Tuning H₂S/CH₄ competitive reactions in sulfur fuel chemical looping combustion via oxygen vacancy engineering of Ca/Zn spinel ferrite oxygen carriers

The interactions between CH₄ and H₂S in oxygen carriers (OCs) during chemical looping combustion (CLC) are a vital research focus, especially for enhancing syngas generation and reducing unwanted byproducts. This research thoroughly examines the behavior of CH₄ and H₂S on both pristine and oxygen-deficient surfaces of CaFe₂O₄ (CAF) and ZnFe₂O₄ (ZNF) OCs through a combination of experimental studies and density functional theory (DFT) simulations. Experimental findings show that the temperature during synthesis significantly affects the properties of OCs. Those created at 850 °C exhibited enhanced resistance to sulfur, while OCs synthesized at 950 °C and 1000 °C reached maximum H₂S uptake levels of 108 mg/g and 142 mg/g, while still achieving CH₄ conversion rates above 80% after 10 redox cycles. DFT simulation further clarifies that H₂S preferentially occupies the Ca and Fe sites and dominates in co-adsorption with CH₄. Oxygen vacancies reduce H₂S adsorption while promoting CH₄ binding, although the presence of oxygen vacancies weakens the repulsion between CH₄ and H₂S, the inhibitory effect of H₂S on CH₄ remains. These results offer essential insights for designing sulfur-resistant Ca-Fe/Zn-Fe spinel OCs and understanding the mechanisms behind the competitive reactions of CH₄ and H₂S, thus contributing to the advancement of effective CLC systems with enhanced carbon capture efficiency.