Direct Photocatalytic Synthesis of Acetic Acid from Methane and CO at Ambient Temperature Using Water as Oxidant

Electron Delocalization and Transfer Across Polyoxometalates-Based Subnanomaterials in Catalytic Reactions

Due to their identical building blocks and high surface-to-volume ratio, subnanomaterials exhibit significant properties compared to their bulk nanomaterial counterparts. The interactions between these building blocks can result in either equal or unequal sharing of electrons, leading to electron transfer in heterojunctions or electron delocalization within symmetric structures. Clusters, possessing electronic properties akin to atoms, can serve as reservoirs of electrons to stabilize crucial intermediates in catalytic reactions. This perspective provides a novel understanding of well-defined subnanomaterials with distinct architectures, such as cluster-based constructions and co-assembled heterojunctions, emphasizing the relationship between electronic structures and catalytic properties. The objective is to provide novel perspectives on the realm of subnanomaterials and cluster-based architectures.

Single-cluster Functionalized TiO2 Nanotube Array for Boosting Water Oxidation and CO2 Photoreduction to CH3OH

Solar-driven CO2 reduction and water oxidation to liquid fuels represents a promising solution to alleviate energy crisis and climate issue, but it remains a great challenge for generating CH3OH and CH3CH2OH dominated by multi-electron transfer. Single-cluster catalysts with super electron acceptance, accurate molecular structure, customizable electronic structure and multiple adsorption sites, have led to greater potential in catalyzing various challenging reactions. However, accurately controlling the number and arrangement of clusters on functional supports still faces great challenge. Herein, we develop a facile electrosynthesis method to uniformly disperse Wells-Dawson- and Keggin-type polyoxometalates on TiO2 nanotube arrays, resulting in a series of single-cluster functionalized catalysts P2M18O62@TiO2 and PM12O40@TiO2 (M=Mo or W). The single polyoxometalate cluster can be distinctly identified and serves as electronic sponge to accept electrons from excited TiO2 for enhancing surface-hole concentration and promote water oxidation. Among these samples, P2Mo18O62@TiO2-1 exhibits the highest electron consumption rate of 1260 μmol g-1 for CO2-to-CH3OH conversion with H2O as the electron source, which is 11 times higher than that of isolated TiO2 nanotube arrays. This work supplied a simple synthesis method to realize the single-dispersion of molecular cluster to enrich surface-reaching holes on TiO2, thereby facilitating water oxidation and CO2 reduction.

Forked Vein Structure W/WO3- x with Dual Active Sites in W and Oxygen Vacancies to Enhance Methylene Self-Coupling for Efficient Conversion of Methane to Ethylene

The directional conversion of methane to ethylene is challenging due to the dissociation of the C─H bond and the self-coupling of methyl intermediates. Herein, a novel W/WO3- x catalyst with the fork vein structure consisting of an alternating arrangement of WO3- x and W is developed. Impressively, the catalyst achieves an unprecedented C2H4 yield of 1822.73 µmol g-1 h-1, with a selectivity of 82.49%. The enhanced catalytic activity is ascribed to the multifunctional synergistic effect induced by oxygen vacancies and W sites in W/WO3- x. Oxygen vacancies provide abundant coordination of unsaturation sites, which promotes the adsorption and activation of CH4, thus reducing the dissociation energy barrier of the C─H bond. The CH2 coupling barrier on the metal W surface is significantly lower compared to WO3, so CH2 can migrate to the W site for coupling. Importantly, the W/WO3- x with high periodicity provides multiple ordered local microelectric fields, and CH2 intermediates with dipole moments undergo orientation polarization and displacement polarization driven by the electric field, thus enabling CH2 migration. This work opens a new avenue for the structural design and modulation of photocatalysts, and provides new perspectives on the migration of methylene between multiple active sites.

Polyoxometalates Based Catalysts for Carbonylation Reactions: A Review

As a type of diverse and structurally adjustable metal-oxo clusters, polyoxometalates (POMs) based materials have been extensively applied as a catalysis in various valuable reactions. This review summarized recent progress in the application of POMs-based catalysts for various carbonylation reactions including (1). Carbonylation of olefins, (2). Carbonylation of formaldehyde, (3). Carbonylation of methanol or dimethyl ether, (4). Oxidative carbonylation of methane, (5). Oxidative carbonylation of phenol and (6). Reductive carbonylation of nitrobenzene. A brief perspective on POMs-based catalysts for the carbonylation reactions is proposed.

