Photocatalytic Methane Oxidation to Carbonyl Products

ABSTRACT

Direct photocatalytic oxidation of methane (CH₄) to value‐added carbonyl compounds, such as formaldehyde (HCHO), formic acid (HCOOH), acetaldehyde (CH₃CHO), and acetic acid (CH₃COOH), represents an attractive and promising alternative to conventional indirect routes via syngas, which suffer from high energy consumption and carbon emissions. By utilizing light energy to activate the C─H bond under mild conditions, photocatalysis effectively prevents the overoxidation of the target compound, thereby alleviating the long‐standing trade‐off between activity and selectivity imposed by thermodynamic constraints. A deep understanding of the reaction mechanism underlying photocatalytic CH₄ conversion to carbonyl compounds is therefore a prerequisite for designing high‐performance photocatalytic systems; yet, this topic remains unexplored to date. In this review, we systematically discuss diverse reaction pathways from CH₄ to C1 and C2 carbonyl compounds and summarize a comprehensive toolbox for mechanistic investigation, including basic characterizations, in situ spectroscopies, necessary control experiments, probing and trapping experiments, isotope labelling, and theoretical calculations. We then assess recent advances in catalytic systems and identify the key factors governing carbonyl formation, selectivity, and productivity. Finally, we highlight the central challenges that remain for practical implementation, particularly regarding catalyst performance, rational reaction system design, efficient product separation, and possible cascade reactions.

This review summarizes recent advances in the photocatalytic oxidation of methane to carbonyl products. A comprehensive toolkit of experimental and theoretical methods is presented to elucidate the underlying reaction mechanism. The reported pathways for converting methane to C1 and C2 carbonyls are systematically classified. The review further discusses the knowledge gaps hindering the transition from laboratory to industrial‐scale production and explores promising future research directions.

1. Introduction

Methane (CH₄), the main constituent of natural gas, shale gas, and combustible ice, is one of the most abundant hydrocarbon resources on Earth [1]. Currently, CH₄ is primarily burned for heat and power generation rather than used as a feedstock for high‐value chemical conversion [2]. Besides, a large fraction of gaseous CH₄ reserves remains underutilized as they are located in remote regions where conventional liquefaction and transport infrastructure are economically unviable [3]. Furthermore, CH₄ is a potent greenhouse gas, and its leakage during transportation poses significant climate risks [4, 5]. In contrast, the on‐site conversion of CH₄ into value‐added chemicals offers an attractive alternative that not only enhances product value but also addresses economic and environmental challenges associated with long‐distance transportation [6].

Traditionally, the industrial conversion of CH₄ to value‐added chemicals relies on the indirect pathways, beginning with the steam reforming of methane (SRM). SRM operates at extreme temperatures (typically > 1000 K) to produce syngas (a mixture of CO and H₂) [7], which is then employed as a feedstock for Fischer–Tropsch synthesis or methanol production [8]. However, such high‐temperature requirement incurs substantial operating costs, high energy consumption, and considerable CO₂ emissions, limiting the sustainability of this approach. Additionally, the multi‐step nature of the indirect conversion pathway restricts the overall atom economy. These limitations have motivated sustained interest in direct pathways for selectively converting CH₄ into value‐added chemicals under milder conditions [9].

In thermocatalysis, the efficient direct conversion of CH₄ into value‐added chemicals remains a longstanding challenge because of the inherent trade‐off between activity and selectivity. The CH₄ molecule is of tetrahedral symmetry and possesses strong C─H bonds (439 kJ mol⁻¹) [10], rendering it kinetically inert under mild conditions. Although high temperatures can facilitate C─H bond activation, they also promote the overoxidation of desired products. As a result, conventional thermocatalytic systems often struggle to deliver both high selectivity and high productivity. Considerable efforts have therefore been devoted to developing alternative strategies for the efficient and sustainable conversion of CH₄ into value‐added chemicals [11].

Photocatalysis has emerged as a promising strategy for driving chemical transformations using solar power [12]. In principle, light provides a reaction‐driving force decoupled from temperature, offering opportunities to overcome the activity–selectivity trade‐off that constrains conventional thermocatalysis. In photocatalytic systems, photogenerated carriers can activate CH₄ under mild conditions either directly or indirectly via the in situ generation of reactive oxygen species (ROS) [13]. This decoupling of activation from temperature enables precise control over reaction depth, allowing oxidation steps to be halted at desired intermediates, provided that the identity, concentration, and lifetime of ROS are rationally modulated. Furthermore, the compatibility of photocatalysis with low‐intensity, diffuse solar irradiation makes it particularly suitable for decentralized CH₄ utilization, with the potential to reduce the capital costs associated with centralized infrastructure for CH₄ upgrading and transport [14]. Compared with other green conversion technologies, such as electrocatalysis [15] and photoelectrocatalysis [16, 17, 18], photocatalysis bypasses the need for complicated electrochemical cells and can be carried out with a simpler reactor architecture. It not only supports liquid‐phase reaction systems (gas–liquid–solid systems) but also accommodates gas‐solid systems [19]. Furthermore, the absence of external electrical input renders photocatalysis economically attractive as it requires lower operating costs and infrastructure investment, making it more suitable in decentralized CH₄ utilization scenarios.

Over the past decade, considerable effort has been devoted to the photocatalytic CH₄ conversion, yielding remarkable progress in the photosynthesis of value‐added products such as alcohols [20, 21], aldehydes [22, 23], carboxylic acids [24, 25], and multicarbon products [26, 27, 28]. Compared with carbonyl compounds with complex reaction pathways, alcohols (e.g., methanol and ethanol) and hydrocarbons (e.g., ethane and ethene) have simple conversion pathways and often serve as intermediate species for subsequent conversion. Furthermore, research progress on the photocatalytic conversion of CH₄ to alcohols [29, 30, 31] and hydrocarbons [32, 33, 34] has been systematically and thoroughly summarized. However, there is currently a lack of systematic reviews on the photocatalytic conversion of CH₄ to carbonyl products, such as formaldehyde (HCHO), formic acid (HCOOH), acetaldehyde (CH₃CHO), and acetic acid (CH₃COOH). Notably, these carbonyl compounds are important platform chemicals because the polar carbonyl group (C═O) can participate in a wide range of industrial synthetic transformations [35]. For instance, HCHO is a key precursor for resins and plastics [36]; HCOOH has emerged as a promising energy carrier and hydrogen storage medium in renewable energy systems [37]; CH₃CHO represents a major commodity chemical applied in the manufacture of perfumes, dyes, and pharmaceutical synthesis [38]; and CH₃COOH constitutes a cornerstone of the chemical industry, serving as a precursor for vinyl acetate, terephthalic acid, and numerous other derivatives [39]. Direct photocatalytic conversion of CH₄ into these carbonyl compounds provides important advantages over conventional indirect routes by bypassing syngas production, thereby improving atom economy and simplifying process design.

This review aims to guide the ongoing research by systematically summarizing key recent advances in the photocatalytic CH₄ oxidation to carbonyl compounds, excluding alcohols and hydrocarbons. The content encompasses the following aspects. First, the fundamental understandings of CH₄ conversion and the suite of analytical tools are briefly outlined. Subsequently, we review and discuss the reaction pathways for the conversion of CH₄ to C1 and C2 carbonyl compounds, respectively. Finally, we discuss the existing knowledge gap in this field, with a focus on the scale‐up process from the laboratory to industrial‐scale production, and share our perspectives on the future directions.

