Photocatalytic Aerobic Conversion of Methane

Kong, Yuxiang; Yang, Chunxiang; Cai,  Yiyu; Mu, Xiaowei; Li, Lu

doi:10.35534/prp.2024.10005

Photocatalysis: Research and Potential

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Photocatalytic Aerobic Conversion of Methane

Yuxiang Kong ¹ Chunxiang Yang ¹ Yiyu Cai ² Xiaowei Mu ^3,* Lu Li ^1,*

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State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China

Macao Institute of Materials Science and Engineering (MIMSE), Faculty of Innovation Engineering, Macau University of Science and Technology, Taipa, Macau SAR 999078, China

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Changchun 130022, China

Authors to whom correspondence should be addressed.

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Photocatalysis: Research and Potential 2024, 1 (3), 10005; https://doi.org/10.35534/prp.2024.10005

Received: 28 May 2024 Accepted: 09 July 2024 Published: 11 July 2024

ABSTRACT: The direct conversion of methane into high-value chemicals has been a persistent research focus in the fields of chemical engineering and energy. Photocatalysis, as an innovative technology, not only circumvents the issues of catalyst sintering and carbon deposition associated with traditional thermal catalysis but also transcends thermodynamic limitations by providing new reaction pathways. Utilizing molecular oxygen as an oxidant generates various reactive oxygen species, offering unique thermodynamic advantages for methane conversion. This review summarizes the advancements in photocatalytic partial oxidation (PPOM) and oxidative coupling of methane (POCM) using oxygen as an oxidant. It discusses the activation mechanisms and reaction pathways of methane and oxygen in different systems, as well as the application of photochemical cycling strategies in methane conversion. Finally, it addresses the challenges in this field, proposes potential solutions, and offers perspectives on the future development of photocatalytic systems.

Keywords: Photocatalysis; Methane; Oxygen; Partial Oxidation; Oxidative Coupling; Photochemical looping

1. Introduction

As petroleum reserves continue to dwindle and environmental issues become increasingly severe, efforts are being made to expand and innovate within the traditional petrochemical industry while seeking new primary carbon sources to meet the demands of chemical production. Methane, which is abundant and cost-effective, is widely found in natural gas, coalbed methane, shale gas, and combustible ice, making it a promising alternative to the depleting petroleum resources [1,2,3]. However, as shown in Figure 1A, the methane molecule is a tetrahedral structure with a bond angle of 109.5°, exhibiting high symmetry and low polarizability. It is a typical non-polar molecule, with a C—H bond dissociation energy of 439 kJ·mol⁻¹. The molecular orbitals of methane reveal a very low energy for the highest occupied molecular orbital (HOMO) and a very high energy for the lowest unoccupied molecular orbital (LUMO), indicating that both donating and accepting an electron require substantial energy, making methane exceedingly difficult to activate [4]. Additionally, most methane reserves are located in remote areas, and the transportation and storage of gaseous methane are costly and prone to leakage, potentially causing significant environmental problems [5,6]. Therefore, converting methane into transportable and higher-value chemicals is a promising yet challenging approach [7]. Methane conversion can be categorized into two main types: direct and indirect conversion [8]. The indirect conversion of methane, also known as the syngas (H₂ and CO) route [9,10], involves reforming methane to syngas, which is then further converted into high-value chemical feedstocks such as ammonia, methanol, and olefins [11]. This is currently the primary commercial route for large-scale methane conversion [12,13,14]. However, indirect conversion methods such as dry reforming of methane (DRM) and steam reforming of methane (SRM) are thermodynamically unfavorable at room temperature (Table 1 and Figure 1B), requiring high temperatures and pressures, resulting in high energy consumption and costs, which are detrimental to sustainable development. Compared to these methane reforming reactions, partial oxidation of methane (POM) to syngas is a spontaneous reaction that can reduce energy consumption and produce a H₂/CO ratio of 2/1, which is more suitable for subsequent processes such as methanol synthesis and Fischer-Tropsch synthesis. Direct conversion of methane involves converting methane directly into high-value chemicals, such as liquid oxygenates and hydrocarbons [15,16]. Compared to indirect conversion, direct conversion bypasses intermediate steps, saving costs and utilizing methane more efficiently. Direct conversion of methane includes oxidative and non-oxidative pathways. The non-oxidative pathway, or non-oxidative coupling of methane (NOCM), is an endothermic reaction typically requiring extremely high temperatures (>1000 °C) to activate the inert C—H bond [17], which can easily cause catalyst deactivation due to carbon deposition [18]. Generally, introducing oxidants (e.g., O₂, H₂O₂) can effectively lower the Gibbs free energy of the reaction (Table 1 and Figure 1B), allowing the reaction to proceed under milder conditions. Oxidative pathways such as partial oxidation of methane (POM) and oxidative coupling of methane (OCM) are of particular interest due to their low energy consumption and good atom economy. Despite this, in industrial applications of thermal catalytic direct conversion of methane, it remains challenging to couple methane into C₂₊ hydrocarbons at very low temperatures and avoid the formation of a large amount of thermodynamically favored by-products such as CO₂ [19]. Partial oxidation of methane also struggles to selectively oxidize methane into oxygenates and typically requires expensive oxidants such as H₂O₂ [20].

Figure 1. (A) Structure (left) and molecular orbital diagrams (right) of methane. * represents the antibonding orbitals. (B) Thermodynamically unfavourable (ΔG > 0) (left) and favourable (ΔG < 0) (right) reaction pathways for methane conversion operated at room temperature. (C) Schematic diagram of direct and indirect methane activation over a semiconductor photocatalyst.

Table 1. Change of Gibbs free energy for various methane conversion reactions.

Photocatalysis, as an emerging technology, utilizes widely available and environmentally friendly solar energy to activate alkane’s inert C—H bond under mild conditions [21,22,23]. As illustrated in Figure 1B, the introduction of photon energy significantly lowers the activation barrier for methane, breaking the thermodynamic equilibrium, which is crucial for achieving efficient low-temperature methane conversion [24,25,26,27,28]. Since the initial report by Kaliaguine et al. in 1978 on the photocatalytic conversion of methane, where CH₃O⁻ and C₂H₅O⁻ species were observed on TiO₂ surfaces under UV irradiation [29], photocatalytic aerobic conversion of methane has garnered increasing attention. This process can be divided into two pathways: direct activation and indirect activation of methane [30]. As depicted in Figure 1C, when light irradiates the surface of a photocatalyst, photons with energy greater than the semiconductor band gap are absorbed, causing electrons to transition from the valence band (VB) to the conduction band (CB), leaving holes in the VB. The photogenerated electrons and holes migrate to the catalyst surface, where they can react with additional reactants such as H₂O, O₂, and H₂O₂ to generate reactive oxygen species (·OH, ·OOH, ·O₂⁻), which assist in methane activation (indirect activation pathway). Photogenerated holes can also accumulate on lattice oxygen, forming O⁻, thereby directly activating methane to generate ·CH₃ (direct activation pathway). Activated methane can then couple to form C₂₊ hydrocarbons (POCM) or combine with reactive oxygen species to form oxygenates (PPOM). Commonly used oxidants include H₂O₂, H₂O, and O₂. H₂O₂ is expensive, difficult to store and transport, and prone to decomposition under high pressure and heating, making it unsuitable for industrial applications. H₂O has low reactivity and is thermodynamically unfavorable (Table 1). In contrast, O₂ is economical, environmentally friendly, and thermodynamically favorable, providing unique advantages in photocatalytic aerobic conversion [31,32]. However, the complex nature of the reactive oxygen species generated by O₂ presents a significant challenge in controlling product selectivity for researchers. This review summarizes recent advances in the photocatalytic conversion of methane using O₂ as the oxidant, focusing on the photocatalytic partial oxidation of methane (PPOM) and the oxidative coupling of methane (POCM). The reaction mechanisms in various systems, including C—H bond activation, O₂ reduction, and radical intermediate pathways, are examined, followed by an introduction to photochemical cycling strategies. Finally, the challenges and future prospects in the field of photocatalytic aerobic conversion of methane are discussed.