Best Practices for Experiments and Reports in Photocatalytic Methane Conversion

Efficiently converting methane into valuable chemicals via photocatalysis under mild condition represents a sustainable route to energy storage and value-added manufacture. Despite continued interest in this area, the achievements have been overshadowed by the absence of standardized protocols for conducting photocatalytic methane oxidation experiments as well as evaluating the corresponding performance. In this review, we present a structured solution aimed at addressing these challenges. Firstly, we introduce the norms underlying reactor design and outline various configurations in the gas-solid and gas-solid-liquid reaction systems. This discussion helps choosing the suitable reactors for methane conversion experiments. Subsequently, we offer a comprehensive step-by-step protocol applicable to diverse methane-conversion reactions. Emphasizing meticulous verification and accurate quantification of the products, this protocol highlights the significance of mitigating contamination sources and selecting appropriate detection methods. Lastly, we propose the standardized performance metrics crucial for evaluating photocatalytic methane conversion. By defining these metrics, the community could obtain the consensus of assessing the performance across different studies. Moving forward, the future of photocatalytic methane conversion necessitates further refinement of stringent experimental standards and evaluation criteria. Moreover, development of scalable reactor is essential to facilitate the transition from laboratory proof-of-concept to potentially industrial production.

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.

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.

Boosting Gas-Phase TiO2 Photocatalysis with Weak Electric Field Strengths of Volt/Centimeter

Among semiconductor nanomaterials, titanium dioxide is at the forefront of heterogeneous photocatalysis, but its catalytic activity greatly suffers from the loss of photoexcited charge carriers through deleterious recombination processes. Here, we investigate the impact of an external electric field (EEF) applied to conventional P25 TiO2 nanopowder with or without Au nanoparticles (NPs) to circumvent this issue. The study of two redox reactions in the gas phase, water splitting and toluene degradation, reveals an enhancement of the photocatalytic activity with rather modest electric fields of a few volt/centimeters only. Such an improvement arises from the electric-field-induced quenching of the green emission in anatase, allowing the photoexcited charge carriers to be transferred to the adsorbed reactants instead of pointless radiative recombinations. Applying an EEF across a trap-rich metal oxide material, such as TiO2, which, when impregnated with Au NPs, leads, respectively, to 12- and 6-fold enhancements in the production of hydrogen and the oxidation of toluene for an electric field of 8 V/cm, without any electrolysis, is a simple and elegant strategy to meet higher photocatalytic efficiencies.

{Ru(C6 H6 )}-Decorating Heteropolymolybdate for Highly Activity Photocatalytic Oxidation of Benzyl Alcohol to Benzaldehyde

An unclassical structure of {Ru(C6 H6 )}-based polyoxometalate, Cs6 H4 [Te2 Mo12 O46 {Ru(C6 H6 )}] ⋅ 16.5H2 O (1), has been successfully constructed from {Te2 Mo12 O46 }-type heteropolymolybdate and {Ru(C6 H6 )} group, which structure type was discovered for the first time. Compound 1 not only possesses strong light-harvesting ability, but also exhibits high carrier separation efficiency and lower charge transfer resistance. Under visible light irradiation, compound 1 displayed excellent catalytic activity and circularity in the conversion of benzyl alcohol to benzaldehyde (yield=94 %; turnover number=500; turnover frequency=20.8 h-1 ). Finally, the electron paramagnetic resonance measurement and energy level matching analysis provide theoretical basis for the derivation of the reaction mechanism.

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.

Nitrogen-Doped Titanium Dioxide for Selective Photocatalytic Oxidation of Methane to Oxygenates

Photocatalytic conversion of methane (CH4) to value-added chemicals using H2O as the oxidant under mild conditions is a desired sustainable pathway for synthesizing commodity chemicals. However, controlling product selectivity while maintaining high product yields is greatly challenging. Herein, we develop a highly efficient strategy, based on the precise control of the types of nitrogen dopants, and the design of photocatalysts, to achieve high selectivity and productivity of oxygenates via CH4 photocatalytic conversion. The primary product (methanol) is obtained in a high yield of 159.8 μmol·g-1·h-1 and 47.7% selectivity, and the selectivity of oxygenate compounds reached 92.5%. The unique hollow porous structure and substituted nitrogen sites of nitrogen-doped TiO2 synergistically promote its photo-oxidation performance. Furthermore, in situ attenuated total reflectance Fourier transform infrared spectroscopy provides direct evidence of the key intermediates and their evolution for producing methanol and multicarbon oxygenates. This study provides insights into the mechanism of photocatalytic CH4 conversion.

Mild Methane Electrochemical Oxidation Boosted via Plasma Pre-Activation

Obtaining partial methane oxidation reaction (MOR) with various oxygenates via a mild electrochemical method is practically difficult because of activation of stable C─H bond and consequent reaction pathway regulation. Here, a real-time tandem MOR with cascaded plasma and electrocatalysis to activate and convert the methane (CH4 ) synergistically is reported for the first time. Boosted CH4 conversion is demonstrated toward value-added products including, alcohols, carboxylates, and ketone via use of commercial Pd-based electrocatalysts. Compared with hash industrial processes, a mild condition, that is, anode potential < 1.0 V versus RHE (reversible hydrogen electrode) is used that mitigates overoxidation of oxygenates and obviates competing reaction(s). One evidence that Pd(II) sites and surface adsorbed hydroxyls are important in facilitating activated-CH4 species conversion, and establish a reaction mechanism for conversion(s) that involves coupling reactions between adsorbed hydroxyls, carbon monoxide and C1 /C2 alkyls. One conclude that pre-activation is important in boosting electrochemical partial MOR under mild conditions and will be of benefit in the development of sustainable CH4 conversion technology.

