Catalytic Methane Monofunctionalization by an Electrogenerated High-Valent Pd Intermediate

Selective C-H Bond Activation in Propane with Molecular Oxygen over Cu(I)-ZSM-5 at Ambient Conditions

Selective activation of C-H bonds in light alkanes under mild conditions is challenging but holds the promise of efficient upgrading of abundant hydrocarbons. In this work, we report the conversion of propane to propylene with ∼95% selectivity on Cu(I)-ZSM-5 with O2 at room temperature and pressure. The intraporous Cu(I) species was oxidized to Cu(II) during the reaction but could be regenerated with H2 at 220 °C. Diffuse reflectance ultraviolet spectroscopy indicated the presence of both Cu+-O2 and Cu2(μ-O2)2+ species in the zeolite pores during the reaction, and electron paramagnetic resonance results showed that propane activation occurred via a radical-mediated pathway distinct from that with H2O2 as the oxidant. Correlation between spectroscopic and reactivity results on Cu(I)-ZSM-5 with different Cu loadings suggests that the isolated intraporous Cu(I) species is the main active species in propane activation.

Electrocatalytic functional group conversion-based carbon resource upgrading

The conversions of carbon resources, such as alcohols, aldehydes/ketones, and ethers, have been being one of the hottest topics most recently for the goal of carbon neutralization. The emerging electrocatalytic upgrading has been regarded as a promising strategy aiming to convert carbon resources into value-added chemicals. Although exciting progress has been made and reviewed recently in this area by mostly focusing on the explorations of valuable anodic oxidation or cathodic reduction reactions individually, however, the reaction rules of these reactions are still missing, and how to purposely find or rationally design novel but efficient reactions in batches is still challenging. The properties and transformations of key functional groups in substrate molecules play critically important roles in carbon resources conversion reactions, which have been paid more attention to and may offer hidden keys to achieve the above goal. In this review, the properties of functional groups are addressed and discussed in detail, and the reported electrocatalytic upgrading reactions are summarized in four categories based on the types of functional groups of carbon resources. Possible reaction pathways closely related to functional groups will be summarized from the aspects of activation, cleavage and formation of chemical bonds. The current challenges and future opportunities of electrocatalytic upgrading of carbon resources are discussed at the end of this review.

Unraveling the potential-dependent structure evolution in CuO for electrocatalytic biomass valorization

Electrocatalytic oxidation of renewable biomass (such as glucose) into high-value-added chemicals provides an effective approach to achieving carbon neutrality. CuO-derived materials are among the most promising electrocatalysts for biomass electrooxidation, but the identification of their active sites under electrochemical conditions remains elusive. Herein, we report a potential-dependent structure evolution over CuO in the glucose oxidation reaction (GOR). Through systematic electrochemical and spectroscopic characterizations, we unveil that CuO undergoes Cu2+/Cu+ and Cu3+/Cu2+ redox processes at increased potentials with successive generation of Cu(OH)2 and CuOOH as the active phases. In addition, these two structures have distinct activities in the GOR, with Cu(OH)2 being favorable for aldehyde oxidation, and CuOOH showed faster kinetics in carbon-carbon cleavage and alcohol/aldehyde oxidation. This work deepens our understanding of the dynamic reconstruction of Cu-based catalysts under electrochemical conditions and may guide rational material design for biomass valorization.

Fundamental Limitation in Electrochemical Methane Oxidation to Alcohol: A Review and Theoretical Perspective on Overcoming It

The direct conversion of gaseous methane to energy-dense liquid derivatives such as methanol and ethanol is of profound importance for the more efficient utilization of natural gas. However, the thermo-catalytic partial oxidation of this simple alkane has been a significant challenge due to the high C-H bond energy. Exploiting electrocatalysis for methane activation via active oxygen species generated on the catalyst surface through electrochemical water oxidation is generally considered as economically viable and environmentally benign compared to energy-intensive thermo-catalysis. Despite recent progress in electrochemical methane oxidation to alcohol, the competing oxygen evolution reaction (OER) still impedes achieving high faradaic efficiency and product selectivity. In this review, an overview of current progress in electrochemical methane oxidation, focusing on mechanistic insights on methane activation, catalyst design principles based on descriptors, and the effect of reaction conditions on catalytic performance are provided. Mechanistic requirements for high methanol selectivity, and limitations of using water as the oxidant are discussed, and present the perspective on how to overcome these limitations by employing carbonate ions as the oxidant.

