Abstract
The conversion of photocatalytic methane into methanol in high yield with selectivity remains a huge challenge due to unavoidable overoxidation. Here, the photocatalytic oxidation of CH4 into CH3OH by O2 is carried out on Ag-decorated facet-dominated TiO2. The {001}-dominated TiO2 shows a durable CH3OH yield of 4.8 mmol g−1 h−1 and a selectivity of approximately 80%, which represent much higher values than those reported in recent studies and are better than those obtained for {101}-dominated TiO2. Operando Fourier transform infrared spectroscopy, electron spin resonance, and nuclear magnetic resonance techniques are used to comprehensively clarify the underlying mechanism. The straightforward generation of oxygen vacancies on {001} by photoinduced holes plays a key role in avoiding the formation of •CH3 and •OH, which are the main factors leading to overoxidation and are generally formed on the {101} facet. The generation of oxygen vacancies on {001} results in distinct intermediates and reaction pathways (oxygen vacancy → Ti–O2• → Ti–OO–Ti and Ti–(OO) → Ti–O• pairs), thus achieving high selectivity and yield for CH4 photooxidation into CH3OH.
Highlights
The conversion of photocatalytic methane into methanol in high yield with selectivity remains a huge challenge due to unavoidable overoxidation
Bare TiO2 and that with variable Ag loading were evaluated for photocatalytic CH4 oxidation with molecular O2 as an oxidant (CH4:O2 ratio = 20:1)
We report the photocatalytic oxidation of CH4 into CH3OH by molecular O2 on anatase TiO2
Summary
The conversion of photocatalytic methane into methanol in high yield with selectivity remains a huge challenge due to unavoidable overoxidation. High temperatures and pressures are normally required to activate C–H bonds, which greatly increases capital investment and gives rise to operational risks and environmental problems[1]. CH4 oxidation by utilizing photon energy instead of thermal energy. Upon excitation of semiconducting photocatalysts by photons, a series of highly active oxygen-containing radicals formed in photocatalytic CH4 oxidation can readily activate the C–H bond at room temperature[2,3,4,5,6]. The activation energy of the C–H bond in CH4 is much higher than that in the product (CH3OH)[7]. For gas-phase CH4 oxidation, the product CH3OH can adsorb onto the surface of photocatalyst (such as TiO2 and ZnO loaded with various cocatalysts) and become overoxidized to CO and CO29–11 It has been reported that the intermediate photocatalytic species are closely related to the arrangement and coordination of the surface atoms on different crystal facets[20,21,22,23,24]
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