Evolution Of Molecular Hydrogen Research Articles

Electrocatalytic Hydrogen Evolution from a Cobaloxime-Based Metal–Organic Framework Thin Film

Molecular hydrogen evolution catalysts (HECs) are synthetically tunable and often exhibit high activity, but they are also hampered by stability concerns and practical limitations associated with their use in the homogeneous phase. Their incorporation as integral linker units in metal–organic frameworks (MOFs) can remedy these shortcomings. Moreover, the extended three-dimensional structure of MOFs gives rise to high catalyst loadings per geometric surface area. Herein, we report a new MOF that exclusively consists of cobaloximes, a widely studied HEC, that act as metallo-linkers between hexanuclear zirconium clusters. When grown on conducting substrates and under applied reductive potential, the cobaloxime linkers promote electron transport through the film as well as function as molecular HECs. The obtained turnover numbers are orders of magnitude higher than those of any other comparable cobaloxime system, and the molecular integrity of the cobaloxime catalysts is maintained for at least 18 h of electrocatalysis. Being one of the very few hydrogen evolving electrocatalytic MOFs based on a redox-active metallo-linker, this work explores uncharted terrain for greater catalyst diversity and charge transport pathways.

Geometry of the Catalytic Active Site in [FeFe]-Hydrogenase Is Determined by Hydrogen Bonding and Proton Transfer

[FeFe]-hydrogenases are efficient metalloenzymes that catalyze the oxidation and evolution of molecular hydrogen, H2. They serve as a blueprint for the design of synthetic H2-forming catalysts. [FeFe]-hydrogenases harbor a six-iron cofactor that comprises a [4Fe-4S] cluster and a unique diiron site with cyanide, carbonyl, and hydride ligands. To address the ligand dynamics in catalytic turnover and upon carbon monoxide (CO) inhibition, we replaced the native aminodithiolate group of the diiron site by synthetic dithiolates, inserted into wild-type and amino acid variants of the [FeFe]-hydrogenase HYDA1 from Chlamydomonas reinhardtii. The reactivity with H2 and CO was characterized using in situ and transient infrared spectroscopy, protein crystallography, quantum chemical calculations, and kinetic simulations. All cofactor variants adopted characteristic populations of reduced species in the presence of H2 and showed significant changes in CO inhibition and reactivation kinetics. Differences were attribut...

Graphite Conjugation Eliminates Redox Intermediates in Molecular Electrocatalysis.

The efficient interconversion of electrical and chemical energy requires the intimate coupling of electrons and small-molecule substrates at catalyst active sites. In molecular electrocatalysis, the molecule acts as a redox mediator which typically undergoes oxidation or reduction in a separate step from substrate activation. These mediated pathways introduce a high-energy intermediate, cap the driving force for substrate activation at the reduction potential of the molecule, and impede access to high rates at low overpotentials. Here we show that electronically coupling a molecular hydrogen evolution catalyst to a graphitic electrode eliminates stepwise pathways and forces concerted electron transfer and proton binding. Electrochemical and X-ray absorption spectroscopy data establish that hydrogen evolution catalysis at the graphite-conjugated Rh molecule proceeds without first reducing the metal center. These results have broad implications for the molecular-level design of energy conversion catalysts.

A Molecular Cobalt Hydrogen Evolution Catalyst Showing High Activity and Outstanding Tolerance to CO and O2.

There is a demand to develop molecular catalysts promoting the hydrogen evolution reaction (HER) with a high catalytic rate and a high tolerance to various inhibitors, such as CO and O2 . Herein we report a cobalt catalyst with a penta-dentate macrocyclic ligand (1-Co), which exhibits a fast catalytic rate (TOF=2210 s-1 ) in aqueous pH 7.0 phosphate buffer solution, in which proton transfer from a dihydrogen phosphate anion (H2 PO4 - ) plays a key role in catalytic enhancement. The electrocatalyst exhibits a high tolerance to inhibitors, displaying over 90 % retention of its activity under either CO or air atmosphere. Its high tolerance to CO is concluded to arise from the kinetically labile character of undesirable CO-bound species due to the geometrical frustration posed by the ligand, which prevents an ideal trigonal bipyramid being established.

