H2-evolving Activity Research Articles

Controllable construction of direct Z-scheme Mo2C-CdxZn1−xIn2S4 heterojunction via defect-mediated modulation for enhanced visible-light photocatalysis

Direct Z-scheme semiconductor heterostructures possess fascinating merits for solar photocatalysis, including increased light harvesting, spatially isolated photogenerated charge carriers, and well-preserved strong redox capability. However, steering Z-scheme charge transfer of semiconductor heterojunction in a controllable manner is still a challenging task. In this work, unique direct Z-scheme Mo2C-CdxZn1−xIn2S4 heterojunction is constructed via a defect-mediated modulating strategy, where Cd-doping switches the charge transfer mode of Mo2C-CdxZn1−xIn2S4 heterojunction from type-I to Z-scheme through increasing S vacancies, up-shifting conduction band level, and narrowing bandgap of CdxZn1−xIn2S4 component. Moreover, the defect-induced Mo-S bond affords fast channels for built-in electric field-induced Z-scheme photo-electrons transporting from Mo2C conduction band to CdxZn1−xIn2S4 valence band, supporting by X-ray photoelectron spectroscopy, surface photovoltage spectroscopy, and radical testing results. Noticeably, under irradiation of visible light (λ > 400 nm), Mo2C-CdxZn1−xIn2S4 Z-scheme heterojunction displays an excellent and stable photocatalytic H2-evolving activity, reaching an outstanding H2 generation rate of 28.12 mmol·g−1·h−1 with the corresponding high apparent quantum efficiency of 21.1% at 400 nm, far surpassing that of Pt-loaded C0.25ZIS and most documented ZnIn2S4-containing photocatalysts. The present work could inspire the crafty design of efficient Z-scheme photocatalysts through defect-mediated heterostructure construction and energy band engineering.

Unique porous ZnS-CdS-CoSx Reuleaux triangle nanosheets: Highly promoted visible-light photocatalytic H2 evolution via synergistic effect of Z-scheme heterojunction and vacancy defects

Photocatalytic water-splitting employing Z-scheme semiconductor heterojunction is a promising avenue to realize the efficient conversion and storage of renewable solar energy because of its space-separated reduction and oxidation sites, effective charge separation and transportation, as well as the stronger redox abilities of photogenerated carriers. Nevertheless, the efficiency of Z-scheme photocatalysts is often weakened by the insufficient coupling of components due to the stepwise hybridization processes. In this work, unique ZnS-CdS-CoSx porous Reuleaux triangle nanosheets with intimate Z-scheme hetero-interface and S, Zn vacancies were produced by using Zn2Co3(OH)10·2H2O as the bimetallic precursor for subsequent sulfurization and Cd2+-exchange reaction. Noticeably, such a novel structure shows desirable features which suggest a promising photocatalyst for visible-light photocatalytic H2 evolution, including superior charge-separating capability enabled by the Z-scheme charge transfer and synergetic charge-trapping effect of S, Zn vacancies, excellent light-harvesting capacity, abundant S22− species as H2-evolving active sites, a large surface area, and good stability without obvious decline in activity after multiple cycling and long-term tests. The ZnS-CdS-CoSx heterojunction exhibits an outstanding visible-light-driven H2-evolving activity as high as 53.43 mmol·g−1·h−1, corresponding to a high apparent quantum efficiency of 30.8 % at 420 nm, far better than that of Pt-loaded CdS and a great deal of CdS-based photocatalysts reported in the literatures. The present work may shed new light on the designed synthesis of high-performance semiconductor heterojunction for sustainable solar utilization and environmental remediation.

An Ancient Respiratory System in the Widespread Sedimentary Archaea Thermoprofundales.

Thermoprofundales, formerly Marine Benthic Group D (MBG-D), is a ubiquitous archaeal lineage found in sedimentary environments worldwide. However, its taxonomic classification, metabolic pathways, and evolutionary history are largely unexplored because of its uncultivability and limited number of sequenced genomes. In this study, phylogenomic analysis and average amino acid identity values of a collection of 146 Thermoprofundales genomes revealed five Thermoprofundales subgroups (A–E) with distinct habitat preferences. Most of the microorganisms from Subgroups B and D were thermophiles inhabiting hydrothermal vents and hot spring sediments, whereas those from Subgroup E were adapted to surface environments where sunlight is available. H2 production may be featured in Thermoprofundales as evidenced by a gene cluster encoding the ancient membrane-bound hydrogenase (MBH) complex. Interestingly, a unique structure separating the MBH gene cluster into two modular units was observed exclusively in the genomes of Subgroup E, which included a peripheral arm encoding the [NiFe] hydrogenase domain and a membrane arm encoding the Na+/H+ antiporter domain. These two modular structures were confirmed to function independently by detecting the H2-evolving activity in vitro and salt tolerance to 0.2 M NaCl in vivo, respectively. The peripheral arm of Subgroup E resembles the proposed common ancestral respiratory complex of modern respiratory systems, which plays a key role in the early evolution of life. In addition, molecular dating analysis revealed that Thermoprofundales is an early emerging archaeal lineage among the extant MBH-containing microorganisms, indicating new insights into the evolution of this ubiquitous archaea lineage.

