MnxOy/NC and CoxOy/NC Nanoparticles Embedded in a Nitrogen‐Doped Carbon Matrix for High‐Performance Bifunctional Oxygen Electrodes

Reversible interconversion of water into H₂ and O₂, and the recombination of H₂ and O₂ to H₂O thereby harnessing the energy of the reaction provides a completely green cycle for sustainable energy conversion and storage. The realization of this goal is however hampered by the lack of efficient catalysts for water splitting and oxygen reduction. We report exceptionally active bifunctional catalysts for oxygen electrodes comprising Mn₃O₄ and Co₃O₄ nanoparticles embedded in nitrogen‐doped carbon, obtained by selective pyrolysis and subsequent mild calcination of manganese and cobalt N₄ macrocyclic complexes. Intimate interaction was observed between the metals and nitrogen suggesting residual M–N_x coordination in the catalysts. The catalysts afford remarkably lower reversible overpotentials in KOH (0.1 M ) than those for RuO₂, IrO₂, Pt, NiO, Mn₃O₄, and Co₃O₄, thus placing them among the best non‐precious‐metal catalysts for reversible oxygen electrodes reported to date.     

Characterization and Activity of Cu‐MnOx/γ‐Al2O3 Catalyst for Hydrogenation of Carbon Dioxide

The effect of manganese on the dispersion, reduction behavior and active states of surface of supported copper oxide catalysts have been investigated by XRD, temperature‐programmed reduction and XPS. The activity of methanol synthesis from CO₂/H₂ was also investigated. The catalytic activity over CuO‐MnO_x/γ‐Al₂O₃ catalyst for CO₂ hydrogenation is higher than that of CuO/γ‐Al₂O₃. The adding of manganese is beneficial in enhancing the dispersion of the supported copper oxide and make the TPR peak of the CuO‐MnK_x/γ‐Al₂O₃ catalyst different from the individual supported copper and manganese oxide catalysts, which indicates that there exists strong interaction between the copper and manganese oxide. For the CuO/γ‐Al₂O₃ catalyst there are two reducible copper oxide species; α and β peaks are attributed to the reduction of highly dispersed copper oxide species and bulk CuO species, respectively. For the CuO‐MnO_x/γ‐Al₂O₃ catalyst, four reduction peaks are observed, α peak is attributed to the dispersed copper oxide species; β peak is ascribed to the bulk CuO; γ peak is attributed to the reduction of high dispersed CuO interacting with manganese; δ peak may be the reduction of the manganese oxide interacting with copper oxide. XPS results show that Cu⁺ mostly existed on the working surface of the Cu‐Mn/γ‐Al₂O₃ catalysts. The activity was promoted by Cu with positive charge which was formed by means of long path exchange function between Cu? O? Mn. These results indicate that there is synergistic interaction between the copper and manganese oxide, which is responsible for the high activity of CO₂ hydrogenation.     

Oxygen‐Atom Transfer Chemistry and Thermolytic Properties of a Di‐tert‐Butylphosphate‐Ligated Mn4O4 Cubane

[Mn₄O₄{O₂P(OtBu)₂}₆] ( 1 ), an Mn₄O₄ cubane complex combining the structural inspiration of the photosystem II oxygen‐evolving complex with thermolytic precursor ligands, was synthesized and fully characterized. Core oxygen atoms within complex 1 are transferred upon reaction with an oxygen‐atom acceptor (PEt₃), to give the butterfly complex [Mn₄O₂{O₂P(OtBu)₂}₆(OPEt₃)₂]. The cubane structure is restored by reaction of the latter complex with the O‐atom donor PhIO. Complex 1 was investigated as a precursor to inorganic Mn metaphosphate/pyrophosphate materials, which were studied by X‐ray absorption spectroscopy to determine the fate of the Mn₄O₄ unit. Under the conditions employed, thermolyses of 1 result in reduction of the manganese to Mn^II species. Finally, the related butterfly complex [Mn₄O₂{O₂P(pin)}₆(bpy)₂] (pin=pinacolate) is described.     

