Catalytic Activity and Stability of Oxides: The Role of Near-Surface Atomic Structures and Compositions

Boosting Electrocatalytic Nitrogen Reduction on Cobalt-Based Perovskite via Regulating Reaction Pathway Through Donation-Back-Donation Modulation

The electrocatalytic approach of combining N2 and H2O to produce ammonia, known as the electrocatalytic N2 reduction reaction (eNRR), has garnered significant attention due to its environmental benefits and potential for supporting a decentralized agricultural economy. However, the underlying chemistry governing the reaction pathways remains poorly understood, hindering the design of low-cost and efficient eNRR catalysts. Here we report the enhancement of the electrocatalytic eNRR activity of perovskite oxides by tuning the reaction pathway through a "donation-back-donation" mechanism. This is achieved by controlling the spin state via adjusting the distribution of d orbital electrons in low-cost transition metals, such as cobalt. Specifically, the cobalt in perovskite SrCoO3 (SC) with a low-spin state demonstrates an 18 times higher ammonia yield rate compared to that in Co3O4 and 1.5 times higher than cobalt in perovskite LaCoO3 (LC). The low spin states of cobalt in SC enable better control of the eNRR reaction pathway over the transformation of *N2H to *NHNH or *NNH2, resulting in alternating hydrogenation in SC rather than distal hydrogenation in LC with a high spin state. The unprecedented improvement in eNRR by regulating the spin state of Co demonstrates the bright of low-cost Co-based electrocatalysts for ammonia production.

Enhancing Catalytic Removal of Autoexhaust Soot Particles via the Modulation of Interfacial Oxygen Vacancies in Cu/CeO2 Catalysts

The purification efficiency of autoexhaust carbon strongly depends on the heterogeneous interface structure between active metal and oxide, which can modulate the local electronic structure of defect sites to promote the activation of reactant molecules. Herein, the high-dispersion CuO clusters supported on the well-defined CeO2 nanorods were prepared using the complex deposition slow method. The formation of heteroatomic Cu+-Ov-Ce3+ interfacial structural units as active sites can capture electrons to achieve activation of the NO and O2 molecules. Among all of the synthesized catalysts, the Cu10/CeO2 catalyst exhibits superior catalytic performance (T50 = 351 °C) along with remarkable tolerance to H2O and SO2 in the removal of soot particles. Through a combination of comprehensive characterizations and density functional theory calculations, it is proposed that the interfacial Cu+-Ov-Ce3+ site, acting as an electron enrichment center, can capture electrons from the Cu d-band and Ce d/f-band to obtain high delocalized electron density, and then enhance the oxidation of NO to NO2, which plays a crucial role in the NOx-assisted catalytic mechanism for soot oxidation. This study presents a novel strategy for developing highly efficient catalysts that exhibit resistance to H2O and SO2, aimed at enhancing the removal of soot particles.

Ho3Fe5O12 nanoparticles immobilized on FPS for production of a biopolymer from CO2 and limonene epoxide

Herein, we present the synthesis of nanocatalysts with a large surface area. This was achieved through an interaction involving tetraethyl orthosilicate (TEOS) and tripolyphosphate (TPP), followed by the coupling of a ruthenium acetate complex with the click-transformed ligand of filamentous phosphosilicate (FPS). As a result, Ho3Fe5O12 nanoparticles were uniformly distributed without aggregation over FPS, forming Ho3Fe5O12@FPS. This substance was subsequently employed as a green nanocatalyst for the synthesis of cyclic carbonate from carbon dioxide and limonene epoxide whilst adhering to eco-friendly conditions. In the next step, we attempted to synthesize a polymer from synthesized natural cyclic carbonate. The incorporation of threadlike FPS divisions increased the ability to adsorb and aided the retrieval of the adsorbent without notably diminishing its effectiveness. The formed products were easily separated from the eco-friendly medium, and the catalyst was reused many times without a noticeable decrease in its activity and specificity.

