Low Temperature H2S Removal with Metal-Doped Nanostructure ZnO Sorbents: Study of the Origin of Enhanced Reactivity in Cu-Containing Materials

Sulfidation of pure and metal-doped ZnO nanostructure sorbents (M0.03Zn0.97O, M = Fe, Co, Ni, Cu) was studied in order to clarify the effect of metal on the transformation kinetics at 200−350 °C. The solids were prepared by coprecipitation from metal nitrate solution followed by calcination at 400 °C. Reaction with H2S was studied by thermal gravimetric analysis (TGA) using a gas mixture containing 0.2 vol % H2S in equimolar H2−N2. It was found that at 350 °C the TGA sulfidation profiles of all studied samples are similar, with the interface reaction being the main rate-determining step. After lowering the temperature to 250 °C the transformation of Cu0.03Zn0.97O continues to be controlled by the interface reaction with only a slightly decreased rate. In contrast, for all other samples the diffusion resistance appears, provoking a significant drop of their transformation rates. This finding shows that during sulfidation of Cu-doped ZnO the diffusion is faster than for all other sorbents. The same effect was observed for the sample prepared by impregnation of ZnO powder and containing supported Cu species. In order to understand the origin of this effect, the sulfided sorbents were characterized by XRD and N2 physisorption, and no correlation was found between the sulfidation rate and textural properties of formed sulfides. This result indicates that sulfur transport during sulfidation occurs by solid state rather than gas phase diffusion. Also XPS has shown that Cu2S−ZnS solid solution is formed during sulfidation of the Cu-doped solids. We thus suggest that diffusion enhancement in the presence of copper is brought about by sulfur vacancies created through charge compensation of Cu+ replacing Zn2+.