Molecular tuning of CO2-to-ethylene conversion.

The electrocatalytic reduction of carbon dioxide, powered by renewable electricity, to produce valuable fuels and feedstocks provides a sustainable and carbon-neutral approach to the storage of energy produced by intermittent renewable sources1. However, the highly selective generation of economically desirable products such as ethylene from the carbon dioxide reduction reaction (CO2RR) remains a challenge2. Tuning the stabilities of intermediates to favour a desired reaction pathway can improve selectivity3-5, and this has recently been explored for the reaction on copper by controlling morphology6, grain boundaries7, facets8, oxidation state9 and dopants10. Unfortunately, the Faradaic efficiency for ethylene is still low in neutral media (60 per cent at a partial current density of 7 milliamperes per square centimetre in the best catalyst reported so far9), resulting in a low energy efficiency. Here we present a molecular tuning strategy-the functionalization of the surface of electrocatalysts with organic molecules-that stabilizes intermediates for more selective CO2RR to ethylene. Using electrochemical, operando/in situ spectroscopic and computational studies, we investigate the influence of a library of molecules, derived by electro-dimerization of arylpyridiniums11, adsorbed on copper. We find that the adhered molecules improve the stabilization of an 'atop-bound' CO intermediate (that is, an intermediate bound to a single copper atom), thereby favouring further reduction to ethylene. As a result of this strategy, we report the CO2RR to ethylene with a Faradaic efficiency of 72 per cent at a partial current density of 230 milliamperes per square centimetre in a liquid-electrolyte flow cell in a neutral medium. We report stable ethylene electrosynthesis for 190 hours in a system based on a membrane-electrode assembly that provides a full-cell energy efficiency of 20 per cent. We anticipate that this may be generalized to enable molecular strategies to complement heterogeneous catalysts by stabilizing intermediates through local molecular tuning.

Pyridinethiolate-Capped CdSe Quantum Dots for Red-Light-Driven H2 Production in Water.

The utilization of low-energy sunlight to produce renewable fuels is a subject of great interest. Here we report the first example of metal chalcogenide quantum dots (QDs) capped with a pyridinethiolate carboxylic acid (pyS-COOH) for red-light-driven H2 production in water. The precious-metal-free system is robust over 240 h, and achieves a turnover number (TON) of 43910 ± 305 (vs Ni) with a rate of 31570 ± 1690 mmol g-1 h-1 for hydrogen production. In contrast to the inactive QDs capped with other thiolate ligands, the CdSe-pyS-COOH QDs give a significantly higher singlet oxygen quantum yield [ΦΔ (1O2)] in solution.

Molecular Nickel Thiolate Complexes for Electrochemical Reduction of CO2 to C1-3 Hydrocarbons.

We report the unprecedented electrocatalytic activity of a series of molecular nickel thiolate complexes (1-5) in reducing CO2 to C1-3 hydrocarbons on carbon paper in pH-neutral aqueous solutions. Ni(mpo)2 (3, mpo=2-mercaptopyridyl-N-oxide), Ni(pyS)3 - (4, pyS=2-mercaptopyridine), and Ni(mp)2 - (5, mp=2-mercaptophenolate) were found to generate C3 products from CO2 for the first time in molecular complex. Compound 5 exhibits Faradaic efficiencies (FEs) of 10.6 %, 7.2 %, 8.2 % for C1 , C2 , C3 hydrocarbons respectively at -1.0 V versus the reversible hydrogen electrode. Addition of CO to the system significantly promotes the FEC1-C3 to 41.1 %, suggesting that a key Ni-CO intermediate is associated with catalysis. A variety of spectroscopies have been performed to show that the structures of nickel complexes remain intact during CO2 reduction.

Combination of Organic Dye and Iron for CO2 Reduction with Pentanuclear Fe2Na3 Purpurin Photocatalysts.

Molecular photocatalysts designed with earth-abundant elements are rare and challenging in artificial photosynthesis study. Herein, we report a multimetallic Fe2Na3 purpurin (1) complex for the reduction of CO2 in DMF under visible-light irradiation. The photocatalytic system achieves 91% selectivity and 2625 ± 334 turnovers of CO in 120 h, which is among the highest reported for a noble-metal-free catalyst without an additional photosensitizer. UV-vis and electrochemical studies suggest that the mechanism involves subsequent reductions and protonations of 1 to generate [FeII2Na3((H)2PP)6]5- and [FeIII2Na3((H)2PP)6]3- as the active photocatalysts in CO2 reduction.

Efficient Red-Light-Driven Hydrogen Evolution with an Anthraquinone Organic Dye.

