Hydrogen Evolution In Aqueous Solution Research Articles

The Influence of Reaction Conditions on the Rate of Hydrogen Evolution in Aqueous Solutions of Glycerol over Pt/TiO2 Photocatalysts

The Influence of Reaction Conditions on the Rate of Hydrogen Evolution in Aqueous Solutions of Glycerol over Pt/TiO2 Photocatalysts

Ti-based MOFs: New insights on the impact of ligand composition and hole scavengers on stability, charge separation and photocatalytic hydrogen evolution

We synthesized a series of isostructural MIL-125-Ti with various terephthalic: amino-terephthalic acid ratios and evaluated their photostability and performance for photocatalytic hydrogen evolution in aqueous solutions with different hole scavengers. We demonstrate that the commonly used triethanolamine corroded the MOFs under photocatalytic conditions, in contrast to methanol. The combination of MIL-125-Ti and methanol outperformed the photocatalytic activity of NH2-MIL-125-Ti by 45 times under broad UV irradiation. Photoluminescence spectroscopy under simulated photocatalytic conditions and in solid-state as well as time-resolved emission studies reveal that the holes residing in the metal-oxo cluster can be effectively scavenged by methanol, leading to high HER activity, while the holes residing in the NH2-groups in the ligand favour charge recombination and are thus detrimental to the photocatalytic performance. The mixed-ligand MOFs follow a dual-excitation pathway, introducing new states and transitions. These results are expected to advance our understanding of MOF-based photocatalysis.

Self-Assembly of Albumin and [FeFe]-Hydrogenase Mimics for Photocatalytic Hydrogen Evolution.

To imitate the protein microenvironment surrounding the Fe2S2 active site, we established a convenient method to prepare self-assembled ovalbumin (OVA) nanogels that noncovalently incorporated hydrophobic FeFe-H2ase mimic (FeFe-1) in situ with an excellent loading efficiency up to 53.9%. The OVA@FeFe-1 composite nanogels (CG) were applied to photocatalytic hydrogen evolution in aqueous solution and exhibited improved catalytic efficiency with a turnover number amplified over 15 times relative to free FeFe-1 catalysts. The enhanced activity was attributed to the enrichment of the FeFe-1 catalyst and changes of protein secondary structures and microenvironment.

Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts

Photocatalytic decomposition of organic materials-contained aqueous solution is assessed using a plasma discharged into the liquid directly. The correlation of H2 generation and optical emission spectroscopy is discussed in terms of photocatalytic H2 production using plasma and photocatalysts. Variations of the active species are evaluated according to the conditions of the plasma in the liquid phase. The optical emission spectra vary according to the plasma discharging conditions in the liquid phase. The intensities of the OH· peaks at 309 nm increase with the addition of ethanol or acetaldehyde in water. The highest intensities and rate of H2 evolution are observed at a 10% acetaldehyde concentration in the aqueous solution. The rates of H2 evolution in the ethanol or acetaldehyde solution correspond to the concentration of OH· in the solution. The photocatalytic reaction using liquid plasma generates hydrogen at the same time as the decomposition of the organic chemicals. The rate of hydrogen evolution in aqueous solutions containing the organic chemicals is higher than that in pure water. This is because hydrogen is further generated due to hydrogen generation by photolysis of the organic chemicals. CaTiO3 perovskite photocatalyst shows better photocatalytic activity than TiO2. Ni loading on the photocatalyst lead to an increase in H2 production.

Diiron Dithiolate Complex Induced Helical Structure of Histone and Application in Photochemical Hydrogen Generation.

Very-lysine-rich calf thymus histone proteins form disordered structure and hydrophobic interaction-driven aggregates in weakly acidic solution. We reported that the conjugation of diiron dithiolate complex to the lysine residues induced formation of helical conformation and condensed nanoassemblies with a high loading capacity up to 18.7 wt %. The incorporated diiron dithiolate complex showed photocatalytic activity for hydrogen evolution in aqueous solutions, with a turnover number (based on [FeFe] catalyst moiety) up to 359 that was more than 6 times that of the free catalyst. The increase of helical conformation in proteins was well correlated to the increasing enhancement of photocatalytic activity. We demonstrated that the [FeFe]-hydrogenase-mimic biohybrid system based on the photocatalyst-induced protein conformational conversion and reassembly is efficient for hydrogen generation regardless of the relatively large size.

