Evolution Of Molecular Hydrogen Research Articles

Expanding the scope of resonance Raman spectroscopy in hydrogenase research: New observable states and reporter vibrations

Oxygen-tolerant [NiFe] hydrogenases are valuable blueprints for the activation and evolution of molecular hydrogen under application-relevant conditions. Vibrational spectroscopic techniques play a key role in the investigation of these metalloenzymes. For instance, resonance Raman spectroscopy has been introduced as a site-selective approach for probing metal-ligand coordinates of the [NiFe] active site and FeS clusters. Despite its success, this approach is still challenged by a limited number of detectable active-site states – due to missing resonance enhancement or intrinsic light sensitivity – and difficulties in their assignment. Utilizing two oxygen-tolerant [NiFe] hydrogenases as model systems, we illustrate how these challenges can be met by extending excitation and detection wavelength regimes in resonance Raman spectroscopic studies. Specifically, we observe that this technique does not only probe low-frequency metal-ligand vibrations but also high-frequency intra-ligand modes of the diatomic CO/CN− ligands at the active site of [NiFe] hydrogenases. These reporter vibrations are routinely probed by infrared absorption spectroscopy, so that direct comparison of spectra from both techniques allows an unambiguous assignment of states detected by resonance Raman spectroscopy. Moreover, we find that a previously undetected state featuring a bridging hydroxo ligand between Ni and Fe can be probed using higher excitation wavelengths, as photoconversion occurring at lower wavelengths is avoided. In summary, this study expands the applicability of resonance Raman spectroscopy to hydrogenases and other complex metalloenzymes by introducing new strategies for probing and assigning redox-structural states of the active site.

Copper complexation of rosarin: formation of bis-copper rosarin and mono-copper linear tridipyrrin complexes.

A novel rosarin di-Cu complex 2Cu-1 and a linear six-pyrrolic mono-copper complex 1Cu-1 were synthesized using rosarin as the ligand. The molecular conformations of these complexes were confirmed by X-ray crystallography. The optical study of 1Cu-1 indicated NIR-II absorption due to the long six-pyrrolic ligand and the ICT effect. The 2Cu-1 complex exhibited a very narrow electronic reduction-oxidation gap of 0.50 eV, attributed to the antiaromatic characteristics of the rosarin ring. The first HER study of the di-copper rosarin complex 2Cu-1 indicated that the multi-metal poly-pyrrolic complexes are promising molecular hydrogen evolution reaction catalysts.

The Radial Distribution and Excitation of H2 around Young Stars in the HST-ULLYSES Survey

The spatial distribution and evolution of gas in the inner 10 au of protoplanetary disks form the basis for estimating the initial conditions of planet formation. Among the most important constraints derived from spectroscopic observations of the inner disk are the radial distributions of the major gas phase constituents, how the properties of the gas change with inner disk dust evolution, and how the chemical abundances and excitation conditions are influenced by the high-energy radiation from the central star. We present a survey of the radial distribution, excitation, and evolution of inner disk molecular hydrogen (H2) obtained as part of the Hubble Space Telescope-ULLYSES program. We analyze far-UV spectroscopy of 71 (63 accreting) pre-main-sequence systems in ULLYSES DR5 to characterize the H2 emission lines, H2 dissociation continuum emission, and major photochemical/disk evolution driving the UV emissions (Lyα, UV continuum, and C iv). We use the widths of the H2 emission lines to show that most fluorescent H2 arises between 0.1 and 1.4 au from the parent star, and show positive correlations of the average emitting radius with the accretion luminosity and with the dust disk mass. We find a strong correlation between H2 dissociation emission and both the accretion-dominated Lyα luminosity and the inner disk dust clearing, painting a picture where water molecules in the inner 3 au are exposed to and dissociated by strong Lyα emission as the opacity of the inner disk declines with time.

