Thomas J. Meyer

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/cm(2) 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.

Rapid Selective Electrocatalytic Reduction of Carbon Dioxide to Formate by an Iridium Pincer Catalyst Immobilized on Carbon Nanotube Electrodes

An iridium pincer dihydride catalyst was immobilized on carbon nanotube-coated gas diffusion electrodes (GDEs) by using a non-covalent binding strategy. The as-prepared GDEs are efficient, selective, durable, gas permeable electrodes for electrocatalytic reduction of CO2 to formate. High turnover numbers (ca. 54,000) and turnover frequencies (ca. 15u2005s(-1)) were enabled by the novel electrode architecture in aqueous solutions saturated in CO2 with added HCO3(-).

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

The six-coordinate Fe(III)-aqua complex [Fe(III)(dpaq)(H2O)](2+) (1, dpaq is 2-[bis(pyridine-2-ylmethyl)]amino-N-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 ko = 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.

Photoinduced electron transfer in a chromophore-catalyst assembly anchored to TiO2.

Photoinduced formation, separation, and buildup of multiple redox equivalents are an integral part of cycles for producing solar fuels in dye-sensitized photoelectrosynthesis cells (DSPECs). Excitation wavelength-dependent electron injection, intra-assembly electron transfer, and pH-dependent back electron transfer on TiO(2) were investigated for the molecular assembly [((PO(3)H(2)-CH(2))-bpy)(2)Ru(a)(bpy-NH-CO-trpy)Ru(b)(bpy)(OH(2))](4+) ([TiO(2)-Ru(a)(II)-Ru(b)(II)-OH(2)](4+); ((PO(3)H(2)-CH(2))(2)-bpy = ([2,2-bipyridine]-4,4-diylbis(methylene))diphosphonic acid); bpy-ph-NH-CO-trpy = 4-([2,2:6,2″-terpyridin]-4-yl)-N-((4-methyl-[2,2-bipyridin]-4-yl)methyl) benzamide); bpy = 2,2-bipyridine). This assembly combines a light-harvesting chromophore and a water oxidation catalyst linked by a synthetically flexible saturated bridge designed to enable long-lived charge-separated states. Following excitation of the chromophore, rapid electron injection into TiO(2) and intra-assembly electron transfer occur on the subnanosecond time scale followed by microsecond-millisecond back electron transfer from the semiconductor to the oxidized catalyst, [TiO(2)(e(-))-Ru(a)(II)-Ru(b)(III)-OH(2)](4+)→[TiO(2)-Ru(a)(II)-Ru(b)(II)-OH(2)](4+).

Water oxidation by an electropolymerized catalyst on derivatized mesoporous metal oxide electrodes.

A general electropolymerization/electro-oligomerization strategy is described for preparing spatially controlled, multicomponent films and surface assemblies having both light harvesting chromophores and water oxidation catalysts on metal oxide electrodes for applications in dye-sensitized photoelectrosynthesis cells (DSPECs). The chromophore/catalyst ratio is controlled by the number of reductive electrochemical cycles. Catalytic rate constants for water oxidation by the polymer films are similar to those for the phosphonated molecular catalyst on metal oxide electrodes, indicating that the physical properties of the catalysts are not significantly altered in the polymer films. Controlled potential electrolysis shows sustained water oxidation over multiple hours with no decrease in the catalytic current.

Synthesis of Phosphonic Acid Derivatized Bipyridine Ligands and Their Ruthenium Complexes

Water-stable, surface-bound chromophores, catalysts, and assemblies are an essential element in dye-sensitized photoelectrosynthesis cells for the generation of solar fuels by water splitting and CO2 reduction to CO, other oxygenates, or hydrocarbons. Phosphonic acid derivatives provide a basis for stable chemical binding on metal oxide surfaces. We report here the efficient synthesis of 4,4-bis(diethylphosphonomethyl)-2,2-bipyridine and 4,4-bis(diethylphosphonate)-2,2-bipyridine, as well as the mono-, bis-, and tris-substituted ruthenium complexes, [Ru(bpy)2(Pbpy)](2+), [Ru(bpy)(Pbpy)2](2+), [Ru(Pbpy)3](2+), [Ru(bpy)2(CPbpy)](2+), [Ru(bpy)(CPbpy)2](2+), and [Ru(CPbpy)3](2+) [bpy = 2,2-bipyridine; Pbpy = 4,4-bis(phosphonic acid)-2,2-bipyridine; CPbpy = 4,4-bis(methylphosphonic acid)-2,2-bipyridine].

An amide-linked chromophore-catalyst assembly for water oxidation.