2. Fundamental Understandings

2.1. Reaction Routes of Photocatalytic Methane Oxidation to Carbonyl Products

Since the average oxidation states of carbonyl products (HCHO: 0; HCOOH: +2; CH₃CHO: −1; CH₃COOH: 0) are higher than those of common products like methanol (CH₃OH: −2) and ethane (C₂H₆: −3), the reaction routes for photocatalytic CH₄ oxidation to carbonyl products are more diverse. For C1 carbonyl products, the reaction routes are summarized in Figure 1. HCHO can be produced through three distinct pathways: methyl hydroperoxide (CH₃OOH) decomposition (red line), continuous oxidation (yellow line), and lattice oxygen participation (sky blue line). The CH₃OOH decomposition mechanism suggests that CH₃OOH or CH₃OO•, which originates from the coupling of the methyl radical (•CH₃) with either the hydroperoxide radical (•OOH) or molecular oxygen (O₂), is the key intermediate for HCHO production. The continuous oxidation pathway proceeds via methanol (CH₃OH) as an intermediate, representing a stepwise one‐electron oxidation sequence. In the lattice oxygen participation pathway, the surface lattice oxygen is deeply involved, yielding intermediates such as CH₃O_L and CH₂O_L. This pathway, analogous to the Mars‐van Krevelen (MvK) mechanism [40], involves the incorporation of lattice oxygen into the product HCHO, followed by the regeneration of surface oxygen vacancies via oxygen reduction. As for the production of HCOOH, the reaction routes can be categorized into continuous oxidation (orange line) and deep dehydrogenation (royal blue line). While the continuous oxidation route takes HCHO as the intermediate, the deep dehydrogenation route posits that CH₄ is deeply dehydrogenated to •CH at the photocatalyst surface, which then couples with ROS to generate HCOOH. Besides these routes, there are some inter‐pathway transformations that allow mixed processes to take place.

FIGURE 1.

Schematic diagram for photocatalytic CH₄ oxidation to C1 carbonyl products.

The reaction routes to C2 carbonyl products are more complicated due to the limited literature and the diversity of reaction systems. Therefore, we will discuss them later in section 4, “Methane to C2 carbonyl products.”

2.2. Toolbox for Reaction Route Elucidation

Given the complexity of the reaction pathways, a brief introduction to the experimental and theoretical toolbox for elucidating them is necessary here.

2.2.1. Basic Characterizations

The ability to absorb light is a key parameter for evaluating the performance of a photocatalyst. Ultraviolet–visible (UV–vis) spectroscopy provides information on the light absorption ability of a material at each specific wavelength. For semiconductor photocatalysts, UV–vis can be used to determine the bandgap. Together with x‐ray photoelectron spectroscopy (XPS), the band structure of a photocatalyst can be established [41]. The positions of the conduction band and valence band determine the reduction and oxidation abilities of photogenerated electrons and holes, respectively. Once the band structure is known, certain reaction pathways can be included or ruled out based on their alignment with the reduction or oxidation potentials of the corresponding half‐reaction [8].

In addition to light absorption, the separation efficiency and recombination kinetics of photocarriers are also crucial for the performance of semiconductor photocatalysis [42]. Photocurrent tests reflect the separation efficiency of a material [43]. However, the method of preparing the photoanode and the electrolyte used in photocurrent tests should be carefully considered, as they influence the results. Photoluminescence spectroscopy (PL) is another method for measuring photocarrier separation efficiency [44]. Nevertheless, when nonradiative recombination is significant, PL results may not accurately reflect the separation efficiency. The recombination kinetics of photocarriers are also critical for the activity of a photocatalyst because the longer the lifetime of photocarriers, the more likely they are to migrate to the surface of the photocatalyst and participate in redox half‐reactions [45]. To probe photocarrier recombination kinetics, transient absorption spectroscopy (TAS) is a powerful tool [46]. By tracking the decay kinetics of transient absorption signals, researchers can evaluate the efficiency of carrier separation and recombination. More importantly, by monitoring distinct transient absorption signals at their characteristic wavelengths, TAS can be used to independently discern the lifetime difference between photogenerated electrons and holes [47]. This capability is instrumental in determining which carrier participates in the rate‐determining step of the surface redox reaction, as the carrier with the shorter effective lifetime typically indicates the slower half‐reaction that governs the overall kinetic bottleneck.

Apart from photocarrier behavior, the behavior of oxygen species also affects the reaction pathways. High‐resolution XPS and electron paramagnetic resonance spectroscopy (EPR) can detect the existence of oxygen vacancies (O_V), which play important roles in many catalytic processes [48]. In addition, O₂ temperature‐programmed desorption (TPD) can profile the strength of O₂ adsorption and the stability of lattice oxygen [26]. As for the O₂ adsorption configuration, low‐temperature Fourier transform infrared spectroscopy (LT‐FTIR) can inspect whether O₂ preferentially adsorbs at the active site in an “end‐on” or “side‐on” manner, which determines its fate after activation [41]. Moreover, the ability of O₂ activation can be examined by linear sweep voltammetry (LSV) [49].

2.2.2. In Situ Spectroscopic Methods

In situ spectroscopies are powerful techniques that enable time‐resolved observation of the photocatalysts under light irradiation. In situ XPS and in situ x‐ray absorption fine structure (XAFS) reveal the transfer direction of photocarriers [41], making them highly suitable for the study of composite materials.

Identifying the generation behaviors of radicals can be achieved using in situ EPR [50]. With appropriate trapping agent [51], such as 5,5‐dimethyl‐1‐pyrroline N‐oxide (DMPO) or 2,2,6,6‐tetramethylpiperidine (TEMP), and solvent, ROS like singlet oxygen (¹O₂), superoxide ion (O₂ ⁻), •OOH, •OH, and O_V are distinguished.

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), attenuated total reflection infrared Fourier (ATR‐IR), and Raman spectroscopy offer opportunities for monitoring the evolution of intermediates [27, 52, 53]. However, the high sensitivity of these characterization methods is a double‐edged sword. Similar to XPS, even trace adventitious carbon may display signals in the IR or Raman spectrum, which is easily mistaken for genuine signals. To rule out adventitious carbon signals, it is recommended to pretreat the materials with in situ irradiation under aerobic conditions. Besides, control experiments using deuterated or ¹³C‐labeled CH₄ should be conducted, and if the signal indeed comes from CH₄ conversion, the peak positions will shift due to the isotopic effects [54].

2.2.3. Control Experiments

Carefully designed control experiments are essential for exploring reaction pathways. Blank control experiments conducted without light, photocatalyst, CH₄, oxidant, or any other possible reactant are necessary to prove that the catalysis occurs as intended. Additionally, control experiments performed in the absence of light and/or photocatalyst but in the presence of Fenton's reagent can reveal the role of •OH in the reaction route [22]. Furthermore, kinetic analysis can be carried out using control experiments. By examining the correlation between each factor and the reaction rate, the reliability of the proposed mechanism can be further validated.

2.2.4. Probing and Trapping Experiments

Probing experiments can uncover the presence of unstable intermediates. For instance, coumarin probes •OH generation by forming an adduct that is inspected by PL spectroscopy [41]. Nitro blue tetrazolium (NBT) may indicate the generation of •OOH when monitored by UV–vis spectroscopy [55]. Besides, in situ generated H₂O₂ is detected by UV–vis spectroscopy using potassium titanium oxalate as a probing reagent [55].

Trapping experiments highlight the importance of each possible reactive species in catalysis. The frequently used scavengers and the corresponding trapped reactive species are outlined in Table 1.

TABLE 1.

Frequently used scavengers in photocatalytic CH₄ conversion systems.

Species	Scavengers
h+	Na2C2O4 [60], Na2S [61], Na2SO3 [62], EDTA‐2Na [24]
e−	AgNO3 [63], K2Cr2O7 [64]
1O2	E160A [61]
O2−	p‐BQ a [61]
•OH	t BuOH [61], SA b [60], i PrOH [24]

^a p‐BQ = para‐benzoquinone.

^b SA = salicylic acid.

