Decoding fuels and oxygen carriers interaction: Insights into site-specific adsorption mechanisms and driving forces of H2, CH4 and CO on doped iron-based oxygen carriers during chemical looping combustion

Abstract

Oxygen carriers (OCs) in chemical looping combustion (CLC) system can catalyze fuel decomposition and regulate the reactions kinetics. Adsorption of gaseous fuel on the surface of the OCs during CLC is of vital importance, as the core of catalytic reactions begins with the adsorption mechanism and even largely determines the enrichment, activation and selectivity of the reactants. However, the iron-based OCs with transition metal doping made minute structural differences could be largely divergent in the adsorption behavior during CLC, which seems not to raise much concern. Herein, the surface interactions for adsorption of representative gaseous fuels, H₂, CH₄, and CO onto Mn/Co/Ni/Cu/Zn-doped iron-based OCs during CLC were explored in-depth based on DFT calculations. The nature of the interaction between fuels and OCs was revealed from both quantitative views and direct pictures. Stable adsorption configurations were identified. H₂ and CO prefer bridge sites, while CH₄ favors hollow sites. Energy decomposition analysis quantitatively revealed distinct dominant interactions: orbital forces (55–65 %) for H₂, electrostatic interactions (40–50 %) for CH₄, and orbital forces for CO physisorption (45–57 %) versus combined orbital/electrostatic forces (50–56/42–49 %) for chemisorption. Reduced density gradient and interaction region indicator analyses visually confirmed van der Waals-dominated interactions, with Cu/Ni/Co doping enhancing adsorption. Electronic structure analysis (density of states, d-band center, work function) demonstrated that doping modulates OCs reactivity via TM···O/Fe···O interactions, upward d-band shifts, and reduced work functions, driven by dopant electronic structure, surface charge imbalance, and lattice distortion.