Understanding the catalysis of noble metals in reduction of iron oxide by hydrogen: insights from DFT calculations

Abstract

Fe/FeO_x redox couples have been widely used as an oxygen carrier for redox devices such as chemical looping reactors and solid oxide iron–air batteries (SOIABs) because of their low cost and high oxygen capacity. However, a critical challenge is the sluggish reduction kinetics of FeO_x in the intermediate temperature range, significantly limiting the devices' achievable efficiency and service life. Here, we report on a combined theoretical and experimental study on the catalytic effect of noble metals (Ir, Ru, Rh, Pd and Pt) on the H₂-reduction kinetics of FeO_x. We first use density functional theory (DFT) to calculate the electron projected density of states (PDOS) near the Fermi level (E_F) of several noble metal (Ir, Pd, Ru, Rh, Pd)/Fe₃O₄ systems. We reveal that Ir offers the highest PDOS near E_F among all noble metals studied, which provides abundant electrons for efficient cleavage of O–Fe bonds and low-energy dissociation of H₂ molecules, thus resulting in significantly boosted reduction kinetics of Fe₃O₄. Experimentally, the results of temperature programmed reduction and SOIAB performance testing corroborate the theoretical predictions.