Molecular oxygen enhances H2O2 utilization for the photocatalytic conversion of methane to liquid-phase oxygenates

Buffered Hydroxyl Radical for Photocatalytic Non-Oxidative Methane Coupling

Hydroxy radical (⋅OH) is a prestigious oxidant that allows the cleavage of strong chemical bonds of methane but is untamed, leading to over-oxidation of methane and waste of oxidants, especially at high methane conversion. Here, we managed to buffer ⋅OH in an aqueous solution of photo-irradiated Fe3+, where ⋅OH almost participates in methane oxidation. Due to the interaction between Fe3+ and SO4 2-, the electron transfer from OH- to excited-state Fe3+ for ⋅OH generation is retarded, while excessive ⋅OH is consumed by generated Fe2+ to restore Fe3+. When combined with a Ru/SrTiO3:Rh photocatalyst, the buffered ⋅OH converts methane to C2+ hydrocarbons and H2 with formation rates of 246 and 418 μmol h-1, respectively. The apparent quantum efficiency reaches 13.0±0.2 %, along with 10.2 % methane conversion and 81 % C2+ selectivity after 80 hours of reaction. Overall, this work presents a strategy for controlling active radicals for selective and efficient photocatalysis.

Direct Photocatalytic Oxidation of Methane to Formic Acid with High Selectivity via a Concerted Proton-Electron Transfer Process

Light-driven direct conversion of methane to formic acid is a promising approach to convert methane to value-added chemicals and promote sustainability. However, this process remains challenging due to the complex requirements for multiple protons and electrons. Herein, we report the design of WO3-based photocatalysts modified with Pt active sites to address this challenge. We demonstrate that modulating the dimensional effect of Pt on the WO3 support is key to enhancing the catalytic performance of selective CH4-to-HCOOH conversion. The Pt nanoparticles on WO3 exhibit superior conversion rate, selectivity and durability in the production of HCOOH compared to the Pt-free sample and WO3 decorated with Pt single atoms. The optimal PtNPs-WO3 catalyst achieves a HCOOH conversion rate of 17.7 mmol g-1, with 84% selectivity and stability maintained for up to 48 h. Mechanistic studies show that the protonation of O2 to hydroxyl radicals is the limiting step for HCOOH yield. Pt nanoparticles can facilitate electron transfer and promote O2 dissociation, generating hydroxyl radicals via a proton-coupled electron transfer process. This process provides sufficient protons to lower the formation barrier for •OH radicals, thereby promoting the activation of CH4. In addition, Pt nanoparticles regulate the adsorption of oxygenated hydrocarbon intermediates, increasing the selectivity of the reaction. This work advances our understanding of catalyst design for methane conversion and the effective regulation of complex reaction pathways.

Efficient Photocatalytic CH4-to-Ethanol Conversion by Limiting Interfacial Hydroxyl Radicals Using Gold Nanoparticles

Photocatalytic CH4 oxidation to ethanol with high selectivity is attractive but substantially challenging. The activation of inert C-H bonds at ambient conditions requires highly reactive oxygen species like hydroxyl radicals (⋅OH), while the presence of those oxidative species also facilitates fast formation of C1 products, instead of the kinetically sluggish C-C coupling to produce ethanol. Herein, we developed a BiVO4 photocatalyst with surface functionalization of Au nanoparticles (BiVO4@Au), which not only enables photogeneration of ⋅OH to activate CH4 into ⋅CH3, but also in situ consumes those ⋅OH species to retard their further attack on ⋅CH3, resulting in an enhanced ⋅CH3/⋅OH ratio and facilitating C-C coupling toward ethanol. The ⋅CH3/⋅OH ratio is further improved by transporting CH4 via a gas-diffusion layer to the photocatalytic interface, leading to even higher ethanol selectivity and production rates. At ambient conditions and without photosensitizers or sacrificial agents, the BiVO4@Au photocatalyst exhibited an outstanding CH4-to-ethanol conversion performance, including a peak ethanol yield of 680 μmol ⋅ g-1 ⋅ h-1, a high selectivity of 86 %, and a stable photoconversion of >100 h, substantially exceeding most of the previous reports. Our work suggests an attractive approach of in situ generation and modulation of the ⋅OH levels for photocatalytic CH4 conversion toward multi-carbon products.

