Activation of Methane on 2D Platinum-Group Metal Oxide Nanosheets

Methane is a promising feedstock for the synthesis of value-added chemicals; however, its activation requires cleavage of the C–H bond, which possesses the highest bond dissociation energy among alkanes. Here, we investigate the methane oxidation activity of a series of two-dimensional platinum-group metal oxide nanosheets–─RuO2, IrO2, and PtO2─whose three-dimensional bulk analogues are known to exhibit high hydrocarbon oxidation activity in both theoretical and experimental studies. In situ electrical resistance measurements during methane oxidation revealed a decrease in resistance, attributed to the consumption of lattice oxygen. Ex situ Raman scattering spectroscopy measurements showed an increase in the intensity of bands assigned to amorphous carbon deposited on the nanosheet surfaces. In both measurements, the observed sequence was RuO2 < IrO2 < PtO2. Ambient-pressure X-ray photoelectron spectroscopy measurements further revealed that the temperature required for methane-induced nanosheet reduction followed the order PtO2 < IrO2 < RuO2, consistent with the trends seen in electrical and Raman characterizations. These findings support a theoretically proposed mechanism in which methane oxidation activity correlates with the energy proximity between the dz2 orbital of surface metal atoms and the nonbonding orbital of dissociated CH3– fragments.