Facile preparation and photocatalytic hydrogen production of WS2 and its composites

Two-dimensional transition metal dichalcogenides, MoS2 and WS2 have been widely considered as promising materials for photocatalytic hydrogen production. However, compared with the widely investigated MoS2, researches on WS2 are much less. Besides, for the synthesis of WS2, methods suitable for large-scale preparation are still rare. Then in this paper, a facile method for the preparation of WS2 was developed based on the liquid-phase precipitation-calcination method reported previously. In specific, thiourea was introduced in the calcination process to realize the in-situ conversion of impurity WO3 to WS2. As a result, WS2 was successfully prepared. More interestingly, a series of photocatalysts with different compositions and performances were easily obtained only through changing the thiourea amount. When no thiourea was used, a WS2/WO3 heterostructure was constructed, while when excessive thiourea was introduced, an efficient WS2/g-C3N4 heterostructure (g-C3N4 = graphitic carbon nitride) was fabricated. Moreover, their hydrogen production performances were investigated with Erythrosine B (ErB) and triethanolamine (TEOA) as photosensitizer and sacrificial agent, respectively. The results showed that the as-obtained WS2 has a comparable H2-evolving activity (1686.3 μmol h−1 g−1) to the WS2/WO3 (1637.8 μmol h−1 g−1), and the WS2/g-C3N4 owns the highest performance (2428.7 μmol h−1 g−1). This work provides a facile and feasible route for the preparation of efficient WS2-based photocatalysts.

Cobalt(II) Pentaaza-Macrocyclic Schiff Base Complex as Catalyst for Light-Driven Hydrogen Evolution in Water: Electrochemical Generation and Theoretical Investigation of the One-Electron Reduced Species.

We previously reported that the tetraazamacrocyclic Schiff base complex [CoIII(CR14)(X)2]n+ (CR14 = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene, X = Cl (n = 1) (1-Cl2) or H2O (n = 3) (1-(H2O)2)) is a very efficient H2-evolving catalyst (HEC) in fully aqueous solutions at pH 4.0-4.5 when used in a photocatalytic system including a photosensitizer and ascorbate as sacrificial electron donor. The excellent H2-evolving activity of this complex, compared to other cobalt and rhodium catalysts studied in the same photocatalytic conditions, can be related to the high stability of its two-electron reduced form, the putative "Co(I)" state. These very interesting results led us to investigate the H2-evolving performances of a series of compounds from a close-related family, the pentaaza-macrocyclic cobalt [CoII(CR15)(H2O)2]Cl2 complex (2, CR15 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), which comprises a larger macrocycle with five nitrogen atoms instead of four. Electrochemical as well as spectroscopic investigations in CH3CN coupled to density functional theory (DFT) calculations point to decoordination of one of the amine upon reduction of Co(II) to the low-valent "Co(I)" form. The resulting unchelated amine could potentially act as a proton relay promoting the H2 formation via proton-coupled-electron transfer (PCET) reactions. Besides, the iron, manganese, and zinc analogues, [FeII(CR15)(X)2]n+ (X = Cl (n = 0) or H2O (n = 2)) (3), [MnII(CR15)(CH3CN)2](PF6)2 (4), and {[ZnII(CR15)Cl](PF6)}n (5) were also synthesized and investigated. The photocatalytic activity of 2-5 toward proton reduction was then evaluated in a tricomponent system containing the [RuII(bpy)3]Cl2 photosensitizer and ascorbate, in fully aqueous solution. The photocatalytic activity of 2 was also compared with that of 1 in the same experimental conditions. It was found that the number of catalytic cycles versus catalyst for 2 are slightly lower than that for 1, suggesting that if the amine released upon reduction of 2 plays a role in promoting the H2-evolving catalytic activity, other factors balance this effect. Finally, photophysical and nanosecond transient absorption spectroscopies were used to investigate the photocatalytic system.