V-Integration Modulates t2g -Electrons of a Single Crystal Ir1- x (Ir0.8 V0.2 O2 )x -BHC for Boosted and Durable OER in Acidic Electrolyte

Realizing efficacious π-donation from the O 2p orbital to electron-deficient metal (t_2g ) d-orbitals along with separately tuned adsorption of *O and *OOH, is an imperious pre-requisite for an electrocatalyst design to demonstrate boosted oxygen evolution reaction (OER) performance. To regulate the π-donation and the adsorption ability for *O and *OOH, herein, a facile strategy to modulate the electron transfer from electron-rich t_2g -orbitals to electron-deficient t_2g -orbitals, via strong π-donation from the π-symmetry lone pairs of the bridging O^2- , and the d-band center of a biomimetic honeycomb (BHC)-like nanoarchitecture (Ir_{1- x} (Ir_0.8 V_0.2 O₂ )_x -BHC) is introduced. The suitable integration of V heteroatoms in the single crystal system of IrO₂ decreases the electron density on the neighboring Ir sites, and causes an upshift in the d-band center of Ir_{1- x} (Ir_0.8 V_0.2 O₂ )_x -BHC, weakening the adsorption of *O while strengthening that of *OOH, lowers the energy barrier for OER. Therefore, BHC design demonstrates excellent OER performance (shows a small overpotential of 238 mV at 10 mA cm^-2 and a Tafel slope of 39.87 mV dec^-1 ) with remarkable stability (130 h) in corrosive acidic electrolyte. This work opens a new corridor to design robust biomimetic nanoarchitectures of modulated π-symmetry (t_2g ) d-orbitals and the band structure, to achieve excellent activity and durability in acidic environment.

Tailoring Oxygen Reduction Reaction Pathway on Spinel Oxides via Surficial Geometrical-Site Occupation Modification Driven by the Oxygen Evolution Reaction

The oxygen reduction reaction (ORR) has been demonstrated as a critical technology for both energy conversion technologies and hydrogen peroxide intermediate production. Herein, an in situ oxygen evolution reaction (OER) surface evolution strategy is applied for changing the surface structure of MnCo₂ O₄ oxide with tetrahedral and octahedral cations vacancies to realize reaction pathway switching from 2e^- ORR and 4e^- ORR. Interestingly, the as-synthesized MnCo₂ O₄ -pristine (MnCo₂ O₄ -P) with the highest surficial Mn/Co octahedron occupation favors two electrons reaction routes exhibiting high H₂ O₂ selectivity (≈80% and reaches nearly 100% at 0.75 V vs RHE); after surface atoms reconstruction, MnCo₂ O₄ -activation (MnCo₂ O₄ -A) with the largest Mn/Co tetrahedron occupation present excellent ORR performance through the four-electron pathway with an ultrahigh onset potential and half-wave potential of 0.78 and 0.92 V, ideal mass activity (MA), and turnover frequencies (TOF) values. Density functional theory (DFT) calculations reveal the concurrent modulations of both Co and Mn by the surface reconstructions, which improve the electroactivity of MnCo₂ O₄ -A toward the 4e^- pathway. This work provides a new perspective to building correlation of OER activation-ORR property, bringing detailed understating for reaction route transformation, and thus guiding the development of certain electrocatalysts with specific purposes.

The Restructuring-Induced CoO x Catalyst for Electrochemical Water Splitting

Restructuring is an important yet less understood phenomenon in the catalysis community. Recent studies have shown that a group of transition metal sulfide catalysts can completely or partially restructure during electrochemical reactions which then exhibit high activity even better than the best commercial standards. However, such restructuring processes and the final structures of the new catalysts are elusive, mainly due to the difficulty from the reaction-induced changes that cannot be captured by ex situ characterizations. To establish the true structure-property relationship in these in situ generated catalysts, we use multimodel operando characterizations including Raman spectroscopy, X-ray absorption spectroscopy, and X-ray reflectivity to investigate the restructuring of a representative catalyst, Co₉S₈, that shows better activity compared to the commercial standard RuO₂ during the oxygen evolution reaction (OER), a key half reaction in water-splitting for hydrogen generation. We find that Co₉S₈ ultimately converts to oxide cluster (CoO _x ) containing six oxygen coordinated Co octahedra as the basic unit which is the true catalytic center to promote high OER activity. The density functional theory calculations verify the in situ generated CoO _x consisting of edge-sharing CoO₆ octahedral clusters as the actual active sites. Our results also provide insights to design other transition-metal-based materials as efficient electrocatalysts that experience a similar restructuring in OER.