Laser flash photolysis study of Nb2O5/g-C3N4 heterostructures as efficient photocatalyst for molecular H2 evolution

Improvements of visible light activity, slow recombination rate, stability, and efficiency are major challenges facing photocatalyst technologies today. Utilizing heterostructures of g-C3N4 (bandgap ∼2.7eV) with Nb2O5 (bandgap ∼3.4eV) as an alternative materials for the first time, we tried to overcome such challenges in this work. Heterostructures of Nb2O5/g-C3N4 have been synthesized via hydrothermal technique. And then a time-resolved laser flash photolysis of those heterostructures has been analyzed, focusing on seeking how to improve photocatalytic efficiency for molecular hydrogen (H2) evolution. The transient absorption spectra and the lifetime of charge carriers at different wavelengths have been observed for Nb2O5/g-C3N4, where g-C3N4 was used for a control. The role of hole scavenger (methanol) has also been investigated for the purpose of boosting charge trapping and H2 evolution. The long lifetime of Nb2O5/g-C3N4 heterostructures (6.54165 μs) compared to g-C3N4 (3.1651897 μs) has successfully supported the increased H2 evolution of 75 mmol/h.g. An enhancement in the rate of H2 evolution (160 mmol/h.g) in the presence of methanol has been confirmed. This study not only deepens our understanding of the role of scavenger, but also enables a rigorous quantification of the recombination rate crucial for photocatalytic applications in relation with efficient H2 production.

Thermally Activated vs. Photochemical Hydrogen Evolution Reactions-A Tale of Three Metals.

Molecular processes behind hydrogen evolution reactions can be quite complex. In macroscopic electrochemical cells, it is extremely difficult to elucidate and understand their mechanism. Gas phase models, consisting of a metal ion and a small number of water molecules, provide unique opportunities to understand the reaction pathways in great detail. Hydrogen evolution in clusters consisting of a singly charged metal ion and one to on the order of 50 water molecules has been studied extensively for magnesium, aluminum and vanadium. Such clusters with around 10-20 water molecules are known to eliminate atomic or molecular hydrogen upon mild activation by room temperature black-body radiation. Irradiation with ultraviolet light, by contrast, enables hydrogen evolution already with a single water molecule. Here, we analyze and compare the reaction mechanisms for hydrogen evolution on the ground state as well as excited state potential energy surfaces. Five distinct mechanisms for evolution of atomic or molecular hydrogen are identified and characterized.

Electrochemical Strategy for Proton Relay Installation Enhances the Activity of a Hydrogen Evolution Electrocatalyst.

A new method to install a proton relay that enhances the reactivity near an active catalytic site for H2 production is reported, afforded by the electrochemical reduction and protonation of one of the ligands in the paddlewheel Rh2(II,II) hydrogen evolution complex, cis-[Rh2(DPhF)2(bncn)2]2+ (Rh-bncn; DPhF = N,N'-diphenylformamidinate, bncn = benzo[c]cinnoline). An electrochemical reversible prewave is observed for Rh-bncn at potentials more positive than the first bncn-centered reduction couple in the presence of strong acids, observed at -0.72 V vs Fc+/0 (Fc = ferrocene) in the cyclic voltammograms (CVs) in DMF (0.1 M TBAPF6). The origin of this prewave is shown to arise from a precatalytic transformation that originates from a concerted proton-electron transfer (CPET) event occurring at one of the bridging bncn ligands. Through electrochemical analysis, CV simulations, and electronic structure calculations, a reaction mechanism is elucidated. In this system, the electrochemically formed N-H bond on the reduced bncn ligand serves as a proton relay in the H2 formation reaction through a cooperative interligand pathway involving one of the bridging DPhF ligands after a second reduction step, accessible at approximately -1.15 V vs Fc+/0. Since calculations show that hydrogen evolution takes place at the bridging ligands and does not involve the dirhodium core, it is predicted that more abundant metal centers can be incorporated into this ligand scaffold, leading to new candidates for electrocatalytic hydrogen reduction. As such, this work delineates a new design strategy to incorporate proton relays in molecular bimetallic hydrogen evolution electrocatalysts to achieve higher efficiency.

Hydrogen Evolution Electrocatalysis with a Molecular Cobalt Bis(alkylimidazole)methane Complex in DMF: a Critical Activity Analysis.

