The effect of copper on the redox behaviour of iron oxide for chemical-looping hydrogen production probed by in situ X-ray absorption spectroscopy

Abstract

The production of high purity hydrogen with the simultaneous capture of CO₂, can be achieved through a chemical looping (CL) cycle relying on an iron oxide-based oxygen carrier. Indeed, the availability of active and cyclically stable oxygen carriers is a key criterion for the practical implementation of this technology. In this regard, improving our understanding of the reduction pathway(s) of iron-based oxygen carriers and the development of concepts to increase the reduction kinetics are important aspects. The aim of this work is to evaluate the effect of the addition of copper on the redox behaviour of iron oxide based oxygen carriers stabilized on ZrO₂. In situ pulsed-H₂ XANES (Fe K-edge) experiments allowed for the determination of the reduction pathways in these materials, viz. the reduction of both Fe₂O₃ and CuFe₂O₄ proceeded via a Fe²⁺ intermediate: Fe₂O₃ (CuFe₂O₄) → Fe₃O₄ (Cu⁰) → FeO (Cu⁰) → Fe⁰ (Cu⁰). In the first step CuFe₂O₄ is reduced to Cu⁰ and Fe₃O₄, whereby Cu⁰ promotes the further reduction of iron oxide, increasing their rate of formation. In particular, the rate of reduction of FeO → Fe⁰ is accelerated most dramatically by Cu⁰. This is an encouraging result as the FeO → Fe⁰ transition is the slowest reduction reaction.