Structural Transformation of Heterogeneous Materials for Electrocatalytic Oxygen Evolution Reaction

Electrochemical water splitting for hydrogen generation is a promising pathway for renewable energy conversion and storage. One of the most important issues for efficient water splitting is to develop cost-effective and highly efficient electrocatalysts to drive sluggish oxygen-evolution reaction (OER) at the anode side. Notably, structural transformation such as surface oxidation of metals or metal nonoxide compounds and surface amorphization of some metal oxides during OER have attracted growing attention in recent years. The investigation of structural transformation in OER will contribute to the in-depth understanding of accurate catalytic mechanisms and will finally benefit the rational design of catalytic materials with high activity. In this Review, we provide an overview of heterogeneous materials with obvious structural transformation during OER electrocatalysis. To gain insight into the essence of structural transformation, we summarize the driving forces and critical factors that affect the transformation process. In addition, advanced techniques that are used to probe chemical states and atomic structures of transformed surfaces are also introduced. We then discuss the structure of active species and the relationship between catalytic performance and structural properties of transformed materials. Finally, the challenges and prospects of heterogeneous OER electrocatalysis are presented.

Defect-Driven Oxide Transformations and the Electrochemical Interphase

ConspectusThe redox reaction pathway is crucial to the sustainable production of the fuels and chemicals required for a carbon-neutral society. Our society is becoming increasingly dependent on devices using batteries and electrolyzers, all of which rely on a series of redox reactions. The overall properties of oxide materials make them very well suited for such electrochemical and catalytic applications due to their associated cationic redox properties and the static site-adsorbate interactions. As these technologies have matured, it has become apparent that defect-driven redox reactions, defect-coupled diffusion, and structural transformations that are both time- and rate-dependent are also critical materials processes. This change in focus, considering not only redox properties but also more complex, dynamic behaviors, represents a new research frontier in the molecular sciences as they are strongly linked to device operation and degradation and lie at the heart of various phenomena that take place at electrochemical interfaces. Fundamental studies of the structural, electronic, and chemical transformation mechanisms are key to the advancement of materials and technological innovations that could be implemented in various electrochemical systems.In this Account, we focus on recent studies and advances in characterizing and understanding the dynamic redox evolution and structural transformations that take place in model perovskites and layered oxides under reactive conditions and correlate them with degradation mechanisms and operations in electrolyzers and batteries. We show that the dynamic evolution of oxygen vacancies and cationic migration in the surface or bulk occurs at the solid-liquid interface, using a combination of different synchrotron-based X-ray spectroscopies and scattering probes. Detailed redox-structure-reactivity correlation studies show how defects and diffusion processes can be tailored to drive various physical and chemical transformations in electrolyzers and batteries. We also highlight a strong correlation between oxygen redox reactivity and structural reorganization in both model thin films and particles, helping to bridge the gap between fundamental studies of the reaction mechanism and device applications. On the basis of these findings, we discuss strategies to probe and tune the redox reactivity and structural stability of the redox-active oxide interphase toward devising efficient pathways for energy and chemical harvesting.

Copper oxide/graphitic carbon nitride composite for bisphenol a degradation by boosted peroxymonosulfate activation: Mechanism of Cu-O covalency governs

Surface structure can govern heterogeneous catalysis, resulting in its critical role in nonradical reactions. Here, we explored whether Cu-O covalency plays a critical role in controlling the inherent properties of copper oxide/graphitic carbon nitride (CuO-CN). Experiments and theoretical calculations show that, in contrast to the traditional concept of low-valent metal control activity, surface modification enlarges Cu-O covalency, and high-valent copper species at the surface easily bind peroxymonosulfate (PMS, (HSO₅^-)) anions. Therefore, optimized CuO-CN corresponds to a 14.8-fold higher kinetic reaction rate (0.10392 min^-1) for PMS activation and pollutant degradation over those of unoptimized CuO-CN. Based on two-dimensional Fourier transform infrared correlation spectroscopy (2D-FT-IR-COS), Cu-O was determined to be the main active site. Cu-O is more active than other groups and acts before other groups. Benefiting from this electron transfer mechanism, CuO-CN shows good environmental tolerance (pH, anions, humic acid and actual water bodies such as tap water and groundwater). The established empirical kinetic model shows a strong linear correlation with the experimental kinetic reaction rate (> 0.94). CuO-CN/PMS can degrade organic pollutants efficiently for up to 30 days in a filter reactor. This work provides an understanding of the key role of the surface electronic structure in the nonradical activation of PMS and may provide support for improving the design of PMS catalysts.