Analysis of the Scale of Global Human Needs and Opportunities for Sustainable Catalytic Technologies

We analyzed the enormous scale of global human needs, their carbon footprint, and how they are connected to energy availability. We established that most challenges related to resource security and sustainability can be solved by providing distributed, affordable, and clean energy. Catalyzed chemical transformations powered by renewable electricity are emerging successor technologies that have the potential to replace fossil fuels without sacrificing the wellbeing of humans. We highlighted the technical, economic, and societal advantages and drawbacks of short- to medium-term decarbonization solutions to gauge their practicability, economic feasibility, and likelihood for widespread acceptance on a global scale. We detailed catalysis solutions that enhance sustainability, along with strategies for catalyst and process development, frontiers, challenges, and limitations, and emphasized the need for planetary stewardship. Electrocatalytic processes enable the production of solar fuels and commodity chemicals that address universal issues of the water, energy and food security nexus, clothing, the building sector, heating and cooling, transportation, information and communication technology, chemicals, consumer goods and services, and healthcare, toward providing global resource security and sustainability and enhancing environmental and social justice.

Methane Oxidation to Methanol

The direct transformation of methane to methanol remains a significant challenge for operation at a larger scale. Central to this challenge is the low reactivity of methane at conditions that can facilitate product recovery. This review discusses the issue through examination of several promising routes to methanol and an evaluation of performance targets that are required to develop the process at scale. We explore the methods currently used, the emergence of active heterogeneous catalysts and their design and reaction mechanisms and provide a critical perspective on future operation. Initial experiments are discussed where identification of gas phase radical chemistry limited further development by this approach. Subsequently, a new class of catalytic materials based on natural systems such as iron or copper containing zeolites were explored at milder conditions. The key issues of these technologies are low methane conversion and often significant overoxidation of products. Despite this, interest remains high in this reaction and the wider appeal of an effective route to key products from C-H activation, particularly with the need to transition to net carbon zero with new routes from renewable methane sources is exciting.

Bisulfate as a redox-active ligand in vanadium-based electrocatalysis for CH4 functionalization

The roles of unforgiving H₂SO₄ solvent in CH₄ activation with molecular catalysts have not been experimentally well-illustrated despite computational predictions. Here, we provide experimental evidence that metal-bound bisulfate ligand introduced by H₂SO₄ solvent is redox-active in vanadium-based electrocatalytic CH₄ activation discovered recently. Replacing one of the two terminal bisulfate ligands with redox-inert dihydrogen phosphate in the pre-catalyst vanadium (V)-oxo dimer completely quenches its activity towards CH₄, which may inspire environmentally benign catalysis with minimal use of H₂SO₄.

Partial Methane Oxidation in Fuel Cell-Type Reactors for Co-Generation of Energy and Chemicals: A Short Review

The conversion of methane into chemicals is of interest to achieve a decarbonized future. Fuel cells are electrochemical devices commonly used to obtain electrical energy but can be utilized either for chemicals’ production or both energy and chemicals cogeneration. In this work, the partial oxidation of methane in fuel cells for electricity generation and valuable chemicals production at the same time is reviewed. For this purpose, we compile different types of methane-fed fuel cells, both low- and high-temperature fuel cells. Despite the fact that few studies have been conducted on this subject, promising results are driving the development of fuel cells that use methane as a fuel source for the cogeneration of power and valuable chemicals.

