Atomically Dispersed Cu on In2O3 for Relay Electrocatalytic Conversion of Nitrate and CO2 to Urea.

Urea electrosynthesis from coelectrolysis of NO3- and CO2 (UENC) holds a significant prospect to achieve efficient and sustainable urea production. Herein, atomically dispersed Cu on In2O3 (Cu1/In2O3) is designed as an effective and robust catalyst for the UENC. Combined theoretical calculations and in situ spectroscopic analysis reveal the synergistic effect of the Cu1-O2-In site and the In site to boost the UENC energetics via a relay catalysis pathway, where the Cu1-O2-In site drives *NO3 → *NH2 and the In site catalyzes *CO2 → *CO. The generated *CO is then migrated from the In site to the Cu1-O2-In site, followed by C-N coupling with *NH2 on the Cu1-O2-In site to generate urea. Consequently, Cu1/In2O3 assembled within a flow cell exhibits an impressive urea yield rate of 28.97 mmol h-1 g-1 with a urea-Faradaic efficiency (FEurea) of 50.88%.

Metal Atom-Support Interaction in Single Atom Catalysts toward Hydrogen Peroxide Electrosynthesis.

Single metal atom catalysts (SACs) have garnered considerable attention as promising agents for catalyzing important industrial reactions, particularly the electrochemical synthesis of hydrogen peroxide (H2O2) through the two-electron oxygen reduction reaction (ORR). Within this field, the metal atom-support interaction (MASI) assumes a decisive role, profoundly influencing the catalytic activity and selectivity exhibited by SACs, and triggers a decade-long surge dedicated to unraveling the modulation of MASI as a means to enhance the catalytic performance of SACs. In this comprehensive review, we present a systematic summary and categorization of recent advancements pertaining to MASI modulation for achieving efficient electrochemical H2O2 synthesis. We start by introducing the fundamental concept of the MASI, followed by a detailed and comprehensive analysis of the correlation between the MASI and catalytic performance. We describe how this knowledge can be harnessed to design SACs with optimized MASI to increase the efficiency of H2O2 electrosynthesis. Finally, we distill the challenges that lay ahead in this field and provide a forward-looking perspective on the future research directions that can be pursued.

Atomically Dispersed Iridium on Polyimide Support for Acidic Oxygen Evolution.

Designing a high-performing iridium (Ir) single-atom catalyst is desired for acidic water electrolysis, which shows enormous potential given its high catalytic activity toward acidic oxygen evolution reaction (OER) with minimum usage of precious Ir metal. However, it still remains a substantial challenge to stabilize the Ir single atoms during the OER operation without sacrificing the activity. Here, we report a high-performing OER catalyst by immobilizing Ir single atoms on a polyimide support, which exhibits a high mass activity on a carbon paper electrode while simultaneously achieving outstanding stability with negligible decay for 360 h. The resulting electrode (denoted as Ir1-PI@CP) reaches a 49.7-fold improvement in mass activity compared to the counterpart electrode prepared without polyimide support. Both our experimental and theoretical results suggest that, owing to the strong metal-support interactions, the polyimide support can enhance the Ir 5d states of Ir single atoms in Ir1-PI@CP, which can tailor the adsorption energies of intermediates and decrease the thermodynamic barrier at the rate-determining step of the OER, but also facilitate the proton-electron-transfer process and improve the reaction kinetics. This work offers an alternative avenue for developing single-atom catalysts with superior activity and durability toward various catalytic systems and beyond.

Zinc Assisted Thermal Etching for Rich Edge-Located Fe-N4 Active Sites in Defective Carbon Nanofiber for Activity Enhancement of Oxygen Electroreduction.