One-Pot Synthesis of Mixed-Phase MoVTeNbOx Catalysts for Selective Oxidation of Propane to Acrylic Acid

One-pot hydrothermal synthesis assisted by sodium citrate and citric acid is developed to fabricate mixed-phased MoVTeNbOx catalysts with different phase compositions for selective oxidation of propane to acrylic acid. Structures of various catalysts are comprehensively characterized. Interfacial interactions occur among the M1 and M2 phases of mixed-phase MoVTeNbOx catalysts to exert synergistic effects on the selective production of acrylic acid. Various mixed-phase MoVTeNbOx catalysts exhibit the same type of active site on the M1-phase component, with a significantly enhanced ability to adsorb and activate propane. The propane conversion increases linearly with the density of surface site for propane adsorption, while the propene selectivity increases linearly with the density of surface site for propene adsorption. Among the synthesized catalysts, a MoVTeNbOx catalyst composed of 35.3 wt % M1 phase, 19.2 wt % M2 phase, and 45.7 wt % amorphous phase exhibits the largest density of active site and consequently the best catalytic performance with 38.9% propane conversion and 71.2% acrylic acid selectivity at 380 °C.

Electrochemical Oxidation of Low-Concentration Methane on Pt/Pt and Pt/CP under Ambient Conditions

Methane is a potent greenhouse gas, and its rapid conversion at low concentrations under ambient conditions is a challenging process where combustion is not an option. Herein, we report an electrochemical method to address this problem. It was achieved by applying an oxidation potential to electrochemically activate methane followed by conducting an anodic cyclic voltammogram to fully oxidize activated methane to carbon dioxide on platinized Pt mesh (Pt/Pt) and carbon paper (Pt/CP). This "dynamic potential" oxidation approach enabled methane conversion with low energy consumption, thanks to the low activation potential. Effects of various experimental conditions (applied potential, reaction time, and methane concentration) were investigated. Pure methane and methane/nitrogen gas mixtures containing a series of low concentrations of methane were tested. It was found that methane conversion is independent of its concentration on both Pt/Pt and Pt/CP. Compared to Pt/Pt electrocatalysis, Pt/CP displayed approximately 10 times higher catalytic activity, which can be attributed to the stronger binding of intermediate CO* to Pt, leading to easier CO* activation in the presence of a carbon substrate. Carbon dioxide was the only compound detected during the electro-oxidation phase for Pt/Pt, while for Pt/CP, carbon dioxide and a small amount of formic acid (after 15 h reaction) were observed. Electrocatalytic conversion of methane to carbon dioxide on Pt/CP using 0.5% methane was measured, giving a methane conversion rate of 7.5 × 10-8 mol L-1 s-1 m-2, while the methane conversion rate on Pt/Pt with 1% methane was only 8.3 × 10-9 mol L-1 s-1 m-2.

Role of H2O2 as Oxidant and H2O as Co-Catalyst Over Anatase-TiO2(101) for Conversion of Methane to Methanol

Computations based on density functional theory are carried out to examine the mechanism of photocatalytic oxidation of methane to methanol with H2O2 as oxidant and water as co-catalyst over anatase TiO2(101) surface. The reaction proceeds with hydrogen abstraction from methane followed by the formation of surface methoxy species, which is reduced to methanol. We compare the reaction energetics for C-H dissociation in the presence and absence of surface defect, but find no discernible impact of O-vacancy on methane oxidation. In comparison, hydroxyl produced as a result of H2O2 or H2O photo-decomposition dramatically reduces the barrier for CH3-H bond cleavage. The reaction proceeds further by the reduction of surface methoxy group and is the rate-limiting step for methanol formation. Additionally, we find that methyl can also react with water to form methanol with a considerably lower barrier, suggesting an active involvement of water for methanol formation. We also study the role of water as a co-catalyst and observe significant reduction in barriers, facilitated by alternate pathway for proton transfer. The reaction pathways presented provide valuable insights into the mechanism of methane oxidation in the presence of H2O2 as oxidant and demonstrate the rate-enhancing role of water for these steps.