Template-based electrodeposition and characterization of niobium nanowires

Template-based electrodeposition is a versatile technique for the fabrication of freestanding nanowires. However, nanowires of refractory metals, like niobium, have remained inaccessible via this technique due to preferential hydrogen evolution in aqueous solutions. In this work, we have developed a synthetic strategy for template-based electrodeposition of niobium nanowires using an ionic liquid. Optimal growth conditions have been determined using cyclic voltammetry. Structural characterization using transmission electron microscopy, electron diffraction, and energy dispersive spectroscopy showed that the niobium nanowires were polycrystalline with an average grain size of 14 nm. The use of ionic liquid-based electrolytes for nanowire deposition could be expanded to other metals improving material quality due to their high stability, good conductivity, and wide electrochemical window.

Photofunctions of iridium(iii) complexes in vesicles: long-lived excited states and visible-light sensitization for hydrogen evolution in aqueous solution.

Recently, the photochemistry of cationic bis-cyclometalated Ir(iii) complexes has gained increasing attention in interdisciplinary studies owing to the characteristic photofunctions of the complexes, including long-lived and strong phosphorescence, visible-light absorption, and photosensitizing properties. Herein, two Ir(iii) complexes, Ir2 and Ir3, composed of coumarin 6 and different ancillary ligands (i.e., 4,4'-dimethyl-2,2'-bipyridine and 2-(2-pyrazolyl)pyridine) were prepared and characterized to investigate their photophysical properties in several solvents. These compounds were incorporated into DPPC vesicles to investigate their photosensitizing properties in an aqueous solution. Ir2 and Ir3 absorbed light strongly in the visible region and exhibited similar phosphorescence spectral profiles dominated by the coumarin ligands in all tested environments. In contrast, the ancillary ligands and the surrounding environment influenced the excited-state deactivation pathways. For instance, the phosphorescence of Ir2 was weak in highly polar solvents and the vesicle membrane, and in contrast, Ir3 exhibited intense and surprisingly long-lived phosphorescence in these media (Φ = 25-38% and τ = 32-41 μs). Visible-light illumination of Ir2 and Ir3 bound to the membrane surface resulted in H2 evolution when an electron donor and a water-soluble Ni(ii) catalyst were present in the outer aqueous phase. Under these conditions, the reaction system incorporating Ir2 performed much better than the Ir3 system in terms of photosensitizer stability and turnover frequency.

Efficient photocatalytic hydrogen evolution with ligand engineered all-inorganic InP and InP/ZnS colloidal quantum dots

Photocatalytic hydrogen evolution is a promising technique for the direct conversion of solar energy into chemical fuels. Colloidal quantum dots with tunable band gap and versatile surface properties remain among the most prominent targets in photocatalysis despite their frequent toxicity, which is detrimental for environmentally friendly technological implementations. In the present work, all-inorganic sulfide-capped InP and InP/ZnS quantum dots are introduced as competitive and far less toxic alternatives for photocatalytic hydrogen evolution in aqueous solution, reaching turnover numbers up to 128,000 based on quantum dots with a maximum internal quantum yield of 31%. In addition to the favorable band gap of InP quantum dots, in-depth studies show that the high efficiency also arises from successful ligand engineering with sulfide ions. Due to their small size and outstanding hole capture properties, sulfide ions effectively extract holes from quantum dots for exciton separation and decrease the physical and electrical barriers for charge transfer.

Photocatalytic Hydrogen Production with Conjugated Polymers as Photosensitizers.