The direct utilization of the full solar spectrum to obtain renewable fuels remains a challenge because the conversion of the low-energy light (red and near-infrared) is difficult. Current light-driven systems show activity for hydrogen generation with the high-energy part of sunlight. Here we report the use of a simple anthraquinone organic dye in an artificial photosynthetic system that promotes efficient red-light-driven production of hydrogen. The system contains no noble metal and exhibits a turnover number greater than 0.78 million and a quantum yield of 30.6% at 630 nm. A mechanistic study revealed that the excited-state and redox properties of the chromophore are critical to achieving high activity and stability.

Improving Photocatalytic Hydrogen Production through Incorporating Copper to Organic Photosensitizers.

Organic dyes have been investigated extensively as promising photosensitizers in noble-metal-free photocatalytic systems for hydrogen production. However, other than functional group optimization, there are very few methods reported to be effective in improving their photocatalytic activity. Herein, we report the incorporation of Cu2+ into purpurin and gallein dyes for visible-light-driven hydrogen production. These Cu-dye chromophores significantly promote the photocatalytic activity of homogeneous systems when paired with a series of molecular Ni or Fe catalysts. Under optimal conditions, the Cu-purpurin and Cu-gallein photosensitizers exhibit more than 20-fold increases in turnover frequencies for hydrogen evolution when compared with purpurin and gallein. Catalytic systems with the Cu-purpurin chromophore show no decrease in activity over 120 h. Based on electrochemical and fluorescence quenching experiments, the enhancement of photocatalytic activity is likely due to the fact that Cu2+ can facilitate the transfer of electrons from the photosensitizers to the catalysts through creating highly reducing centers.

Tetranuclear Manganese Models of the OEC Displaying Hydrogen Bonding Interactions: Application to Electrocatalytic Water Oxidation to Hydrogen Peroxide.

Toward the development of structural and functional models of the oxygen evolving complex (OEC) of photosystem II, we report the synthesis of site-differentiated tetranuclear manganese complexes featuring three six-coordinate and one five-coordinate Mn centers. To incorporate biologically relevant second coordination sphere interactions, substituents capable of hydrogen bonding are included as pyrazolates with arylamine substituents. Complexes with terminal anionic ligands, OH- or Cl-, bound to the lower coordinate metal center are supported through the hydrogen-bonding network in a fashion reminiscent of the enzymatic active site. The hydroxide complex was found to be a competent electrocatalyst for O-O bond formation, a key transformation pertinent to the OEC. In an acetonitrile-water mixture, at neutral pH, electrochemical water oxidation to hydrogen peroxide was observed, albeit with low (15%) Faradaic yield, likely due to competing reactions with organics. In agreement, 9,10-dihydroanthracene is electrochemically oxidized in the presence of this cluster both via H-atom abstraction and oxygenation with ∼50% combined Faradaic yield.

Photocatalytic Hydrogen Generation by CdSe/CdS Nanoparticles.

The photocatalytic hydrogen (H2) production activity of various CdSe semiconductor nanoparticles was compared including CdSe and CdSe/CdS quantum dots (QDs), CdSe quantum rods (QRs), and CdSe/CdS dot-in-rods (DIRs). With equivalent photons absorbed, the H2 generation activity orders as CdSe QDs ≫ CdSe QRs > CdSe/CdS QDs > CdSe/CdS DIRs, which is surprisingly the opposite of the electron-hole separation efficiency. Calculations of photoexcited surface charge densities are positively correlated with the H2 production rate and suggest the size of the nanoparticle plays a critical role in determining the relative efficiency of H2 production.

Photogeneration of hydrogen from water using CdSe nanocrystals demonstrating the importance of surface exchange.

Unique tripodal S-donor capping agents with an attached carboxylate are found to bind tightly to the surface of CdSe nanocrystals (NCs), making the latter water soluble. Unlike that in similarly solubilized CdSe NCs with one-sulfur or two-sulfur capping agents, dissociation from the NC surface is greatly reduced. The impact of this behavior is seen in the photochemical generation of H2 in which the CdSe NCs function as the light absorber with metal complexes in aqueous solution as the H2-forming catalyst and ascorbic acid as the electron donor source. This precious-metal-free system for H2 generation from water using [Co(bdt)2](-) (bdt, benzene-1,2-dithiolate) as the catalyst exhibits excellent activity with a quantum yield for H2 formation of 24% at 520 nm light and durability with >300,000 turnovers relative to catalyst in 60 h.

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 author's 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 QD's 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.