2. Photocatalytic Partial Oxidation of Methane (PPOM)

Total oxidation of methane produces low value CO₂, a greenhouse gas with adverse environmental impacts, making it unsustainable. In contrast, photocatalytic partial oxidation of methane (PPOM) can proceed under mild conditions to yield higher-value oxygenates (CO, CH₃OH, HCHO, CH₃CH₂OH, etc.). The choice of oxidant is crucial for the reactivity and selectivity of PPOM, and molecular oxygen, being inexpensive and environmentally friendly, is often used as the oxidant in the PPOM process. The coupling of O₂ reduction and CH₄ oxidation is thermodynamically favorable, and the reactive oxygen species (ROS) formed from O₂ reduction facilitate CH₄ activation and product formation. Therefore, understanding the role of O₂ in the PPOM process is vital. Research in this area can be divided into gas-phase and liquid-phase systems. 2.1. Gas-Phase Systems In gas-phase systems, methane can be photo-oxidized by O₂ to produce oxygenates such as CH₃OH, HCHO, and CO. In 1987, Brazdil’s research team first achieved the photocatalytic conversion of CH₄ to CH₃OH on CuMoO₄ [33]. Under visible light irradiation and at 100 °C, using O₂ as the oxidant, a CH₃OH yield of 6 μmol·h⁻¹ was achieved. The doping of Cu²⁺ extended the catalyst’s visible light activity and prolonged the lifespan of O⁻. Additionally, Shuben Li’s research team achieved CH₄ photo-oxidation to CH₃OH at temperatures below 350 K and atmospheric pressure using Mo-doped porous TiO₂ catalysts pre-adsorbed with water [34]. In 2019, Xiaoyong Wu’s research team reported a g-C₃N₄-modified Cs_0.33WO₃ photocatalyst (g-C₃N₄@Cs_0.33WO₃) that selectively photo-oxidized low-concentration CH₄ (1000 ppm) to CH₃OH at room temperature, with a yield of 4.38 μmol·g⁻¹·h⁻¹ [35]. As shown in Figure 2A, O₂ was activated to ·O²⁻, which oxidized CH₄ on the g-C₃N₄ surface to methoxy radicals. Figure 2B illustrates two methoxy radical reaction pathways: in the selective oxidation pathway, photogenerated electrons from Cs_0.33WO₃ rapidly transfer to g-C₃N₄, preventing overoxidation of methoxy radicals, while the other pathway produces a small amount of CO_x, possibly due to overoxidation of CH₄ on g-C₃N₄ not bound to Cs_0.33WO₃. However, in gas-phase systems, product desorption is crucial, and the desorption of CH₃OH requires relatively high temperatures, which can lead to overoxidation. Therefore, producing liquid oxygenates (CH₃OH and HCHO) in gas-phase PPOM systems is challenging. In contrast, gas-phase systems are more suitable for selective CO production. In 1988, Grätzel’s research team achieved the photocatalytic conversion of CH₄ to CO at room temperature and atmospheric pressure using TiO₂-supported molybdenum oxide (TiO₂/MoO₃) [36]. Buxing Han’s research team developed an Ag/AgCl@SiO₂ photocatalyst that selectively photo-oxidized CH₄ to CO, with a CO yield of 2.3 μmol·h⁻¹ and a selectivity of 73% [37]. Mechanistic studies revealed that singlet ¹O₂ generated in situ from O₂ could activate methane to form the key intermediate COOH^∗, which further dehydrates to form CO. Notably, this catalyst effectively photo-oxidized CH₄ to CO using sunlight in outdoor tests, demonstrating its practical application potential. In the above works, the hydrogen products were H₂O. In contrast, converting CH₄ to hydrogen, especially syngas (CO and H₂), is more valuable and meaningful. In 2019, Miyauchi’s research team proposed a PPOM scheme by loading Pd nanoparticles onto ultra-wide bandgap (UWBG) strontium tantalate (Sr₂Ta₂O₇) [38]. Compared to dark conditions, light irradiation reduced the starting temperature for syngas formation to below 423 K, and under external temperatures of 623 K and UV irradiation, CO and H₂ production rates reached 46.8 mmol·g⁻¹·min⁻¹ and 54.9 mmol·g⁻¹·min⁻¹, respectively. The photothermal carriers generated by interband excitation of Pd nanoparticles drove the photocatalytic reaction, with separated hot electrons and holes promoting the activation of O₂ and CH₄, respectively, while the thermal relaxation of carriers increased the catalyst surface temperature, further facilitating the reaction. The research team also incorporated a series of noble metals (Rh, Pd, Ru, and Pt) into MCM-41 molecular sieves, with Rh/MCM performing best, achieving efficient syngas production (CO selectivity ~50%) in a flow system at temperatures as low as 423 K, with a CH₄ quantum yield of 1.8% (λ ≥ 250 nm) [39]. Similarly, the hot electrons and holes generated by interband excitation of metal nanoparticles directly activated adsorbed O₂ and CH₄, and the photothermal effect from carrier relaxation further promoted the reaction. Recently, Ordomsky’s research team used conventional Cu(In,Ga)Se₂ (CIGS) absorbers to selectively photo-oxidize CH₄ to CO and H₂ at room temperature [40]. The CIGS films coated on Mo showed the best performance, with a CO yield of 2.4 mmol·g⁻¹ and a CO/H₂ ratio of approximately 2:1, with CO selectivity exceeding 80%. As shown in Figure 2C, the reaction involves stepwise dissociation and coupling of CH₄ to form hydrocarbons, followed by dehydrogenation to form disordered carbon, and finally partial oxidation of carbon to CO. While photocatalytic partial oxidation of methane to CO and H₂ reduces reaction temperature and mitigates the explosion risk of premixed CH₄/O₂, the reaction still tends to produce more stable H₂O and CO₂, resulting in low selectivity for CO and H₂, and the CO/H₂ ratio is not 1:2, making it difficult to use directly for methanol or Fischer-Tropsch synthesis. Therefore, to apply photocatalytic partial oxidation of methane to syngas in industrial production, it is necessary to improve the selectivity and ratio of CO and H₂ and further reduce the reaction temperature.

Figure 2. (A) Proposed O2 activation mechanism and (B) Schematic illustration for PPOM over g-C3N4@Cs0.33WO3 [35]. (C) Schematic diagram of CH4 partial oxidation to syngas over CIGS [40]. Note: Step 2 in Figure 2B represents selective oxidation and Step 2 * represents peroxidation.

2.2. Liquid-Phase Systems Compared to gas-phase systems, using H₂O as a solvent in liquid-phase systems offers unique advantages. Firstly, H₂O can promote catalyst dispersion. Secondly, the presence of H₂O facilitates the desorption of liquid oxygenates from the catalyst surface, significantly inhibiting the overoxidation of liquid products and thus improving reaction selectivity. Additionally, the introduction of H₂O makes O₂ activation into reactive oxygen species (ROS) easier, while H₂O itself can also serve as a source of ROS. These ROS can promote the activation of CH₄ and the formation of products, although confirming the sources of ROS adds to the complexity of research. Based on the different ROS generated from O₂ activation, current research can be categorized into several types. 2.2.1. O₂ Activation to ·OOH In 2019, JinHua Ye’s research team first reported the use of O₂ as an oxidant to convert CH₄ into oxygenates in a liquid-phase system [41]. Under ambient conditions and light irradiation, different cocatalysts (Pt, Pd, Au, Ag) loaded on ZnO achieved efficient production of liquid oxygenates (CH₃OH and HCHO), with Au−ZnO showing the best performance, achieving a yield of 125 μmol·h⁻¹ and a selectivity over 95%. Under light excitation, photogenerated holes and electrons on ZnO were separated, with CH₄ oxidized to ·CH₃ by photogenerated holes, while O₂, assisted by protonation in water, was reduced to ·OOH by photogenerated electrons on the cocatalyst. Subsequently, ·CH₃ and ·OOH combined to form the initial product CH₃OOH, which was further reduced to CH₃OH. HCHO could be produced by the photooxidation of CH₃OH by photogenerated holes or ·OH, or directly by the decomposition of CH₃OOH (Figure 3A). Isotope labeling experiments further confirmed that the O in CH₃OH originated from CH₃OOH, rather than from the coupling of ·CH₃ and ·OH generated from the photooxidation of water. Subsequently, the research team developed a dual-cocatalyst modified titanium dioxide photocatalyst (Au−CoO_x/TiO₂), achieving a primary product (CH₃OH and HCHO) yield of 2540 μmol·g⁻¹·h⁻¹ and a selectivity of 95% under ambient conditions [42]. Mechanistic studies indicated that the excellent activity and selectivity stemmed from the synergistic effect of Au nanoparticles and CoO_x. Upon illumination, Au nanoparticles facilitated the separation of photogenerated carriers and the reduction of O₂, while ·OH generated from water oxidation could over-oxidize CH₃OH to HCHO and CO₂. CoO_x modulated the oxidative capacity of the photocatalyst, inhibiting the formation of highly oxidative ·OH, thus improving selectivity. Junwang Tang’s research team reported an Au−Cu alloy-modified ZnO (Au_0.2Cu_0.15−ZnO) achieving a primary product (CH₃OH, CH₃OOH, and HCHO) yield of 11,225 μmol·g⁻¹·h⁻¹, with nearly 100% selectivity and an apparent quantum efficiency of 14.1% at 365 nm [43]. As shown in Figure 3B, Cu acted as the electron acceptor, reducing O₂ to ·OOH, while Au accepted photogenerated holes to oxidize H₂O to ·OH, synergistically promoting charge separation and methane conversion. Both ·OH and photogenerated holes could oxidize methane to ·CH₃, and in ROS quenching experiments, the introduction of salicylic acid as a sacrificial agent for ·OH nearly halted methane conversion, proving that ·OH was the primary active species for methane activation. They also developed Pd−def−In₂O₃ [44], Pd−def−TiO₂ [45], Pd−def−WO₃ [46], and Cu−def−WO₃ [47] photocatalysts with similar PPOM mechanisms. Under light excitation, semiconductor supports generated photogenerated electron-hole pairs, with O₂ reduced to ·OOH radicals by photogenerated electrons and H₂O oxidized to ·OH by holes, further activating methane. These studies provide guidance for the rational design of future catalysts.

