Photocatalytic Hydrogen Production System Research Articles

Enhancement of photocatalytic H2 evolution on pyrene-based polymer promoted by MoS2 and visible light

Conjugated polymers have gathered particular interests in photocatalysis because of their excellent properties such as optimization flexibility, cost-effective, and robust stability. The pyrene-based polymer (PyP), as one of the conjugated polymers, holds a great potential for water splitting because of their suitable band structure, visible light absorption, and high surface area. However, because of the fast charge carrier recombination confined by Frenkel excitons with high exciton binding energy, PyP in its pristine form remains barely active for photoctalytic hydrogen evolution. The concise construction of heterojunction is a feasible way to accelerate the charge separation, increase the lifetime of the photogenerated e−/h+ pair and decrease the activation barriers of hydrogen evolution reaction. In this work, a series of transition metal sulfides as cocatalysts have been deposited on PyP to construct heterojunctions for photocatalytic H2 evolution reaction (HER). The loading techniques (immersion, in-situ and photodeposition) and the loading contents of cocatalysts have been investigated. The optimized MoS2/PyP sample exhibited a 10-fold increase in comparison with pure PyP without modification. The extended π-conjugation, high surface area, and widely exposed 2D interface highlight the importance of PyP as effective supports for stabilizing the homogeneously dispersed MoS2, thereby resulting in an efficient photocatalytic activity. This study provides a new idea to construct low-cost, sustainable and efficient hybrid system for photocatalytic hydrogen production.

Electrochemical Generation and Spectroscopic Characterization of the Key Rhodium(III) Hydride Intermediates of Rhodium Poly(bipyridyl) H2-Evolving Catalysts.

We previously reported that the [RhIII(dmbpy)2Cl2]+ (dmbpy = 4,4'-dimethyl-2,2'-bipyridine) complex is an efficient H2-evolving catalyst in water when used in a molecular homogeneous photocatalytic system for hydrogen production with [RuII(bpy)3]2+ (bpy = 2,2'-bipyridine) as photosensitizer and ascorbic acid as sacrificial electron donor. The catalysis is believed to proceed via a two-electron reduction of the Rh(III) catalyst into the square-planar [RhI(dmbpy)2]+, which reacts with protons to form a Rh(III) hydride intermediate that can, in turn, release H2 following different pathways. To improve the current knowledge of these key intermediate species for H2 production, we performed herein a detailed electrochemical investigation of the [RhIII(dmbpy)2Cl2]+ and [RhIII(dtBubpy)2Cl2]+ (dtBubpy = 4,4'-di- tert-butyl-2,2'-bipyridine) complexes in CH3CN, which is a more appropriate medium than water to obtain reliable electrochemical data. The low-valent [RhI(Rbpy)2]+ and, more importantly, the hydride [RhIII(Rbpy)2(H)Cl]+ species (R = dm or dtBu) were successfully electrogenerated by bulk electrolysis and unambiguously spectroscopically characterized. The quantitative formation of the hydrides was achieved in the presence of weak proton sources (HCOOH or CF3CO3H), owing to the fast reaction of the electrogenerated [RhI(Rbpy)2]+ species with protons. Interestingly, the hydrides are more difficult to reduce than the initial Rh(III) bis-chloro complexes by ∼310-340 mV. Besides, 0.5 equiv of H2 is generated through their electrochemical reduction, showing that Rh(III) hydrides are the initial catalytic molecular species for hydrogen evolution. Density functional theory calculations were also performed for the dmbpy derivative. The optimized structures and the theoretical absorption spectra were calculated for the initial bis-chloro complex and for the various rhodium intermediates involved in the H2 evolution process.

Relationship between interatomic electron transfer and photocatalytic activity of TiO2

