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952 KiB

Open AccessArticle

Mechanism of Catalytic Water Oxidation by the Ruthenium Blue Dimer Catalyst: Comparative Study in D₂O versus H₂O

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Materials 2013, 6(2), 392-409; https://doi.org/10.3390/ma6020392 - 30 Jan 2013

Cited by 30 | Viewed by 8076

Abstract

Water oxidation is critically important for the development of energy solutions based on the concept of artificial photosynthesis. In order to gain deeper insight into the mechanism of water oxidation, the catalytic cycle for the first designed water oxidation catalyst, cis,cis [...] Read more.

Water oxidation is critically important for the development of energy solutions based on the concept of artificial photosynthesis. In order to gain deeper insight into the mechanism of water oxidation, the catalytic cycle for the first designed water oxidation catalyst, cis,cis-[(bpy)₂(H₂O)Ru^IIIORu^III(OH₂)(bpy)₂]⁴⁺ (bpy is 2,2-bipyridine) known as the blue dimer (BD), is monitored in D₂O by combined application of stopped flow UV-Vis, electron paramagnetic resonance (EPR) and resonance Raman spectroscopy on freeze quenched samples. The results of these studies show that the rate of formation of BD[4,5] by Ce(IV) oxidation of BD[3,4] (numbers in square bracket denote oxidation states of the ruthenium (Ru) centers) in 0.1 M HNO₃, as well as further oxidation of BD[4,5] are slower in D₂O by 2.1–2.5. Ce(IV) oxidation of BD[4,5] and reaction with H₂O result in formation of an intermediate, BD[3,4]′, which builds up in reaction mixtures on the minute time scale. Combined results under the conditions of these experiments at pH 1 indicate that oxidation of BD[3,4]′ is a rate limiting step in water oxidation with the BD catalyst. Full article

(This article belongs to the Special Issue Advanced Materials for Water-Splitting)

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248 KiB

Open AccessArticle

Photocatalytic Water Splitting for Hydrogen Production with Novel Y₂MSbO₇ (M = Ga, In, Gd) under Visible Light Irradiation

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Jingfei Luan

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Jianhui Chen

Materials 2012, 5(11), 2423-2438; https://doi.org/10.3390/ma5112423 - 21 Nov 2012

Cited by 9 | Viewed by 5587

Abstract

Novel photocatalysts Y₂MSbO₇ (M = Ga, In, Gd) were synthesized by the solid state reaction method for the first time. A comparative study on the structural and photocatalytic properties of Y₂MSbO₇ (M = Ga, In, Gd) was [...] Read more.

Novel photocatalysts Y₂MSbO₇ (M = Ga, In, Gd) were synthesized by the solid state reaction method for the first time. A comparative study on the structural and photocatalytic properties of Y₂MSbO₇ (M = Ga, In, Gd) was reported. The results showed that Y₂GaSbO₇, Y₂InSbO₇ and Y₂GdSbO₇ crystallized with the pyrochlore-type structure, cubic crystal system, and space group Fd3m. The lattice parameter for Y₂GaSbO₇ was 10.17981 Å. The lattice parameter for Y₂InSbO₇ was 10.43213 Å. The lattice parameter for Y₂GdSbO₇ was 10.50704 Å. The band gap of Y₂GaSbO₇ was estimated to be 2.245 eV. The band gap of Y₂InSbO₇ was 2.618 eV. The band gap of Y₂GdSbO₇ was 2.437 eV. For the photocatalytic water-splitting reaction, H₂ or O₂ evolution was observed from pure water with Y₂GaSbO₇, Y₂InSbO₇ or Y₂GdSbO₇ as catalyst under visible light irradiation. (Wavelength > 420 nm). Furthermore, H₂ and O₂ were also evolved by using Y₂GaSbO₇, Y₂InSbO₇ or Y₂GdSbO₇as a catalyst from CH₃OH/H₂O and AgNO₃/H₂O solutions, respectively, under visible light irradiation (λ > 420 nm). Y₂GaSbO₇ showed the highest activity compared with Y₂InSbO₇ or Y₂GdSbO₇. At the same time, Y₂InSbO₇ showed higher activity compared with Y₂GdSbO₇. The photocatalytic activities were further improved under visible light irradiation with Y₂GaSbO₇, Y₂InSbO₇ or Y₂GdSbO₇being loaded by Pt, NiO or RuO₂. The effect of Pt was better than that of NiO or RuO₂for improving the photocatalytic activity of Y₂GaSbO₇, Y₂InSbO₇ or Y₂GdSbO₇. Full article

(This article belongs to the Special Issue Advanced Materials for Water-Splitting)

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545 KiB

Open AccessArticle

Investigation of Iron Oxide Morphology in a Cyclic Redox Water Splitting Process for Hydrogen Generation

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Michael M. Bobek

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Richard C. Stehle

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David W. Hahn

Materials 2012, 5(10), 2003-2014; https://doi.org/10.3390/ma5102003 - 23 Oct 2012

Cited by 5 | Viewed by 5772

Abstract

A solar fuels generation research program is focused on hydrogen production by means of reactive metal water splitting in a cyclic iron-based redox process. Iron-based oxides are explored as an intermediary reactive material to dissociate water molecules at significantly reduced thermal energies. With [...] Read more.

