Facile CO2 separation and subsequent H2 production via chemical-looping combustion over ceria–zirconia solid solutions

Abstract

A novel chemical-looping combustion scheme is proposed, where facile gas separation via steam condensation enables the production of sequestrable CO₂ from alkanes, such as CH₄, and pure H₂ from H₂O. This cycle consists of two steps, namely, (1) the endothermic reduction of a ceria-based solid solution via the complete oxidation of CH₄, followed by (2) the exothermic oxidation of the reduced metal oxide via H₂O splitting. Relative to iron oxide-based materials and undoped ceria, ceria–zirconia solid solutions possess favorable partial molar enthalpic and entropic properties; this promotes selective production of complete combustion products, H₂O and CO₂, during the reforming reaction. Thermodynamic predictions suggest that the complete oxidation of CH₄ is possible by increasing the Zr content to 20 mol%, operating below 600 °C, increasing total pressure, or reducing the amount of delivered reactant. Furthermore, any H₂, CO, or unreacted CH₄ that may persist is thermodynamically favored to oxidize if exposed to unreacted oxide downstream, as is typical for a packed-bed or downer reactor configuration. Experiments were performed to validate the thermodynamic trends using isothermal thermogravimetry coupled with residual gas analysis, which confirmed that high selectivity towards H₂O and CO₂ is attainable for methane-driven reduction of Ce_0.9Zr_0.1O₂; selectivities greater than 0.70 were observed at initial reaction extents. Importantly, metal oxide oxidation via H₂O splitting and selective production of H₂ (or CO if CO₂ is the delivered oxidant) is also thermodynamically favored at the operating conditions considered for the first step. This work ultimately presents a viable avenue for the carbon-neutral conversion of CH₄ (or other alkanes) to H₂ if a renewable energy resource, such as solar energy, is leveraged to supply process heat.