We demonstrate the direct observation of interatomic electron excitation and transfer on single-crystal TiO2 (001) surface for water splitting by a synchronous illumination X-ray photoelectron spectroscopy (SI-XPS). The quantitative results clearly reveal that the initially absorbed water molecules on TiO2 surface promote the electron transfer between Ti and O atoms. However, when water molecule was dissociated into OH and H groups on Ti and O sites, respectively, the electron excitation and transfer ability have been gradually decreased and finally terminated, probably due to the reverse electron-attraction of OH group on Ti atoms. Amazingly, when Pt co-catalyst was deposited on TiO2, the electron excitation between Ti and O atoms have been significantly enhanced and no evident decrease has been observed during the entire water splitting process. In addition, the changes of Pt–O and Ti–O chemical bonds at the photocatalytic interfaces have been also detected. Furthermore, based on the above mechanism studies, the photocatalytic hydrogen production system for Pt/R-TiO2 (001) in KOH solution containing CH3OH was proposed, which exhibits a significant enhancement of hydrogen evolution rate (20.78 μmol h−1), which is nearly 2.5 times higher than that of traditional Pt/R-TiO2 (001) in CH3OH system (8.35 μmol h−1). The finding in this work may pave the way for developing other high-efficiency photocatalytic hydrogen production system over TiO2-based photocatalysts.

PtIx/[(CH3)2NH2]3[BiI6] as a well-dispersed photocatalyst for hydrogen production in hydroiodic acid

Constructing a photocatalytic system for hydrogen generation is a promising means for producing clean and renewable energy. However, developing photocatalysts with high efficiency in a wide range of visible light is still a formidable challenge. Here, we report on a highly active system for photocatalytic hydrogen production from hydroiodic acid splitting that contains a soluble organic-inorganic hybrid PtIx/[(CH3)2NH2]3[BiI6] as a newcomer photocatalyst for harvesting a wide range of visible light (up to 630 nm). Under irradiation at 465 nm, the photocatalytic system achieves apparent quantum yield of ~82.8% for photocatalytic hydrogen production along with a stable activity for at least 100 h. We propose that the superior photocatalytic performance is attributed to collisions between [BiI6]3− and Pt ions (Pt2+ or Pt4+), giving rise to efficient charge separation, which could be an effective approach towards designing and fabricating organic-inorganic semiconductors with improved performance in photocatalysis.

Development of the direct solar photocatalytic water splitting system for hydrogen production in Northwest China: Design and evaluation of photoreactor

Abstract A novel CPC reactor for solar photocatalytic hydrogen production was designed and evaluated in the present study. Two operation models, namely the natural circulation model and the gas disturbance model, are proposed and illustrated from the viewpoints of thermodynamics and hydrodynamics. The designed photoreactor is operated under natural circulation for most of the time, with high pressure gas disturbing the sedimentary photocatalysts from time to time. The CPC parameters are designed according to the local meteorological conditions. The reactor performance such as the radiation distribution on the absorber tube, the absorbed solar irradiation, the critical flow rates and the hydrogen productivity are estimated and analyzed. An east-west orientated, north-south angle adjustable and truncated CPC with the concentration ratio of 4.12 is designed for the photoreactor. The required limiting settling velocity is much larger than the natural circulation velocity, which validates the necessity of gas disturbance. The estimated results show that the ideal mean hydrogen productivities are 2.9 L/h and 4.0 L/h in a typical spring and summer week respectively, with the photocatalyst being Cd0.5Zn0.5S.

Optimization of the radiation absorption for a scaled-up photocatalytic hydrogen production system

The effective absorption of photons by photocatalyst in the reactor is a challenge for scaled-up application of a photocatalytic hydrogen production system. The spatial distribution of radiation and photon absorption are proposed to be strongly dependent on the photocatalyst concentration and the structure of the solar collector. However, there have been few reports of the radiation absorption properties of a cylindrical reactor based on high power solar collector that is suitable for a large-scale photocatalytic hydrogen production system. In this study, the distribution of radiation in the reactor as represented by the local volumetric rate of photon absorption was determined by adopting the modified six-flux absorption scattering model based on the compound parabolic concentrator and the surface uniform concentrator. The explanations of the differences of radiation absorption properties were comprehensively analyzed by ray-tracking technique. The structure of the solar collector and the optimum catalyst concentration are recommended based on photon absorption efficiency. This work may serve as potential reference values for the optimal reactor operating parameters for large-scale application.

