Subal Dey

Activating Fe(I) Porphyrins for the Hydrogen Evolution Reaction Using Second-Sphere Proton Transfer Residues

Iron porphyrin complexes with second-sphere distal triazole residues show a hydrogen evolution reaction (HER) catalyzed by the Fe(I) state in both organic and aqueous media, whereas an analogous iron porphyrin complex without the distal residues catalyzes the HER in the formal Fe(0) state. This activation of the Fe(I) state by the second-sphere residues lowers the overpotential of the HER by these iron porphyrin complexes by 50%. Experimental data and theoretical calculations indicate that the distal triazole residues, once protonated, enhance the proton affinity of the iron center via formation of a dihydrogen bond with an Fe(III)-H- intermediate.

An acetate bound cobalt oxide catalyst for water oxidation: role of monovalent anions and cations in lowering overpotential.

A homogeneous solution of Co(II) in acetate buffer at pH 7 is found to be an efficient water oxidation catalyst (WOC) showing significantly greater current density than Co(II) in phosphate buffer (Co-Pi) under identical conditions owing to the higher solubility of the former. When electrodeposited on ITO/FTO electrodes it forms acetate bound cobalt(II)-oxide materials (Co-Ac-WOC) showing a catalytic current density of 0.1 mA cm(-2) at 830 mV and 1 mA cm(-2) at 1 V in a pH 7 buffer solution. The morphology of Co-Ac-WOC and its evolution with time and deposition potential is investigated with AFM, HR-TEM and SEM. The chemical composition of Co-Ac-WOC is investigated using XPS, EDX, ATR-FTIR and combustion analysis which indicate that this material has a CoO core with chloride and acetate anions bound to the Co center. Sodium is found to be integrated in the Co-Ac-WOC. The presence of the sodium and chloride ions lowers the onset potential for the oxygen evolution reaction (OER) by 240 mV relative to the classic Co-Pi at pH 7. The lower onset potential and higher OER current lowers the exchange current density to 10(-6.7) A cm(-2) in Co-Ac-WOC relative to 10(-8)-10(-10) A cm(-2) in Co-Pi and its derivatives.

Electrocatalytic O2 Reduction by [Fe-Fe]-Hydrogenase Active Site Models

The instability of [Fe-Fe]-hydrogenase and its synthetic models under aerobic conditions is an inherent challenge in their development as practical H2 producing electrodes. The electrochemical oxygen reduction reaction of a series of synthetic model complexes of the [Fe-Fe] hydrogenase is investigated, and a dominant role of the bridgehead nitrogen in reducing the amount of partially reduced oxygen species (PROS), which is detrimental to the stability of these complexes, is discovered.

Ammonium Tetrathiomolybdate: A Versatile Catalyst for Hydrogen Evolution Reaction from Water under Ambient and Hostile Conditions

The lack of catalysts that can selectively reduce protons to produce hydrogen from water in the presence of oxygen and other conventional inhibitors of hydrogen evolution reaction (HER) has been a fundamental problem stalling the development of a practical hydrogen economy. Ammonium tetrathiomolybdate (ATM), a common laboratory reagent, spontaneously assembles on Au electrodes. Atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy data indicate formation of multiple layers of ATM which are stable over a wide pH range for days. These assemblies can produce hydrogen with very low onset potentials. It shows a turnover rate of 1.4 s(-1) and turnover number >5 × 10(4) in pH 7 at 180 mV overpotential. The pH dependence of the peak potential suggests that the generation of H2 from water proceeds likely via a ligand based proton coupled electron transfer process which precludes inhibition by O2. The ATM functionalized Au electrodes are found to efficiently catalyze HER in saline rich, CO saturated, and sulfide rich water sources with minimal inhibition of catalytic activity.

Oxygen-Tolerant H2 Production by [FeFe]-H2ase Active Site Mimics Aided by Second Sphere Proton Shuttle

The instability of [FeFe]-H2ases and their biomimetics toward O2 renders them inefficient to implement in practical H2 generation (HER). Previous investigations on synthetic models as well as natural enzymes proved that reactive oxygen species (ROS) generated on O2 exposure oxidatively degrades the 2Fe subcluster within the H-cluster active site. Recent electrochemical studies, coupled with theoretical investigations on [FeFe]-H2ase suggested that selective O2 reduction to H2O could eliminate the ROS, and hence, tolerance against oxidative degradation could be achieved ( Nat. Chem. 2017, 9, 88-95). We have prepared a series of 2Fe subsite mimics with substituted arenes attached to bridgehead N atoms in the S to S linker, (μ-S2(CH2)2NAr)[Fe(CO)3]2. Structural analyses find the nature of the substituent on the arene offers steric control of the orientation of bridgehead N atoms, affecting their proton uptake and translocation ability. The heterogeneous electrochemical studies of these complexes physiadsorbed on edge plane graphite (EPG) electrode show the onset of HER activity at ∼180 mV overpotential in pH 5.5 water. In addition, bridgehead N-protonation and subsequent H-bonding capability are established to facilitate the O-O bond cleavage resulting in selective O2 reduction to H2O. This allows a synthetic [FeFe]-H2ase model to reduce protons to H2 unabated in the presence of dissolved O2 in water at nearly neutral pH (pH 5.5); i.e., O2-tolerant, stable HER activity is achieved.

Activation of Co(I) State in a Cobalt-Dithiolato Catalyst for Selective and Efficient CO2 Reduction to CO

Reduction of CO2 holds the key to solving two major challenges taunting the society-clean energy and clean environment. There is an urgent need for the development of efficient non-noble metal-based catalysts that can reduce CO2 selectively and efficiently. Unfortunately, activation and reduction of CO2 can only be achieved by highly reduced metal centers jeopardizing the energy efficiency of the process. A carbon monoxide dehydrogenase inspired Co complex bearing a dithiolato ligand can reduce CO2, in wet acetonitrile, to CO with ∼95% selectivity over a wide potential range and 1559 s-1 rate with a remarkably low overpotential of 70 mV. Unlike most of the transition-metal-based systems that require reduction of the metal to its formal zerovalent state for CO2 reduction, this catalyst can reduce CO2 in its formal +1 state making it substantially more energy efficient than any system known to show similar reactivity. While covalent donation from one thiolate increases electron density at the Co(I) center enabling it to activate CO2, protonation of the bound thiolate, in the presence of H2O as a proton source, plays a crucial role in lowering overpotential (thermodynamics) and ensuring facile proton transfer to the bound CO2 ensuring facile (kinetics) reactivity. A very covalent Co(III)-C bond in a Co(III)-COOH intermediate is at the heart of selective protonation of the oxygen atoms to result in CO as the exclusive product of the reduction.