FeNi2 S4 -A Potent Bifunctional Efficient Electrocatalyst for the Overall Electrochemical Water Splitting in Alkaline Electrolyte.

For a carbon-neutral society, the production of hydrogen as a clean fuel through water electrolysis is currently of great interest. Since water electrolysis is a laborious energetic reaction, it requires high energy to maintain efficient and sustainable production of hydrogen. Catalytic electrodes can reduce the required energy and minimize production costs. In this context, herein, a bifunctional electrocatalyst made from iron nickel sulfide (FeNi2 S4 [FNS]) for the overall electrochemical water splitting is introduced. Compared to Fe2 NiO4 (FNO), FNS shows a significantly improved performance toward both OER and HER in alkaline electrolytes. At the same time, the FNS electrode exhibits high activity toward the overall electrochemical water splitting, achieving a current density of 10 mA cm-2 at 1.63 V, which is favourable compared to previously published nonprecious electrocatalysts for overall water splitting. The long-term chronopotentiometry test reveals an activation followed by a subsequent stable overall cell potential at around 2.12 V for 20 h at 100 mA cm-2 .

Switching Electrocatalytic Hydrogen Evolution Pathways through Electronic Tuning of Copper Porphyrins.

The electronic structure of metal complexes plays key roles in determining their catalytic features. However, controlling electronic structures to regulate reaction mechanisms is of fundamental interest but has been rarely presented. Herein, we report electronic tuning of Cu porphyrins to switch pathways of the hydrogen evolution reaction (HER). Through controllable and regioselective ß-oxidation of Cu porphyrin 1, we synthesized analogues 2-4 with one or two ß-lactone groups in either a cis or trans configuration. Complexes 1-4 have the same Cu-N4 core site but different electronic structures. Although ß-oxidation led to large anodic shifts of reductions, 1-4 displayed similar HER activities in terms of close overpotentials. With electrochemical, chemical and theoretical results, we show that the catalytically active species switches from a CuI species for 1 to a Cu0 species for 4. This work is thus significant to present mechanism-controllable HER via electronic tuning of catalysts.

A Dynamic Water Channel Affects O2 Stability in [FeFe]-Hydrogenases.

[FeFe]-hydrogenases are capable of reducing protons at a high rate. However, molecular oxygen (O2 ) induces the degradation of their catalytic cofactor, the H-cluster, which consists of a cubane [4Fe4S] subcluster (4FeH ) and a unique diiron moiety (2FeH ). Previous attempts to prevent O2 -induced damage have focused on enhancing the protein's sieving effect for O2 by blocking the hydrophobic gas channels that connect the protein surface and the 2FeH . In this study, we aimed to block an O2 diffusion pathway and shield 4FeH instead. Molecular dynamics (MD) simulations identified a novel water channel (WH ) surrounding the H-cluster. As this hydrophilic path may be accessible for O2 molecules we applied site-directed mutagenesis targeting amino acids along WH in proximity to 4FeH to block O2 diffusion. Protein film electrochemistry experiments demonstrate increased O2 stabilities for variants G302S and S357T, and MD simulations based on high-resolution crystal structures confirmed an enhanced local sieving effect for O2 in the environment of the 4FeH in both cases. The results strongly suggest that, in wild type proteins, O2 diffuses from the 4FeH to the 2FeH . These results reveal new strategies for improving the O2 stability of [FeFe]-hydrogenases by focusing on the O2 diffusion network near the active site.

Electroplated Electrodes for Continuous and Mass-Efficient Electrochemical Hydrogenation.