Atomically Dispersed Iron-Copper Dual-Metal Sites Synergistically Boost Carbonylation of Methane

The direct liquid-phase oxidative carbonylation of methane, utilizing abundant natural gas, offers a mild and straightforward alternative. However, most catalysts proposed for this process suffer from low acetic acid yields due to few active sites and rapid C1 oxygenate generation, impeding their industrial feasibility. Herein, we report a highly efficient 0.1Cu/Fe-HZSM-5-TF (TF denotes template-free synthesis) catalyst featuring exclusively mononuclear Fe and Cu anchored in the ZSM-5 channels. Under optimized conditions, the catalyst achieved an unprecedented acetic acid yield of 40.5 mmol gcat -1 h-1 at 50 °C, tripling the previous records of 12.0 mmol gcat -1 h-1. Comprehensive characterization, isotope-labeled experiments and density functional theory (DFT) calculations reveal that the homogeneous mononuclear Fe sites are responsible for the activation and oxidation of methane, while the neighboring Cu sites play a key role in retarding the oxidation process, promoting C-C coupling for effective acetic acid synthesis. Furthermore, the methyl-group carbon in acetic acid originates solely from methane, while its carbonyl-group carbon is derived exclusively from CO, rather than the conversion of other C1 oxygenates. The proposed bimetallic catalyst design not only overcomes the limitations of current catalysts but also generalizes the oxidative carbonylation of other alkanes, demonstrating promising advancements in sustainable chemical synthesis.

Pseudo-Jahn-Teller Effect Breaks the pH Dependence in Two-Electron Oxygen Electroreduction

The hydrogenation of small molecules (like O2 and CO2) often exhibits strong activity dependence on pHs because of discrepant proton donor environments. However, some catalysts can show seldom dependence on two-electron oxygen electroreduction, a sustainable route of O2 hydrogenation to hydrogen peroxide (H2O2). In this work, a pH-resistant oxygen electroreduction system arising from the pseudo-Jahn-Teller effect is demonstrated. Thorough operando Raman spectra, local environment analyses and density function theory simulations, the lattice distortion of TiOxFy that introduces the pseudo-Jahn-Teller effect contributing to regulating local pHs at electrode-electrolyte interfaces and the absorption/desorption of key *OOH intermediate is revealed. Consequently, as comparison to 78.6% activity attenuation for common catalyst, the TiOxFy displays minor activity decay (3.2%) in the pH range of 1-13 with remarkable Faradaic efficiencies (93.4-96.4%) and H2O2 yield rates (595-614 mg cm-2 h-1) in the current densities of 100-1000 mA cm-2. Further techno-economics analyses display the H2O2 production cost dependent on pHs, giving the lowest H2O2 price of $0.37 kg-1. The present finding is expected to provide an additional dimension to pseudo-Jahn-Teller effect that leverages systems beyond traditional conception.

Boosting Reactive Oxygen Species Formation Over Pd and VOδ Co-Modified TiO2 for Methane Oxidation into Valuable Oxygenates

Direct photocatalytic methane oxidation into value-added products provides a promising strategy for methane utilization. However, the inefficient generation of reactive oxygen species (ROS) partly limits the activation of CH4. Herein, it is reported that Pd and VOδ co-modified TiO2 enables direct and selective methane oxidation into liquid oxygenates in the presence of O2 and H2. Due to the extra ROS production from the in situ formed H2O2, a highly improved yield rate of 5014 µmol g-1 h-1 for liquid oxygenates with a selectivity of 89.3% is achieved over the optimized Pd0.5V0.2-TiO2 catalyst at ambient temperature, which is much better than those (2682 µmol g-1 h-1, 77.8%) without H2. Detailed investigations also demonstrate the synergistic effect between Pd and VOδ species for enhancing the charge carrier separation and transfer, as well as improving the catalytic activity for O2 reduction and H2O2 production.