2.2.5. Isotopic Labeling Experiments

Isotopic labeling experiments are essential for clarifying the source of atoms in products. The ¹³C isotope experiment helps to exclude the possibility of adventitious carbon transformation. Similarly, the source of oxygen in the products could be traced using ¹⁸O₂ and H₂ ¹⁸O as a source. The labeled products are distinguished using mass spectroscopy (MS) [56]. To discern whether the lattice oxygen of photocatalysts participates in the generation of products, ¹⁸O exchange experiments are the first performed on the photocatalyst, followed by conducting the catalysis in an ¹⁸O‐free environment. If the products are ¹⁸O‐labeled, the lattice oxygen is then considered to contribute to the product formation. Of note, if the catalysis is carried out in aqueous solution, discussing the oxygen source of HCHO is redundant, as HCHO readily hydrates and exchanges oxygen atoms with the solvent H₂O [57].

In addition to tracing the source of atoms, isotopic labelling experiments can also be used to analyze the rate‐determining step. Owing to the kinetic isotope effect (KIE) [58], the reaction would be greatly slowed when using deuterated CH₄ as the reactant if CH₄ activation is the rate‐determining step.

2.2.6. Density Functional Theory Calculations

Density functional theory (DFT) calculations provide further support for the feasibility of reaction pathways [59]. The actual active sites are identified by comparing the adsorption energy (E _a) of the reactant at each possible site. Besides, a comparison analysis of the reaction energy (ΔE) for overoxidation and the desorption energy (–E _a) reveals the propensity for overoxidation to occur. Finally, projected density of states (PDOS) and integrated crystal orbital Hamilton population (ICOHP) analyses provide the molecular‐level insight into the underlying catalytic mechanism.

3. Methane to C1 Carbonyl Products

3.1. Formaldehyde (HCHO)

3.1.1. CH₃OOH Decomposition Pathway

Although the actual oxidation state of C in CH₃OOH is −2, its average oxidation state (0) is identical to that in HCHO. Therefore, the decomposition of CH₃OOH is recognized as a dehydration process. Given that the main route for CH₃OOH formation involves the coupling of •CH₃ and •OOH radicals, the key to controlling this pathway lies in regulating the generation of •OOH radicals, which could be achieved by introducing a single‐site cocatalyst.

The first application of the CH₃OOH decomposition strategy (Figure 2a) was demonstrated by our group using cubic indium oxide‐supported gold single atoms (Au₁‐In₂O₃) [41]. Because the valence band position of cubic In₂O₃ lies above the reduction potential of •OH/H₂O but below that of •CH₃/CH₄, photogenerated holes on In₂O₃ are only capable of activating CH₄ but cannot oxidize H₂O to •OH. Meanwhile, in situ high‐resolution XPS, LT‐FTIR, EPR, and DFT results disclosed that Au single atoms preferentially accepted photogenerated electrons, adsorbed O₂ in an end‐on configuration, and selectively produced •OOH radicals. Consequently, CH₃OOH was efficiently generated and decomposed, giving rise to a remarkable HCHO of 6.09 mmol g⁻¹ with 97.62% selectivity. Subsequently, Zhang et al. constructed single‐copper sites within the metal‐organic frameworks (MOFs) UIO66‐NH₂ (UION) [65]. Through a sequential deprotonation‐metallization‐modification process, the MOFs were decorated with uniformly coordinated ─Cu(OH) single sites. This synthetic approach avoided the formation of randomly distributed low‐activity sites, thereby promoting selective O₂ activation and •OOH formation. The resulting UION‐Cu(OH) photocatalyst exhibited near 100% HCHO selectivity with a space‐time yield (STY) of 2.75 mmol g⁻¹ h⁻¹. More recently, Hou and co‐workers adopted a similar strategy by introducing platinum single atoms onto mesoporous In₂O_{3‐ x} (OH)_y (Pt_SAs‐mIn₂O_{3‐ x} (OH) _y ) [66]. As a derivative of In₂O₃, mIn₂O_{3‐ x} (OH)_y retained the same band structure and carrier redox capability. Notably, it possessed a high density of frustrated Lewis pairs (Lewis‐acidic In(L) and Lewis‐basic In–OH), which facilitated CH₄ activation. The Pt_SAs‐mIn₂O_{3‐ x} (OH) _y catalyst delivered an excellent HCHO yield of 10.18 mmol g⁻¹ h⁻¹ with 84.9% selectivity.

FIGURE 2.

Schematic diagrams for CH₃OOH decomposition (a and b) and continuous oxidation (c and g) pathways to HCHO. [O]: Oxidants.

A variation of the CH₃OOH decomposition pathway proposes that CH₃OO• also serves as a key intermediate (Figure 2b). For example, Li et al. designed a series of control experiments to clarify the role of each ROS in the photocatalytic oxidation of CH₄ over Pt single‐atom‐decorated zinc oxide (Pt₁‐ZnO) [61]. Using p‐benzoquinone (p‐BQ), tert‐butanol (TBA), E160A, and sodium sulfite (Na₂SO₃) as quenchers for O₂ ⁻, •OH, ¹O₂, and photogenerated holes (h⁺), respectively, they found that O₂ ⁻ and h⁺ contributed significantly to CH₄ conversion, •OH showed little influence on HCHO formation, and ¹O₂ had a limited effect on the reaction. Based on these results, the authors proposed a reaction scheme in which •CH₃ reacts with O₂ or O₂ ⁻ to form CH₃OO• or CH₃OOH, which subsequently decomposes to form HCHO. Despite achieving a high HCHO of 2.98 mmol g⁻¹ h⁻¹, the selectivity remained moderate (61.2%) due to insufficient suppression of •OH. Recently, following the CH₃OOH decomposition route, Fan and co‐workers introduced a “dual‐gas co‐adsorption domain” architecture for producing an HCHO solution [67]. Edge‐rich porous ZnO (pZnO) was selected as a platform for light‐harvesting and O₂ enrichment, while the cocatalyst NiO_{1‐ x} theoretically screened from 37 metals and metal oxides promoted CH₄ adsorption and activation. DFT calculations uncovered that co‐adsorbed CH₄ and O₂ were converted into *CH₃OO, which then decomposed to HCHO. This configuration yielded a 3 mM HCHO solution, which could directly feed into an alkaline HCHO fuel cell, generating 0.2 kWh of electricity with simultaneous valuable formate formation.

As a peroxide, CH₃OOH is sensitive to temperature. Tang et al. exploited the instability of CH₃OOH and achieved a record‐high HCHO productivity of 401.5 µmol h⁻¹ through a photon‐phonon‐driven cascade reaction [68]. They observed that the ZnO modified with single ruthenium atoms (0.5Ru‐ZnO) primarily produced CH₃OOH at 30°C, whereas elevating the reaction temperature to 150°C enhanced the CH₄ conversion rate and accelerated the kinetics of CH₃OOH decomposition. A set of mechanistic exploring experiments disclosed that changing the solvent nearly suppressed the generation of both CH₃OOH and HCHO. While introducing h⁺ scavenger only reduced the production of CH₃OOH and HCHO, the addition of the •OH sacrificial agent isopropanol almost completely quenched the reaction. Additionally, ¹⁸O labeling experiments confirmed that the oxygen in HCHO was mainly derived from H₂O rather than O₂. Based on the above results, the authors proposed a CH₃OOH decomposition pathway for HCHO generation, in which CH₃OOH is produced via CH₃OH as an intermediate, and •OH, rather than •OOH, plays a crucial role in the oxidation process.