Synthesis and Characterization of β—Cyclodextrin Bonded Metal Porphyrins

Reactions of 5-(p-aminophenyl)-10,15,20-triphenyl porphyrin (1) with Ru3(CO)12 or M(OCOCH3)2 (M=Ni,Mn) afforded metalloporphyrins(4-6),respectively.6-Deoxy-6-io-do-β-cyclodextrin(2) and mono(6-O-trifluoromethanesulfonyl) permethylated β-cyclodextrin(3) reacted with complexes 4-6 to give β-cyclodextrin bonded metal porphyrins (7-9) and permethylated β-cyclodextrin bonded me-tal porphyrins (10-12) respectively.These new complexes were identified by MS,IR,UV-visible and ^1H NMR spectra,and elemental analysis.     

Nitrogen‐Rich Manganese Oxynitrides with Enhanced Catalytic Activity in the Oxygen Reduction Reaction

The catalytic activity of manganese oxynitrides in the oxygen reduction reaction (ORR) was investigated in alkaline solutions to clarify the effect of the incorporated nitrogen atoms on the ORR activity. These oxynitrides, with rock‐salt‐like structures with different nitrogen contents, were synthesized by reacting MnO, Mn₂O₃, or MnO₂ with molten NaNH₂ at 240–280 °C. The anion contents and the Mn valence states were determined by combustion analysis, powder X‐ray diffraction, and X‐ray absorption near‐edge structure analysis. An increase in the nitrogen content of rock‐salt‐based manganese oxynitrides increases the valence of the manganese ions and reinforces the catalytic activity for the ORR in 1 m KOH solution. Nearly single‐electron occupancy of the antibonding e_g states and highly covalent Mn?N bonding thus enhance the ORR activity of nitrogen‐rich manganese oxynitrides.     

On the Influence of Oxygen on the Degradation of Fe‐N‐C Catalysts

Fe‐N‐C catalysts containing atomic FeN_x sites are promising candidates as precious‐metal‐free catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. The durability of Fe‐N‐C catalysts in fuel cells has been extensively studied using accelerated stress tests (AST). Herein we reveal stronger degradation of the Fe‐N‐C structure and four‐times higher ORR activity loss when performing load cycling AST in O₂‐ vs. Ar‐saturated pH 1 electrolyte. Raman spectroscopy results show carbon corrosion after AST in O₂, even when cycling at low potentials, while no corrosion occurred after any load cycling AST in Ar. The load‐cycling AST in O₂ leads to loss of a significant fraction of FeN_x sites, as shown by energy dispersive X‐ray spectroscopy analyses, and to the formation of Fe oxides. The results support that the unexpected carbon corrosion occurring at such low potential in the presence of O₂ is due to reactive oxygen species produced between H₂O₂ and Fe sites via Fenton reactions.     

Magnetic properties of two double‐layer structures built from hydroxy­naphthoic acids and manganese

Double‐layer structures consisting of alternating polar and non‐polar layers have been prepared using Mn²⁺ ions and o‐hydroxy­naphthoic acids. The polar layers contain the Mn²⁺ ions, carboxylate groups, hydr­oxy groups and water mol­ecules. The non‐polar layers are built up from the naphthalene moieties. In catena‐poly[[diaqua­manganese(II)]bis­(μ‐3‐hy­droxy‐2‐naphthoato‐κ²O:O′)] (also called manganese 3‐hy­droxy‐2‐naphthoate dihydrate), [Mn(C₁₁H₇O₃)₂(H₂O)₂]_n, (I), the Mn²⁺ ions are connected by carboxylate groups to form two‐dimensional networks. This compound shows distinct antiferromagnetic inter­actions and long‐range ordering at low temperature. In contrast, tetra­aqua­bis(1‐hydr­oxy‐2‐naph­thoato‐κO)manganese(II), [Mn(C₁₁H₇O₃)₂(H₂O)₄], (II), which lacks a close linkage between the Mn²⁺ ions, reveals purely paramagnetic behaviour. In (II), the Mn²⁺ ion lies on an inversion centre.     