Breaking the Activity-Selectivity Trade-off for CH4-to-C2H6 Photoconversion

Photocatalytic conversion of methane (CH4) to ethane (C2H6) has attracted extensive attention from academia and industry. Typically, the traditional oxidative coupling of CH4 (OCM) reaches a high C2H6 productivity, yet the inevitable overoxidation limits the target product selectivity. Although the traditional nonoxidative coupling of CH4 (NOCM) can improve the product selectivity, it still encounters unsatisfied activity, arising from being thermodynamically unfavorable. To break the activity-selectivity trade-off, we propose a conceptually new mechanism of H2O2-triggered CH4 coupling, where the H2O2-derived ·OH radicals are rapidly consumed for activating CH4 into ·CH3 radicals exothermically, which bypasses the endothermic steps of the direct CH4 activation by photoholes and the interaction between ·CH3 and ·OH radicals, affirmed by in situ characterization techniques, femtosecond transient absorption spectroscopy, and density-functional theory calculation. By this pathway, the designed Au-WO3 nanosheets achieve unprecedented C2H6 productivity of 76.3 mol molAu-1 h-1 with 95.2% selectivity, and TON of 1542.7 (TOF = 77.1 h-1) in a self-designed flow reactor, outperforming previously reported photocatalysts regardless of OCM and NOCM pathways. Also, under outdoor natural sunlight irradiation, the Au-WO3 nanosheets exhibit similar activity and selectivity toward C2H6 production, showing the possibility for practical applications. Interestingly, this strategy can be applied to other various photocatalysts (Au-WO3, Au-TiO2, Au-CeO2, Pd-WO3, and Ag-WO3), showing a certain universality. It is expected that the proposed mechanism adds another layer to our understanding of CH4-to-C2H6 conversion.

Construction of an Electron Capture and Transfer Center for Highly Efficient and Selective Solar-Light-Driven CO2 Conversion

Exploring high-efficiency photocatalysts for selective CO2 reduction is still challenging because of the limited charge separation and surface reactions. In this study, a noble-metal-free metallic VSe2 nanosheet was incorporated on g-C3N4 to serve as an electron capture and transfer center, activating surface active sites for highly efficient and selective CO2 photoreduction. Quasi in situ X-ray photoelectron spectroscopy (XPS), soft X-ray absorption spectroscopy (sXAS), and femtosecond transient absorption spectroscopy (fs-TAS) unveiled that VSe2 could capture electrons, which are further transferred to the surface for activating active sites. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations revealed a kinetically feasible process for the formation of a key intermediate and confirmed the favorable production of CO on the VSe2/PCN (protonated C3N4) photocatalyst. As an outcome, the optimized VSe2/PCN composite achieved 97% selectivity for solar-light-driven CO2 conversion to CO with a high rate of 16.3 μmol·g-1·h-1, without any sacrificial reagent or photosensitizer. This work offers new insights into the photocatalyst design toward highly efficient and selective CO2 conversion.

Photocatalytic Conversion of Methane to Ethanol at a Three-Phase Interface with Concentration-Matched Hydroxyl and Methyl Radicals

