Defects-Induced Single-Atom Anchoring on Metal-Organic Frameworks for High-Efficiency Photocatalytic Nitrogen Reduction

Aiming to improve the photocatalytic activity in N2 fixation to produce ammonia, herein, we proposed a photochemical strategy to fabricate defects, and further deposition of Ru single atoms onto UiO-66 (Zr) framework. Electron-metal-support interactions (EMSI) were built between Ru single atoms and the support via a covalently bonding. EMSI were capable of accelerating charge transfer between Ru SAs and UiO-66, which was favorable for highly-efficiently photocatalytic activity. The photocatalytic production rate of ammonia improved from 4.57 μmol g-1  h-1 to 16.28 μmol g-1  h-1 with the fabrication of defects onto UiO-66, and further to 53.28 μmol g-1  h-1 with Ru-single atoms loading. From the DFT results, it was found that d-orbital electrons of Ru were donated to N2 π✶-antibonding orbital, facilitating the activation of the N≡N triple bond. A distal reaction pathway was probably occurred for the photocatalytic N2 reduction to ammonia on Ru1 /d-UiO-66 (single Ru sites decorated onto the nodes of defective UiO-66), and the first step of hydrogenation of N2 was the reaction determination step. This work shed a light on improving the photocatalytic activity via feasibly anchoring single atoms on MOF, and provided more evidences to understand the reaction mechanism in photocatalytic reduction of N2 .

Construction of Low-Coordination Cu-C2 Single-Atoms Electrocatalyst Facilitating the Efficient Electrochemical CO2 Reduction to Methane

Constructing Cu single-atoms (SAs) catalysts is considered as one of the most effective strategies to enhance the performance of electrochemical reduction of CO2 (e-CO2 RR) towards CH4 , however there are challenges with activity, selectivity, and a cumbersome fabrication process. Herein, by virtue of the meta-position structure of alkynyl in 1,3,5-triethynylbenzene and the interaction between Cu and -C≡C-, a Cu SAs electrocatalyst (Cu-SAs/HGDY), containing low-coordination Cu-C2 active sites, was synthesized through a simple and efficient one-step method. Notably, this represents the first achievement of preparing Cu SAs catalysts with Cu-C2 coordination structure, which exhibited high CO2 -to-CH4 selectivity (72.1 %) with a high CH4 partial current density of 230.7 mA cm-2 , and a turnover frequency as high as 2756 h-1 , dramatically outperforming currently reported catalysts. Comprehensive experiments and calculations verified the low-coordination Cu-C2 structure not only endowed the Cu SAs center more positive electricity but also promoted the formation of H•, which contributed to the outstanding e-CO2 RR to CH4 electrocatalytic performance of Cu-SAs/HGDY. Our work provides a novel H⋅-transferring mechanism for e-CO2 RR to CH4 and offers a protocol for the preparation of two-coordinated Cu SAs catalysts.

Breaking Local Charge Symmetry of Iron Single Atoms for Efficient Electrocatalytic Nitrate Reduction to Ammonia

The electrochemical conversion of nitrate pollutants into value-added ammonia is a feasible way to achieve artificial nitrogen cycle. However, the development of electrocatalytic nitrate-to-ammonia reduction reaction (NO3 - RR) has been hampered by high overpotential and low Faradaic efficiency. Here we develop an iron single-atom catalyst coordinated with nitrogen and phosphorus on hollow carbon polyhedron (denoted as Fe-N/P-C) as a NO3 - RR electrocatalyst. Owing to the tuning effect of phosphorus atoms on breaking local charge symmetry of the single-Fe-atom catalyst, it facilitates the adsorption of nitrate ions and enrichment of some key reaction intermediates during the NO3 - RR process. The Fe-N/P-C catalyst exhibits 90.3 % ammonia Faradaic efficiency with a yield rate of 17980 μg h-1  mgcat -1 , greatly outperforming the reported Fe-based catalysts. Furthermore, operando SR-FTIR spectroscopy measurements reveal the reaction pathway based on key intermediates observed under different applied potentials and reaction durations. Density functional theory calculations demonstrate that the optimized free energy of NO3 - RR intermediates is ascribed to the asymmetric atomic interface configuration, which achieves the optimal electron density distribution. This work demonstrates the critical role of atomic-level precision modulation by heteroatom doping for the NO3 - RR, providing an effective strategy for improving the catalytic performance of single atom catalysts in different electrochemical reactions.

Atomic Insights into Synergistic Nitroarene Hydrogenation over Nanodiamond-Supported Pt1 -Fe1 Dual-Single-Atom Catalyst

Fundamental understanding of the synergistic effect of bimetallic catalysts is of extreme significance in heterogeneous catalysis, but a great challenge lies in the precise construction of uniform dual-metal sites. Here, we develop a novel method for constructing Pt1 -Fe1 /ND dual-single-atom catalyst, by anchoring Pt single atoms on Fe1 -N4 sites decorating a nanodiamond (ND) surface. Using this catalyst, the synergy of nitroarenes selective hydrogenation is revealed. In detail, hydrogen is activated on the Pt1 -Fe1 dual site and the nitro group is strongly adsorbed on the Fe1 site via a vertical configuration for subsequent hydrogenation. Such synergistic effect decreases the activation energy and results in an unprecedented catalytic performance (3.1 s-1 turnover frequency, ca. 100 % selectivity, 24 types of substrates). Our findings advance the applications of dual-single-atom catalysts in selective hydrogenations and open up a new way to explore the nature of synergistic catalysis at the atomic level.