NHC‐Containing Manganese(I) Electrocatalysts for the Two‐Electron Reduction of CO2

The synthesis and characterization of the first catalytic manganese N‐heterocyclic carbene complexes are reported: MnBr(N‐methyl‐N′‐2‐pyridylbenzimidazol‐2‐ylidine)(CO)₃ and MnBr(N‐methyl‐N′‐2‐pyridylimidazol‐2‐ylidine)(CO)₃. Both new species mediate the reduction of CO₂ to CO following two‐electron reduction of the Mn^I center, as observed with preparative scale electrolysis and verified with ¹³CO₂. The two‐electron reduction of these species occurs at a single potential, rather than in two sequential steps separated by hundreds of millivolts, as is the case for previously reported MnBr(2,2′‐bipyridine)(CO)₃. Catalytic current enhancement is observed at voltages similar to MnBr(2,2′‐bipyridine)(CO)₃.     

Enhanced Catalytic Activity and Unexpected Products from the Oxidation of Cyclohexene by Organic Nanoparticles of 5,10,15,20‐Tetrakis‐(2,3,4,5,6‐pentafluorophenyl)porphyrinatoiron(III) in Water by Using O2

The catalytic oxidation of alkenes by most iron porphyrins using a variety of oxygen sources, but generally not dioxygen, yields the epoxide with minor quantities of other products. The turnover numbers for these catalysts are modest, ranging from a few hundred to a few thousand depending on the porphyrin structure, axial ligands, and other reaction conditions. Halogenation of substituents increases the activity of the metalloporphyrin catalyst and/or makes it more robust to oxidative degradation. Oxidation of cyclohexene by 5,10,15,20‐tetrakis‐(2,3,4,5,6‐pentafluorophenyl)porphyrinato iron(III), ([Fe^III(tppf₂₀)]) and H₂O₂ is typical of the latter: the epoxide is 99 % of the product and turnover numbers are about 350. 1 – 4 Herein, we report that dynamic organic nanoparticles (ONPs) of [Fe^III(tppf₂₀)] with a diameter of 10 nm, formed by host–guest solvent methods, catalytically oxidize cyclohexene with O₂ to yield only 2‐cyclohexene‐1‐one and 2‐cyclohexene‐1‐ol with approximately 10‐fold greater turnover numbers compared to the non‐aggregated metalloporphyrin in acetonitrile/methanol. These ONPs facilitate a greener reaction because the reaction solvent is 89 % water and O₂ is the oxidant in place of synthetic oxygen sources. This reactivity is unexpected because the metalloporphyrins are in close proximity and oxidative degradation of the catalyst should be enhanced, thus causing a significant decrease in catalytic turnovers. The allylic products suggest a different oxidative mechanism compared to that of the solvated metalloporphyrins. These results illustrate the unique properties of some ONPs relative to the component molecules or those attached to supports.     

Amperometric Flow‐Biosensor for Cyanide Based on an Inhibitory Effect upon Bioelectrocatalytic Reduction of Oxygen by Peroxidase‐Modified Carbon‐Felt

A novel, simple and relative highly sensitive amperometric flow biosensor for cyanide was developed by using horseradish peroxidase (HRP)‐adsorbed carbon‐felt (CF), based on an inhibitory effect on the HRP‐catalyzed O₂ reduction. The HRP‐CF showed a sufficient bioelecrocatalytic activity for O₂ reduction in the potential region from 0 to ?0.5 V at pH 5.0, due to a direct electron transfer‐based O₂ reduction process via ferrous‐HRP and compound III. This HRP‐catalyzed O₂ reduction was reversibly inhibited by cyanide, which enabled to fabricate a novel and simple reagentless (i.e., no requirement of the ordinary substrate, H₂O₂, and the electron transfer mediators) flow‐biosensor for cyanide. When air‐saturated 0.1 M phosphate buffer (pH 5.0) was used as a carrier under the applied potential of ?0.2 V vs. Ag/AgCl, the steady‐state base‐current due to the HRP‐catalyzed O₂ reduction was reversibly inhibited by the cyanide injection (200 µL), resulting in peak‐shape current responses. The magnitude of the inhibition peak currents linearly increased with increasing concentrations of cyanide up to 1 µM, and the detection limit was found to be 0.04 µM (S/N=2). The apparent inhibition constant K_i′ was estimated to be 0.87 µM.     