Recent advances in organic electrosynthesis employing transition metal complexes as electrocatalysts

Organic electrosynthesis has been widely used as an environmentally conscious alternative to conventional methods for redox reactions because it utilizes electric current as a traceless redox agent instead of chemical redox agents. Indirect electrolysis employing a redox catalyst has received tremendous attention, since it provides various advantages compared to direct electrolysis. With indirect electrolysis, overpotential of electron transfer can be avoided, which is inherently milder, thus wide functional group tolerance can be achieved. Additionally, chemoselectivity, regioselectivity, and stereoselectivity can be tuned by the redox catalysts used in indirect electrolysis. Furthermore, electrode passivation can be avoided by preventing the formation of polymer films on the electrode surface. Common redox catalysts include N-oxyl radicals, hypervalent iodine species, halides, amines, benzoquinones (such as DDQ and tetrachlorobenzoquinone), and transition metals. In recent years, great progress has been made in the field of indirect organic electrosynthesis using transition metals as redox catalysts for reaction classes including C-H functionalization, radical cyclization, and cross-coupling of aryl halides-each owing to the diverse reactivity and accessible oxidation states of transition metals. Although various reviews of organic electrosynthesis are available, there is a lack of articles that focus on recent research progress in the area of indirect electrolysis using transition metals, which is the impetus for this review.

Electrocatalytic Methane Functionalization with d0 Early Transition Metals Under Ambient Conditions

The undesirable loss of methane (CH₄ ) at remote locations welcomes approaches that ambiently functionalize CH₄ on-site without intense infrastructure investment. Recently, we found that electrochemical oxidation of vanadium(V)-oxo with bisulfate ligand leads to CH₄ activation at ambient conditions. The key question is whether such an observation is a one-off coincidence or a general strategy for electrocatalyst design. Here, a general scheme of electrocatalytic CH₄ activation with d⁰ early transition metals is established. The pre-catalysts' molecular structure, electrocatalytic kinetics, and mechanism were detailed for titanium (IV), vanadium (V), and chromium (VI) species as model systems. After a turnover-limiting one-electron electrochemical oxidation, the yielded ligand-centered cation radicals activate CH₄ with low activation energy and high selectivity. The reactivities are universal among early transition metals from Period 4 to 6, and the reactivities trend for different early transition metals correlate with their d orbital energies across periodic table. Our results offer new chemical insights towards developing advanced ambient electrocatalysts of natural gas.

Photoelectrochemical Conversion of Methane into Value-Added Products

Methane has been reported to be directly converted into value-added products through various methods. Among them, photoelectrochemical (PEC) methane conversion is considered an eco-friendly method because it utilizes solar light and is able to control the selectivity to different products by means of application of an external bias. Recently, some PEC methane conversion systems have been reported, but their performance efficiencies are relatively lower than those of other existing thermal, photocatalytic, and electrochemical systems. The detailed mechanism of methane activation is not clear at this stage. In this review, various catalytic materials and their roles in the reaction pathways are summarized and discussed. Furthermore, promising semiconductor materials, co-catalysts, and oxidants have also been proposed. Finally, direct and indirect pathways in the design of the PEC methane conversion system have been discussed.

AgII -Mediated Electrocatalytic Ambient CH4 Functionalization Inspired by HSAB Theory

Although most class (b) transition metals have been studied in regard to CH₄ activation, divalent silver (Ag^II ), possibly owing to its reactive nature, is the only class (b) high-valent transition metal center that is not yet reported to exhibit reactivities towards CH₄ activation. We now report that electrochemically generated Ag^II metalloradical readily functionalizes CH₄ into methyl bisulfate (CH₃ OSO₃ H) at ambient conditions in 98 % H₂ SO₄ . Mechanistic investigation experimentally unveils a low activation energy of 13.1 kcal mol^-1 , a high pseudo-first-order rate constant of CH₄ activation up to 2.8×10³  h^-1 at room temperature and a CH₄ pressure of 85 psi, and two competing reaction pathways preferable towards CH₄ activation over solvent oxidation. Reaction kinetic data suggest a Faradaic efficiency exceeding 99 % beyond 180 psi CH₄ at room temperature for potential chemical production from widely distributed natural gas resources with minimal infrastructure reliance.

