Fractal Design of Hierarchical PtPd with Enhanced Exposed Surface Atoms for Highly Catalytic Activity and Stability.

Hierarchical assembly of arc-like fractal nanostructures not only has its unique self-similarity feature for stability enhancement but also possesses the structural advantages of highly exposed surface-active sites for activity enhancement, remaining a great challenge for high-performance metallic nanocatalyst design. Herein, we report a facile strategy to synthesize a novel arc-like hierarchical fractal structure of PtPd bimetallic nanoparticles (h-PtPd) by using pyridinium-type ionic liquids as the structure-directing agent. Growth mechanisms of the arc-like nanostructured PtPd nanoparticles have been fully studied, and precise control of the particle sizes and pore sizes has been achieved. Due to the structural features, such as size control by self-similarity growth of subunits, structural stability by nanofusion of subunits, and increased numbers of exposed active atoms by the curved homoepitaxial growth, h-PtPd displays outstanding electrocatalytic activity toward oxygen reduction reaction and excellent stability during hydrothermal treatment and catalytic process.

Interfacial Carbon Makes Nano-Particulate RuO2 an Efficient, Stable, pH-Universal Catalyst for Splitting of Seawater.

An electrocatalyst composed of RuO2 surrounded by interfacial carbon, is synthesized through controllable oxidization-calcination. This electrocatalyst provides efficient charge transfer, numerous active sites, and promising activity for pH-universal electrocatalytic overall seawater splitting. An electrolyzer with this catalyst gives current densities of 10 mA cm-2 at a record low cell voltage of 1.52 V, and shows excellent durability at current densities of 10 mA cm-2 for up to 100 h. Based on the results, a mechanism for the catalytic activity of the composite is proposed. Finally, a solar-driven system is assembled and used for overall seawater splitting, showing 95% Faraday efficiency.

Hierarchically Fractal PtPdCu Sponges and their Directed Mass- and Electron-Transfer Effects.

Fractal Pt-based materials with hierarchical structures and high self-similarity have attracted more and more attention due to their bioinspiring maximum optimization of energy utilization and mass transfer. However, their high-efficiency design of the mass- and electron-transfer still remains to be a great challenge. Herein, fractal PtPdCu hollow sponges (denoted as PtPdCu-HS) facilitating both directed mass- and electron-transfer are presented. Such directed transfer effects greatly promote electrocatalytic activity, regarded as 3.9 times the mass activity, 7.3 times the specific activity, higher poison tolerance, and higher stability than commercial Pt/C for the methanol oxidation reaction (MOR). A new "directed mass- and electron-transfer" concept, characteristics, and mechanism are proposed at the micro/nanoscale to clarify the structural design and functional enhancement of fractal electrocatalyst. This work displays new possibilities for designing novel nanomaterials with high activity and superior stability toward electrocatalysis or other practical applications.

Anion-π Catalysis Enabled by the Mechanical Bond.

We report a series of rotaxane-based anion-π catalysts in which the mechanical bond between a bipyridine macrocycle and an axle containing an NDI unit is intrinsic to the activity observed, including a [3]rotaxane that catalyses an otherwise disfavoured Michael addition in >60 fold selectivity over a competing decarboxylation pathway that dominates under Brønsted base conditions. The results are rationalized by detailed experimental investigations, electrochemical and computational analysis.

Host-Guest-Induced Electron Transfer Triggers Radical-Cation Catalysis.

Modifying the reactivity of substrates by encapsulation is a fundamental principle of capsule catalysis. Here we show an alternative strategy, wherein catalytic activation of otherwise inactive quinone "co-factors" by a simple Pd2L4 capsule promotes a range of bulk-phase, radical-cation cycloadditions. Solution electron-transfer experiments and cyclic voltammetry show that the cage anodically shifts the redox potential of the encapsulated quinone by a significant 1 V. Moreover, the capsule also protects the reduced semiquinone from protonation, thus transforming the role of quinones from stoichiometric oxidants into catalytic single-electron acceptors. We envisage that the host-guest-induced release of an "electron hole" will translate to various forms of non-encapsulated catalysis that involve other difficult-to-handle, highly reactive species.