Precipitation of dopants on acceptor-doped LaMnO3±δ revealed by defect chemistry from first principles

Perovskite oxides degrade at elevated temperatures while precipitating dopant-rich particles on the surface. A knowledge-based improvement of surface stability requires a fundamental and quantitative understanding of the dopant precipitation mechanism on these materials. We propose that dopant precipitation is a consequence of the variation of dopant solubility between calcination and operating conditions in solid oxide fuel cells (SOFCs) and electrolyzer cells (SOECs). To study dopant precipitation, we use 20% (D = Ca, Sr, Ba)-doped LaMnO_3+δ (LDM20) as a model system. We employ a defect model taking input from density functional theory calculations. The defect model considers the equilibration of LDM20 with a reservoir consisting of dopant oxide (DO), peroxide (DO₂), and O₂ in the gas phase. The equilibrated non-stoichiometry of the A-site and B-site as a function of temperature, T, and oxygen partial pressure, p(O₂), reveals three regimes for LDM20: A-site deficient (oxidizing conditions), A-site rich (atmospheric conditions), and near-stoichiometric (reducing conditions). Assuming an initial A/B non-stoichiometry, we compute the dopant precipitation boundaries in a p-T phase diagram. Our model predicts precipitation both under reducing (DO) and under highly oxidizing conditions (DO₂). We found precipitation under anodic, SOEC conditions to be promoted by large dopant size, while under cathodic, SOFC conditions precipitation is promoted by initial A-site excess. The main driving forces for precipitation are oxygen uptake by the condensed phase under oxidizing conditions and oxygen release assisted by B-site vacancies under reducing conditions. Possible strategies for mitigating dopant precipitation under in electrolytic and fuel cell conditions are discussed.

Mn-O Covalency Governs the Intrinsic Activity of Co-Mn Spinel Oxides for Boosted Peroxymonosulfate Activation

Transition metal (TM)-based bimetallic spinel oxides can efficiently activate peroxymonosulfate (PMS) presumably attributed to enhanced electron transfer between TMs, but the existing model cannot fully explain the efficient TM redox cycling. Here, we discover a critical role of TM-O covalency in governing the intrinsic catalytic activity of Co_3-x Mn_x O₄ spinel oxides. Experimental and theoretical analysis reveals that the Co sites significantly raises the Mn valence and enlarges Mn-O covalency in octahedral configuration, thereby lowering the charge transfer energy to favor Mn_Oh -PMS interaction. With appropriate Mn^IV /Mn^III ratio to balance PMS adsorption and Mn^IV reduction, the Co_1.1 Mn_1.9 O₄ exhibits remarkable catalytic activities for PMS activation and pollutant degradation, outperforming all the reported TM spinel oxides. The improved understandings on the origins of spinel oxides activity for PMS activation may inspire the development of more active and robust metal oxide catalysts.

Cation insertion to break the activity/stability relationship for highly active oxygen evolution reaction catalyst

The production of hydrogen at a large scale by the environmentally-friendly electrolysis process is currently hampered by the slow kinetics of the oxygen evolution reaction (OER). We report a solid electrocatalyst α-Li₂IrO₃ which upon oxidation/delithiation chemically reacts with water to form a hydrated birnessite phase, the OER activity of which is five times greater than its non-reacted counterpart. This reaction enlists a bulk redox process during which hydrated potassium ions from the alkaline electrolyte are inserted into the structure while water is oxidized and oxygen evolved. This singular charge balance process for which the electrocatalyst is solid but the reaction is homogeneous in nature allows stabilizing the surface of the catalyst while ensuring stable OER performances, thus breaking the activity/stability tradeoff normally encountered for OER catalysts.

Renewable hydrogen production from water will require understanding and improving the oxygen evolution reaction (OER) on catalyst surfaces. Here, authors report α-Li₂IrO₃ to transform into a hydrated birnessite phase under OER conditions that exhibits enhanced OER performances and durabilities.