Electrocatalytic Methane Oxidation Greatly Promoted by Chlorine Intermediates

Renewable energy-powered methane (CH₄ ) conversion at ambient conditions is an attractive but highly challenging field. Due to the highly inert character of CH₄ , the selective cleavage of its first C-H bond without over-oxidation is essential for transforming CH₄ into value-added products. In this work, we developed an efficient and selective CH₄ conversion approach at room temperature using intermediate chlorine species (*Cl), which were electrochemically generated and stabilized on mixed cobalt-nickel spinels with different Co/Ni ratios. The lower overpotentials for *Cl formation enabled an effective activation and conversion of CH₄ to CH₃ Cl without over-oxidation to CO₂ , and Ni³⁺ at the octahedral sites in the mixed cobalt-nickel spinels allowed to stabilize surface-bound *Cl species. The CoNi₂ O_x electrocatalyst exhibited an outstanding yield of CH₃ Cl (364 mmol g^-1  h^-1 ) and a high CH₃ Cl/CO₂ selectivity of over 400 at room temperature, with demonstrated capability of direct CH₄ conversion under seawater working conditions.

Electrocatalysis as an enabling technology for organic synthesis

Electrochemistry has recently gained increased attention as a versatile strategy for achieving challenging transformations at the forefront of synthetic organic chemistry. Electrochemistry's unique ability to generate highly reactive radical and radical ion intermediates in a controlled fashion under mild conditions has inspired the development of a number of new electrochemical methodologies for the preparation of valuable chemical motifs. Particularly, recent developments in electrosynthesis have featured an increased use of redox-active electrocatalysts to further enhance control over the selective formation and downstream reactivity of these reactive intermediates. Furthermore, electrocatalytic mediators enable synthetic transformations to proceed in a manner that is mechanistically distinct from purely chemical methods, allowing for the subversion of kinetic and thermodynamic obstacles encountered in conventional organic synthesis. This review highlights key innovations within the past decade in the area of synthetic electrocatalysis, with emphasis on the mechanisms and catalyst design principles underpinning these advancements. A host of oxidative and reductive electrocatalytic methodologies are discussed and are grouped according to the classification of the synthetic transformation and the nature of the electrocatalyst.

Directing transition metal-based oxygen-functionalization catalysis

This review presents the recent progress of oxygen functionalization reactions based on non-electrochemical (conventional organic synthesis) and electrochemical methods. Although both methods have their advantages and limitations, the former approach has been used to synthesize a broader range of organic substances as the latter is limited by several factors, such as poor selectivity and high energy cost. However, because electrochemical methods can replace harmful terminal oxidizers with external voltage, organic electrosynthesis has emerged as greener and more eco-friendly compared to conventional organic synthesis. The progress of electrochemical methods toward oxygen functionalization is presented by an in-depth discussion of different types of electrically driven-chemical organic synthesis, with particular attention to recently developed electrochemical systems and catalyst designs. We hope to direct the attention of readers to the latest breakthroughs of traditional oxygen functionalization reactions and to the potential of electrochemistry for the transformation of organic substrates to useful end products.

Photocatalytic C-H activation and the subtle role of chlorine radical complexation in reactivity

The functionalization of methane, ethane, and other alkanes derived from fossil fuels is a central goal in the chemical enterprise. Recently, a photocatalytic system comprising [Ce^IVCl₅(OR)]^2- [Ce^IV, cerium(IV); OR, -OCH₃ or -OCCl₂CH₃] was disclosed. The system was reportedly capable of alkane activation by alkoxy radicals (RO•) formed by Ce^IV-OR bond photolysis. In this work, we present evidence that the reported carbon-hydrogen (C-H) activation of alkanes is instead mediated by the photocatalyst [NEt₄]₂[CeCl₆] (NEt₄ ⁺, tetraethylammonium), and RO• are not intermediates. Spectroscopic analyses and kinetics were investigated for C-H activation to identify chlorine radical (Cl•) generation as the rate-limiting step. Density functional theory calculations support the formation of [Cl•][alcohol] adducts when alcohols are present, which can manifest a masked RO• character. This result serves as an important cautionary note for interpretation of radical trapping experiments.

