Design of Ni-promoted Ca2Fe2O5/modified ceria as oxygen carrier for fast and carbon-resistant chemical-looping steam methane reforming

Chemical looping steam methane reforming enables the production of high-purity syngas and hydrogen while eliminating the energy-intensive downstream separation steps typically required by conventional technologies. Ca2Fe2O5 is a promising oxygen carrier thanks to its high redox stability and low equilibrium pO2. However, its practical application is hindered by sluggish reduction kinetics. Here, we have overcome this limitation by preparing nickel-promoted composite oxygen carriers (OC) through the physical mixing of Ca2Fe2O5 and modified CeO2. The selective deposition of Ni on the surface of the CeO2-based phase significantly increases the OC reduction rate, outperforming samples in which Ni is in close contact with Ca2Fe2O5 (either on the surface or within the lattice). SiO2 modified ceria was found to be more effective than CeO2-Al2O3 and Ce0.8Zr0.2O2 in promoting the microstructural stability of CeO2 within the composite, ensuring consistent performance over multiple redox cycles. The co-presence of Ca2Fe2O5 and the catalyst in the same reactive bed effectively limits carbon formation at a low OC conversion degree. In situ synchrotron X-ray diffraction and isothermal packed-bed tests revealed a gas-mediated synergistic mechanism whereby carbon deposition was inhibited by H2O and CO2 generated locally from the reduction of Ca2Fe2O5. This led to fast oxygen exchange and stable syngas production. After 50 reaction cycles, the composite OC produced 13.0 mmol/g of syngas in the reduction step with 88 % CH4 conversion, and 6.8 mmol/g of H2 in the water splitting step with > 99 % purity, demonstrating the potential of this OC design strategy.