Electrocatalytic hydrogenations (ECH) enable the reduction of organic substrates upon usage of electric current and present a sustainable alternative to conventional processes if green electricity is used. Opposed to most current protocols for electrode preparation, this work presents a one-step binder- and additive-free production of silver- and copper-electroplated electrodes. Controlled adjustment of the preparation parameters allows for the tuning of catalyst morphology and its electrochemical properties. Upon optimization of the deposition protocol and carbon support, high faradaic efficiencies of 93 % for the ECH of the Vitamin A- and E-synthon 2-methyl-3-butyn-2-ol (MBY) are achieved that can be maintained at current densities of 240 mA cm-2 and minimal catalyst loadings of 0.2 mg cm-2, corresponding to an unmatched production rate of 1.47 kgMBE gcat -1 h-1. For a continuous hydrogenation process, the protocol can be directly transferred into a single-pass operation mode giving a production rate of 1.38 kgMBE gcat -1 h-1. Subsequently, the substrate spectrum was extended to a total of 17 different C-C-, C-O- and N-O-unsaturated compounds revealing the general applicability of the reported process. Our results lay an important groundwork for the development of electrochemical reactors and electrodes able to directly compete with the palladium-based thermocatalytic state of the art.

A CuICoII cryptate for the visible light-driven reduction of CO2.

Among the rare bimetallic complexes known for the reduction of CO2, CoIICoII and ZnIICoII hexamine cryptates are described as efficient photocatalysts. In close relation to the active sites of natural, CO2-reducing enzymes, we recently reported the asymmetric cryptand {NSNN}m ({NSNN}m = N[(CH2)2SCH2(m-C6H4)CH2NH(CH2)2]3N) comprising distinct sulphur- and nitrogen-rich binding sites and the corresponding CuIMII (MII = CoII, NiII, CuII) complexes. To gain insight into the effect of metals in different oxidation states and sulphur-incorporation on the photocatalytic activity, we herein investigate the CuICoII complex of {NSNN}m as catalyst for the visible light-driven reduction of CO2. After 24 h irradiation with LED light of 450 nm, CuICoII-{NSNN}m shows a high efficiency for the photocatalytic CO2-to-CO conversion with 9.22 µmol corresponding to a turnover number of 2305 and a high selectivity of 98% over the competing H2 production despite working in an acetonitrile/water (4 : 1) mixture. Experiments with mononuclear counterparts and computational studies show that the high activity can be attributed to synergistic catalysis between Cu and Co. Furthermore, it was shown that an increase of the metal distance results in the loss of synergistic effects and rather single-sited Co catalysis is observed.

Insights into the Molecular Mechanism of Formaldehyde Inhibition of [FeFe]-Hydrogenases.

[FeFe]-hydrogenases are efficient H2 converting biocatalysts that are inhibited by formaldehyde (HCHO). The molecular mechanism of this inhibition has so far not been experimentally solved. Here, we obtained high-resolution crystal structures of the HCHO-treated [FeFe]-hydrogenase CpI from Clostridium pasteurianum, showing HCHO reacts with the secondary amine base of the catalytic cofactor and the cysteine C299 of the proton transfer pathway which both are very important for catalytic turnover. Kinetic assays via protein film electrochemistry show the CpI variant C299D is significantly less inhibited by HCHO, corroborating the structural results. By combining our data from protein crystallography, site-directed mutagenesis and protein film electrochemistry, a reaction mechanism involving the cofactor's amine base, the thiol group of C299 and HCHO can be deduced. In addition to the specific case of [FeFe]-hydrogenases, our study provides additional insights into the reactions between HCHO and protein molecules.

Mechanochemical one-pot synthesis of heterostructured pentlandite-carbon composites for the hydrogen evolution reaction.

We have utilized carbon sources as milling additives to enable a direct mechanochemical one-pot synthesis of Fe3Co3Ni3S8/carbon (Pn/C) materials using elemental reaction mixtures. The obtained Pn/C materials are thoroughly characterized and their carbon content could be adjusted up to 50 wt%. In addition to carbon black (CB) as an additive, Pn/C materials were produced using graphite, reduced graphene oxide (rGO), and carbon nanotubes (CNTs), which allows the overall physicochemical properties of materials for energy storage applications to be adjusted. By employing the Pn/C materials as electrocatalysts for the HER in a zero-gap proton exchange membrane (PEM) electrolyzer, we were able to reach a current density of 1 A cm-2 at a cell potential as low as 2.12 V using Pn, which was synthesized with 25 wt% CB. Furthermore, electrolysis at an applied current density of 1 A cm-2 for 100 h displays a stable performance, thus providing a sustainable synthesis procedure for potential future energy storage applications. Herein, we show that catalyst supports play an important role in the overall performance.

