Plasmon-enhanced light-driven water oxidation by a dye-sensitized photoanode

Dye-sensitized photoelectrosynthesis cells (DSPECs) provide a basis for artificial photosynthesis and solar fuels production. By combining molecular chromophores and catalysts with high surface area, transparent semiconductor electrodes, a DSPEC provides the basis for light-driven conversion of water to O2 and H2 or for reduction of CO2 to carbon-based fuels. The incorporation of plasmonic cubic silver nanoparticles, with a strongly localized surface plasmon absorbance near 450 nm, to a DSPEC photoanode induces a great increase in the efficiency of water oxidation to O2 at a DSPEC photoanode. The improvement in performance by the molecular components in the photoanode highlights a remarkable advantage for the plasmonic effect in driving the 4e-/4H+ oxidation of water to O2 in the photoanode

Polymer-supported CuPd nanoalloy as a synergistic catalyst for electrocatalytic reduction of carbon dioxide to methane

Photo- and electrochemical CO2 reduction to carbon fuels is not only an attractive solution to the greenhouse effect, but could also become an integral part of a global energy storage strategy with renewable electrical energy sources used to store energy in the chemical bonds of carbon fuels. A novel electrodeposition strategy is reported here for the preparation of highly dispersed, ultrafine metal nanoparticles and nanoalloys on an electroactive polymeric film. It is shown that a bimetallic Cu–Pd nanoalloy exhibits a greater than twofold enhancement in Faradaic efficiency for CO2 reduction to methane compared with a state-of-the-art nanoCu catalyst. The fabrication procedure for the alloy nanoparticles is straightforward and applicable as a general procedure for catalytic electrodes for integrated electrolysis devices

A donor-chromophore-catalyst assembly for solar CO2 reduction

We describe here the preparation and characterization of a photocathode assembly for CO2 reduction to CO in 0.1 M LiClO4 acetonitrile. The assembly was formed on 1.0 μm thick mesoporous films of NiO using a layer-by-layer procedure based on Zr(IV)–phosphonate bridging units. The structure of the Zr(IV) bridged assembly, abbreviated as NiO|-DA-RuCP22+-Re(I), where DA is the dianiline-based electron donor (N,N,N′,N′-((CH2)3PO3H2)4-4,4′-dianiline), RuCP2+ is the light absorber [Ru((4,4′-(PO3H2CH2)2-2,2′-bipyridine)(2,2′-bipyridine))2]2+, and Re(I) is the CO2 reduction catalyst, ReI((4,4′-PO3H2CH2)2-2,2′-bipyridine)(CO)3Cl. Visible light excitation of the assembly in CO2 saturated solution resulted in CO2 reduction to CO. A steady-state photocurrent density of 65 μA cm−2 was achieved under one sun illumination and an IPCE value of 1.9% was obtained with 450 nm illumination. The importance of the DA aniline donor in the assembly as an initial site for reduction of the RuCP2+ excited state was demonstrated by an 8 times higher photocurrent generated with DA present in the surface film compared to a control without DA. Nanosecond transient absorption measurements showed that the expected reduced one-electron intermediate, RuCP+, was formed on a sub-nanosecond time scale with back electron transfer to the electrode on the microsecond timescale which competes with forward electron transfer to the Re(I) catalyst at t1/2 = 2.6 μs (kET = 2.7 × 105 s−1)

Base-enhanced catalytic water oxidation by a carboxylate–bipyridine Ru(II) complex

Development of rapid, robust water oxidation catalysts remains an essential element in solar water splitting by artificial photosynthesis. We report here dramatic rate enhancements with added buffer bases for a robust Ru(II) polypyridyl catalyst with a calculated half-time for water oxidation of ∼7 μs in 1.0 M phosphate. The results of detailed kinetic studies provide insight into the water oxidation mechanism and an important role for added buffer bases in accelerating water oxidation by concerted atom–proton transfer

Light-Driven Water Splitting Mediated by Photogenerated Bromine

Light-driven water splitting was achieved using a dye-sensitized mesoporous oxide film and the oxidation of bromide (Br−) to bromine (Br2) or tribromide (Br3−). The chemical oxidant (Br2 or Br3−) is formed during illumination at the photoanode and used as a sacrificial oxidant to drive a water oxidation catalyst (WOC), here demonstrated using [Ru(bda)(pic)2], (1; pic=picoline, bda=2,2′-bipyridine-6,6′-dicarboxylate). The photochemical oxidation of bromide produces a chemical oxidant with a potential of 1.09 V vs. NHE for the Br2/Br− couple or 1.05 V vs. NHE for the Br3−/Br− couple, which is sufficient to drive water oxidation at 1 (RuV/IV≈1.0 V vs. NHE at pH 5.6). At pH 5.6, using a 0.2 m acetate buffer containing 40 mm LiBr and the [Ru(4,4′-PO3H2-bpy)(bpy)2]2+ (RuP2+, bpy=2,2′-bipyridine) chromophore dye on a SnO2/TiO2 core–shell electrode resulted in a photocurrent density of around 1.2 mA cm−2 under approximately 1 Sun illumination and a Faradaic efficiency upon addition of 1 of 77 % for oxygen evolution

