Heterojunction WO₃–BiVO₄ Photoanodes for TEMPO-Mediated Benzyl Alcohol Dehydrogenation in Organic Media

Photoelectrosynthetic approaches to fine chemical production offer a means of directly integrating renewable energy into processes that rely on fossilized carbon-based energy. This work investigates the use of heterojunction semiconductor photoanodes comprised of a tungsten oxide (WO3) base layer and bismuth vanadate (BiVO4) photoactive layer to drive the production of benzaldehyde via benzyl alcohol dehydrogenation. Illumination of the WO3|BiVO4 photoanode drives interfacial oxidation of solution-dissolved TEMPO which mediates oxidation of benzyl alcohol to benzaldehyde. Detailed photoelectrochemical and impedimetric studies show the importance of TEMPO as a mediator and the influence of other solution components for achieving overall dehydrogenation. Optimized reaction conditions result in a Faradaic efficiency of 66% (19 ± 2% product yield) for benzaldehyde and a 43% Faradaic efficiency (10 ± 4% product yield) for cathodic hydrogen production with added bias. As opposed to using oxygen as a terminal acceptor, this study focuses on the photoelectrochemical production of two value-added products and reveals some of the compound challenges presented when considering a complete cell reaction as opposed to isolated half-cell chemistry

Inner Layer Control of Performance in a Dye-Sensitized Photoelectrosynthesis Cell

Interfacial charge transfer and core-shell structures play important roles in dye-sensitized photoelectrosynthesis cells (DSPEC) for water splitting into H2 and O2. An important element in the design of the photoanode in these devices is a core/shell structure which controls local electron transfer dynamics. Here, we introduce a new element, an internal layer of Al2O3 lying between the Sb:SnO2/TiO2 layers in a core/shell electrode which can improve photocurrents by up to 300%. In these structures, the results of photocurrent, transient absorption, and linear scan voltammetry measurements point to an important role for the Al2O3 layer in controlling internal electron transfer within the core/shell structure

Electron Transfer Mediator Effects in the Oxidative Activation of a Ruthenium Dicarboxylate Water Oxidation Catalyst

The mechanism of electrocatalytic water oxidation by the water oxidation catalyst, ruthenium 2,2′-bipyridine-6,6′-dicarboxylate (bda) bis-isoquinoline (isoq), [Ru­(bda)­(isoq)₂], <b>1</b>, at metal oxide electrodes has been investigated. At indium-doped tin oxide (ITO), diminished catalytic currents and increased overpotentials are observed compared to glassy carbon (GC). At pH 7.2 in 0.5 M NaClO₄, catalytic activity is enhanced by the addition of [Ru­(bpy)₃]²⁺ (bpy = bipyridine) as a redox mediator. Enhanced catalytic rates are also observed at ITO electrodes derivatized with the surface-bound phosphonic acid derivative [Ru­(4,4′-(PO₃H₂)₂bpy)­(bpy)₂]²⁺, <b>RuP</b>²⁺. Controlled potential electrolysis with measurement of O₂ at ITO with and without surface-bound RuP²⁺ confirm that water oxidation catalysis occurs. Remarkable rate enhancements are observed with added acetate and phosphate, consistent with an important mechanistic role for atom-proton transfer (APT) in the rate-limiting step as described previously at GC electrodes

Ru(II) Polypyridyl-Modified TiO₂ Nanoparticles for Photocatalytic C–C/C–O Bond Cleavage at Room Temperature

Bond cleavage reactions including that of C–C and C–O bonds are important to the chemical industry and organic chemistry. Performing this chemical transformation under mild conditions (e.g., room temperature, solar light) can benefit both the selectivity and yield of the targeted products. This manuscript describes a simple one-pot approach used to carry out Cα–Cβ/Cβ–O σ-bond cleavage using photocatalytic nanoparticles that afford the cleavage products in excellent yields at room temperature with visible-light illumination. We synthesized a carboxylic acid-functionalized RuII polypyridyl complex (RuC) and TiO2 nanoparticles (TiO2 NPs) with the average dimensions of 6.6 nm width and 14.7 nm length using a hydrothermal method. The photocatalyst RuC was immobilized onto TiO2 NPs (RuC-TiO2 NPs) to perform a photocatalytic cleavage reaction with a nonphenolic lignin model compound, 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)­propane-1,3-diol (DMP-2ol), under simulated solar illumination. Photophysical studies of RuC and TiO2 NPs reveal that intermolecular energy/electron transfer from the photoexcited RuC to TiO2 NPs occurs in acetonitrile solution. Under ambient temperature and aerobic conditions, the photocatalytic reaction with RuC-TiO2 NPs and DMP-2ol generates the main Cα–Cβ/Cβ–O bond cleavage products of 3,4-dimethoxybenzaldehyde (1, 82%) and 2-methoxyphenol (2, 90%) in excellent yields. This study successfully performed the C–C/C–O bond cleavage reaction using a homogeneously dispersed photocatalytic system at room temperature, under solar illumination, and without the need for additional mediators or oxidizing/reducing agents. This system presents a possible approach to support light-driven lignin depolymerization under mild conditions, which is a target of future work

Photochemical Synthesis of a Water Oxidation Catalyst Based on Cobalt Nanostructures

New cobalt-based nanocomposites have been prepared by photoreduction of Co²⁺ salts to generate cobalt nanoparticles deposited on carbon-based materials such as nanocyrstalline diamond and carbon felt. Spontaneous air oxidation converts the metal to Co₂O₃ which has been tested as a water oxidation catalyst. This work demonstrates that the cobalt oxide nanostructures can be deposited on various carbon surfaces and can catalyze the four-electron oxidation of water to oxygen under anodic bias

