Showing papers on "Artificial photosynthesis published in 2008"

Catalysis: the art of splitting water.

Abstract: Plants produce oxygen from water, but the same chemical reaction is hard to achieve synthetically. A new family of catalysts could breathe fresh life into the quest for artificial photosynthesis.

Bioinspired Energy Conversion Systems for Hydrogen Production and Storage

Abstract: Recent developments in photocatalytic hydrogen production by using artificial photosynthesis systems is described, together with those in hydrogen storage through the fixation of CO2 with H2. Hydrogen can be stored in the form of formic acid, which can be converted back to H2 in the presence of an appropriate catalyst. Electron donor–acceptor dyads are utilized as efficient photocatalysts to reduce methyl viologen (MV2+) by NADH (β-nicotinamide adenine dinucleotide, reduced form) analogues to produce the methyl violgen radical cation that acts as an electron mediator for the production of hydrogen. Porphyrin-monolayer-protected gold clusters that enhance the light harvesting efficiency can also be used for the photocatalytic reduction of methyl viologen by NADH analogues. The use of a simple electron donor–acceptor dyad, the 9-mesityl-10-methylacridinium ion (Acr+–Mes), enables the construction of a highly efficient photocatalytic hydrogen-evolution system without an electron mediator such as MV2+, with poly(N-vinyl-2-pyrrolidone)-protected platinum nanoclusters (Pt–PVP) and NADH as a hydrogen-evolution catalyst and an electron donor, respectively. Hydrogen thus produced can be stored in the form of formic acid (liquid) by fixation of CO2 with H2 in water by using ruthenium aqua complexes [RuII(η6-C6Me6)(L)(OH2)]2+ [L = 2,2′-bipyridine (bpy), 4,4′-dimethoxy-2,2′-bipyridine (4,4′-OMe-bpy)] and iridium aqua complexes [IrIIICp*(L)(OH2)]2+ (Cp* = η5-C5Me5, L = bpy, 4,4′-OMe-bpy) as catalysts at pH 3.0. Catalytic systems for the decomposition of HCOOH to H2 are also described. The combination of photocatalytic hydrogen generation with the catalytic fixation of CO2 with H2 and the decomposition of HCOOH back to H2 provides an excellent system for cutting CO2 emission.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Evidence for a terminal Pt(iv)-oxo complex exhibiting diverse reactivity

Abstract: Many important biological and chemical processes, including photocatalytic water oxidation to molecular oxygen (of interest as a route to artificial photosynthesis) and the activation of dioxygen on metal surfaces, are thought to involve transition metal terminal oxo complexes. Poverenov et al. now report the synthesis of a platinum based oxidizing reagent with potentially useful characteristics. It's a d6 Pt(IV) terminal oxo complex that is not stabilized by an electron accepting ligand framework, and so exhibits reactivity as both an inter- and intra-molecular oxygen donor and as an electrophile. It also undergoes water activation to produce a terminal dihydroxo complex, which may be of relevance to the mechanism of water oxidation and other catalytic reactions. Terminal oxo complexes of transition metals are important in biological and chemical processes, for example, the catalytic oxidation of organic molecules and the activation of dioxygen on metal surfaces are thought to involve oxo complexes. This paper explored the reactivity of a d6 Pt(IV) complex, a dn (n > 5) terminal oxo complex that is not stabilized by an electron withdrawing ligand framework. The complex exhibits reactivity as an inter- and intra-molecular oxygen donor and as an electrophile. Terminal oxo complexes of transition metals have critical roles in various biological and chemical processes1,2. For example, the catalytic oxidation of organic molecules3,4, some oxidative enzymatic transformations5,6,7, and the activation of dioxygen on metal surfaces8 are all thought to involve oxo complexes. Moreover, they are believed to be key intermediates in the photocatalytic oxidation of water to give molecular oxygen, a topic of intensive global research aimed at artificial photosynthesis and water splitting9,10,11,12,13. The terminal oxo ligand is a strong π-electron donor, so it readily forms stable complexes with high-valent early transition metals. As the d orbitals are filled up with valence electrons, the terminal oxo ligand becomes destabilized2. Here we present evidence for a dn (n > 5) terminal oxo complex that is not stabilized by an electron withdrawing ligand framework. This d6 Pt(iv) complex exhibits reactivity as an inter- and intramolecular oxygen donor and as an electrophile. In addition, it undergoes a water activation process leading to a terminal dihydroxo complex, which may be relevant to the mechanism of catalytic reactions such as water oxidation.