A Molecular Cobalt Hydrogen Evolution Catalyst Showing High Activity and Outstanding Tolerance to CO and O2

AbstractThere is a demand to develop molecular catalysts promoting the hydrogen evolution reaction (HER) with a high catalytic rate and a high tolerance to various inhibitors, such as CO and O2. Herein we report a cobalt catalyst with a penta‐dentate macrocyclic ligand (1‐Co), which exhibits a fast catalytic rate (TOF=2210 s−1) in aqueous pH 7.0 phosphate buffer solution, in which proton transfer from a dihydrogen phosphate anion (H2PO4−) plays a key role in catalytic enhancement. The electrocatalyst exhibits a high tolerance to inhibitors, displaying over 90 % retention of its activity under either CO or air atmosphere. Its high tolerance to CO is concluded to arise from the kinetically labile character of undesirable CO‐bound species due to the geometrical frustration posed by the ligand, which prevents an ideal trigonal bipyramid being established.

Ag/Ag2O as a Co-Catalyst in TiO2 Photocatalysis: Effect of the Co-Catalyst/Photocatalyst Mass Ratio

Mixtures and composites of Ag/Ag2O and TiO2 (P25) with varying mass ratios of Ag/Ag2O were prepared, employing two methods. Mechanical mixtures (TM) were obtained by the sonication of a suspension containing TiO2 and Ag/Ag2O. Composites (TC) were prepared by a precipitation method employing TiO2 and AgNO3. Powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) confirmed the presence of Ag(0) and Ag2O. The activity of the materials was determined employing methylene blue (MB) as the probe compound. Bleaching of MB was observed in the presence of all materials. The bleaching rate was found to increase with increasing amounts of TiO2 under UV/vis light. In contrast, the MB bleaching rate decreased with increasing TiO2 content upon visible light illumination. XRD and XPS data indicate that Ag2O acts as an electron acceptor in the light-induced reaction of MB and is transformed by reduction of Ag+, yielding Ag(0). As a second light-induced reaction, the evolution of molecular hydrogen from aqueous methanol was investigated. Significant H2 evolution rates were only determined in the presence of materials containing more than 50 mass% of TiO2. The experimental results suggest that Ag/Ag2O is not stable under the experimental conditions. Therefore, to address Ag/Ag2O as a (photo)catalytically active material does not seem appropriate.

Influence of Samarium on Hydrogen Absorption during the Nickel Electrodeposition on Palladium

During the nickel electrodeposition process, secondary reactions corresponding to the reduction of a proton to Hads and the evolution of molecular hydrogen occur. The Hads could be absorbed and diffused in the substrate. In this work, we studied the effect of samarium on the process of hydrogen permeation that carries out during the electrodeposition of nickel from a nickel sulfamate bath which also contains the tensioactive SDS (sodium dodecyl sulfate). The experiments were carried out at pH 3 and T=25 ° C, with a Devanathan Starchuski cell and with different concentrations of SDS and Sm. According to the results obtained from the permeation current and the hydrogen diffusion in the metal, we observe that additions of SDS and Sm at concentrations of 0.1g/l and 2g/l respectively will decrease the hydrogen absorption in the substrate. The influence of these additives on the mechanism of hydrogen absorption will be discussed.