Adapting Synthetic Models of Heme/Cu Sites to Energy-Efficient Electrocatalytic Oxygen Reduction Reaction.

In nature, cytochrome c oxidases catalyze the 4e- oxygen reduction reaction (ORR) at the heme/Cu site, in which CuI is used to assist O2 activation. Because of the thermodynamic barrier to generate CuI , synthetic Fe-porphyrin/Cu complexes usually show moderate electrocatalytic ORR activity. We herein report on a Co-corrole/Co complex 1-Co for energy-efficient electrocatalytic ORR. By hanging a CoII ion over Co corrole, 1-Co realizes electrocatalytic 4e- ORR with a half-wave potential of 0.89 V versus RHE, which is outstanding among corrole-based electrocatalysts. Notably, 1-Co outperforms Co corrole hanged with CuII or ZnII . We revealed that the hanging CoII ion can provide an electron to improve O2 binding thermodynamically and dynamically, a function represented by the biological CuI ion of the heme/Cu site. This work is significant to present a remarkable ORR electrocatalyst and to show the vital role of a second-sphere redox-active metal ion in promoting O2 binding and activation.

Developing electrochemical hydrogenation towards industrial application.

Electrochemical hydrogenation reactions gained significant attention as a sustainable and efficient alternative to conventional thermocatalytic hydrogenations. This tutorial review provides a comprehensive overview of the basic principles, the practical application, and recent advances of electrochemical hydrogenation reactions, with a particular emphasis on the translation of these reactions from lab-scale to industrial applications. Giving an overview on the vast amount of conceivable organic substrates and tested catalysts, we highlight the challenges associated with upscaling electrochemical hydrogenations, such as mass transfer limitations and reactor design. Strategies and techniques for addressing these challenges are discussed, including the development of novel catalysts and the implementation of scalable and innovative cell concepts. We furthermore present an outlook on current challenges, future prospects, and research directions for achieving widespread industrial implementation of electrochemical hydrogenation reactions. This work aims to provide beginners as well as experienced electrochemists with a starting point into the potential future transformation of electrochemical hydrogenations from a laboratory curiosity to a viable technology for sustainable chemical synthesis on an industrial scale.

Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide-Dioxygen Interconversion.

Through nitrosylation of [Fe-S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2-). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2- by a series of DNICs [(NO)2Fe(µ-MePyr)2Fe(NO)2] (1) and [(NO)2Fe(µ-SEt)2Fe(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)][Fe(µ-MePyr)4(µ-O)2(Fe(NO)2)4] (2-K-crown) and a [Fe3+(MePyr)x(NO2)y(O)z]n adduct, stoichiometric NO monooxygenation yielding NO2- occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe(NO)2}9-{Fe(NO)2}10 electronic structure, [K-18-crown-6-ether][(NO)2Fe(µ-MePyr)2Fe(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO-)2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether][(NO)2Fe(µ-SEt)2Fe(NO)2] (3-red) display a reversible equilibrium of "3 + O2- ⇋ 3-red + O2(g)", which is ascribed to the covalent [Fe(µ-SEt)2Fe] core and redox-active [Fe(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2- and O2 during reaction with O2- (or O2).

Coordination Tuning of Metal Porphyrins for Improved Oxygen Evolution Reaction.

The nucleophilic attack of water or hydroxide on metal-oxo units forms an O-O bond in the oxygen evolution reaction (OER). Coordination tuning to improve this attack is intriguing but has been rarely realized. We herein report on improved OER catalysis by metal porphyrin 1-M (M=Co, Fe) with a coordinatively unsaturated metal ion. We designed and synthesized 1-M by sterically blocking one porphyrin side with a tethered tetraazacyclododecane unit. With this protection, the metal-oxo species generated in OER can maintain an unoccupied trans axial site. Importantly, 1-M displays a higher OER activity in alkaline solutions than analogues lacking such an axial protection by decreasing up to 150-mV overpotential to achieve 10 mA/cm2 current density. Theoretical studies suggest that with an unoccupied trans axial site, the metal-oxo unit becomes more positively charged and thus is more favoured for the hydroxide nucleophilic attack as compared to metal-oxo units bearing trans axial ligands.

