Phase engineering of Fe2O3 nanocrystals for the direct oxidation of CH4 to HCOOH

The direct oxidation of methane (CH4) under mild conditions is a highly desirable technology to achieve the low-carbon and atom-economic production of high-value-added C1 oxygenates. However, the difficult activation of the C-H bond of CH4 and the overoxidation of C1 oxygenates to carbon dioxide (CO2) are still the major bottlenecks. Herein, we systematically investigate the phase engineering of Fe2O3 in dependence on the performance of the direct oxidation of CH4. The cubic phase of Fe2O3 (Fe2O3-c) nanocrystals enables the direct oxidation of CH4 to HCOOH (DOMF) under mild aqueous conditions, and the HCOOH yield and selectivity could reach 6.315 mmol gcat-1 h-1 and 89.4%, which are 69.3 and 1.34 times higher than those of the hexagonal phase of Fe2O3 (Fe2O3-h) nanocrystals. Mechanism experiments indicate that Fe2O3-c nanocrystals can optimal CH4 adsorption behavior, favoring to the occurrence of the DOMF. In addition, in situ DRIFTS demonstrates that the DOMF is the radical reaction process that the activation of C-H bond is caused by the •OH radical. Subsequently, the unstable CH3OOH and stable CH3OH intermediates undergo further oxidation to form HCOOH. This work offers the phase engineering strategy to enhance the catalytic performance and reveal the reaction mechanism for the direct oxidation of CH4.