Sulfurized Two-Dimensional Conductive Metal-Organic Framework as a High-Performance Cathode Material for Rechargeable Mg Batteries.

Rechargeable Mg batteries are a promising energy storage technology to overcome the limitations inherent to Li ion batteries. A critical challenge in advancing Mg batteries is the lack of suitable cathode materials. In this work, we report a cathode design that incorporates S functionality into two-dimensional metal-organic-frameworks (2D-MOFs). This new cathode material enables good Mg2+ storage capacity and outstanding cyclability. It was found that upon the initial Mg2+ insertion and disinsertion, there is an apparent structural transformation that crumbles the layered 2D framework, leading to amorphization. The resulting material serves as the active material to host Mg2+ through reduction and/or oxidation of S and, to a limited extent, O. The reversible nature of S and O redox chemistry was confirmed by spectroscopic characterizations and validated by density functional calculations. Importantly, during the Mg2+ insertion and disinsertion process, the 2D nature of the framework was maintained, which plays a key role in enabling the high reversibility of the MOF cathode.

Highly Selective Electrochemical Baeyer-Villiger Oxidation through Oxygen Atom Transfer from Water.

The Baeyer-Villiger oxidation of ketones is a crucial oxygen atom transfer (OAT) process used for ester production. Traditionally, Baeyer-Villiger oxidation is accomplished by thermally oxidizing the OAT from stoichiometric peroxides, which are often difficult to handle. Electrochemical methods hold promise for breaking the limitation of using water as the oxygen atom source. Nevertheless, existing demonstrations of electrochemical Baeyer-Villiger oxidation face the challenges of low selectivity. We report in this study a strategy to overcome this challenge. By employing a well-known water oxidation catalyst, Fe2O3, we achieved nearly perfect selectivity for the electrochemical Baeyer-Villiger oxidation of cyclohexanone. Mechanistic studies suggest that it is essential to produce surface hydroperoxo intermediates (M-OOH, where M represents a metal center) that promote the nucleophilic attack on ketone substrates. By confining the reactions to the catalyst surfaces, competing reactions (e.g., dehydrogenation, carboxylic acid cation rearrangements, and hydroxylation) are greatly limited, thereby offering high selectivity. The surface-initiated nature of the reaction is confirmed by kinetic studies and spectroelectrochemical characterizations. This discovery adds nucleophilic oxidation to the toolbox of electrochemical organic synthesis.

Polysulfides in Magnesium-Sulfur Batteries.

Mg-S batteries hold great promise as a potential alternative to Li-based technologies. Their further development hinges on solving a few key challenges, including the lower capacity and poorer cycling performance when compared to Li counterparts. At the heart of the issues is the lack of knowledge on polysulfide chemical behaviors in the Mg-S battery environment. In this Review, a comprehensive overview of the current understanding of polysulfide behaviors in Mg-S batteries is provided. First, a systematic summary of experimental and computational techniques for polysulfide characterization is provided. Next, conversion pathways for Mg polysulfide species within the battery environment are discussed, highlighting the important role of polysulfide solubility in determining reaction kinetics and overall battery performance. The focus then shifts to the negative effects of polysulfide shuttling on Mg-S batteries. The authors outline various strategies for achieving an optimal balance between polysulfide solubility and shuttling, including the use of electrolyte additives, polysulfide-trapping materials, and dual-functional catalysts. Based on the current understanding, the directions for further advancing knowledge of Mg polysulfide chemistry are identified, emphasizing the integration of experiment with computation as a powerful approach to accelerate the development of Mg-S battery technology.

Infrared Spectroscopic Observation of Oxo- and Superoxo-Intermediates in the Water Oxidation Cycle of a Molecular Ir Catalyst.