3.1.2. Continuous Oxidation Pathway

The continuous oxidation pathway to HCHO takes CH₃OH as an intermediate, presenting a distinct challenge for selectivity control. Because CH₃OH is thermodynamically more stable than CH₃OOH, its oxidation might lead to overoxidation. To tackle this challenge, our group developed a quantum‐sized bismuth vanadate (q‐BiVO₄) photocatalyst for CH₄ oxidation [56]. By optimizing the reaction time, wavelength, and light intensity, an HCHO selectivity of 86.7% was achieved. A continuous oxidation pathway (Figure 2c) was suggested because CH₃OH, rather than HCHO, was the main product at shorter reaction times, and almost no CH₃OOH was observed. CH₃OOH was believed to be the intermediate for CH₃OH formation because O₂ was found to be the main oxygen source of CH₃OH. The subsequent CH₃OH oxidation is mediated by •OH. After a dehydrogenation and radical coupling process, hydrated formaldehyde (HOCH₂OH) is formed. Finally, the dehydrogenation of HOCH₂OH yields HCHO. More recently, we conducted crystal‐phase engineering on commercial anatase titanium oxide (TiO₂) and found that the anatase‐to‐rutile phase distribution played a decisive role in determining HCHO production [69]. TiO₂ with 90% anatase phase (90% A‐TiO₂) enabled a high HCHO production of 24.27 mmol g_cat ⁻¹ with 97.4% selectivity, on account of its high phase interface concentration, which facilitated photocarrier separation. Anaerobic control experiment and characterizations demonstrated that lattice oxygen participated in the formation of HCHO. Accordingly, a slightly modified continuous oxidation pathway was proposed, in which the oxygen in CH₃OH comes from lattice oxygen (Figure 2d). To scale up the HCHO production, we designed a gas–liquid–solid pause‐flow microtube reactor, which elevated HCHO production from 0.26 mmol g⁻¹ h⁻¹ L⁻¹ in a batch reactor to 13.94 mmol g⁻¹ h⁻¹ L⁻¹. Khodakov and co‐workers further improved the performance of our pause‐flow reactor [70]. They used hydrofluoric acid (HF) to regulate the junction between anatase {001} and {010} facets. The best catalyst, HF4‐TiO₂, with a {001}/{010} facet ratio of 62:38, exhibited a HCHO productivity of 280 mmol g⁻¹ h⁻¹ L⁻¹ in the pause‐flow microtube reactor. Mechanistic investigations indicated that CH₄ was converted to HCHO via a continuous oxidation process mediated by lattice oxygen.

Surface O_V contributes to the adsorption of reactants and the modulation of the band structure of the photocatalyst [48], serving as an important strategy to suppress the overoxidation of CH₄. Lu and co‐workers engineered an O_V‐rich surface‐disorder layer on TiO₂ and achieved a HCHO production rate of 3.16 mmol g⁻¹ h⁻¹ with 81.2% selectivity [71]. O_V was created by annealing TiO₂ under a H₂ atmosphere. The optimal photocatalyst was obtained after treatment at 600°C (H₂‐600). LSV measurement manifested that O_V increased the efficacy of O₂ activation. EPR trapping experiments indicated that both •OH and •OOH were generated from O₂ reduction, with •OH considered the main contributor to HCHO production due to its strong correlation. Accordingly, the authors proposed a continuous oxidation mechanism for CH₄ conversion (Figure 2e). Tang and co‐workers integrated O_V with a highly dispersed redox cocatalyst Cu single atoms to synergistically improve charge separation efficiency and surface reaction dynamics [72]. Under visible light irradiation, a HCHO yield of 4979 µmol g⁻¹ with nearly 100% selectivity was obtained using defective tungsten loaded with 0.029% Cu (Cu_0.029‐def‐WO₃) as the photocatalyst. EPR measurements under argon atmosphere showed that •OH came from H₂O oxidation. ¹⁸O labeling experiments confirmed that both H₂O and O₂ contributed to HCHO formation. The proposed mechanism is presented in Figure 2f. Later in 2024, Zuo's group reported an efficient Au/ZnO catalyst for HCHO generation from CH₄ [63]. An HCHO production of 19.14 µmol h⁻¹ with 87.94% selectivity was attained using optimized 0.1 wt% Au on ZnO (Au_0.1/ZnO). DFT results indicated that the cocatalyst Au increased the surface concentration of O_V. Moreover, mechanistic investigations uncovered that in situ formed H₂O₂ from O₂ reduction played a major role in •OH generation (O₂ → •OOH → H₂O₂ → •OH). HCHO was ultimately produced via the rapid oxidation of intermediate CH₃OH (Figure 2g).

3.1.3. Lattice Oxygen Participation Pathway

The characteristic feature of the lattice oxygen participation pathway is the intermediate CH₂O_L. The direct interaction between CH₄ and surface oxygen atoms offers unique opportunities for selectivity control by bypassing the formation of CH₃OOH or CH₃OH. In 2021, Han and co‐workers first introduced the lattice‐oxygen participation pathway into the photocatalytic CH₄ oxidation system [73]. A nearly 100% selectivity for HCHO was achieved with Au‐decorated cuboid‐shaped WO₃ (Au/c‐WO₃). The authors conjectured a one‐step transformation mechanism to account for the conversion of CH₄ to HCHO (Figure 3a). It should be noted, however, that this mechanistic speculation has limitations, specifically, the postulated six‐membered ring intermediate is chemically implausible because the radius of C is smaller than that of W. Nevertheless, it offered a possible explanation for the observed high HCHO selectivity. Recently, we further investigated the origin of the selectivity of WO₃ catalysts [74]. WO₃ crystals dominated by {001} and {110} facets (WO₃{001} and WO₃{110}) were chosen as the model catalysts. While WO₃{001} readily exhibited 100% selectivity towards HCHO, WO₃{110} hardly achieved a high HCHO selectivity. A series of in situ characterizations and DFT calculations revealed that WO₃{001} followed a lattice‐oxygen participation pathway (Figure 3b), whereas WO₃{110} experienced a CH₃OOH decomposition pathway.

FIGURE 3.

Schematic diagrams for lattice oxygen participation pathways (a and b) and mixed process (c and d) to HCHO. [O]: Oxidants.

The lattice oxygen participation pathway also exists in gas–solid systems. Wu and co‐workers pioneered a continuous‐flow gas–solid photothermal catalytic system for CH₄ oxidation to HCHO [75]. Silver single atoms were anchored on ZnO (Ag₁‐ZnO) to manipulate the mobility of lattice oxygen, thereby promoting the generation and desorption of the CH₂O_L intermediate. The synergy of heat and light boosted the efficiency of photocarrier separation. Instead of using liquid water as in a conventional three‐phase system, H₂O was introduced in vapor form, enabling the collection of a highly concentrated HCHO solution. After 12 h of adsorption, a concentrated HCHO solution of 514.2 µmol mL⁻¹ was obtained. Inspired by methane monooxygenase, the same group subsequently developed a diatomic iron pair anchored on ZnO photocatalyst (Fe₂‐ZnO) for gas‐solid photothermal production of HCHO [76]. Under optimized conditions, Fe₂‐ZnO exhibited an HCHO productivity of 31.8 mmol g⁻ h⁻¹ with 71.7% selectivity and excellent stability over 50 h (with <10% activity loss).

3.1.4. Mixed Process

Apart from the distinct pathways above, the conversion of CH₄ to HCHO may also occur in a mixed mechanism. However, the selectivity is usually low due to the coexistence of multiple ROS‐driven pathways. For example, in the first work on gas–liquid–solid aerobic photocatalytic CH₄ conversion, the selectivity of HCHO was only 32.8% with 0.1 wt% Au/ZnO [22]. The results of coumarin probing experiments and control experiments with Fenton's reagent suggested that •OH is not the main contributor to CH₄ conversion. The authors raised a mechanism in which CH₄ is activated by photogenerated holes and then couples with •OOH from O₂ reduction (Figure 3c). The transformation from CH₃OOH to HCHO was considered to occur either via direct decomposition or via indirect oxidation of CH₃OH. This mechanism was later adopted by Wei and co‐workers in a photocatalytic system using ZIF‐8 derived Ag‐decorated ZnO (3.0Ag/Z‐450) [51]. They further improved the performance by introducing an O_V‐enriched amorphous surface layer [77]. The optimized Ag_0.5‐Ovs/ZnO‐z achieved 70% HCHO selectivity through a more complex mechanism (Figure 3d). Afterward, Zhang et al. replaced Ag with Au and improved the performance, achieving an HCHO production rate of 8.65 mmol g⁻¹ h⁻¹ with 76.3% selectivity [78].