Mechanisms of Photocatalytic Molecular Hydrogen and Molecular Oxygen Evolution over La-Doped NaTaO3 Particles: Effect of Different Cocatalysts and Their Specific Activity

A better understanding of the mechanisms of H2 and O2 evolution over cocatalyst-loaded photocatalysts is an essential step in constructing efficient artificial systems for the overall water splitting. In this paper, La-doped NaTaO3 particles loaded with different cocatalysts (i.e., noble metals and metal oxides) have been synthesized and used as model photocatalysts to study the mechanisms of photocatalytic H2 and/or O2 evolution from pure water, aqueous methanol solution, and aqueous silver nitrate solution. It was found that the photocatalytic activity and selectivity toward H2 and/or O2 evolution strongly depend on the nature of the cocatalyst and the investigated system. For pure water and aqueous silver nitrate systems, the affinity of the cocatalyst nanoparticles to react with the photogenerated charge carriers (electrons or holes) was found to be the main reason for the observed selective behavior for H2 and O2 evolution. The creation of active sites and subsequent decrease in activation energy is ...

Application of a Water‐Soluble Matrix‐Stabilized Palladium Nanoparticle Catalyst for Photocatalytic Hydrogen Generation with High Activity and Stability

AbstractPd@(BiPy‐PEG‐OMe) is a catalyst comprised of palladium nanoparticles (Pd‐NPs) stabilized and made water soluble by 2,2′‐bipyridine‐end‐functionalized polyethylene glycol monomethyl ether (BiPy‐PEG‐OMe). The catalyst has been used in the past for nitrile hydrogenation. In this work, we prove that it is also very active for photocatalytic hydrogen generation, which might be true for more catalysts of its kind and therefore worthy of further investigation. Using the inexpensive photosensitizer Eosin Y, high turnover numbers (TONs) of over 4 500 were achieved for the evolution of molecular hydrogen from pure water under visible‐light irradiation. Replacing Eosin Y, which showed only a short lifetime under experimental conditions (i.e. a few hours), by a novel osmium‐based metal complex, which is also characterized by its crystal structure, the longevity of the system can be boosted to over one and a half months with a maximum TON of 1 500. Combining excellent yield and stability is a clear goal for further research.

A family of molecular nickel hydrogen evolution catalysts providing tunable overpotentials using ligand-centered proton-coupled electron transfer paths.

Two new nickel dithiolate derivatives have been examined for their electrocatalytic activity for the hydrogen evolution reaction (HER) in attempts to clarify whether the overpotential for the HER can be tuned upon varying the ligand-centered reduction potential that triggers the HER by the catalysts. We demonstrate the validity of this approach to achieve desirable tunability in the overpotential for the HER.

Active Cocatalysts for Photocatalytic Hydrogen Evolution Derived from Nickel or Cobalt Amine Complexes.

A cost-effective and robust strategy for the anchoring of molecular hydrogen evolution cocatalysts onto semiconductors has recently been reported. The composite materials were highly efficient and stable towards photocatalytic H2 evolution. This study provides guidance for the design and construction of highly active heterogeneous photocatalysts.

Copper underpotential deposition on boron nitride nanomesh

The boron nitride nanomesh is a corrugated monolayer of hexagonal boron nitride (h-BN) on Rh(111), which so far has been studied mostly under ultrahigh vacuum conditions. Here, we investigate how copper underpotential deposition (upd) can be used to quantify defects in the boron nitride monolayer and to assess the potential window of the nanomesh, which is important to explore its functionality under ambient and electrochemical conditions. In dilute sulfuric acid, the potential window of h-BN/Rh(111) is close to 1volt, i.e. larger than that of the Rh substrate, and is limited by molecular hydrogen evolution on the negative and by oxidative removal on the positive side. From copper upd on pristine h-BN/Rh(111) wafer samples, we estimate a collective defect fraction on the order of 0.08–0.7% of the geometric area, which may arise from line and point defects in the h-BN layer that are created during its chemical vapour deposition. Overpotential deposition (opd) is demonstrated to have significant consequences on the defect area. We hypothesise that this non-innocent Cu electrodeposition involves intercalation originating at initial defects, causing irreversible delamination of the h-BN layer; this effect may be used for 2D material nanoengineering. On the relevant timescale, upd itself does not alter the defect area on repeated cycling; therefore, metal upd may find use as a general tool to determine the collective defect area in hybrids between 2D materials and various substrate metals.

