A Multi-Heme Flavoenzyme as a Solar Conversion Catalyst

The enzyme flavocytochrome <i>c</i>₃ (fcc₃), which catalyzes hydrogenation across a CC double bond (fumarate to succinate), is used to carry out the fuel-forming reaction in an artificial photosynthesis system. When immobilized on dye-sensitized TiO₂ nanoparticles, fcc₃ catalyzes visible-light-driven succinate production in aqueous suspension. Solar-to-chemical conversion using neutral water as the oxidant is achieved with a photoelectrochemical cell comprising an fcc₃-modified indium tin oxide cathode linked to a cobalt phosphate-modified BiVO₄ photoanode. The results reinforce new directions in the area of artificial photosynthesis, in particular for solar-energy-driven synthesis of organic chemicals and commodities, moving away from simple fuels as target molecules

Visible Light-Induced Hole Injection into Rectifying Molecular Wires Anchored on Co₃O₄ and SiO₂ Nanoparticles

Tight control of charge transport from a visible light sensitizer to a metal oxide nanoparticle catalyst for water oxidation is a critical requirement for developing efficient artificial photosynthetic systems. By utilizing covalently anchored molecular wires for hole transport from sensitizer to the oxide surface, the challenge of high rate and unidirectionality of the charge flow can be addressed. Functionalized hole conducting molecular wires of type p-oligo­(phenylenevinylene) (3 aryl units, abbreviated PV3) with various anchoring groups for the covalent attachment to Co3O4 catalyst nanoparticles were synthesized and two alternative methods for attachment to the oxide nanoparticle surface introduced. Covalent anchoring of intact PV3 molecules on Co3O4 nanoparticles (and on SiO2 nanoparticles for control purposes) was established by FT-Raman, FT-IR, and optical spectroscopy including observation, in some cases, of the vibrational signature of the anchored functionality. Direct monitoring of the kinetics of hole transfer from a visible light sensitizer in aqueous solution ([Ru­(bpy)3]2+ (and derivatives) light absorber, [Co­(NH3)5Cl]2+ acceptor) to wire molecules on inert SiO2(12 nm) particles by nanosecond laser absorption spectroscopy revealed efficient, encounter controlled rates. For wire molecules anchored on Co3O4 nanoparticles, the recovery of the reduced sensitizer at 470 nm indicated similarly efficient hole transfer to the attached PV3, yet no transient hole signal was detected at 600 nm. This implies hole injection from the anchored wire molecule into the Co3O4 particle within 1 μs or shorter, indicating efficient charge transport from the visible light sensitizer to the oxide catalyst particle

Selective Visible-Light-Driven CO₂ Reduction on a p‑Type Dye-Sensitized NiO Photocathode

We present a photocathode assembly for the visible-light-driven selective reduction of CO₂ to CO at potentials below the thermodynamic equilibrium in the dark. The photoelectrode comprises a porous p-type semiconducting NiO electrode modified with the visible-light-responsive organic dye P1 and the reversible CO₂ cycling enzyme carbon monoxide dehydrogenase. The direct electrochemistry of the enzymatic electrocatalyst on NiO shows that in the dark the electrocatalytic behavior is rectified toward CO oxidation, with the reactivity being governed by the carrier availability at the semiconductor–catalyst interface

How Formaldehyde Inhibits Hydrogen Evolution by [FeFe]-Hydrogenases: Determination by ¹³C ENDOR of Direct Fe–C Coordination and Order of Electron and Proton Transfers

Formaldehyde (HCHO), a strong electrophile and a rapid and reversible inhibitor of hydrogen production by [FeFe]-hydrogenases, is used to identify the point in the catalytic cycle at which a highly reactive metal-hydrido species is formed. Investigations of the reaction of Chlamydomonas reinhardtii [FeFe]-hydrogenase with formaldehyde using pulsed-EPR techniques including electron–nuclear double resonance spectroscopy establish that formaldehyde binds close to the active site. Density functional theory calculations support an inhibited super-reduced state having a short Fe–¹³C bond in the 2Fe subsite. The adduct forms when HCHO is available to compete with H⁺ transfer to a vacant, nucleophilic Fe site: had H⁺ transfer already occurred, the reaction of HCHO with the Fe-hydrido species would lead to methanol, release of which is not detected. Instead, Fe-bound formaldehyde is a metal-hydrido mimic, a locked, inhibited form analogous to that in which two electrons and only one proton have transferred to the H-cluster. The results provide strong support for a mechanism in which the fastest pathway for H₂ evolution involves two consecutive proton transfer steps to the H-cluster following transfer of a second electron to the active site

How Light-Harvesting Semiconductors Can Alter the Bias of Reversible Electrocatalysts in Favor of H₂ Production and CO₂ Reduction

The most efficient catalysts for solar fuel production should operate close to reversible potentials, yet possess a bias for the fuel-forming direction. Protein film electrochemical studies of Ni-containing carbon monoxide dehydrogenase and [NiFeSe]-hydrogenase, each a reversible electrocatalyst, show that the electronic state of the electrode strongly biases the direction of electrocatalysis of CO₂/CO and H⁺/H₂ interconversions. Attached to graphite electrodes, these enzymes show high activities for both oxidation and reduction, but there is a marked shift in bias, in favor of CO₂ or H⁺ reduction, when the respective enzymes are attached instead to n-type semiconductor electrodes constructed from CdS and TiO₂ nanoparticles. This catalytic rectification effect can arise for a reversible electrocatalyst attached to a semiconductor electrode if the electrode transforms between semiconductor- and metallic-like behavior across the same narrow potential range (<0.25 V) that the electrocatalytic current switches between oxidation and reduction