Hydrothermal synthesis of MoS2-NiS/CdS with enhanced photocatalytic hydrogen production activity and stability

CdS is one of the most well-known photocatalysts for hydrogen production, while proper cocatalyst is indispensable to realize high hydrogen production performance. In this work, a three-component composite MoS2-NiS/CdS with dual cocatalysts was synthesized by a relatively facile and environmentally friendly three-step hydrothermal method and characterized by series of techniques. The as-prepared MoS2-NiS/CdS possesses good photostability and excellent photocatalytic hydrogen production performance. Under visible light irradiation of a 300 W Xe lamp (λ ≥ 420 nm), the H2 production rate reached a high value of 2525.2 μmol h−1 and the total amount of H2 evolved in 22 h achieved 40.5 mmol, further confirming that the photocatalysts with dual reduction cocatalysts possess higher photocatalytic performance than those with single cocatalyst. Especially, first study about the effect of pH of lactic acid aqueous solution on hydrogen production activity was carried out and the results indicated that increasing the pH from 1.5 to 3.0 leads to an obvious increase in H2-evolving activity. More importantly, an attempt on the photocatalytic hydrogen production under the irradiation of a 30 W LED lamp demonstrated that the H2 evolved was also up to 32.1 mmol in 48 h, showing the feasibility of using cheap and long lifetime LED lamp as light source for photocatalytic hydrogen production of CdS-based photocatalysts.

Site-directed mutagenesis of Campylobacter concisus respiratory genes provides insight into the pathogen\u2019s growth requirements

Campylobacter concisus is an emerging human pathogen found throughout the entire human oral-gastrointestinal tract. The ability of C. concisus to colonize diverse niches of the human body indicates the pathogen is metabolically versatile. C. concisus is able to grow under both anaerobic conditions and microaerophilic conditions. Hydrogen (H2) has been shown to enhance growth and may even be required. Analysis of several C. concisus genome sequences reveals the presence of two sets of genes encoding for distinct hydrogenases: a H2-uptake-type (“Hyd”) complex and a H2-evolving hydrogenase (“Hyf”). Whole cells hydrogenase assays indicate that the former (H2-uptake) activity is predominant in C. concisus, with activity among the highest we have found for pathogenic bacteria. Attempts to generate site-directed chromosomal mutants were partially successful, as we could disrupt hyfB, but not hydB, suggesting that H2-uptake, but not H2-evolving activity, is an essential respiratory pathway in C. concisus. Furthermore, the tetrathionate reductase ttrA gene was inactivated in various C. concisus genomospecies. Addition of tetrathionate to the medium resulted in a ten-fold increase in cell yield for the WT, while it had no effect on the ttrA mutant growth. To our knowledge, this is the first report of mutants in C. concisus.

SWCNT Photocatalyst for Hydrogen Production from Water upon Photoexcitation of (8, 3) SWCNT at 680-nm Light

Single-walled carbon nanotubes (SWCNTs) are potentially strong optical absorbers with tunable absorption bands depending on their chiral indices (n, m). Their application for solar energy conversion is difficult because of the large binding energy (>100 meV) of electron-hole pairs, known as excitons, produced by optical absorption. Recent development of photovoltaic devices based on SWCNTs as light-absorbing components have shown that the creation of heterojunctions by pairing chirality-controlled SWCNTs with C60 is the key for high power conversion efficiency. In contrast to thin film devices, photocatalytic reactions in a dispersion/solution system triggered by the photoexcitation of SWCNTs have never been reported due to the difficulty of the construction of a well-ordered surface on SWCNTs. Here, we show a clear-cut example of a SWCNT photocatalyst producing H2 from water. Self-organization of a fullerodendron on the SWCNT core affords water-dispersible coaxial nanowires possessing SWCNT/C60 heterojunctions, of which a dendron shell can act as support of a co-catalyst for H2 evolution. Because the band offset between the LUMO levels of (8, 3)SWCNT and C60 satisfactorily exceeds the exciton binding energy to allow efficient exciton dissociation, the (8, 3)SWCNT/fullerodendron coaxial photocatalyst shows H2-evolving activity (QY = 0.015) upon 680-nm illumination, which is E22 absorption of (8, 3) SWCNT.

Synthesis, Characterization, and Photocatalytic H2-Evolving Activity of a Family of [Co(N4Py)(X)](n+) Complexes in Aqueous Solution.

