A Ce–Fe Oxygen Carrier with a Core–Shell Structure for Chemical Looping Steam Methane Reforming

Abstract

Chemical looping steam methane reforming (CLSMR) is capable of both syngas and hydrogen generation, and an oxygen carrier is a key issue for CLSMR. In this work, a core–shell-structured Fe@Ce oxygen carrier is presented. With iron oxide nanocores covered by a ceria shell, this oxygen carrier integrates the advantages of both superior ion conductivity of ceria and high oxygen storage capacity of iron oxide. The Fe@Ce oxygen carrier was investigated for CLSRM in a fixed bed with the uniform mixing structural Fe–Ce as a reference. The test results showed that the Fe@Ce oxygen carrier exhibited a higher yield of syngas and hydrogen, CO selectivity, and methane conversion in the long-time reduction process compared with the Fe–Ce oxygen carrier. This indicates that the core–shell oxygen carrier was capable of providing more selective oxygen, resulting from the CeO2 shell that could facilitate the migration and transport of oxygen ions between the iron oxide nanocore and the shell surface. CO selectivity and methane conversion were further improved along with the redox cycles for the core–shell oxygen carrier attributed to the enhancement of the conduction of oxygen ion mainly resulting from the increase of phase proportion of CeFeO3. The average amount of carbon deposition for Fe@Ce was only 28.7% of that for Fe–Ce, which led to higher-quality syngas (H2/CO close to 2) and higher-purity hydrogen. Superior resistance toward carbon deposition is attributed to excellent oxygen ion conductivity of the ceria shell. Rapid migration and supplement of oxygen ion prevent depletion of oxygen on the surface of particles. Besides, an Fe0 metal active site, which promotes decomposition of methane, is scarce on the surface of particles due to coverage of the CeO2 shell, leading to higher resistance toward carbon deposition.