Selective CO Production by Photoelectrochemical Methane Oxidation on TiO₂

Abstract

The inertness of the C–H bond in CH4 poses significant challenges to selective CH4 oxidation, which often proceeds all the way to CO2 once activated. Selective oxidation of CH4 to high-value industrial chemicals such as CO or CH3OH remains a challenge. Presently, the main methods to activate CH4 oxidation include thermochemical, electrochemical, and photocatalytic reactions. Of them, photocatalytic reactions hold great promise for practical applications but have been poorly studied. Existing demonstrations of photocatalytic CH4 oxidation exhibit limited control over the product selectivity, with CO2 as the most common product. The yield of CO or other hydrocarbons is too low to be of any practical value. In this work, we show that highly selective production of CO by CH4 oxidation can be achieved by a photoelectrochemical (PEC) approach. Under our experimental conditions, the highest yield for CO production was 81.9%. The substrate we used was TiO2 grown by atomic layer deposition (ALD), which features high concentrations of Ti3+ species. The selectivity toward CO was found to be highly sensitive to the substrate types, with significantly lower yield on P25 or commercial anatase TiO2 substrates. Moreover, our results revealed that the selectivity toward CO also depends on the applied potentials. Based on the experimental results, we proposed a reaction mechanism that involves synergistic effects by adjacent Ti sites on TiO2. Spectroscopic characterization and computational studies provide critical evidence to support the mechanism. Furthermore, the synergistic effect was found to parallel heterogeneous CO2 reduction mechanisms. Our results not only present a new route to selective CH4 oxidation, but also highlight the importance of mechanistic understandings in advancing heterogeneous catalysis.

Efficient Photoelectrochemical Conversion of Methane into Ethylene Glycol by WO3 Nanobar Arrays

“…Over the past several decades, various semiconductors have been employed for constituting such a PEC system [11, 12] . Among them, tungsten oxide (WO 3 ) has been known as one of the most promising candidates because of its non‐toxicity, high oxidation ability and suitable band gap [8, 13] .…”

Solar- versus Thermal-Driven Catalysis for Energy Conversion

“…Very recently, a new PEC approach for CH 4 oxidation to CO over Ti 3+ doped atomic-layer-deposition-grown TiO 2 has been reported for the CO production with over 80% selectivity in all products (CO, O 2 , and carbonate, etc.). 39 Details of typical activities can be found in Table 1. It is extremely promising that, using sunlight without any further energy, CH 4 can be directly converted into high-value products, although the reaction rate is still several orders of magnitude lower than thermally catalyzed processes.…”

Atomically Thin 2D Transition Metal Oxides: Structural Reconstruction, Interaction with Substrates, and Potential Applications

“…Han et al reported that (001)‐terminated TiO 2 nanosheet exhibits an excellent photocatalytic activity, which is even better than commercial Degussa P25 TiO 2 . It is noteworthy that apart from widely investigated photocatalytic water splitting, 2D TMOs are emerging as promising photocatalysts for other value‐added chemical reactions in recent years . For instance, Chen et al synthesized ultrathin, single‐crystal WO 3 nanosheets by 2D oriented attachment .…”

“…They found that the dimensionality reduction alters the bandgap of WO 3 such that the WO 3 nanosheet becomes active in the photocatalytic reduction of CO 2 into CH 4 in the presence of water. More recently, Li et al discovered that on anatase (101)‐terminated TiO 2 , CO can be produced via photocatalytic oxidation of CH 4 with a remarkably high yield of 81.9% . They proposed that such high selectivity for CO formation stems from the existence of surface Ti 3+ sites.…”