Artificial photosynthesis is a chemical process that aims to capture energy from sunlight to produce solar fuels. Light absorption by a robust and efficient photosensitizer is one of the key steps in solar energy conversion. However, common photosensitizers, including [Ru(bpy)3]2+ (RuP), remain far from the ideal. In this work, we exploited the performance of conjugated polymers (CPs) as photosensitizers in photodriven hydrogen evolution in aqueous solution (pH 6). Interestingly, CPs, such as poly(fluorene- co-phenylene) derivative (429 mmolH2·gCP-1·h-1), exhibit steady and high reactivity toward hydrogen evolution; this performance can rival that of a phosphonated RuP under the same conditions, indicating that CPs are promising metal-free photosensitizers for future applications in photocatalysis.

Influence of graphitic materials microstructure in the hydrogen evolution in aqueous solution of tetra-alkylammonium-sulfonic acid ionic liquid

Vitreous carbon (VC), pyrolytic carbon (PC) and moulded graphite (MG) were tested as cathodic materials in hydrogen production by water electrolysis in the presence of the ionic liquid tetrafluoroborate of 3-triethylammonium-propane sulfonic acid (TEA-PS.BF4). The physical characterization of the carbon materials indicated large differences in the microstructure of VC, PC, and MG and this significantly affected their electrochemical response. The mechanism presented for all the materials studied in the hydrogen evolution reaction (HER) was Volmer–Heyrovsky, where the H2 desorption at the catalytic surface is the determining step. The MG electrode presented an unfavourable performance due to the formation of nanobubbles that coalesced without H2 desorption and led to the deactivation of the catalytic sites. This behaviour is attributed to the presence of large and ordered crystallites at the material surface, with a greater number of hydrophobic domains in comparison with VC and PC material surfaces. The VC and PC electrodes presented a higher performance compared to the Pt cathode, showing lower activation energy, higher cathodic exchange current and lower charge transfer resistance. The set of results indicates VC and PC as promising alternative materials to constitute cathodes for the electrolysis of water using TEA-PS.BF4 aqueous solution as electrolyte.

Photocatalysts fabricated by depositing plasmonic Ag nanoparticles on carbon quantum dots/graphitic carbon nitride for broad spectrum photocatalytic hydrogen generation

To make the best and highest use of broad spectrum solar energy remains a tremendous challenge and the main target in the photocatalytic area. A novel promising photocatalyst supported on surface plasmon resonance of Ag nanoparticles (NPs) and upconversion photoluminescence property from carbon quantum dots (CQDs) is reported to improve broad spectrum absorption and photoinduced charge transfer of graphitic carbon nitride (g-C3N4) in the photo-driven H2 production for the first time. Here the new-styled nanocomposites not only have more prominent UV–vis photocatalytic ability, also can harness near-infrared light to trigger hydrogen evolution in aqueous solution. Meanwhile, Ag NPs and CQDs serve as electron-reservoirs, which stimulate the separation of photo-generated electron-holes, enhancing quantum efficiency of g-C3N4. Remarkably, the most notable photocatalytic hydrogen generation as high as 626.93μmolg−1h−1 under visible light, which is about 6.7 and 2.8 times higher than pure g-C3N4 and the best CQDs/g-C3N4 composite respectively, was achieved upon 6mL CQDs/g-C3N4 (6CCN) loaded with 3wt% Ag. Moreover, a facile method is designed to prepare Ag/CQDs/g-C3N4 photocatalysts, and their chemical composition, morphologies, optical properties and stability were characterized methodically.

Mixed Valence Tin Oxide Sn3O4: A Visible-Light Driven Semiconductor for Photocatalytic Water Splitting Under Visible Light Irradiation