A solar fuels generation research program is focused on hydrogen production by means of reactive metal water splitting in a cyclic iron-based redox process. Iron-based oxides are explored as an intermediary reactive material to dissociate water molecules at significantly reduced thermal energies. With a goal of studying the resulting oxide chemistry and morphology, chemical assistance via CO is used to complete the redox cycle. In order to exploit the unique characteristics of highly reactive materials at the solar reactor scale, a monolithic laboratory scale reactor has been designed to explore the redox cycle at temperatures ranging from 675 to 875 K. Using high resolution scanning electron microscope (SEM) and electron dispersive X-ray spectroscopy (EDS), the oxide morphology and the oxide state are quantified, including spatial distributions. These images show the change of the oxide layers directly after oxidation and after reduction. The findings show a significant non-stoichiometric O/Fe gradient in the atomic ratio following oxidation, which is consistent with a previous kinetics model, and a relatively constant, non-stoichiometric O/Fe atomic ratio following reduction. Full article

(This article belongs to the Special Issue Advanced Materials for Water-Splitting)

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688 KiB

Open AccessArticle

A Series of Supramolecular Complexes for Solar Energy Conversion via Water Reduction to Produce Hydrogen: An Excited State Kinetic Analysis of Ru(II),Rh(III),Ru(II) Photoinitiated Electron Collectors

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Travis A. White

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Jessica D. Knoll

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Shamindri M. Arachchige

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Karen J. Brewer

Materials 2012, 5(1), 27-46; https://doi.org/10.3390/ma5010027 - 27 Dec 2011

Cited by 18 | Viewed by 6374

Abstract

Mixed-metal supramolecular complexes have been designed that photochemically absorb solar light, undergo photoinitiated electron collection and reduce water to produce hydrogen fuel using low energy visible light. This manuscript describes these systems with an analysis of the photophysics of a series of six [...] Read more.

Mixed-metal supramolecular complexes have been designed that photochemically absorb solar light, undergo photoinitiated electron collection and reduce water to produce hydrogen fuel using low energy visible light. This manuscript describes these systems with an analysis of the photophysics of a series of six supramolecular complexes, [{(TL)₂Ru(dpp)}₂RhX₂](PF₆)₅ with TL = bpy, phen or Ph₂phen with X = Cl or Br. The process of light conversion to a fuel requires a system to perform a number of complicated steps including the absorption of light, the generation of charge separation on a molecular level, the reduction by one and then two electrons and the interaction with the water substrate to produce hydrogen. The manuscript explores the rate of intramolecular electron transfer, rate of quenching of the supramolecules by the DMA electron donor, rate of reduction of the complex by DMA from the ³MLCT excited state, as well as overall rate of reduction of the complex via visible light excitation. Probing a series of complexes in detail exploring the variation of rates of important reactions as a function of sub-unit modification provides insight into the role of each process in the overall efficiency of water reduction to produce hydrogen. The kinetic analysis shows that the complexes display different rates of excited state reactions that vary with TL and halide. The role of the MLCT excited state is elucidated by this kinetic study which shows that the ³MLCT state and not the ³MMCT is likely that key contributor to the photoreduction of these complexes. The kinetic analysis of the excited state dynamics and reactions of the complexes are important as this class of supramolecules behaves as photoinitiated electron collectors and photocatalysts for the reduction of water to hydrogen. Full article

(This article belongs to the Special Issue Advanced Materials for Water-Splitting)

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Review

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709 KiB

Open AccessReview

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Materials 2012, 5(11), 2015-2054; https://doi.org/10.3390/ma5112015 - 24 Oct 2012

Cited by 128 | Viewed by 11336

Abstract

Thermochemical multistep water- and CO₂-splitting processes are promising options to face future energy problems. Particularly, the possible incorporation of solar power makes these processes sustainable and environmentally attractive since only water, CO₂ and solar power are used; the concentrated solar [...] Read more.

Thermochemical multistep water- and CO₂-splitting processes are promising options to face future energy problems. Particularly, the possible incorporation of solar power makes these processes sustainable and environmentally attractive since only water, CO₂ and solar power are used; the concentrated solar energy is converted into storable and transportable fuels. One of the major barriers to technological success is the identification of suitable active materials like catalysts and redox materials exhibiting satisfactory durability, reactivity and efficiencies. Moreover, materials play an important role in the construction of key components and for the implementation in commercial solar plants. The most promising thermochemical water- and CO₂-splitting processes are being described and discussed with respect to further development and future potential. The main materials-related challenges of those processes are being analyzed. Technical approaches and development progress in terms of solving them are addressed and assessed in this review. Full article

(This article belongs to the Special Issue Advanced Materials for Water-Splitting)

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1282 KiB

Open AccessReview

Controlling Morphological Parameters of Anodized Titania Nanotubes for Optimized Solar Energy Applications

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Andrew Haring

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Amanda Morris

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Michael Hu

Materials 2012, 5(10), 1890-1909; https://doi.org/10.3390/ma5101890 - 19 Oct 2012

Cited by 59 | Viewed by 7601

Abstract

Anodized TiO₂ nanotubes have received much attention for their use in solar energy applications including water oxidation cells and hybrid solar cells [dye-sensitized solar cells (DSSCs) and bulk heterojuntion solar cells (BHJs)]. High surface area allows for increased dye-adsorption and photon absorption. [...] Read more.

