Nanostructured Tin Catalysts for Selective Electrochemical Reduction of Carbon Dioxide to Formate

High surface area tin oxide nanocrystals prepared by a facile hydrothermal method are evaluated as electrocatalysts toward CO₂ reduction to formate. At these novel nanostructured tin catalysts, CO₂ reduction occurs selectively to formate at overpotentials as low as ∼340 mV. In aqueous NaHCO₃ solutions, maximum Faradaic efficiencies for formate production of >93% have been reached with high stability and current densities of >10 mA/cm² on graphene supports. The notable reactivity toward CO₂ reduction achieved here may arise from a compromise between the strength of the interaction between CO₂^•– and the nanoscale tin surface and subsequent kinetic activation toward protonation and further reduction

Nonaqueous Electrocatalytic Oxidation of the Alkylaromatic Ethylbenzene by a Surface Bound Ru^V(O) Catalyst

The catalyst [Ru­(Mebimpy)­(4,4′-((HO)₂OPCH₂)₂bpy­(OH₂)]²⁺, where Mebimpy is 2,6-bis­(1-methylbenzimidazol-2-yl)­pyridine and 4,4′-((HO)₂OPCH₂)₂bpy is 4,4′-bis-methlylenephosphonato-2,2′-bipyridine, attached to nanocrystalline Sn­(IV)-doped In₂O₃ (nanoITO) electrodes (nanoITO|Ru^II–OH₂²⁺) has been utilized for the electrocatalytic oxidation of the alkylaromatics ethylbenzene, toluene, and cumene in propylene carbonate/water mixtures. Oxidative activation of the surface site to nanoITO|Ru^V(O)³⁺ is followed by hydrocarbon oxidation at the surface with a rate constant of 2.5 ± 0.2 M^–1 s^–1 (<i>I</i> = 0.1 M LiClO₄, <i>T</i> = 23 ± 2 °C) for the oxidation of ethylbenzene. Electrocatalytic oxidation of ethylbenzene to acetophenone occurs with a faradic efficiency of 95%. H/D kinetic isotope effects determined for oxidation of ethylbenzene point to a mechanism involving oxygen atom insertion into a C–H bond of ethylbenzene followed by further 2e^–/2H⁺ oxidation to acetophenone

Electrocatalysis on Oxide-Stabilized, High-Surface Area Carbon Electrodes

A procedure is described for preparing and derivatizing novel, high surface area electrodes consisting of thin layers of nanostructured ITO (Sn­(IV)-doped indium tin oxide, <i>nano</i>ITO) on reticulated vitreous carbon (RVC) to give RVC|<i>nano</i>ITO. The resulting hybrid electrodes are highly stabilized oxidatively. They were surface-derivatized by phosphonate binding of the electrocatalyst, [Ru­(Mebimpy)­(4,4′-((HO)₂OPCH₂)₂bpy)­(OH₂)]²⁺ (Mebimpy = 2,6-bis­(1-methylbenzimidazol-2-yl)­pyridine; bpy = 2,2′-bipyridine) (<b>1-PO</b>_<b>3</b><b>H</b>_<b>2</b>) to give RVC|<i>nano</i>ITO-Ru^II-OH₂²⁺. The redox properties of the catalyst are retained on the electrode surface. Electrocatalytic oxidation of benzyl alcohol to benzaldehyde occurs with a 75% Faradaic efficiency compared to 57% on <i>nano</i>ITO. Electrocatalytic water oxidation at 1.4 V vs SCE on derivatized RVC|<i>nano</i>ITO electrode with an internal surface area of 19.5 cm² produced 7.3 μmoles of O₂ in 70% Faradaic yield in 50 min

A Half-Reaction Alternative to Water Oxidation: Chloride Oxidation to Chlorine Catalyzed by Silver Ion

Chloride oxidation to chlorine is a potential alternative to water oxidation to oxygen as a solar fuels half-reaction. Ag­(I) is potentially an oxidative catalyst but is inhibited by the high potentials for accessing the Ag­(II/I) and Ag­(III/II) couples. We report here that the complex ions AgCl₂^– and AgCl₃^2– form in concentrated Cl^– solutions, avoiding AgCl precipitation and providing access to the higher oxidation states by delocalizing the oxidative charge over the Cl^– ligands. Catalysis is homogeneous and occurs at high rates and low overpotentials (10 mV at the onset) with μM Ag­(I). Catalysis is enhanced in D₂O as solvent, with a significant H₂O/D₂O inverse kinetic isotope effect of 0.25. The results of computational studies suggest that Cl^– oxidation occurs by 1e^– oxidation of AgCl₃^2– to AgCl₃^– at a decreased potential, followed by Cl^– coordination, presumably to form AgCl₄^2– as an intermediate. Adding a second Cl^– results in “redox potential leveling”, with further oxidation to {AgCl₂(Cl₂)}⁻ followed by Cl₂ release

