The chemical looping reforming (CLR) process, which partially oxidizes methane to syngas through cyclic redox reaction of a transition metal oxide based redox catalyst, represents an alternative and potentially efficient approach for methane valorization. Unlike conventional partial oxidation schemes which require cryogenic air separation, the CLR process inherently avoids air separation by replacing gaseous oxygen with regeneratable ionic oxygen (O2-) from the redox catalyst lattice. Recent studies show that a Fe2O3@LSF core-shell redox catalyst is effective for CLR, as it combines the high selectivity of an LSF surface and the high oxygen capacity of an iron oxide core. These studies also indicate that the reaction between methane and the redox catalyst being highly dynamic, resulting from changes in lattice oxygen availability and catalyst surface properties.
In the present study, a transient pulse injection approach is used to investigate the underlying mechanisms of methane partial oxidation over the Fe2O3@LSF redox catalyst. Results confirm that the methane-redox catalyst reaction undergoes three reaction regions with markedly different mechanisms. This is followed with depletion of active lattice oxygen and onset of coke formation. Throughout the reduction reaction, O2- diffusion is the rate-limiting step. Using pulsed isotope exchange experiments, it is revealed that the redox catalyst goes through mechanism changes as it transitions between reduction regions. While the utilization of oxygen atoms maintains a modified Mars-van Krevlen mechanism throughout the reaction, the mechanism of methane conversion changes from an Eley-Rideal type of mechanism in the first region (in which methane weakly interacts with catalyst surface) to a Langmuir-Hinshelwood-like mechanism in the third region (in which methane is strongly adsorbed and dissociated on the catalyst surface). Availability of surface oxygen, which is determined by total lattice oxygen available in the catalyst and the O2- diffusion rate, controls the reduction scheme of the catalyst and the reaction mechanism.