Facile CO separation and subsequent H production via chemical-looping combustion over ceria-zirconia solid solutions.

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 Ce0.9Zr0.1O2; 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.