Ni/Al co-doping induces FeO6 octahedral distortion to activate lattice oxygen in Ca2Fe2O5 for enhanced chemical looping hydrogen generation

Abstract

Chemical looping steam methane reforming (CLSMR) is an efficient technology for syngas and hydrogen production, but its progress is hindered by catalyst activity and stability limitations. This study develops highly reactive Ca₂Ni_xAl_yFe_2−x−yO₅ oxygen carriers, with Ni and Al doping significantly enhancing performance. At 850 °C, the Ca₂Ni_0.1Al_0.2Fe_1.7O₅ carrier achieves excellent results: CO selectivity of 89.75%, syngas yield of 6.18 mmol g⁻¹, and pure hydrogen yield of 4.40 mmol g⁻¹ during the steam oxidation stage, with no carbon deposition and stable performance over 15 cycles. Ni doping enhances the catalytic activity, whereas Al doping promotes the formation of oxygen vacancies. Their synergistic effect enhances active site density and oxygen transport, boosting overall catalytic efficiency and enabling high-yield syngas and hydrogen co-production. Density functional theory (DFT) calculations indicate that the upward shift of the O-2p band center reflects enhanced reactivity of lattice oxygen, while adjustments in the Fe–O–Fe bond angles within FeO₄ and FeO₆ groups further optimize the migration pathways of oxygen ions. Co-doping with Ni and Al reduces the oxygen vacancy formation energy of the Ca₂Fe₂O₅ oxygen carrier from 3.35 eV to 2.04 eV, and decreases the oxygen migration energy from 1.36 eV to 1.02 eV, demonstrating its facilitation in oxygen mobility and migration. Additionally, the synergistic interaction between oxygen vacancies and Brønsted acid sites further enhances reaction efficiency.