The direct oxidation of CH4 to C2H5OH is attractive but challenging owing to the intricate processes involving carbon-chain growth and hydroxylation simultaneously. The inherent difficulty arises from the strong tendency of CH4 to overoxidize in the commonly used pressurized powder suspension systems rich in reactive oxygen radicals (ROR), which are specifically designed for CH4 concentration and activation. Meanwhile, the strong tendency of nucleophilic attack of potent ROR on the C-C bond of the resulting product C2H5OH ultimately leads to a higher selectivity for C1 oxygenates. This study addresses this multifaceted issue by designing a three-phase interface based on a hydrophilic floating Fe(III)-cross-linked macroporous alginate hydrogel film encapsulated with C3N4 [Fe(III)@ACN] to simultaneously enhance the accessibility of H2O and CH4 molecules to the active sites and species within the macroporous channel. The hydrophilic properties of Fe(III)@ACN allow the in situ production of H2O2 from C3N4 through the water oxidation reaction under irradiation. The concurrent photoinduced Fe(II) triggers Fenton reaction with H2O2 to produce •OH. The enhanced mass transfer of CH4 at the three-phase interface ensures the efficient formation of •CH3 by reacting with •OH, ultimately facilitating carbon-chain growth in the conversion pathway from CH4 to CH3OH and finally to C2H5OH with •CH3 and •OH present in comparable concentrations. Thus, the Fe(III)@ACN catalyst exhibits a remarkable 96% selectivity for alcohol, achieving a 90% selectivity for C2H5OH in the alcohol products. The C2H5OH production rate reaches 171.7 μmol g-1 h-1 without the need for precious-metal additive.

Tailored Exfoliation of Polymeric Carbon Nitride for Photocatalytic H2O2 Production and CH4 Valorization Mediated by O2 Activation

The exfoliation of bulk C3N4 (BCN) into ultrathin layered structure is an effective strategy to boost photocatalytic efficiency by exposing interior active sites and accelerating charge separation and transportation. Herein, we report a novel nitrate anion intercalation-decomposition (NID) strategy that is effective in peeling off BCN into few-layer C3N4 (fl-CN) with tailored thickness down to bi-layer. This strategy only involves hydrothermal treatment of BCN in diluted HNO3 aqueous solution, followed by pyrolysis at various temperatures. The decomposition of the nitrate anions not only exfoliates BCN and changes the band structure, but also incorporates oxygen species onto fl-CN, which is conducive to O2 adsorption and hence relevant chemical processes. In photocatalytic O2 reduction under visible light irradiation, the H2O2 production rate over the optimal fl-CN-530 catalyst is 952 μmol g-1 h-1, which is 8.8 times that over BCN. More importantly, under full arc irradiation and in the absence of hole scavenger, CH4 can be photocatalytically oxidized by on-site formed H2O2 and active oxygen species to generate value-added C1 oxygenates with high selectivity of 99.2 % and record-high production rate of 1893 μmol g-1 h-1 among the metal-free C3N4-based photocatalysts.

Catalyzing Artificial Photosynthesis with TiO2 Heterostructures and Hybrids: Emerging Trends in a Classical yet Contemporary Photocatalyst

Titanium dioxide (TiO2) stands out as a versatile transition-metal oxide with applications ranging from energy conversion/storage and environmental remediation to sensors and optoelectronics. While extensively researched for these emerging applications, TiO2 has also achieved commercial success in various fields including paints, inks, pharmaceuticals, food additives, and advanced medicine. Thanks to the tunability of their structural, morphological, optical, and electronic characteristics, TiO2 nanomaterials are among the most researched engineering materials. Besides these inherent advantages, the low cost, low toxicity, and biocompatibility of TiO2 nanomaterials position them as a sustainable choice of functional materials for energy conversion. Although TiO2 is a classical photocatalyst well-known for its structural stability and high surface activity, TiO2-based photocatalysis is still an active area of research particularly in the context of catalyzing artificial photosynthesis. This review provides a comprehensive overview of the latest developments and emerging trends in TiO2 heterostructures and hybrids for artificial photosynthesis. It begins by discussing the common synthesis methods for TiO2 nanomaterials, including hydrothermal synthesis and sol-gel synthesis. It then delves into TiO2 nanomaterials and their photocatalytic mechanisms, highlighting the key advancements that have been made in recent years. The strategies to enhance the photocatalytic efficiency of TiO2, including surface modification, doping modulation, heterojunction construction, and synergy of composite materials, with a specific emphasis on their applications in artificial photosynthesis, are discussed. TiO2-based heterostructures and hybrids present exciting opportunities for catalyzing solar fuel production, organic degradation, and CO2 reduction via artificial photosynthesis. This review offers an overview of the latest trends and advancements, while also highlighting the ongoing challenges and prospects for future developments in this classical yet rapidly evolving field.

