Perovskite-type (ABO₃) oxygen carriers (OCs) with non-stoichiometric lattice oxygen release have been widely investigated in chemical looping partial oxidation processes (CLPO), and improving the syngas yield depends crucially on tailoring their elemental composition. Nevertheless, how their composition affects the electronic structure and finally the syngas yield remains elusive. Herein, by means of experiments and density functional theory calculations, we report that A-site lanthanide affects the Fe–O covalency in LnFeO₃ (Ln = La, Pr, Sm, and Gd), controlling the syngas yield of OCs. Specifically, a small A-site cation radius induces severe geometric tilting of FeO₆ octahedra, which weakens the Fe–O orbital hybridization in real space, and thus the Fe–O covalency in k space. The decrease in Fe–O covalency in k space can be gauged by the increasing charge-transfer energy, i.e., GdFeO₃ > SmFeO₃ > PrFeO₃ > LaFeO₃. The increase in the charge-transfer energy increases the formation energy of oxygen vacancies and the migration energy of oxygen anions, finally deteriorating oxygen mobility and surface oxygen activity. The activity and stability tests show that the syngas yields decreased in the order of LaFeO₃ > PrFeO₃ > SmFeO₃ > GdFeO₃, corresponding to the Fe–O covalency intensity. Our findings unlock a useful tool, i.e., charge-transfer energy, for rationally designing OCs and deepening our understanding of the modulating mechanism of lattice oxygen in chemical looping technologies.

To cope with global warming and increasing carbon emissions, the chemical looping process has attracted attention due to its excellent ability to convert fossil fuel and capture CO₂. In this case, chemical looping partial oxidation technology has become the focus of attention due to its advantages in the production of syngas and hydrogen, especially with respect to the design and selection of oxygen carriers, which directly affect the efficiency of the production of syngas and hydrogen. In particular, the conversion of methane can reach 95% in the chemical looping partial oxidation of methane, and the selectivity of syngas, in the range of 700 °C to 900 °C at atmospheric pressure, can reach 99% for twenty or more cycles. In this review, from the perspective of metal oxide selection and structure regarding the chemical looping partial oxidation process, we discuss the role of oxygen carriers in the chemical looping partial oxidation cycle, in which the specific surface area, the lattice oxygen mobility, and the thermal stability are understood as the important factors affecting reactivity. We hope to summarize the design and development of efficient oxygen carriers with high oxygen-carrying capacity and syngas selectivity, as well as contribute to the selection, design, optimization, and redox reaction mechanism of redox catalysts.