Cytotoxicity, mutagenicity, oxidative stress and mitochondrial impairment in human hepatoma (HepG2) cells exposed to copper oxide, copper-iron oxide and carbon nanoparticles

The increasing application of nanomaterials in various fields such as drug delivery, cosmetics, disease detection, cancer treatment, food preservation etc. has resulted in high levels of engineered nanoparticles in the environment, thus leading to higher possibility of direct or indirect interactions between these particles and biological systems. In this study, the toxic effects of three commercially available nanomaterials; copper oxide nanoparticles, copper-iron oxide nanopowders and carbon nanopowders were determined in the human hepatoma HepG2 cells using various toxicological assays which are indicative of cytotoxicity (MTT and neutral red assays), mutagenicity (cytokinesis-block micronucleus assay), oxidative stress (total reactive oxygen species and superoxide anion production) and mitochondrial impairment (cellular oxygen consumption). There was increased cytotoxicity, mutagenicity, and mitochondrial impairment in the cells treated with higher concentrations of the nanomaterials, especially the copper oxide nanoparticles. The fold production of reactive oxygen species was similar at the concentrations tested in this study but longer exposure duration resulted in production of more superoxide anions. The results of this study showed that copper oxide nanoparticles are highly toxic to the human HepG2 cells, thus implying that the liver is a target organ in human for copper oxide nanoparticles toxicity.

Impurity Segregation and Nanoparticle Reorganization of Indium Doped MgO Cubes

Metal oxide nanocomposites are non-equilibrium solids and promising precursors for functional materials. Annealing of such materials can provide control over impurity segregation and, depending on the level of consolidation, represents a versatile approach to engineer free surfaces, particle-particle interfaces and grain boundaries. Starting with indium-magnesium-oxide nanoparticle powders obtained via injection of an indium organic precursor into the magnesium combustion flame and subsequent particle quenching in argon, we investigated the stability of the trivalent In3+ ions in the host lattice of MgO nanoparticles by determining grain growth, morphology evolution and impurity segregation. The latter process is initiated by vacuum annealing at 873 K and can be tracked at 1173 K on a time scale of minutes. In the first instance the surface segregated indium wets the nanoparticle interfaces. After prolonged annealing indium evaporates and leaves the powder via the gas phase. Resulting MgO nanocubes are devoid of residual indium, regain their high morphological definition and show spectroscopic fingerprints (UV Diffuse Reflectance and Photoluminescence emission) that are characteristic of electronically unperturbed MgO cube corner and edge features. The results of this combined XRD, TEM, and spectroscopy study reveal the parameter window within which control over indium segregation is used to introduce a semiconducting metal oxide component into the intergranular region between insulating MgO nanograins.

Enlarged CoO Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction

Cobalt-containing spinel oxides are promising electrocatalysts for the oxygen evolution reaction (OER) owing to their remarkable activity and durability. However, the activity still needs further improvement and related fundamentals remain untouched. The fact that spinel oxides tend to form cation deficiencies can differentiate their electrocatalysis from other oxide materials, for example, the most studied oxygen-deficient perovskites. Here, a systematic study of spinel ZnFe_x Co_2-x O₄ oxides (x = 0-2.0) toward the OER is presented and a highly active catalyst superior to benchmark IrO₂ is developed. The distinctive OER activity is found to be dominated by the metal-oxygen covalency and an enlarged CoO covalency by 10-30 at% Fe substitution is responsible for the activity enhancement. While the pH-dependent OER activity of ZnFe_0.4 Co_1.6 O₄ (the optimal one) indicates decoupled proton-electron transfers during the OER, the involvement of lattice oxygen is not considered as a favorable route because of the downshifted O p-band center relative to Fermi level governed by the spinel's cation deficient nature.

Controlling cation segregation in perovskite-based electrodes for high electro-catalytic activity and durability

Solid oxide cell (SOC) based energy conversion systems have the potential to become the cleanest and most efficient systems for reversible conversion between electricity and chemical fuels due to their high efficiency, low emission, and excellent fuel flexibility. Broad implementation of this technology is however hindered by the lack of high-performance electrode materials. While many perovskite-based materials have shown remarkable promise as electrodes for SOCs, cation enrichment or segregation near the surface or interfaces is often observed, which greatly impacts not only electrode kinetics but also their durability and operational lifespan. Since the chemical and structural variations associated with surface enrichment or segregation are typically confined to the nanoscale, advanced experimental and computational tools are required to probe the detailed composition, structure, and nanostructure of these near-surface regions in real time with high spatial and temporal resolutions. In this review article, an overview of the recent progress made in this area is presented, highlighting the thermodynamic driving forces, kinetics, and various configurations of surface enrichment and segregation in several widely studied perovskite-based material systems. A profound understanding of the correlation between the surface nanostructure and the electro-catalytic activity and stability of the electrodes is then emphasized, which is vital to achieving the rational design of more efficient SOC electrode materials with excellent durability. Furthermore, the methodology and mechanistic understanding of the surface processes are applicable to other materials systems in a wide range of applications, including thermo-chemical photo-assisted splitting of H₂O/CO₂ and metal-air batteries.