The Effects of Ultrasound on the Electro-Oxidation of Sulfate Solutions at Low pH.

The electro-oxidation of sulfate solutions is a well-established route for the generation of powerful oxidants such as persulfate. Despite this, the effects of simultaneous ultrasound irradiation during this process has attracted little attention. Herein, we investigate the effects of a low-intensity ultrasonic field on the generation of solution-phase oxidants during the electro-oxidation of sulfate solutions. Our results show that at high current densities and high sulfate concentrations, ultrasound has little effect on the Faradaic and absolute yields of solution-phase oxidants. However, at lower current densities and sulfate concentrations, the amount of these oxidants in solution appears to decrease under ultrasonic irradiation. A mechanism explaining these results is proposed (and validated), whereby anodically-generated sulfate and hydroxyl radicals are more effectively transported into bulk solution (where they are quenched) during sonication, whereas in the absence of an ultrasonic field these radicals combine with one another to form more persistent species (such as persulfate) that can be detected by iodometry.

Solar-Driven Water Oxidation and Decoupled Hydrogen Production Mediated by an Electron-Coupled-Proton Buffer.

Solar-to-hydrogen photoelectrochemical cells (PECs) have been proposed as a means of converting sunlight into H2 fuel. However, in traditional PECs, the oxygen evolution reaction and the hydrogen evolution reaction are coupled, and so the rate of both of these is limited by the photocurrents that can be generated from the solar flux. This in turn leads to slow rates of gas evolution that favor crossover of H2 into the O2 stream and vice versa, even through ostensibly impermeable membranes such as Nafion. Herein, we show that the use of the electron-coupled-proton buffer (ECPB) H3PMo12O40 allows solar-driven O2 evolution from water to proceed at rates of over 1 mA cm(-2) on WO3 photoanodes without the need for any additional electrochemical bias. No H2 is produced in the PEC, and instead H3PMo12O40 is reduced to H5PMo12O40. If the reduced ECPB is subjected to a separate electrochemical reoxidation, then H2 is produced with full overall Faradaic efficiency.

Converting between the oxides of nitrogen using metal-ligand coordination complexes.

The oxides of nitrogen (chiefly NO, NO3(-), NO2(-) and N2O) are key components of the natural nitrogen cycle and are intermediates in a range of processes of enormous biological, environmental and industrial importance. Nature has evolved numerous enzymes which handle the conversion of these oxides to/from other small nitrogen-containing species and there also exist a number of heterogeneous catalysts that can mediate similar reactions. In the chemical space between these two extremes exist metal-ligand coordination complexes that are easier to interrogate than heterogeneous systems and simpler in structure than enzymes. In this Tutorial Review, we will examine catalysts for the inter-conversions of the various nitrogen oxides that are based on such complexes, looking in particular at more recent examples that take inspiration from the natural systems.

Efficient Electrocatalytic Water Oxidation at Neutral and High pH by Adventitious Nickel at Nanomolar Concentrations.

Electrolytic water oxidation using earth-abundant elements is a key challenge in the quest to develop cheap, large surface area arrays for solar-to-hydrogen conversion. There have been numerous studies in this area in recent years, but there remains an imperative to demonstrate that the current densities reported are indeed due to the species under consideration and not due to the presence of adventitious (yet possibly highly active) contaminants at low levels. Herein, we show that adventitious nickel at concentrations as low as 17 nM can act as a water oxidation catalyst in mildly basic aqueous solutions, achieving stable (tens of hours) current densities of 1 mA cm(-2) at overpotentials as low as 540 mV at pH 9.2 and 400 mV at pH 13. This nickel was not added to the electrolysis baths deliberately, but it was found to be present in the electrolytes as an impurity by ICP-MS. The presence of nickel on anodes from extended-time bulk electrolysis experiments was confirmed by XPS. In showing that such low levels of nickel can perform water oxidation at overpotentials comparable to many recently reported water oxidation catalysts, this work serves to raise the burden of proof required of new materials in this field: contamination by adventitious metal ions at trace loadings must be excluded as a possible cause of any observed water oxidation activity.

