Effect of Mo dopants on improving hydrogen production by redox of iron oxide: catalytic role of Mo cation and kinetic study

Abstract

Iron oxide (Fe₂O₃) is a promising substitute for other hydrogen storage materials, which can store/produce hydrogen by a reversible redox process (Fe₃O₄ + 4H₂ ↔ 3Fe + 4H₂O). In order to evaluate the effectiveness of iron oxide for this purpose, a series of Mo-doped iron oxides (Fe₂O–5%Mo, –8%Mo and –10%Mo) were prepared by a hydrothermal synthesis. The catalytic effect of Mo cations on hydrogen production was investigated. The results show that the addition of an Mo ion to Fe₂O₃ could improve the hydrogen production in water decomposition at a relatively modest temperature (300–400 °C) and the Fe₂O₃–8%Mo sample was the most effective one. A reasonable explanation for this result is that the repeated change (Mo⁴⁺ ↔ Mo³⁺ + Mo⁰) of valence of the Mo ion during the redox process resulted in the change of composition and microstructure of the doped samples, which suppressed sintering. The apparent activation energy of Fe₂O₃ and Fe₂O₃–8%Mo at the H₂ production step is about 55.5–65.3 and 36.2–45.2 kJ mol⁻¹, respectively. The cooperative effect of Mo ions and active Fe on water decomposition is the main reason for lowering the activation energy, which was also the reason for enhancing hydrogen production.