3.2. Formic Acid (HCOOH)

3.2.1. Continuous Oxidation Pathway

Similar to HCHO, HCOOH can be produced via a continuous oxidation pathway. This pathway typically involves CH₃OH and/or HCHO as intermediates. The further activation of these intermediates and suppression of HCOOH overoxidation are key to obtaining high HCOOH selectivity.

The early examples of photocatalytic CH₄ oxidation to HCOOH used H₂O₂ as the oxidant. In 2019, Hu's group intended to convert CH₄ to CH₃OH by mesoporous graphitic carbon nitride (g‐CN) in the presence of H₂O₂. Unexpectedly, HCOOH emerged with a higher productivity and 39% selectivity under visible light irradiation [24]. The authors proposed a mechanism in which •OH governs the activation of CH₄ and the oxidation of the intermediate CH₃OH to HCOOH (Figure 4a). Because the photogenerated holes of g‐CN are incapable of oxidizing H₂O₂ to •OH, •OH can only be produced via an electron‐mediated pathway, which further limits overoxidation. To suppress the competing O₂ generation that wasted H₂O₂, Wu et al. fabricated a Fe‐based photocatalyst integrating Fe‐N _x sites and Fe/Fe₃C nanoparticles (FeN _x /C‐5‐700) for catalyzing Photo‐Fenton‐like •OH production [79]. The speculated reaction pathway considers CH₃OH and HCHO as intermediates (Figure 4b). FeN _x /C‐5‐700 demonstrated high selectivity (>90%) toward HCOOH production.

FIGURE 4.

Schematic diagrams for continuous oxidation (a–c) and deep dehydrogenation (c and d) pathways to HCOOH. [O]: Oxidants.

It is widely believed that the strongly oxidative •OH is the key species in the continuous oxidation pathway to HCOOH. Although H₂O₂ is a good source of •OH, its instability and high cost make it an unfavorable oxidant for practical industrial production [62]. Consequently, increasing attention has been paid to replacing H₂O₂ with O₂ in the photo‐oxidation of CH₄. In 2022, Zhong's group doped TiO₂ with Cu and W (Cu‐W‐TiO₂) and gained a mixture of liquid oxygenates from CH₄ photooxidation [80]. Compared with pristine TiO₂, Cu‐W‐TiO₂ exhibited a stronger tendency to reduce O₂ to •OH under illumination, owing to its downshift of the conduction band. HCOOH emerged as the second‐most abundant product, with 33.6% selectivity. The low selectivity was attributed to the uncontrolled continuous oxidation pathway (Figure 4c). In subsequent work, TiO₂ was combined with SrWO₄ to form an I‐type heterojunction, and the HCOOH selectivity increased to 48.3% [81]. To boost •OH generation, Xiong and co‐workers adjusted the pH of the liquid phase by adding H₂SO₄ [64]. Using platinum‐nanoparticle‐decorated WO₃ (Pt_NPs‐WO₃) as the photocatalyst, an HCOOH productivity of 17.7 mmol g⁻¹ and a selectivity of 70% were achieved.

To increase HCOOH selectivity, Khodakov and co‐workers designed a photo‐thermocatalytic cascade process [62]. The authors loaded a cesium salt of phosphotungstic acid onto TiO₂ (CsPW‐TiO₂) to form a heterojunction, thereby enhancing the production of C1 oxygenates. The thermocatalyst Ru/Al₂O₃ was introduced into the system through a careful designed one‐pot reactor, where CsPW‐TiO₂ and Ru/Al₂O₃ were separated by a sand filter. This integration system selectively converted C1 oxygenates to HCOOH, achieving 85% selectivity.

Besides •OH, the continuous oxidation pathway can also be driven by surface‐bounded oxygen (*O). Our group conducted DFT calculations to obtain a priori screening of noble metals (Pd, Pt) loaded metal oxides (TiO₂, ZnO, WO₃) for HCOOH production [82]. We found that when Pd serves as the cocatalyst of WO₃, HCOOH production is more favorable because the overoxidation barrier is higher than the desorption barrier. For Pd, the oxidizing ability of *O is stronger than that of surface‐bounded •OH (*OH). Therefore, *O is more likely to be the major contributor to the continuous oxidation pathway. The optimal photocatalyst, Pd_0.25/WO₃, exhibited an HCOOH selectivity of 62%.

Although W‐based materials are common photocatalysts for the aerobic conversion of CH₄ to HCOOH, there are still some examples using other materials. For example, Li et al. reported that GaN delivered a HCOOH selectivity of 79.74% in the presence of H₂O and O₂ [55]. Moreover, silicon‐supported Au nanoparticles (Au NPs/SiO₂) could drive CH₄ oxidation to HCOOH with a selectivity of 98% under the irradiation of 3.6 eV photons [25].

3.2.2. Deep Dehydrogenation Pathway

The deep dehydrogenation pathway was proposed by Hu and co‐workers in 2021 [83]. Briefly, this mechanism proposes that CH₄ is deeply dehydrogenated to *CH by photogenerated holes on the catalyst surface, and the subsequent reaction of *CH with ROS gives birth to HCOOH (Figure 4d). The designed catalyst, silicomolybdic‐acid‐loaded TiO₂ (HSiMo‐TiO₂), could convert CH₄ into a range of liquid oxygenates (CH₃OH, HCHO, HCOOH, CH₃CHO), but the selectivity towards HCOOH was low (38%). The authors then modified the cocatalyst to another Keggin‐type polyoxometalate (SiW₁₂O_x/TiO₂) and got a slightly improved HCOOH selectivity of 47% [84].

4. Methane to C2 Carbonyl Products

4.1. Acetaldehyde (CH₃CHO)

The formation of CH₃CHO from CH₄ requires C─C bond formation via coupling of C1 intermediates, which presents even greater challenges than the formation of C1 products. Consequently, only a few reports have observed CH₃CHO production, and its yield and selectivity were generally low (< 50%). Furthermore, CH₃CHO is typically generated under anaerobic conditions.

Sun and co‐workers treated ceria nanoparticles at high temperature to create O_V [85]. While the production rate of ethanol (C₂H₅OH) increased with the calcination temperature, the yield of CH₃CHO remained almost constant (∼1 µmol g⁻¹ h⁻¹). The highest selectivity (49.8%) toward CH₃CHO was obtained with ceria treated at 500°C (CeO₂‐500). For C₂H₅OH production, the proposed reaction pathway involves CH₄ coupling to C₂H₆, followed by C₂H₆ activation and hydroxylation. CH₃CHO likely arises from the subsequent oxidation of C₂H₅OH (Figure 5a).

FIGURE 5.

Schematic diagrams for photocatalytic CH₄ oxidation to CH₃CHO (a and b) and CH₃COOH (c and d). [O]: Oxidants.

TiO₂ was modified with reduced graphene oxide (RGO/TiO₂) to enable the photocatalytic conversion of CH₄ to C2+ oxygenates [86]. The optimized catalyst achieved a CH₃CHO production rate of 289.6 µmol g⁻¹ h⁻¹ with 48% selectivity in a humid CH₄ atmosphere. In addition, other products such as CO, CH₃OH, and acetone were also detected. The authors proposed a CO‐coupling mechanism to account for the formation of CH₃CHO.

CH₃CHO was also produced via the coupling of CH₄ and CO₂. Liu et al. loaded Ru nanoparticles onto a Zn‐doped g‐C₃N₄ photocatalyst (0.5% Ru/Zn‐g‐C₃N₄) and attained this transformation [87]. The proposed reaction pathway begins with the reduction of CO₂ to CO on Ru nanoparticles, followed by CO coupling with •CH₃ to yield CH₃CHO (Figure 5b). Because photogenerated electrons can further reduce CH₃CHO, the selectivity towards CH₃CHO is only 20.7%.