Electronically Stabilized Nonplanar Phenalenyl Radical and Its Planar Isomer.

Stable phenalenyl radicals have great potential as the basis for new materials for applications in the field of molecular electronics. In particular, electronically stabilized phenalenyl species that do not require steric shielding are molecules of fundamental interest, but are notoriously difficult to synthesize. Herein, the synthesis and characterization of two phenalenyl-type cations is reported: planar benzo[i]naphtho[2,1,8-mna]xanthenium (8(+)) and helical benzo[a]naphtho[8,1,2-jkl]xanthenium (9(+)), which can be reduced to the corresponding radicals. Radical 9 represents the first stable, helical phenalenyl radical which does not rely on bulky substituents to ensure its stability. Both cations are water-soluble, and the radicals are stable for weeks at room temperature under air. These compounds were characterized crystallographically, and also by NMR, EPR, electrochemistry, and electronic spectra. The synthesis of the previously reported compound benzo[5,6]naphthaceno[1,12,11,10-jklmna]xanthylium (5(+)), the largest oxygen-containing polycyclic hydrocarbon, was undertaken for comparison with 8(+) and 9(+), allowing us to report its crystal structure here for the first time. The different properties of these compounds and their radicals are explained by considering their differing aromaticities using in-depth computational methods.

Low pH electrolytic water splitting using earth-abundant metastable catalysts that self-assemble in situ.

Typical catalysts for the electrolysis of water at low pH are based on precious metals (Pt for the cathode and IrO2 or RuO2 for the anode). However, these metals are rare and expensive, and hence lower cost and more abundant catalysts are needed if electrolytically produced hydrogen is to become more widely available. Herein, we show that electrode-film formation from aqueous solutions of first row transition metal ions at pH 1.6 can be induced under the action of an appropriate cell bias and that in the case of cobalt voltages across the cell in excess of 2 V lead to the formation of a pair of catalysts that show functional stability for oxygen evolution and proton reduction for over 24 h. We show that these films are metastable and that if the circuit is opened, they redissolve into the electrolyte bath with concomitant O2 and H2 evolution, such that the overall Faradaic efficiency for charge into the system versus amounts of gases obtained approaches unity for both O2 and H2. This work highlights the ability of first row transition metals to mediate heterogeneous electrolytic water splitting in acidic media by exploiting, rather than trying to avoid, the natural propensity of the catalysts to dissolve at the low pHs used. This in turn we hope will encourage others to examine the promise of metastable electrocatalysts based on abundant elements for a range of reactions for which they have traditionally been overlooked on account of their perceived instability under the prevailing conditions.

Toward metal complexes that can directionally walk along tracks: controlled stepping of a molecular biped with a palladium(II) foot.

We report on the design, synthesis, and operation of a bimetallic molecular biped on a three-foothold track. The "walker" features a palladium(II) complex "foot" that can be selectively stepped between 4-dimethylaminopyridine and pyridine ligand sites on the track via reversible protonation while the walker remains attached to the track throughout by means of a kinetically inert platinum(II) complex foot. The substitution pattern of the three ligand binding sites, together with the kinetic stability of the metal-ligand coordination bonds, affords the two positional isomers a high degree of metastability, meaning that altering the chemical state of the track does not automatically instigate stepping in the absence of an additional stimulus (heat in the presence of a coordinating solvent). The use of metastable metal complexes for foot-track interactions offers a promising alternative to dynamic covalent chemistry for the design of small-molecule synthetic molecular walkers.

Enhanced ultrasonic degradation of methylene blue using a catalyst-free dual-frequency treatment.

