Ca₂Fe₂O₅: A promising oxygen carrier for CO/CH₄ conversion and almost-pure H₂ production with inherent CO₂ capture over a two-step chemical looping hydrogen generation process

Chemical looping hydrogen generation (CLHG) is a promising technology for high-purity hydrogen production with inherent CO₂ separation. The selection of a high-performance oxygen carrier capable of being reduced and oxidized over multiple redox cycles against deactivation is a key issue for CLHG technology. In this work, a two-step chemical looping hydrogen generation (TCLHG) process is proposed by using a novel calcium ferrite, Ca₂Fe₂O₅, as an oxygen carrier which is synthesized with applied a citric acid assisted sol–gel method. The experimental results indicate that the reduced oxygen carrier achieves one-step oxidation from Fe⁰ to Fe³⁺ by using steam as an oxidizing agent. Thus, higher yields of hydrogen could be generated compared with Fe₂O₃. The fresh and reacted Ca-Fe based oxygen carriers were characterized using different methods such as XRD, SEM/EDS, TEM, N₂ adsorption, H₂-TPR, XPS, and Mossbauer spectroscopy test etc. The oxygen release and storage capacity, cyclic stability, and carbon deposition characteristics of the Ca-Fe based oxygen carriers were investigated using TGA and a fixed bed reactor with multicycles of CO/CH₄ reduction and H₂O/O₂ oxidation. Ca₂Fe₂O₅ is proved to be a more stable formation of the calcium ferrite compounds and a promising oxygen carrier for TCLHG process which shows perfect reducibility, oxidation activity, and cyclic stability. The existence of Ca appears to perform a significant effect on the Fe³⁺ reduction and Fe⁰ oxidation and the reduction from Fe³⁺ to Fe⁰ was concluded to be a simple one-step reaction.