Photocatalytic oxidation of methane to methanol by tungsten trioxide-supported atomic gold at room temperature

Highlights

• Tungsten trioxide-supported atomic-scale gold (Au₁/WO₃) catalyst was prepared with a facile route.

• A tip-enhanced localized electrons effect is found in the Au₁/WO₃ catalyst.

• The Au₁/WO₃ exhibits enhanced photocatalytic oxidation of CH₄ to CH₃OH with solar energy.

• Up to 589 µmol g⁻¹ of CH₃OH in one hour is generated at room temperature.

Abstract

Atomic-scale metals as active center have been widely investigated for efficient photocatalysis. Understanding the specific electronic structure of atomic-scale center is of profound fundamental importance for superior catalytic performance. Here, we report an atomically dispersed gold on tungsten trioxide (Au₁/WO₃) catalyst for photocatalytic oxidation of methane toward value-added methanol. The Au₁ species reveal a specific tip-enhanced local electrons field which favors the C-H dehydrogenation of methane and thus form methanol (up to 589 µmol g⁻¹ h⁻¹). Both experimental and theoretical results demonstrate such tip-enhanced effect enhance the catalytic activity of methane oxidation. The theoretical calculations further reveal a lower adsorption energy of product methanol on Au₁, in contrast to Au particles, which suppresses the overoxidation of methanol, and thus promotes its selectivity. Establishing the relationship between electronic density and catalytic activity may create a platform for designing efficient atomic-scale catalysts for C1 catalysis and green chemistry.

Introduction

Methane (CH₄), as the principal constituent of natural gas, is widely used as an important fuel and feedstock material in industry [1], [2], [3]. However, emission of CH₄ may cause global warming as a result of greenhouse effect, which is around ~23 times higher than that of carbon dioxide (CO₂) [4], [5], [6], [7]. Therefore, CH₄ conversion to value-added hydrocarbon chemicals is significant to realize sustainable energy and environment [8], [9], [10], [11]. The preferable product of CH₄ conversion is methanol (CH₃OH) which can be directly used as fuel source [12], [13]. Given the intrinsic inertness of CH₄ with strong C-H bond (434 kJ mol⁻¹), and negligible electron affinity, harsh reactive conditions of thermal catalysis such as high temperature (> 700 °C) are required to convert CH₄ to CH₃OH, leading to energy consumption and low selectivity of CH₃OH [10], [14] Photocatalysis using solar energy could drive many tough reactions at ambient conditions, such as water splitting and CO₂ reduction. Most recently, photocatalytic methane conversion has attracted increasing interest by using solar energy at room temperature. Typical semiconductors catalysts, such as TiO₂ and ZnO supported co-catalysts i.e., Ag, have been demonstrated to exhibit good photocatalytic activity in gas-phase CH₄ oxidation [15], [16]. In such systems, O₂ is generally activated into active oxygen species, such as •O₂^-, ultimately leading to serious over-oxidation of CH₄ into HCHO, CO₂, or CO as the main products [17].

Aqueous-phase photocatalysis enables oxidation of CH₄ to generate more liquid oxygenates, such as CH₃OH. For instance, oxidation of CH₄ to CH₃OH via photocatalysis has been recently reported over FeO_x/TiO₂ with oxidant H₂O₂ [18]. As the crucial point to selectively generate CH₃OH is hydrogen abstraction of CH₄, [19] •OH radical is remarkably responsible for dehydrogenation of CH₄ to form •CH₃ radical [20], [21]. In nature, the dimeric Fe species as active sites for direct and selective oxidation of CH₄ to CH₃OH by methane monooxygenase enzymes in aqueous condition [22], [23]. Furthermore, TiO₂-based Fe species as an efficient catalyst has recently been reported for photocatalytic selective oxidation of CH₄ to CH₃OH in aqueous solution using H₂O₂ as oxidant to generate •OH [18], [24]. Thereby, Fe species co-catalyst-H₂O₂ oxidant system plays important roles in determining CH₃OH generation.

Worth noting is that employing co-catalysts is crucial to improve the efficiency of CH₃OH generation, such as noble metals (Au, Ag, Pt, etc.) on semiconductors (i.e., TiO₂, WO₃) [25]. Interestingly, Au is remarkably active for selective hydrocarbon oxidation because of its high electronegativity [26], [27]. Notably, dehydrogenation of CH₄ to form methyl (•CH₃) radical is favored on Au [28]. Owing to high cost and natural scarcity of Au, however, utilizing Au in cost-effective way remains the major challenging circumstance. Interestingly, atomic scale metals contribute to very distinguished catalytic behavior from their bulk or nanoparticles because of maximized utilization of active sites [29], [30], [31], [32]. Furthermore, the cationic character of atomically dispersed metals is expected to have a minimal number of choices of binding sites for reactants or intermediates [29]. Therefore, atomic scale Au potentially exhibits higher reactive activity than their bulk nanoparticles, owning to multiple types of reactive sites [33], [34]. On the other hand, atomic scale Au with low coordination number can prohibit the successive dehydrogenation of CH₄, stabilizing the formed •CH₃ radical which is a key intermediate for selective generation of CH₄OH. Consequently, manufacturing Au into atomic scale is a promising route to activate CH₄ during the photocatalytic process.

Herein, we applied tungsten trioxides-supported Au atoms (Au₁/WO₃) as catalyst for oxidation of CH₄ with visible light. The Au₁/WO₃ material exhibits specific electronic structure and tip-enhanced local electric field which are favorable for activating CH₄ based on theoretical calculations. Further, the •OH radical formed is crucial to promote the selectivity of methanol product. Based on experimental and theoretical results, radicals-pathway mechanism and four-step reaction routs are suggested to account for the methane oxidation reaction.