Deactivation Mechanism of COS Hydrolysis over Potassium Modified Alumina

Abstract: Carbonyl sulfide (COS) is commonly found in conventional fossil fuels, such as nature gas, oil-associated gas, and blast-furnace gas, and its untreated emission not only corrodes pipelines and poisons catalysts but will also inevitably pollute the environment and endanger human health. Catalytic hydrolysis is recognized as the most promising strategy to eliminate COS because it can be performed under mild reaction conditions with a high removal efficiency. Notably, alkali metals promote catalytic COS hydrolysis over Al2O3 owing to their electron donor properties, basicity, and electrostatic adsorption. However, despite the significant attraction of using potassium-promoted Al2O3 (K2CO3/Al2O3) as conventional catalysts for COS hydrolysis, the mechanism of COS hydrolysis over K2CO3/Al2O3 remains unclear and is controversial owing to the complex composition of the K species. In this study, commercial Al2O3 modified with potassium and sodium salts were synthesized using the wet impregnation method and characterized by various techniques. Based on the results of the activity measurements, the K2CO3-, K2C2O4-, NaHCO3-, Na2CO3-, and NaC2O4-modified catalysts had a positive effect on COS hydrolysis. Among them, the K2CO3/Al2O3 catalyst exhibited the highest COS conversion. Notably, the K2CO3/Al2O3 catalyst exhibited an excellent catalytic performance (~93%, 20 h), which is significantly better than that of pristine Al2O3 (~58%). Furthermore, this study provides strong evidence for the role of H2O during catalytic hydrolysis over K2CO3/Al2O3 using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS). The in situ DRIFTS analysis revealed that hydrogen thiocarbonate formed as an intermediate during COS hydrolysis over K2CO3/Al2O3. Meanwhile, the XPS findings suggested that sulfates and elemental sulfur accumulated on the catalyst surface, which may have contributed to catalyst poisoning. Additionally, the effect of water vapor content in the reaction pathway of COS hydrolysis over K2CO3/Al2O3 was investigated. The presence of excess water resulted in a reduction in catalytic activity owing to competitive adsorption between H2O and COS molecules on the catalyst surface. The enhancement in the catalytic activity over K2CO3/Al2O3 may be attributed to the formation of HO-Al-O-K interfacial sites. More importantly, all the catalysts were used under industrially relevant conditions, which provides valuable theoretical guidance for practical applications in the future. Thus, this detailed mechanistic study reveals new insights into the roles of the interfacial K co-catalyst, which provides a new opportunity for the rational design of stable and efficient catalysts for COS hydrolysis.