Keto-anthraquinone covalent organic framework for H2O2 photosynthesis with oxygen and alkaline water

Hydrogen peroxide photosynthesis suffers from insufficient catalytic activity due to the high energy barrier of hydrogen extraction from H2O. Herein, we report that mechanochemically synthesized keto-form anthraquinone covalent organic framework which is able to directly synthesize H2O2 (4784 μmol h-1 g-1 at λ > 400 nm) from oxygen and alkaline water (pH = 13) in the absence of any sacrificial reagents. The strong alkalinity resulted in the formation of OH-(H2O)n clusters in water, which were adsorbed on keto moieties within the framework and then dissociated into O2 and active hydrogen, because the energy barrier of hydrogen extraction was largely lowered. The produced hydrogen reacted with anthraquinone to generate anthrahydroquinone, which was subsequently oxidized by O2 to produce H2O2. This study ultimately sheds light on the importance of hydrogen extraction from H2O for H2O2 photosynthesis and demonstrates that H2O2 synthesis is achievable under alkaline conditions.

Heterogeneous Photocatalytic Strategies for C(sp3 )-H Activation

Activation of ubiquitous C(sp3 )-H bonds is extremely attractive but remains a great challenge. Heterogeneous photocatalysis offers a promising and sustainable approach for C(sp3 )-H activation and has been fast developing in the past decade. This Minireview focuses on mechanism and strategies for heterogeneous photocatalytic C(sp3 )-H activation. After introducing mechanistic insights, heterogeneous photocatalytic strategies for C(sp3 )-H activation including precise design of active sites, regulation of reactive radical species, improving charge separation and reactor innovations are discussed. In addition, recent advances in C(sp3 )-H activation of hydrocarbons, alcohols, ethers, amines and amides by heterogeneous photocatalysis are summarized. Lastly, challenges and opportunities are outlined to encourage more efforts for the development of this exciting and promising field.

A multicomponent triformylphoroglucinol-based covalent organic framework for overall hydrogen peroxide photosynthesis

A multicomponent covalent organic framework (COF-Tfp-BpyDaaq) integrating bipyridine with diaminoanthraquinone through a triformylphoroglucinol linkage was synthesized for the first time as a photocatalyst for overall H2O2 photosynthesis. It exhibits enhanced photo-charge separation and H2O2 production rate over its two-component counterparts, demonstrating the pivotal role of multicomponent synthesis in designing efficient photocatalysts.

Photodriven Methane Conversion on Transition Metal Oxide Catalyst: Recent Progress and Prospects

Methane as the main component in natural gas is a promising chemical raw material for synthesizing value-added chemicals, but its harsh chemical conversion process often causes severe energy and environment concerns. Photocatalysis provides an attractive path to active and convert methane into various products under mild conditions with clean and sustainable solar energy, although many challenges remain at present. In this review, recent advances in photocatalytic methane conversion are systematically summarized. As the basis of methane conversion, the activation of methane is first elucidated from the structural basis and activation path of methane molecules. The study is committed to categorizing and elucidating the research progress and the laws of the intricate methane conversion reactions according to the target products, including photocatalytic methane partial oxidation, reforming, coupling, combustion, and functionalization. Advanced photocatalytic reactor designs are also designed to enrich the options and reliability of photocatalytic methane conversion performance evaluation. The challenges and prospects of photocatalytic methane conversion are also discussed, which in turn offers guidelines for methane-conversion-related photocatalyst exploration, reaction mechanism investigation, and advanced photoreactor design.