Atomic Layer Deposition of Ultrathin Nickel Sulfide Films and Preliminary Assessment of Their Performance as Hydrogen Evolution Catalysts.

Transition metal sulfides show great promise for applications ranging from catalysis to electrocatalysis to photovoltaics due to their high stability and conductivity. Nickel sulfide, particularly known for its ability to electrochemically reduce protons to hydrogen gas nearly as efficiently as expensive noble metals, can be challenging to produce with certain surface site compositions or morphologies, e.g., conformal thin films. To this end, we employed atomic layer deposition (ALD), a preeminent method to fabricate uniform and conformal films, to construct thin films of nickel sulfide (NiSx) using bis(N,N'-di-tert-butylacetamidinato)nickel(II) (Ni(amd)2) vapor and hydrogen sulfide gas. Effects of experimental conditions such as pulse and purge times and temperature on the growth of NiSx were investigated. These revealed a wide temperature range, 125-225 °C, over which self-limiting NiSx growth can be observed. In situ quartz crystal microbalance (QCM) studies revealed conventional linear growth behavior for NiSx films, with a growth rate of 9.3 ng/cm2 per cycle being obtained. The ALD-synthesized films were characterized using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) methods. To assess the electrocatalyitic activity of NiSx for evolution of molecular hydrogen, films were grown on conductive-glass supports. Overpotentials at a current density of 10 mA/cm2 were recorded in both acidic and pH 7 phosphate buffer aqueous reaction media and found to be 440 and 576 mV, respectively, with very low NiSx loading. These results hint at the promise of ALD-grown NiSx materials as water-compatible electrocatalysts.

(Invited) Molecular Hydrogen-Evolution Electrocatalysts for Dye-Sensitized Solar Fuels

Proton reduction is one of the most fundamental and important reactions in nature. MoS2 edges have been identified as the active sites for hydrogen evolution reaction (HER) electrocatalysis. Designing molecular mimics of MoS2 edge sites is an attractive strategy to understand the underlying catalytic mechanism of different edge sites and improve their activities. Herein I will discuss our recent progress in this aspect. The first example is a dimeric molecular analog [Mo2S12]2-, as the smallest unit possessing both the terminal and bridging disulfide ligands. Our electrochemical tests show that [Mo2S12]2- is a superior heterogeneous HER catalyst under acidic conditions. Computations suggest that the bridging disulfide ligand of [Mo2S12]2- exhibits a hydrogen adsorption free energy near zero (-0.05 eV). The second example is a group of compounds with the general formular of MoO(S2)2bpy. By varying substitution on the bpy ligand, we demonstrate the induction effect on the S—S ligand, thereby tuning the rate constants for HER and overpotentials. We have studied homogenous electrocatalytic hydrogen production performance metrics of three catalysts with different bipyridine substitutions. By varying the electron-donating abilities, we present the first demonstration of using the ligand to tune catalytic properties of the S—S bond in molecular MoS2 edge site mimics. These works can shed light on the relationship between structure and electrocatalytic activity of molecular MoS2 catalysts and thus is of broad importance from catalytic hydrogen production to biological enzyme functions. Lastly, I will discuss how to integrate the molecular catalysts into dye-sensitized photoeletrochemical platforms for solar hydrogen production.