The limited number of works reporting CH₃CHO formation likely stems from inherent mechanistic constraints. The formation of CH₃CHO requires both the C–C coupling and the carbonyl formation. When C–C coupling occurs prior to carbonyl formation, with C₂H₅OH as the key intermediate, the subsequent carbonyl formation step involves oxidation, which is incompatible with the anaerobic environment. If carbonyl formation is before or simultaneous to C─C coupling, CO is an important intermediate. Nevertheless, existing studies that generate CO from CH₄ or CO₂ exhibit low production efficiency. Directly employing CO as the reactant offers a promising strategy to boost CH₃CHO yield, though product selectivity must be carefully considered, as other carbonyl byproducts such as CH₃COOH and acetone may also be formed concurrently.

4.2. Acetic Acid (CH₃COOH)

4.2.1. Methane as Carbon Source

Direct oxidation of CH₄ to CH₃COOH using only CH₄ as the carbon source is a straightforward approach, but it poses significant challenges in achieving both C─C bond formation and controlled oxidation. Wang et al. synthesized a composite photocatalyst (Ag/AgCl‐WO_{3‐ x} ) and obtained this transformation with a production rate of 188.5 µmol g⁻¹ h⁻¹ and a selectivity of 62.7% under an atmosphere of CH₄ and O₂ [88]. The authors found that introducing CO₂ into the reaction system increased the productivity rate to 290.5 µmol g⁻¹ h⁻¹, suggesting that CO₂ was a crucial intermediate in CH₃COOH formation. In the proposed mechanism (Figure 5c), the Ag/AgCl cocatalyst enhances the generation of reactive species such as •OH and •Cl, thereby accelerating CH₄ activation. A portion of the generated •CH₃ is oxidized to CO₂ by •OH. The CO₂ is then captured by O_V and coupled with •CH₃ to yield CH₃COOH.

Lou, Cao, and co‐workers elevated CH₃COOH production to 3006 µmol g⁻¹ h⁻¹ with 86.8% selectivity by constructing a ZSM‐5 bearing Fe‐(μ‐O)‐Zn dual‐atom sites [89]. They investigated the reaction pathway (Figure 5d) using DFT calculations. It was proposed that the Fe‐(μ‐O)‐Zn sites dissociate O₂ under light irradiation to form Fe═O species. The highly reactive Fe═O then activates CH₄, and the resulting HCHO intermediate migrates to the Zn site. After the activation of a second CH₄ molecule, *HCHO couples with •CH₃ to yield *OCH₂CH₃. Finally, dehydrogenation and hydroxylation of *OCH₂CH₃ give birth to CH₃COOH.

Xiong et al. improved the selectivity of CH₃COOH to 91.6% using a carefully designed chemical looping process [39]. A PdO/Pd heterointerface was established on a WO₃ photocatalyst to provide active sites for CH₄ activation and C─C coupling. In situ DRIFTS revealed the existence of a *CO intermediate. Isotope labeling experiments demonstrated that PdO underwent the Mar–van Krevelen process, supplying oxygen atoms to *CO. The coupling of •CH₃, *CO, and •OH then delivered CH₃COOH. The authors subsequently designed a photochemical flow reaction device that successfully sustained CH₃COOH production for 45 h.

4.2.2. Methane Coupling With CO

Co‐feeding of CO with CH₄ provides an alternative route to CH₃COOH, potentially enabling better selectivity control and higher efficiency by introducing a second carbon source.

Ordomsky, Khodakov, and co‐workers reported the first example [60]. TiO₂ was decorated with ammonium phosphotungstic polyoxometalate anchoring Pt single atoms ((Pt/NPW)/TiO₂), enabling stable production of CH₃COOH for 60 h with 66% selectivity. The reaction was carried out under anaerobic conditions, and hydrogen gas (H₂) was detected. After the reaction, the color of the photocatalyst changed to dark blue, which is the color of reduced NPW. This observation suggested that the lattice oxygen of NPW might participate in CH₃COOH synthesis. Based on these findings, a mechanism involving the depletion of lattice oxygen was proposed (Figure 5e).

The coupling of CH₄ and CO in the absence of an external oxidant is highly atom‐economical, yet the endothermic nature of this reaction hinders efficient production. Cheng et al. used H₂O₂ as the oxidant and reported a higher CH₃COOH productivity (52.7 µmol g⁻¹ h⁻¹) with Pd nanoparticles and O_V co‐decorated on WO₃ nanosheets (Pd/V_O‐WO₃) [90]. In this system, the oxygen in the hydroxyl group no longer originates from lattice oxygen of the catalyst but from the •OH produced via H₂O₂ reduction or H₂O oxidation (Figure 5f). Mechanistic studies revealed that Ov served as an active site for CH₄ adsorption, while Pd nanoparticles facilitated H₂O₂ activation.

Wang and co‐workers enhanced CH₃COOH synthesis using a rhodium (Rh) single‐atom catalyst at an elevated temperature (150°C) and under an aerobic atmosphere [91]. The well‐defined framework structure of porphyrin‐based MOFs afforded a Rh loading of up to 5.04 wt%, offering abundant active sites for CH₃COOH formation. A benchmark performance of 23.62 mmol g⁻¹ h⁻¹ was achieved with this architecture.

Homogeneous catalysts also contributed to the synthesis of CH₃COOH. Recently, Zuo and co‐workers investigated the performance of iron‐based homogeneous catalysts in direct CH₄ carbonylation [92]. The optimal catalyst yielded 1028 µmol of C2 products (CH₃COOH and CH₃CO₂CH(CF₃)₂) over an 8‐h test, with an exceptional C2/C1 selectivity of 26:1 and a turnover number (TON) of up to 10680.

5. Conclusions and Perspectives

In this review, we summarize recent advances in photocatalytic CH₄ oxidation to carbonyl products with a focus on elucidating the reaction pathways leading to HCHO, HCOOH, CH₃CHO, and CH₃COOH. Although this field has witnessed remarkable progress (Table 2), significant challenges remain for practical implementation. Therefore, we offer our perspectives on future directions, hoping to inspire the translation of laboratory discoveries into industrial applications.

TABLE 2.

Performance comparison among the state‐of‐the‐art works on photocatalytic CH₄ oxidation to carbonyl products.