Figure 3. (A) Schematic diagram of photocatalytic CH4 oxidation over cocatalyst/ZnO [41]. (B) Schematic illustration of photocatalytic methane conversion over Au0.2Cu0.15−ZnO [43]. (C) Proposed photocatalytic mechanism for the selective oxidation of CH4 over Au9Pd1/ZnO with O2 [48]. (D) Proposed mechanism for PPOM on Au1/In2O3 (left) and AuNPs/In2O3 (right) [49].

Jun Wang’s research team reported AuPd nanoparticle-loaded defect ZnO nanosheets (Au₉Pd₁/ZnO) for efficient photocatalytic methane oxidation to oxygenates under ambient conditions, achieving a maximum liquid oxygenate yield of 152.2 mM·g⁻¹·h⁻¹ with a selectivity of 86.7% and an apparent quantum efficiency of 16.5% at 380 nm [48]. The excellent photocatalytic performance was attributed to the synergistic effect between the defect ZnO substrate and the AuPd cocatalyst. The former promoted CH₄ adsorption, while the latter enhanced light absorption, charge separation, and O₂ activation to ROS. As illustrated in Figure 3C, under illumination, electrons were excited from the valence band to the conduction band of ZnO, then transferred to Au₉Pd₁ nanoparticles, reducing O₂ to ·OOH and ·OH. Photogenerated holes remaining in the ZnO valence band could oxidize CH₄ to ·CH₃ and H₂O to ·OH, although this process was less efficient. Thus, ROS generated from O₂ reduction played a dominant role in activating CH₄ adsorbed on ZnO to ·CH₃. ·CH₃ could then interact with ·OOH and ·OH to form CH₃OOH and CH₃OH. In the presence of Au₉Pd₁ nanoparticles, CH₃OOH could be partially converted to CH₃OH via a two-electron reduction process, with further oxidation by holes or ·OH leading to HCHO and CO₂ formation. Zhiyong Tang’s research team achieved selective generation of ·OOH and ·OH by adjusting the band structure and active site size of Au/In₂O₃, enabling efficient and selective production of HCHO and CH₃OH [49]. After three hours of photocatalytic CH₄ oxidation at room temperature, the HCHO yield on Au single-atom-loaded In₂O₃ (Au₁/In₂O₃) reached 6.09 mmol·g⁻¹ with a selectivity of 97.62%, while Au nanoparticle-loaded In₂O₃ (AuNPs/In₂O₃) achieved a CH₃OH yield of 5.95 mmol·g⁻¹ with a selectivity of 89.42%. Figure 3D summarizes the entire process of selective photocatalytic oxidation of CH₄ on Au/In₂O₃. For Au₁/In₂O₃, its valence band potential is more negative than the H₂O/·OH oxidation potential, preventing ·OH generation during the reaction. Instead, photogenerated holes oxidize CH₄ to ·CH₃, while photogenerated electrons transferred to Au reduce adsorbed O₂, with end-on adsorbed O₂ favoring reduction to ·OOH. ·OOH subsequently combines with ·CH₃ to form CH₃OOH, which decomposes to form HCHO. On the surface of Au nanoparticles, side-on adsorbed O₂ is more easily reduced to ·OH, which combines with ·CH₃ to form CH₃OH. This demonstrates that rational design of photocatalysts can precisely control the types of radicals formed. 2.2.2. O₂ Activation to ·OH In liquid-phase PPOM systems, not only can H₂O be oxidized to produce ·OH, but O₂ can also be activated to ·OH, which participates in the reaction. The ·OH radicals promote methane activation and product formation and participate in the oxidation of intermediates. However, an excess of ·OH can lead to overoxidation of products, so controlling ·OH formation is crucial for improving reaction activity and selectivity. In 2020, Zhiyong Tang’s research team designed Au nanoparticle-modified ZnO photocatalysts that, for the first time, activated O₂ to ·OH in a liquid-phase PPOM system [50]. Under ambient conditions and light irradiation, the Au/ZnO catalyst exhibited excellent performance, with a CH₃OH yield of 1371 μmol·g⁻¹ and a selectivity of 99.1%. Isotope experiments with ¹⁸O demonstrated that the O atoms in the product CH₃OH originated from both O₂ and H₂O, not just O₂. As shown in Figure 4A, Au nanoparticles act as electron conductors, effectively extracting electrons from the conduction band of ZnO and injecting them into O₂, producing ·OH through the O₂→H₂O₂→·OH pathway. Simultaneously, holes remaining in the valence band of ZnO oxidize H₂O to ·OH. Subsequently, CH₄ is activated by the generated ·OH to form ·CH₃, and finally, ·OH and ·CH₃ directly combine to produce CH₃OH. They also developed quantum-sized BiVO₄ (q−BiVO₄) capable of oxidizing methane to liquid oxygenates under visible light irradiation [51]. It was found that the selectivity for CH₃OH and HCHO could be regulated by altering the amounts of O₂ and H₂O in the reaction system, reaction time, and the wavelength and intensity of light irradiation. Shorter wavelengths and longer reaction times enhanced the oxidation ability towards CH₄, favoring HCHO formation under prolonged UV irradiation. Conversely, visible light irradiation and the introduction of large amounts of water to increase dissolved CH₄ content inhibited overoxidation, thereby increasing CH₃OH selectivity. In this system, O₂ is activated to ·OH, which activates CH₄ to form ·CH₃. ·CH₃ then combines with O₂, protons, and an electron to form CH₃OOH, which decomposes to produce CH₃OH, with further oxidation yielding HCHO (Figure 4B). JinHua Ye’s research team reported P-doped g-C₃N₄ (CNP) achieving a CH₃CH₂OH yield of 51 μmol·g⁻¹·h⁻¹ at 25 °C and 1 atm [52]. As shown in Figure 4C, P doping enhanced the process of O₂ activation through H₂O₂ (O₂→H₂O₂→·OH) to generate ·OH, which activated CH₄ to form ·CH₃. ·CH₃ further formed CH₃CH₂OH along with small amounts of HCOOH and CO₂. Wenting Wu’s research team constructed a sulfone-modified conjugated organic polymer that achieved photocatalytic conversion of CH₄ to CH₃OH and HCOOH under ambient light irradiation [53]. Mechanistic studies showed that light irradiation induced homolysis of S=O bonds on the catalyst, generating ·O and ·S. ·O could adsorb and activate CH₄, while ·S provided electrons to ¹O₂, generating H₂O₂, which then decomposed into ·OH, further oxidizing CH₄. Recently, the team also reported an Au−Pd alloy-modified ZnO photocatalyst (Au−Pd_0.5/ZnO) for CH₄ conversion to CH₃OH, achieving a CH₃OH yield and selectivity of 81.0 μmol·h⁻¹ and 88.2%, respectively [54]. Unlike the traditional O₂→H₂O₂→·OH pathway, they proposed a strategy for efficiently generating ·OH directly from O₂→·OOH→·OH, improving CH₃OH yield and selectivity. As shown in Figure 4D, under light irradiation, photogenerated holes oxidize CH₄ to ·CH₃, while O₂ adsorbed on the Au−Pd alloy is reduced to ·OOH by photogenerated electrons. The Au−Pd alloy facilitates O₂ adsorption and the cleavage of the O−O bond in ·OOH, quickly and directly converting ·OOH to ·OH. Finally, ·CH₃ combines with ·OH to form CH₃OH. This work not only provides a new strategy for efficiently generating ·OH directly but also offers guidance for the precise design of composite photocatalysts for PPOM reactions.

