Morphology and Microstructure of Li/MgO Catalysts for the Oxidative Coupling of Methane

A series of catalysts for the oxidative coupling of methane (OCM) based on MgO with a varying content of Li have been synthesized by the gel-combustion method. The resulting catalytically active systems are studied by a combination of TEM and SEM methods. Samples with a low abundance of Li exhibit a hierarchical pore system built from tubular structures made from primary MgO particles. Upon calcination at 1073 K, these particles undergo a change in shape from cubic via truncated octahedral to platelet morphologies, depending on the Li content of the precursor. Morphological indications have been found for the role of Li as flux in this transformation. The modification of the primary particle morphology leads to a drastic change in secondary structure from open sponges to compact sintered plates upon addition of Li at loadings above 10 wt%, with respect to the precursor. The microstructure of the primary particles reveals two families of high-energy structures, namely edge-and-step structures and protrusions on flat terraces. A relation was found between catalytic function in OCM and the transformation from cubic to complex-terminated particles. Based on these findings, it is suggested that sites active for the coupling reaction of methane are related to the protrusions arising from segregation of oxygen vacancies to the surface of MgO.