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The sticking probability of Hz on the surface of polycrystalline V covered with oxygen was examined as a function of oxygen concentration in the bulk metal (0.4 to 3.3 at.%) at specimen temperatures from 200 to 1000℃. At all temperatures examined, the sticking probability was strongly dependent on the oxygen concentration in the bulk.Such correlation between the reaction rate of Hz on the surface and the oxygen concentration in the bulk was attributed to the dependence of concentration of oxygen vacancies (holes in surface oxygen coverage) on the oxygen bulk concentration. The values of sticking probability were relatively high even at the oxygen bulk concentration of 3.3 at.%; the oxygen coverage on vanadium was rather "holey" even at the oxygen concentration close to the solubility limit. The activation energy for H_2 dissociation on such holes in the coverage was evaluated to be 10 kJ/mol H_2.

Sticking coefficient of H2, α, on a vanadium surface was measured after heat treatments in vacuum at 673, 873, 973 and 1273 K. The values of α were comparable after heat treatments at 673, 873 and 973 K, while significant reduction was observed after heat treatment at 1273 K. Arrhenius plots of α showed that the pre-exponential factor, α0, was far smaller than unity for all heat treatment conditions. This observation indicated that only minor portions of surface sites were active for hydrogen ingress. Change in chemical surface state of V by heat treatments in vacuum at 673, 873, 1073 and 1273 K was also investigated by means of X-ray photoelectron spectroscopy. The specimen surface was mainly covered by VO at 673 K and by an oxygen adlayer at 873 K. Sulfur appeared at 1073 K and completely substituted for oxygen at 1273 K. Namely, sulfur became the dominant surface impurity in the temperature region where α showed significant reduction. It was therefore concluded that the barrier effect of oxygen, including formation of a VO layer, was much weaker than that of sulfur under the present conditions.