Study on deactivation and reaction mechanism of Co thiolate complexes in photocatalytic hydrogen production system

Hydrogen is an environment-friendly source with high efficiency, which is also an important alternative to traditional energy sources. In this paper, noble-metal-free photocatalyst Co thiolate complexes Co(bpy)(pyS)2 (M1) and Co(phen)(pyS)2 (M2) were synthesized and used in photocatalytic decomposition of water to generate hydrogen. The deactivation and reaction mechanism of Co thiolate complexes in photocatalytic hydrogen generation system were studied. The results of photocatalytic hydrogen generation with regeneration of system by re-addition of catalyst and fluorescein exhibited that the main reasons for the deactivation of system were probably the deactivation of catalyst and small part of decomposition of photosensitizer. The results of UV-vis spectra of the system after irradiation revealed that the electron transfer mode between fluorescein, catalyst, and triethylamine was favorable for the stability and life of fluorescein. Furthermore, the electron transfer rates from fluorescein to M1 and M2 were 2.1 × 1012M−1 S−1 and 1.5 × 1012M−1 S−1. This investigation may lay the theories foundation for further research on hydrogen evolution of noble-metal-free catalysts and also may provide a novel approach for building a feasible and more environmental-friendly photocatalytic hydrogen production system.

Preparation, characterization and photocatalytic properties of diiron mimic modified Nano Silica

Diiron mimic modified Nano Silica (1) was synthesized in three steps. The intermediate (2, 3) and final product (1) were characterized by SEM (EDX), elemental analysis, FT-IR, XRD, TG-DTA and AAS. The diiron mimic modified Nano Silica (1) could be considered as a new heterogeneous [FeFe]-hydrogenase model. In addition, 1 was applied to a photocatalytic hydrogen production system which made up of photosensitizer ([Ir(ppy)2(bpy)]PF6), electron sacrifice (TEA), proton source (H2O) and solvent (acetonitrile). The turnover number was 324 for surface modified catalyst and 60 for photosensitizer in 5h of illumination. The study of the recovered catalyst found that most of the catalysts were broken down within 5h of catalytic reaction.

Photocatalytic water splitting solar-to-hydrogen energy conversion: Perovskite-type hydride XBeH3 (X = Li or Na) as active photocatalysts

A highly enhanced photocatalytic hydrogen production system has been achieved, by substitution of Na by Li and moving from cubic to orthorhombic phase in XBeH3 system. Ab-initio calculations from first- to second-principles methods were performed to investigate the suitability of the perovskite-type hydride namely; NaBeH3 and LiBeH3 in cubic phase and LiBeH3 in orthorhombic phase to be used as active photocatalysts. We found significant increases in the fundamental energy band gap when we move from NaBeH3-cubic (0.94eV)→LiBeH3-cubic (1.34eV)→LiBeH3-orthorhombic (2.44eV). The obtained energy band gap's values show good agreement with the previous reported results. Enlarging the fundamental energy band gap from 0.94→1.34→2.44eV shows the investigated materials to be promising candidates for light-driven photocatalysts and highly enhanced photocatalytic H2 production systems. The absorption level of NaBeH3-cubic, LiBeH3-cubic and LiBeH3-orthorhombic exhibited an obvious enhancement in the UV-light region (absorption edge λ=343.4nm)→visible light region (λ=431.9nm)→UV-light region (λ=349.2nm), respectively.

Calix[4]arene based dye-sensitized Pt@UiO-66-NH2 metal-organic framework for efficient visible-light photocatalytic hydrogen production

A cone-calix[4]arene dye Calix-3 with four D-π-A units has been utilized to sensitize Zr-containing metal-organic framework (MOF) embedded with Pt particles, denoted as Pt@UiO-66-NH2, for photocatalytic H2 production under visible light irradiation. The structures of UiO-66-NH2, Pt-loadings, and dye-adsorbed amounts of these photocatalysts are optimized. Comparatively, Calix-3-sensitized Pt@UiO-66-NH2 catalysts with 0.65wt% of Pt loading and 15.7μmol/g of Calix-3 dye amount exhibit much higher hydrogen production activity (1528μmolg−1h−1 based on the mass of MOF or 236mmolg−1[Pt] h−1 based on Pt mass) than that sensitized by single D-π-A M-3 dyes (516μmolg−1h−1 or 24mmolg−1[Pt] h−1) under the similar photocatalytic conditions, and perform excellent stability during the long-term tests simultaneously. In view of the extremely low Pt loading, the activity based on Pt mass is one of the highest among all the reported MOF-based photocatalytic hydrogen production systems. The enhancement in hydrogen evolution efficiency and stability could be ascribed to lower tendency for aggregation, higher molar absorption coefficients, more efficient electron transfer, and better intrinsic and adsorbed stability of Calix-3 dyes. This work provides useful insights for future design and synthesis of new functional dye sensitization MOF system for photocatalytic hydrogen production.

