Background Introduction
Methane, as the main component of natural gas, is not only a clean energy source but also an essential raw material for synthesizing various high-value-added chemicals. However, due to the stability of its molecular structure, the chemical conversion of methane typically requires high-temperature and high-pressure conditions, which not only consumes a large amount of energy but may also lead to environmental issues. To address this challenge, photocatalytic technology has emerged, utilizing solar energy to activate methane under mild conditions, providing a sustainable solution for chemical synthesis. Although this field holds great potential, it still faces numerous technical challenges.
In traditional methane conversion processes, such as steam methane reforming, although large-scale industrial application of methane has been achieved, this process not only has high energy consumption but also generates a large amount of carbon dioxide emissions. Therefore, exploring methods for the direct conversion of methane under mild conditions is of great significance for improving energy efficiency and reducing environmental pollution.[1-3]
In recent years, photocatalytic technology has attracted widespread attention due to its ability to activate methane under mild conditions. Photocatalytic methane conversion uses light energy to excite the catalyst, generating high-energy electron-hole pairs. These high-energy electron-hole pairs can break the chemical inertness of methane molecules, achieving efficient conversion of methane at lower temperatures and pressures.[4-6] This process can not only reduce energy consumption but also reduce greenhouse gas emissions, playing a strategic role in promoting green chemical synthesis.
Methane is known for its high chemical stability, which stems from its symmetrical tetrahedral geometry, where four equivalent C-H bonds impart high bond energy, low electron and proton affinity, and low polarizability (see Figure 1a).[7,8] Additionally, the large energy gap between methane’s highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) increases its difficulty to gain or lose electrons at the molecular level.[1,9] This inherent chemical inertness means that in thermal catalytic conversion processes, overcoming a high activation energy barrier (Ea) is required, usually at temperatures ranging from 700 to 1000 degrees Celsius to achieve effective conversion. Despite persistent efforts by scientists over the years to develop new catalysts to improve the efficiency of methane conversion, high energy consumption and carbon emissions remain issues in thermal catalytic processes. Currently, the only large-scale application for methane conversion is its indirect conversion to syngas through steam reforming, further producing olefins, methanol, and liquid hydrocarbons. Efficiently converting methane into a range of carbon-based chemicals, including alcohols, aromatics, long-chain alkanes, and olefins, has long been a goal in the field of catalytic chemistry.[10]
Heterogeneous photocatalysts involve processes occurring at the catalyst-medium interface, including the adsorption and activation of reactants, the formation of intermediates, and the desorption of products, following electron-hole separation and transfer.[11-13] The complex reaction process at the catalytic interface presents challenges for the precise regulation of the overall efficiency of catalytic reactions.[14,15]Methane activation is typically considered the rate-determining step in methane conversion reactions.[16,17]Photocatalytic methane activation based on heterogeneous catalysts can be categorized into two types:
1) Direct activation, where methane is directly adsorbed and activated on the photocatalyst surface under light conditions (Figure 1d);
2) Indirect activation, where methane is activated with the help of photogenerated active radicals from other reactants such as water and oxygen molecules (Figure 1e).
Although there are differences between these two pathways for methane activation, photogenerated active oxygen, such as surface active sites (O⁻ on oxides) and active radicals (•OH and •O²⁻), plays a key role in both pathways described above.
Figure 1 a) Molecular structure of methane. b) Energy changes in ground-state and excited-state reactions. c) Basic principles of photocatalytic reactions. d) Direct activation of methane on the photocatalyst surface. e) Indirect activation of methane by photo-induced active radicals.