Molecular Ir catalysts have emerged as an important class of model catalysts for understanding structure-activity relationships in water oxidation, a reaction that is central to renewable fuel synthesis. Prior efforts have mostly focused on controlling and elucidating the emergence of active species from prepared precursors. However, the development of efficient and stable molecular Ir catalysts also necessitates probing of reaction intermediates. To date, relatively little is known about the key intermediates in the cycles of the molecular Ir catalysts. Herein, we probed the catalytic cycle of a homogeneous Ir catalyst ("blue dimer") at a Au electrode/aqueous electrolyte interface by combining surface-enhanced infrared absorption spectroscopy (SEIRAS) with phase-sensitive detection (PSD). Cyclic voltammograms (CVs) from 1.4 to 1.7 VRHE (RHE = reversible hydrogen electrode) give rise to a band at ∼818 cm-1, whereas CVs from 1.4 to ≥1.85 VRHE generate an additional band at ∼1146 cm-1. Isotope labeling experiments indicate that the bands at ∼818 and ∼1146 cm-1 are attributable to oxo (IrV═O) and superoxo (IrIV-OO•) moieties, respectively. This study establishes PSD-SEIRAS as a sensitive tool for probing water oxidation cycles at electrode/electrolyte interfaces and demonstrates that the relative abundance of two key intermediates can be tuned by the thermodynamic driving force of the reaction.

Low Catalyst Loading Enhances Charge Accumulation for Photoelectrochemical Water Splitting.

Solar water oxidation is a critical step in artificial photosynthesis. Successful completion of the process requires four holes and releases four protons. It depends on the consecutive accumulation of charges at the active site. While recent research has shown an obvious dependence of the reaction kinetics on the hole concentrations on the surface of heterogeneous (photo)electrodes, little is known about how the catalyst density impacts the reaction rate. Using atomically dispersed Ir catalysts on hematite, we report a study on how the interplay between the catalyst density and the surface hole concentration influences the reaction kinetics. At low photon flux, where surface hole concentrations are low, faster charge transfer was observed on photoelectrodes with low catalyst density compared to high catalyst density; at high photon flux and high applied potentials, where surface hole concentrations are moderate or high, slower surface charge recombination was afforded by low-density catalysts. The results support that charge transfer between the light absorber and the catalyst is reversible; they reveal the unexpected benefits of low-density catalyst loading in facilitating forward charge transfer for desired chemical reactions. It is implied that for practical solar water splitting devices, a suitable catalyst loading is important for maximized performance.

Atomically dispersed Ir catalysts exhibit support-dependent water oxidation kinetics during photocatalysis.

Heterogeneous water oxidation catalysis is central to the development of renewable energy technologies. Recent research has suggested that the reaction mechanisms are sensitive to the hole density at the active sites. However, these previous results were obtained on catalysts of different materials featuring distinct active sites, making it difficult to discriminate between competing explanations. Here, a comparison study based on heterogenized dinuclear Ir catalysts (Ir-DHC), which feature the same type of active site on different supports, is reported. The prototypical reaction was water oxidation triggered by pulsed irradiation of suspensions containing a light sensitizer, Ru(bpy)32+, and a sacrificial electron scavenger, S2O82-. It was found that at relatively low temperatures (288-298 K), the water oxidation activities of Ir-DHC on indium tin oxide (ITO) and CeO2 supports were comparable within the studied range of fluences (62-151 mW cm-2). By contrast, at higher temperatures (310-323 K), Ir-DHC on ITO exhibited a ca. 100% higher water oxidation activity than on CeO2. The divergent activities were attributed to the distinct abilities of the supporting substrates in redistributing holes. The differences were only apparent at relatively high temperatures when hole redistribution to the active site became a limiting factor. These findings highlight the critical role of the supporting substrate in determining the turnover at active sites of heterogeneous catalysts.

Tuning Electrode Reactivity through Organometallic Complexes.

The use of molecularly modified electrodes in catalysis heralds a new paradigm in designing chemical transformations by allowing control of catalytic activity. Herein, we provide an overview of reported methods to develop electrodes functionalized with organometallic complexes and a summary of commonly used techniques for characterizing the electrode surface after immobilization. In addition, we highlight the implications of surface functionalization in catalysis to emphasize the key aspects that should be considered during the development and optimization of functionalized electrodes. Particularly, surface-molecule electronic coupling and electrostatic interactions within a hybrid system are discussed to present effective handles in tuning catalytic activity. We envision that this emerging type of hybrid catalytic system has the potential to combine the advantages of homogeneous catalysis and heterogeneous supports and could be applied to an expanded range of transformations beyond energy conversion.