Figure 4. (A) Schematic illustration of photocatalytic CH4 conversion on Au/ZnO [50]. (B) Proposed mechanism of photocatalytic CH4 oxidation over q−BiVO₄ [51]. (C) Schematic diagram of photocatalytic methane conversion over CNP [52]. (D) Proposed mechanism of photocatalytic CH4 oxidation on AuPd0.5/ZnO [54].

2.2.3. O₂ Activation to ·O₂⁻ O₂ can also be activated to ·O₂⁻. Qin Kuang’s research team developed a hollow porous Pd/H−TiO₂ photocatalyst, where the unique hollow structure and strong metal-support interaction synergistically promoted the photocatalytic conversion of CH₄ to CH₃OH [55]. As shown in Figure 5A, ·OH and ·O₂⁻ are the primary active species involved in the photocatalytic process. Here, ·OH primarily activates methane to ·CH₃, while ·O₂⁻ further converts ·CH₃ to CH₃OH. Liangshu Zhong’s research team reported a W-doped TiO₂ photocatalyst [56]. The W doping modified the electronic and band structure of TiO₂, achieving a yield of liquid oxygenates of 12.2 mmol·g⁻¹ with a selectivity of 99.4%. Subsequently, they developed a Cu and W co-doped TiO₂ (Cu−W−TiO₂) photocatalyst, achieving an oxygenate yield of 34.5 mmol·g⁻¹ with a selectivity of 97.1% [57]. Under light irradiation, O₂ was reduced to ·O₂⁻ by photogenerated electrons or electrons captured in the W⁶⁺/W⁵⁺ cycle, while CH₄ was activated to ·CH₃ by Cu⁺. The ·O₂⁻ combined with H⁺ to form ·OOH, which reacted with ·CH₃ to form CH₃OOH, which was further reduced to CH₃OH. The synergistic effect of hole and electron capture processes on Cu−W−TiO₂ reduced the recombination of photogenerated carriers, promoting methane activation and efficient conversion. They also developed a SrWO₄/TiO₂ heterojunction catalyst [58]. The formation of the heterostructure facilitated the separation and transfer of photogenerated carriers, achieving an oxygenate yield of 13,365 μmol·g⁻¹ and a selectivity of 98.7%. Yunhang Hu’s research team reported an Au−Pd/TiO₂ photocatalyst, achieving a CH₃OH yield of 12.6 mmol·g⁻¹·h⁻¹ in the presence of O₂ and H₂O [59]. As shown in Figure 5B, photogenerated electrons reduced O₂ to ·O₂⁻, which was then converted to ·OOH. Meanwhile, H₂O was oxidized to ·OH by photogenerated holes, activating CH₄ to ·CH₃. ·CH₃ coupled with ·OOH to form CH₃OOH, which was further reduced to CH₃OH. TiO₂ absorbed UV light, generating electrons and holes, while Au−Pd nanoparticles not only facilitated the transfer of photogenerated electrons but also absorbed visible light, increasing the catalyst temperature. The increased temperature enhanced the process of H₂O oxidation to ·OH and drove the reduction of O₂ to ·O₂⁻ and the conversion of CH₃OOH to CH₃OH. The synergistic effect of Au−Pd nanoparticles and TiO₂ facilitated the efficient conversion of CH₄ to CH₃OH. Li Niu’s research team prepared Au nanoparticle-modified cubic WO₃ (c−WO₃) for the selective photo-oxidation of CH₄ to HCHO [60]. ¹⁸O₂ isotope tests indicated that the O in HCHO originated from lattice oxygen on the exposed (002), (020), and (200) planes of c−WO₃, and the surface-consumed lattice oxygen could be regenerated by the reduction of O₂. Under light irradiation, W⁵⁺ and O⁻ were generated on the c−WO₃ surface. O⁻ cleaved the C—H bond, activating CH₄ to −OCH₃, which was further dehydrogenated by adjacent terminal O⁻ to form HOCH₂OH, and finally dehydrated to form HCHO. Au nanoparticles captured photogenerated electrons, further facilitating the formation of O⁻. JinHua Ye’s research team achieved a CH₃OH yield of 4.8 mmol·g⁻¹·h⁻¹ with a selectivity of about 80% using Ag-modified TiO₂ with a dominant (001) facet to inhibit PPOM overoxidation [61]. As shown in Figure 5C, on the TiO₂(001) surface, photogenerated holes oxidized surface oxygen to form oxygen vacancies. O₂ reduced by photogenerated electrons formed ·O₂⁻, which were stabilized by oxygen vacancies, forming surface superoxides (Ti−O₂·). These could capture photogenerated electrons, forming surface peroxides (Ti−OO−Ti and Ti−(OO)), which dissociated into Ti−O· pairs. These pairs could directly activate CH₄ to form CH₃OH, effectively avoiding the formation of ·CH₃ and ·OH, thus inhibiting overoxidation. Zhiyong Tang’s research team used a “pause-flow” reactor with a TiO₂ dual-phase catalyst (anatase 90% and rutile 10%) (anatase/rutile) for the highly selective conversion of CH₄ to HCHO, achieving an HCHO yield of 8.09 mmol·g⁻¹·h⁻¹ with a selectivity of 97.4% [62]. Under light irradiation, carriers were generated inside A/R−TiO₂, with O₂ activated to ·O₂⁻ on R−TiO₂. The ·O₂⁻ was further activated to O⁻ species, activating CH₄ to ^∗CH₃O. ^∗CH₃O was converted to CH₃OH, desorbed with the assistance of H₂O, and finally oxidized to HCHO by ·OH. The O⁻ species originated from TiO₂ lattice oxygen, with O₂ filling the oxygen vacancies in TiO₂ (Figure 5D). This work guides the rational design of catalysts and reactors for industrial photocatalytic conversion of low-carbon feedstocks and demonstrates the feasibility of large-scale formaldehyde production. In the PPOM process, controlling the selectivity for single oxygenate products remains a significant challenge. O₂ activation can produce various ROS, each with different oxidation capabilities and mechanisms for activating CH₄. Future research should focus on the rational design of photocatalysts to precisely control the type of ROS generated, thereby controlling intermediate formation to enhance reaction efficiency and selectivity. Additionally, current photocatalytic methane partial oxidation reactions primarily occur in closed systems. While some progress has been made, the limitations of closed systems restrict large-scale production of oxygenates. Therefore, future research should focus on developing reaction systems and designing reactors to achieve efficient methane conversion on a larger scale, paving the way for industrial photocatalytic methane conversion.

Figure 5. (A) Schematic illustration for the oxidation of methane to methanol over Pd/H−TiO2 [55]. (B) Schematic diagram of photocatalytic methane conversion over Au–Pd/TiO2 [59]. (C) Proposed mechanism for CH4 oxidation by O2 on the (001) facets of TiO2 [61]. (D) Proposed mechanism of photocatalytic methane oxidation on A/R−TiO2 [62].

3. Photocatalytic Oxidative Coupling of Methane (POCM)

The oxidative coupling of methane (OCM) refers to the conversion of methane into C₂₊ hydrocarbons in the presence of an oxidant (typically O₂) [63]. Since Keller and Bhasin first reported the OCM production of valuable chemicals like ethylene and ethane in 1982 [64], the OCM reaction has garnered increasing attention. However, while the introduction of O₂ lowers the Gibbs free energy of the reaction, it still requires relatively high reaction temperatures and inevitably generates overoxidized products such as CO and CO₂. Therefore, photocatalytic oxidative coupling of methane (POCM) under mild conditions has become a new research hotspot. Researchers have deeply studied catalyst design, reactor optimization, and reaction mechanisms, achieving some significant results (Table 2).

Table 2. Representative works on photocatalytic OCM reaction with O₂ as the oxidant.