Water splitting using a particle photocatalyst with visible light has attracted interest, because this will realize efficient conversion of the solar energy to hydrogen fuels for sustainable energy managements. Many water-splitting photocatalysts, such as metal-doped oxides, metal-(oxy)nitrides, and metal-oxysulfides, exhibit water reduction and oxidation ability when absorbing visible light. However, in such water-splitting photocatalysts, solely doping and/or replacement of foreign element at host material has been attempted to prepare visible-light-driven photocatalysts. The doping and/or replacement induces defect sites and charge imbalance into photocatalyst, resulting in the formation of recombination centers between excited electron and holes, in return to create new valence state. Therefore, the development of new native visible-light-sensitive photocatalyst for water splitting without doping of foreign elements is a major challenge for photocatalyst research. Herein, we report that an undoped metal oxide, Sn3O4, can efficiently catalyze Hydrogen (H2)-evolution in the presence of a sacrifice agent and, furthermore, can split water to H2 and O2 under irradiation of visible light by two-step photoexcitation (Z-scheme) systems using Sn3O4 as H2-evolving photocatalyst, combined with an appropriate O2-evolving photocatalyst in the presence of iodate/iodide (IO3 −/I−) as an electron mediator. We prepared nanocrystals of Sn3O4 by hydrothermal synthesis using sodium citrate as a ligand [1]. The Sn3O4 significantly catalyzed hydrogen evolution in aqueous solution in the presence of sacrifice agent under irradiation of visible light (λ>420nm), even without Pt co-catalyst. Pt co-catalyst was critical for Sn3O4 material to exhibit higher photocatalytic activity, so far in this study. The apparent quantum yield was 0.2% for high purity Sn3O4 and 1.5% for Pt loaded-Sn3O4, competitive to that of visible-light-sensitive doped metal oxide and (oxy)nitrides photocatalyst. Simultaneous evolution of H2 and O2 was attempted by using Sn3O4 as H2-evolving photocatalyst, combined with an appropriate O2-evolving photocatalyst (BiVO4, WO3 etc.,). By employing the appropriate combination of Sn3O4 and the redox couple (IO3 −/I−), simultaneous evolution of H2 and O2 stably proceeded with higher rates. [1] M. Maidhily, T. Tanabe, et al., ACS Appl. Mater. Interfaces 2014, 6, 3790.

The Influence of Homogeneous Reactions on the Impedance Response of a Rotating Disk Electrode

For a number of systems, the impedance response at an electrode surface may be influenced by homogeneous reactions in the electrolyte. Hauser and Newman have discussed the influence of homogenous consumption of cuprous ion on the impedance response associated with dissolution of a copper rotating disk electrode [1]. Remita et al. have shown that, for a deaerated aqueous electrolyte containing dissolved carbon dioxide, the hydrogen evolution is enhanced by the homogeneous dissociation of CO2 [2]. Similarly, homogeneous dissociation of acetic acid enhances cathodic reduction of hydronium ions [3]. Coupled electrochemical and homogeneous reactions are also involved in sensors used to monitor glucose concentrations for management of diabetes [4].A mathematical model has been developed for the impedance response associated with the coupling of homogeneous and heterogeneous electrochemical reactions. The model accounts for a homogeneous reaction in the electrolyte where species AB reacts reversibly to form A- and B+. The B+ species then reacts electrochemically on a rotating disk electrode to produce B. An analytic expression for velocity was employed that combined a three-term velocity expansion near the electrode surface to a three-term expansion that applied far from the electrode. The nonlinear expression for the homogeneous reaction was employed in which the concentrations of both A- and B+ were assumed to be dependent on position.The model development required two steps. The nonlinear coupled differential equations governing this system were solved under the assumption of a steady state. The concentrations resulting from the steady-state simulation were used in the solution of the linearized set of differential equations describing the sinusoidal steady state. The diffusion impedance showed two capacitive loops. The low-frequency loop has a characteristic dimensionless frequency K=2.5, which is in agreement with the characteristic frequency associated with diffusion in the absence of homogeneous reactions. The characteristic frequency of the high-frequency loop could be expressed in terms of a reaction layer thickness that was a function of the rate of the homogeneous reaction [5]. The diffusion impedance accounting for a homogeneous reaction was smaller in magnitude than the diffusion impedance in the absence of a homogeneous reaction.Reference K. Hauser and J. Newman, “Singular Perturbation Analysis of the Faradaic Impedance of Copper Dissolution Accounting for the Effects of Finite Rates of a Homogeneous Reaction," Journal of The Electrochemical Society, 136 (1989) 2820-2831.E. Remita, B. Tribollet, E. Sutter, V. Vivier, F. Ropital, and J. Kittel, “Hydrogen Evolution in Aqueous Solutions Containing Dissolved CO2: Quantitative Contribution of the Buffering Effect," Corros. Sci., 50 (2008) 1433-1440.T. Thu, B. Brown, S. Nešić, and B. Tribollet, “Investigation of the Electrochemical Mechanisms for Acetic Acid Corrosion of Mild Steel,” Corrosion, 70 (2014) 223–229.A. Heller and B. Feldman, “Electrochemical Glucose Sensors and Their Applications in Diabetes Management,” Chemical Reviews, 108 (2008) 2482–2505.V. G. Levich, Physicochemical Hydrodynamics, Prentice Hall, Englewood Cliffs, NJ, 1962.T. von Kármán, “Über Laminaire und Turbulente Reibung,” Zeitschrift für angewandte Mathematik und Mechanik, 1 (1921) 233–252.