[Co(HBMIMPh2 )2 ](BF4 )2 (1) [HBMIMPh2 =bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methane] was investigated for its electrocatalytic hydrogen evolution performance in DMF using voltammetry and during controlled potential/current electrolysis (CPE/CCE) in a novel in-line product detection setup. Performances were benchmarked against three reported molecular cobalt hydrogen evolution reaction (HER) electrocatalysts, [Co(dmgBF2 )2 (solv)2 ] (2) (dmgBF2 =difluoroboryldimethylglyoximato), [Co(TPP)] (3) (TPP=5,10,15,20-tetraphenylporphyrinato), and [Co(bapbpy)Cl](Cl) (4) [bapbpy=6,6'-bis-(2-aminopyridyl)-2,2'-bipyridine], showing distinct performances differences with 1 being the runner up in H2 evolution during CPE and the best catalyst in terms of overpotential and Faradaic efficiency during CCE. After bulk electrolysis, for all of the complexes, a deposit on the glassy carbon electrode was observed, and post-electrolysis X-ray photoelectron spectroscopy (XPS) analysis of the deposit formed from 1 demonstrated only a minor cobalt contribution (0.23 %), mainly consisting of Co2+ . Rinse tests on the deposits derived from 1 and 2 showed that the initially observed distinct activity was (partly) preserved for the deposits. These observations indicate that the molecular design of the complexes dictates the features of the formed deposit and therewith the observed activity.

Pyridine vs. Imidazole Axial Ligation on Cobaloxime Grafted Graphene: Hydrogen Evolution Reaction Insights.

While cobaloximes have been protagonists in the molecular (photo)catalytic hydrogen evolution reaction field, researchers originally shed light on the catalytically active metallic center. However, the specific chemical environment of cobalt, including equatorial and axial ligation, has also a strong impact on the catalytic reaction. In this article, we aim to demonstrate how pyridine vs. imidazole axial ligation of a cobaloxime complex covalently grafted on graphene affects the hydrogen evolution reaction performance in realistic acidic conditions. While pyridine axial ligation mirrors a drastically superior electrocatalytic performance, imidazole exhibits a remarkable long-term stability.

Temperature programmed desorption comparison of lunar regolith to lunar regolith simulants LMS-1 and LHS-1

Water and molecular hydrogen evolution from Apollo sample 14163 and lunar regolith simulants LMS-1, a mare simulant, and LHS-1, a highlands simulant, were examined using Temperature Programmed Desorption (TPD) in ultra-high vacuum. LMS-1, LHS-1, and Apollo 14163 released water upon heating, whereas only the Apollo sample directly released measurable quantities of molecular hydrogen. The resulting H2O and H2 TPD curves were fit using a model which considers desorption at the vacuum grain interface, transport in the void space between grain-grain boundaries, molecule formation via recombination reactions and sub-surface diffusion. The model yielded a most probable H2O formation and desorption effective activation energy of ∼150 kJmol−1 for all samples. The probability distribution widths of the effective activation energies were ∼100–400, ∼100–350, and ∼100–300 kJmol−1 for LMS-1, LHS-1, and Apollo 14163, respectively. In addition to having the narrowest energy distribution width, the Apollo sample released the least amount to water (103 ppm) relative to LMS-1 (176 ppm) and LHS-1 (195 ppm). Since essentially no molecular hydrogen was observed from the simulants, the results indicate that LMS-1 and LHS-1 display water surface formation, binding, and transport interactions similar to actual regolith but not the desorption chemistry associated with the implanted hydrogen from the solar wind. Overall, these terrestrial surrogates are useful for understanding the surface and interface interactions of lunar regolith grains, which are largely dominated by the terminal hydroxyl sites under both solar wind bombardment and terrestrial preparation conditions.

Electrochemical interfaces on chalcogenides: Some structural perspectives and synergistic effects of single-surface active sites

The use of single-atom metals (SAM) as catalysts of energy conversion reactions is a recent topic, which has gained popularity in the last two decades. Transition metal dichalcogenides emerged as important electrocatalysts since it was discovered that their chalcogenide edge sites are active towards the electrocatalytic hydrogen evolution reaction (HER) and could also serve as supports for other metals within the same applications. Currently, several groups have reported a novel metal−chalcogenide arrangement, with the possibility of isolating metals at specific sites on chalcogenides to enhance their properties resulting in a synergistic effect in which both chalcogenide and single-atom metal features are exploited, either as promoters or active sites. Theoretical studies have been the basis of these reports. Engineering based on transition metal chalcogenides (TMC) and single-atom metal (SAM) is considered an effective strategy to improve the kinetics of molecular hydrogen evolution (HER). This strategy combines three effects, namely, EMSI, TMC and charge transfer • The combination of single metal atoms with 2D TMCs activates inert sites of the support for electrocatalytic reactions. • The transition from theoretical modeling of SAM materials to experimental studies is an imminent challenge. • The position of the SAM on the chalcogenide is crucial to understand the effects in electrocatalytic activity.