Anodized TiO₂ nanotubes have received much attention for their use in solar energy applications including water oxidation cells and hybrid solar cells [dye-sensitized solar cells (DSSCs) and bulk heterojuntion solar cells (BHJs)]. High surface area allows for increased dye-adsorption and photon absorption. Titania nanotubes grown by anodization of titanium in fluoride-containing electrolytes are aligned perpendicular to the substrate surface, reducing the electron diffusion path to the external circuit in solar cells. The nanotube morphology can be optimized for the various applications by adjusting the anodization parameters but the optimum crystallinity of the nanotube arrays remains to be realized. In addition to morphology and crystallinity, the method of device fabrication significantly affects photon and electron dynamics and its energy conversion efficiency. This paper provides the state-of-the-art knowledge to achieve experimental tailoring of morphological parameters including nanotube diameter, length, wall thickness, array surface smoothness, and annealing of nanotube arrays. Full article

(This article belongs to the Special Issue Advanced Materials for Water-Splitting)

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582 KiB

Open AccessReview

Hydrogen

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John O’M. Bockris

Materials 2011, 4(12), 2073-2091; https://doi.org/10.3390/ma4122073 - 30 Nov 2011

Cited by 10 | Viewed by 7935

Abstract

The idea of a “Hydrogen Economy” is that carbon containing fuels should be replaced by hydrogen, thus eliminating air pollution and growth of CO₂ in the atmosphere. However, storage of a gas, its transport and reconversion to electricity doubles the cost of [...] Read more.

The idea of a “Hydrogen Economy” is that carbon containing fuels should be replaced by hydrogen, thus eliminating air pollution and growth of CO₂ in the atmosphere. However, storage of a gas, its transport and reconversion to electricity doubles the cost of H₂ from the electrolyzer. Methanol made with CO₂ from the atmosphere is a zero carbon fuel created from inexhaustible components from the atmosphere. Extensive work on the splitting of water by bacteria shows that if wastes are used as the origin of feed for certain bacteria, the cost for hydrogen becomes lower than any yet known. The first creation of hydrogen and electricity from light was carried out in 1976 by Ohashi et al. at Flinders University in Australia. Improvements in knowledge of the structure of the semiconductor-solution system used in a solar breakdown of water has led to the discovery of surface states which take part in giving rise to hydrogen (Khan). Photoelectrocatalysis made a ten times increase in the efficiency of the photo production of hydrogen from water. The use of two electrode cells; p and n semiconductors respectively, was first introduced by Uosaki in 1978. Most photoanodes decompose during the photoelectrolysis. To avoid this, it has been necessary to create a transparent shield between the semiconductor and its electronic properties and the solution. In this way, 8.5% at 25 °C and 9.5% at 50 °C has been reached in the photo dissociation of water (GaP and InAs) by Kainthla and Barbara Zeleney in 1989. A large consortium has been funded by the US government at the California Institute of Technology under the direction of Nathan Lewis. The decomposition of water by light is the main aim of this group. Whether light will be the origin of the post fossil fuel supply of energy may be questionable, but the maximum program in this direction is likely to come from Cal. Tech. Full article

(This article belongs to the Special Issue Advanced Materials for Water-Splitting)

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533 KiB

Open AccessReview

Manganese-based Materials Inspired by Photosynthesis for Water-Splitting

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Harvey J.M. Hou

Materials 2011, 4(10), 1693-1704; https://doi.org/10.3390/ma4101693 - 28 Sep 2011

Cited by 61 | Viewed by 11666

Abstract

In nature, the water-splitting reaction via photosynthesis driven by sunlight in plants, algae, and cyanobacteria stores the vast solar energy and provides vital oxygen to life on earth. The recent advances in elucidating the structures and functions of natural photosynthesis has provided firm [...] Read more.

In nature, the water-splitting reaction via photosynthesis driven by sunlight in plants, algae, and cyanobacteria stores the vast solar energy and provides vital oxygen to life on earth. The recent advances in elucidating the structures and functions of natural photosynthesis has provided firm framework and solid foundation in applying the knowledge to transform the carbon-based energy to renewable solar energy into our energy systems. In this review, inspired by photosynthesis robust photo water-splitting systems using manganese-containing materials including Mn-terpy dimer/titanium oxide, Mn-oxo tetramer/Nafion, and Mn-terpy oligomer/tungsten oxide, in solar fuel production are summarized and evaluated. Potential problems and future endeavors are also discussed. Full article

(This article belongs to the Special Issue Advanced Materials for Water-Splitting)

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