A series of [Co(III)(N4Py)(X)](ClO4)n (X = Cl(-), Br(-), OH(-), N3(-), NCS(-)-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co(II)(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3](2+) by ascorbate followed by an electron transfer from [Ru(II)(bpy)2(bpy(•-))](+) to the [Co(II)(N4Py)(HA)](+) catalyst. The reduced catalyst then enters into the H2-evolution cycle.

Efficient Electrochemical and Photoelectrochemical H2 Production from Water by a Cobalt Dithiolene One-Dimensional Metal-Organic Surface.

Solar-driven hydrogen evolution from water has emerged as an important methodology for the storage of renewable energy in chemical bonds. Efficient and practical clean-energy devices for electrochemical or photoelectrochemical splitting of water require the immobilization of stable and active hydrogen-evolving catalysts onto electrode or photocathode materials, which remains a significant challenge. Here we show that cobalt(II) reacts with benzene-1,2,4,5-tetrathiol in the presence of base to form a cobalt dithiolene polymer 1. The generated polymer is immobilized onto glassy carbon electrodes (GCE) to generate a metal-organic surface (MOS 1|GCE), which displays efficient H2-evolving activity and stability in acidic aqueous solutions. Moreover, the generated polymer is integrated with planar p-type Si to generate very efficient photocathode materials (MOS 1|Si) for solar-driven hydrogen production from water. Photocurrents up to 3.8 mA/cm(2) at 0 V vs RHE were achieved under simulated 1 Sun illumination. MOS 1|Si photocathodes operate at potentials 550 mV more positive than MOS 1|GCE cathodes to reach the same activity for H2 evolution from water (1 mA/cm(2)).

Activation of formate hydrogen-lyase via expression of uptake [NiFe]-hydrogenase in Escherichia coli BL21(DE3).

BackgroundSeveral recent studies have reported successful hydrogen (H2) production achieved via recombinant expression of uptake [NiFe]-hydrogenases from Hydrogenovibrio marinus, Rhodobacter sphaeroides, and Escherichia coli (hydrogenase-1) in E. coli BL21(DE3), a strain that lacks H2-evolving activity. However, there are some unclear points that do not support the conclusion that the recombinant hydrogenases are responsible for the in vivo H2 production.ResultsUnlike wild-type BL21(DE3), the recombinant BL21(DE3) strains possessed formate hydrogen-lyase (FHL) activities. Through experiments using fdhF (formate dehydrogenase-H) or hycE (hydrogenase-3) mutants, it was shown that H2 production was almost exclusively dependent on FHL. Upon expression of hydrogenase, extracellular formate concentration was changed even in the mutant strains lacking FHL, indicating that formate metabolism other than FHL was also affected. The two subunits of H. marinus uptake [NiFe]-hydrogenase could activate FHL independently of each other, implying the presence of more than two different mechanisms for FHL activation in BL21(DE3). It was also revealed that the signal peptide in the small subunit was essential for activation of FHL via the small subunit.ConclusionsHerein, we demonstrated that the production of H2 was indeed induced via native FHL activated by the expression of recombinant hydrogenases. The recombinant strains with [NiFe]-hydrogenase appear to be unsuitable for practical in vivo H2 production due to their relatively low H2 yields and productivities. We suggest that an improved H2-producing cell factory could be designed by constructing a well characterized and overproduced synthetic H2 pathway and fully activating the native FHL in BL21(DE3).

Light-Driven Proton Reduction in Aqueous Medium Catalyzed by a Family of Cobalt Complexes with Tetradentate Polypyridine-Type Ligands.

A series of tetradentate 2,2':6',2″:6″,2‴-quaterpyridine-type ligands related to ppq (ppq = 8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2'-yl)quinoline) have been synthesized. One ligand replaces the 1,10-phenanthroline (phen) moiety of ppq with 2,2'-bipyridine and the other two ligands have a 3,3'-polymethylene subunit bridging the quinoline and pyridine. The structural result is that both the planarity and flexibility of the ligand are modified. Co(II) complexes are prepared and characterized by ultraviolet-visible light (UV-vis) and mass spectroscopy, cyclic voltammetry, and X-ray analysis. The light-driven H2-evolving activity of these Co complexes was evaluated under homogeneous aqueous conditions using [Ru(bpy)3](2+) as the photosensitizer, ascorbic acid as a sacrificial electron donor, and a blue light-emitting diode (LED) as the light source. At pH 4.5, all three complexes plus [Co(ppq)Cl2] showed the fastest rate, with the dimethylene-bridged system giving the highest turnover frequency (2125 h(-1)). Cyclic voltammograms showed a significant catalytic current for H2 production in both aqueous buffer and H2O/DMF medium. Combined experimental and theoretical study suggest a formal Co(II)-hydride species as a key intermediate that triggers H2 generation. Spin density analysis shows involvement of the tetradentate ligand in the redox sequence from the initial Co(II) state to the Co(II)-hydride intermediate. How the ligand scaffold influences the catalytic activity and stability of catalysts is discussed, in terms of the rigidity and differences in conjugation for this series of ligands.