3.1. Batch Systems In 2018, Jinlin Long’s research team achieved room-temperature photocatalytic coupling of methane to ethane using Au nanoparticles supported on porous ZnO nanosheets (Au/m−ZnO−4.8) [65]. The plasmonic field formed at the Au/ZnO interface effectively induced charge separation in photo-excited ZnO, initiating methane activation at the Zn sites. Mechanistic studies revealed that the rate-determining step for methane coupling was the reduction of protons by hot electrons induced by Au plasmonics. The introduction of O₂ generated reactive oxygen species (ROS), accelerating proton consumption and increasing the ethane production rate from 11.0 µmol·g⁻¹·h⁻¹ to 19.5 µmol·g⁻¹·h⁻¹. Tierui Zhang’s research team systematically elucidated the photocatalytic OCM mechanism by designing a series of transition metal (Au, Ag, Pd, Cu, Ni, Ru, and Pt)-supported ZnO nanoparticles (M/ZnO) [66]. They found that whether ^∗CH₃ underwent C—C coupling or deep dehydrogenation on the metal surface was closely related to the d−σ interaction between ^∗CH₃ and the metal. Au/ZnO achieved the highest C₂−C₄ yield of 683 µmol·g⁻¹·h⁻¹ (selectivity 83%) due to strong d-σ hybridization between ^∗CH₃ and Au, which reduced the Au—C—H bond angle, decreasing the spatial hindrance for ^∗CH₃ coupling along the C—C pathway. As shown in Figure 6A, photoexcited Zn⁺−O⁻ active sites on ZnO efficiently activated methane to ^∗CH₃ and adsorbed H₂O, followed by selective coupling of ^∗CH₃ on the metal surface to form ethane or overoxidation to CO₂. Meanwhile, O₂ was reduced by photogenerated electrons, following the pathway O₂→O₂⁻→O₂²⁻→2O⁻→2O²⁻, replenishing the Ov created by H₂O desorption on ZnO. Scholten’s research team synthesized ZnO nanostructures with photocatalytic OCM activity using the hydrolysis of imidazole zinc chloride ionic liquids (ILs). The size and shape (irregular particles, nanorods) of the ZnO nanostructures depended on the synthesis conditions [67]. When the ionic liquid to ZnCl₂ ratio was equimolar, the resulting irregular ZnO particles had the highest C₂H₆ yield (787.3 µmol·g⁻¹·h⁻¹) and a selectivity of about 93%. This result provides a new approach to designing efficient POCM catalysts under room temperature and non-metal conditions. In addition to using classic ZnO semiconductors as POCM supports, researchers have explored other high-activity supports, achieving some progress. In 2023, Li Li’s research team used Au nanoparticle-modified ZnGa₂O₄ nanosheets (Au/ZnGa₂O₄) for the photocatalytic oxidative coupling of methane, achieving an ethane yield of 1315.3 µmol·g⁻¹·h⁻¹ and a selectivity of 53% [68]. The results showed that the reaction activity significantly increased compared to pure ZnGa₂O₄ due to Au nanoparticles promoting O₂ adsorption and activation, producing O₂⁻. O₂⁻ was further reduced by photogenerated electrons to O⁻, which cracked CH₄ to ·CH₃. Au nanoparticles effectively stabilized ·CH₃ and prevented its overoxidation to CO₂. Similarly, Dunwei Wang’s research team achieved an ethane yield of 1690 µmol·g⁻¹·h⁻¹ and a selectivity of 85% by constructing an Au_2.6%/Bi₂WO₆ model to regulate ROS [69]. The study found that OCM performance was highly sensitive to the properties of the photocatalyst, which should facilitate the release of surface lattice oxygen, forming oxygen vacancies more easily. Au cocatalysts enhanced lattice oxygen activity, promoting Ov formation. Bi₂WO₆ has tunable surface oxygen, and under light irradiation, lattice oxygen was excited, activating the C—H bond in CH₄. The resulting ·CH₃ migrated to Au sites, where another CH₄ molecule was activated, producing a desorbable H₂O molecule and leaving an Ov. O₂ filled the Ov (Figure 6B). Throughout the process, O₂⁻ was the active species, so ·CH₃ was more likely to couple at Au sites to form C2H6 rather than over-oxidize to CO₂. Zizhong Zhang’s research team achieved high activity and selectivity for photocatalytic OCM by constructing dual-active sites (N and oxygen vacancies) on TiO₂ nanosheets, regulating O₂ activation pathways [70]. Due to the different O₂ activation sites on TiO₂, the alkane yield on TiO₂ nanosheets with N and Ov dual-active sites (Au/TiO₂⁻NV_o) increased from 1600 µmol·g⁻¹·h⁻¹ to 3200 µmol·g⁻¹·h⁻¹, and the selectivity improved from 61% to 93% compared to regular Au/TiO₂ nanosheets. For Au/TiO₂, O₂ easily captured photogenerated electrons on TiO₂, forming O₂⁻, which tended to react with ·CH₃ intermediates to form ⁻OOCH₃, which further decomposed into H₂O and CO₂, leading to CH₄ overoxidation. For Au/TiO₂⁻NVo, as shown in Figure 6C, O₂ adsorbed on oxygen vacancies was reduced to O₂⁻, and under the action of excited N atoms, the O−O bond of O₂⁻ cleaved, forming milder O^·− active species. O^·− could cleave the C—H bond in CH₄ to form ·CH₃, and hydroxyl radicals formed by the photolysis of Ti−O bonds activated the second CH₄. ·CH₃ coupled on Au NPs to form C₂H₆. After O atoms refilled the oxygen vacancies, the process continued with another two CH₄ molecules, inhibiting CH₄ overoxidation and enhancing OCM activity and selectivity. Unlike traditional oxide catalysts for POCM reactions, Yufei Song’s research team achieved an ethane yield of 81.6 µmol·g⁻¹·h⁻¹ and a selectivity of 90.3% by constructing Au-loaded ZnAl layered double hydroxide with oxygen vacancies (Au/ZnAl-v) [71]. In a batch system, the catalytic activity was stable for 100 h in a flow system, with a CH₄ conversion rate of 8.5 mmol·g⁻¹·h⁻¹ and a C₂H₆ selectivity of 92%. The study found that introducing Ov into ZnAl-LDH significantly promoted the efficiency and selectivity of photocatalytic OCM. O₂ first adsorbed and activated on Ov, then the activated O₂ cleaved the C—H bond in CH₄. The resulting ·CH₃ migrated to Au sites, coupling to form ethane. This report provides a new approach to designing efficient, selective, and stable photocatalytic materials for OCM using non-traditional oxide supports.

Figure 6. (A) Schematic diagram of the proposed reaction mechanism for photocatalytic OCM on metal/ZnO [66]. (B) Schematic description of Au-modified Bi2WO6 nanosheets for POCM [69]. (C) Proposed reaction mechanism of the POCM on Au/TiO2−NVo [70].