Methylene blue is one of the most common pollutants found in wastewater, primarily due to its widespread use in the dye industry. Consequently, it is imperative to explore environmentally friendly and efficient methods for degrading this pollutant into non-toxic byproducts. While ultrasonic degradation methods in combination with additives or catalysts have proven effective, such additives or catalysts may inadvertently contribute to secondary pollution. Moreover, the preparation of these catalysts imposes an additional burden in terms of effort and cost. To address these issues, this paper introduces a catalyst-free dual-frequency ultrasound degradation approach for methylene blue. The sonochemical quality of the cavitation bubbles is improved using this technique because the bulk solution is populated with two types of bubbles, whose mean sizes are determined by the dual ultrasound frequencies. The findings demonstrate that, under identical acoustic power density conditions, dual-frequency ultrasound consistently outperforms single-frequency modes across all investigated parameters. Furthermore, the larger the difference between the dual frequencies used, the more effective the degradation of methylene blue. Finally, after just 20 min of sonication, a degradation efficiency of 91% was achieved with dual frequencies of 20 and 80 kHz at an acoustic power density of 209.63 ± 6.94 W/L. Consequently, this technique offers an environmentally friendly, catalyst-free, and highly effective method for degrading methylene blue.

Retraction: Synthesis of a chiral dinuclear Cu(II)-benzothiazolamine complex: evidence of cuprophilic interaction in its structure and exploration of its electrochemical properties and catalytic performance.

Retraction of 'Synthesis of a chiral dinuclear Cu(II)-benzothiazolamine complex: evidence of cuprophilic interaction in its structure and exploration of its electrochemical properties and catalytic performance' by O. Stephen Ojo et al., Dalton Trans., 2024, https://doi.org/10.1039/d3dt02994h.

Evaluating the native oxide of titanium as an electrocatalyst for oxalic acid reduction.

Herein, we show that unmodified titanium electrodes bearing the naturally-forming native TixOy coating display superior activity for the electroreduction of oxalic acid to glyoxylic acid and glycolic acid compared to Ti-based electrodes that have been deliberately modified for this purpose, in terms of both oxalic acid conversion and overall yields of reduced products.

Assessing Challenges of 2D-Molybdenum Ditelluride for Efficient Hydrogen Generation in a Full-Scale Proton Exchange Membrane (PEM) Water Electrolyzer.

Proton exchange membrane (PEM) water electrolyzers are critical enablers for sustainable green hydrogen production due to their high efficiency. However, nonplatinum catalysts are rarely evaluated under actual electrolyzer operating conditions, limiting knowledge of their feasibility for H2 production at scale. In this work, metallic 1T'-MoTe2 films were synthesized on carbon cloth supports via chemical vapor deposition and tested as cathodes in PEM electrolysis. Initial three-electrode tests revealed that at 100 mA cm-2, the overpotential of 1T'-MoTe2 approached that of leading 1T'-MoS2 systems, confirming its promise as a hydrogen evolution catalyst. However, when tested in a full-scale PEM electrolyzer, 1T'-MoTe2 delivered only 150 mA cm-2 at 2 V, far below expectations. Postelectrolysis analysis revealed an unexpected passivating tellurium layer, likely inhibiting catalytic sites. While initially promising, the unanticipated passivation caused 1T'-MoTe2 to underperform in practice. This highlights the critical need to evaluate emerging electrolyzer catalysts in PEM electrolyzers, revealing limitations of the idealized three-electrode configuration. Moving forward, validation of model systems in actual electrolyzers will be key to identifying robust nonplatinum catalysts for sustainable green hydrogen production.

A bio-inspired, small molecule electron-coupled-proton buffer for decoupling the half-reactions of electrolytic water splitting.

Electron-coupled-proton buffers (ECPBs) allow H2 and O2 evolution to be separated from each other in time during the electrolysis of water. Natural photosynthetic systems achieve an analogous feat during water splitting and employ a range of intermediate redox mediators such as quinone derivatives to aid this process. Drawing on this natural example, we show that a low molecular weight quinone derivative is capable of decoupling H2 evolution from O2 evolution at scale during electrochemical water splitting. This work could significantly lower the cost of ECPBs, paving the way for their more widespread adoption in water splitting.