Towards atomic precision in HMF and methane oxidation electrocatalysts

With the increasing emphasis on transitioning to a sustainable society, electrosynthetic routes to generate fuels and chemicals are rapidly gaining traction. While the electrolysis of water and CO2 has been heavily investigated over the last decade, electrocatalysis of other abundant resources such as biomass and methane is now increasingly coming into focus. As this area is relatively less mature, much work remains to be done. In particular, efforts to decipher reaction mechanisms and extract the fundamental insights are necessary to develop economically competitive electrosynthetic routes using biomass and methane. Against this backdrop, this feature article focuses on the recent developments within the community using atomically precise catalysts, both homogeneous and heterogeneous, as model systems to understand these reactions.

Selective Activation of Propane Using Intermediates Generated during Water Oxidation

Electrochemical conversion of light alkanes to high-value oxygenates provides an attractive avenue for eco-friendly utilization of these hydrocarbons. However, such conversion under ambient conditions remains exceptionally challenging due to the high energy barrier of C-H bond cleavage. Herein, we investigated theoretically the partial oxidation of propane on a series of single atom alloys by using active intermediates generated during water oxidation as the oxidant. We show that by controlling the potential and pH, stable surface oxygen atoms can be maintained under water oxidation conditions. The free energy barrier for C-H bond cleavage by the surface oxygen can be as small as 0.54 eV, which can be surmounted easily at room temperature. Our calculations identified three promising surfaces as effective propane oxidation catalysts. Our complementary experiments demonstrated the partial oxidation of propane to acetone on Ni-doped Au surfaces. We also investigated computationally the steps leading to acetone formation. These studies show that the concept of exploiting intermediates generated in water oxidation as oxidants provides a fruitful strategy for electrocatalyst design to efficiently convert hydrocarbons into value-added chemicals.

Rapid Electrochemical Methane Functionalization Involves Pd-Pd Bonded Intermediates

High-valent Pd complexes are potent agents for the oxidative functionalization of inert C-H bonds, and it was previously shown that rapid electrocatalytic methane monofunctionalization could be achieved by electro-oxidation of Pd^II to a critical dinuclear Pd^III intermediate in concentrated or fuming sulfuric acid. However, the structure of this highly reactive, unisolable intermediate, as well as the structural basis for its mechanism of electrochemical formation, remained elusive. Herein, we use X-ray absorption and Raman spectroscopies to assemble a structural model of the potent methane-activating intermediate as a Pd^III dimer with a Pd-Pd bond and a 5-fold O atom coordination by H_xSO₄^(x-2) ligands at each Pd center. We further use EPR spectroscopy to identify a mixed-valent M-M bonded Pd₂^II,III species as a key intermediate during the Pd^II-to-Pd^III₂ oxidation. Combining EPR and electrochemical data, we quantify the free energy of Pd dimerization as <-4.5 kcal/mol for Pd₂^II,III and <-9.1 kcal/mol for Pd^III₂. The structural and thermochemical data suggest that the aggregate effect of metal-metal and axial metal-ligand bond formation drives the critical Pd dimerization reaction in between electrochemical oxidation steps. This work establishes a structural basis for the facile electrochemical oxidation of Pd^II to a M-M bonded Pd^III dimer and provides a foundation for understanding its rapid methane functionalization reactivity.