Mediator-assisted water oxidation by the ruthenium “blue dimer” cis,cis-[(bpy)2(H2O)RuORu(OH2)(bpy)2]4+

Abstract: Light-driven water oxidation occurs in oxygenic photosynthesis in photosystem II and provides redox equivalents directed to photosystem I, in which carbon dioxide is reduced. Water oxidation is also essential in artificial photosynthesis and solar fuel-forming reactions, such as water splitting into hydrogen and oxygen (2 H2O + 4 hν → O2 + 2 H2) or water reduction of CO2 to methanol (2 H2O + CO2 + 6 hν → CH3OH + 3/2 O2), or hydrocarbons, which could provide clean, renewable energy. The “blue ruthenium dimer,” cis,cis-[(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+, was the first well characterized molecule to catalyze water oxidation. On the basis of recent insight into the mechanism, we have devised a strategy for enhancing catalytic rates by using kinetically facile electron-transfer mediators. Rate enhancements by factors of up to ≈30 have been obtained, and preliminary electrochemical experiments have demonstrated that mediator-assisted electrocatalytic water oxidation is also attainable.

Computational study of the lowest triplet state of ruthenium polypyridyl complexes used in artificial photosynthesis.

Abstract: The potential energy surfaces of the first excited triplet state of some ruthenium polypyridyl complexes were investigated by means of density functional theory. Focus was placed on the interaction between the geometrical changes accompanying the photoactivity of these complexes when used as antenna complexes in artificial photosynthesis and dye-sensitized solar cells and the accompanying changes in electronic structure. The loss process (3)MLCT --> (3)MC can be understood by means of ligand-field splitting, traced down to the coordination of the central ruthenium atom.

Coupled electron transfers in artificial photosynthesis.

Abstract: Light-induced charge separation in molecular assemblies has been widely investigated in the context of artificial photosynthesis. Important progress has been made in the fundamental understanding of electron and energy transfer and in stabilizing charge separation by multi-step electron transfer. In the Swedish Consortium for Artificial Photosynthesis, we build on principles from the natural enzyme photosystem II and Fe-hydrogenases. An important theme in this biomimetic effort is that of coupled electron-transfer reactions, which have so far received only little attention. (i) Each absorbed photon leads to charge separation on a single-electron level only, while catalytic water splitting and hydrogen production are multi-electron processes; thus there is the need for controlling accumulative electron transfer on molecular components. (ii) Water splitting and proton reduction at the potential catalysts necessarily require the management of proton release and/or uptake. Far from being just a stoichiometric requirement, this controls the electron transfer processes by proton-coupled electron transfer (PCET). (iii) Redox-active links between the photosensitizers and the catalysts are required to rectify the accumulative electron-transfer reactions, and will often be the starting points of PCET.

Oxygen catalysis: The other half of the equation.

Abstract: Artificial photosynthesis — splitting water with light — is an attractive way to make hydrogen, but what happens to the oxygen? A catalyst that aids in the efficient production of gaseous oxygen improves the viability of this approach.

Carbon and fuel production from atmospheric CO2 and H2O by artificial photosynthesis and method of operation thereof

Abstract: The present invention relates generally to reduction of atmospheric carbon dioxide and to production of carbon therefrom for further use as, for example, fuel and morespecifically, to the process of dissolving atmospheric carbon dioxide into a suitable preferably alkali metal salt flux for electrolysis thereof into carbon and oxygen.