Selective Formation of Acetic Acid and Methanol by Direct Methane Oxidation Using Rhodium Single-Atom Catalysts.

Atomically dispersed catalysts such as single-atom catalysts have been shown to be effective in selectively oxidizing methane, promising a direct synthetic route to value-added oxygenates such as acetic acid or methanol. However, an important challenge of this approach has been that the loading of active sites by single-atom catalysts is low, leading to a low overall yield of the products. Here, we report an approach that can address this issue. It utilizes a metal-organic framework built with porphyrin as the linker, which provides high concentrations of binding sites to support atomically dispersed rhodium. It is shown that up to 5 wt% rhodium loading can be achieved with excellent dispersity. When used for acetic acid synthesis by methane oxidation, a new benchmark performance of 23.62 mmol·gcat-1·h-1 was measured. Furthermore, the catalyst exhibits a unique sensitivity to light, producing acetic acid (under illumination, up to 66.4% selectivity) or methanol (in the dark, up to 65.0% selectivity) under otherwise identical reaction conditions.

Methane Carboxylation Using Electrochemically Activated Carbon Dioxide.

Direct synthesis of CH3 COOH from CH4 and CO2 is an appealing approach for the utilization of two potent greenhouse gases that are notoriously difficult to activate. In this Communication, we report an integrated route to enable this reaction. Recognizing the thermodynamic stability of CO2 , our strategy sought to first activate CO2 to produce CO (through electrochemical CO2 reduction) and O2 (through water oxidation), followed by oxidative CH4 carbonylation catalyzed by Rh single atom catalysts supported on zeolite. The net result was CH4 carboxylation with 100 % atom economy. CH3 COOH was obtained at a high selectivity (>80 %) and good yield (ca. 3.2 mmol g-1 cat in 3 h). Isotope labelling experiments confirmed that CH3 COOH is produced through the coupling of CH4 and CO2 . This work represents the first successful integration of CO/O2 production with oxidative carbonylation reaction. The result is expected to inspire more carboxylation reactions utilizing preactivated CO2 that take advantage of both products from the reduction and oxidation processes, thus achieving high atom efficiency in the synthesis.

Characterizing Grain Boundary Effects on Mg2+ Conduction in Metal-Organic Frameworks.

Next-generation materials for fast ion conduction have the potential to revolutionize battery technology. Metal-organic frameworks (MOFs) are promising candidates for achieving this goal. Given their structural diversity, the design of efficient MOF-based conductors can be accelerated by a detailed understanding and accurate prediction of ion conductivity. However, the polycrystalline nature of solid-state materials requires consideration of grain boundary effects, which is complicated by challenges in characterizing grain boundary structures and simulating ensemble transport processes. To address this, we have developed an approach for modeling ion transport at grain boundaries and predicting their contribution to conductivity. Mg2+ conduction in the Mg-MOF-74 thin film was studied as a representative system. Using computational techniques and guided by experiments, we investigated the structural details of MOF grain boundary interfaces to determine accessible Mg2+ transport pathways. Computed transport kinetics were input into a simplified MOF nanocrystal model, which combines ion transport in the bulk structure and at grain boundaries. The model predicts Mg2+ conductivity in the MOF-74 film within chemical accuracy (<1 kcal/mol activation energy difference), validating our approach. Physically, Mg2+ conduction in MOF-74 is inhibited by strong Mg2+ binding at grain boundaries, such that only a small fraction of grain boundary alignments allow for fast Mg2+ transport. This results in a 2-3 order-of-magnitude reduction in conductivity, illustrating the critical impact of the grain boundary contribution. Overall, our work provides a computation-aided platform for molecular-level understanding of grain boundary effects and quantitative prediction of ion conductivity. Combined with experimental measurements, it can serve as a synergistic tool for characterizing the grain boundary composition of MOF-based conductors.