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

Dye-Sensitized Hydrobromic Acid Splitting for Hydrogen Solar Fuel Production

Hydrobromic acid (HBr) has significant potential as an inexpensive feedstock for hydrogen gas (H₂) solar fuel production through HBr splitting. Mesoporous thin films of anatase TiO₂ or SnO₂/TiO₂ core–shell nanoparticles were sensitized to visible light with a new Ru^II polypyridyl complex that served as a photocatalyst for bromide oxidation. These thin films were tested as photoelectrodes in dye-sensitized photoelectrosynthesis cells. In 1 N HBr (aq), the photocatalyst undergoes excited-state electron injection and light-driven Br^– oxidation. The injected electrons induce proton reduction at a Pt electrode. Under 100 mW cm⁻² white-light illumination, sustained photocurrents of 1.5 mA cm^–2 were measured under an applied bias. Faradaic efficiencies of 71 ± 5% for Br^– oxidation and 94 ± 2% for H₂ production were measured. A 12 μmol h^–1 sustained rate of H₂ production was maintained during illumination. The results demonstrate a molecular approach to HBr splitting with a visible light absorbing complex capable of aqueous Br^– oxidation and excited-state electron injection

Bias-Dependent Oxidative or Reductive Quenching of a Molecular Excited-State Assembly Bound to a Transparent Conductive Oxide

Visible light induced electron or hole injection by the surface-bound molecular assembly [(4,4′-(Me)₂bpy)­(4,4′-(CH₂PO₃H₂)₂bpy)­Ru^II(MebpyCH₂CH₂bpyMe)­Re^I(CO)₃Br]²⁺ (Me = CH₃, bpy =2,2′-bipyridine) into In₂O₃:Sn nanoparticles (<i>nano</i>ITO) has been investigated as a function of applied bias by transient absorption spectroscopy. The metallic properties of degenerately doped <i>nano</i>ITO allowed the driving force for electron or hole injection to be varied systematically by controlling the Fermi level of the oxide through an applied bias. At <i>E</i>_app > 0.4 V vs SCE, electron injection occurred by oxidative quenching of the Ru-based metal-to-ligand charge-transfer (MLCT) excited state to yield oxidized Ru^III. At <i>E</i>_app < 0.4 V, hole injection by reductive quenching of the MLCT excited state yielded reduced Ru^II(bpy^•–) followed by rapid intra-assembly electron transfer to generate Re^I(bpy^•–)

Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes

The hole-injection and recombination photophysics for NiO sensitized with RuP ([Ru^II(bpy)₂(4,4′-(PO₃H₂)₂-bpy)]²⁺) are explored. Ultrafast transient absorption (TA) measurements performed with an external electrochemical bias reveal the efficiency for productive hole-injection, that is, quenching of the dye excited state that results in a detectable charge-separated electron–hole pair, is linearly dependent on the electronic occupation of intragap states in the NiO film. Population of these states via a negative applied potential increases the efficiency from 0% to 100%. The results indicate the primary loss mechanism for dye-sensitized NiO is rapid nongeminate recombination enabled by the presence of latent holes in the surface of the NiO film. Our findings suggest a new design paradigm for NiO photocathodes and devices centered on the avoidance of this recombination pathway

Amplified Luminescence Quenching of Phosphorescent Metal–Organic Frameworks

Amplified luminescence quenching has been demonstrated in metal–organic frameworks (MOFs) composed of Ru­(II)-bpy building blocks with long-lived, largely triplet metal-to-ligand charge-transfer excited states. Strong non-covalent interactions between the MOF surface and cationic quencher molecules coupled with rapid energy transfer through the MOF microcrystal facilitates amplified quenching with a 7000-fold enhancement of the Stern–Völmer quenching constant for methylene blue compared to a model complex

Electrocatalytic Water Oxidation with a Copper(II) Polypeptide Complex

A self-assembly-formed tri­glycyl­glycine macro­cyclic ligand (TGG^4–) complex of Cu­(II), [(TGG^4–)­Cu^II–OH₂]^2–, efficiently catalyzes water oxidation in a phosphate buffer at pH 11 at room temperature by a well-defined mechanism. In the mechanism, initial oxidation to Cu­(III) is followed by further oxidation to a formal “Cu­(IV)” with formation of a peroxide intermediate, which undergoes further oxidation to release oxygen and close the catalytic cycle. The catalyst exhibits high stability and activity toward water oxidation under these conditions with a high turnover frequency of 33 s^–1