Richard Eisenberg

Catalysts made of earth-abundant elements (Co, Ni, Fe) for water splitting: Recent progress and future challenges

This article reviews recent significant advances in the field of water splitting. Catalysts play very important roles in two half reactions of water splitting - water reduction and water oxidation. Considering potential future applications, catalysts made of cheap and earth abundant element(s) are especially important for economically viable energy conversion. This article focuses only on catalysts made of cobalt (Co), nickel (Ni) and iron (Fe) elements for water reduction and water oxidation. Different series of catalysts that can be applied in electrocatalytic and photocatalytic water spitting are discussed in detail and their catalytic mechanisms are introduced. Finally, the future outlook and perspective of catalysts made of earth abundant elements will be discussed.

Robust Photogeneration of H2 in Water Using Semiconductor Nanocrystals and a Nickel Catalyst

Robust Reduction A major challenge in the design of artificial photosynthesis catalysts has been their instability under the reaction conditions—a problem that plants and other autotrophs address by perpetually reproducing their biochemical machinery. Han et al. (p. 1321, published online 8 November) now demonstrate a system for photoreductive hydrogen generation in water that manifests undiminished activity for weeks at a time. Semiconductor nanoparticles for light absorption were combined with a soluble nickel complex for the catalytic chemistry. The system currently requires a sacrificial electron donor, but its robustness shows promise for future pairing with an integrated oxidation catalyst. A photoreduction system combining nanoparticulate light absorbers with a soluble molecular catalyst proves stable for weeks. Homogeneous systems for light-driven reduction of protons to H2 typically suffer from short lifetimes because of decomposition of the light-absorbing molecule. We report a robust and highly active system for solar hydrogen generation in water that uses CdSe nanocrystals capped with dihydrolipoic acid (DHLA) as the light absorber and a soluble Ni2+-DHLA catalyst for proton reduction with ascorbic acid as an electron donor at pH = 4.5, which gives >600,000 turnovers. Under appropriate conditions, the precious-metal–free system has undiminished activity for at least 360 hours under illumination at 520 nanometers and achieves quantum yields in water of over 36%.

Making Hydrogen from Water Using a Homogeneous System Without Noble Metals

A photocatalytic noble metal-free system for the generation of hydrogen has been constructed using Eosin Y (1) as a photosensitizer, the complex [Co(dmgH)(2)pyCl](2+) (5, dmgH = dimethylglyoximate, py = pyridine) as a molecular catalyst, and triethanolamine (TEOA) as a sacrificial reducing agent. The system produces H(2) with an initial rate of approximately 100 turnovers per hour upon irradiation with visible light (lambda > 450 nm). Addition of free dmgH(2) greatly increases the durability of the system addition of 12 equiv of dmgH(2) (vs cobalt) to the system produces approximately 900 turnovers of H(2) after 14 h of irradiation. The rate of H(2) evolution is maximum at pH = 7 and decreases sharply at more acidic or basic pH. Spectroscopic study of photolysis solutions suggests that hydrogen production occurs through protonation of a Co(I) species to give a Co(III) hydride, which then reacts further by reduction and protolysis to give Co(II) and molecular hydrogen.

A Homogeneous System for the Photogeneration of Hydrogen from Water Based on a Platinum(II) Terpyridyl Acetylide Chromophore and a Molecular Cobalt Catalyst

The complex [Co(dmgH)2pyCl]2+ (1, dmgH = dimethylglyoximate, py = pridine) has been used as a molecular catalyst for visible light driven hydrogen production in the presence of [Pt(tolylterpyridine)(phenylacetylide)]+ (3) as a photosensitizer and triethanolamine (TEOA) as a sacrificial reducing agent. Complex 3 is quenched oxidatively by [Co(dmgH)pyCl]2+ (1) with a rate constant kq of 1.27 x 10(9) M(-1) s(-1). Photogeneration of H2 is only seen when 1 + 3 + TEOA are all present. H2 production is maximized for this system at pH 8.5 and declines to very low levels at pH < 7 and pH > 12. Irradiation of the reaction solution initially containing 1.61 x 10(-2) M TEOA, 1.11 x 10(-5) M of 3, and 1.99 x 10(-4) M of Co catalyst 1 in MeCN/water (3:2 v/v) at pH = 8.5 for 10 h with lambda > 410 nm yields 400 turnovers of H2. When TEOA is 0.27 M, approximately 1000 turnovers are obtained after 10 h of irradiation. Spectroscopic study of the photolyses solutions suggests that H2 formation proceeds via Co(I) and protonation to form Co(III) hydride species.