Superexchange Effects on Oxygen Reduction Activity of Edge-Sharing [Cox Mn1-x O6 ] Octahedra in Spinel Oxide

Mn-Co containing spinel oxides are promising, low-cost electrocatalysts for the oxygen reduction reaction (ORR). Most studies are devoted to the design of porous Mn-Co spinels or to strongly coupled hybrids (e.g., MnCo₂ O₄ /N-doped-rmGO) to maximize the mass efficiency. The lack of analyses by metal oxide intrinsic activity (activity normalized to catalysts' surface area) hinders the development of fundamental understanding of the physicochemical principles behind the catalytic activities. A systematic study on the composition dependence of ORR in ZnCo_x Mn_2-x O₄ (x = 0.0-2.0) spinel is presented here with special attention to the role of edge sharing [Co_x Mn_1-x O₆ ] octahedra in the spinel structure. The ORR specific activity of ZnCo_x Mn_2-x O₄ spans across a potential window of 200 mV, indicating an activity difference of ≈3 orders of magnitude. The curve of composition-dependent ORR specific activity as a function of Co substitution exhibits a volcano shape with an optimum Mn/Co ratio of 0.43. It is revealed that the modulated e_g occupancy of active Mn cations (0.3-0.9), as a consequence of the superexchange effect between edge sharing [CoO₆ ] and [MnO₆ ], reflects the ORR activity of edge sharing [Co_x Mn_1-x O₆ ] octahedra in the ZnCo_x Mn_2-x O₄ spinel oxide. These findings offer crucial insights in designing spinel oxide catalysts with fine-tuned e_g occupancy for efficient catalysis.

Layer-by-Layer Epitaxial Growth of Defect-Engineered Strontium Cobaltites

Control over structure and composition of (ABO₃) perovskite oxides offers exciting opportunities since these materials possess unique, tunable properties. Perovskite oxides with cobalt B-site cations are particularly promising, as the range of the cation's stable oxidation states leads to many possible structural frameworks. Here, we report growth of strontium cobalt oxide thin films by molecular beam epitaxy, and conditions necessary to stabilize different defect concentration phases. In situ X-ray scattering is used to monitor structural evolution during growth, while in situ X-ray absorption near-edge spectroscopy is used to probe oxidation state and measure changes to oxygen vacancy concentration as a function of film thickness. Experimental results are compared to kinetically limited thermodynamic predictions, in particular, solute trapping, with semiquantitative agreement. Agreement between observations of dependence of cobaltite phase on oxidation activity and deposition rate, and predictions indicates that a combined experimental/theoretical approach is key to understanding phase behavior in the strontium cobalt oxide system.

Active sites of copper-complex catalytic materials for electrochemical carbon dioxide reduction

Restructuring-induced catalytic activity is an intriguing phenomenon of fundamental importance to rational design of high-performance catalyst materials. We study three copper-complex materials for electrocatalytic carbon dioxide reduction. Among them, the copper(II) phthalocyanine exhibits by far the highest activity for yielding methane with a Faradaic efficiency of 66% and a partial current density of 13 mA cm⁻² at the potential of – 1.06 V versus the reversible hydrogen electrode. Utilizing in-situ and operando X-ray absorption spectroscopy, we find that under the working conditions copper(II) phthalocyanine undergoes reversible structural and oxidation state changes to form ~ 2 nm metallic copper clusters, which catalyzes the carbon dioxide-to-methane conversion. Density functional calculations rationalize the restructuring behavior and attribute the reversibility to the strong divalent metal ion–ligand coordination in the copper(II) phthalocyanine molecular structure and the small size of the generated copper clusters under the reaction conditions.

The catalytic conversion of carbon dioxide into value-added products requires an understanding of the active species present under working conditions. Here, the authors discover copper-containing complexes to reversibly transform during electrocatalysis into methane-producing copper nanoclusters.

Trends in adsorption of electrocatalytic water splitting intermediates on cubic ABO3 oxides

The reactivity of solid oxide surfaces towards adsorption of oxygen and hydrogen is a key metric for the design of new catalysts for electrochemical water splitting.