Synthesis and Photophysical and Electrochemical Study of Tyrosine Covalently Linked to High‐Valent Copper(III) and Manganese(IV) Complexes

As bio‐inspired chemical model of the oxygen‐evolving complex (OEC) in photosystem II, a new tyrosine‐modified corrole ligand 3 and its high‐valent copper and manganese complexes 3a and 3b were synthesized and characterized. The copper complexes 1a and 2a of corrole 1 and 2 were also prepared for comparison. The emission property indicates that the emission of ligands 2 and 3 is located at 670 nm, but no emission is observed for their metal complexes due to its suppression by the metal center. The electrochemical study shows that 3a might dimerize at the first two reversible oxidations, a behavior which was not observed in the case of 1a and 2a . The corrolato manganese(IV) complex 3b shows one reversible reduction and one quasireversible oxidation at ?0.17 and 0.77 V vs. Ag/Ag⁺, respectively.     

Structural comparison of group 7 tricarbonyl complexes of 2‐{[2‐(1H‐imidazol‐4‐yl)ethyl]iminomethyl}‐5‐methylphenolate

Two tricarbonyl complexes of rhenium(I) and manganese(I) coordinated by the ligand 2‐{[2‐(1H‐imidazol‐4‐yl)ethyl]iminomethyl}‐5‐methylphenolate are reported, viz. fac‐tricarbonyl(2‐{[2‐(1H‐imidazol‐4‐yl‐κN³)ethyl]iminomethyl‐κN}‐5‐methylphenolato‐κO)rhenium(I) methanol monosolvate, [Re(C₁₆H₁₄N₃O₄)(CO)₃]·CH₃OH, (I), and fac‐tricarbonyl(2‐{[2‐(1H‐imidazol‐4‐yl‐κN³)ethyl]iminomethyl‐κN}‐5‐methylphenolato‐κO)manganese(I), fac‐[Mn(C₁₆H₁₄N₃O₄)(CO)₃], (II), display facial coordination in a distorted octahedral environment. The crystal structure of (I) is stabilized by O—H...O, N—H...O and C—H...O hydrogen‐bond interactions, while that of (II) is stabilized by N—H...O hydrogen‐bond interactions only. These interactions result in two‐dimensional networks and π–π stacking for both structures.     

Long‐Lived Excited States of Zwitterionic Copper(I) Complexes for Photoinduced Cross‐Dehydrogenative Coupling Reactions

Four heteroleptic copper(I) complexes containing phenanthroline and monoanionic nido‐carborane‐diphosphine ligands have been prepared and structurally characterized by various spectroscopic techniques and X‐ray diffraction. These complexes exhibit intense absorptions in the visible range and excited‐state lifetimes on the microsecond scale. Their application in visible‐light‐induced cross‐dehydrogenative coupling reactions was investigated. Preliminary studies showed that one of the four copper(I) complexes is an efficient catalyst for photoinduced oxidative C?H functionalization using oxygen as oxidant. Furthermore, α‐functionalized tertiary amines were obtained in good‐to‐excellent yields by light irradiation (λ>420 nm) of a mixture of our Cu^I complex, tertiary amines, and a variety of nucleophiles (nitroalkane, acetone, or indoles) under aerobic conditions. Electron paramagnetic resonance measurements provided evidence for the formation of superoxide radical anions (O₂^??) rather than singlet oxygen (¹O₂) during these photocatalytic reactions.     

