Abstract
Methane is a promising source molecule for the production of value-added chemicals. However, their inertness, which originates from the most stable C-H bond among alkanes, hinders their utilization because high temperatures are generally required for the oxidation/dehydrogenation of methane [1]. Recently, it was reported that methane is readily activated on the (110) surface of IrO2, a platinum group metal oxide (PGMO), even at temperatures below room temperature [2]. This high activity is attributed to the strong bonding interactions between the IrO2 surface and CH3 fragments formed after C-H bond cleavage [3]. Such PGMO surface-CH3 interactions correlate with the activation barrier, and PtO2 has been identified as more favorable for the activation of methane [3]. Our group has focused on the inherently high surface sensitivity of nanosheet structures and has successfully observed methane oxidation on IrO2 nanosheets at room temperature [4]. In this study, a series of PGMO nanosheets (IrO2, RuO2, and PtO2) was systematically studied in terms of their methane activation ability.
The nanosheet films were fabricated on SiO2/Si substrates by the layer-by-layer deposition or the transfer of vacuum-filtrated films. The change in the electrical resistance of the fabricated films upon methane exposure was measured in situ, and two factors affecting the electrical resistance were spectroscopically investigated: the bonding state of the nanosheets and deposition of reaction products. The change ratio of the electrical resistance was found to be in the order RuO2 < IrO2 < PtO2 (Figure 1). In the case of PtO2, monolayered films were found to be unnecessary for observing the resistance decrease caused by the reduction of PtO2 to Pt during methane oxidation because PtO2 is an insulator, unlike metallic IrO2 and RuO2. The consumption of lattice oxygen for methane oxidation was confirmed by near ambient pressure X-ray photoelectron spectroscopy (AP-XPS). The reduction of PGMO did not occur in RuO2 without a temperature increase, whereas partial and most reductions were observed even at room temperature in IrO2 and PtO2, respectively. The presence of amorphous carbon deposited on nanosheet surfaces as a product of the methane oxidation reaction was confirmed by Raman scattering spectroscopy. The peak enhancement ratios before and after methane exposure were in the order of RuO2 < IrO2 < PtO2, which is consistent with the electrical and AP-XPS results shown above. In conclusion, the high oxidation activity of PtO2 for methane was confirmed even in the nanosheet form. The material dependence of the methane activation ability of PGMO nanosheets found in this study supports the theoretically determined mechanism based on the PGMO-CH3 interaction, which may provide a guideline for the development of catalytic materials with high methane activation abilities.
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[2] Z. Liang et al., Science. 356, 299 (2017).
[3] Y. Tsuji et al., J. Phys. Chem. C. 122, 15359 (2018).
[4] Y. Ishihara et al., Adv. Mater. Interfaces. 10, 2300258 (2023).