Cobalt(III) tetraaza-macrocyclic complexes as efficient catalyst for photoinduced hydrogen production in water: Theoretical investigation of the electronic structure of the reduced species and mechanistic insight

We recently reported a very efficient homogeneous system for visible-light driven hydrogen production in water based on the cobalt(III) tetraaza-macrocyclic complex [Co(CR)Cl2]+ (1) (CR=2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) as a noble metal-free catalyst, with [RuII(bpy)3]2+ (Ru) as photosensitizer and ascorbate/ascorbic acid (HA−/H2A) as a sacrificial electron donor and buffer (PhysChemChemPhys 2013, 15, 17544). This catalyst presents the particularity to achieve very high turnover numbers (TONs) (up to 1000) at pH 4.0 at a relative high concentration (0.1mM) generating a large amount of hydrogen and having a long term stability. A similar activity was observed for the aquo derivative [CoIII(CR)(H2O)2]3+ (2) due to substitution of chloro ligands by water molecule in water. In this work, the geometry and electronic structures of 2 and its analog [ZnII(CR)Cl]+ (3) derivative containing the redox innocent Zn(II) metal ion have been investigated by DFT calculations under various oxidation states. We also further studied the photocatalytic activity of this system and evaluated the influence of varying the relative concentration of the different components on the H2-evolving activity. Turnover numbers versus catalyst (TONCat) were found to be dependent on the catalyst concentration with the highest value of 1130 obtained at 0.05mM. Interestingly, the analogous nickel derivative, [NiII(CR)Cl2] (4), when tested under the same experimental conditions was found to be fully inactive for H2 production. Nanosecond transient absorption spectroscopy measurements have revealed that the first electron-transfer steps of the photocatalytic H2-evolution mechanism with the Ru/cobalt tetraaza/HA−/H2A system involve a reductive quenching of the excited state of the photosensitizer by ascorbate (kq=2.5×107M−1s−1) followed by an electron transfer from the reduced photosensitizer to the catalyst (ket=1.4×109M−1s−1). The reduced catalyst can then enter into the cycle of hydrogen evolution.

The ferredoxin Rr-HydB is required for the H2-evolving activity of Rr-HydA, a [FeFe]-hydrogenase of Rhodospirillum rubrum

The [FeFe]-hydrogenase of Rhodospirillum rubrum (Rr-HydA; Rru_A0310) was expressed in an Escherichia coli strain co-expressing the hydrogenase maturation proteins HydE, HydF, and HydG from Clostridium acetobutylicum (Ca-HydEFG). Rr-HydA, which purified as a 50 kDa protein, showed an in vitro H2-evolving activity in the presence of dithionite-reduced methyl viologen. The ability of Rr-HydA to carry out H2-evolving activity when heterologously expressed in a Rhodobacter sphaeroides nifDK hupSL mutant containing the genes coding for Ca-HydEFG was examined. Photoheterotrophic H2 evolution was only observed in the presence of Rr-HydB [Rru_A0309 (22 kDa)], which is encoded by a gene located immediately upstream from the gene encoding Rr-HydA. Rr-HydB contains Fe–S cluster(s). Consistently, in a reaction mixture containing spinach ferredoxin oxidoreductase and NADPH, H2 was evolved when both Rr-HydA and Rr-HydB were present together, and the rate of H2 evolution by Rr-HydA was proportional to the level of Rr-HydB until an equimolar ratio of the two proteins was reached. Thus, it is proposed that Rr-HydB acts as Rr-HydA-specific ferredoxin that donates electrons to Rr-HydA through a direct molecular interaction.