Insights into the Surface Reactivity of Cermet and Perovskite Electrodes in Oxidizing, Reducing, and Humid Environments

Understanding the surface chemistry of electrode materials under gas environments is important in order to control their performance during electrochemical and catalytic applications. This work compares the surface reactivity of Ni/YSZ and La_0.75Sr_0.25Cr_0.9Fe_0.1O₃, which are commonly used types of electrodes in solid oxide electrochemical devices. In situ synchrotron-based near-ambient pressure photoemission and absorption spectroscopy experiments, assisted by theoretical spectral simulations and combined with microscopy and electrochemical measurements, are used to monitor the effect of the gas atmosphere on the chemical state, the morphology, and the electrical conductivity of the electrodes. It is shown that the surface of both electrode types readjusts fast to the reactive gas atmosphere and their surface composition is notably modified. In the case of Ni/YSZ, this is followed by evident changes in the oxidation state of nickel, while for La_0.75Sr_0.25Cr_0.9Fe_0.1O₃, a fine adjustment of the Cr valence and strong Sr segregation is observed. An important difference between the two electrodes is their capacity to maintain adsorbed hydroxyl groups on their surface, which is expected to be critical for the electrocatalytic properties of the materials. The insight gained from the surface analysis may serve as a paradigm for understanding the effect of the gas environment on the electrochemical performance and the electrical conductivity of the electrodes.

Chaperonin-Nanocaged Hemin as an Artificial Metalloenzyme for Oxidation Catalysis

Taking inspiration from biology's effectiveness in functionalizing protein-based nanocages for chemical processes, we describe here a rational design of an artificial metalloenzyme for oxidations with the bacterial chaperonin GroEL, a nanocage for protein folding in nature, by supramolecular anchoring of catalytically active hemin in its hydrophobic central cavity. The promiscuity of the chaperonin cavity is an essential element of this design, which can mimic the hydrophobic binding pocket in natural metalloenzymes to accept cofactor and substrate without requiring specific ligand-protein interactions. The success of this approach is manifested in the efficient loading of multiple monomeric hemin cofactors to the GroEL cavity by detergent dialysis and good catalytic oxidation properties of the resulting biohybrid in tandem with those of the clean oxidant of H₂O₂. Investigation of the mechanism of hemin-GroEL-catalyzed oxidation of two-model substrates reveals that the kinetic behavior of the complex follows a ping-pong mechanism in both cases. Through comparison with horseradish peroxidase, the oxidative activity and stability of hemin-GroEL were observed to be similar to those found in natural peroxidases. Adenosine 5'-triphosphate (ATP)-regulated partial dissociation of the biohybrid, as assessed by the reduction of its catalytic activity with the addition of the nucleotide, raises the prospect that ATP may be used to recycle the chaperonin scaffold. Moreover, hemin-GroEL can be applied to the chromogenic detection of H₂O₂, which (or peroxide in general) is commonly contained in industrial wastes. Considering the rich chemistry of free metalloporphyrins and the ease of production of GroEL and its supramolecular complex with hemin, this work should seed the creation of many new artificial metalloenzymes with diverse reactivities.

Cations in Octahedral Sites: A Descriptor for Oxygen Electrocatalysis on Transition-Metal Spinels

Exploring efficient and low-cost electrocatalysts for the oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER) is critical for developing renewable energy technologies such as fuel cells, metal-air batteries, and water electrolyzers. A rational design of a catalyst can be guided by identifying descriptors that determine its activity. Here, a descriptor study on the ORR/OER of spinel oxides is presented. With a series of MnCo₂ O₄ , the Mn in octahedral sites is identified as an active site. This finding is then applied to successfully explain the ORR/OER activities of other transition-metal spinels, including Mn_x Co_3-x O₄ (x = 2, 2.5, 3), Li_x Mn₂ O₄ (x = 0.7, 1), XCo₂ O₄ (X = Co, Ni, Zn), and XFe₂ O₄ (X = Mn, Co, Ni). A general principle is concluded that the e_g occupancy of the active cation in the octahedral site is the activity descriptor for the ORR/OER of spinels, consolidating the role of electron orbital filling in metal oxide catalysis.

Advanced hybrid battery with a magnesium metal anode and a spinel LiMn2O4 cathode

Two Mg–Li dual salt hybrid electrolytes are developed, which exhibit excellent oxidative stability up to around 3.8 V (vs. Mg/Mg²⁺) on Al, and were successfully applied in hybrid Mg–Li cells with lithium-ion intercalation cathodes and magnesium metal anodes.