Dual-Function Reaction Center for Simultaneous Activation of CH₄ and O₂ via Oxygen Vacancies during Direct Selective Oxidation of CH₄ into CH₃OH

Abstract

The direct oxidation of methane (CH 4 ) to methanol (CH 3 OH) has been a focus of global concern and is quite challenging due to the thermodynamically stable CH 4 and uncontrolled overoxidation of the products. Here, we provided a new viewpoint on the role of oxygen vacancies that induce a dual-function center in enhancing the adsorption and activation of both CH 4 and O 2 reactants for the subsequent selective formation of a CH 3 OH liquid fuel on two-dimensional BiOCl photocatalysts at atmospheric pressure. The key for the favorable activity lies in the simultaneous ability of the electron-trapped Bi atom in activating CH 4 and the formation of • O 2 − radicals due to the activation of O 2 at the adjacent oxygen vacancy site, which immediately attack the activated CH 4 to directly produce CH 3 OH, in initiating the oxidation reaction. What is more, the relatively low reaction barriers and the easy desorption of CH 3 OH concertedly facilitate the highly selective conversion of CH 4 up to 85 μmol of CH 3 OH, with a small amount of CO 2 and CO as the byproducts over the BiOCl nanosheets with an oxygen vacancy concentration of 2.4%. This work could broaden the avenue toward the application of oxygen-defective metal oxides in the photocatalytic selective conversion of CH 4 to CH 3 OH and offer a disparate perspective on the role of oxygen vacancy acting as the surface electron transfer channel in enhancing the photocatalytic performance.

Ultrahigh efficiency CH4 photocatalytic conversion to C1 liquid products over cheap and vacancy-rich CeO2 at 30 °C

“…With illumination time of 2 h, a high yield rate of C1 liquid product of 3.88 mmol (g cat h) −1 was reached, equivalent to a space time yield of 928 g (kg cat h) −1 (an important industrial production index). This is 1–3 orders of magnitude larger than previous reports 18,19,21,22,34,41–43 (see Fig. 3f and Table S1 in the ESI†), which is accompanied by a high C1 selectivity of 97.73% (HCHO and HCOOH selectivity of 76.33%).…”

Simultaneously Accelerating Carrier Transfer and Enhancing O2/CH4 Activation via Tailoring the Oxygen-Vacancy-Rich Surface Layer for Cocatalyst-Free Selective Photocatalytic CH4 Conversion

“…To further resolve the defective structure of the surface disorder layer, surface-sensitive characterizations including XPS and Raman spectroscopy techniques were employed. As shown in Figure b, the O 1s spectra can be deconvoluted into three peaks at 529.6, 531.6, and 533.2 eV, corresponding to the lattice oxygen, oxygen vacancy, and physically adsorbed H 2 O, respectively. − Evidently, the proportion of oxygen vacancies increases with increasing temperature from 400 to 800 °C (specific values are listed in Table S2). Meanwhile, the peak of the Ti 4+ 2p 1/2 shifts to lower binding energy (Figure c), suggesting the increased electron density of Ti atoms due to the formation of oxygen vacancies. , As shown in Figure S3, six first-order Raman-active vibrations (3E g + 2B 1g + A 1g ) belonging to only anatase TiO 2 are observed in the Raman spectra (Figure S9).…”

Nonoxidative Coupling of Methane over Ceria-Supported Single-Atom Pt Catalysts in DBD Plasma

“…2 Consequently, the chemical conversion of methane to value-added chemicals is attracting increasing interests. 3,4 Currently, the dominant method of methane upgradation is the indirect conversion of methane to hydrocarbons/oxygenates via formation of syngas (H 2 + CO) by steam reforming, CO 2 reforming, or partial oxidation, followed by Fisher−Tropsch synthesis. 5,6 Direct methane conversion to light hydrocarbons by oxidative and nonoxidative coupling of methane provides prospects for more efficient and one-step methane upgradation by circumventing the energy-intensive syngas production step.…”

“…7,8 Nonoxidative coupling of methane (NCM) is preferable due to its unique capability of generating C 2+ hydrocarbons and H 2 from methane without the unselective production of undesirable CO x . 9 Despite intensive studies, NCM (eq 1) still remains a challenging issue due to the particularly strong C−H bond strength (434 kJ/mol) in CH 4 , along with its nonpolar structure and high ionization energy. 10 As a result, the conventional thermal activation of methane usually requires high temperatures above 1000 °C, leading to a high operation cost.…”

“…Methane is set to become the most abundant hydrocarbon feedstock for the synthesis of petrochemical derivatives, with the depletion of oil reserves . Consequently, the chemical conversion of methane to value-added chemicals is attracting increasing interests. , Currently, the dominant method of methane upgradation is the indirect conversion of methane to hydrocarbons/oxygenates via formation of syngas (H 2 + CO) by steam reforming, CO 2 reforming, or partial oxidation, followed by Fisher–Tropsch synthesis. , Direct methane conversion to light hydrocarbons by oxidative and nonoxidative coupling of methane provides prospects for more efficient and one-step methane upgradation by circumventing the energy-intensive syngas production step. , Nonoxidative coupling of methane (NCM) is preferable due to its unique capability of generating C 2+ hydrocarbons and H 2 from methane without the unselective production of undesirable CO x …”