The strong metal-support interactions induced electrocatalytic three-electron oxygen reduction to hydroxyl radicals for water treatment

Efficient Electrochemical-Enzymatic Conversion of PET to Formate Coupled with Nitrate Reduction Over Ru-Doped Co3O4 Catalysts

Electrochemical reforming presents a sustainable route for the conversion of nitrate (NO3 -) and polyethylene terephthalate (PET) into value-added chemicals, such as ammonia (NH3) and formic acid (HCOOH). However, its widespread application has been constrained by low selectivity due to the complexity of reduction processes and thus energy scaling limitations. In this study, the atomically dispersed Ru sites in Co3O4 synergistically interact with Co centers, facilitating the adsorption and activation of hydroxyl radicals (OH*) and ethylene glycol (EG), resulting in a remarkable HCOOH selectivity of 99% and a yield rate of 11.2 mmol h-1 cm-2 surpassing that of pristine Co3O4 (55% and 3.8 mmol h-1 cm-2). Furthermore, when applied as a bifunctional cathode catalyst, Ru-Co3O4 achieves a remarkable Faradaic efficiency (FE) of 98.5% for NH3 production (3.54 mmol h-1 cm-2) at -0.3 V versus RHE. Additionally, we developed a prototype device powered by a commercial silicon photovoltaic cell, enabling on-site solar-driven production of formate and NH3 through enzyme-catalyzed PET and NO3 - conversion. This study offers a viable approach for waste valorization and green chemical production, paving the way for sustainable energy applications.

Combining a Pd Cluster and a Built-in Electric Field as a Biomimic for Stable C-Cl Bond Polarization

Adopting the essence of enzyme catalysis, the strong binding of substrates into the active site pocket for their selective activation through multiple noncovalent interactions in the reactive site design can effectively enhance the electrocatalysis process. However, mimicking the enzyme catalytic process, particularly the introduction of reactant activation mechanisms, remains a significant challenge. Herein, we present a Pd cluster inside the Fe2N-Fe3O4-based built-in electric field (BEF), denoted as Pd/Fe2N-Fe3O4, to serve as an enzyme mimic to activate stable C-Cl bonds. Theoretical calculations and in situ Raman indicate that the probe molecule 2,4-dichlorophenol (2,4-DCP) adsorbs onto the Pd site and rotates inside the BEF with the C4-Cl bond being selectively activated and elongated from 1.73 to 1.82 Å. This makes Pd/Fe2N-Fe3O4 an excellent electrocatalytic hydrodechlorination catalyst, with Pd usage down to 2.5 μg cm-2, which is 32.7-360 times less than that of conventional catalysts like Pd/C, and achieving a Faradaic efficiency exceeding 20%. We reveal that besides H*-mediated electrochemical reduction, Pd/Fe2N-Fe3O4 also hydrodechlorinates activated 2,4-DCP via the proton-electron coupled transfer pathway. This understanding of the role of BEF in reactant activation, along with the strategy of integrating BEF and noble metals to mimic enzymes, provides a direction for the design of advanced electrocatalysts.

Unlocking single-atom induced electronic metal-support interactions in electrocatalytic one-electron water oxidation for wastewater purification

Electro-oxidation is a promising green technology for decentralized wastewater purification. However, its efficacy is primarily constrained by the selectivity and efficiency of hydroxyl radical (•OH) generation through one-electron water oxidation. In this study, we elucidate the mechanism of electronic metal-support interactions (EMSI) of Ni single-atoms on antimony-doped tin oxide anode (Ni/ATO) to enhance •OH production and overall water treatment efficiency. We experimentally and theoretically investigate both the structural evolution process and micro-interface mechanisms associated with the EMSI effects induced by Ni single-atoms. The optimized electronic structures in the interfacial catalysts under EMSI conditions and the co-catalytic role of Ni single-atoms synergistically facilitate selective and efficient •OH generation, resulting in over a fivefold increase in its steady-state concentration and tenfold enhancement in pseudo-first-order rate constant of sulfamethoxazole degradation compared to those on bare ATO. With the EMSI, rapid electron transfer channels were established for a marked enhancement in the adsorption, conversion, and dissociation of interfacial H2O molecules. Notably, it is revealed that Ni single-atoms serve as co-catalytic sites, exhibiting a "H-pulling effect" that is crucial for •OH generation. The Ni/ATO anode demonstrates great efficiency in degrading various refractory organic pollutants, and effectively treats real pharmaceutical wastewater with low energy consumption. Furthermore, it presents remarkable stability and adaptability, while maintaining a minimal environmental footprint during wastewater treatment processes. This work addresses the theoretical gaps between EMSI effects and co-catalysis in electro-oxidation systems, while providing a robust technological solution for wastewater purification.

