Catalytic reduction of U(Ⅵ) to U(Ⅳ) using hydrogen as the sole reducing agent.

Through the analysis of the reaction process, the mechanism of the above abnormal phenomena was clarified. This study systematically studied the catalytic hydrogen reduction process and clarified the reaction mechanism, which is an important reference for the engineering application of this process. [Display omitted] • Uranium tetravalent (U(IV)) is the most important reductant in PUREX process of spent fuel reprocessing. • At present, hydrazine nitrate is usually used as reductant to achieve high conversion of uranium (Ⅵ) to uranium (IV) under the action of catalyst. • However, the existing studies show that the reaction kinetics is slow, and the organic impurities in hydrazine nitrate may reduce the life of the catalyst. • Here, we have developed a catalyst with hydrogen as sole reductant to achieve rapid reaction and high conversion from uranium(VI) to uranium(IV). • It was found that the particle size of catalyst support affected the diffusion of macromolecular uranium in the channel, thus affecting the reaction kinetics. • In addition, this study also found the abnormal phenomenon of the reaction system, that is, the reaction rate decreased with the increase of temperature. Using hydrogen gas as a sole reducing agent to catalyze the reduction of hexavalent uranium to prepare tetravalent uranium can avoid the problem of pollutant poisoning catalysts caused by hydrazine reducing agents. However, research using only hydrogen as the sole reducing agent is very scarce. Herein, catalyst has been developed for catalytic hydrogen reduction to prepare uranium(Ⅳ) and a systematic study of the influence of process parameters such as catalyst carrier particle size, temperature, pressure, and acidity on the catalytic hydrogen reduction process for the preparation of uranium(Ⅳ) is presented. The effect of catalyst carrier particle size is particularly pronounced due to the differing rates of molecular diffusion. Fine particle-sized catalyst carriers exhibit faster catalytic reaction kinetics. Temperature exhibits an unusual phenomenon in its impact on the reaction process. As temperature increases, the reaction rate decreases, and an upper limit reaction temperature is observed. Furthermore, at excessively high reaction temperatures, an anomalous occurrence of uranium(Ⅳ) formation followed by disappearance was observed. Through an analysis of the reaction mechanism, the underlying reasons for this abnormal phenomenon are elucidated. Additionally, this paper also reveals that reaction rates increase with increasing reaction pressure and decrease with decreasing raw material acidity. Process parameters such as reaction temperature, pressure, and acidity exhibit coupled effects on the reaction process. Different reaction upper limit temperatures are observed under varying pressures and raw material acidities. [ABSTRACT FROM AUTHOR]