Synthesis, crystal structure, spectroscopic and electrochemical properties, and H2-evolving activity of a new [PtCl(terpyridine)]+ derivative with viologen-like redox properties

A new Pt(II)-based metalloviologen analog, [PtCl(Mepytpy)](2+) (abbreviated as PV(2+); Mepytpy(+) = 4'-(4-methyl-pyridinio)-2,2':6',2''-terpyridine), was synthesized and its spectroscopic and electrochemical properties were investigated. The structures of Mepytpy(+) and PV(2+) were solved by single-crystal X-ray diffraction analysis. Electrochemical studies in DMF (N,N-dimethylformamide) revealed that both the first and second reduction potentials of PV(2+) (E = -0.87 V vs. Fc/Fc(+) for PV(2+)/PV(+)˙; E = -1.31 V vs. Fc/Fc(+) for PV(+)˙/PV(0)) are nearly consistent with those of methylviologen (N,N'-dimetyl-4,4'-bipyridinium, MV(2+)) (E = -0.90 V vs. Fc/Fc(+) for MV(2+)/MV(+)˙; E = -1.28 V vs. Fc/Fc(+) for MV(+)˙/MV(0)), where the first reduction of PV(2+) is assigned as the reduction at the Mepytpy(+) ligand bound to the Pt(ii) ion. Upon the addition of a reducing agent Na(2)S(2)O(4) into an aqueous acetate buffer solution (pH = 5.0) of PV(2+), the generation of one-electron-reduced radical species (PV(+)˙) was spectrophotometrically observed and was assigned based on the time-dependent density functional theory (TD-DFT) calculations. The H(2)-evolving activity of PV(2+) was evaluated in the presence of a sacrificial electron donor (EDTA) in an aqueous acetate buffer solution (pH = 5.0). It was found that the stability of the metalloviologen PV(2+) during the photolysis is much higher than that of the parent compound [PtCl(tpy)](+).

Promoting Effect of Electrostatic Interaction between a Cobalt Catalyst and a Xanthene Dye on Visible-Light-Driven Electron Transfer and Hydrogen Production

The readily obtained noble-metal-free molecular catalyst systems, with xanthene dyes (Rose Bengal, RB2–; Eosin Y, EY2–; and Eosin B, EB2–) as photosensitizers, [Co(bpy)3]Cl2 as catalyst, and triethylamine as sacrificial electron donor, are highly active for visible-light-driven (λ > 450 nm) hydrogen production from water. The turnover frequency is up to 54 TON/min versus RB2– with a RB2–/[Co(bpy)3]Cl2 molar ratio of 1:10 in CH3CN/H2O under optimal conditions in the first half hour of irradiation (λ > 450 nm), and the turnover number is up to 2076 versus RB2–. Comparative studies show the following: (1) The photocatalytic H2-evolving activity of the cationic cobalt complex [Co(bpy)3]Cl2 is apparently higher than the neutral cobaloxime complexes with xanthene dyes as potosensitizers, and also much higher than the analogous system of [Ru(bpy)3]Cl2/[Co(bpy)3]Cl2. (2) The UV–vis absorptions of xanthene dyes are red-shifted to different extents upon addition of [Co(bpy)3]Cl2 to the aqueous or CH3CN/H2O solution...

Frontier orbital engineering of photo-hydrogen-evolving molecular devices: a clear relationship between the H2-evolving activity and the energy level of the LUMO

Two new Ru(II)Pt(II) dimers, [Ru(bpy)(2)(mu-L2)PtCl(2)](2+) (5) and [Ru(bpy)(2)(mu-L3)PtCl(2)](2+) (6), were synthesized and characterized, and their electrochemical and spectroscopic properties together with their photo-hydrogen-evolving activities were evaluated (bpy = 2,2'-bypridine; L2 = 4'-[1,10]phenanthrolin-5-ylcarbamoyl)-[2,2']bipyridinyl-4-carboxylic acid ethyl ester; L3 = 4'-methyl-[2,2']bipyridinyl-4-carboxylic acid [1,10]phenanthrolin-5-ylamide). The structures of 5 and 6 are basically identical with that of the first active model of a photo-hydrogen-evolving molecular device developed in our group, [Ru(bpy)(2)(mu-L1)PtCl(2)](2+) (4) (L1 = 4'-([1,10]phenanthrolin-5-ylcarbamoyl)-[2,2']bipyridinyl-4-carboxylic acid), except for the difference in the substituent group at the 4-position of the bpy moiety bound to Pt(II) (-COOH for 4; -COOEt for 5; -CH(3) for 6). Electrochemical studies revealed that the first reduction potential of 5 (E(1/2) = -1.23 V) is nearly consistent with that of 4 (E(1/2) = -1.20 V) but is more positive than that of 6 (E(1/2) = -1.39 V), where the first reduction is associated with the reduction of the bpy moiety bound to Pt(II), consistent with a general tendency that the first reduction of bpy shows an anodic shift upon introduction of electron-withdrawing group. Density functional theory (DFT) calculations for 5 and 6 also show that the lowest unoccupied molecular orbital (LUMO) corresponds to the pi* orbital of the bpy moiety bound to Pt(II) for all the Ru(II)Pt(II) dimers, and the energy level of the LUMO of 6 is destabilized compared with those of 4 and 5, consistent with the results of the electrochemical studies. The photochemical hydrogen evolution from water driven by 4-6 in the presence a sacrificial electron donor (EDTA) was investigated. 5 was found to be active as an H(2)-evolving catalyst, while 6 shows no activity at all. However, 6 was found to drive photochemical H(2) evolution in the presence of both EDTA and methyl viologen (N,N'-dimethyl-4,4'-bipyridinium, MV(2+)), indicating that the (3)MLCT excited state of the Ru(bpy)(2)(phen)(2+) moiety is once oxidatively quenched by MV(2+) to give MV(+) and then hydrogen evolution from water by MV(+*) proceeds as a dark reaction. Emission decays and transient absorption spectra also show that the intramolecular electron transfer (IET) is accelerated in the active Ru(II)Pt(II) dimers 4 and 5, while such acceleration is not realized for the inactive Ru(II)Pt(II) dimer 6. The driving forces (DeltaG degrees(ET)) for the IET processes are estimated to be -0.16 eV for 4, -0.09 eV for 5 and 0.03 eV for 6, indicating that the IET process in 6 is uphill. It is concluded that efficient IET is required to drive the photochemical H(2) evolution from water with these Ru(II)Pt(II)-based molecular devices.