Ref.	Year	Catalyst	p(O2) /bar	p(CH4) /bar	p(CO) /bar	p(CO2) /bar	p(total) /bar	Flow Rate /mL min−1	RH /%	Light Source	λ /nm	LPD /mW cm−2	T /°C	m cat /mg	t /h	V solution /mL	Solution	Product	STY /µmol g−1 h−1	SY /µmol g−1	Rate /µmol h−1	Yield /µmol	c /mM	Sel. /%
[41]	2023	Au1/In2O3	10	20	0	0	30	0	N/A	300 W Xe lamp	300–1100 nm	250	25	10	3	180	H2O	HCHO	2.03E+03	6.09E+03	2.03E+01	6.09E+01	3.38E‐01	97.6
[65]	2024	UION‐Cu(OH)	1	9	0	0	10	0	N/A	300 W Xe lamp	300–1100 nm	N/A	20	5	10	10	H2O	HCHO	2.75E+03	2.75E+04	1.38E+01	1.38E+02	1.38E+01	100
[68]	2024	0.5Ru‐ZnO	0.75	20	0	0	20.75	0	N/A	LED	365 nm	75	150	10	1	180	H2O	HCHO	4.02E+04	4.02E+04	4.02E+02	4.02E+02	2.23E+00	90.4
[61]	2025	Pt1/ZnO	1	19	0	0	20	0	N/A	300 W Xe lamp	N/A	N/A	25	10	4	75	H2O	HCHO	2.98E+03	1.19E+04	2.98E+01	1.19E+02	1.59E+00	61.2
[67]	2026	NiO1‐x/pZnO	1	19	0	0	20	0	N/A	300 W Xe lamp	300–780 nm	N/A	25	10	10	100	H2O	HCHO	2.85E+03	2.85E+04	2.85E+01	2.85E+02	2.85E+00	88.5
[66]	2026	PtSAs‐mIn2O3‐x(OH)y	1	29	0	0	30	0	N/A	LED	365 nm	30	25	10	0.67	25	H2O	HCHO	1.02E+04	6.79E+03	1.02E+02	6.79E+01	2.72E+00	84.9
[56]	2021	q‐BiVO4	10	10	0	0	20	0	N/A	Hg lamp	300‐400 nm	170	25	10	7	10	H2O	HCHO	1.87E+03	1.31E+04	1.87E+01	1.31E+02	1.31E+01	86.7
[69]	2022	90% A‐TiO2	5	20	0	0	25	0	N/A	300 W Xe lamp	300‐1100 nm	130	25	2	3	180	H2O	HCHO	8.09E+03	2.43E+04	1.62E+01	4.85E+01	2.70E‐01	97.4
[70]	2026	HF4‐TiO2	1	20	0	0	21	0	N/A	LED	365 nm	800	25	1	2	50	H2O	HCHO	6.86E+03	1.37E+04	6.86E+00	1.37E+01	2.74E‐01	80
[71]	2022	Ov‐rich TiO2 (H2‐600)	1	19	0	0	20	0	N/A	300 W Xe lamp	300–450 nm	50	25	5	1	10	H2O	HCHO	3.16E+03	3.16E+03	1.58E+01	1.58E+01	1.58E+00	81.2
[72]	2023	Cu0.029‐def‐WO3	1	19	0	0	20	0	N/A	LED	420 nm	N/A	25	5	2	120	H2O	HCHO	2.49E+03	4.98E+03	1.24E+01	2.49E+01	2.07E‐01	100
[63]	2024	Au0.1/ZnO	6	24	0	0	30	0	N/A	300 W Xe lamp	350–780 nm	N/A	25	10	1	30	H2O	HCHO	1.91E+03	1.91E+03	1.91E+01	1.91E+01	6.38E‐01	87.9
[23]	2024	0.28% N‐TiO2 (A0.75/R0.25)	1	19	0	0	20	0	N/A	LED	365nm	N/A	25	2	2	100	H2O	HCHO	2.35E+04	4.69E+04	4.69E+01	9.39E+01	9.39E‐01	90.2
[102]	2021	cWO3	7	13	0	0	20	0	N/A	Xe lamp	N/A	150	25	10	30	150	H2O	HCHO	1.54E+02	4.61E+03	1.54E+00	4.61E+01	3.07E‐01	99.4
[73]	2021	Au0.3/c‐WO3	1	20	0	0	21	0	N/A	300 W Xe lamp	300–780 nm	100	25	2	24	150	H2O	HCHO	3.00E+02	7.20E+03	6.00E‐01	1.44E+01	9.60E‐02	100
[74]	2024	WO3{001}	7	13	0	0	20	0	N/A	Xe lamp	N/A	150	25	10	3	150	H2O	HCHO	7.58E+01	2.27E+02	7.58E‐01	2.27E+00	1.52E‐02	100
			7	13	0	0	20	0	N/A	Xe lamp	N/A	150	50	10	3	150	H2O	HCHO	1.65E+02	4.96E+02	1.65E+00	4.96E+00	3.31E‐02	73.7
[75]	2025	Ag1‐ZnO	0.01	0.9	0	0	1	50	20	300 W Xe lamp	200–1000 nm	1000	228	4	12	50	H2O	HCHO	2.95E+04	3.53E+05	1.18E+02	1.41E+03	2.83E+01	71.2
			0.01	0.9	0	0	1	50	20	300 W Xe lamp	200‐1000 nm	1000	228	4	12	2	H2O	HCHO	2.14E+04	2.57E+05	8.57E+01	1.03E+03	5.14E+02	N/A
[76]	2025	Fe2‐ZnO	0.96	0.008	0	0	1	50	40	300 W Xe lamp	200–1000 nm	1000	221	4	50	2	H2O	HCHO	3.18E+04	1.59E+06	1.27E+02	6.36E+03	3.18E+03	71.7
			0.96	0.008	0	0	1	50	40	300 W Xe lamp	200–1000 nm	1000	221	4	50	2	H2O	HCHO	3.18E+04	1.59E+06	2.32E+01	1.16E+03	5.80E+02	N/A
[22]	2019	0.1 wt% Au/ZnO	1	20	0	0	21	0	N/A	300 W Xe lamp	300–500 nm	100	25	10	2	100	H2O	HCHO	8.63E+01	1.73E+02	8.63E‐01	1.73E+00	1.73E‐02	32.8
[51]	2024	3.0Ag/Z‐450	1	29	0	0	30	0	N/A	300 W Xe lamp	N/A	450	25	2	8	75	H2O	HCHO	4.33E+03	3.47E+04	8.66E+00	6.93E+01	9.24E‐01	57.7
[78]	2025	0.2Au‐ZnO	2	18	0	0	20	0	N/A	300 W Xe lamp	N/A	N/A	25	5	1	50	H2O	HCHO	8.65E+03	8.65E+03	4.33E+01	4.33E+01	8.65E‐01	76.3
[77]	2025	Ag0.5‐OVs/ZnO‐z	1	29	0	0	30	0	N/A	300 W Xe lamp	N/A	450	25	5	8	75	H2O	HCHO	2.29E+03	1.83E+04	1.14E+01	9.16E+01	1.22E+00	70
[24]	2020	g‐CN	0	30	0	0	30	0	N/A	300 W Xe lamp	>420 nm	N/A	35	15	36	25	50 mM H2O2	HCOOH	3.06E+01	1.10E+03	4.58E‐01	1.65E+01	6.60E‐01	39
[79]	2020	Fe/Fe3C	0	1	0	0	1	0	N/A	Xe lamp	N/A	150	25	8	4	4	100 mM H2O2	HCOOH	1.16E+03	4.66E+03	9.32E+00	3.73E+01	9.32E+00	>90
[80]	2022	Cu‐W‐TiO2	6	20	0	0	26	0	N/A	300 W Xe lamp	350–760 nm	200	30	10	2	50	H2O	HCOOH	5.00E+03	1.00E+04	5.00E+01	1.00E+02	2.00E+00	33.6
[81]	2023	SrWO4/TiO2	2	30	0	0	32	0	N/A	300 W Xe lamp	300–420 nm	N/A	25	10	2	50	H2O	HCOOH	3.03E+03	6.06E+03	3.03E+01	6.06E+01	1.21E+00	48.3
[64]	2025	PtNPs‐WO3	1	19	0	0	20	0	N/A	LED	365 nm	60	26	10	12	120	450 mM H2SO4	HCOOH	1.48E+03	1.77E+04	1.48E+01	1.77E+02	1.48E+00	70
[62]	2023	Cs3PW12O40(CsPW)‐TiO2 + Ru/Al2O3	0.2	6	0	0	7	0	N/A	400 W Hg‐Xe lamp	N/A	N/A	20	10	2	100	H2O	HCOOH	2.53E+03	5.06E+03	2.53E+01	5.06E+01	5.06E‐01	85
[82]	2024	Pd0.25/WO3	10	20	0	0	30	0	N/A	300 W Xe lamp	300–780 nm	140	25	10	5	180	H2O	HCOOH	9.34E+02	4.67E+03	9.34E+00	4.67E+01	2.59E‐01	62
[55]	2023	GaN	0.73	0.27	0	0	1	0	N/A	300 W Xe lamp	N/A	N/A	25	30	20	1	H2O	HCOOH	8.01E‐01	1.60E+01	2.40E‐02	4.80E‐01	4.80E‐01	79.7
[25]	2024	Au NPs/SiO2	0.023	0.042	0	0	1	0	N/A	300 W Xe arc lamp	344 nm	1000	25	100	8	0	N/A	HCOOH	1.15E+04	9.20E+04	1.15E+03	9.20E+03	N/A	98
[83]	2021	HSiMo‐TiO2	N/A	N/A	0	0	N/A	0	N/A	300 W Xe lamp	AM 1.5G	N/A	150	20	2	20	H2O	HCOOH	6.80E+02	1.36E+03	1.36E+01	2.72E+01	1.36E+00	38
[84]	2022	SiW12Ox/TiO2	N/A	N/A	0	0	N/A	0	N/A	300 W Xe lamp	AM 1.5G	100	140	15	2	20	H2O	HCOOH	3.06E+03	6.12E+03	4.59E+01	9.18E+01	4.59E+00	47
[85]	2020	CeO2‐500	0	0.2	0	0	0.2	0	N/A	300 W Xe lamp	AM 1.5G	100	25	2	2	15	H2O	CH3CHO	9.00E‐01	1.80E+00	1.80E‐03	3.60E‐03	2.40E‐04	49.8
[86]	2022	RGO/TiO2	0	0.1	0	0	1	4	100	300 W Xe lamp	AM 1.5	100	60	1	4	0	N/A	CH3CHO	2.90E+02	1.16E+03	2.90E‐01	1.16E+00	N/A	48
[87]	2019	0.5% Ru/Zn‐g‐C3N4‐1/20	0	0.0375	0	0.0375	0.5	0	N/A	150 W Xe lamp	N/A	N/A	N/A	100	3	0	N/A	CH3CHO	1.10E+02	3.30E+02	1.10E+01	3.30E+01	N/A	20.7
[88]	2023	Ag/AgCl‐WO3‐x	0.5	0.1	0	0	1	0	N/A	500 W Xe lamp	N/A	N/A	25	20	1	20	H2O	CH3COOH	1.89E+02	1.89E+02	3.77E+00	3.77E+00	1.89E‐01	62.7
[89]	2025	Fe1‐Zn1/ZSM‐5	0.333	0.667	0	0	1	75	N/A	500 W Hg lamp	220–1100 nm	N/A	25	20	20	100	H2O	CH3COOH	3.01E+03	6.01E+04	6.01E+01	1.20E+03	1.20E+01	86.8
[39]	2023	PdO/Pd‐WO3	0	1	0	0	1	0	N/A	300 W Xe lamp	N/A	200	25	10	1	10	H2O	CH3COOH	9.07E+01	9.07E+01	9.07E‐01	9.07E‐01	9.07E‐02	91.6
[60]	2022	(Pt/NPW)/TiO2	0	10	1	0	11	0	N/A	400 W Hg‐Xe lamp	N/A	N/A	20	50	60	10	H2O	CH3COOH	1.94E+01	1.17E+03	9.72E‐01	5.83E+01	5.83E+00	66
[90]	2025	Pd/VO‐WO3	0	0.5	0.5	0	1	0	N/A	300 W Xe lamp	N/A	N/A	25	15	3	15	50 mM H2O2	CH3COOH	5.27E+01	1.58E+02	7.91E‐01	2.37E+00	1.58E‐01	63.7
[91]	2023	Rh1/pMOF	4	15	5	0	24	0	N/A	Xe lamp	AM 1.5G	N/A	150	20	3	20	H2O	CH3COOH	2.36E+04	7.09E+04	4.72E+02	1.42E+03	7.09E+01	66.4
[92]	2025	Fe(terpy)Cl3	0.2	2	5	0	8	0	N/A	LED	400 nm	N/A	25	N/A	8	10	HFIP	CH3COOH	N/A	N/A	3.01E+01	2.41E+02	2.41E+01	67.9