Cooperative Ni(Co)-Ru-P Sites Activate Dehydrogenation for Hydrazine Oxidation Assisting Self-powered H2 Production

Water electrolysis for H2 production is restricted by the sluggish oxygen evolution reaction (OER). Using the thermodynamically more favorable hydrazine oxidation reaction (HzOR) to replace OER has attracted ever-growing attention. Herein, we report a twisted NiCoP nanowire array immobilized with Ru single atoms (Ru1 -NiCoP) as superior bifunctional electrocatalyst toward both HzOR and hydrogen evolution reaction (HER), realizing an ultralow working potential of -60 mV and overpotential of 32 mV for a current density of 10 mA cm-2 , respectively. Inspiringly, two-electrode electrolyzer based on overall hydrazine splitting (OHzS) demonstrates outstanding activity with a record-high current density of 522 mA cm-2 at cell voltage of 0.3 V. DFT calculations elucidate the cooperative Ni(Co)-Ru-P sites in Ru1 -NiCoP optimize H* adsorption, and enhance adsorption of *N2 H2 to significantly lower the energy barrier for hydrazine dehydrogenation. Moreover, a self-powered H2 production system utilizing OHzS device driven by direct hydrazine fuel cell (DHzFC) achieve a satisfactory rate of 24.0 mol h-1  m-2 .

Second Sphere Effects Promote Formic Acid Dehydrogenation by a Single-Atom Gold Catalyst Supported on Amino-Substituted Graphdiyne

Regulating the second sphere of homogeneous molecular catalysts is a common and effective method to boost their catalytic activities, while the second sphere effects have rarely been investigated for heterogeneous single-atom catalysts primarily due to the synthetic challenge for installing functional groups in their second spheres. Benefiting from the well-defined and readily tailorable structure of graphdiyne (GDY), an Au single-atom catalyst on amino-substituted GDY is constructed, where the amino group is located in the second sphere of the Au center. The Au atoms on amino-decorated GDY displayed superior activity for formic acid dehydrogenation compared with those on unfunctionalized GDY. The experimental studies, particularly the proton inventory studies, and theoretical calculations revealed that the amino groups adjacent to an Au atom could serve as proton relays and thus facilitate the protonation of an intermediate Au-H to generate H₂ . Our study paves the way to precisely constructing the functional second sphere on single-atom catalysts.

Operando Stability of Single-Atom Electrocatalysts

Single-atom catalysts (SACs) are appealing next-generation catalysts for various electrochemical technologies. Along with significant breakthroughs in their initial activity, SACs now face the next challenge for their viable applications, insufficient operational stability. In this Minireview, we summarize the current knowledge of SAC degradation mechanisms mainly based on Fe-N-C SACs, some of the most investigated SACs. Recent studies on isolated metal, ligand, and support degradations are introduced, and the underlying fundamentals of each degradation path are categorized into active site density (SD) and turnover frequency (TOF) losses. Finally, we discuss the challenges and prospects for the future outlook of stable SACs.

Single-Atom-Mediated Spinel Octahedral Structures for Elevated Performances of Li-Oxygen Batteries

Li-oxygen batteries have attracted much attention due to its ultra-high theoretical specific capacity, but the discharge product Li₂ O₂ is easy to accumulate, leading to low battery stability. Here, we demonstrate a series of high-efficiency cathode catalysts of Co₃ O₄ loaded with single-atomic metals (M=Ru, Pd, Pt, Au, Ir). The single-atomic metal could substitute the central Co atom in the octahedral coordination structure and maintain the structural stability; benefiting from the electron promoter effect, rendering more highly active Co³⁺ exposed, providing rich nucleation sites for Li₂ O₂ deposition. And the loaded M atoms could separate the active Co³⁺ centers, thereby regulating the dispersion of Li₂ O₂ to obtained a sheet-like morphology, which could facilitate its decomposition in the subsequent charge cycle. Our work found that the single atoms could effectively modulate the active metal oxide with which it is coordinated, thus collectively boosting the catalytic performance.

Super-Coordinated Nickel N4 Ni1 O2 Site Single-Atom Catalyst for Selective H2 O2 Electrosynthesis at High Current Densities

Electrochemical production of hydrogen peroxide (H₂ O₂ ) from O₂ on single-atom catalysts has attracted great attention, yet the quest for robust catalysts is driven by achieving >90 % Faradaic efficiency (FE) under industrial-relevant current densities (>100 mA cm^-2 ). Herein we synthesize a catalyst that contains single nickel site coordinated by four nitrogen and two oxygen atoms (i.e., N₄ Ni₁ O₂ ) via involving carboxyl functionalized multiwall carbon nanotubes as a substrate to provide extra O coordination to the regular NiN₄ site. It has a cathodic energy efficiency of approximately 82 % and a H₂ O₂ FE of around 96 % at 200 mA cm^-2 current density, outperforming the reported single-atom catalysts for H₂ O₂ electrosynthesis.

Tuning the Spin State of the Iron Center by Bridge-Bonded Fe-O-Ti Ligands for Enhanced Oxygen Reduction

Exploring functional substrates and precisely regulating the electronic structures of atomic metal active species with moderate spin state are of great importance yet remain challenging. Hereon, we provide an axial Fe-O-Ti ligand regulated spin-state transition strategy to improve the oxygen reduction reaction (ORR) activity of Fe centers. Theoretical calculations indicate that Fe-O-Ti ligands in FeN₃ O-O-Ti can induce a low-to-medium spin-state transition and optimize O₂ adsorption by FeN₃ O. As a proof-of-concept, the oriented catalyst was prepared from atomic-Fe-doped polymer-like quantum dots and ultrathin o-terminated MXene. The optimal catalyst exhibits an intrinsic activity that is almost 5 times higher than the control sample (without axial Fe-O-Ti ligands). It also delivers a superior performance in Zn-air batteries and H₂ /O₂ anion exchange membrane fuel cells in a wide-temperature range.