For the direct activation pathway, modification strategies to optimize photocatalytic methane activation typically include acid/base site modification and crystal defect engineering. [18-20] Basic sites on metal oxides are considered critical in methane activation as they exhibit relatively strong electronic interactions with weakly acidic methane.[21-23] Methane adsorbed on metal oxides forms negatively charged groups coordinated with metal cations and positively charged groups coordinated with the basic lattice oxygen of the metal oxide, resulting in polarization of the C–H bond on the metal oxide surface (Figure 2a). Depending on the basicity of the metal oxide, these two parts will exhibit Lewis acidic (positive) or Lewis basic (negative) characteristics.[24] Additionally, defects in nanocatalysts disrupt the translational symmetry of the crystal cell, including three-dimensional volume defects (such as pores), two-dimensional planar defects (such as grain boundaries), one-dimensional line defects (dislocations), and zero-dimensional point defects (such as vacancies). These defects directly influence the coordination structure of metal oxides and their corresponding catalytic properties.[25] For example, Pd-modified ZnO-Au composites have a unique interfacial defect structure that facilitates the dissociation of CH₄ molecules into methoxy and methyl, promoting photocatalytic methane-to-ethylene conversion (Figure 2b).[26]
In the indirect pathway, hydrogen atom transfer is an effective strategy for activating the C–H bond in methane, involving the transfer of a hydrogen atom through the separation of protons and electrons in free radical reactions (Figure 2c).[27,28] Photocatalytic hydrogen atom transfer uses photogenerated radical intermediates to activate C–H bonds.[29] In the gas phase, exposed (CuO)⁺ cations can interact with oxygen-centered radicals to facilitate hydrogen atom transfer and methane activation.[30] In situ photocatalytic mass spectrometry based on synchrotron radiation has been reported to capture gas-phase active intermediates during the photocatalytic oxidation of methane.[31] Besides detecting stable substances like CO₂, H₂O, C₂H₆, and CH₃OH, it can also detect active methyl radicals (•CH₃) and methoxy radicals (CH₃O•) (Figure 2d). Furthermore, mass spectrometry combined with quantum chemical calculations reveals that (AuO)⁺ selectively transfers oxygen atoms from (AuO)⁺ rather than extracting hydrogen atoms from methane to activate it into methanol, whereas the latter case was observed in homologous compounds (CuO)⁺ and (AgO)⁺.
Figure 2. a) Adsorption and C–H bond polarization of methane on the surface of basic metal oxides and schematic of different positions on the MgO surface.[21,22] b) In the case of ZnO-AuPd mixed catalyst, photocatalytic conversion of CH₄ to C₂H₄ via surface alkoxy intermediates. [26] c) Schematic of methane activation through hydrogen atom transfer.[30] d) Detection of reaction intermediates during photocatalytic methane oxidation via in situ synchrotron radiation photoionization mass spectrometry.[31]
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Oxidative Dehydrogenation of Methane is another widely studied indirect pathway for activating and converting methane under mild conditions.[32] Depending on the form of active oxygen species involved, the oxidative dehydrogenation of methane mainly follows three mechanisms: Rideal-Eley (R-E) mechanism and Langmuir-Hinshelwood (L-H) mechanism, both primarily involving surface adsorbed oxygen, as well as the Mars-van Krevelen (Mv-K) mechanism, which mainly involves lattice oxygen. Below is a brief introduction to these reaction mechanisms:
1) R-E Mechanism: CH₄ molecules initially bind with lattice oxygen in transition metal oxides, leading to the cleavage of C–H bonds in CH₄ and the formation of free methyl radicals, which are subsequently oxidized.[33] At the same time, this binding process consumes oxygen from the lattice, resulting in weak electronic oxygen vacancies. These vacancies absorb molecular oxygen to replenish the lost lattice oxygen, thus completing the catalytic cycle (Figure 3a).
2) L-H Mechanism: The chemical adsorption of molecular oxygen on the catalyst surface is significantly easier than that of methane, which leads to molecular oxygen preferentially adsorbing on the catalyst surface when exposed to oxygen-containing atmosphere.[34] Compared to molecular oxygen, these adsorbed oxygen species exhibit higher reactivity when combining with CH₄ molecules, resulting in the elongation of C–H bonds in methane. This elongation subsequently disrupts the tetrahedral structure of CH₄, generating reactive methyl radicals and facilitating the oxidative dehydrogenation of CH₄ (Figure 3b). The L-H mechanism is primarily used in noble metal-supported metal oxide catalysts.[35]
3) Mv-K Mechanism involves reactions of surface oxygen and migration of lattice oxygen.[36] Initially, gaseous CH₄ molecules are adsorbed onto the active sites of the catalyst. Then, the adsorbed CH₄ reacts with surface lattice oxygen to generate CH₃⁺ ions, which are subsequently oxidized to products such as CO₂ and H₂O, leading to the creation of oxygen vacancies. This step can be referred to as the reduction of the catalyst. Finally, the adsorbed products are desorbed, and internal lattice oxygen migrates to the surface to refill the oxygen vacancies with the surface-adsorbed oxygen, which is the reoxidation of the catalyst (Figure 3c).
Figure 3. a) Schematic of the Superficial Rideal-Eley Mechanism Proposed for Lanthanum and Cobalt-based Perovskite Oxide Catalysts.[33] b) Methane catalytic combustion reaction pathway via the Langmuir-Hinshelwood mechanism.[35] c) Methane oxidation reaction via the Mars-van Krevelen mechanism, with Pd doped in Pd-NiCo₂O₄ and Pd/CeO₂.[36,37]
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