Electrolyte Engineering Stabilizes Photoanodes Decorated with Molecular Catalysts.

Invited for this month's cover is the group of Dunwei Wang from Boston College and Serhiy Cherevko from the Helmholtz Institute Erlangen-Nürnberg for Renewable Energy. The image illustrates the impact of different electrolyte environments on the stability of hematite decorated with an iridium molecular catalyst used for solar water splitting. The Research Article itself is available at 10.1002/cssc.202202319.

Selective Methane Oxidation by Heterogenized Iridium Catalysts.

Oxidative methane (CH4) carbonylation promises a direct route to the synthesis of value-added oxygenates such as acetic acid (CH3COOH). Here, we report a strategy to realize oxidative CH4 carbonylation through immobilized Ir complexes on an oxide support. Our immobilization approach not only enables direct CH4 activation but also allows for easy separation and reutilization of the catalyst. Furthermore, we show that a key step, methyl migration, that forms a C-C bond, is sensitive to the electrophilicity of carbonyl, which can be tuned by a gentle reduction to the Ir centers. While the as-prepared catalyst that mainly featured Ir(IV) preferred CH3COOH production, a reduced catalyst featuring predominantly Ir(III) led to a significant increase of CH3OH production at the expense of the reduced yield of CH3COOH.

Photocatalytic Depolymerization of Native Lignin toward Chemically Recyclable Polymer Networks.

As an inedible component of biomass, lignin features rich functional groups that are desired for chemical syntheses. How to effectively depolymerize lignin without compromising the more valuable cellulose and hemicellulose has been a significant challenge. Existing biomass processing procedures either induce extensive condensation in lignin that greatly hinders its chemical utilization or focus on fully depolymerizing lignin to produce monomers that are difficult to separate for subsequent chemical synthesis. Here, we report a new approach to selective partial depolymerization, which produces oligomers that can be readily converted to chemically recyclable polymer networks. The process takes advantage of the high selectivity of photocatalytic activation of the ß-O-4 bond in lignin by tetrabutylammonium decatungstate (TBADT). The availability of exogenous electron mediators or scavengers promotes cleavage or oxidation of this bond, respectively, enabling high degrees of control over the depolymerization and the density of a key functional group, C=O, in the products. The resulting oligomers can then be readily utilized for the synthesis of polymer networks through reactions between C=O and branched -NH2 as a dynamic covalent cross-linker. Importantly, the resulting polymer network can be recycled to enable a circular economy of materials directly derived from biomass.

Electrolyte Engineering Stabilizes Photoanodes Decorated with Molecular Catalysts.

Molecular catalysts are promising oxygen evolution promoters in conjunction with photoanodes for solar water splitting. Maintaining the stability of both photoabsorber and cocatalyst is still a prime challenge, with many efforts tackling this issue through sophisticated material designs. Such approaches often mask the importance of the electrode-electrolyte interface and overlook easily tunable system parameters, such as the electrolyte environment, to improve efficiency. We provide a systematic study on the activity-stability relationship of a prominent Fe2 O3 photoanode modified with Ir molecular catalysts using in situ mass spectroscopy. After gaining detailed insights into the dissolution behavior of the Ir cocatalyst, a comprehensive pH study is conducted to probe the impact of the electrolyte on the performance. An inverse trend in Fe and Ir stability is found, with the best activity-stability synergy obtained at pH 9.7. The results bring awareness to the overall photostability and electrolyte engineering when advancing catalysts for solar water splitting.

Highly stable preferential carbon monoxide oxidation by dinuclear heterogeneous catalysts.

Atomically dispersed catalysts have been shown highly active for preferential oxidation of carbon monoxide in the presence of excess hydrogen (PROX). However, their stability has been less than ideal. We show here that the introduction of a structural component to minimize diffusion of the active metal center can greatly improve the stability without compromising the activity. Using an Ir dinuclear heterogeneous catalyst (DHC) as a study platform, we identify two types of oxygen species, interfacial and bridge, that work in concert to enable both activity and stability. The work sheds important light on the synergistic effect between the active metal center and the supporting substrate and may find broad applications for the use of atomically dispersed catalysts.

