Beyond the Active Site: Mechanistic Investigations of the Role of the Secondary Coordination Sphere and Beyond in Multi-electron Electrocatalytic Reactions

Construction of Fe/Co-N4 Single-Atom Sites for the Oxygen Reduction Reaction in Zinc-Air Batteries

Insight into the modulation effect of oxygen reduction reaction (ORR) active centers is of profound significance but remains a great challenge. Here, we designed Co, Fe dual-metal single-atom sites (CoFe-DSAs/NC) uniformly anchored on nitrogen-doped multiwalled carbon nanotubes for boosting ORR performance through regulating the 4d electronic orbitals of the Co-N4 active site. Mechanism studies revealed that for the first time the neighboring Fe-N4 atomic sites were able to regulate the d-band center of Co-N4 single-atom active centers while maintaining the balance of adsorption-desorption affinity for O2 and oxygen-containing species on Co-N4, thereby resulting in a superior ORR performance with a positive half-wave potential (0.90 V vs RHE). The assembled zinc-air battery based on CoFe-DSAs/NC exhibited an increased open-circuit voltage (1.48 V) and an elevated specific capacity (782.33 mAh·g-1). The work provides a new clue for reasonably designing high-performance ORR catalysts through adjusting the d-band center of active sites.

NaCl template-assisted construction of a CoP-MoP heterostructured electrocatalyst for electrocatalytic nitrogen reduction

The electrocatalytic nitrogen reduction reaction (NRR) to ammonia is a promising technology to store renewable energy and mitigate greenhouse gas emissions. However, it usually suffers from low ammonia yield and selectivity because of the lack of efficient electrocatalysts. Herein, we report that the construction of metal phosphide heterojunctions is an efficient strategy for NRR activity enhancement. A CoP-MoP heterojunction electrocatalyst, which is fabricated by a facile NaCl template-assisted strategy, exhibits a favorable ammonia yield rate of 77.8 μg h-1 mgcat-1 (38.9 μg h-1 cm-2) and a high faradaic efficiency of 11.16% at -0.50 V versus the reversible hydrogen electrode. The high NRR electrocatalytic activity can be attributed to the electronic coupling effects and interfacial synergistic effects of CoP and MoP at the heterojunction interface, which accelerates the electron transfer rate. Moreover, Mo doping changes the d-band centers of metal sites on the CoP surface, which is conducive to N2 adsorption and promotes N2* adsorption in the competition of occupying active sites, thus inhibiting the HER. This work manifests the high potential of phosphide electrocatalysts and opens an alternative route toward NRR electrocatalysis.

Metallocavitins as Advanced Enzyme Mimics and Promising Chemical Catalysts

The supramolecular approach is becoming increasingly dominant in biomimetics and chemical catalysis due to the expansion of the enzyme active center idea, which now includes binding cavities (hydrophobic pockets), channels and canals for transporting substrates and products. For a long time, the mimetic strategy was mainly focused on the first coordination sphere of the metal ion. Understanding that a highly organized cavity-like enzymatic pocket plays a key role in the sophisticated functionality of enzymes and that the activity and selectivity of natural metalloenzymes are due to the effects of the second coordination sphere, created by the protein framework, opens up new perspectives in biomimetic chemistry and catalysis. There are two main goals of mimicking enzymatic catalysis: (1) scientific curiosity to gain insight into the mysterious nature of enzymes, and (2) practical tasks of mankind: to learn from nature and adopt from its many years of evolutionary experience. Understanding the chemistry within the enzyme nanocavity (confinement effect) requires the use of relatively simple model systems. The performance of the transition metal catalyst increases due to its retention in molecular nanocontainers (cavitins). Given the greater potential of chemical synthesis, it is hoped that these promising bioinspired catalysts will achieve catalytic efficiency and selectivity comparable to and even superior to the creations of nature. Now it is obvious that the cavity structure of molecular nanocontainers and the real possibility of modifying their cavities provide unlimited possibilities for simulating the active centers of metalloenzymes. This review will focus on how chemical reactivity is controlled in a well-defined cavitin nanospace. The author also intends to discuss advanced metal–cavitin catalysts related to the study of the main stages of artificial photosynthesis, including energy transfer and storage, water oxidation and proton reduction, as well as highlight the current challenges of activating small molecules, such as H2O, CO2, N2, O2, H2, and CH4.

Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines

Pendant amines play an invaluable role in chemical reactivity, especially for molecular catalysts based on earth-abundant metals. As inspired by [FeFe]-hydrogenases, which contain a pendant amine positioned for cooperative bifunctionality, synthetic catalysts have been developed to emulate this multifunctionality through incorporation of a pendant amine in the second coordination sphere. Cyclic diphosphine ligands containing two amines serve as the basis for a class of catalysts that have been extensively studied and used to demonstrate the impact of a pendant base. These 1,5-diaza-3,7-diphosphacyclooctanes, now often referred to as "P₂N₂" ligands, have profound effects on the reactivity of many catalysts. The resulting [Ni(P^R₂N^R'₂)₂]²⁺ complexes are electrocatalysts for both the oxidation and production of H₂. Achieving the optimal benefit of the pendant amine requires that it has suitable basicity and is properly positioned relative to the metal center. In addition to the catalytic efficacy demonstrated with [Ni(P^R₂N^R'₂)₂]²⁺ complexes for the oxidation and production of H₂, catalysts with diphosphine ligands containing pendant amines have also been demonstrated for several metals for many different reactions, both in solution and immobilized on surfaces. The impact of pendant amines in catalyst design continues to expand.

Mirror-plane disorder in a nickel chloride Schiff base complex: a suitable case study for crystallographic instruction

The nickel chloride complex of the Schiff base N 2,N 2′-propanediylbis(2,3-butanedione-2-imine-3-oxime), namely, chlorido(3,9-dimethylundeca-3,8-diene-2,10-dione 10-oxime 2-oximato-κ4 N,N′,N′′,N′′′)nickel(II), [NiCl(C11H19N4O2)], at 100 K crystallizes in the orthorhombic space group Cmce. The structure exhibits mirror disorder of the main molecule that is not present in the bromide analogue. The relatively small number of unique reflections in the data set and the disorder imposed by the crystallographic mirror plane present a challenging educational case study.

Electrocatalytic Hydrogen Evolution by Cobalt Complexes with a Redox Non-Innocent Polypyridine Ligand

Novel cobalt and zinc complexes with the tetradentate ppq (8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2'-yl)quinoline) ligand have been synthesized and fully characterized. Electrochemical measurements have shown that the formal monovalent complex [Co(ppq)(PPh₃)]⁺ (2) undergoes two stepwise ligand-based electroreductions in DMF, affording a [Co(ppq)DMF]^-1 species. Theoretical calculations have described the electronic structure of [Co(ppq)DMF]^-1 as a low-spin Co(II) center coupling with a triple-reduced ppq radical ligand. In the presence of triethylammonium as the proton donor, the cobalt complex efficiently drives electrocatalytic hydrogen evolution with a maximum turnover frequency of thousands per second. A mechanistic investigation proposes an EECC H₂-evolving pathway, where the second ligand-based redox process (E), generating the [Co(ppq)DMF]^-1 intermediate, initiates proton reduction, and the second proton transfer process (C) is the rate-determining step. This work provides a unique example for understanding the role of redox-active ligands in electrocatalytic H₂ evolution by transition metal sites.