Single-atom catalysts (SACs) with edge-located metal active sites exhibit superior oxygen reduction reaction (ORR) performance due to their narrower energy gap and higher electron density. However, controllably designing such active sites to fully reveal their advantages remains challenging. Herein, rich edge-located Fe-N4 active sites anchored in hierarchically porous carbon nanofibers (denoted as e1-Fe-N-C) are fabricated via an in situ zinc-assisted thermal etching strategy. The e1-Fe-N-C catalyst demonstrates superior alkaline ORR activity compared to counterparts with fewer edge-located Fe-N4 sites and commercial Pt/C. Density functional theory calculations show that the accumulation of more negative charges near the Fe-N and the formation of partially reduced Fe state in the edge-located Fe-N4 sites reduce the energy barrier for the ORR process. Additionally, the unique hierarchically porous structures with mesopores and macropores facilitate full utilization of the active sites and enhance long-range mass transfer. The zinc-air battery (ZAB) assembled with e1-Fe-N-C has a peak power density of 198.9 mW cm-2, superior to commercial Pt/C (152.3 mW cm-2). The present strategy by facile controlling the amount of the zinc acetate template systematically demonstrates the superiority of edge-located Fe-N4 sites, providing a new design avenue for rational defect engineering to achieve high-performance ORR.

Curved Structure Regulated Single Metal Sites for Advanced Electrocatalytic Reactions.

Curved surface with defined local electronic structures and regulated surface microenvironments is significant for advanced catalytic engineering. Since single-atom catalysts are highly efficient and active, they have attracted much attention in recent years. The curvature carrier has a significant effect on the electronic structure regulation of single-atom sites, which effectively promote the catalytic efficiency. Here, the effect of the curvature structure with exposed metal atoms for catalysis is comprehensively summarized. First, the substrates with curvature features are reviewed. Second, the applications of single-atom catalysts containing curvature in a variety of different electrocatalytic reactions are discussed in depth. The impact of curvature effects in catalytic reactions is further analyzed. Finally, prospects and suggestions for their application and future development are presented. This review paves the way for the construction of high curvature-containing surface carriers, which is of great significance for single-atom catalysts development.

Recent advancements and prospects in noble and non-noble electrocatalysts for materials methanol oxidation reactions.

The direct methanol fuel cell (DMFC) represents a highly promising alternative power source for small electronics and automobiles due to its low operating temperatures, high efficiency, and energy density. The methanol oxidation process (MOR) constitutes a fundamental chemical reaction occurring at the positive electrode of a DMFC. Pt-based materials serve as widely utilized MOR electrocatalysts in DMFCs. Nevertheless, various challenges, such as sluggish reaction rates, high production costs primarily attributed to the expensive Pt-based catalyst, and the adverse effects of CO poisoning on the Pt catalysts, hinder the commercialization of DMFCs. Consequently, endeavors to identify an alternative catalyst to Pt-based catalysts that mitigate these drawbacks represent a critical focal point of DMFC research. In pursuit of this objective, researchers have developed diverse classes of MOR electrocatalysts, encompassing those derived from noble and non-noble metals. This review paper delves into the fundamental concept of MOR and its operational mechanisms, as well as the latest advancements in electrocatalysts derived from noble and non-noble metals, such as single-atom and molecule catalysts. Moreover, a comprehensive analysis of the constraints and prospects of MOR electrocatalysts, encompassing those based on noble metals and those based on non-noble metals, has been undertaken.

Ultrahigh Single Au Atoms Loaded Porous Aromatic Frameworks for Enhanced Photocatalytic Hydrogen Evolution.

Supported single-atom catalysts (SACs) are promising in heterogeneous catalysis because of their atom economy, unusual transformations, and mechanistic clarity. The metal SAs loading, however, limits the catalytic efficiency. Herein, an in situ pre-metallated monomer-based preparation strategy is shown to achieve ultrahigh Au SAs loading in catalyst formations. The polymerization of single-atom loaded monomers yield a new porous aromatic framework (PAF-164) with Au SAs loading up to a record high 45.3 wt.%. SACs of Au-PAFs exhibit excellent photocatalytic activity in hydrogen (H2) evolution, and the H2 evolution rate of Au100%-SAs-PAF-164 can reach 4.82 mmol g-1 h-1 with great recyclability.

Single-atom catalysts for electrocatalytic nitrate reduction into ammonia.