Accurate Measurements of NH3 Differential Adsorption Heat Unveil Structural Sensitivity of Brønsted Acid and Brønsted/Lewis Acid Synergy in Zeolites

Differential adsorption heats of NH3 on a series of zeolites, including MOR, MFI, FER, and BEA, are accurately measured to probe their acidity using flow-pulse adsorption microcalorimetry. Initial adsorption heats of NH3 at Brønsted acid sites (BAS) vary between 105 to 136 kJ/mol, depending on framework aluminum amounts and topography structures of zeolites. A Brønsted/Lewis acid synergy between BAS and proximate tricoordinated framework-associated aluminum species is identified to generate super acid sites with initial adsorption heats of NH3 around 150 kJ/mol, but occurs only in the MFI zeolites and sensitively depends on the Si/Al ratio. These accurate data of NH3 differential adsorption heats unveil structural sensitivity of BAS and Brønsted/Lewis acid synergy in zeolites and provide experimental benchmark data for fundamental understanding of acidity and acid-catalysis of zeolites.

PdCu nanoalloy decorated photocatalysts for efficient and selective oxidative coupling of methane in flow reactors

Methane activation by photocatalysis is one of the promising sustainable technologies for chemical synthesis. However, the current efficiency and stability of the process are moderate. Herein, a PdCu nanoalloy (~2.3 nm) was decorated on TiO2, which works for the efficient, stable, and selective photocatalytic oxidative coupling of methane at room temperature. A high methane conversion rate of 2480 μmol g-1 h-1 to C2 with an apparent quantum efficiency of ~8.4% has been achieved. More importantly, the photocatalyst exhibits the turnover frequency and turnover number of 116 h-1 and 12,642 with respect to PdCu, representing a record among all the photocatalytic processes (λ > 300 nm) operated at room temperature, together with a long stability of over 112 hours. The nanoalloy works as a hole acceptor, in which Pd softens and weakens C-H bond in methane and Cu decreases the adsorption energy of C2 products, leading to the high efficiency and long-time stability.

Photocatalytic Conversion of Methane: Current State of the Art, Challenges, and Future Perspectives

With 28-34 times the greenhouse effect of CO2 over a 100-year period, methane is regarded as the second largest contributor to global warming. Reducing methane emissions is a necessary measure to limit global warming to below 1.5 °C. Photocatalytic conversion of methane is a promising approach to alleviate the atmospheric methane concentrations due to its low energy consumption and environmentally friendly characteristics. Meanwhile, this conversion process can produce valuable chemicals and liquid fuels such as CH3OH, CH3CH2OH, C2H6, and C2H4, cutting down the dependence of chemical production on crude oil. However, the development of photocatalysts with a high methane conversion efficiency and product selectivity remains challenging. In this review, we overview recent advances in semiconductor-based photocatalysts for methane conversion and present catalyst design strategies, including morphology control, heteroatom doping, facet engineering, and cocatalysts modification. To gain a comprehensive understanding of photocatalytic methane conversion, the conversion pathways and mechanisms in these systems are analyzed in detail. Moreover, the role of electron scavengers in methane conversion performance is briefly discussed. Subsequently, we summarize the anthropogenic methane emission scenarios on earth and discuss the application potential of photocatalytic methane conversion. Finally, challenges and future directions for photocatalytic methane conversion are presented.

Cu and Zn Bimetallic Co-Modified H-MOR Catalyst for Direct Oxidation of Low-Concentration Methane to Methanol

