution as the hydrolysis proceeds with increasing pH. On the presentation, the results will be also discussed with the corresponding An(IV) hydrolytic species.