Investigation of Highly Active Carbon-, Cobalt-, and Noble Metal-Free MnO2/NiO/Ni-Based Bifunctional Air Electrodes for Metal-Air Batteries with an Alkaline Electrolyte.

Compared to other battery technologies, metal-air batteries offer high specific capacities because the active material at the cathode side is supplied by ambient atmosphere. To secure and further extend this advantage, the development of highly active and stable bifunctional air electrodes is currently the main challenge that needs to be resolved. Herein, a highly active carbon-, cobalt-, and noble-metal-free MnO2/NiO-based bifunctional air electrode is presented for metal-air batteries in alkaline electrolytes. Notably, while electrodes without MnO2 reveal stable current densities over 100 cyclic voltammetry cycles, MnO2 containing samples show a superior initial activity and an elevated open circuit potential. Along this line, the partial substitution of MnO2 by NiO drastically increases the cycling stability of the electrode. X-ray diffractograms, scanning electron microscopy images, and energy-dispersive X-ray spectra are obtained before and after cycling to investigate structural changes of the hot-pressed electrodes. XRD results suggest that MnO2 is dissolved or transformed into an amorphous phase during cycling. Furthermore, SEM micrographs show that the porous structure of a MnO2 and NiO containing electrode is not maintained during cycling.

Triflate vs Acetonitrile: Understanding the Iron(II)-Based Coordination Chemistry of Tri(quinolin-8-yl)amine.

In this study, a synthesis route of tri(quinolin-8-yl)amine (L), a recent member of the tetradentate tris(2-pyridylmethyl)amine (TPA) ligand family, is reported. With the neutral ligand L bound to an iron(II) center in κ4 mode, two cis-oriented coordination sites remain vacant. These can be occupied by coligands such as counterions and solvent molecules. How sensitive this equilibrium can be is most evident if both triflate anions and acetonitrile molecules are available. All three combinations─bis(triflato), bis(acetonitrile), and mixed coligand species─could be characterized by single-crystal X-ray diffraction (SCXRD), which is unique so far for this class of ligand. While at room temperature, the three compounds tend to crystallize concomitantly, the equilibrium can be shifted in favor of the bis(acetonitrile) species by lowering the crystallization temperature. Removed from their mother liquor, the latter is very sensitive to evaporation of the residual solvent, which was observed by powder X-ray diffraction (PXRD) and Mössbauer spectroscopy. The solution behavior of the triflate and acetonitrile species was studied in detail using time- and temperature-resolved UV/vis spectroscopy, Mössbauer spectroscopy of frozen solution, NMR spectroscopy, and magnetic susceptibility measurements. The results indicate a bis(acetonitrile) species in acetonitrile showing a temperature-dependent spin-switching behavior between high- and low-spin. In dichloromethane, the results reveal a high-spin bis(triflato) species. In pursuit of understanding the coordination environment equilibria of the [Fe(L)]2+ complex, a series of compounds with different coligands was prepared and analyzed with SCXRD. The crystal structures indicate that the spin state can be controlled by changing the coordination environment─all of the {N6}-coordinated complexes display geometries expected for low-spin species, while any other donor atom in the coligand position induces a shift to the high-spin state. This fundamental study sheds light on the coligand competition of triflate and acetonitrile, and the high number of crystal structures allows further insights into the influence of different coligands on the geometry and spin state of the complexes.

The [4Fe-4S]-Cluster of HydF is not Required for the Binding and Transfer of the Diiron Site of [FeFe]-Hydrogenases.

