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, 50/5 0, 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 mon olayers, and is therefore largely confined to the surface of the oxides. Th e sulfur uptake was found to be proportional to the surface area of the oxi des 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 m ain 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 surf ace spinel at Co/Zn ratios less than or equal to 70/30 to a monophasic zinc ian or pure Co3O4 structure at higher cobalt loadings. Analysis of the sulf ided mixed oxides showed that microcrystalline membraneous sheets containin g cobalt, zinc and sulfur developed on sulfiding. XPS studies of the sulfid ed oxides indicated that H2S reduced the surface spinel found at Co/Zn rati os less than or equal to 30/70 and the zincian/pure Co3O4 found at higher c obalt concentrations to CoO and ZnO prior to the formation of their sulfide s. The results are interpreted in terms of a surface reconstruction occurri ng during sulfiding.