Cobalt-zinc oxide absorbents for low temperature gas desulfurisation

The hydrogen sulfide absorption capacity of a series of cobalt-zinc oxides with nominal Co/Zn atomic ratios of 0/100, 10/90, 20/80, 30/70, 40/60, 50/50, 70/30, 90/10 and 100/0 was determined using a continuous flow absorption apparatus. The reaction of the mixed oxides with H2S amounted to ca. 3 monolayers, and is therefore largely confined to the surface of the oxides. The sulfur uptake was found to be proportional to the surface area of the oxides with a Co/Zn ratio less than or equal to 40/60, indicating that lattice diffusion played a major role in the rate determining step, and that the main function of the cobalt was to increase the surface area. At high cobalt concentrations, the sulfur uptake increased more than proportionately with surface area and the reaction was virtually :stoichiometric for the oxide with a Co/Zn ratio of 100/0. This was associated with a change in the oxide structure from a bulk biphasic ZnO and Co3O4 absorbent with a ZnCo2O4 surface spinel at Co/Zn ratios less than or equal to 70/30 to a monophasic zincian or pure Co3O4 structure at higher cobalt loadings. Analysis of the sulfided mixed oxides showed that microcrystalline membraneous sheets containing cobalt, zinc and sulfur developed on sulfiding. XPS studies of the sulfided oxides indicated that H2S reduced the surface spinel found at Co/Zn ratios less than or equal to 30/70 and the zincian/pure Co3O4 found at higher cobalt concentrations to CoO and ZnO prior to the formation of their sulfides. The results are interpreted in terms of a surface reconstruction occurring during sulfiding.