3.2. Flow Systems In batch systems, although gas-catalyst contact is sufficient, prolonged residence time inevitably leads to overoxidation of CH₄, resulting in lower selectivity. Additionally, scaling up batch reactors to industrial production is challenging. Therefore, developing flow reaction systems is essential for achieving efficient and selective photocatalytic OCM. In 2020, Junwang Tang’s research team first reported photocatalytic OCM in a flow reaction system [72]. They introduced Pt nanoparticles and CuO_x clusters onto TiO₂ (PC-50) (Cu_0.1Pt_0.5/PC-50), achieving a C₂ hydrocarbon yield of 68 µmol·g⁻¹·h⁻¹ and a selectivity of 60% under ambient conditions. The synergistic effect of Pt nanoparticles and CuO_x clusters increased the C₂ hydrocarbon yield by 3.5 times compared to PC-50 and more than twice the combined activity of Pt/PC-50 and Cu/PC-50. Under light irradiation, electrons were excited from the valence band to the conduction band of TiO₂ and migrated to Pt, while photogenerated holes transferred to CuO_x clusters. This process delayed carrier recombination and lowered the oxidation potential of photogenerated holes, preventing overoxidation of CH₄. CH₄ was activated by photogenerated holes on CuO_x clusters, forming ·CH₃ and H⁺, with ·CH₃ coupling to form C₂H₆. Pt nanoparticles reduced O₂ and combined H⁺ to remove it as H₂O. Similarly, the research team used Pd nanoparticle-modified anatase TiO₂ (Pd_1.8−TiO₂) to achieve a C₂H₆ yield of 818 µmol·g⁻¹·h⁻¹ under mild conditions, which was 13 times that of pure TiO₂, with a selectivity of 80.4% [73]. Pd nanoparticles acted as photogenerated hole acceptors, participating in CH₄ activation and ·CH₃ coupling, effectively inhibiting photogenerated carrier recombination, thereby significantly improving catalyst performance. They further reported a highly efficient and stable PdCu nanoalloy-modified TiO₂ (PdCu/TiO₂) photocatalyst for OCM in a mild flow system, achieving a C₂ hydrocarbon yield of 1240 µmol·g⁻¹·h⁻¹ and the photocatalyst exhibits the turnover frequency and turnover number of 116 h⁻¹ and 12,642 with respect to PdCu. [74]. As shown in Figure 7A, under light irradiation, photogenerated electrons from TiO₂ reduced O₂ to superoxide radicals, while photogenerated holes transferred to the PdCu alloy to activate adsorbed CH₄, generating ·CH₃ and H⁺. ·CH₃ further coupled to form C₂H₆, with H⁺ consumed by superoxide radicals to form H₂O. Introducing Pd nanoparticles into TiO₂ led to effective charge transfer, weakening the C—H bond in CH₄ and facilitating its activation, producing more ·CH₃. Cu reduced the adsorption energy of the target product C₂H₆, preventing catalyst coking. Thus, the synergistic effect of PdCu nanoalloy achieved efficient, selective, and stable photocatalytic OCM. Additionally, they designed an Ag−AgBr/TiO₂ ternary catalyst and studied the effect of reaction pressure on photocatalytic OCM performance in a pressurized flow reactor [75]. When the reaction pressure increased from 1 bar to 6 bar, the C₂H₆ yield increased from 18.2 µmol·g⁻¹·h⁻¹ to 354 µmol·g⁻¹·h⁻¹, with a C₂₊ selectivity of 79%. Higher reaction pressure enhanced the mass transfer efficiency of reactants and products, improving reaction efficiency and selectivity. Ag nanoparticles, as electron acceptors, facilitated charge transfer, while the AgBr and TiO₂ heterostructure reduced oxidation capability, preventing overoxidation. Thus, designing high-performance catalysts and rational reaction systems is crucial for photocatalytic OCM. Au NPs, as a cocatalyst, not only promote charge separation but also stabilize ·CH₃, preventing CH₄ overoxidation to CO₂, thereby improving photocatalytic OCM activity and selectivity. Moreover, the electromagnetic decay of localized surface plasmon resonance (LSPR) on Au nanoparticles can generate hot carriers. Hot carrier relaxation can induce a photothermal effect and the hot carrier separation not only promotes O₂ reduction and CH₄ activation, but also prolongs carrier lifetime. Therefore, using Au NPs as cocatalysts in photocatalytic OCM is favored by researchers. In 2021, JinHua Ye’s research team efficiently and selectively coupled CH₄ to C₂H₆ using Au NP-loaded ZnO/TiO₂ hybrids (Au−ZnO/TiO₂(4/1)), achieving a C₂H₆ yield of over 5000 µmol·g⁻¹·h⁻¹ with 90% selectivity [76]. The study found that modifying ZnO with TiO₂ and Au formed a ZnO/TiO₂⁻Au heterojunction, enhancing photocatalytic activity while maintaining ZnO’s mild C—H bond overoxidation capability in CH₄. Introducing Au cocatalysts promoted O₂ adsorption and activation, facilitating the desorption of ^∗CH₃ as ·CH₃ in the gas phase, thus promoting C₂H₆ formation and inhibiting overoxidation to CO₂. As shown in Figure 7B, under light irradiation, the photocatalyst generated photogenerated electrons and holes, with the heterojunction allowing rapid electron transfer to Au NPs, reducing O₂ to ^∗O₂⁻, while photogenerated holes transferred to ZnO, aiding CH₄ activation to produce ^∗CH₃. ^∗CH₃ desorbed as ·CH₃ on Au and coupled to form C₂H₆. C₂H₆ further reacted with holes to form ·C₂H₅, which coupled with ·CH₃ to form C₃H₈ or further oxidized to C₂H₄, with H₂O as a byproduct. Unlike Au, Pt, with stronger O₂ reduction capability, tended to form ^∗OCH₃, ultimately oxidizing to CO₂. A lower O₂/CH₄ ratio meant less collision between ^∗O and ^∗CH₃, inhibiting CH₄ overoxidation. Therefore, the O₂/CH₄ ratio in feed gas significantly controls product selectivity. Subsequently, Khodakov’s research team studied the role of 6–60 nm plasmonic Au NPs supported on TiO₂ in methane oxidative coupling. Under optimized conditions with 14 nm Au NPs (14−nm−Au/TiO₂), an ethane yield of 819 µmol·g⁻¹·h⁻¹ and a selectivity of 86% were achieved [77]. They found that the size (6–60 nm) and amount (>0.5 wt%) of Au NPs did not significantly affect methane coupling. Under UV excitation, TiO₂ generated oxygen vacancies, activating methane at these sites, while Au NPs activated O₂ and facilitated charge separation. Using a rapid sputtering method, Junwang Tang’s research team synthesized highly efficient Au60s/TiO₂ catalysts on glass fiber filters, achieving a C₂ hydrocarbon yield of 23,950 µmol·g⁻¹·h⁻¹ with 86% selectivity, the highest reported efficiency to date [78]. Introducing Au NPs as cocatalysts extended the lifetime of TiO₂ photoelectrons by 66 times, forming more superoxide radicals, promoting the photocatalytic methane conversion cycle. As catalytic centers and photogenerated hole acceptors, Au NPs facilitated methane adsorption and increased photogenerated hole numbers, favoring selective C—H bond cleavage and C—C bond coupling. Under light irradiation, TiO₂ generated photogenerated electrons and holes, with photogenerated holes transferring to Au NPs while long-lived photogenerated electrons on TiO₂ reduced oxygen. Au efficiently adsorbed and activated CH₄, generating ·CH₃ and H⁺, with ·CH₃ coupling to form C₂H₆ and H⁺ combining with superoxide radicals to form H₂O (Figure 7C). Recently, Yujie Xiong’s research team loaded Au NPs onto TiO₂ nanosheets (Au_2.0%/TiO₂), achieving a C₂₊ hydrocarbon yield of 19,280 µmol·g⁻¹·h⁻¹ with 90% selectivity in a custom 3D-printed multi-point injection flow reactor, maintaining stability for over 240 h [79]. Unlike pure TiO₂, where O₂⁻ induced CH₄ overoxidation, the presence of Au NPs stabilized ^∗CH₃ intermediates and constructed an Au−TiO₂ interface, regulating O₂ activation to produce mild O₂²⁻ species, avoiding ^∗CH₃ overoxidation. The localized electric field induced by Au NPs’ LSPR promoted the polarization and dissociation of C—H bonds in CH₄. As shown in Figure 7D, under light irradiation, photogenerated electrons in the TiO₂ conduction band were captured by Au NPs, achieving O₂ activation at the Au−TiO₂ interface, generating mild O₂²⁻ species. These mild O₂²⁻ species dissociated adsorbed CH₄ on Au NPs to produce ^∗CH₃. Another CH₄ underwent a similar process to generate ^∗CH₃, which coupled to form C₂H₆. The two formed ^∗OH radicals continued to abstract hydrogen atoms from two other CH₄ molecules, generating a second C₂H₆ and two H₂O molecules, regenerating O₂ activation sites, completing the photocatalytic OCM cycle. This work guides the synergistic design of reactors and photocatalysts to simultaneously regulate mass transfer and reactant activation for high-performance flow systems. In summary, future research should focus on reaction system and photocatalyst design. For reaction systems, flow systems have improved reaction efficiency and target product selectivity, but issues such as low mass transfer efficiency and product separation remain. Designing more efficient reactors to control the catalytic conversion process is necessary. Additionally, the CH₄/O₂ ratio, flow rate, and reaction pressure significantly impact photocatalytic OCM performance. Therefore, ensuring safety requires finer control of reaction conditions. For photocatalyst design, rational catalysts should generate mild active oxygen species to promote methane activation while stabilizing methyl intermediates to facilitate C—C bond coupling. Currently, most cocatalysts are Au or other precious metals, and the main product is ethane, which has lower commercial value. Considering economic feasibility, developing non-precious metal catalysts and novel catalysts to regulate CH₄ activation and carbon intermediate conversion to produce higher-value chemicals such as ethylene and propane is essential.

Figure 7. (A) Schematic diagram of photoexcitation and reaction over PdCu/TiO2 [74]. (B) Schematic illustration of the reaction pathways for photocatalytic OCM over Au−ZnO/TiO2(4/1) [76]. (C) Proposed POCM process over Au60s/TiO2 [78]. (D) Schematic illustration of the proposed mechanism for photocatalytic OCM on Au2.0%/TiO2 [79].