The active site of [FeFe]-hydrogenases contains a cubane [4Fe-4S]-cluster and a unique diiron cluster with biologically unusual CO and CN- ligands. The biogenesis of this diiron site, termed [2FeH ], requires the maturation proteins HydE, HydF and HydG. During the maturation process HydF serves as a scaffold protein for the final assembly steps and the subsequent transfer of the [2FeH ] precursor, termed [2FeP ], to the [FeFe]-hydrogenase. The binding site of [2FeP ] in HydF has not been elucidated, however, the [4Fe-4S]-cluster of HydF was considered as a possible binding partner of [2FeP ]. By targeting individual amino acids in HydF from Thermosipho melanesiensis using site directed mutagenesis, we examined the postulated binding mechanism as well as the importance and putative involvement of the [4Fe-4S]-cluster for binding and transferring [2FeP ]. Surprisingly, our results suggest that binding or transfer of [2FeP ] does not involve the proposed binding mechanism or the presence of a [4Fe-4S]-cluster at all.

Increasing the O2 Resistance of the [FeFe]-Hydrogenase CbA5H through Enhanced Protein Flexibility.

The high turnover rates of [FeFe]-hydrogenases under mild conditions and at low overpotentials provide a natural blueprint for the design of hydrogen catalysts. However, the unique active site (H-cluster) degrades upon contact with oxygen. The [FeFe]-hydrogenase fromClostridium beijerinckii (CbA5H) is characterized by the flexibility of its protein structure, which allows a conserved cysteine to coordinate to the active site under oxidative conditions. Thereby, intrinsic cofactor degradation induced by dioxygen is minimized. However, the protection from O2 is only partial, and the activity of the enzyme decreases upon each exposure to O2. By using site-directed mutagenesis in combination with electrochemistry, ATR-FTIR spectroscopy, and molecular dynamics simulations, we show that the kinetics of the conversion between the oxygen-protected inactive state (cysteine-bound) and the oxygen-sensitive active state can be accelerated by replacing a surface residue that is very distant from the active site. This sole exchange of methionine for a glutamate residue leads to an increased resistance of the hydrogenase to dioxygen. With our study, we aim to understand how local modifications of the protein structure can have a crucial impact on protein dynamics and how they can control the reactivity of inorganic active sites through outer sphere effects.

Substrate-Gated Transformation of a Pre-Catalyst into an Iron-Hydride Intermediate [(NO)2(CO)Fe(µ-H)Fe(CO)(NO)2]- for Catalytic Dehydrogenation of Dimethylamine Borane.

Continued efforts are made on the development of earth-abundant metal catalysts for dehydrogenation/hydrolysis of amine boranes. In this study, complex [K-18-crown-6-ether][(NO)2Fe(µ-MePyr)(µ-CO)Fe(NO)2] (3-K-crown, MePyr = 3-methylpyrazolate) was explored as a pre-catalyst for the dehydrogenation of dimethylamine borane (DMAB). Upon evolution of H2(g) from DMAB triggered by 3-K-crown, parallel conversion of 3-K-crown into [(NO)2Fe(N,N'-MePyrBH2NMe2)]- (5) and an iron-hydride intermediate [(NO)2(CO)Fe(µ-H)Fe(CO)(NO)2]- (A) was evidenced by X-ray diffraction/nuclear magnetic resonance/infrared/nuclear resonance vibrational spectroscopy experiments and supported by density functional theory calculations. Subsequent transformation of A into complex [(NO)2Fe(µ-CO)2Fe(NO)2]- (6) is synchronized with the deactivated generation of H2(g). Through reaction of complex [Na-18-crown-6-ether][(NO)2Fe(η2-BH4)] (4-Na-crown) with CO(g) as an alternative synthetic route, isolated intermediate [Na-18-crown-6-ether][(NO)2(CO)Fe(µ-H)Fe(CO)(NO)2] (A-Na-crown) featuring catalytic reactivity toward dehydrogenation of DMAB supports a substrate-gated transformation of a pre-catalyst [(NO)2Fe(µ-MePyr)(µ-CO)Fe(NO)2]- (3) into the iron-hydride species A as an intermediate during the generation of H2(g).