Mechanism of the Palladium‐Catalyzed Carbohydroxylation of Allene‐Substituted Conjugated Dienes: Rationalization of the Recently Observed Nucleophilic Attack by Water on a (π‐Allyl)palladium Intermediate

Abstract: This thesis describes the development and study of catalysts for redox reactions, which either utilize oxygen or hydrogen peroxide for the purpose of selectively oxidizing organic substrates, or produce oxygen as the necessary byproduct in the production of hydrogen by artificial photosynthesis.The first chapter gives a general introduction about the use of environmentally friendly oxidants in the field of organic synthesis, and about the field of artificial photosynthesis. The second chapter describes a computational study of the mechanism of palladium-catalyzed oxidative carbohydroxylation of allene-substituted conjugated dienes. The proposed mechanism, which was supported by DFT calculations, involves an unusual water attack on a (π-allyl)palladium complex. The third chapter describes a computational study of the oxidation of unfunctionalized hydrocarbons, ethers and alcohols with hydrogen peroxide, catalyzed by methyltrioxorhenium (MTO). The mechanism was found to proceed via rate-limiting hydride abstraction followed by hydroxide transfer in a single concerted, but highly asynchronous, step as shown by intrinsic reaction coordinate (IRC) scans. The fourth chapter describes the use of a new hybrid (hydroquinone-Schiff base)cobalt catalyst as electron transfer mediator (ETM) in the palladium-catalyzed aerobic carbocyclization of enallenes. Covalently linking the two ETMs gave a fivefold rate increase compared to the use of separate components. The fifth chapter describes an improved synthetic route to the (hydroquinone-Schiff base)cobalt catalysts. Preparation of the key intermediate 5-(2,5-hydroxyphenyl)salicylaldehyde was improved by optimization of the key Suzuki coupling and change of protecting groups from methyl ethers to easily cleaved THP groups. The catalysts could thus be prepared in good overall yield from inexpensive starting materials.Finally, the sixth chapter describes the preparation and study of two catalysts for water oxidation, both based on ligands containing imidazole groups, analogous to the histidine residues present in the oxygen evolving complex (OEC) and in many other metalloenzymes. The first, ruthenium-based, catalyst was found to catalyze highly efficient water oxidation induced by visible light. The second catalyst is, to the best of our knowledge, the first homogeneous manganese complex to catalyze light-driven water oxidation.

Chlorin-sensitized High-efficiency Photovoltaic Cells that Mimic Spectral Response of Photosynthesis

Abstract: Chlorin e6, a metal-free hydrophilic derivative of chlorophyll, was used as a sensitizer of mesoporous TiO2 film to construct a high-efficiency solar cell as a model for artificial photosynthesis. The cell exhibited wide spectral responsivity in the visible light region that resembles action spectrum of photosynthesis. Optimization of dye adsorption method by using co-adsorbing surfactant agents that suppress intermolecular aggregation of the dye achieved high energy conversion efficiency up to 4.3% under simulated sunlight of 100 mW cm−2 intensity.

Bioinspired Energy Conversion Systems for Hydrogen Production and Storage

Abstract: Recent developments in photocatalytic hydrogen production by using artificial photosynthesis systems is described, together with those in hydrogen storage through the fixation of CO2 with H2. Hydrogen can be stored in the form of formic acid, which can be converted back to H2 in the presence of an appropriate catalyst. Electron donor–acceptor dyads are utilized as efficient photocatalysts to reduce methyl viologen (MV2+) by NADH (β-nicotinamide adenine dinucleotide, reduced form) analogues to produce the methyl violgen radical cation that acts as an electron mediator for the production of hydrogen. Porphyrin-monolayer-protected gold clusters that enhance the light harvesting efficiency can also be used for the photocatalytic reduction of methyl viologen by NADH analogues. The use of a simple electron donor–acceptor dyad, the 9-mesityl-10-methylacridinium ion (Acr+–Mes), enables the construction of a highly efficient photocatalytic hydrogen-evolution system without an electron mediator such as MV2+, with poly(N-vinyl-2-pyrrolidone)-protected platinum nanoclusters (Pt–PVP) and NADH as a hydrogen-evolution catalyst and an electron donor, respectively. Hydrogen thus produced can be stored in the form of formic acid (liquid) by fixation of CO2 with H2 in water by using ruthenium aqua complexes [RuII(η6-C6Me6)(L)(OH2)]2+ [L = 2,2′-bipyridine (bpy), 4,4′-dimethoxy-2,2′-bipyridine (4,4′-OMe-bpy)] and iridium aqua complexes [IrIIICp*(L)(OH2)]2+ (Cp* = η5-C5Me5, L = bpy, 4,4′-OMe-bpy) as catalysts at pH 3.0. Catalytic systems for the decomposition of HCOOH to H2 are also described. The combination of photocatalytic hydrogen generation with the catalytic fixation of CO2 with H2 and the decomposition of HCOOH back to H2 provides an excellent system for cutting CO2 emission.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Designing photosystem II: molecular engineering of photo-catalytic proteins