4. Photochemical Looping

Chemical looping involves decomposing a reaction into multiple sub-reactions, which are carried out in isolated spaces to achieve in situ separation of products [80,81,82,83]. In methane aerobic conversion reactions, the use of oxygen can lead to deep oxidation of methane and poses an explosion risk, significantly limiting the industrial application of this reaction. Applying chemical looping techniques to methane conversion, dividing the reaction into two half-reactions, can greatly improve the selectivity and safety of the reaction. This technology typically uses metal oxides as oxygen carriers. In the methane oxidation reaction, lattice oxygen is used instead of traditional O₂. Subsequently, the reduced oxygen carrier reacts with air to replenish the lattice oxygen, completing one reaction loop. Yong Lu’s research team achieved methane oxidative coupling at lower temperatures through chemical looping to activate O₂ [84]. They developed a TiO₂-doped Mn₂O₃−Na₂WO₄/SiO₂ catalyst system, generating MnTiO₃ in the reaction flow, triggering the MnTiO₃↔Mn₂O₃ chemical loop to activate O₂, achieving a 22% CH₄ conversion rate and 62% C₂−C₃ selectivity at 650 °C. Liang-Shih Fan’s research team embedded Fe₂O₃ nanoparticles in a mesoporous SiO₂ carrier (Fe₂O₃@SBA-15), significantly suppressing CO₂ production in methane partial oxidation, achieving nearly 100% CO selectivity in a cyclic redox system at 750–935 °C [85]. Theoretical calculations indicated that low-coordination Fe atoms favored CH₄ adsorption and activation, while low-coordination lattice oxygen atoms significantly promoted Fe−O bond cleavage and CO formation, thereby enhancing CO selectivity. Based on the characteristics of chemical looping, researchers have applied it to photocatalytic systems, yielding valuable results. In 2012, Jiesheng Chen’s research team first reported a Ga³⁺-modified ETS-10 molecular sieve material that utilized its oxygen centers and metal center active sites for strong activation of methane C—H bonds, achieving efficient coupling of methane to ethane at room temperature [86]. Under light irradiation, photogenerated electrons reduced Ti⁴⁺ to Ti³⁺, while photogenerated holes oxidized surface hydroxyl groups to hydroxyl radicals, thus activating the C—H bonds of methane. As the reaction proceeded, Ti−OH groups were gradually consumed, leading to catalyst deactivation, which could be restored to photocatalytic activity by simple heat treatment in humid air. In 2020, Khodakov’s research team proposed a photochemical looping strategy using highly dispersed silver ions in a silver-phosphotungstic acid-titanium dioxide nanocomposite (Ag−HPW/TiO₂) to achieve high selectivity and nearly quantitative ethane production [87]. As Ag⁺ was continuously reduced to metallic Ag, the catalyst’s color changed from light gray to deep black, gradually deactivating the catalyst. To continuously synthesize ethane, the catalyst was regenerated by exposing it to air under light for 7 h. As shown in Figure 8A, under light irradiation, the photocatalyst generated photogenerated electrons and holes. The electrons reduced Ag⁺ to metallic Ag, while the holes oxidized CH₄ to ·CH₃, which coupled to form C₂H₆. Exposing the catalyst to air under light regenerated Ag to its oxidized state, forming an Ag⁺↔Ag⁰ redox loop. This strategy separated the CH₄ oxidation step from the O₂ reduction step, effectively inhibiting methane overoxidation. Similarly, Yongfu Sun’s research team achieved efficient CH₄→C₂H₆ photocatalytic conversion using Au/ZnO porous nanosheets with dual active species of Au^δ− and O⁻ [88]. During photoexcitation, lattice oxygen in ZnO was easily oxidized to O⁻ active species. The generated Au^δ− and O⁻ could polarize the inert C—H bond and stabilize the resulting active ^∗CH₃ intermediate, thus avoiding overoxidation. The consumed lattice oxygen was replenished through the Mars-van Krevelen mechanism. Ye Wang’s research team constructed a solar-driven Fe³⁺/Fe²⁺ redox loop, combining CH₄ photochemical coupling with electrochemical H₂ production. This system achieved high selectivity for CH₄ coupling to C₂H₆ while reducing the potential for electrochemical hydrogen production [89]. Fe³⁺ hydrolyzed to form [Fe(H₂O)₅OH]²⁺, which absorbed UV light, transferring electrons from the OH to Fe³⁺, resulting in free ^∗OH and Fe²⁺. ^∗OH oxidized CH₄ to ^∗CH₃, which coupled to form C₂H₆. Fe²⁺ was oxidized back to Fe³⁺ at the anode, replacing the traditional oxygen evolution reaction (OER), and H₂ was produced at the cathode, completing the redox loop (Figure 8B). Yujie Xiong’s research team proposed a methoxy- and ethoxy-intermediate-mediated pathway, directly photocatalytically converting CH₄ to C₂H₄ under mild conditions using ZnO−AuPd_2.7% [90]. Zn sites on ZnO served as the adsorption and activation sites for CH₄, activating it to ^∗CH₃. The resulting ^∗CH₃ preferred to combine with ZnO lattice oxygen to form methoxy intermediates. Assisted by Pd, methoxy could dehydrogenate to −CH₂O, which then reacted with another activated CH₄ to form ethoxy intermediates. Ethoxy further dehydrogenated to form C₂H₄, avoiding the overoxidation of CH₄ to CO_x. After 8 h, 3.68% of ZnO total lattice oxygen was consumed, reducing catalytic activity. The formed oxygen vacancies were easily replenished by washing with water. They also designed Pd−Zn modified WO₃ nanosheets (Pd₅/Zn_0.35−WO₃) to achieve efficient conversion of CH₄ to C₂H₄ [91]. Zn sites promoted the adsorption and activation of CH₄, forming methyl and methoxy intermediates with the help of lattice oxygen. Pd sites facilitated methoxy dehydrogenation to methylene radicals, forming C₂H₄ and inhibiting overoxidation. The consumed lattice oxygen could be replenished by photochemical looping with air exposure. Additionally, they reported a Pd single-atom modified TiO₂ photocatalyst (Pd₁/TiO₂) for methane coupling to ethane [92]. For TiO₂, the significant contribution of O atoms to the valence band made lattice oxygen directly involved in methane activation. The activated ^∗CH₃ was difficult to desorb from O sites, leading to overoxidation. In Pd1/TiO₂, the Pd−O4 unit contributed most to the surface TiO₂ valence band maximum (VBM), accumulating photogenerated holes and facilitating CH₄ dissociation on Pd, inhibiting overoxidation with lattice oxygen. However, this process still required lattice oxygen consumption, leading to performance degradation after 6 h. The consumed lattice oxygen could be replenished by heating in air. Recently, they loaded Au NPs on Nb3O7(OH) with abundant surface lattice hydroxyl groups, achieving efficient photocatalytic coupling of CH₄ to C₂H₆ under mild conditions [93]. Lattice hydroxyls on Nb3O7(OH) facilitated CH₄ activation, forming key methoxy intermediates. The consumed lattice oxygen could be replenished by photochemical looping, either by washing with water or air exposure. Photochemical looping strategies can also be applied to the partial oxidation of methane to produce oxygenates. In 2019, Khodakov’s research team highly dispersed zinc on phosphotungstic acid/titanium dioxide (Zn−HPW/TiO₂), selectively photocatalytically oxidizing methane to carbon monoxide under ambient conditions [94]. Under UV excitation, ZnO formed Zn⁺−O⁻ pairs that adsorbed and activated methane to form Zn-methyl species. Surface methyl zinc reacted with zinc carbonate (formed from ZnO and gas-phase CO₂) to form methyl zinc carbonate, which decomposed to produce CO. After 12 h, the formation rates of CO and CO₂ slowed due to insufficient oxygen content and gradual reduction of Zn²⁺ to Zn⁰. The Zn²⁺ was regenerated and lattice oxygen replenished through the Mars-van Krevelen mechanism by irradiation in air. Photochemical looping can also occur in gas-solid-liquid systems. Yujie Xiong’s research team constructed a PdO/Pd−WO₃ heterojunction nanocomposite with CH₄ activation and C—C coupling active sites, directly converting CH₄ to CH₃COOH without introducing additional carbon sources [95]. As shown in Figure 8C, CH₄ adsorbed on Pd sites was activated by ·OH, with ^∗CH₃ gradually converting to Pd−CO intermediates under the assistance of O atoms in PdO and ·OH dehydrogenation. ^∗CO and ^∗CH₃ coupled to form Pd−COCH₃ intermediates, which further hydrolyzed to CH₃COOH. The formation of CH₃COOH consumed lattice oxygen in PdO, leading to performance decay after 3 h. The consumed lattice oxygen could be replenished by heating in air. Currently, photochemical looping strategies for methane conversion are applied only in closed systems and not connected to air systems. Most applications are limited to batch systems. Future research should focus on reactor design, especially flow reaction systems. Additionally, the time-consuming Mars-van Krevelen mechanism for catalyst regeneration needs to be made more efficient for industrial applications. Moreover, hydrogen products are mostly water. Adjusting the reaction pathways of active species to produce more valuable hydrogen gas would be beneficial. Combining photochemical looping strategies with methane conversion could solve existing challenges and realize the industrialization of photocatalytic methane conversion.

