Adsorption and decomposition of H2S on UO2(0 0 1)

The adsorption and decomposition of H2S on UO2(0 0 1) are studied using ultrahigh vacuum methods, including X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), electron stimulated desorption (ESD), and temperature programmed desorption (TPD), over the temperature range 100–800 K. Our work is motivated by the large stockpiles of depleted uranium in the US, and the interest in possible uses of UO2 in catalytic applications, e.g., hydrodesulfurization processes to remove impurity sulfur from petroleum. Hydrogen sulfide (H2S) is adsorbed molecularly at 100 K on the surface, and a desorption peak for molecular H2S is observed at ∼130 K in TPD, corresponding to a binding energy of ∼0.3 eV. Adsorption properties as a function of H2S exposure are measured at different temperatures using XPS; the S2p intensity and lineshapes demonstrate that the saturation coverage of S-containing species is ∼1 monolayer (ML) at 100 K, and decreases with increasing temperature to ∼0.2 ML of dissociation fragments at 300 K. Moreover, LEIS and XPS show that a small amount of S remains chemically bonded to the UO2 surface after TPD measurements. We use complementary ESD techniques to measure the adsorption rates of H2S at different coverages and temperatures, for comparison with the XPS data. We suggest that oxygen vacancies are of importance in the adsorption of S-containing molecules, as proposed recently by several groups for the S/TiO2 system.