Balancing electron transfer rate and driving force for efficient photocatalytic hydrogen production in CdSe/CdS nanorod–[NiFe] hydrogenase assemblies

We describe a hybrid photocatalytic system for hydrogen production consisting of nanocrystalline CdSe/CdS dot-in-rod (DIR) structures coupled to [NiFe] soluble hydrogenase I (SHI) fromPyrococcus furiosus.

MgTiO3/MgTi2O5/TiO2 heterogeneous belt-junctions with high photocatalytic hydrogen production activity

An effective photocatalytic hydrogen production catalyst comprising MgTiO3/MgTi2O5/TiO2 heterogeneous belt-junctions was prepared using magnesium ions by a thermally driven doping method. The tri-phase heterogeneous junction was confirmed by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). The as-prepared MgTiO3/MgTi2O5/TiO2 heterojunctions exhibited a very high photocatalytic hydrogen production activity (356.1 mol·g0.1gcat·h-1) and an apparent quantum efficiency (50.69% at 365 nm) that is about twice of that of bare TiO2 nanobelts (189.4 mol·g0.1gcat·h-1). Linear sweep voltage and transient photocurrent characterization as well as analysis of the electrochemical impedance spectra and Mott–Schottky plots revealed that the high photocatalytic performance is caused by the one-dimensional structure, which imparts excellent charge transportation characteristic, and the MgTiO3/MgTi2O5/TiO2 tri-phase heterojunction, which effectively drives the charge separation through the inherent electric field. This titanate-based tri-phase heterogeneous junction photocatalyst further enriches the catalyst system for photocatalytic hydrogen production.

Synergistic Effect of a Molecular Cocatalyst and a Heterojunction in a 1 D Semiconductor Photocatalyst for Robust and Highly Efficient Solar Hydrogen Production.

Photocatalytic production of hydrogen by water splitting is a promising pathway for the conversion of solar energy into chemical energy. However, the photocatalytic conversion efficiency is often limited by the sluggish transfer of the photogenerated charge carriers, charge recombination, and subsequent slow catalytic reactions. Herein, we report a highly active noble-metal-free photocatalytic system for hydrogen production in water. The system contains a water-soluble nickel complex as a molecular cocatalyst and zinc sulfide on 1D cadmium sulfide as the heterojunction photocatalyst. The complex can efficiently transport photogenerated electrons and holes over a heterojunction photocatalyst to hamper charge recombination, leading to highly improved catalytic efficiency and durability of a heterojunction photocatalyst- molecular cocatalyst system. The results show that under optimal conditions, the average apparent quantum yield was approximately 58.3 % after 7 h of irradiation with monochromatic 420 nm light. In contrast, the value is only 16.8 % if the molecular cocatalyst is absent. Such a remarkable performance in a molecular cocatalyst-based photocatalytic system without any noble metal loading has, to the best of our knowledge, not been reported to date.

Cobalt thiolate complexes catalyst in noble-metal-free system for photocatalytic hydrogen production

Thiolate complexes Co(bpy)(pyS)2 (M) were synthesized, the effect of reaction conditions (such as pH values, solvents, electron donor, photosensitizers, catalyst, and concentrations) on catalytic performance of catalyst M was studied and the optimum reaction conditions were selected. The results of catalytic performance imply that complexes M showed good catalytic activity when concentration of catalyst M was 2.5 μM, concentration of fluorescein was 2.0 mM, pH value was 11.6, electron donor was triethylamine (5%), and the solvent was EtOH/H2O (1: 1 V/V). The catalysis system presented the best performance and hydrogen production reached 369.2 μmol h–1 after reacting for 15 h; the life of catalyst M was 41 h investigated under the optimum conditions.

Iron Polypyridyl Complexes for Photocatalytic Hydrogen Generation.

A series of Fe(III) complexes were recently reported that are stable and active electrocatalysts for reducing protons into hydrogen gas. Herein, we report the incorporation of these electrocatalysts into a photocatalytic system for hydrogen production. Hydrogen evolution is observed when these catalysts are paired with fluorescein (chromophore) and triethylamine (sacrificial electron source) in a 1:1 ethanol:water mixture. The photocatalytic system is highly active and stable, achieving TONs > 2100 (with respect to catalyst) after 24 h. Catalysis proceeds through a reductive quenching pathway with a quantum yield of over 3%.