Synthesis, crystal structure, solution and spectroscopic properties, and hydrogen-evolving activity of [K(18-crown-6)][Pt(ii)(2-phenylpyridinato)Cl2

A water-soluble ortho-metalated platinum(II) complex [K(18-crown-6)][Pt(ppy)Cl2]·0.5H2O (1) (ppy = phenylpyridinate, 18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane) was prepared from a 1 : 1 reaction of [K(18-crown-6)]2[PtCl4] and 2-phenylpyridine in methanol. The compound crystallizes in the triclinic system, space groupP (No. 2). A solvolysis product of 1, Pt(ppy)Cl(DMF) (DMF = N,N-dimethylformamide) (10), was also isolated and structurally characterized by X-ray diffraction (monoclinic system, space groupP21/n (No. 14)), revealing the lability of the coordination bond trans to the C(ppy) donor. Ligand substitution reactions of 1 in water were investigated by means of UV-visible absorption spectroscopy and time-dependent DFT calculations. The initial hydrolysis product in aqueous media was suggested to be Pt(ppy)Cl(OH2), in which the chloride ion trans to the C(ppy) donor in [Pt(ppy)Cl2]− is replaced with an aqua ligand due to the stronger trans effect originated by the C(ppy) donor. It was found that the ligand substitution reactions of [Pt(ppy)Cl2]− is extremely complicated. Leaving the complex in solution for a prolonged time resulted in deposition of a yellowish green chloride-bridged dimer Pt2(ppy)2(μ-Cl)2. Moreover, an unidentified blue species, ascribable to a Pt(II)/Pt(III) mixed-valence species, started to form while the initial hydrolysis reactions proceeded. Photochemical hydrogen production from water catalyzed by [Pt(ppy)Cl2]− was examined using a photosystem made up of Ru(bpy)32+ (bpy = 2,2′-bipyridine), methylviologen, and a sacrificial electron donor (EDTA) in an acetate buffer (pH 5). It is shown that the H2-evolving activity of [Pt(ppy)Cl2]− is higher that those of the hydrolysis products of [Pt(ppy)Cl2]−, but are comparable to those of other common Pt(II) complexes having a cis-Pt(II)Cl2 unit, such as cis-PtCl2(NH3)2, PtCl2(4,4′-dicarboxy-bpy), and PtCl2(2,2′-bipyrimidine).