Graded ZnS/ZnSxO1−x heterostructures produced by oxidative photolysis of zinc sulfide: Structure, optical properties and photocatalytic evolution of molecular hydrogen

Abstract The photolysis of microcrystalline and single crystal ZnS on the ambient air was found to result in the formation of metallic zinc clusters and radical sulfur species additionally to the elimination of sulfur in reactions with oxygen and generation of S vacancies. The latter process results in the formation of a graded alloyed ZnO x S 1−x layer with a continuously decreasing content of sulfur and increasing content of oxygen from the bulk to the surface of the crystals. These photolytic changes are observed within the surface layer with a thickness of 50–80 nm which is comparable to the depth of light penetration into the ZnS crystals. Coupling of ZnS to ZnO, both via pre-photolysis thermal oxidation of ZnS in the dark or by in situ formation of ZnO via the oxidative photocorrosion of ZnS accelerates the photolysis considerably, most probably, as a result of directed and opposite flow of the electrons and holes within the graded ZnO x S 1−x layer and suppression of the electron-hole recombination. The ZnS/ZnS x O 1−x heterostructures revealed a relatively high photocatalytic activity in hydrogen evolution from water/ethanol mixtures under illumination with the UV light with a quantum yield of up to 1.6% in the absence of any co-catalysts. The dependence between the rate of photocatalytic hydrogen formation and the duration of ZnS photolysis was found to be dome-shaped one as a result of the interplay of two factors—(i) an increase of the probability of the spatial separation of the electron and hole within the ZnO x S 1−x layer and (ii) a decrease of the probability of ethanol oxidation by the holes with an increase in the ZnS x O 1−x layer thickness (or photolysis duration as an equivalent). In the optimal conditions the rate of photocatalytic H 2 evolution was 30 times higher than that for original non-photolyzed ZnS owing to the unique graded structure of the alloyed zinc oxysulfide layer produced by the oxidative photolysis.

Reactions of N+(3P) ions with H2and HD molecules at low temperatures

Context. This work is motivated by the necessity to take account of both the nuclear spin symmetries of H 2 and the spin-orbit interaction of N + ions in order to investigate gas phase reactions in interstellar chemistry, leading to the formation of nitrogenous and deuterated compounds. Aims. The main objective in this work is to determine the rate coefficients for each possible initial quantum state of the reactants N + (3 P j) + H 2 (J) (and their isotopic variants). Only in this way does it become possible both to analyse experimental data and to develop realistic applications to interstellar chemical models to constrain the gas phase chemistry of ammonia and its isotopologues. Methods. A statistical treatment is presented of state selective reactive collisions involving N + ions in fine structure state j with H 2 or HD molecules in a rotation level J of the ground vibration state, leading either to the production of NH + ions and H in the case of the H 2 reactant, and to the production of either NH + ions or ND + in the case of the HD reactant. The energies of fine structure states (j = 0, 1, 2) of the N + ions are treated on an equal footing with the other energies of internal motions. All fine structure states are considered to be reactive. Results. Cross sections for state-to-state collisions are calculated for collision energies ranging from 0.1–30 meV. These cross sections are then averaged over the kinetic energies of the reactants for each (J, j) to obtain the rate coefficients for a range of kinetic temperatures 10–200 K. The exo/endothermicity of the reactions involving N + (3 P j) + H 2 (J) (and isotopic variants) is derived from the difference ∆E e between the dissociation energies of the electronic molecular potentials of NH + and H 2. The value ∆E e = 101 meV is found to satisfactorily reproduce the experiments performed with ortho-H 2 and to a lesser extent with para-H 2. This value is used to determine the rate coefficient of the N + + HD reaction leading to the formation of ND +. The calculated value is consistent with the available experimental data. Conclusions. The present results allow for the determination of reaction rate coefficients for any given distribution of specific fine structure and rotational state populations of the reactants. In interstellar conditions, where N + is in its 3 P 0 state and para-and ortho-H 2 respectively in J = 0 and J = 1. Our results enable a study of the influence of the ortho/para evolution of molecular hydrogen on the formation of nitrogen compounds.