Ammonia (NH3) is a versatile and important compound with a wide range of uses, which is currently produced through the demanding Haber-Bosch process. Electrocatalytic nitrate reduction into ammonia (NRA) has recently emerged as a sustainable approach for NH3synthesis under ambient conditions. However, the NRA catalysis is a complex multistep electrochemical process with competitive hydrogen evolution reaction that usually results in poor selectivity and low yield rate for NH3synthesis. With maximum atom utilization and well-defined catalytic sites, single atom catalysts (SACs) display high activity, selectivity and stability toward various catalytic reactions. Very recently, a number of SACs have been developed as promising NRA electrocatalysts, but systematical discussion about the key factors that affect their NRA performance is not yet to be summarized to date. This review focuses on the latest breakthroughs of SACs toward NRA catalysis, including catalyst preparation, catalyst characterization and theoretical insights. Moreover, the challenges and opportunities for improving the NRA performance of SACs are discussed, with an aim to achieve further advancement in developing high-performance SACs for efficient NH3synthesis.

A Metal-Sulfur-Carbon Catalyst Mimicking the Two-Component Architecture of Nitrogenase.

The production of ammonia (NH3) from nitrogen sources involves competitive adsorption of different intermediates and multiple electron and proton transfers, presenting grand challenges in catalyst design. In nature nitrogenases reduce dinitrogen to NH3 using two component proteins, in which electrons and protons are delivered from Fe protein to the active site in MoFe protein for transfer to the bound N2. We draw inspiration from this structural enzymology, and design a two-component metal-sulfur-carbon (M-S-C) catalyst composed of sulfur-doped carbon-supported ruthenium (Ru) single atoms (SAs) and nanoparticles (NPs) for the electrochemical reduction of nitrate (NO3-) to NH3. The catalyst demonstrates a remarkable NH3 yield rate of ~37 mg L-1 h-1 and a Faradaic efficiency of ~97% for over 200 hours, outperforming those consisting solely of SAs or NPs, and even surpassing most reported electrocatalysts. Our experimental and theoretical investigations reveal the critical role of Ru SAs with the coordination of S in promoting the formation of the HONO intermediate and the subsequent reduction reaction over the NP-surface nearby. This study proves a better understanding of how M-S-Cs act as a synthetic nitrogenase mimic during ammonia synthesis, and contributes to the future mechanism-based catalyst design.

Molecular Engineering of Porous Fe-N-C Catalyst with Sulfur Incorporation for Boosting CO2 Reduction and Zn-CO2 Battery.

Transition metal-nitrogen-carbon (M-N-C) catalysts have emerged as promising candidates for electrocatalytic CO2 reduction reaction (CO2RR) due to their uniform active sites and high atomic utilization rate. However, poor efficiency at low overpotentials and unclear reaction mechanisms limit the application of M-N-C catalysts. In this study, Fe-N-C catalysts are developed by incorporating S atoms onto ordered hierarchical porous carbon substrates with a molecular iron thiophenoporphyrin. The well-prepared FeSNC catalyst exhibits superior CO2RR activity and stability, attributes to an optimized electronic environment, and enhances the adsorption of reaction intermediates. It displays the highest CO selectivity of 94.0% at -0.58 V (versus the reversible hydrogen electrode (RHE)) and achieves the highest partial current density of 13.64 mA cm-2 at -0.88 V. Furthermore, when employed as the cathode in a Zn-CO2 battery, FeSNC achieves a high-power density of 1.19 mW cm-2 and stable charge-discharge cycles. Density functional theory calculations demonstrate that the incorporation of S atoms into the hierarchical porous carbon substrate led to the iron center becoming more electron-rich, consequently improving the adsorption of the crucial reaction intermediate *COOH. This study underscores the significance of hierarchical porous structures and heteroatom doping for advancing electrocatalytic CO2RR and energy storage technologies.

Boosting the Electrocatalytic Oxygen Reduction Activity of MnN4-Doped Graphene by Axial Halogen Ligand Modification.