Syntheses, characterization, and photo-hydrogen-evolving properties of tris(2,2′-bipyridine)ruthenium(ii) derivatives tethered to a cis-Pt(ii)Cl2unit: insights into the structure–activity relationship

The photo-hydrogen-evolving activity (activity to enhance the photochemical EDTA-reduction of water into molecular hydrogen) was evaluated for three different Ru(II)Pt(II) dimers with a general formula of [(bpy)2Ru(micro-bridge)PtCl2]2+(bpy = 2,2'-bipyridine; bridge = 4,4'-bis(N-(3-aminopropyl)carbamoyl)-2,2'-bipyridine (L1), 2,3-bis(2-pyridyl)pyrazine (L2), and 4,4'-bis(N-(4-pyridyl)methylcarbamoyl)-2,2'-bipyridine (L3); EDTA = ethylenediaminetetraacetic acid disodium salt). A new Ru(II)Pt(II) complex, [(bpy)2Ru(micro-L3)PtCl2]2+, was synthesized and characterized. It was confirmed that all three compounds are ineffective towards photochemical H2 production. In each case, an acetate-buffer solution (pH = 5) containing the Ru(II)Pt(II) dimer and EDTA was photolysed using a 350-W Xe lamp under an Ar atmosphere, during which the amount of H2 evolved was analysed by gas chromatography. Additional photolysis experiments were carried out by adding [Ru(bpy)3]2+ and methylviologen (N,N'-dimethyl-4,4'-bipyridinium) to the photolysis solutions described above to test the H2-evolving activity of the Pt(II) unit involved in these Ru(II)Pt(II) dimers. As a result, the Pt(II) units involved in the L1 and L2 compounds were found to be active as an H2-evolving catalyst, while that of the L3 compound was found to show no activity at all. The extent of intramolecular electron-transfer quenching from the 3MLCT excited state of the [Ru(bpy)3]2+ derivative to the tethering Pt(II) catalyst centre was investigated by comparison of the luminescence spectra of these compounds, together with the related compounds. The results showed that the quenching of the 3MLCT luminescence is not at all enhanced in either the L1 or the L3 compounds. On the other hand, the L2 compound is strongly quenched as previously reported. In addition to the above studies, the H2-evolving activity of some Pt(II) monomers, cis-PtCl2(NH3)2, PtCl2(en)(en = ethylenediamine), cis-PtCl2(4-methylpyridine)2, PtCl2(2,2'-bipyrimidine), PtCl2(4,4'-dicarboxy-2,2'-bipyridine), and [PtCl(terpy)]+(terpy = 2,2':6',2''-terpyridine), were similarly investigated in the presence of EDTA, [Ru(bpy)3]2+ and methylviologen, since they were regarded as structural analogues of the Pt(II) units involved in the L1-L3 compounds. The compounds having a cis-Pt(II)Cl2 unit were generally found to show high H2-evolving activity. This was interpreted in terms of the ligation of negatively charged chloride anions leading to the destabilization of the Pt(II) dz2 orbital responsible for the hydrogenic activation. Importantly, cis-PtCl2(4-methylpyridine)2 exhibited relatively high activity as an H2-evolving catalyst, suggesting the importance of the flexible rotation of the pyridyl ligands for efficient hydrogenic activation at the axial site of the Pt(II) ion. The DFT calculations also showed the validity of the structure-activity relationship discussed above for the L3 compound.

Purification and catalytic properties of a CO-oxidizing:H2-evolving enzyme complex from Carboxydothermus hydrogenoformans.

From the membrane fraction of the Gram-positive bacterium Carboxydothermus hydrogenoformans, an enzyme complex catalyzing the conversion of CO to CO2 and H2 was purified. The enzyme complex showed maximal CO-oxidizing:H2-evolving enzyme activity with 5% CO in the headspace (450 U per mg protein). Higher CO concentrations inhibited the hydrogenase present in the enzyme complex. For maximal activity, the enzyme complex had to be activated by either CO or strong reductants. The enzyme complex also catalyzed the CO- or H2-dependent reduction of methylviologen at 5900 and 180 U per mg protein, respectively. The complex was found to be composed of six hydrophilic and two hydrophobic polypeptides. The amino-terminal sequences of the six hydrophilic subunits were determined allowing the identification of the encoding genes in the preliminary genome sequence of C. hydrogenoformans. From the sequence analysis it was deduced that the enzyme complex is formed by a Ni-containing carbon monoxide dehydrogenase (CooS), an electron transfer protein containing four [4Fe-4S] clusters (CooF) and a membrane bound [NiFe] hydrogenase composed of four hydrophilic subunits and two membrane integral subunits. The hydrogenase part of the complex shows high sequence similarity to members of a small group of [NiFe] hydrogenases with sequence similarity to energy conserving NADH:quinone oxidoreductases. The data support a model in which the enzyme complex is composed of two catalytic sites, a CO-oxidizing site and a H2-forming site, which are connected via a different iron-sulfur cluster containing electron transfer subunits. The exergonic redox reaction catalyzed by the enzyme complex in vivo has to be coupled to energy conservation, most likely via the generation of a proton motive force.