Enhanced Fe(III)/Fe(II) Cycle by Lattice Sulfur for Continuous Fenton Reactions

The Fenton reaction is usually limited by the sluggish regeneration of Fe(II). In this article, we developed a Fenton system that uses metal sulfides (MSx) and diluted Fe(III) to activate H2O2, and the enhanced mechanism of the Fe(III)/Fe(II) cycle in the presence of sulfides was investigated. The lattice sulfur of MSx can donate electrons to reduce Fe(III) into Fe(II) and is partially oxidized to SO42- during H2O2 activation. •OH and 1O2 are the primary reactive oxygen species for pollutant removal. Meanwhile, low-cost iron-based sulfide (FeSx) is selected for scale-up experiments in a fixed-bed reactor, which can maintain 100% atrazine degradation over 240 h. Additionally, the Fukui function is employed to analyze the selective degradation pathway of atrazine, and the biological toxicity of the organic intermediates is also assessed. The novel FeSx/Fe(III) system provides a potential alternative to the traditional Fenton reaction.

Noble-Metal-Free Electrocatalysts for Selective Hydrogen Peroxide Generation via Oxygen Reduction Reaction

Hydrogen peroxide (H2O2) is a versatile chemical widely used in various industries. The traditional anthraquinone method for H2O2 synthesis has environmental and safety concerns due to the use of organic solvents and hazardous by-products. The direct synthesis of H2O2 from H2 and O2 poses risks of flammability and explosion. Recently, the 2-electron oxygen reduction reaction (2e- ORR) method has emerged as a promising alternative, offering safety, environmental friendliness, and cost-effectiveness. This method utilizes gas diffusion electrodes to efficiently generate H2O2 without the need for additional dilution. In this review, we focus on the recent advancements in noble-metal-free materials for 2e- ORR electrocatalysis, which play a crucial role in the efficient production of H2O2. These materials, including transition metal compounds, macrocyclic complexes, carbon-based catalysts, framework materials, and MXenes catalysts, demonstrate significant advantages in enhancing H2O2 yield. The development of these non-precious metal catalysts can reduce costs and improve sustainability and promote the commercialization of related technologies. The review concludes with an outlook on the future trends of 2e- ORR electrocatalysts.

Stability challenges of transition metal-modified cathodes for electro-Fenton process: A mini-review

Electro-Fenton (EF) process with transition-metal (TM) modified cathode has been regarded as a green and promising technology for wastewater treatment. Recently, breakthroughs in boosting catalyst activity for both two-electron oxygen reduction reaction (2e- ORR) and Fenton's reaction have gained intensive attention. However, achieving long-term stability of catalysts remains challenging, but is decisive for large-scale applications. This minireview provides fundamental understanding on the activity-stability correlation and the deactivation mechanisms of TM-based catalysts in EF systems, focusing on physical and chemical evolution, metal dissolution, catalyst detachment and structure collapse during long-term electrolysis. Subsequently, ongoing efforts from catalyst design to electrode engineering to stabilize the metal active sites are highlighted. Finally, the challenges and future perspectives in developing active and durable TM-modified cathodes are discussed, serving as a roadmap towards the large-scale application of EF process for wastewater treatment.