The direct oxidation of low-concentration methane to value-added chemicals can not only reduce carbon emission but also provide an alternative production route for fossil fuels. Herein, we proposed a novel catalyst for the direct oxidation of low-concentration methane to methanol via the impregnation method, which selected copper and zinc as co-modifiers to modify the MOR catalyst. The highest methanol yield of 71.35 μmol·gcat-1·h-1 was obtained over a bimetallic Cu0.5Zn0.35-MOR catalyst. The catalyst retained good activity after three cycles of testing experiments, indicating good recyclability. Based on the results of performance tests and characterization studies, it was confirmed that Cu species bound to the zeolite framework were the main active sites for methane oxidation. The introduction of Zn decreased the generation of the octahedrally coordinated extra-framework aluminum, which promoted the dispersion of Cu within the zeolite framework. In other words, more tetrahedrally coordinated FAl-stabilized Cu species were presented in our CuZn-MOR catalyst system in comparison to the monometallic Cu-MOR catalyst. Benefiting from the aforementioned modification, the agglomerative sintering of the metal during the reaction was effectively prevented. This work may provide a feasible guide for the future optimization of Cu-based catalysts designed for the selective oxidation of methane.

Hierarchical triphase diffusion photoelectrodes for photoelectrochemical gas/liquid flow conversion

Photoelectrochemical device is a versatile platform for achieving various chemical transformations with solar energy. However, a grand challenge, originating from mass and electron transfer of triphase-reagents/products in gas phase, water/electrolyte/products in liquid phase and catalyst/photoelectrode in solid phase, largely limits its practical application. Here, we report the simulation-guided development of hierarchical triphase diffusion photoelectrodes, to improve mass transfer and ensure electron transfer for photoelectrochemical gas/liquid flow conversion. Semiconductor nanocrystals are controllably integrated within electrospun nanofiber-derived mat, overcoming inherent brittleness of semiconductors. The mechanically strong skeleton of free-standing mat, together with satisfactory photon absorption, electrical conductivity and hierarchical pores, enables the design of triphase diffusion photoelectrodes. Such a design allows photoelectrochemical gas/liquid conversion to be performed continuously in a flow cell. As a proof of concept, 16.6- and 4.0-fold enhancements are achieved for the production rate and product selectivity of methane conversion, respectively, with remarkable durability.

CuxOy nanoparticles and Cu-OH motif decorated ZSM-5 for selective methane oxidation to methyl oxygenates

Copper decorated zeolites are promising candidates for the partial oxidation of methane to generate methanol with elevated energy density, nevertheless, the modulation and possible synergism between multiple Cu active sites still need to be delved in depth. Here, ZSM-5 catalysts with modulated Cu motifs were proposed using copper oxysalts as precursors through a calcination process. By modifying the contents of copper oxysalts precursors, the Cu active sites varied, and a unique M shaped trend of CH₃OH productivity emerged. Attributed to the synergetic effects of Cu_xO_y nanoparticles (adsorbing CH₄ and generating *OCH₃ species) and Cu-OH motif (binding CH₄ and forming Si···CH₃), a maximum CH₃OH yield of 15975.73 μmol/g_cat/h (with CH₃OOH yield of 2155.59 μmol/g_cat/h) and methyl oxygenates selectivity up to 72.79 % could be achieved. This work paved an efficient, low cost, and succinct way for the manufacture of catalysts with tunable active sites and high performance over methane to methanol conversion.

Electrochemical sensing mechanism of ammonium ions over an Ag/TiO2 composite electrode modified by hematite

Here, we demonstrate a new electrochemical sensing mechanism of ammonium ions (NH₄⁺) involving a two-electron oxygen reduction reaction (ORR) and a hydrazine reaction. The NH₄⁺ are electrooxidized to hydrazine by H₂O₂ derived from the ORR over a self-supporting Ag/TiO₂ nanotube array composite electrode modified by hematite (Ag/Fe₂O₃/TNTs). The Ag/Fe₂O₃/TNT sensor exhibits a high sensitivity of 1876 µA mM^-1 cm^-2 with a detection limit of 0.18 µM under non-alkaline conditions, a short response time of 3 s, good reproducibility, and fine selectivity among various interferents, and is also successfully used in real water bodies to display high accuracy. Furthermore, this new mechanism has a certain universality in a range of Ag (main catalyst)/transition metal oxide (cocatalyst)/TNT sensing systems. This work offers a new design basis for the urgently needed electrochemical ammonia nitrogen sensors.