Ref.: reference; p: partial pressure; RH: relative humidity; λ: wavelength; LPD: light power density; T: temperature; m _cat: mass of catalyst; t: reaction time; V _solution: volume of solution; STY: space time yield; SY: space yield; c: concentration of solution; Sel.: selectivity; N/A: not applicable.

First of all, continuous efforts should be made to improve photocatalyst performance. Despite a few excellent works [23, 25, 66, 68, 75, 76, 91] reporting space‐time yield (STY) on the order of 10⁴ µmol g⁻¹ h⁻¹, the catalyst loading is still on the mg scale. In practical industrial production, much larger amounts of catalyst are required, which would significantly lower the apparent STY. Moreover, the stability of the photocatalyst must be improved. The current record for CH₄ photooxidation to carbonyl products (60 h) [60] is far short of the requirements of the chemical industry, which typically demands catalyst lifetimes on the order of years. To improve the intrinsic activity and stability of photocatalysts, designing materials with proper band structure, excellent light‐harvesting efficiency, enriched active sites, and high dynamic lattice stability is a promising direction. Strategies such as heterojunction engineering [93], single‐atom modification [94], and defect engineering [95] can be considered during catalyst design.

In addition to enhancing the intrinsic performance of photocatalysts, it is worthwhile to rationally design the reaction systems. The decline in catalyst performance during scale‐up and the poor stability are mainly related to insufficient optimization of transfer processes within the reactor [96]. Simply enlarging the reactor size without rationally considering these transfer processes inevitably leads to inhomogeneity in substrate concentrations, pressure, and temperature distributions throughout the reaction system, which constitutes the primary cause of catalyst performance degradation. For batch reactors, the stirring speed profoundly influences external mass transfer coefficients and boundary layer thickness, while the geometric parameters of the reactor, such as the aspect ratio, headspace volume, and illumination window area, determine the efficiency of light utilization and the uniformity of photon flux density. Compared to batch reactors, flow reactors are better suited for industrial production. In continuous‐flow reactors, parameters such as flow rate, flow direction, channel geometry, and illumination methods all have a decisive impact on the photocatalyst performance. Specifically, the flow rate determines the residence time and interfacial mass transfer coefficient; the reactor geometry determines the light penetration depth and available catalytic surface area; and the illumination strategy determines the uniformity of the photon flux density. Poorly designed flow patterns may lead to axial dispersion, channeling effects, or dead zones, resulting in a widened residence time. This can cause incomplete conversion or over‐oxidation, thereby simultaneously compromising yield, selectivity, and the long‐term stability of the catalyst. Regarding the reaction temperature, although room‐temperature catalysis appears attractive, fluctuations in ambient temperature are inevitable and unacceptable for industrial synthesis. A slightly elevated temperature (e.g., < 200°C) is, in our opinion, more appropriate, as it not only enables stable production but also enhances the reaction kinetics.

The light source is a pivotal factor in photocatalysis. Sunlight is undoubtedly a green light source, but it is intermittent and suffers from low light power density (LPD). While the intermittency is unavoidable, the LPD can be increased by using a concentrator such as a Fresnel lens. In contrast, light emission diode (LED) provides a stable light source with tunable LPD, making them highly suitable for industrial production.

One important step in industrial production that is usually overlooked in photocatalytic CH₄ conversion is the separation and purification of products. These processes consume a large amount of energy [97] and must be carefully considered before translating laboratory discoveries into practice. When evaluating the industrialization prospects of a photocatalytic system, a fixed‐bed‐like gas–solid production configuration is preferable to gas–liquid–solid triple‐phase systems, because obtaining a sufficiently concentrated solution is challenging in the latter.

Last but not least, cascade reactions provide opportunities for fixing CH₄ into higher‐value chemicals. For example, the diluted solution obtained from gas–liquid–solid systems could directly serve as the electrolyte for a fuel cell [67] or as the feed for biosynthesis [98]. Beyond independent methane oxidation, combining photocatalytic conversion with downstream organic synthesis through tandem reactions represents a highly attractive strategy for maximizing carbon utilization and energy efficiency. Organic conversion reactions that use aldehydes or carboxylic acids directly as feedstocks, such as reductive amination [99], Mannich reaction [100], and Stetter reaction [101], are all highly promising candidates for tandem reactions. From the perspective of tandem processes, the ideal design would allow these reactions to directly utilize the products from the photocatalytic step, eliminating the need for energy‐intensive separation and purification. This not only circumvents the separation bottlenecks currently hindering industrial feasibility but also significantly improves overall economic efficiency. Therefore, future research should focus on developing compatible catalyst systems and reaction conditions to enable photocatalytic methane oxidation and subsequent C─N or C─C coupling reactions in a tandem reactor.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

Data sharing not applicable to this article as no datasets were generated or analysed during the current study.