Mechanistic studies of hydrogen evolution in aqueous solution catalyzed by a tertpyridine-amine cobalt complex.

The ability of cobalt-based transition metal complexes to catalyze electrochemical proton reduction to produce molecular hydrogen has resulted in a large number of mechanistic studies involving various cobalt complexes. While the basic mechanism of proton reduction promoted by cobalt species is well-understood, the reactivity of certain reaction intermediates, such as Co(I) and Co(III)-H, is still relatively unknown owing to their transient nature, especially in aqueous media. In this work we investigate the properties of intermediates produced during catalytic proton reduction in aqueous solutions promoted by the [(DPA-Bpy)Co(OH2)](n+) (DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,20-bipyridine-6-methanamine) complex ([Co(L)(OH2)](n+) where L is the pentadentate DPA-Bpy ligand or [Co(OH2)](n+) as a shorthand). Experimental results based on transient pulse radiolysis and laser flash photolysis methods, together with electrochemical studies and supported by density functional theory (DFT) calculations indicate that, while the water ligand is strongly coordinated to the metal center in the oxidation state 3+, one-electron reduction of the complex to form a Co(II) species results in weakening the Co-O bond. The further reduction to a Co(I) species leads to the loss of the aqua ligand and the formation of [Co(I)-VS)](+) (VS = vacant site). Interestingly, DFT calculations also predict the existence of a [Co(I)(κ(4)-L)(OH2)](+) species at least transiently, and its formation is consistent with the experimental Pourbaix diagram. Both electrochemical and kinetics results indicate that the Co(I) species must undergo some structural change prior to accepting the proton, and this transformation represents the rate-determining step (RDS) in the overall formation of [Co(III)-H](2+). We propose that this RDS may originate from the slow removal of a solvent ligand in the intermediate [Co(I)(κ(4)-L)(OH2)](+) in addition to the significant structural reorganization of the metal complex and surrounding solvent resulting in a high free energy of activation.

Hydrogen production on a hybrid photocatalytic system composed of ultrathin CdS nanosheets and a molecular nickel complex.

The production of clean and renewable hydrogen through water splitting by using solar energy has received much attention due to the increasing global energy demand. We report an economic and artificial photosynthetic system free of noble metals, consisting of ultrathin CdS nanosheets as a photosensitizer and nickel-based complex as a molecular catalyst. Emission quenching and flash photolysis studies reveal that this hybrid system allows for effective electron transfer from the excited CdS nanosheets to the nickel-based complex to generate reduced intermediate species for efficient hydrogen evolution. Notably, the unique morphological and structural features of the ultrathin CdS nanosheets contribute to the highly efficient photocatalytic performance. As a consequence, the resulting system shows exceptional activity and stability for photocatalytic hydrogen evolution in aqueous solution with a turnover number (TON) of about 28,000 versus catalyst and a lifetime of over 90 h under visible light irradiation.

Photoreduction of iron(III) to iron(0) nanoparticles for simultaneous hydrogen evolution in aqueous solution.