Exploring highly active electrocatalysts as platinum (Pt) substitutes for the oxygen reduction reaction (ORR) remains a significant challenge. In this work, single Mn embedded nitrogen-doped graphene (MnN4) with and without halogen ligands (F, Cl, Br, and I) modifying were systematically investigated by density functional theory (DFT) calculations. The calculated results indicated that these ligands can transform the dyz and dxz orbitals of Mn atom in MnN4 near the Fermi-level into dz2 orbital, and shift the d-band center away from the Fermi-level to reduce the adsorption capacity for reaction intermediates, thus enhancing the ORR catalytic activity of MnN4. Notably, Br and I modified MnN4 respectively with the lowest overpotentials of 0.41 and 0.39 V, possess superior ORR catalytic activity. This work is helpful for comprehensively understanding the ligand modification mechanism of single-atom catalysts and develops highly active ORR electrocatalysts.

Synthesis of Mo-Pt/CeO2 Dual Single-Atom Nanozyme for Multifunctional Biochemical Detection Applications.

Elaborated structural modulation of Pt-based artificial nanozymes can efficiently improve their catalytic activity and expand their applications in clinical diagnosis and biochemical sensing. Herein, a highly efficient dual-site peroxidase mimic composed of highly dispersed Pt and Mo atoms is reported. The obtained Mo-Pt/CeO2 exhibits exceptional peroxidase-like catalytic activity, with a Vmax as high as 34.16 × 10-8 m s-1, which is 37.5 times higher than that of the single-site counterpart. Mechanism studies suggest that the Mo atoms can not only serve as adsorption and activation sites for the H2O2 substrate but also regulate the charge density of Pt centers to promote the generation ability of •OH. As a result, the synergistic effect between the dual active sites significantly improves the catalytic efficiency. Significantly, the application of the Mo-Pt/CeO2 catalyst's excellent peroxidase-like activity is extended to various biochemical detection applications, including the trace detection of glucose and cysteine, as well as the assessment of antioxidants' antioxidant capacity. This work reveals the great potential of rational design dual-site active centers for constructing high-performance artificial nanozymes.

Nickel-based dual single atom electrocatalysts for the nitrate reduction reaction.

Electrochemical nitrate (NO3-) reduction reaction (NO3-RR) to ammonium (NH4+) or nitrogen (N2) provides a green route for nitrate remediation. However, nitrite generation and hydrogen evolution reactions hinder the feasibility of the process. Herein, dual single atom catalysts were rationally designed by introducing Ag/Bi/Mo atoms to atomically dispersed NiNC moieties supported by nitrogen-doped carbon nanosheet (NCNS) for the NO3-RR. Ni single atoms loaded on NCNS (Ni/NCNS) tend to reduce NO3- to valuable NH4+ with a high selectivity of 77.8 %. In contrast, the main product of NO3-RR catalyzing by NiAg/NCNS, NiBi/NCNS, and NiMo/NCNS was changed to N2, giving rise to N2 selectivity of 48.4, 47.1 and 47.5 %, respectively. Encouragingly, Ni/NCNS, NiBi/NCNS, and NiAg/NCNS showed excellent durability in acidic electrolytes, leading to nitrate conversion rates of 70.3, 91.1, and 93.2 % after a 10-h reaction. Simulated wastewater experiments showed that NiAg/NCNS could remove NO3- up to 97.8 % at -0.62 V after 9-h electrolysis. This work afforded a new strategy to regulate the reaction pathway and improve the conversion efficiency of the NO3-RR via engineering the dual atomic sites of the catalysts.

Enhancing Oxygen Reduction Reaction of Single-Atom Catalysts by Structure Tuning.

Deciphering the fine structure has always been a crucial approach to unlocking the distinct advantages of high activity, selectivity, and stability in single-atom catalysts (SACs). However, the complex system and unclear catalytic mechanism have obscured the significance of exploring the fine structure. Therefore, we endeavored to develop a three-component strategy to enhance oxygen reduction reaction (ORR), delving deep into the profound implications of the fine structure, focusing on central atoms, coordinating atoms, and environmental atoms. Firstly, the mechanism by which the chemical state and element type of central atoms influence catalytic performance is discussed. Secondly, the significance of coordinating atoms in SACs is analyzed, considering both the number and type. Lastly, the impact of environmental atoms in SACs is reviewed, encompassing existence state and atomic structure. Thorough analysis and summarization of how the fine structure of SACs influences the ORR have the potential to offer valuable insights for the accurate design and construction of SACs.