Figure 8. (A) Schematic diagram of a photochemical looping process on Ag−HPW/TiO2 [87]. (B) Schematic diagram of the Fe3+−Fe2+ cycle pathway and mechanism [89]. (C) Schematic illustration for photochemical conversion of CH4 to CH3COOH over PdO/Pd−WO3 [95].

5. Summary and Outlook

Directly converting methane into value-added chemicals offers a promising alternative to the energy-intensive industrial methane reforming processes. Compared to the thermodynamically unfavorable non-oxidative coupling of methane (NOCM), the introduction of oxygen not only lowers the Gibbs free energy of the reaction, making it spontaneous, but also produces various reactive oxygen species that facilitate methane activation. Photocatalysis can reduce the activation barrier for methane, activating the inert C—H bond and enabling the reaction under milder conditions. This review has summarized the latest research progress in the photocatalytic partial oxidation of methane (PPOM) and oxidative coupling of methane (POCM) using oxygen as an oxidant. It primarily covers the activation mechanisms of methane and oxygen in different reaction systems, evaluation of methane aerobic conversion efficiency, and elucidation of reaction mechanisms. Additionally, the unique advantages of photochemical cycling in methane conversion are introduced. These insights aim to deepen the understanding of methane and oxygen activation mechanisms to design more efficient catalysts and reaction systems. Despite substantial progress in photocatalytic methane conversion, significant gaps remain before industrial application can be realized. For example, the catalytic performance obtained experimentally is far from the requirements of industrial production; the high-carbon products is difficult to be synthesized efficiently and directionally; the poor selectivity of the products caused by peroxidation makes it difficult to separate the products; the reaction system is relatively elementary and the reactor is difficult to scale up; the studies of the reaction mechanism is insufficient, especially the activation mechanism of the C—H bond as well as the monitoring of the reaction intermediates and the reactive oxygen species. To address these issues, future research should focus on the following aspects: (1) Designing Efficient Catalysts: Currently, most photocatalysts for methane conversion are UV-responsive. Therefore, designing narrow-bandgap semiconductors or black photocatalysts that absorb the entire solar spectrum, including visible and infrared light, is essential. Photocatalysts should also possess good electron-hole separation capabilities and suitable redox potentials. They should have moderate oxidation ability for methane and strong activation capacity for oxygen to promote the formation of reactive oxygen species. Typically, precious metals are introduced as cocatalysts to promote charge separation and oxygen activation. However, for economic viability, doping, defect engineering, heterojunction construction, or developing non-precious metal catalysts to replace precious metal catalysts should be explored. Traditional inorganic semiconductor materials often limit photocatalysts, so developing new photocatalytic materials, including covalent organic frameworks and inorganic-organic or metal-organic frameworks, may show excellent performance in methane conversion. Moreover, hot-carrier mediated photocatalysis can be induced by the Landau damping of surface plasmon resonance (SPR) in plasmonic metals or direct interband electron transitions in other noble metals [96,97]. Hot carriers can not only undergo relaxation to increase the catalyst surface temperature, but also directly participate in the photochemical reaction process, which greatly promotes the reaction efficiency. Therefore, by establishing a plasmonic or non-plasmonic hot carrier-based photocatalytic system can make isolating support materials exhibit photocatalytic activity, which undoubtedly enriches the photocatalyst system and has significant meaning for finding new photocatalyst materials. (2) Hot carriers’ effect: The role of metal nanoparticles is usually considered to be the promotion of light absorption, charge separation, activation of reactants, and stabilization of reaction intermediates. However, metal nanoparticles can simultaneously utilize the photo and thermal energy to promote reaction. The hot carriers’ effect induced by metal nanoparticles should not be ignored for photocatalytic conversion of methane. More attention should be paid to how the hot carriers’ effect promotes the reduction of O₂ and the activation of CH₄ to improve the efficiency and selectivity of the reaction. The electromagnetic decay of localized surface plasmon resonance (LSPR) on plasmonic nanoparticles such as Au, Ag and Cu or interband electron transition from the d-band to the s-band in non-plasmonic nanoparticles such as Pt, Pd, Rh, and Ru can produce hot carriers, which are different from those carriers generated by traditional semiconductor bandgap excitation [98,99]. In general, bandgap excitation and hot carrier generation compete with each other. Therefore, it is possible to promote the generation of hot carriers by choosing a suitable metal as well as by regulating the supports. Hot carrier relaxation can induce a photothermal effect to increase the catalyst surface temperature. Moreover, hot carriers can directly interact with gas molecules adsorbed on the surface of metal nanoparticles and thus participate in photochemical processes. Meanwhile, the process of hot carrier charge separation not only prolongs the carrier lifetime, but also promotes the reduction of O₂ and the activation of CH₄, which further improves the efficiency and selectivity of the reaction. Therefore, it is necessary to fully understand and utilize the hot carriers’ effect of metal nanoparticles. (3) Expanding to C₂₊ Products: The products of photocatalytic methane aerobic conversion are mostly C₁ oxygenates or C₂H₆, which have low commercial value and require further conversion for industrial significance. Reports on C₂ oxygenates, C₂H₄, and C₃H₈ are rare and yields are low. Methane activation energy is usually higher than that of oxidation products, leading to overoxidation of products. Oxidation and coupling reactions compete, making it challenging to form C₂₊ products. This necessitates deeper mechanistic studies to reveal possible reaction pathways and precise control of reaction sites and intermediates to promote C—C bond coupling. (4) Developing Efficient Reaction Systems: The reaction system is critical for reaction activity and product selectivity. Although flow systems have advantages over batch systems in terms of gas-solid or gas-solid-liquid mass transfer efficiency, reaction stability, and selectivity, the complexity of methane conversion and the costs and safety of reactions should be considered. Drawing from mature flow systems in thermal catalysis, developing novel photocatalytic flow reactors is essential. Currently reported photocatalytic methane conversion flow systems mainly apply to gas-solid two-phase reactions. Gas-liquid-solid three-phase systems, such as those in methane partial oxidation, largely remain in batch reactors. Developing new reactors, such as membrane reactors, can further enhance methane reaction efficiency and target product selectivity. Reaction temperature, pressure, flow rate, CH₄/O₂ ratio, light source, and reaction time all affect yield and product selectivity. For example, an appropriate CH₄/O₂ ratio can prevent overoxidation in methane coupling, and pressurized reactors can increase the solubility of CH₄ and O₂ in H₂O. Additionally, due to the low selectivity of products in methane aerobic conversion, the cost of subsequent product separation is high. Rationally designing porous metal-organic frameworks can replace energy-intensive distillation for separation and purification. (5) Utilizing Advanced In Situ Characterization: Research on the reaction mechanisms of photocatalytic methane aerobic conversion heavily relies on molecular and atomic-level characterization techniques, especially in situ and operando characterization. Since oxygen activation can produce various reactive oxygen species, in situ EPR can detect these species to understand the mechanism of O₂ activation. Isotope labeling can determine the source of liquid oxygen. Advanced techniques like in situ IR spectroscopy, in situ XPS, and X-ray absorption spectroscopy can characterize the chemical states of reaction intermediates and active sites. Time-resolved spectroscopy can provide precise information on methane activation and conversion processes. Theoretical calculations can simulate reaction processes at the molecular level, exploring the reaction mechanism of methane conversion and determining the energies of different reaction steps, thus revealing the correct reaction pathway to guide the development of high-performance photocatalytic methane conversion systems. In summary, abundant and inexpensive methane not only holds the potential to replace petroleum and other fossil fuels but can also serve as a raw material for synthesizing value-added chemicals. Due to methane’s stability, activating and converting methane molecules is highly challenging. Activating methane’s inert C—H bond using solar energy is strategically significant for energy development and sunlight utilization. Although there is still a gap between photocatalytic methane conversion and industrial application, achieving this will bring immense benefits to society.

Author Contributions

Conceptualization, Y.K. and L.L.; Methodology, Y.K.; Writing—Original Draft Preparation, Y.K. and C.Y.; Writing—Review & Editing, X.M. and L.L.; Project Administration, X.M., Y.C. and L.L.; Funding Acquisition, L.L.

Ethics Statement

Not applicable.

Informed Consent Statement

Funding

This work was financially supported by the Natural Science Foundation of Jilin Province (grant nos. 20240101177JC and 20240302098GX), the National Natural Science Foundation (NSFC) of China (grant no. 22379050), the National Key Research and Development Program of China (2023YFA1506303), and the Fundamental Research Funds for the Central Universities.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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