Modulation of Single-Iron-Atom Coordination Environment Toward Three-Electron Oxygen Reduction for Photocatalytic CH4 Conversion to CH3OH

By modifying the coordination environment of single-Fe-atom active site, effective regulation of the photocatalytic oxygen reduction pathway can be achieved to attain high activity for photocatalytic oxidation of CH4 to CH3OH in an aqueous solution. A comprehensive investigation is conducted to study the impact of different coordination numbers of single Fe atoms on photocatalytic CH4 oxidation reaction over carbon nitride. Among which, Fe1/C3-xN4 with a Fe-N3 coordination exhibit an exceptional photocatalytic performance in CH4 oxidation, reaching a remarkable methanol yield of 928.27 µmol gcat -1, much higher than Fe1/C3N4 and Fe1/C3N4-x (308.47 and 473.26 µmol gcat -1, respectively). Based on a collection of in situ characterizations and time-dependent density functional theory calculations, it is determined that Fe1/C3-xN4 with an optimal coordination number possesses the optimized electronic configuration that enables three-electron oxygen reduction to generate hydroxyl radicals for photocatalytic conversion of CH4 to CH3OH.

Activating Oxygen via the 3-Electron Pathway to Hydroxyl Radical by La-O4 Single-atom on WO3 for Water Purification

Shifting photocatalytic oxygen activation towards 3-electron (e-) pathway to hydroxyl radicals, instead of the normal 1 e- to superoxide radicals, becomes increasingly important for pollution degradation since hydroxyl radicals possess a high oxidation potential (2.80 V). Here, we demonstrate a selective oxygen activation to hydroxyl radicals via 3 e- pathway using single atomically dispersed La based on WO3 fabricated with oxygen vacancies to weaken La-O coordination strength (denoted as LaO4-WOv). Unsaturated-state La overcomes the rate-limited step of 2 e- oxygen reduction reaction towards to hydrogen peroxide via tuning binding free energy of key reaction intermediate *OOH, stepped by easy generation of hydroxyl radicals via 1 e-. This strategy alters the activation of oxygen process from 1 e- to 3 e-. Hydrogen peroxide and hydroxyl radicals over LaO4-WOv produces 3.8 and 3.3 mmol L-1 h-1, 35 and 8 times that of detected over WO3, respectively. Rapid and complete removal of tetracycline realizes over LaO4-WOv with 0.077 min-1 of rate constant, 38 times that of WO3. Degradation efficiencies keep greater than 98 % following five repeats, revealing a practical-utilization-level robust stability. This work builds an unsaturated-state transition metal site for manipulating oxygen activation towards an effective water purification as a representative application.

Mineral-Mediated Epitaxial Growth of CoO Nanoparticles for Efficient Electrochemical H2O2 Activation

Solution-phase epitaxy is a versatile method to synthesize functional nanomaterials with customized properties, where supports play a central role as they not only serve as nucleation templates but also greatly affect the local electronic structures. However, developing functional supports remains a great challenge. Herein, inspired by the commonly observed epitaxy of minerals in the natural environment, we report using calcination-modified kaolinite as the support for the epitaxial growth of hexagonal CoO nanoparticles (h-CoO NPs), which enables over 40 times higher mass-specific activity toward H2O2 electrochemical activation than the counterpart without the support. High-resolution electron microscopy, magic-angle spinning nuclear magnetic resonance, and X-ray absorption fine structure results prove that the Al sites in kaolinite play a crucial role in the formation of h-CoO NPs. Moreover, the five-coordinate Al (AlV) sites produced by the dehydration of kaolinite are indispensable for forming the epitaxial interface. Theoretical calculations reveal that the local electron densities around AlV sites are lower than those of general six-coordinate Al sites, which render AlV sites with strong adsorption capability that facilitates the nucleation of h-CoO NPs. Also, the AlV sites induce the electron transfer from h-CoO to the kaolinite support that results in the upshift of the Co 3d band center and hence improve the H2O2 activation kinetics. Our results demonstrate the superiority of nanoclay as functional supports and could offer a more benign strategy to the solution-phase epitaxy production of functional nanomaterials for diverse applications.