Solar concentrator with uniform irradiance for particulate photocatalytic hydrogen production system

The solar concentrator is an essential part of the particulate photocatalytic hydrogen production system (PPHS). In this study, a mathematical model was developed and applied to avoid non-uniform concentration of solar intensity on the receiving surface of a compound parabolic concentrator (CPC). The reflector profile curve was calculated via a first-order differential equation. The geometric concentration ratio and bottom interval parameters were changed to optimize the surface uniform concentrator (SUC). The overall performance of the concentrator was then assessed based on the optical efficiency and uniformity on the receiver in ray tracing software. A prototype of a SUC was manufactured to validate the calculation results. It is found that the conversion efficiency of solar energy to hydrogen with the SUC was 8.57% higher than that of the CPC.

Performance of four various shapes of photocatalytic reactors with respect to hydrogen and sulphur recovery from sulphide containing wastestreams

A more economically viable way of producing hydrogen from sulphide wastewater is under active investigation by many researchers, to convert solar energy into chemical energy with higher efficiency. In this paper the photocatalytic hydrogen production from sulphide waste stream was compared using four different configuration of liquid-phase photocatalytic hydrogen production systems, viz., cylindrical inner irradiated (LR1(inner)), and cylindrical outer irradiated (LR1(outer)), tubular outer irradiated (LR2) and trapezoidal outer irradiated (LR3) photocatalytic reactors. CdS and ZnS coated iron oxide core shell particles (CdS + ZnS)/Fe2O3) was used as novel photocatalyst to study the reactor performance. Results are presented and discussed to evaluate how the system parameters affect the hydrogen production rate. The hydrogen production for the above reactors were found to be 7857, 938, 580 and 1116 μmol/h with the quantum yield of 21%, 6%, 2.5% and 7.5% respectively.

Enhanced photocatalytic H2 production on CdS nanorods with simple molecular bidentate cobalt complexes as cocatalysts under visible light.

Photocatalytic hydrogen production via water splitting has attracted much attention for future clean energy application. Herein we report a noble-metal-free photocatalytic hydrogen production system containing a simple bidentate cobalt Schiff base complex as the molecular cocatalyst, CdS nanorods as the photosensitizer, and ascorbic acid as the electron donor. The system shows highly enhanced photocatalytic activity compared to pure CdS NRs under visible light (λ > 420 nm). Under optimal conditions, the turnover numbers (TONs) for hydrogen production reached ∼15 200 after 12 hours of irradiation, and an apparent quantum yield of ∼27% was achieved at 420 nm monochromatic light. Steady-state photoluminescence (PL) spectra indicated efficient charge transfer between the excited CdS NRs and the cobalt cocatalyst for improved hydrogen production. Spectroscopic studies of the photocatalytic reaction revealed the reduction of the Co(ii) complex to Co(i) species, which are probably active intermediates for hydrogen evolution. On the basis of the spectroscopic studies, we propose a reaction mechanism for hydrogen production in the present photocatalytic system.

Sustainable hydrogen production from electroplating wastewater over a solar light responsive photocatalyst

An environmentally friendly and sustainable photocatalytic hydrogen production system was successfully developed using EDTA in the wastewater as the photo-excited hole scavenger and a solar light responsive material as the photocatalyst.

Visible-light-driven hydrogen production from water in a noble-metal-free system catalyzed by zinc porphyrin sensitized MoS2/ZnO

Photocatalytic hydrogen production system using zinc oxide as a photocatalyst typically suffer from low efficiency because of the wide band gap of ZnO. Despite tremendous efforts, it is still a great challenge to design efficient visible-light-responsive ZnO-based photocatalysts for hydrogen production. Herein, a new system for visible-light-driven hydrogen evolution was constructed by using Zn(II)-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin dye-sensitized MoS2/ZnO as a photocatalyst and triethanolamine as the sacrificial reductant. Even without any noble metal, the as-prepared composite photocatalyst loaded with 0.50 wt% MoS2 has a H2 production rate of 75 μmol h−1 g−1 under visible light irradiation (λ > 420 nm), and the turnover number with respect to zinc dye reaches 516 after 9 h visible light irradiation. This study might offer a new paradigm for constructing efficient, visible-light-responsive and noble-metal-free solar-to-H2 conversion system by using dye-sensitized semiconductor as the photocatalyst.