In-Situ characterizations to investigate the nature of Co3+ coordination environment to activate surface adsorbed oxygen for methane oxidation

In this work, we synthesized accurately the three spinel Co3O4 samples with different exposed crystal planes, and the methane oxidation activities following an order of Co3O4-(2 2 0) > Co3O4-(3 1 1) > Co3O4-(1 1 1). The mutual transformation of Co3+Oh and Co2+Oh is considered as the source of activity, and surface adsorbed oxygen is activated to reactive oxygen species react with methane. Highly reactive oxygen species play an important role in the reaction process. The (2 2 0) crystal surfaces exhibit the highest methane catalytic activity because of the high exposure of Co3+ and surface lattice oxygen atoms. [Display omitted] •Co3O4 nanowires showed higher performance for oxidations of methane.•Co3O4 nanowires mainly expose (2 2 0) crystal planes.•The (2 2 0) crystal planes expose more Co3+ and surface lattice oxygen atoms.•The mutual transformation of Co3+Oh and Co2+Oh is the source of activity.•Surface adsorbed oxygen species are induced to convert into reactive oxygen. Identifying the activity origin of spinel Co3O4 catalysts is extremely important for fundamental research and practical application. It is reported that octahedrally coordinated Co3+ (Co3+Oh) is considered as active sites in spinel Co3O4, but there is still a lack of sufficient evidence to prove the effect of Co species. In this work, we synthesized accurately the three spinel Co3O4 samples with different exposed crystal planes to investigate the reaction mechanism between methane molecule and Co species. The activity results show that methane oxidation activities follow an order of Co3O4-(2 2 0) > Co3O4-(3 1 1) > Co3O4-(1 1 1). A series of in-situ characterization analyses are performed to explore the evolution process of Co species and the transform of lattice oxygen species for methane combustion. The results indicate that the high catalytic activity is assigned to the exposed state of surface lattice oxygen atoms and the mutual transformation of Co3+Oh and Co2+Oh. Therefore, it explained that the high activity of the (2 2 0) crystal plane is mainly involved with the high exposure of Co3+ and surface lattice oxygen. Adsorption oxygen species were induced to reactive oxygen species during the transformation of Co3+Oh stable structure and Co2+Oh unstable structure.