(180a) Mechanistic Investigations into the Hydrolysis of Carbonyl Sulfide on Metal Oxides By Ab-Initio Simulations

Coal-derived syngas contains carbonyl sulfide (COS), a contaminant capable of poisoning the downstream catalytic processes. Metal oxide-based catalysts such as alumina and titania are used for the hydrolysis of COS and the products CO₂ and H₂S are removed using various solvent-based absorption processes. In previous studies, the role of local structures of the metal oxides on the COS hydrolysis activity and the major reaction pathway have not been considered. Accordingly, this work aims at obtaining the energetics of COS hydrolysis on metal oxides by DFT calculations. Using γ-Al₂O₃ (110) as the representative surface, the feasibility of two possible mechanistic pathways is explored based on the reaction energy diagrams and degree of rate control (DORC) analysis.

The calculations show that the presence of surface hydroxyl groups is crucial for COS hydrolysis, and gas-phase COS reacts with the surface hydroxyl groups resulting in the formation of different types of surface hydrogen thiocarbonate (HTC) intermediates. Using data from spectroscopy studies, two pathways, i.e., direct and HCO₃-mediated pathways are proposed (Fig. A). A comparison of the reaction energy diagrams shows that the direct pathway is favored as compared to the HCO₃-mediated pathway (Fig. B). DORC analysis is used to further establish the dominance of the direct pathway over the HCO₃-mediated pathway (Fig. C). It is found that at temperatures above 100°C, the formation of the surface HTC species has the highest influence on the overall rate of COS hydrolysis. Moreover, the contribution of this reaction to the overall reaction increases with temperature. The abstraction of surface hydrogen species by the HTC intermediate is also found to have an influence on the overall reaction rate. A kinetic model is proposed, the details of which will be given in the full manuscript.