Stabilisation of tri-valent ions with a vacant coordination site at a corrole-metal interface

By exploiting an established on-surface metallation strategy, we address the ability of the corrolic macrocycle to stabilise transition metal ions in high-valent (III) oxidation states in metal-supported molecular layers. This approach offers a route to engineer adsorbed metal complexes that cannot be easily fabricated by organic synthesis methods and bear a vacant axial coordination site for catalytic conversions.

Ambient methane functionalization initiated by electrochemical oxidation of a vanadium (V)-oxo dimer

The abundant yet widely distributed methane resources require efficient conversion of methane into liquid chemicals, whereas an ambient selective process with minimal infrastructure support remains to be demonstrated. Here we report selective electrochemical oxidation of CH₄ to methyl bisulfate (CH₃OSO₃H) at ambient pressure and room temperature with a molecular catalyst of vanadium (V)-oxo dimer. This water-tolerant, earth-abundant catalyst possesses a low activation energy (10.8 kcal mol^‒1) and a high turnover frequency (483 and 1336 hr⁻¹ at 1-bar and 3-bar pure CH₄, respectively). The catalytic system electrochemically converts natural gas mixture into liquid products under ambient conditions over 240 h with a Faradaic efficiency of 90% and turnover numbers exceeding 100,000. This tentatively proposed mechanism is applicable to other d⁰ early transition metal species and represents a new scalable approach that helps mitigate the flaring or direct emission of natural gas at remote locations.

The undesirable geological release of methane at remote locations can be lessened through an efficient methane conversion process. Here, the authors report selective ambient functionalization of methane by a vanadium (V)-oxo electrocatalyst with a low activation energy and a high turnover frequency.

AxCe9(IO3)36 (A = K, La): the effects of a change in the intermediate valence on the second-harmonic generation response

Two new isostructural non-centrosymmetric cerium iodates K_xCe₉(IO₃)₃₆ (x≈ 3) and La_xCe₉(IO₃)₃₆ (x≈ 0.3) were synthesized via a hydrothermal method and systematically characterized. They are intermediate valence compounds with the same band gap, showing similar microscopic net dipole moments per unit volume, but different macroscopic second-harmonic generation (SHG) responses under 1064 nm laser irradiation. This research gives a new view for understanding the effects of electronic structure on the SHG response.

Effective methane conversion to methanol on bi-functional graphene-oxide-supported platinum nanoclusters (Pt5) - a DFT study

Nowadays identifying a high-performance catalyst for converting methane to methanol is crucial because methanol serves as an excellent energy source and has wide chemical applications. In the present study, we used DFT, a computational chemistry method, to investigate the reaction mechanism of methanol production by conversion of methane on Pt₅ nanoparticles supported on graphene oxide (GO) substrates. Computational results predicted that the Pt₅/GO system exhibits excellent catalysis efficiency, compared with those of the previously examined Pt₂/GO and Pt₂O₂/GO systems. Energetics of examined molecular species and the reaction mechanism showed that the Pt₅/GO system exhibits high stability in this catalysis reaction and catalyzes the reaction efficiently. Moreover, between the two investigated surfaces GO and UGO, GO performed better and should be a promising catalyst support to convert methane into methanol.

Solution Catalytic Cycle of Incompatible Steps for Ambient Air Oxidation of Methane to Methanol

Direct chemical synthesis from methane and air under ambient conditions is attractive yet challenging. Low-valent organometallic compounds are known to activate methane, but their electron-rich nature seems incompatible with O₂ and prevents catalytic air oxidation. We report selective oxidation of methane to methanol with an O₂-sensitive metalloradical as the catalyst and air as the oxidant at room temperature and ambient pressure. The incompatibility between C-H activation and O₂ oxidation is reconciled by electrochemistry and nanomaterials, with which a concentration gradient of O₂ within the nanowire array spatially segregated incompatible steps in the catalytic cycle. An unexpected 220 000-fold increase of the apparent reaction rate constants within the nanowire array leads to a turnover number up to 52 000 within 24 h. The synergy between nanomaterials and organometallic chemistry warrants a new catalytic route for CH₄ functionalization.