The co-evolution of molecular hydrogen and the grain size distribution in an isolated galaxy

ABSTRACT Understanding the evolution of dust and molecular hydrogen (H2) is a critical aspect of galaxy evolution, as they affect star formation and the spectral energy distribution of galaxies. We use the N-body/smoothed particle hydrodynamics code gadget4-osaka to compute the evolution of dust and H2 in a suite of numerical simulations of an isolated Milky Way-like galaxy. The evolution of the full grain size distribution (GSD) is solved by sampling the grain size on a logarithmically spaced grid with 30 bins. The evolution of a primordial chemistry network with 12 species is solved consistently with the hydrodynamic evolution of the system, including star formation, metal and energy ejections from stars into the interstellar medium through supernova feedback, and stellar winds. The formation model for H2 considers the GSD and photodissociation through the UV radiation of young stars. We identify the processes needed for producing a sizeable amount of H2, verify that the resulting star formation law in the later stages of galaxy evolution is consistent with observations of local spirals, and show that our model manages to produce a galactic molecular gas fraction in line with observations of Milky Way-like galaxies. We stress the importance of the co-evolution of the GSD and H2, as models assuming a fixed MRN shape for the GSD overestimate the production of H2 in regimes where the dust abundance is dominated by large grains and underestimate it in the regime where the dust is dominated by small grains, both of which are realized in simulations of dust evolution.

Metal-organic framework (MOF)-derived plate-shaped CoS1.097 nanoparticles for an improved hydrogen evolution reaction.

Metal-organic framework (MOF)-derived transition metal sulfides are viewed as reliable, cost-effective, and alternative hydrogen evolution reaction (HER)-efficient electrocatalysts. They have been used to replace platinum (and their alloys) for production of renewable energy carriers such as hydrogen. Progress towards development of non-precious transition-metal sulfides through different synthetic routes to obtain unique morphological nanostructures with enhanced HER activity is challenging. We introduced a transition-metal sulfide, cobalt sulfide (CoS1.097), derived from a cobalt MOF [Co-BPY-DDE] by following facile, one-step solvothermal sulfurization. By varying the sulfurization temperature (from 140 °C to 180 °C) during the solvothermal method, three cobalt-sulfide products were obtained: CoS1.097-140, CoS1.097-160, and CoS1.097-180, respectively. Temperature variation had a vital role in optimizing the HER activity of the electrocatalyst. Besides, notable plate-shaped cobalt sulfide nanoparticles (CoS1.097-160) required overpotential of 163 mV to deliver a current density of 10 mA cm-2 with a low Tafel slope of 53 mV dec-1, thereby demonstrating faster reaction kinetics during the evolution of molecular hydrogen. Furthermore, 25 h of long-term stability of the electrocatalyst reflected its practical applicability in acidic media. CoS1.097-160 had uniform plate-shaped morphology and large electrochemical active surface area, which contributed to enhanced electrochemical performance through water electrolysis.

Bioinspired Artificial Photosynthetic Systems.

Mimicking photosynthesis using artificial systems, as a means for solar energy conversion and green fuel generation, is one of the holy grails of modern science. This perspective presents recent advances towards developing artificial photosynthetic systems. In one approach, native photosystems are interfaced with electrodes to yield photobioelectrochemical cells that transform light energy into electrical power. This is exemplified by interfacing photosystem I (PSI) and photosystem II (PSII) as an electrically contacted assembly mimicking the native Z-scheme, and by the assembly of an electrically wired PSI/glucose oxidase biocatalytic conjugate on an electrode support. Illumination of the functionalized electrodes led to light-induced generation of electrical power, or to the generation of photocurrents using glucose as the fuel. The second approach introduces supramolecular photosensitizer nucleic acid/electron acceptor complexes as functional modules for effective photoinduced electron transfer stimulating the subsequent biocatalyzed generation of NADPH or the Pt-nanoparticle-catalyzed evolution of molecular hydrogen. Application of the DNA machineries for scaling-up the photosystems is demonstrated. A third approach presents the integration of artificial photosynthetic modules into dynamic nucleic acid networks undergoing reversible reconfiguration or dissipative transient operation in the presence of auxiliary triggers. Control over photoinduced electron transfer reactions and photosynthetic transformations by means of the dynamic networks is demonstrated.