Enzyme‐Inspired Room‐Temperature Lithium–Oxygen Chemistry via Reversible Cleavage and Formation of Dioxygen Bonds

Li‐O₂ batteries are promising energy storage systems due to their ultra‐high theoretical capacity. However, most Li‐O₂ batteries are based on the reduction/oxidation of Li₂O₂ and involve highly reactive superoxide and peroxide species that would cause serious degradation of cathodes, especially carbon‐based materials. It is important to explore lithium‐oxygen reactions and find new Li‐O₂ chemistry which can restrict or even avoid the negative influence of superoxide/peroxide species. Here, inspired by enzyme‐catalyzed oxygen reduction/oxidation reactions, we introduce a copper(I) complex 3 N‐Cu^I (3 N=1,4,7‐trimethyl‐1,4,7‐triazacyclononane) to Li‐O₂ batteries and successfully modulate the reaction pathway to a moderate one on reversible cleavage/formation of O?O bonds. This work demonstrates that the reaction pathways of Li‐O₂ batteries could be modulated by introducing an appropriate soluble catalyst, which is another powerful choice to construct better Li‐O₂ batteries.     

Tuning a Single Ligand System to Stabilize Multiple Spin States of Manganese: A First Example of a Hydrazone‐Based Manganese(III) Spin‐Crossover Complex

A series of bis‐chelate pseudo‐octahedral mononuclear coordination complexes of manganese with the chromophore [MnN₄O₂]ⁿ⁺ (n=0, 1) have been generated in all three principal oxidation states of this transition‐metal center under ambient conditions by utilizing a readily tunable, versatile phenolic pyridylhydrazone ligand system (i.e., H₂(3,5‐R¹,R²)‐L; L=ligand). Strategic combinations of the nature and position of a variety of substituent groups afforded selective, spontaneous stabilization of multiple spin states of the manganese center, which, upon close crystallographic scrutiny, appears to be in part due to the occurrence or absence of hydrogen‐bonding interactions that involve the phenolate/phenolic oxygen atom. The divalent complexes are isolable in two forms, namely, molecular [Mn^II{H(3,5‐R¹,R²)‐L}₂] and ionic [Mn^II{H₂(3,5‐R¹,R²)‐L}{H(3,5‐R¹,R²)‐L}]ClO₄, with the latter complex converting easily into the former complex on deprotonation. Accessibility of the higher‐valent states is achievable only when the phenolate oxygen atom is sterically hindered from participation in hydrogen bonding. The [Mn^III{H(3,5‐tBu₂)‐L}₂]ClO₄ complex is the first example of a hydrazone‐based Mn^III complex to exhibit spin crossover. Formation of the tetravalent complexes [Mn^IV{(3,5‐R¹,R²)‐L}₂] (R¹=tBu, R²=H; R¹=R²=tBu) necessitates base‐assisted abstraction of the hydrazinic proton.     

Electrocatalytic Reduction of Oxygen at Multi‐Walled Carbon Nanotubes and Cobalt Porphyrin Modified Glassy Carbon Electrode

The multi‐walled carbon nanotubes (MWNTs) modified glassy carbon electrode exhibited electrocatalytic activity to the reduction of oxygen in 0.1 M HAc‐NaAc (pH 3.8) buffer solution. Further modification with cobalt porphyrin film on the MWNTs by adsorption, the resulted modified electrode showed more efficient catalytic activity to O₂ reduction. The reduction peak potential of O₂ is shifted much more positively to 0.12 V (vs. Ag/AgCl), and the peak current is increased greatly. Cyclic voltammetry (CV), transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), were used to characterize the material and the modified film on electrode surface. Electrochemical experiments gave the total number of electron transfer for oxygen reduction as about 3, which indicated a co‐exist process of 2 electrons and 4 electrons for reduction of oxygen at this modified electrode. Meanwhile, the catalytic activities of the multilayer film (MWNTs/CoTMPyP)_n prepared by layer‐by‐layer method were investigated, and the results showed that the peak current of O₂ reduction increased and the peak potential shifted to a positive direction with the increase of layer numbers.     

混合价双核锰[Mn2(cyclen)2(μ-O)2](ClO4)3·4H2O的晶体结构和表征

The mixed-valence manganese(Ⅲ/Ⅳ) complex [Mn2(cyclen)2(μ-O)2](ClO4)3-4H2O (1) (cyclen=1,4,7,10-tetraazacyclododecan) with chemical formula C16H48Cl3Mn2N8O18 has been synthesized and characterized by single crystal X-ray diffraction analysis, elemental analysis, IR and electronic spectra. The results showed that the manganese(Ⅲ/Ⅳ) ions were six-coordinated by four nitrogen atoms from cyclen and two oxygen atoms from the oxygen bridge, forming a distorted octahedron geometry. There were two very strong peaks in the range of 400-700 nm in electronic spectrum, which was similar to Mn catalase and Mn ribonucleotide reductase extracted from organisms.Electrochemical study indicated that the complex underwent a quasi-reversible one-electron reduction and oxidation at E1/2=0.827 V in acetonitrile.     

