The effect of surface state on the kinetics of cerium-hydride formation

We studied the effects of surface oxidation on the kinetics of hydrogen (H) absorption into two commercial samples of Ce: bulk samples and thin rolled foils. A Sieverts' apparatus was used to measure the H absorption kinetics and X-ray photoelectron spectroscopy (XPS) was used to determine the surface oxidation state and the presence of impurities, notably carbon (C). In the as-received condition, the bulk Ce specimen was covered by a Ce₂O₃ film (sesquioxide) whereas the as-received foil was covered with a CeO₂ film (dioxide). In this condition, the H absorption rate at room temperature was approximately an order of magnitude faster in the bulk sample than in the foil. Vacuum annealing at P ≤ 1 × 10^-4 Torr and T = 773 K significantly increased the rate of H uptake, making it similar in both samples. This annealing reduced the Ce₂O₃ in the bulk samples so that the samples became more metallic, but produced little change in the XPS signal from the CeO₂ film on the foil sample. The study suggests that the differences in H uptake kinetics are affected not only by the nature of the oxide (sesquioxide versus dioxide) but also by the integrity of the surface oxide film, which may be affected by the thermal treatment.