Strong Metal-Support Interactions in Heterogeneous Oxygen Electrocatalysis

Molecular oxygen redox electrocatalysis involves oxygen reduction and evolution as core reactions in various energy conversion and environmental technology fields. Strong metal-support interactions (SMSIs) based nanomaterials are regarded as desirable and state-of-the-art heterogeneous electrocatalysts due to their exceptional physicochemical properties. Over the past decades, considerable advancements in theory and experiment have been achieved in related studies, especially in modulating the electronic structure and geometrical configuration of SMSIs to enable activity, selectivity, and stability. In this focuses on the concept of SMSI, explore their various manifestations and mechanisms of action, and summarizes recent advances in SMSIs for efficient energy conversion in oxygen redox electrocatalysis applications. Additionally, the correlation between the physicochemical properties of different metals and supports is systematically elucidated, and the potential mechanisms of the structure-activity relationships between SMSIs and catalytic performance are outlined through theoretical models. Finally, the obstacles confronting this burgeoning field are comprehensively concluded, targeted recommendations and coping strategies are proposed, and future research perspectives are outlined.

Strong metal-support interaction (SMSI) in environmental catalysis: Mechanisms, application, regulation strategies, and breakthroughs

The strong metal-support interaction (SMSI) in supported catalysts plays a dominant role in catalytic degradation, upgrading, and remanufacturing of environmental pollutants. Previous studies have shown that SMSI is crucial in supported catalysts' activity and stability. However, for redox reactions catalyzed in environmental catalysis, the enhancement mechanism of SMSI-induced oxygen vacancy and electron transfer needs to be clarified. Additionally, the precise control of SMSI interface sites remains to be fully understood. Here we provide a systematic review of SMSI's catalytic mechanisms and control strategies in purifying gaseous pollutants, treating organic wastewater, and valorizing biomass solid waste. We explore the adsorption and activation mechanisms of SMSI in redox reactions by examining interfacial electron transfer, interfacial oxygen vacancy, and interfacial acidic sites. Furthermore, we develop a precise regulation strategy of SMSI from systematical perspectives of interface effect, crystal facet effect, size effect, guest ion doping, and modification effect. Importantly, we point out the drawbacks and breakthrough directions for SMSI regulation in environmental catalysis, including partial encapsulation strategy, size optimization strategy, interface oxygen vacancy strategy, and multi-component strategy. This review article provides the potential applications of SMSI and offers guidance for its controlled regulation in environmental catalysis.

Ov-rich γ-MnO2 enhanced electrocatalytic three-electron oxygen reduction to hydroxyl radicals for sterilization in neutral media

Marine biofouling severely limits the development of the marine economy, and reactive oxygen species (ROS) produced by electrocatalytic antifouling techniques could inactivate marine microorganisms and inhibit the formation of marine biofouling. Compared with an electro-Fenton reaction, a three-electron oxygen reduction reaction (3e- ORR) could generate a hydroxyl radical (˙OH) in situ without the limitation of pH and iron mud pollutants. Herein, Ov-rich γ-MnO2 is designed to enhance the 3e- ORR performance in neutral media and exhibits excellent sterilization performance for typical marine bacteria. DFT calculation reveals that Ov is beneficial to the "end-on" adsorption and activation of O2, and the Mn site could accept the electrons from *OOH and promote its further reduction to form ˙OH; Ov and Mn sites together guarantee the high 3e- ORR efficiency. In addition, liquid chromatography-tandem mass spectrometry (LC-MS/MS) proves the vast formation of ˙OH in the primary reaction stage, which is the key to sterilization. This work explores the reaction mechanism of the 3e- ORR in neutral media and provides the possibility for the application of electrocatalysis technology in the treatment of marine biofouling pollution.