Unveiling the Aggregation of M-N-C Single-Atom Electrocatalysts into Highly Efficient MOOH Nanoclusters during Alkaline Water Oxidation.

M-N-C-type single-atom catalysts (SACs) are highly efficient for the electrocatalytic oxygen evolution reaction (OER). And the isolated metal atoms are usually considered real active sites. However, the oxidative structural evolution of coordinated N during the OER will probably damage the structure of M-N-C, hence resulting in a completely different reaction mechanism. Here, we reveal the aggregation of M-N-C materials during the alkaline OER. Taking Ni-N-C as an example, multiple characterizations show that the coordinated N on the surface of Ni-N-C is almost completely dissolved in the form of NO3-, accompanied by the generation of abundant O functional groups on the surface of the carbon support. Accordingly, the Ni-N bonds are broken. Through a dissolution-redeposition mechanism and further oxidation, the isolated Ni atoms are finally converted to NiOOH nanoclusters supported by carbon as the real active sites for the enhanced OER. Fe-N-C and Co-N-C also have similar aggregation mechanism. Our findings provide unique insight into the structural evolution and activity origin of M-N-C-based catalysts under electrooxidative conditions.

Copper single-atom catalysts for broad-spectrum antibiotic-resistant bacteria (ARBs) antimicrobial: Activation of peroxides and mechanism of ARBs inactivation.

Antibiotic-resistant bacteria (ARBs) have been widely detected in wastewater and become a potential threat to human health. This work found that low-load single-atom copper (0.1 wt%) anchored on g-C3N4 (SA-Cu/g-C3N4) exhibited excellent ability to activate H2O2 and inactivate ARBs during the photo-Fenton process. The presence of SA-Cu/g-C3N4 (0.4 mg/mL) and H2O2 (0.1 mM) effectively inactivated ARBs. More than 99.9999 % (6-log) of methicillin-resistant Staphylococcus aureus (MRSA), and carbapenem-resistant Acinetobacter baumannii (CRAB) could be inactivated within 5 min. Extended-spectrum ß-lactamase-producing pathogenic Escherichia coli (ESBL-E) and vancomycin-resistant Enterococcus faecium (VRE) were killed within 10 and 30 min, respectively. In addition, more than 5-log of these ARBs were killed within 60 min in real wastewater. Furthermore, D2O-labeling with Raman spectroscopy revealed that SA-Cu/g-C3N4 completely suppressed the viable but nonculturable (VBNC) state and reactivation of bacteria. Electron paramagnetic resonance spectroscopy results demonstrated that g-C3N4 mainly produced 1O2, while SA-Cu/g-C3N4 simultaneously produced both 1O2 and •OH. The •OH and 1O2 cause lipid peroxidation damage to the cell membrane, resulting in the death of the bacteria. These findings highlight that the SA-Cu/g-C3N4 catalyst is a promising photo-Fenton catalyst for the inactivation of ARBs in wastewater.

Main group metal elements enhance electrochemical nitrogen reduction reaction of vanadium-based single atom catalysts through d-p orbital hybridization.

Developing active sites with flexibility and diversity is crucial for single atom catalysts (SACs) towards sustainable nitrogen fixation at ambient conditions. Herein, the effects of doping main group metal elements (MGM) on the stability, catalytic activity, and selectivity of vanadium-based SACs is systematically investigated based on density functional theory calculations. It is found that the catalytic activity of V site can be significantly enhanced by the synergistic effect between MGM and vanadium atoms. More importantly, a volcano curve between the catalytic activity and the adsorption free energy of NNH* can be established, in which V-Pb dimer embedded on N-coordinated graphene (VPb-NG) exhibits optimal NRR activity due to its location at the top of volcano. Further analysis of electronic structures reveals that the unoccupancy ratio (eg/t2g) of V site is dramatically increased by the strong d-p orbital hybridization between V and Pb atoms, subsequently, N2 is activated to a larger extent. These interesting findings may provide a new path for designing active sites in SACs with excellent performance.