A light-driven selective protocol for on-demand methanol and formic acid syntheses with a recyclable GaN catalyst

Herein, we present a protocol for the on-demand preparation of methanol and formic acid via selective photo-oxidation of methane with H2O and O2 catalyzed by GaN. The detailed photosyntheses of methanol or formic acid from CH4/H2O or CH4/H2O/O2 are described, respectively. In addition, we provide experimental details for the accurate quantifications of the final gas/liquid products and photoexcited oxygenated radicals. Finally, we deliver the procedure for scaling up the transformation. For complete details on the use and execution of this protocol, please refer to Han et al. (2023).1.

Solar-Driven Reforming of Methane and Nitrogen to Methanol and Ammonium on Iron-Modified Zeolite under Ambient Conditions in Water

Artificial photosynthesis from selective methane oxidation or nitrogen reduction to value-added chemicals provides a promising pathway for the sustainable chemical industry, while still remaining a great challenge due to the extreme difficulty in C-H and N≡N bond cleavage under ambient conditions. Catalysts that can cocatalyze these two reactions simultaneously are rarely reported. Here, Fe-ZSM-5 with highly dispersed extra-framework Fe-oxo species enables efficient and selective photocatalytic conversion of methane and nitrogen to coproduce methanol and ammonia using H2O as the redox reagent under ambient conditions. The optimized Fe-ZSM-5 photocatalyst achieves up to 0.88 mol/molFe·h of methanol products with 97% selectivity. Meanwhile, the productivity of ammonia is 0.61 mol/molFe·h. In situ EPR and DRIFT studies disclose that water serves as a redox reagent to provide hydroxyl radicals for methane oxidation and protons for nitrogen hydrogenation. Quantum chemical calculations revealed that Fe-oxo species play a significant role in the coactivation of methane and nitrogen molecules, which lowers the energy barriers of rate-determining steps for methanol and ammonia generation.

Fabrication of Polyoxometalate-Based Metal-Organic Frameworks Integrating Paddlewheel Rh2(OAc)4 for Visible-Light-Driven Oxidative Coupling of Amines

The development of visible-light-responsive, environmentally friendly, and reusable photocatalysts for organic oxidation reactions is of vital significance. Herein, four polyoxometalate-based metal-organic frameworks (POMOFs) were synthesized and systematically characterized by assembling the paddlewheel complex Rh2(OAc)4 and various polyoxometalates (POMs). Single-crystal X-ray diffraction analysis revealed that the four POMOFs were isomorphic and possessed rare structural features among the POMOFs, with POMs as nodes and Rh2(OAc)4 as linkers. As expected, the activities of the four POMOFs for the photocatalytic oxidative coupling of benzylamine were better than that of Rh2(OAc)4 or POMs individually, which was ascribed to the synergistic effect between them, and the intrinsic reasons for the difference in the activity were explained via electrochemical measurements. In particular, the product imine yield reached 96.1% with NaRh-SiW12 as the catalyst and a turnover number and a turnover frequency of 480.5 and 120.5 h-1, respectively, while the product yield remained as high as 92% after three repetitions, evidencing its high stability. Moreover, the higher activities of the four POMOFs for the selective epoxidation of various alkenes reaffirm the synergistic effect between Rh2(OAc)4 and POMs.

Light-driven flow synthesis of acetic acid from methane with chemical looping

Oxidative carbonylation of methane is an appealing approach to the synthesis of acetic acid but is limited by the demand for additional reagents. Here, we report a direct synthesis of CH₃COOH solely from CH₄ via photochemical conversion without additional reagents. This is made possible through the construction of the PdO/Pd-WO₃ heterointerface nanocomposite containing active sites for CH₄ activation and C-C coupling. In situ characterizations reveal that CH₄ is dissociated into methyl groups on Pd sites while oxygen from PdO is the responsible for carbonyl formation. The cascade reaction between the methyl and carbonyl groups generates an acetyl precursor which is subsequently converted to CH₃COOH. Remarkably, a production rate of 1.5 mmol g_Pd^-1 h^-1 and selectivity of 91.6% toward CH₃COOH is achieved in a photochemical flow reactor. This work provides insights into intermediate control via material design, and opens an avenue to conversion of CH₄ to oxygenates.