Application of the Rotating Ring-Disc-Electrode Technique to Water Oxidation by Surface-Bound Molecular Catalysts

We report here the application of a simple hydrodynamic technique, linear sweep voltammetry with a modified rotating-ring-disc electrode, for the study of water oxidation catalysis. With this technique, we have been able to reliably obtain turnover frequencies, overpotentials, Faradaic conversion efficiencies, and mechanistic information from single samples of surface-bound metal complex catalysts

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

Multiple Pathways in the Oxidation of a NADH Analogue

Oxidation of the NADH analogue, <i>N</i>-benzyl-1,4-dihydronicotinamide (BNAH), by the 1e^– acceptor, [Os­(dmb)₃]³⁺, and 2e^–/2H⁺ acceptor, benzoquinone (Q), has been investigated in aqueous solutions over extended pH and buffer concentration ranges by application of a double-mixing stopped-flow technique in order to explore the redox pathways available to this important redox cofactor. Our results indicate that oxidation by quinone is dominated by hydride transfer, and a pathway appears with added acids involving concerted hydride-proton transfer (HPT) in which synchronous transfer of hydride to one O-atom at Q and proton transfer to the second occurs driven by the formation of the stable H₂Q product. Oxidation by [Os­(dmb)₃]³⁺ occurs by outer-sphere electron transfer including a pathway involving ion-pair preassociation of HPO₄^2– with the complex that may also involve a concerted proton transfer

Concerted Electron–Proton Transfer (EPT) in the Oxidation of Cysteine

Cysteine is the most acidic of the three common redox active amino acids with p<i>K</i>_a = 8.2 for the thiol compared to p<i>K</i>_a = 10.1 for the phenol in tyrosine and p<i>K</i>_a ≈ 16 for the indole proton in tryptophan. Stopped-flow and electrochemical measurements have been used to explore the role of proton-coupled electron transfer (PCET) and concerted electron–proton transfer (EPT) in the oxidations of <i>L</i>-cysteine and <i>N</i>-acetyl-cysteine by the polypyridyl oxidants M­(bpy)₃³⁺ (M = Fe, Ru, Os) and Ru­(dmb)₃³⁺ (bpy is 2,2′-bipyridine, and dmb is 4,4′-dimethyl-2,2′-bipyridine). Oxidation is rate limited by initial 1e^– electron transfer to M­(bpy)₃³⁺, with added proton acceptor bases, by multiple pathways whose relative importance depends on reaction conditions. The results of these studies document important roles for acetate (AcO^–) and phosphate (HPO₄^2–) as proton acceptor bases in concerted electron–proton transfer (EPT) pathways in the oxidation of <i>L</i>-cysteine and <i>N</i>-acetyl-cysteine with good agreement between rate constant data obtained by electrochemical and stopped-flow methods

Role of Proton-Coupled Electron Transfer in the Redox Interconversion between Benzoquinone and Hydroquinone

Benzoquinone/hydroquinone redox interconversion by the reversible Os­(dmb)₃^3+/2+ couple over an extended pH range with added acids and bases has revealed the existence of seven discrete pathways. Application of spectrophotometric monitoring with stopped-flow mixing has been used to explore the role of PCET. The results have revealed a role for phosphoric acid and acetate as proton donor and acceptor in the concerted electron–proton transfer reduction of benzoquinone and oxidation of hydroquinone, respectively

Water Oxidation Intermediates Applied to Catalysis: Benzyl Alcohol Oxidation

Four distinct intermediates, Ru^IVO²⁺, Ru^IV(OH)³⁺, Ru^VO³⁺, and Ru^V(OO)³⁺, formed by oxidation of the catalyst [Ru­(Mebimpy)­(4,4′-((HO)₂OPCH₂)₂bpy)­(OH₂)]²⁺ [Mebimpy = 2,6-bis­(1-methylbenzimidazol-2-yl) and 4,4′-((HO)₂OPCH₂)₂bpy = 4,4′-bismethylenephosphonato-2,2′-bipyridine] on <i>nano</i>ITO (1-PO₃H₂) have been identified and utilized for electrocatalytic benzyl alcohol oxidation. Significant catalytic rate enhancements are observed for Ru^V(OO)³⁺ (∼3000) and Ru^IV(OH)³⁺ (∼2000) compared to Ru^IVO²⁺. The appearance of an intermediate for Ru^IVO²⁺ as the oxidant supports an O-atom insertion mechanism, and H/D kinetic isotope effects support net hydride-transfer oxidations for Ru^IV(OH)³⁺ and Ru^V(OO)³⁺. These results illustrate the importance of multiple reactive intermediates under catalytic water oxidation conditions and possible control of electrocatalytic reactivity on modified electrode surfaces

Competing Pathways in the <i>photo-</i>Proton-Coupled Electron Transfer Reduction of <i>fac</i>-[Re(bpy)(CO)₃(4,4′-bpy]^+* by Hydroquinone