Kinetics of the Autoreduction of Hexavalent Americium in Aqueous Nitric Acid

The rate of reduction of hexavalent ²⁴³Am due to self-radiolysis was measured across a range of total americium and nitric acid concentrations. These so-called autoreduction rates exhibited zero-order kinetics with respect to the concentration of hexavalent americium, and pseudo-first-order kinetics with respect to the total concentration of americium. However, the rate constants did vary with nitric acid concentration, resulting in values of 0.0048 ± 0.0003, 0.0075 ± 0.0005, and 0.0054 ± 0.0003 h^–1 for 1.0, 3.0, and 6.5 M HNO₃, respectively. This indicates that reduction is due to reaction of hexavalent americium with the radiolysis products of total americium decay. The concentration changes of Am­(III), Am­(V), and Am­(VI) were determined by UV–vis spectroscopy. The Am­(III) molar extinction coefficients are known; however, the unknown values for the Am­(V) and Am­(VI) absorbances across the studied range of nitric acid concentrations were determined by sensitivity analysis in which a mass balance with the known total americium concentration was obtained. The new extinction coefficients and reduction rate constants have been tabulated here. Multiscale radiation chemical modeling using a reaction set with both known and optimized rate coefficients was employed to achieve excellent agreement with the experimental results, and indicates that radiolytically produced nitrous acid from nitric acid radiolysis and hydrogen peroxide from water radiolysis are the important reducing agents. Since these species also react with each other, modeling indicated that the highest concentrations of these species available for Am­(VI) reduction occurred at 3.0 M HNO₃. This is in agreement with the empirical finding that the highest rate constant for autoreduction occurred at the intermediate acid concentration

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

Light‐Driven Water Splitting Mediated by Photogenerated Bromine

Light-driven water splitting was achieved using a dye-sensitized mesoporous oxide film and the oxidation of bromide (Br−) to bromine (Br2) or tribromide (Br3−). The chemical oxidant (Br2 or Br3−) is formed during illumination at the photoanode and used as a sacrificial oxidant to drive a water oxidation catalyst (WOC), here demonstrated using [Ru(bda)(pic)2], (1; pic=picoline, bda=2,2′-bipyridine-6,6′-dicarboxylate). The photochemical oxidation of bromide produces a chemical oxidant with a potential of 1.09 V vs. NHE for the Br2/Br− couple or 1.05 V vs. NHE for the Br3−/Br− couple, which is sufficient to drive water oxidation at 1 (RuV/IV≈1.0 V vs. NHE at pH 5.6). At pH 5.6, using a 0.2 m acetate buffer containing 40 mm LiBr and the [Ru(4,4′-PO3H2-bpy)(bpy)2]2+ (RuP2+, bpy=2,2′-bipyridine) chromophore dye on a SnO2/TiO2 core–shell electrode resulted in a photocurrent density of around 1.2 mA cm−2 under approximately 1 Sun illumination and a Faradaic efficiency upon addition of 1 of 77 % for oxygen evolution

Base-enhanced catalytic water oxidation by a carboxylate–bipyridine Ru(II) complex

In aqueous solution above pH 2.4 with 4% (vol/vol) CH(3)CN, the complex [Ru(II)(bda)(isoq)(2)] (bda is 2,2′-bipyridine-6,6′-dicarboxylate; isoq is isoquinoline) exists as the open-arm chelate, [Ru(II)(CO(2)-bpy-CO(2)(−))(isoq)(2)(NCCH(3))], as shown by (1)H and (13)C-NMR, X-ray crystallography, and pH titrations. Rates of water oxidation with the open-arm chelate are remarkably enhanced by added proton acceptor bases, as measured by cyclic voltammetry (CV). In 1.0 M PO(4)(3–), the calculated half-time for water oxidation is ∼7 μs. The key to the rate accelerations with added bases is direct involvement of the buffer base in either atom–proton transfer (APT) or concerted electron–proton transfer (EPT) pathways