Coordination Desymmetrization of Copper Single-Atom Catalyst for Efficient Nitrate Reduction.

Coordination engineering strategy for optimizing the catalytic performance of single-atom catalysts (SACs) has been rapidly developed over the last decade. However, previous reports on copper SACs for nitrate reduction reactions (NO3RR) have mostly focused on symmetric coordination configurations such as Cu-N4 and Cu-N3. In addition, the mechanism in terms of the regulation of coordination environment and catalytic properties of SACs has not been well demonstrated. Herein, we disrupted the local symmetric structure of copper atoms by introducing unsaturated heteroatomic coordination of Cu-O and Cu-N to achieve the coordination desymmetrization of Cu-N1O2 SACs. The Cu-N1O2 SACs exhibit an efficient nitrate-to-ammonia conversion with a high FE of ~96.5 % and a yield rate of 3120 µg NH3 h-1 cm-2 at -0.60 V vs RHE. As indicated by in situ Raman spectra, the catalysts facilitate the accumulation of NO3 - and the selective adsorption of *NO2, which were further confirmed by the theoretical study of surface dipole moment and orbital hybridization. Our work illustrated the correlation between the coordination desymmetrization and the catalytic performance of copper SACs for NO3RR.

Machine learning-driven shortening the screening process towards high-performance nitrogen reduction reaction electrocatalysts with four-step screening strategy.

The urgent need to prepare clean energy by environmentally friendly and efficient methods, which has led to widespread attention on electrocatalytic nitrogen reduction reaction (NRR) for ammonia production. At present, single atom catalytic nitrogen reduction has become the earliest promising method for industrial production due to its high atomic utilization rate, high selectivity, high controllability, and high stability. However, how to quickly screen catalysts with high catalytic efficiency and selectivity in single-atom catalysts (SACs) remains a challenge. Herein, the 29 SACs are constructed from C6N2 nanosheets doped with transition metals (TM@C6N2), which are analyzed for stability, adsorption performance, NRR catalytic activity, electronic properties, and competitiveness using first-principles calculations. The results show that Mo@C6N2 and Re@C6N2 exhibit the most outstanding catalytic performances, with limiting potentials (UL) of -0.29 and -0.31 V, respectively, in the solvent model. Machine learning is used to derive descriptors from the intrinsic features to predict the free energy changes for the potential-determining step. The importance of features is calculated, with the first ionisation energy (IE1) being the most significant influencing factor. Based on the guidance of machine learning and considering that IE1 is related to the ability of metal atoms to donate electrons, a four-step screening strategy using the Integrated Crystal Orbital Hamilton Populations (ICOHP) to screen catalysts instead of the traditional five-step screening not only improves the screening efficiency but also obtains completely consistent screening results. This work presents a new approach to predicting the catalytic performance of SACs and provides new insights into the influence of intrinsic properties on catalytic activity.

Engineering Lewis-Acid Defects on ZnO Quantum Dots by Trace Transition-Metal Single Atoms for High Glycerol-to-Glycerol Carbonate Conversion.

Efficient conversion of biomass wastes into valuable chemicals has been regarded as a sustainable approach for green and circular economy. Herein, a highly efficient catalytic conversion of glycerol (Gly) into glycerol carbonate (GlyC) by carbonylation with the commercially available urea is presented using low-cost transition metal single atoms supported on zinc oxide quantum dots (M1-ZnO QDs) as a catalyst without using any solvent. A facile one-step wet chemical synthesis allows various types of metal single atoms to simultaneously dope and introduce Lewis-acid defects in the ZnO QD structure. It is found that doping with a trace amount of isolated metal atoms greatly boosts the catalytic activity with Gly conversion of 90.7%, GlyC selectivity of 100.0%, and GlyC yield of 90.6%. Congruential results from both Density Functional Theory (DFT) and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (in situ DRIFTS) studies reveal that the superior catalytic performance can be attributed to the enriched Lewis acid sites that endow optimal adsorption, formation of the intermediate for coupling between urea and Gly, and desorption of GlyC. Moreover, the tiny size of ZnO QDs efficiently promotes the accessibility of these active sites to the reactants.