Cobalt-dithiolene complexes for the photocatalytic and electrocatalytic reduction of protons in aqueous solutions.

Artificial photosynthesis (AP) is a promising method of converting solar energy into fuel (H(2)). Harnessing solar energy to generate H(2) from H(+) is a crucial process in systems for artificial photosynthesis. Widespread application of a device for AP would rely on the use of platinum-free catalysts due to the scarcity of noble metals. Here we report a series of cobalt dithiolene complexes that are exceptionally active for the catalytic reduction of protons in aqueous solvent mixtures. All catalysts perform visible-light-driven reduction of protons from water when paired with Ru(bpy)(3)(2+) as the photosensitizer and ascorbic acid as the sacrificial donor. Photocatalysts with electron withdrawing groups exhibit the highest activity with turnovers up to 9,000 with respect to catalyst. The same complexes are also active electrocatalysts in 11 acetonitrile/water. The electrocatalytic mechanism is proposed to be ECEC, where the Co dithiolene catalysts undergo rapid protonation once they are reduced to CoL(2)(2-). Subsequent reduction and reaction with H(+) lead to H(2) formation. Cobalt dithiolene complexes thus represent a new group of active catalysts for the reduction of protons.

Photocatalytic CO2 reduction with iron porphyrin catalysts and anthraquinone dyes.

Herein we studied visible-light-driven CO2 reduction using a series of tetra-phenylporphyrin iron catalysts and inexpensive anthraquinone dyes. Varying the functional groups on the phenyl moieties of the catalysts significantly enhances the photocatalytic activity, achieving an optimal turnover number (TON) of 10 476 and a selectivity of 100% in the noble-metal-free systems. The highest activity found in a bromo-substituted catalyst is attributed to favorable electron transfer from the photosensitizer to the iron porphyrin.

Fluorinated chlorin chromophores for red-light-driven CO2 reduction.

The utilization of low-energy photons in light-driven reactions is an effective strategy for improving the efficiency of solar energy conversion. In nature, photosynthetic organisms use chlorophylls to harvest the red portion of sunlight, which ultimately drives the reduction of CO2. However, a molecular system that mimics such function is extremely rare in non-noble-metal catalysis. Here we report a series of synthetic fluorinated chlorins as biomimetic chromophores for CO2 reduction, which catalytically produces CO under both 630 nm and 730 nm light irradiation, with turnover numbers of 1790 and 510, respectively. Under appropriate conditions, the system lasts over 240 h and stays active under 1% concentration of CO2. Mechanistic studies reveal that chlorin and chlorinphlorin are two key intermediates in red-light-driven CO2 reduction, while corresponding porphyrin and bacteriochlorin are much less active forms of chromophores.

Nickel pyridinethiolate complexes as catalysts for the light-driven production of hydrogen from aqueous solutions in noble-metal-free systems.

A series of mononuclear nickel(II) thiolate complexes (Et4N)Ni(X-pyS)3 (Et4N = tetraethylammonium; X = 5-H (1a), 5-Cl (1b), 5-CF3 (1c), 6-CH3 (1d); pyS = pyridine-2-thiolate), Ni(pySH)4(NO3)2 (2), (Et4N)Ni(4,6-Y2-pymS)3 (Y = H (3a), CH3 (3b); pymS = pyrimidine-2-thiolate), and Ni(4,4'-Z-2,2'-bpy)(pyS)2 (Z = H (4a), CH3 (4b), OCH3 (4c); bpy = bipyridine) have been synthesized in high yield and characterized. X-ray diffraction studies show that 2 is square planar, while the other complexes possess tris-chelated distorted-octahedral geometries. All of the complexes are active catalysts for both the photocatalytic and electrocatalytic production of hydrogen in 1/1 EtOH/H2O. When coupled with fluorescein (Fl) as the photosensitizer (PS) and triethylamine (TEA) as the sacrificial electron donor, these complexes exhibit activity for light-driven hydrogen generation that correlates with ligand electron donor ability. Complex 4c achieves over 7300 turnovers of H2 in 30 h, which is among the highest reported for a molecular noble metal-free system. The initial photochemical step is reductive quenching of Fl* by TEA because of the latter's greater concentration. When system concentrations are modified so that oxidative quenching of Fl* by catalyst becomes more dominant, system durability increases, with a system lifetime of over 60 h. System variations and cyclic voltammetry experiments are consistent with a CECE mechanism that is common to electrocatalytic and photocatalytic hydrogen production. This mechanism involves initial protonation of the catalyst followed by reduction and then additional protonation and reduction steps to give a key Ni-H(-)/N-H(+) intermediate that forms the H-H bond in the turnover-limiting step of the catalytic cycle. A key to the activity of these catalysts is the reversible dechelation and protonation of the pyridine N atoms, which enable an internal heterocoupling of a metal hydride and an N-bound proton to produce H2.