Crystalline Fe nanoparticles were obtained with fluorescein (Fl) as the photosensitizer in triethylamine (TEA) or triethanolamine (TEOA) aqueous solution with FeCl3 as the Fe precursor under bright visible-light light-emitting diode (LED) irradiation. Photoinduced electron transfer from excited state Fl* and Fl(-) to Fe(3+) produced the Fe nanoparticles, which served as the active catalyst for in situ photocatalytic hydrogen production with Fl and TEA or TEOA as the photosensitizer and electron donors, respectively, in the same system. Robust hydrogen production activities were observed under the Fe nanoparticle photoreduction conditions in basic solution, and tens of milliliters of hydrogen were obtained over prolonged LED irradiation. If inorganic support materials such as NH2 -MCM-41 or reduced graphene oxide were introduced, dispersed nanoparticles with different sizes and shapes were deposited on the supports, which led to variously enhanced hydrogen production activities. The relationships between the morphologies of the Fe/H2 N-MCM-41 or Fe/graphene composites generated in situ and the hydrogen production activities were investigated systematically.

Photo-Assisted Hydrogen Evolution in Aqueous Solution of Formic Acid with Silicon which is Supported with Noble Metals

Silicon is one of the most versatile semiconductors and also displays photocatalytic activity under visible light irradiation. We show that hydrogen evolution from 10% formic acid solution with silicon particles is enhanced by the support of noble metals on the surface of silicon. The most suitable loading metals and corresponding reagents, such as metal salts or metal nanoparticles, are examined for hydrogen evolution. Because carbon dioxide evolution is observed as a by-product and the amount of carbon dioxide is almost the same as evolved hydrogen through all reactions in the presence and absence of irradiation by visible light, we suggest that all evolved hydrogen is produced by the decomposition of formic acid. Furthermore, both the amounts of evolved H 2 and CO 2 in the presence of visible light irradiation are larger than those without irradiation. Semiconductors absorb supra band gap photons to generate photoexcited electron (e) and hole (h+) pairs in the conduction band (CB) and valence band (VB), respectively. Thus, the generated holes are consumed effectively in the oxidation of formic acid. The origin of evolved H 2 is also discussed from the consideration of SEM and XPS measurements of the catalysts through the photoreactions.

Fabrication of cobalt porphyrin. Electrochemically reduced graphene oxide hybrid films for electrocatalytic hydrogen evolution in aqueous solution.

Here we report on an experimental study of an electrocatalyst for the hydrogen evolution reaction (HER) based on cobalt porphyrin and electrochemically reduced graphene oxide (ERGO) functional multilayer films, which are prepared by the alternating layer-by-layer (LBL) assembly of negatively charged graphene oxide (GO) and positively charged [tetrakis (N-methylpyridyl) porphyrinato] cobalt (CoTMPyP) in combination with an electrochemical reduction procedure. The resulting [ERGO@CoTMPyP]n multilayer films display relatively high electrocatalytic activity and superior stability toward HER in alkaline media. Electrochemical studies indicate that CoTMPyP in the multilayer films is the active catalyst for the reduction of protons to dihydrogen.

Recent developments of molybdenum and tungsten sulfides as hydrogen evolution catalysts

Recent work shows that nanoparticulate and amorphous molybdenum and tungsten sulfide materials are active catalysts for hydrogen evolution in aqueous solution. These materials hold promise for applications in clean hydrogen production technologies. In this perspective, the syntheses, structures and catalytic activities of nanoparticulate MoS2 and WS2, incomplete cubane-type [Mo3S4]4+ and amorphous MoSx films are summarized, compared, and discussed.

Hydrogen evolution by cobalt tetraiminecatalysts adsorbed on electrode surfaces

Aryl-substituted tetraimine complexes related to Co(dmgBF(2))(2)(MeCN)(2) (dmg = dimethylglyoxime) were synthesized and are active for hydrogen evolution. Co(dmgBF(2))(2)(MeCN)(2) can be adsorbed to a glassy carbon electrode. The chemically modified electrode is active for hydrogen evolution in aqueous solution at pH < 4.5, with an overpotential of only 100 mV.