Platinum diimine complexes: towards a molecular photochemical device

Abstract Complexes having the general formula PtX2(diimine) where X2=dithiolate, bis(acetylide) and diimine=bipyridine, phenanthroline and derivatives have been investigated for potential use as chromophores in the conversion of light-to-chemical energy. These complexes, like the analogous X=CN systems, are luminescent in fluid solution. Previous studies of the dithiolate derivatives reveal that they possess a charge transfer excited state involving a mixed metal–dithiolate donor orbital and a π*(diimine) acceptor function with excited state properties including emission energies, lifetimes and redox potentials that are tunable by ligand variation. The bis(acetylide) complexes are brightly luminescent in fluid solution, and their excited state is shown to be MLCT in character, consistent with an earlier proposal. Both sets of diimine complexes show evidence of self-quenching, and for Pt(phen)(CCPh)2, weak excimer emission is observed. The mechanism of quenching has been probed through cross-quenching experiments and is thought to involve Pt⋯Pt interactions. Efforts are now focussing on the use of the PtX2(diimine) chromophores in dyads and triads with the goal of constructing a molecular photochemical device for light-to-chemical energy conversion. Connection of the Pt diimine chromophores to both a donor or reductive quencher and an acceptor is envisioned through new ligand bridges currently being synthesized using Pd-catalyzed coupling reactions and carbonyl condensations.

Preface on Making Oxygen

Imagine a planet whose atmosphere is irreversibly changed by the dominant lifeform inhabiting it. The lifeform releases a gas--a waste gas--trillions and trillions of tons of it. The atmosphere becomes so altered that evolution on the planet is forever changed. Science fiction? Not at all! The planet is Earth, but the waste gas is not carbon dioxide and the time is not now. Instead, the unwanted gas is oxygen, the subject of this issue’s Forum, and the time is long, long ago. On primordial Earth, the atmosphere was reducing, most likely made up mainly of nitrogen, methane, ammonia, and water vapor. While oxygen is the most abundant element in the Earth’s crust, it did not exist in the atmosphere in primordial times to even a small fraction of the extent that it does today.

Reductive Side of Water Splitting in Artificial Photosynthesis: New Homogeneous Photosystems of Great Activity and Mechanistic Insight

Rhodamine photosensitizers (PSs) substituting S or Se for O in the xanthene ring give turnover numbers (TONs) as high as 9000 for the generation of hydrogen via the reduction of water using [Co(III)(dmgH)(2)(py)Cl] (where dmgH = dimethylglyoximate and py = pyridine) as the catalyst and triethanolamine as the sacrificial electron donor. The turnover frequencies were 0, 1700, and 5500 mol H(2)/mol PS/h for O, S, and Se derivatives, respectively (Φ(H(2)) = 0%, 12.2%, and 32.8%, respectively), which correlates well with relative triplet yields estimated from quantum yields for singlet oxygen generation. Phosphorescence from the excited PS was quenched by the sacrificial electron donor. Fluorescence lifetimes were similar for the O- and S-containing rhodamines (∼2.6 ns) and shorter for the Se analog (∼0.1 ns). These data suggest a reaction pathway involving reductive quenching of the triplet excited state of the PS giving the reduced PS(-) that then transfers an electron to the Co catalyst. The longer-lived triplet state is necessary for effective bimolecular electron transfer. While the cobalt/rhodamine/triethanolamine system gives unprecedented yields of hydrogen for the photoreduction of water, mechanistic insights regarding the overall reaction pathway as well as system degradation offer significant guidance to developing even more stable and efficient photocatalytic systems.

A cobalt-dithiolene complex for the photocatalytic and electrocatalytic reduction of protons.