Acceptorless dehydrogenation of 1-phenylethanol using Pd/TiO2 catalysts prepared by sol immobilisation

Reversible hydrogenation and acceptorless dehydrogenation of organic molecules would enable pre-existing technologies and infrastructure to be leveraged for the storage and transport of hydrogen. We report the synthesis of active Pd/TiO2 catalysts prepared by sol immobilisation for the acceptorless dehydrogenation of 1-phenylethanol to acetophenone with evolution of molecular hydrogen. The catalyst converts 1-phenylethanol with a TOF of 3600 mol1-PE h−1 molPd−1 while maintaining high selectivity (>95%) at over 80% conversion. This represents a 3.5 x increase in catalytic activity with comparable selectivity over commercially available 5% Pd/C catalysts.

Thermal evolution of water and hydrogen from Apollo lunar regolith grains

The evolution of water and molecular hydrogen from Apollo lunar sample 15221, a mature mare soil, was examined by temperature program desorption (TPD) experiments conducted under ultra-high vacuum conditions. Desorption at the grain/vacuum interface with re-adsorption as water transports though the void space of the grains and activated sub-surface diffusion were found to reproduce the experimental TPD signal. Signal from the grain/vacuum interface yielded the second order desorption activation energies and site probability distributions. Water from sample 15221 exhibited a broad distribution of activation energies peaking at 130 kJmol−1 extending up to 350 kJmol−1 at zero coverage limit with an onset of 110 kJmol−1 at full coverage. Our results suggest that water and hydrogen originating from lunar regolith contributes a minor amount to the observed mass in the LCROSS impact event. The abnormal amount of molecular hydrogen observed in the ejecta plume of the LCROSS impact may indicate that the radiolytic production of H2 from electron and galatic cosmic rays of physisorbed water is a contributor to the vast quantity of molecular hydrogen detected.

Photocatalytic Systems Based on Acridine Salts Derivatives

The possibility of 10-methyl-9-phenylacridine iodide usage as a photocatalyst in the process of molecular hydrogen evolution in various aprotic solvents has been shown. A possible process scheme has been suggested. The influence of the acid nature and its concentration on the efficiency of the photocatalytic process has been revealed. It has been shown that the highest amount of molecular hydrogen is formed in presence of acids with intermediate pK.

In situ constructing intramolecular ternary homojunction of carbon nitride for efficient photoinduced molecular oxygen activation and hydrogen evolution

Abstract Polymeric semiconductors potentially rich in Frenkel excitons are booming in photocatalysis, but most of them only show moderate performance in hydrogen evolution and molecular oxygen activation due to the sluggish electron−hole pair dissociation and charge transfer dynamics. Herein, a novel design of intramolecular g–C3N4–based ternary homojunction is established via one step KSCN/NH4Cl-assisted ionothermal route. The integrated theoretical calculations and experimental results disclose that such a unique ternary structure not only dramatically expedite intralayer exciton splitting and interface charge transfer but also greatly reduce backward charge recombination through the arrangement of HOMO/LUMO levels and consequent internal electric field. As a result, the optimal sample exhibits remarkable enhancements in oxygen (O2) reduction to hydroxyl radicals (•OH), and water splitting with H2 evolution rate (3806.5 μmol h−1 g−1) about 17.8 times higher than that of unmodified melon outperforming most functionalized g-C3N4 and heterojunctions reported thus far. Our study highlights the rational design of chemical structures to modulate the excitonic features and charge-transfer properties of polymeric semiconductors for efficient solar energy utilization.

