Effect of a Support on the Properties of Zinc Oxide Based Sorbents

We present the comparative analysis of three Zn-based sorbents for the process of sulphur removal from hot coal gas. The sorbents were prepared by a slurry impregnation of TiO₂, SiO₂ and Al₂O₃, resulting in complex, multiphase materials, with the dominant phases of Zn₂TiO₄, Zn₂SiO₄ and ZnAl₂O₄, respectively. We have analyzed the effect of supports on the phase composition, texture, reducibility and H₂S sorption. We have found that the phase composition significantly influences the susceptibility of the investigated materials to reduction by hydrogen. Zn₂TiO₄ have been found to be the easiest to reduce which correlates with its ability to adsorb the largest amount of hydrogen sulphide-up to 4.2 gS/100 g-compared to the other sorbents, which absorb up to 2.2 gS/100 g. In the case of Zn₂SiO₄ and ZnAl₂O₄, this effect also correlates with reducibility-these sorbents have been found to be highly resistant to reduction by hydrogen and to absorb much less hydrogen sulphide. In addition, the capacity of ZnAl₂O₄ for H₂S adsorption decreases in the subsequent work cycles-from 2.2 gS/100 g in the first cycle to 0.8 gS/100 g in the third one. Computational analysis on the DFT level has shown that these materials show different thermodynamic stability of sulphur sites within the unit cells of the sorbents. For Zn₂TiO₄ and Zn₂SiO₄, the adsorption is favorable in both the first and second layers of the former and only the top layer of the latter, while for zinc aluminate it is not favorable, which is consistent with the experimental findings.