Solar-Driven Lignin Oxidation via Hydrogen Atom Transfer with a Dye-Sensitized TiO₂ Photoanode

Molecular-based dye-sensitized photoelectrochemical cells (DSPECs) have traditionally targeted solar-driven water splitting for the conversion of solar energy into fuels in aqueous media. This work reports the use of a DSPEC-type photoanode specifically designed to carry out chemoselective oxidation of benzylic alcohol moieties in lignin model compounds and real lignin in organic media. The TiO2-based photoanode incorporates a surface-bound Ru­(II)-based photocatalyst and solution-dissolved hydrogen atom transfer (HAT) co-catalyst to perform solar-driven photocatalytic oxidation of the lignin substrates. Under aerobic conditions with simulated solar illumination, conversion efficiencies in excess of 90% are observed for the formation of the oxidized ketone product from model compounds. The DSPEC half-cell exhibited impressive long-term durability, sustaining photocatalytic oxidation of the lignin model compound over a net illumination period of 80 h. This photoelectrochemical heterogeneous catalytic process provides a unique foundation to perform selective C–O bond cleavage for real lignin conversion technologies

Phosphonate-Derivatized Porphyrins for Photoelectrochemical Applications

A series of phosphonate-derivatized, high redox potential porphyrins with mesityl, pentafluorophenyl, and heptafluoropropyl meso-substituents were synthesized by acid-catalyzed condensation reactions. Ground and excited state redox potentials in the series were varied systematically with the electron-donating or electron-accepting nature of the meso-substitutents. The extent of excitation and injection by porphyrin singlet excited states surface-bound to SnO₂/TiO₂ core/shell metal oxide nanoparticle films varies with the excited state reduction potential, <i>E</i>°^′(P⁺/P*). With the mesityl-substituted porphyrin, high current density and sustained photocurrents are observed at pH 7 with the addition of the electron transfer donor hydroquinone

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

All-in-One Derivatized Tandem p⁺n‑Silicon–SnO₂/TiO₂ Water Splitting Photoelectrochemical Cell

Mesoporous metal oxide film electrodes consisting of derivatized 5.5 μm thick SnO₂ films with an outer 4.3 nm shell of TiO₂ added by atomic layer deposition (ALD) have been investigated to explore unbiased water splitting on p, n, and p⁺n type silicon substrates. Modified electrodes were derivatized by addition of the water oxidation catalyst, [Ru­(bda)­(4-O­(CH₂)₃PO₃H₂)-pyr)₂], <b>1</b>, (pyr = pyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate), and chromophore, [Ru­(4,4′-PO₃H₂-bpy) (bpy)₂]²⁺, <b>RuP</b>²⁺, (bpy = 2,2′-bipyridine), which form 2:1 <b>RuP</b>²⁺/<b>1</b> assemblies on the surface. At pH 5.7 in 0.1 M acetate buffer, these electrodes with a fluorine-doped tin oxide (FTO) back contact under ∼1 sun illumination (100 mW/cm²; white light source) perform efficient water oxidation with a photocurrent of 1.5 mA/cm² with an 88% Faradaic efficiency (FE) for O₂ production at an applied bias of 600 mV versus RHE (ACS Energy Lett., 2016, 1, 231−236). The SnO₂/TiO₂–chromophore–catalyst assembly was integrated with the Si electrodes by a thin layer of titanium followed by an amorphous TiO₂ (Ti/<i>a-</i>TiO₂) coating as an interconnect. In the integrated electrode, p⁺n-Si–Ti/<i>a</i>-TiO₂–SnO₂/TiO₂|-2<b>RuP</b>²⁺/<b>1</b>, the p⁺n-Si junction provided about 350 mV in added potential to the half cell. In photolysis experiments at pH 5.7 in 0.1 M acetate buffer, bias-free photocurrents approaching 100 μA/cm² were obtained for water splitting, 2H₂O → 2H₂ + O₂. The FE for water oxidation was 79% with a hydrogen efficiency of ∼100% at the Pt cathode

Efficient Light-Driven Oxidation of Alcohols Using an Organic Chromophore–Catalyst Assembly Anchored to TiO₂

The ligand 5-PO₃H₂-2,2′:5′,2″-terthiophene-5-trpy, <b>T3</b> (trpy = 2,2′:6′,2″-terpyridine), was prepared and studied in aqueous solutions along with its metal complex assembly [Ru­(<b>T3</b>)­(bpy)­(OH₂)]²⁺ (<b>T3</b>-Ru-OH₂, bpy = 2,2′-bipyridine). <b>T3</b> contains a phosphonic acid group for anchoring to a TiO₂ photoanode under aqueous conditions, a terthiophene fragment for light absorption and electron injection into TiO₂, and a terminal trpy ligand for the construction of assemblies comprising a molecular oxidation catalyst. At a TiO₂ photoanode, <b>T3</b> displays efficient injection at pH 4.35 as evidenced by the high photocurrents (∼350 uA/cm²) arising from hydroquinone oxidation. Addition of [Ru­(bpy)­(OTf)]­[OTf]₂ (bpy = 2,2′-bipyridine, OTf^– = triflate) to <b>T3</b> at the free trpy ligand forms the molecular assembly, <b>T3</b>-Ru-OH₂, with the oxidative catalyst fragment: [Ru­(trpy)­(bpy)­(OH₂)]²⁺. The new assembly, <b>T3</b>-Ru-OH₂, was used to perform efficient light-driven oxidation of phenol (230 μA/cm²) and benzyl alcohol (25 μA/cm²) in a dye-sensitized photoelectrosynthesis cell