Spectroelectrochemistry of Water Oxidation Kinetics in Molecular versus Heterogeneous Oxide Iridium Electrocatalysts.

Water oxidation is the step limiting the efficiency of electrocatalytic hydrogen production from water. Spectroelectrochemical analyses are employed to make a direct comparison of water oxidation reaction kinetics between a molecular catalyst, the dimeric iridium catalyst [Ir2(pyalc)2(H2O)4-(µ-O)]2+ (IrMolecular, pyalc = 2-(2'pyridinyl)-2-propanolate) immobilized on a mesoporous indium tin oxide (ITO) substrate, with that of an heterogeneous electrocatalyst, an amorphous hydrous iridium (IrOx) film. For both systems, four analogous redox states were detected, with the formation of Ir(4+)-Ir(5+) being the potential-determining step in both cases. However, the two systems exhibit distinct water oxidation reaction kinetics, with potential-independent first-order kinetics for IrMolecular contrasting with potential-dependent kinetics for IrOx. This is attributed to water oxidation on the heterogeneous catalyst requiring co-operative effects between neighboring oxidized Ir centers. The ability of IrMolecular to drive water oxidation without such co-operative effects is explained by the specific coordination environment around its Ir centers. These distinctions between molecular and heterogeneous reaction kinetics are shown to explain the differences observed in their water oxidation electrocatalytic performance under different potential conditions.

Interface Engineering Between Multi-Elemental Alloy Nanoparticles and a Carbon Support Toward Stable Catalysts.

Multi-elemental alloy (MEA) nanoparticles have recently received notable attention owing to their high activity and superior phase stability. Previous syntheses of MEA nanoparticles mainly used carbon as the support, owing to its high surface area, good electrical conductivity, and tunable defective sites. However, the interfacial stability issue, such as nanoparticle agglomeration, remains outstanding due to poor interfacial binding between MEA and carbon. Such a problem often causes performance decay when MEA nanoparticles are used as catalysts, hindering their practical applications. Herein, an interface engineering strategy is developed to synthesize MEA-oxide-carbon hierarchical catalysts, where the oxide on carbon helps disperse and stabilize the MEA nanoparticles toward superior thermal and electrochemical stability. Using several MEA compositions (PdRuRh, PtPdIrRuRh, and PdRuRhFeCoNi) and oxides (TiO2 and Cr2 O3 ) as model systems, it is shown that adding the oxide renders superior interfacial stability and therefore excellent catalytic performance. Excellent thermal stability is demonstrated under transmission electron microscopy with in situ heating up to 1023 K, as well as via long-term cycling (>370 hours) of a Li-O2 battery as a harsh electrochemical condition to challenge the catalyst stability. This work offers a new route toward constructing efficient and stable catalysts for various applications.

Electrochemically switchable polymerization from surface-anchored molecular catalysts.

Redox-switchable polymerizations of lactide and epoxides were extended to the solid state by anchoring an iron-based polymerization catalyst to TiO2 nanoparticles. The reactivity of the molecular complexes and their redox-switching characteristics were maintained in the solid-state. These properties resulted in surface-initiated polymerization reactions that produced polymer brushes whose chemical composition is dictated by the oxidation state of the iron-based complex. Depositing the catalyst-functionalized TiO2 nanoparticles on fluorine-doped tin oxide resulted in an electrically addressable surface that could be used to demonstrate spatial control in redox-switchable polymerization reactions. By using a substrate that contained two electrically isolated domains wherein one domain was exposed to an oxidizing potential, patterns of surface-bound polyesters and polyethers were accessible through sequential application of lactide and cyclohexene oxide. The differentially functionalized surfaces demonstrated distinct physical properties that illustrated the promise for using the method to pattern surfaces with multiple, chemically distinct polymer brushes.