Hierarchical Pd/MnO2 nanosheet array supported on Ni foam: An advanced electrode for electrocatalytic hydrodechlorination reaction

Electrocatalytic hydrodechlorination (EHDC) is deemed as one promising approach for efficient and safe detoxification of the trace halogenated organic pollutants in water. Here we prepared one advanced Pd/MnO2-Ni foam electrode via the construction of oxygen-deficient MnO2 nanosheet arrays on Ni foam skeleton, which then served as the support and electron donator to capture and reduce Pd precursor to nanoparticles. With the three-dimensional porous structure, hierarchical skeleton surfaces and improved Pd dispersion, the Pd/MnO2-Ni foam electrode delivered an unprecedented large mass activity (kobs) of 0.883 min−1 mmolPd−1 for EHDC of 2,4-dichlorophenol (2,4-DCP), in comparison to 0.081 min−1 mmolPd−1 of the Pd/Ni foam electrode and those reported in literatures. The Pd/MnO2-Ni foam electrode also displayed high durability during the repeated batch EHDC experiments without the efficiency decay and the leaching of Mn/Ni/Pd, unless some reduced sulfur compounds and nitrite were included. The mechanism study revealed the MnO2 in electrode served as a mediator to transfer H* from Pd to 2,4-DCP, which extended the reactive area beyond Pd and hindered the molecular hydrogen evolution, leading to the enhanced reactions between H* and 2,4-DCP. The Pd/MnO2-Ni foam electrode was also tested in a continuous-flow EHDC system, and displayed the potential and retention time-dependent performances.

Simulations of the star-forming molecular gas in an interacting M51-like galaxy

ABSTRACT We present here the first of a series of papers aimed at better understanding the evolution and properties of giant molecular clouds (GMCs) in a galactic context. We perform high-resolution, three-dimensional arepo simulations of an interacting galaxy inspired by the well-observed M51 galaxy. Our fiducial simulations include a non-equilibrium, time-dependent, chemical network that follows the evolution of atomic and molecular hydrogen as well as carbon and oxygen self-consistently. Our calculations also treat gas self-gravity and subsequent star formation (described by sink particles), and coupled supernova feedback. In the densest parts of the simulated interstellar medium (ISM), we reach sub-parsec resolution, granting us the ability to resolve individual GMCs and their formation and destruction self-consistently throughout the galaxy. In this initial work, we focus on the general properties of the ISM with a particular focus on the cold star-forming gas. We discuss the role of the interaction with the companion galaxy in generating cold molecular gas and controlling stellar birth. We find that while the interaction drives large-scale gas flows and induces spiral arms in the galaxy, it is of secondary importance in determining gas fractions in the different ISM phases and the overall star formation rate. The behaviour of the gas on small GMC scales instead is mostly controlled by the self-regulating property of the ISM driven by coupled feedback.

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation.

The one-step seed-mediated synthesis is widely used for the preparation of ferromagnetic metal nanoparticles (NPs) since it offers a good control of particle morphology. Nevertheless, this approach suffers from a lack of mechanistic studies because of the difficulties of following in real time the heterogeneous nucleation and predicting structure effects with seeds that are generated in situ. Here, we propose a complete scheme of the heteronucleation process involved in one-pot seed-mediated syntheses of cobalt nanoparticles in liquid polyols, relying on geometrical phase analysis (GPA) of high-resolution high-angle annular dark field (HAADF)-STEM images and in situ measurements of the molecular hydrogen evolution. Cobalt particles of different shapes (rods, platelets, or hourglass-like particles) were grown by reducing cobalt carboxylate in liquid polyols in the presence of iridium or ruthenium chloride as the nucleating agent. A reaction scheme was established by monitoring the H2 evolution resulting from the decomposition of metal hydrides, formed in situ by β-elimination of metal alkoxides, and from the polyol dehydrogenation, catalytically activated by the metal particles. This is a very good probe for both the noble metal nucleation and the heterogeneous nucleation of cobalt, showing a good separation of these two steps. Ir and Ru seeds with a size in the range 1-2 nm were found exactly in the center of the cobalt particles, whatever the cobalt particle shape, and high-resolution images revealed an epitaxial growth of the hcp Co on fcc Ir or hcp Ru seeds. The microstructure analysis around the seeds made evident two different ways of relaxing the lattice mismatch between the seeds and the cobalt, with the presence of dislocations around the Ir seeds and compression zones of the cobalt lattice near the Ru seeds. The relationship between the nature of the nucleating agent, the reaction steps, and the microstructure is discussed.