Copper phenanthroline for selective electrochemical CO2 reduction on carbon paper.

We report a series of structurally relevant copper phenanthroline complexes as pre-catalysts for highly selective electrocatalytic reduction of CO2 to C2 products using inexpensive carbon paper electrodes. The Cu complexes with non-substituted phenanthroline promote the production of ethylene with a high faradaic efficiency of 71.2%, while the one with pyridinium-functionalized ligands is more selective for ethanol. The C2 selectivity can be effectively tuned by increasing the number of coordinated phenanthrolines and remains high at a wide range of potentials.

Photocatalytic CO2 reduction with aminoanthraquinone organic dyes.

The direct utilization of solar energy to convert CO2 into renewable chemicals remains a challenge. One essential difficulty is the development of efficient and inexpensive light-absorbers. Here we show a series of aminoanthraquinone organic dyes to promote the efficiency for visible light-driven CO2 reduction to CO when coupled with an Fe porphyrin catalyst. Importantly, high turnover numbers can be obtained for both the photosensitizer and the catalyst, which has not been achieved in current light-driven systems. Structure-function study performed with substituents having distinct electronic effects reveals that the built-in donor-acceptor property of the photosensitizer significantly promotes the photocatalytic activity. We anticipate this study gives insight into the continued development of advanced photocatalysts for solar energy conversion.

Selective CO2 Reduction to Ethylene Using Imidazolium-Functionalized Copper.

Electrochemical CO2 reduction is a promising approach to obtain sustainable chemicals in energy conversion. Improving the selectivity of CO2 reduction toward a particular C2 product such as ethylene remains a significant challenge. Herein, we report a series of imidazolium hexafluorophosphate compounds as surface modifiers for planar Cu foils to boost the Faradaic efficiency (FE) of ethylene from 5 to 73%, which is among the highest reported using polycrystalline Cu. The modified electrodes are convenient to prepare. The structure-function study demonstrates that varying the alkyl or aromatic substituents on the imidazolium nitrogen atoms has significant effects on the morphology of the deposited films and the product selectivity of CO2 reduction. Experimental FEC≥2, FEC2H4, ln(FEC≥2/FECH4), and ln(FEC2H4/FEC2H5OH) values show generally linear relationships with FEH2 while using different imidazolium modifiers, suggesting that factors governing proton reduction may also be directly related to both overall C≥2 generation and ethylene selectivity. This work presents an effective and practical way in tailoring the active sites of metallic surface for selective CO2 reduction.

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.

Impact of ligand exchange in hydrogen production from cobaloxime-containing photocatalytic systems.

Ligand exchange on the Co(dmgH)(2)(py)Cl water reduction catalyst was explored under photocatalytic conditions. The photosensitizer fluorescein was connected to the catalyst through the axially coordinated pyridine. While this two-component complex produces H(2) from water under visible light irradiation in the presence of triethanolamine (TEOA), it is less active than a system containing separate fluorescein and [Co(III)(dmgH)(2)(py)Cl] components. NMR and photolysis experiments show that the Co catalyst undergoes pyridine exchange. Interestingly, glyoximate ligand exchange was also observed photocatalytically and by NMR spectroscopy, thereby showing that integrated systems in which the photosensitizer is linked directly to the Co(dmgH)(2)(py)Cl catalyst may not remain intact during H(2) photogeneration. These studies have also given rise to insights into the catalyst decomposition mechanism.

Promoting photocatalytic CO2 reduction with a molecular copper purpurin chromophore.

CO2 reduction through artificial photosynthesis represents a prominent strategy toward the conversion of solar energy into fuels or useful chemical feedstocks. In such configuration, designing highly efficient chromophores comprising earth-abundant elements is essential for both light harvesting and electron transfer. Herein, we report that a copper purpurin complex bearing an additional redox-active center in natural organic chromophores is capable to shift the reduction potential 540 mV more negative than its organic dye component. When this copper photosensitizer is employed with an iron porphyrin as the catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the sacrificial reductant, the system achieves over 16100 turnover number of CO from CO2 with a 95% selectivity (CO vs H2) under visible-light irradiation, which is among the highest reported for a homogeneous noble metal-free system. This work may open up an effective approach for the rational design of highly efficient chromophores in artificial photosynthesis.