In Situ Production of Hydroxyl Radicals via Three-Electron Oxygen Reduction: Opportunities for Water Treatment

The electro-Fenton (EF) process is an advanced oxidation technology with significant potential; however, it is limited by two steps: generation and activation of H2O2. In contrast to the production of H2O2 via the electrochemical two-electron oxygen reduction reaction (ORR), the electrochemical three-electron (3e-) ORR can directly activate molecular oxygen to yield the hydroxyl radical (⋅OH), thus breaking through the conceptual and operational limitations of the traditional EF reaction. Therefore, the 3e- ORR is a vital process for efficiently producing ⋅OH in situ, thus charting a new path toward the development of green water-treatment technologies. This review summarizes the characteristics and mechanisms of the 3e- ORR, focusing on the basic principles and latest progress in the in situ generation and efficient utilization of ⋅OH through the modulation of the reaction pathway, shedding light on the rational design of 3e- ORR catalysts, mechanistic exploration, and practical applications for water treatment. Finally, the future developments and challenges of efficient, stable, and large-scale utilization of ⋅OH are discussed based on achieving optimal 3e- ORR regulation and the potential to combine it with other technologies.

Constructing Nano-Heterostructure with Dual-Site to Boost H2O2 Activation and Regulate the Transformation of Free Radicals

A major issue with Fenton-like reaction is the excessive consumption of H2O2 caused by the sluggish regeneration rate of low-valent metal, and how to improve the activation efficiency of H2O2 has become a key in current research. Herein, a nano-heterostructure catalyst (1.0-MnCu/C) based on nano-interface engineering is constructed by supporting Cu and MnO on carbon skeleton, and its kinetic rate for the degradation of tetracycline hydrochloride is 0.0436 min-1, which is 2.9 times higher than that of Cu/C system (0.0151 min-1). The enhancement of removal rate results from the introduced Mn species can aggregate and transfer electrons to Cu sites through the electron bridge Mn-N/O-Cu, thus preventing Cu2+ from oxidizing H2O2 to form O2 •-, and facilitating the reduction of Cu2+ and generating more reactive oxygen species (1O2 and ·OH) with stronger oxidation ability, resulting in H2O2 utilization efficiency is 1.9 times as much as that of Cu/C. Additionally, the good and stable practical application capacity in different bodies demonstrates that it has great potential for practical environmental remediation.

Electronic Metal-Support Interactions Boost *OOH Intermediate Generation in Cu/In2Se3 for Electrochemical H2O2 Production

Two-electron oxygen reduction reaction (2e- ORR) is a promising method for the synthesis of hydrogen peroxide (H2O2). However, high energy barriers for the generation of key *OOH intermediates hinder the process of 2e- ORR. Herein, we prepared a copper-supported indium selenide catalyst (Cu/In2Se3) to enhance the selectivity and yield of 2e- ORR by employing an electronic metal-support interactions (EMSIs) strategy. EMSIs-induced charge rearrangement between metallic Cu and In2Se3 is conducive to *OOH intermediate generation, promoting H2O2 production. Theoretical investigations reveal that the inclusion of Cu significantly lowers the energy barrier of the 2e- ORR intermediate and impedes the 4e- ORR pathway, thus favoring the formation of H2O2. The concentration of H2O2 produced by Cu/In2Se3 is ~2 times than In2Se3, and Cu/In2Se3 shows promising applications in antibiotic degradation. This research presents a valuable approach for the future utilization of EMSIs in 2e- ORR.

Enhanced molecular oxygen activation via K/O interfacial modification for boosted electrocatalytic degradation over a broad pH range

Molecular oxygen activation plays an important role in the electrocatalytic degradation of recalcitrant pollutants. And the key lies in the tailoring of electronic structures over catalysts. Herein, carbon nitride with K/O interfacial modification (KOCN) was designed and fabricated for efficient molecular oxygen activation. Theoretical screening results revealed the possible substitution of peripheral N atoms by O atoms and the location of K atoms in the six-fold cavities of g-C3N4 framework. Spectroscopic and experimental results reveal that the existence of K/O promotes charge redistribution over as-prepared catalysts, leading to optimized electronic structures. Therefore, optimized oxygen adsorption was realized over 8 % KOCN, which was further converted into superoxide and singlet oxygen effectively. The rate constant of 8 % KOCN (1.8 × 10-2 min-1) reached 2.2 folds of pristine g-C3N4 (8.1 × 10-3 min-1) counterpart during tetracycline degradation. Moreover, the high electron mobility and excellent structural stability endow the catalyst with remarkable catalytic performance in a broad pH range of 3-11.