Highly Selective Electrocatalytic Reduction of Substituted Nitrobenzenes to Their Aniline Derivatives Using a Polyoxometalate Redox Mediator.

Anilines and substituted anilines are used on the multi-ton scale for producing polymers, pharmaceuticals, dyes, and other important compounds. Typically, these anilines are produced from their corresponding nitrobenzene precursors by reaction with hydrogen at high temperatures. However, this route suffers from a number of drawbacks, including the requirement to handle hydrogen gas, rather harsh reaction conditions that lead to a lack of selectivity and/or toleration of certain functional groups, and questionable environmental sustainability. In light of this, routes to the reduction of nitrobenzenes to their aniline derivatives that operate at room temperature, in aqueous solvent, and without the requirement to use harsh process conditions, hydrogen gas, or sacrificial reagents could be of tremendous benefit. Herein, we report on a highly selective electrocatalytic route for the reduction of nitrobenzenes to their corresponding anilines that works in aqueous solution at room temperature and which does not require the use of hydrogen gas or sacrificial reagents. The method uses a polyoxometalate redox mediator, which reversibly accepts electrons from the cathode and reacts with the nitrobenzenes in solution to reduce them to the corresponding anilines. A variety of substituted nitroarenes are explored as substrates, including those with potentially competing reducible groups and substrates that are difficult to reduce selectively by other means. In all cases, the selectivity for the redox-mediated route is higher than that for the direct reduction of the nitroarene substrates at the electrode, suggesting that redox-mediated electrochemical nitroarene reduction is a promising avenue for the more sustainable synthesis of substituted anilines.

Synthesis of a chiral dinuclear Cu(II)-benzothiazolamine complex: evidence of cuprophilic interaction in its structure and exploration of its electrochemical properties and catalytic performance.

The synthesis of a chiral dinuclear [Cu(OAc)2(L1)]2 complex (A) and its analogues Cu(OAc)2(L1)2 (B), Cu(OAc)2(L1)PPh3 (C), CuBr(L1)PPh3 (D), and Cu(OAc)2(L2) (E) is described. The X-ray structure of A reveals a cuprophilic interaction (2.65 Å) and shows that L1 behaves as a monodentate ligand. The stereogenic centre in L1 aligns the NH group to form non-covalent interactions with the paddle-wheel acetate groups at variable distances (2.4-2.5 Å and 2.2-2.7 Å). Thermogravimetric analysis confirmed our hypothesis that two equivalents of L1 (B) or a combination of L1 and PPh3 (C) would disrupt the cuprophilic interaction. All complexes, except D, showed irreversible redox waves by cyclic voltammetry. Complexes C and E have lower oxidative peaks (at 10 V s-1) than complex A between +0.40 and +0.60 V. This highlights the influence of ligand(s) on the redox behaviour of Cu(II) complexes. The significance of this electrochemical behaviour was evident in the Chan-Lam (CL) coupling reaction, where 2.5 mol% of A successfully facilitated the formation of a C-N bond. This study showcased the structure, thermal stability, electrochemical properties and catalytic performance of a chiral dinuclear copper(II)-benzothiazolamine complex.

Electrochemical reduction of nitrobenzene via redox-mediated chronoamperometry.

Anilines are important feedstocks for pharmaceuticals, dyes, and other materials, but traditional approaches to their syntheses usually lack selectivity and environmental sustainability. Here, we describe the selective reduction of nitrobenzene to aniline under mild conditions, using water as the ultimate source of the required protons and electrons. We describe the electrochemical cell assembly, and detail steps for electrochemical reduction followed by organic extraction and analysis of the extracts using NMR. For complete details on the use and execution of this protocol, please refer to Stergiou and Symes (2022a).