Preparation and characterization of (CeO₂)_x–(Fe₂O₃)_1−x nanocomposites: reduction kinetics and hydrogen storage

A series of nanosized CeO₂–Fe₂O₃ mixed-oxide nanocomposites with different Ce⁴⁺/Fe³⁺ molar ratios were successfully prepared by a co-precipitation technique. The surface area increased with Fe₂O₃ content increasing up to 60 wt% in the composite. However, with further increase in Fe₂O₃ content, the surface area began to decrease. The reduction processes of the CeO₂–Fe₂O₃ nanocomposites were studied in a hydrogen atmosphere at 300–600 °C. The reduction rates increased by increasing both the temperature and Fe₂O₃ content in the nanocomposites. The microstructure of the reduced composites at 500 °C illustrated the presence of a considerable number of macro- and micro-pores. The activation energy values were calculated which were in the range of 3.56–5.37 kJ·mol⁻¹ at the initial stages (up to 35% reduction) and 5.21–10.2 kJ·mol⁻¹ at the final stages (up to 80% reduction) of reduction. The rate-controlling mechanisms in both the initial and final reduction stages were determined, and the initial reaction stage was controlled by combined gaseous diffusion and interfacial chemical reaction mechanisms for all the composites except for pure CeO₂, which was controlled by a chemical reaction mechanism. The final reaction stage was controlled by a gaseous diffusion mechanism for some composites, while for the others it was controlled by combined gaseous diffusion and interfacial chemical reaction mechanisms. The hydrogen sorption properties of the nanocomposites were studied by pressure composition isotherms using a volumetric method. Hydrogen storage in the nanocomposites increased by increasing the temperature because of the formation of oxygen vacancies which enhance atomic H adsorption and function as strong adsorption sites forming more metal hydride covalent bonds.