The complex [Co(bdt)(2)](-) (where bdt = 1,2-benzenedithiolate) is an active catalyst for the visible light driven reduction of protons from water when employed with Ru(bpy)(3)(2+) as the photosensitizer and ascorbic acid as the sacrificial electron donor. At pH 4.0, the system exhibits very high activity, achieving >2700 turnovers with respect to catalyst and an initial turnover rate of 880 mol H(2)/mol catalyst/h. The same complex is also an active electrocatalyst for proton reduction in 1:1 CH(3)CN/H(2)O in the presence of weak acids, with the onset of a catalytic wave at the reversible redox couple of -1.01 V vs Fc(+)/Fc. The cobalt-dithiolene complex [Co(bdt)(2)](-) thus represents a highly active catalyst for both the electrocatalytic and photocatalytic reduction of protons in aqueous solutions.

Luminescent platinum complexes: tuning and using the excited state

Abstract The focus of recent research on square planar Pt(II) diimine dithiolate complexes has been to understand molecular factors that influence their excited state properties and to develop diad and triad systems based on them for use in light-driven reactions. Regarding the former, two series of Pt(diimine)(dithiolate) complexes have been synthesized and studied. All of the compounds display solvatochromic absorption bands and solution luminescence attributable to metal/dithiolate-to-diimine charge transfer excited states of the same orbital parentage. The excited-state energies can be tuned by approximately 1 eV through ligand variation. Excited-state redox potentials have been estimated for all of the complexes from spectroscopic and electrochemical data, and electron transfer quenching rate constants show the expected driving force dependence. Analogous Au(III) systems have been synthesized and characterized including molecular structure determinations of a cationic diimine dithiolate system and a neutral C-deprotonated-2-phenylpyridine derivative. Striking differences exist in the electronic structures of these Au(III) complexes from those of the Pt(II) systems, underscoring the key role of the metal in the excited state structure of the latter. The creation of diads and triads is being undertaken with ligand bridges capable of connecting the Pt(diimine)(dithiolate) moiety with other metal centers. Toward that end, complexes of dipyridocatecholate (dpcat) have been synthesized and characterized. These complexes may serve as models for the linking of chromophore and quencher components of a possible photosynthetic system. The dpcat complexes have been characterized by absorption and steady-state emission spectroscopies. Luminescence and redox properties of these and a related system containing a tetrapyridophenazine (tppz) bridge are described.

Fuel from Water: The Photochemical Generation of Hydrogen from Water

Hydrogen has been labeled the fuel of the future since it contains no carbon, has the highest specific enthalpy of combustion of any chemical fuel, yields only water upon complete oxidation, and is not limited by Carnot considerations in the amount of work obtained when used in a fuel cell. To be used on the scale needed for sustainable growth on a global scale, hydrogen must be produced by the light-driven splitting of water into its elements, as opposed to reforming of methane, as is currently done. The photochemical generation of H2, which is the reductive side of the water splitting reaction, is the focus of this Account, particularly with regard to work done in the senior authors laboratory over the last 5 years. Despite seminal work done more than 30 years ago and the extensive research conducted since then on all aspects of the process, no viable system has been developed for the efficient and robust photogeneration of H2 from water using only earth abundant elements. For the photogeneration of H2 from water, a system must contain a light absorber, a catalyst, and a source of electrons. In this Account, the discovery and study of new Co and Ni catalysts are described that suggest H2 forms via a heterocoupling mechanism from a metal-hydride and a ligand-bound proton. Several complexes with redox active dithiolene ligands are newly recognized to be effective in promoting the reaction. A major new development in the work described is the use of water-soluble CdSe quantum dots (QDs) as light absorbers for H2 generation in water. Both activity and robustness of the most successful systems are impressive with turnover numbers (TONs) approaching 10(6), activity maintained over 15 days, and a quantum yield for H2 of 36% with 520 nm light. The water solubilizing capping agent for the first system examined was dihydrolipoic acid (DHLA) anion, and the catalyst was determined to be a DHLA complex of Ni(II) formed in situ. Dissociation of DHLA from the QD surface proved problematic in assessing other catalysts and stimulated the synthesis of tridentate trithiolate (S3) capping agents that are inert to dissociation. In this way, CdSe QDs having these S3 capping agents were used in systems for the photogeneration of H2 that allowed meaningful comparison of the relative activity of different catalysts for the light-driven production of H2 from water. This new chemistry also points the way to the development of new photocathodes based on S3-capped QDs for removal of the chemical sacrificial electron donor and its replacement electrochemically in photoelectrosynthetic cells.