Effect of temperature and pressure on the reduction of U(VI) to U(IV) by hydrogen over Pt/SiO2: Reaction kinetics and modeling

The kinetics of catalytic reduction of U(VI) by H2 over Pt/SiO2 was investigated as a function of various parameters. The effect of agitation speed and particle size on the reduction of U(VI) was studied initially to eliminate the mass transfer resistance. Later, the effect of catalyst loading, concentration of U(VI), HNO3, N2H4, H2 pressure and temperature on the reduction of U(VI) was studied. Langmuir–Hinshelwood– Hougen–Watson (LHHW) approach was employed to derive kinetic model equations and understand the reaction mechanism. Among the various models, the model – IX corresponding to the adsorption of nitric acid on the catalyst surface was found to be suitable for explaining the concentration profiles of U(VI) and HNO3 in all the temperature and pressure range. The activation energy of the reaction was determined to be around 22.6 kJ mol−1 indicating the reduction of U(VI) with H2 was falling in chemical and diffusion controlled regime.