Dynamics of O2 Chemisorption on a Flat Platinum Surface Probed by an Alignment‐Controlled O2 Beam

O₂ adsorption on Pt surfaces is of great technological importance owing to its relevance to reactions for the purification of car exhaust gas and the oxygen reduction on fuel‐cell electrodes. Although the O₂/Pt(111) system has been investigated intensively, questions still remain concerning the origin of the low O₂ sticking probability and its unusual energy dependence. We herein clarify the alignment dependence of the initial sticking probability (S ₀) using the single spin‐rotational state‐selected [(J ,M )=(2,2)] O₂ beam. The results indicate that, at low translational energy (E ₀) conditions, direct activated chemisorption occurs only when the O₂ axis is nearly parallel to the surface. At high energy conditions (E ₀>0.5 eV), however, S ₀ for the parallel O₂ decreases with increasing E ₀ while that of the perpendicular O₂ increases, accounting for the nearly energy‐independent O₂ sticking probability determined previously by a non‐state‐resolved experiment.     

Synthesis and Magnetic Studies of Copper (II)‐Manganese (II) Heterobinuclear Complexes with an Oxamido Bridge

Four new copper (II)‐manganese (II) heterobinuclear complexes bridged byN, N'‐bis[2‐(dimethylamino)ethyl)]oxamido dianion (dmoxæ) and end‐capped with 1, 10‐phenanthroline (phen), 5‐methyl‐1, 10‐phenanthroline (Mephen), diaminoethane (en) or 1,3‐di‐aminopropane (pn). respectively, namely, [Cu(dmoxae)MnL₂] (CIO₄)₂ (L=phen, Mephen, en, pn), have been synthesized and characterized by elemental analyses, IR, electronic spectral studies, and molar conductivity measurements. The electronic reflectance spectrum indicates the presence of spin exchange‐coupling interaction between bridged copper(II) and manganese (II) ions. The cryomagnetic measurements (4.2‐300 K) of [Cu(dmoxae)Mn(phen)₂](CIO₄)₂ (1) and [Cu(dmoxae)Mn(Mephen)₂](CIO₄)₂(2) complexes demonstrated an antiferromagnetic interaction between the adjacent manganese(II) and copper (II) ions through the oxamido‐bridge within each molecule. On the basis of spin Hamiltonian, H= ‐ 2JS₁. S₂. the magnetic analysis was carried out for the two complexes and the spin‐coupling constant (J) was evaluated as ?35.9 cm^?1 for 1 and ‐ 32.6 cm^?1 for 2. The influence of methyl substitutions in the amine groups of the bridging ligand on magnetic interactions between the metal ions of this kind of complexes is also discussed.     

A Simple Electrochemical Approach Based on Inexpensive Wall‐Jet Screen‐Printed Ring Disk Electrode to Evaluate Oxygen Reduction Catalysts

A simple electrochemical approach to evaluate oxygen reduction catalysts using an inexpensive screen‐printed ring disk carbon electrode system, consisting of a ring electrode deposited with MnO₂ and a disk electrode modified with the catalysts for study, is developed in this study. The as‐prepared MnO₂ is selective and sensitive for H₂O₂ oxidation in the presence of O₂ and is crucial to the proposed approach. By coupling with a wall‐jet electrochemical cell, the product generated from the reduction reaction at the disk electrode can effectively be monitored at the MnO₂‐deposited ring electrode. Model catalysts of nano‐Au and nano‐Pd representing 2e^? reduction of O₂ to H₂O₂ and 4e^? reduction to H₂O, respectively, were evaluated as electrode materials in oxygen reduction reaction to demonstrate the applicability of the proposed method.