Evidence of Sulfur Non-Innocence in [CoII (dithiacyclam)]2+ -Mediated Catalytic Oxygen Reduction Reactions.

In many metalloenzymes, sulfur-containing ligands participate in catalytic processes, mainly via the involvement in electron transfer reactions. In a biomimetic approach, we now demonstrate the implication of S-ligation in cobalt mediated oxygen reduction reactions (ORR). A comparative study between the catalytic ORR capabilities of the four-nitrogen bound [Co(cyclam)]2+ (1; cyclam=1,5,8,11-tetraaza-cyclotetradecane) and the S-containing analog [Co(S2 N2 -cyclam)]2+ (2; S2 N2 -cyclam=1,8-dithia-5,11-diaza-cyclotetradecane) reveals improved catalytic performance once the chalcogen is introduced in the Co coordination sphere. Trapping and characterization of the intermediates formed upon dioxygen activation at the CoII centers in 1 and 2 point to the involvement of sulfur in the O2 reduction process as the key for the improved catalytic ORR capabilities of 2.

Influence of the ZnCrAl Oxide Composition on the Formation of Hydrocarbons from Syngas.

The conversion of syngas into value-added hydrocarbons gains increasing attention due to its potential to produce sustainable platform chemicals from simple starting materials. Along this line, the "OX-ZEO" process that combines a methanol synthesis catalyst with a zeolite, capable of catalyzing the methanol-to-hydrocarbon reaction, was found to be a suitable alternative to the classical Fischer-Tropsch synthesis. Hitherto, understanding the mechanism of the OX-ZEO process and simultaneously optimizing the CO conversion and the selectivity toward a specific hydrocarbon remains challenging. Herein, we present a comparison of a variety of ZnCrAl oxides with different metal ratios combined with a H-ZSM-5 zeolite for the conversion of syngas to hydrocarbons. The effect of aluminum on the catalytic activity was investigated for ZnCrAl oxides with a Zn/Cr ratio of 4:1, 1:1, and 1:2. The product distribution and CO conversion were found to be strongly influenced by the Zn/Cr/Al ratio. Although a ratio of Zn/Cr of 1:2 was best to produce lower olefins and aromatics, with aromatic selectivities of up to 37%, catalysts with a 4:1 ratio revealed high paraffin selectivity up to 52%. Notably, a distinct effect of aluminum in the oxide lattice on the catalytic activity and product selectivity was observed, as a higher Al content leads to a lower CO conversion and a changed product spectrum. We provide additional understanding of the influence of different compositions of ZnCrAl oxides on their surface properties and the catalytic activity in the OX-ZEO process. Furthermore, the variation of the zeolite component supports the important role of the channel topology of the porous support material for the hydrocarbon production. In addition, variation of the gas hourly space velocity showed a correlation of contact time, CO conversion, and hydrocarbon selectivity. At a gas hourly space velocity of 4200 mL/gcat h, CO conversion as high as 44% along with a CO2 selectivity of 42% and a lower paraffin (C2 0-C4 0) selectivity of 41% was observed.

Synthesis and Characterization of Phosphorus-Containing Isocyclam Macrocycles and Their Nickel Complexes.

The tetradentate azamacrocycle cyclam (=1,4,8,11-tetraazacyclotetradecane) was studied profoundly for the coordination of transition metal ions, and the resulting complexes were investigated extensively for their catalytic performance in, e.g., O2 activation and electrocatalytic CO2 reduction. Although the successful synthesis of analogous P4 macrocycles was described earlier, no tetradentate N,P mixed 14-membered macrocycles have been prepared to date and their chemistry remains elusive. Thus, in this work, we showcase the synthesis of phospha-aza mixed cyclam-based macrocycles by selectively "exchanging" one or two secondary amines in the macrocycle isocyclam (=1,4,7,11-tetraazacyclotetradecane) with tertiary phosphines. In addition, we herein present the preparation of the corresponding nickel complexes along with their complex chemical and structural characterization to provide first coordination studies.