Proton-Coupled Electron Transfer Reduction of a Quinone by an Oxide-Bound Riboflavin Derivative

The redox properties of a surface-bound phosphate flavin derivative (flavin mononucleotide, FMN) have been investigated on planar-FTO and <i>nano</i>ITO electrodes under acidic conditions in 1:1 CH₃CN/H₂O (V:V). On FTO, reversible 2e^–/2H⁺ reduction of FTO|-FMN to FTO|-FMNH₂ occurs with the pH and scan rate dependence expected for a 2e^–/2H⁺ surface-bound couple. The addition of tetramethylbenzoquinone (Me₄Q) results in rapid electrocatalyzed reduction to the hydroquinone by a pathway first order in quinone and first order in acid with <i>k</i>_H = (2.6 ± 0.2) × 10⁶ M^–1 s^–1. Electrocatalytic reduction of the quinone also occurs on derivatized, high surface area <i>nano</i>ITO electrodes with evidence for competitive rate-limiting diffusion of the quinone into the mesoporous nanostructure

Electrocatalytic Water Oxidation by a Monomeric Amidate-Ligated Fe(III)–Aqua Complex

The six-coordinate Fe^III-aqua complex [Fe^III(dpaq)­(H₂O)]²⁺ (<b>1</b>, dpaq is 2-[bis­(pyridine-2-ylmethyl)]­amino-<i>N</i>-quinolin-8-yl-acetamido) is an electrocatalyst for water oxidation in propylene carbonate–water mixtures. An electrochemical kinetics study has revealed that water oxidation occurs by oxidation to Fe^V(O)²⁺ followed by a reaction first order in catalyst and added water, respectively, with <i>k</i>_o = 0.035(4) M^–1 s^–1 by the single-site mechanism found previously for Ru and Ir water oxidation catalysts. Sustained water oxidation catalysis occurs at a high surface area electrode to give O₂ through at least 29 turnovers over an 15 h electrolysis period with a 45% Faradaic yield and no observable decomposition of the catalyst

One-Electron Activation of Water Oxidation Catalysis

Rapid water oxidation catalysis is observed following electrochemical oxidation of [Ru^II(tpy)­(bpz)­(OH)]⁺ to [Ru^V(tpy)­(bpz)­(O)]³⁺ in basic solutions with added buffers. Under these conditions, water oxidation is dominated by base-assisted Atom Proton Transfer (APT) and direct reaction with OH^–. More importantly, we report here that the Ru^IVO²⁺ form of the catalyst, produced by 1e^– oxidation of [Ru^II(tpy)­(bpz)­(OH₂)]²⁺ to Ru­(III) followed by disproportionation to [Ru^IV(tpy)­(bpz)­(O)]²⁺ and [Ru^II(tpy)­(bpz)­(OH₂)]²⁺, is also a competent water oxidation catalyst. The rate of water oxidation by [Ru^IV(tpy)­(bpz)­(O)]²⁺ is greatly accelerated with added PO₄^3– with a turnover frequency of 5.4 s^–1 reached at pH 11.6 with 1 M PO₄^3– at an overpotential of only 180 mV

Photocatalytic Conversion of Am(III) to Am(VI) Using a TiO₂ Electrode

Titanium–titania (Ti|TiO2) nanostructured electrodes in 0.1 M HNO3 solutions under UV-light illumination using a 375 nm LED and a 1.55 V vs SCE bias photoelectrochemically oxidize Am­(III) to Am­(VI) (AmVIO22+). Oxidation occurs through photoelectrochemically generated adsorbed hydroxyl radicals (2.81 V vs SCE) and/or direct electron transfer between the excited-state electrode (E(TiO2VB*) = ca. 2.95 V vs SCE) and Am­(III) (E(AmIV/III) = 2.62 V). An electrochemically irreversible but chemically reversible electrochemical process at 1.60 V vs SCE is assigned to the one-electron Am­(VI/V) couple. This system may be applied to used nuclear fuel reprocessing to separate actinides of concern (U, Np, Pu, and Am), where Am is the most significant challenge

Indium Tin-Doped Oxide (ITO) as a High Activity Water Oxidation Photoanode

Photochemical water oxidation was carried out at a mesoporous nanoparticle film composed of indium tin-doped oxide (nanoITO). Annealing nanoITO at temperatures above 250 °C affects both conducting and semiconducting properties. Impressive photoelectrochemical activity was observed at this degenerate n-type semiconductor electrode, outperforming the traditional semiconductor titanium dioxide (TiO2) under the same conditions. In a 0.1 M HNO3 solution, the nanoITO electrode sustained photocurrents of 1.0 mA/cm2 at an Eapplied = 1.5 V vs saturated calomel electrode (SCE) (η = 0.55 V) under a 90 mW/cm2 UV illumination (375 nm). This activity is compared to ∼0.3 mA/cm2 with a traditional TiO2 electrode under the same potential and conditions. Evidence for oxygen generation in the photolysis experiments was quantified using the collector–generator method, and >70% photocurrent efficiency for O2 production was confirmed at this nanoITO photoanode

Water Oxidation and Oxygen Monitoring by Cobalt-Modified Fluorine-Doped Tin Oxide Electrodes

Electrocatalytic water oxidation occurs at fluoride-doped tin oxide (FTO) electrodes that have been surface-modified by addition of Co­(II). On the basis of X-ray photoelectron spectroscopy and transmission electron microscopy measurements, the active surface site appears to be a single site or small-molecule assembly bound as Co­(II), with no evidence for cobalt oxide film or cluster formation. On the basis of cyclic voltammetry measurements, surface-bound Co­(II) undergoes a pH-dependent 1e^–/1H⁺ oxidation to Co­(III), which is followed by pH-dependent catalytic water oxidation. O₂ reduction at FTO occurs at −0.33 V vs NHE, allowing for in situ detection of oxygen as it is formed by water oxidation on the surface. Controlled-potential electrolysis at 1.61 V vs NHE at pH 7.2 resulted in sustained water oxidation catalysis at a current density of 0.16 mA/cm² with 29 000 turnovers per site over an electrolysis period of 2 h. The turnover frequency for oxygen production per Co site was 4 s^–1 at an overpotential of 800 mV at pH 7.2. Initial experiments with Co­(II) on a mesoporous, high-surface-area <i>nano</i>FTO electrode increased the current density by a factor of ∼5