Efficient p-zinc phthalocyanine/n-fullerene organic bilayer electrode for molecular hydrogen evolution induced by the full visible-light energy

A photoelectrode featuring an organic p/n bilayer was used to generate H2 from H+, where n-type fullerene (C60) was used in combination with p-type zinc phthalocyanine (ZnPc). When a Pt catalyst was loaded on the surface of C60 in the organo-photoelectrode, a photocathodic current was successfully generated via H2 evolution at the Pt-coated C60 surface. On the basis of the results of action spectral measurements for photocurrent, the H2 evolution appeared to be induced over the entire visible-light wavelength range of <750 nm. In particular, the visible-light absorption of ZnPc/C60 bilayer efficiently contributed to carrier generation, consequently leading to the H2 evolution at the C60 surface via reducing power generated through a series of photophysical events within the bilayer.

Formation and evolution of molecular hydrogen in disc galaxies with different masses and Hubble types

We investigate the physical properties of molecular hydrogen (H2) in isolated and interacting disk galaxies with different masses and Hubble types by using chemodynamical simulations with H2 formation on dust grains and dust growth and destruction in interstellar medium (ISM). We particularly focus on the dependences of H2 gas mass fractions (f_H2), spatial distributions of HI and H2, and local H2-scaling relations on initial halo masses (M_h), baryonic fractions (f_bary), gas mass fractions (f_g), and Hubble types. The principal results are as follows. The final f_H2 can be larger in disk galaxies with higher M_h, f_bary, and f_g. Some low-mass disk models with M_h smaller than 10^10 M_sun show extremely low f_H2 and thus no/little star formation, even if initial f_g is quite large (>0.9). Big galactic bulges can severely suppress the formation of H2 from HI on dust grains whereas strong stellar bars can not only enhance f_H2 but also be responsible for the formation of H2-dominated central rings. The projected radial distributions of H2 are significantly more compact than those of HI and the simulated radial profiles of H2-to-HI-ratios (R_mol) follow roughly R^-1.5 in MW-type disk models. Galaxy interaction can significantly increase f_H2 and total H2 mass in disk galaxies. The local surface mass densities of H2 can be correlated with those of dust in a galaxy. The observed correlation between R_mol and gas pressure (R_mol ~ P_g^0.92) can be well reproduced in the simulated disk galaxies.

Evolution of dust and molecular hydrogen in the Magellanic System

We investigate the evolution of the interstellar medium (ISM) in self-consistent, chemodynamical simulations of the Magellanic Clouds (MCs) during their recent (z<0.3) past. An explicit modelling of dust and molecular hydrogen lifecycles enables the comparison of our models against the observed properties of the ISM, including elemental depletion from the gas-phase. Combining this model with a tidal-dominated paradigm for the formation for the Magellanic Stream and Bridge, we reproduce the age-metallicity relations, long gas depletion timescales, and presently observed dust and molecular hydrogen masses of the MCs to within their respective uncertainties. We find that these models' enrichment depends sensitively on the processing of dust within the ISM and the dynamical influence of external tides/stellar bars. The ratio of characteristic dust destruction timescales in our SMC and LMC models, a governing parameter of our models' evolution, is consistent with estimates based on observed supernova (SN) rates. Our reference MC models tend to exhibit the disputed universal dust-to-metal ratio, which we argue stems from the adoption of high SNe II condensation efficiencies. Our models are the first to reproduce the one-tenth solar metallicity of the Stream/Leading Arm following tidal stripping of the SMC; the hypothesis that the LMC contributes a metal-rich filament to the Stream, as implied by recent kinematic and abundance analyses, is also appraised in this study.

Bioinspired Iron Sulfide Nanoparticles for Cheap and Long-Lived Electrocatalytic Molecular Hydrogen Evolution in Neutral Water

Alternative materials to platinum-based catalysts are required to produce molecular hydrogen from water at low overpotentials. Transition-metal chalcogenide catalysts have attracted significant interest over the past few years because of their activity toward proton reduction and their relative abundance compared with platinum. We report the synthesis and characterization of a new type of iron sulfide (FeS, pyrrhotite) nanoparticles prepared via a solvothermal route. This material can achieve electrocatalysis for molecular hydrogen evolution with no structural decomposition or activity decrease for at least 6 days at a mild overpotential in neutral water at room temperature.