Etsuko Fujita

Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation

Two major energy-related problems confront the world in the next 50 years. First, increased worldwide competition for gradually depleting fossil fuel reserves (derived from past photosynthesis) will lead to higher costs, both monetarily and politically. Second, atmospheric CO_2 levels are at their highest recorded level since records began. Further increases are predicted to produce large and uncontrollable impacts on the world climate. These projected impacts extend beyond climate to ocean acidification, because the ocean is a major sink for atmospheric CO2.1 Providing a future energy supply that is secure and CO_2-neutral will require switching to nonfossil energy sources such as wind, solar, nuclear, and geothermal energy and developing methods for transforming the energy produced by these new sources into forms that can be stored, transported, and used upon demand.

Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures

Green plants convert CO2 to sugar for energy storage via photosynthesis. We report a novel catalyst that uses CO2 and hydrogen to store energy in formic acid. Using a homogeneous iridium catalyst with a proton-responsive ligand, we show the first reversible and recyclable hydrogen storage system that operates under mild conditions using CO2, formate and formic acid. This system is energy-efficient and green because it operates near ambient conditions, uses water as a solvent, produces high-pressure CO-free hydrogen, and uses pH to control hydrogen production or consumption. The extraordinary and switchable catalytic activity is attributed to the multifunctional ligand, which acts as a proton-relay and strong π-donor, and is rationalized by theoretical and experimental studies. When operating at near-ambient conditions, using water as a solvent, a high-turnover iridium catalyst enables a reversible hydrogen storage system that uses carbon dioxide, formate and formic acid. Proton-responsive ligands in the catalyst allow it to be turned on or off by controlling the pH of the solution.

Recent developments in transition metal carbides and nitrides as hydrogen evolution electrocatalysts

The production of hydrogen by the electrolysis of water, a sustainable and greenhouse-gas-free source, requires an efficient and abundant electrocatalyst that minimizes energy consumption. Interest in transition metal carbides and nitrides has been aroused by their promising properties that make them potential substitutes for Pt-group metals as catalysts for the hydrogen evolution reaction. In this review, we discuss systematically the recent progress in the development of group IV-VI metal carbides and nitrides toward the hydrogen evolution reaction. Some strategies for designing such catalysts and improving their efficiency and reliability, including nanostructuring, optimizing hydrogen binding energy, interaction with the supporting material, and exploiting hybrid structures, are highlighted. We conclude with an outlook on the challenges in designing future HER electrocatalysts.

CO2 Hydrogenation to Formate and Methanol as an Alternative to Photo- and Electrochemical CO2 Reduction

Photoand Electrochemical CO2 Reduction Wan-Hui Wang,*,† Yuichiro Himeda,*,‡,§ James T. Muckerman, Gerald F. Manbeck, and Etsuko Fujita* †School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China ‡National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5-1, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States

PHOTOCHEMICAL CARBON DIOXIDE REDUCTION WITH METAL COMPLEXES

Abstract Transition-metal complexes, CoHMD 2+ (HMD=5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) and Ru(bpy) 2 (CO)X n + (bpy=2,2′-bipyridine, X=CO, Cl, H, etc.), mediate electron transfer in the photochemical reduction of CO 2 . The thermodynamics and kinetics of CO 2 binding to CoHMD + , and spectroscopic characterization of the CO 2 adducts of CoHMD + and [Ru I (bpy)(bpy − )(CO)] are described.

Theoretical studies of the mechanism of catalytic hydrogen production by a cobaloxime

Our theoretical studies of the standard reduction potentials of the molecular complex [Co(II)(dmgBF(2))(2)](0) (dmgBF(2) = difluoro-boryldimethylglyoximate) in acetonitrile solution shed light on its electrocatalytic mechanism for hydrogen production. Three such mechanisms have been proposed, all proceeding through the formation of Co(III)H. Our results indicate that the mechanism involving a Co(II)H intermediate is the most likely.

Photo-Induced Generation of Dihydrogen and Reduction of Carbon Dioxide Using Transition Metal Complexes

Abstract Homogeneous and microheterogeneous transition-metal-based systems that generate dihydrogen and/or reduce carbon dioxide upon irradiation with visible light are considered. Most of the systems involve polypyridine complexes of the d6 centers cobalt(III), rhodium(III), iridium(III), ruthenium(II) and rhenium(I). Complexes with diimine ligands serve as photosensitizers and/or catalyst precursors. The corresponding d8 metal centers and d6 hydrides are important intermediates: bimolecular reactions of the hydrides or their reactions with H2O/H3O+ are responsible for formation of dihydrogen. When carbon dioxide is also present, it may insert into the metal-hydride bond to yield formate. Mechanistic schemes for some dual-acting photoconversion systems that generate both dihydrogen and carbon monoxide or formate are considered.

Effects of a proximal base on water oxidation and proton reduction catalyzed by geometric isomers of [Ru(tpy)(pynap)(OH2)]2+.

Basic difference: The importance of a pendent base in promoting proton-coupled electron-transfer reactions with low activation barriers has been discussed for H(+) reduction or H(2) oxidation in acetonitrile. Investigation of the interaction between a base positioned in the second coordination sphere of a complex and a water ligand in water oxidation reactions using geometric isomers of [Ru(tpy)(pynap)(OH(2))](2+) (see picture) gave intriguing results.

Efficient H2 generation from formic acid using azole complexes in water

Iridium azole-containing complexes are demonstrated to catalyze the dehydrogenation of formic acid into H2–CO2 (1/1) mixtures in aqueous solution in the absence of organic additives, and with a maximum turnover frequency (TOF) of 34000 h−1 at 80 °C.

Nickel(II) macrocycles: highly efficient electrocatalysts for the selective reduction of CO2 to CO

A series of molecular materials that are structurally similar to the NiII macrocycle [Ni(cyclam)]2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) have been used as electrocatalysts for the reduction of CO2 at a mercury pool working electrode in aqueous solution. At pH 5, with an applied potential of −0.96 V vs. NHE (overpotential of −0.55 V), the complexes are highly efficient, having both high rate constants and Faradaic efficiencies (F.E.s) for the selective reduction of CO2 to CO. When the pH is below the pKa (pH < 2) of the Ni(H) species (pKas: 0.5–2), the F.E.s are still high but product selectivity changes to yield predominantly H2 from the reduction of water. At least two of the complexes investigated are better electrocatalysts than [Ni(cyclam)]2+, probably due to: (i) surface geometries that are suitable for adsorption onto the mercury electrode surface, and (ii) electronic effects of methyl groups or cyclohexane rings on the cyclam backbone. Mechanistic studies by pulse radiolysis show evidence of Ni(CO2) adducts for two of the catalysts, with KCO2 ∼ 10 M−1 for the reaction of NiI with CO2 in aqueous solution.