Iron-containing perovskite materials for stable hydrogen production by chemical looping water splitting

Abstract

The purpose of this thesis is to investigate „chemical looping water splitting‟ processes for hydrogen production which involves reducing a solid oxygen carrier material (such as iron oxide) by a reducing agent such as carbon monoxide, syngas or methane. Following this, a second step can be performed where water is used to oxidise the metal, thereby producing free hydrogen (without requiring any separation steps). These two steps (reduction and oxidation) can be repeated to perform redox cycles which would continuously utilise the solid oxygen carrier material. Initial experiments were performed using temperature programmed redox cycles to perform chemical looping water-gas shift and compare the performance of two perovskites (La0.6Sr0.4Co0.2Fe0.8O3-δ and La0.7Sr0.3FeO3-δ) against two supported metal oxides (60% Fe2O3/Al2O3 and 20% NiO/Al2O3). The best performing materials of the perovskites (La0.7Sr0.3FeO3-δ) and support metal oxides (60% Fe2O3/Al2O3) were further tested in isothermal chemical looping water-gas shift cycles over 150 redox cycles to assess material lifetime. The results showed that 60% Fe2O3/Al2O3 under these conditions had deactivated due to the formation of FeAl2O4 which led to lower hydrogen production. La0.7Sr0.3FeO3-δ gave high material stability and steady hydrogen production over more than 100 redox cycles. A nickel-containing perovskite La0.7Sr0.3Fe0.9Ni0.1O3-δ was synthesised and subjected to the same experimental conditions but this material showed no improvement compared to La0.7Sr0.3FeO3-δ in terms of hydrogen production, hydrogen purity and material stability. In an additional experiment 60% Fe2O3/Al2O3 and La0.7Sr0.3FeO3-δ were used as oxygen carrier materials in a three-step chemical looping process involving reduction by methane (to form syngas), water oxidation and air oxidation (to provide heat). 60% Fe2O3/Al2O3 gave good hydrogen purities during water oxidation but showed low material stability, whereas La0.7Sr0.3FeO3-δ showed no loss in material stability but gave low hydrogen purity, possibly due to carbon deposition during the reduction step.