Kinetics of the reduction of uranium oxide catalysts

The reduction of uranium oxide and uranium oxide on alumina catalysts by ethylbenzene and by hydrogen has been studied in a thermobalance. Ethylbenzene mole fractions between 0.0026 and 0.052 and hydrogen mole fractions between 0.1 and 0.6 were applied at temperatures of 425-530 °C. During the reduction the uranium oxides are converted into UO₂. The rate of reduction of pure uranium oxide appears to be constant in the composition region UO_2.6-UO_2.25. The extent of this region is independent of the concentration of the reducing agents and of the reaction temperature. The constant rate is explained in terms of a constant oxygen pressure which is in equilibrium with the two solid phases, U₃O_{8 - x} and U₄O₉. The reduction rate is first order in hydrogen and zero order in ethylbenzene with activation energies of 120 and 190 kJ mol^-1, respectively. Oxygen diffusion through the lattice is probably not rate limiting. The reduction behavior of uranium oxide on alumina is different from that of pure uranium oxide; the rate of reduction continuously decreases with increasing degree of reduction. An explanation for this behavior has been given by visualizing this catalyst as a set of isolated uranium oxide crystallites with a relative wide variation of diameters, in an alumina matrix. At the beginning of the reduction, carbon dioxide and water are the only reaction products. Thereafter, benzene is found as well and, finally, at U O ratios below 2.25, styrene also appears in the reactor outlet.