Electrochemical Reoxidation Enables Continuous Methane-to-Methanol Catalysis with Aqueous Pt Salts

The direct conversion of methane to methanol would enable better utilization of abundant natural gas resources. In the presence of stoichiometric Pt^IV oxidants, Pt^II ions are capable of catalyzing this reaction in aqueous solutions at modest temperatures. Practical implementation of this chemistry requires a viable strategy for replacing or regenerating the expensive Pt^IV oxidant. Herein, we establish an electrochemical strategy for continuous regeneration of the Pt^IV oxidant to furnish overall electrochemical methane oxidation. We show that Cl-adsorbed Pt electrodes catalyze facile oxidation of Pt^II to Pt^IV at low overpotential without concomitant methanol oxidation. Exploiting this facile electrochemistry, we maintain the Pt^II/IV ratio during Pt^II-catalyzed methane oxidation via in situ monitoring of the solution potential coupled with dynamic modulation of the electric current. This approach leads to sustained methane oxidation catalysis with 70% selectivity for methanol.

Templating metastable Pd2 carboxylate aggregates

Evaluation of the potential for metal-metal (M-M) cooperation to enable catalysis requires access to specific polynuclear aggregates that display appropriate geometry and size. In many cases, exerting synthetic control over the aggregation of simple metal salts is a challenge. For example, Pd(ii) acetate self assembles as a trimer (i.e. Pd3(OAc)6) both in the solid state and in solution and does not feature close Pd-Pd interactions. Related carboxylate-supported Pd2 aggregates (i.e. Pd2(OAc)4), which would feature close Pd-Pd interactions, are thermodynamically metastable in solution phase and thus largely unavailable. Here we demonstrate ion metathesis within pre-formed metal-organic frameworks (MOFs) to prepare metastable Pd2 tetracarboxylates sites. The newly synthesized materials are characterized by elemental analysis, PXRD, SCXRD, EXAFS, XANES, and gas adsorption analysis. In addition, the critical role of network solvation on the kinetics of ion metathesis was revealed by coupled TGA-MS and ICP-MS experiments. The demonstration of templated ion metathesis to generate specific metastable coordination sites that are inaccessible in solution phase chemistry represents a new opportunity to interrogate the chemistry of specific polynuclear metal aggregates.

Selective CO Production by Photoelectrochemical Methane Oxidation on TiO2

The inertness of the C-H bond in CH₄ poses significant challenges to selective CH₄ oxidation, which often proceeds all the way to CO₂ once activated. Selective oxidation of CH₄ to high-value industrial chemicals such as CO or CH₃OH remains a challenge. Presently, the main methods to activate CH₄ oxidation include thermochemical, electrochemical, and photocatalytic reactions. Of them, photocatalytic reactions hold great promise for practical applications but have been poorly studied. Existing demonstrations of photocatalytic CH₄ oxidation exhibit limited control over the product selectivity, with CO₂ as the most common product. The yield of CO or other hydrocarbons is too low to be of any practical value. In this work, we show that highly selective production of CO by CH₄ oxidation can be achieved by a photoelectrochemical (PEC) approach. Under our experimental conditions, the highest yield for CO production was 81.9%. The substrate we used was TiO₂ grown by atomic layer deposition (ALD), which features high concentrations of Ti³⁺ species. The selectivity toward CO was found to be highly sensitive to the substrate types, with significantly lower yield on P25 or commercial anatase TiO₂ substrates. Moreover, our results revealed that the selectivity toward CO also depends on the applied potentials. Based on the experimental results, we proposed a reaction mechanism that involves synergistic effects by adjacent Ti sites on TiO₂. Spectroscopic characterization and computational studies provide critical evidence to support the mechanism. Furthermore, the synergistic effect was found to parallel heterogeneous CO₂ reduction mechanisms. Our results not only present a new route to selective CH₄ oxidation, but also highlight the importance of mechanistic understandings in advancing heterogeneous catalysis.