Enhanced Activity of CeO2-ZrO2 Solid Solutions for Chemical-Looping Reforming of Methane via Tuning the Macroporous Structure

Chemical-looping reforming of methane (CLRM) offers a potentially effective approach for the coproduction of syngas and pure hydrogen. Macroporous CeO2-ZrO2 solid solutions with different pore sizes were prepared as oxygen carriers for the CLRM system. The physical and chemical properties of the oxygen carriers were characterized by the techniques of scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), X-ray powder diffraction (XRD), N2 adsorption-desorption, Raman spectra, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and temperature-programmed oxidation (TPO). The relationship among the structural features, the concentration of oxygen defect, and the oxygen mobility of the macroporous CeO2-ZrO2 solid solutions and the nonporous sample were also discussed. It is found that the specific surface area and oxygen mobility of such oxides are closed correlated to their performance for the CLRM process. Compared with the nonporous sample, the macroporous CeO2-ZrO2 solid solutions own lower specific surface area and better oxygen mobility due to the relatively high concentration of the oxygen vacancy. This allows such oxides to own both high conversion and selectivity for the oxidation of methane to syngas using a chemical-looping concept. The presence of the macroporous structure also improves the reoxidation rate and hydrogen yield (the hydrogen yield increased by 50%) in the water splitting step. This can be attributed to the abundant channels in the materials, which may improve the dynamic conditions of the gas-solid reactions, resulting in relatively high utilization rate of oxygen carriers. The pore size also affects the activity of the macroporous oxygen carriers. The increase of the pore size slightly reduces the reactivity of the oxygen carrier due to partial collapse of the macroporous structure. The macroporous Ce-Zr-100 oxygen carrier with a pore size of 100 nm exhibits excellent activity and stability in the coproduction of pure hydrogen and high-quality syngas, even after 40 times successive redox cycles.