The emitting metal-to-ligand charge transfer (MLCT) excited state of <i>fac</i>-[Re^I(bpy)(CO)₃(4,4′-bpy)]⁺ (<b>1</b>) (bpy is 2,2′-bipyridine, 4,4′-bpy is 4,4′-bipyridine), [Re^II(bpy^–•)(CO)₃(4,4′-bpy)]⁺*, is reductively quenched by 1,4-hydroquinone (H₂Q) in CH₃CN at 23 ± 2 °C by competing pathways to give a common electron–proton-transfer intermediate. In one pathway, electron transfer (ET) quenching occurs to give Re^I(bpy^–•)(CO)₃(4,4′-bpy)]⁰ with <i>k</i> = (1.8 ± 0.2) × 10⁹ M^–1 s^–1, followed by proton transfer from H₂Q to give [Re^I(bpy)(CO)₃(4,4′-bpyH^•)]⁺. Protonation triggers intramolecular bpy^•– → 4,4′-bpyH⁺ electron transfer. In the second pathway, preassociation occurs between the ground state and H₂Q at high concentrations. Subsequent Re → bpy MLCT excitation of the adduct is followed by electron–proton transfer from H₂Q in concert with intramolecular bpy^•– → 4,4′-bpyH⁺ electron transfer to give [Re^I(bpy)(CO)₃(4,4′-bpyH^•)]⁺ with <i>k</i> = (1.0 ± 0.4) × 10⁹ s^–1 in 3:1 CH₃CN/H₂O

Visible Photoelectrochemical Water Splitting Based on a Ru(II) Polypyridyl Chromophore and Iridium Oxide Nanoparticle Catalyst

Preparation of Ru­(II) polypyridyl–iridium oxide nanoparticle (IrO_X NP) chromophore–catalyst assemblies on an FTO|<i>nano</i>ITO|TiO₂ core/shell by a layer-by-layer procedure is described for application in dye-sensitized photoelectrosynthesis cells (DSPEC). Significantly enhanced, bias-dependent photocurrents with Lumencor 455 nm 14.5 mW/cm² irradiation are observed for core/shell structures compared to TiO₂ after derivatization with [Ru­(4,4′-PO₃H₂bpy)₂(bpy)]²⁺ (RuP₂) and uncapped IrO_X NPs at pH 5.8 in NaSiF₆ buffer with a Pt cathode. Photocurrents arising from photolysis of the resulting photoanodes, FTO|<i>nano</i>ITO|TiO₂|−RuP₂,IrO₂, are dependent on TiO₂ shell thickness and applied bias, reaching 0.2 mA/cm² at 0.5 V vs AgCl/Ag with a shell thickness of 6.6 nm. Long-term photolysis in the NaSiF₆ buffer results in a marked decrease in photocurrent over time due to surface hydrolysis and loss of the chromophore from the surface. Long-term stability, with sustained photocurrents, has been obtained by atomic layer deposition (ALD) of overlayers of TiO₂ to stabilize surface binding of −RuP₂ prior to the addition of the IrO_X NPs

Redox Mediator Effect on Water Oxidation in a Ruthenium-Based Chromophore–Catalyst Assembly

The synthesis, characterization, and redox properties are described for a new ruthenium-based chromophore–catalyst assembly, [(bpy)₂Ru­(4-Mebpy-4′-bimpy)­Ru­(tpy)­(OH₂)]⁴⁺ (<b>1</b>, [Ru_a^II-Ru_b^II-OH₂]⁴⁺; bpy = 2,2′-bipyridine; 4-Mebpy-4′-bimpy = 4-(methylbipyridin-4′-yl)-<i>N</i>-benzimid-<i>N</i>′-pyridine; tpy = 2,2′:6′,2″-terpyridine), as its chloride salt. The assembly incorporates both a visible light absorber and a catalyst for water oxidation. With added ceric ammonium nitrate (Ce^IV, or CAN), both <b>1</b> and <b>2</b>, [Ru­(tpy)­(Mebim-py)­(OH₂)]²⁺ (Mebim-py = 2-pyridyl-<i>N</i>-methylbenzimidazole), catalyze water oxidation. Time-dependent UV/vis spectral monitoring following addition of 30 equiv of Ce^IV reveals that the rate of Ce^IV consumption is first order both in Ce^IV and in an oxidized form of the assembly. The rate-limiting step appears to arise from slow oxidation of this intermediate followed by rapid release of O₂. This is similar to isolated catalyst <b>2</b>, with redox potentials comparable to the [-Ru_b-OH₂]²⁺ site in <b>1</b>, but <b>1</b> is more reactive than <b>2</b> by a factor of 8 due to a redox mediator effect

A Sensitized Nb₂O₅ Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell

Orthorhombic Nb₂O₅ nanocrystalline films functionalized with [Ru­(bpy)₂(4,4′-(PO₃H₂)₂bpy)]²⁺ were used as the photoanode in dye-sensitized photoelectrosynthesis cells (DSPEC) for hydrogen generation. A set of experiments to establish key propertiesconduction band, trap state distribution, interfacial electron transfer dynamics, and DSPEC efficiencywere undertaken to develop a general protocol for future semiconductor evaluation and for comparison with other wide-band-gap semiconductors. We have found that, for a T-phase orthorhombic Nb₂O₅ nanocrystalline film, the conduction band potential is slightly positive (<0.1 eV), relative to that for anatase TiO₂. Anatase TiO₂ has a wide distribution of trap states including deep trap and band-tail trap states. Orthorhombic Nb₂O₅ is dominated by shallow band-tail trap states. Trap state distributions, conduction band energies, and interfacial barriers appear to contribute to a slower back electron transfer rate, lower injection yield on the nanosecond time scale, and a lower open-circuit voltage (<i>V</i>_oc) for orthorhombic Nb₂O₅, compared to anatase TiO₂. In an operating DSPEC, with the ethylenediaminetetraacetic tetra-anion (EDTA^4–) added as a reductive scavenger, H₂ quantum yield and photostability measurements show that Nb₂O₅ is comparable, but not superior, to TiO₂