Abstract: Biological photosynthesis utilizes membrane-bound pigment/protein complexes to convert light into chemical energy through a series of electron-transfer events. In the unique photosystem II (PSII) complex these electron-transfer events result in the oxidation of water to molecular oxygen. PSII is an extremely complex enzyme and in order to exploit its unique ability to convert sunlight into chemical energy it will be necessary to make a minimal model. Here we will briefly describe how PSII functions and identify those aspects that are essential in order to catalyze the oxidation of water into O(2), and review previous attempts to design simple photo-catalytic proteins and summarize our current research exploiting the E. coli bacterioferritin protein as a scaffold into which multiple cofactors can be bound, to oxidize a manganese metal center upon illumination. Through the reverse engineering of PSII and light driven water splitting reactions it may be possible to provide a blueprint for catalysts that can produce clean green fuel for human energy needs.

Catalytic Reactions Using Transition-Metal-Complexes Toward Solar Fuel Generation

Abstract: Solar energy is a more abundant source of energy than any other source. Many researchers consider solar generation of fuels (stored in the form of chemical bonds such as hydrogen from water and methanol from CO2) as the best and essential solution for saving the Earth, yet sunlight-driven water splitting or CO2 reduction to methanol/ methane remains a formidable problem. In this review we present our personal views about what we need to pursue as chemists who are interested in coordination chemistry and theoretical chemistry in the upcoming decade in order to develop efficient and robust catalysts for water splitting and CO2 reduction, search for sustainable pathways for energy generation, and contribute to the most important yet challenging problem facing the world in this century. We focus our discussion on the following three subjects: (1) Water oxidation by ruthenium molecular catalysts instead of the manganese O2-forming clusters in Photosystem II; (2) H2 production and oxidation by catalysts containing inexpensive metals; and (3) Photogeneration of renewable hydride donors, carriers of a proton and two electrons, to replace the NADP+/NADPH coenzyme for the reduction of CO2 and related species.

Artificial tongues and leaves

Abstract: The objective with synthetic multifunctional nanoarchitecture is to create large suprastructures with interesting functions. For this purpose, lipid bilayer membranes or conducting surfaces have been used as platforms and rigid-rod molecules as shape-persistent scaffolds. Examples for functions obtained by this approach include pores that can act as multicomponent sensors in complex matrices or rigid-rod π-stack architecture for artificial photosynthesis and photovoltaics.

An Attractive Strategy for Solar Energy Conversion: An Oxygen Fuel from Weter

Abstract: A carbon-free fuel can be generated from water by combination of artificial photosynthesis and nanotechnology, in the process of which the main body changed is oxygen atom in water, suggesting that this is an oxygen fuel. The energy stored in the fuel is a form of potential energy that could be obtained by changing the numbers, relative positions and arrangements of atoms within a given substance. Therefore, water molecules through redox reactions are capable of associating in structure with methylene carbene, a building block of hydrocarbon. The building blocks of oxygen fuel are isoelectronic species of methylene carbene that were synthesized by means of the modified photosynthesis, and oxygen fuel is the nanostructured assemblies on nanometer scales that were assembled by means of nanotechnology. The combustion products of oxygen fuel are water and isoelectronic species of carbon dioxide that will be converted into oxygen and/or ozone under the action of solar energy.