The synthesis and analysis of a new amide-linked, dinuclear [Ru(bpy)(2)(bpy-ph-NH-CO-trpy)Ru(bpy)(OH(2))](4+) (bpy = 2,2-bipyridine; bpy-ph-NH-CO-trpy = 4-(2,2:6,2-terpyridin-4-yl)-N-[(4-methyl-2,2-bipyridin-4-yl)methyl]benzamide) assembly that incorporates both a light-harvesting chromophore and a water oxidation catalyst are described. With the saturated methylene linker present, the individual properties of both the chromophore and catalyst are retained including water oxidation catalysis and relatively slow energy transfer from the chromophore excited state to the catalyst.

Blue-green iridium(III) emitter and comprehensive photophysical elucidation of heteroleptic cyclometalated iridium(III) complexes

Synthesis and photophysical properties of the highly emissive complex [Ir(Fppy)2(dmb)](+) are reported along with those of additional heteroleptic cyclometalated Ir(III) complexes, [Ir(ppy)2(NN)](PF6): FppyH = 2-(2,4-difluorophenyl)pyridine; ppyH = 2-phenylpyridine; NN = 4,4-dimethyl-2,2-bipyridine (dmb), 1,10-phenanthroline (phen), or 4,7-diphenyl-1,10-phenanthroline (Ph2phen). TD-DFT calculations and Franck-Condon emission spectral band shape analyses show that the broad and structureless emission from [Ir(Fppy)2(dmb)](+) in acetonitrile at 298 K mainly arises from a triplet metal-to-ligand charge-transfer excited state, (3)MLCTIr(ppy)→NN. The emission maximum varies systematically with variations in electron-donating or -withdrawing substituents on both the NN and the Xppy ligands, and emission efficiencies are high, with an impressive ϕ ≈ 1 for [Ir(Fppy)2(dmb)](+). At 77 K in propionitrile/butyronitrile (4/5, v/v), emission from [Ir(Fppy)2(dmb)](+) is narrow and highly structured consistent with a triplet ligand-centered transition ((3)LCNN) and an inversion in excited-state ordering between the (3)MLCTIr(ppy)→NN and (3)LCNN states. In a semirigid film of the poly(ethyleneglycol)dimethacrylate with nine ethylene glycol spacers, PEG-DMA550, emission from [Ir(Fppy)2(dmb)](+) is MLCT-based. The thermal sensitivity of the photophysical properties of this excited state points to a possible application as a temperature sensor in addition to its more known use in light-emitting devices.

Stabilization of Ruthenium(II) Polypyridyl Chromophores on Nanoparticle Metal-Oxide Electrodes in Water by Hydrophobic PMMA Overlayers

We describe a poly(methyl methacrylate) (PMMA) dip-coating procedure, which results in surface stabilization of phosphonate and carboxylate derivatives of Ru(II)-polypyridyl complexes surface-bound to mesoporous nanoparticle TiO2 and nanoITO films in aqueous solutions. As shown by contact angle and transmission electron microscopy (TEM) measurements, PMMA oligomers conformally coat the metal-oxide nanoparticles changing the mesoporous films from hydrophilic to hydrophobic. The thickness of the PMMA overlayer on TiO2-Ru(II) can be controlled by changing the wt % of PMMA in the dipcoating solution. There are insignificant perturbations in electrochemical or spectral properties at thicknesses of up to 2.1 nm with the Ru(III/II) couple remaining electrochemically reversible and E1/2 values and current densities nearly unaffected. Surface binding by PMMA overlayers results in stable surface binding even at pH 12 with up to a ∼100-fold enhancement in photostability. As shown by transient absorption measurements, the MLCT excited state(s) of phosphonate derivatized [Ru(bpy)2((4,4-(OH)2PO)2bpy)](2+) undergo efficient injection and back electron transfer with pH independent kinetics characteristic of the local pH in the initial loading solution.

Visible Light Driven Benzyl Alcohol Dehydrogenation in a Dye-Sensitized Photoelectrosynthesis Cell

Light-driven dehydrogenation of benzyl alcohol (BnOH) to benzaldehyde and hydrogen has been shown to occur in a dye-sensitized photoelectrosynthesis cell (DSPEC). In the DSPEC, the photoanode consists of mesoporous films of TiO2 nanoparticles or of core/shell nanoparticles with tin-doped In2O3 nanoparticle (nanoITO) cores and thin layers of TiO2 deposited by atomic layer deposition (nanoITO/TiO2). Metal oxide surfaces were coderivatized with both a ruthenium polypyridyl chromophore in excess and an oxidation catalyst. Chromophore excitation and electron injection were followed by cross-surface electron-transfer activation of the catalyst to -Ru(IV)═O(2+), which then oxidizes benzyl alcohol to benzaldehyde. The injected electrons are transferred to a Pt electrode for H2 production. The nanoITO/TiO2 core/shell structure causes a decrease of up to 2 orders of magnitude in back electron-transfer rate compared to TiO2. At the optimized shell thickness, sustained absorbed photon to current efficiency of 3.7% was achieved for BnOH dehydrogenation, an enhancement of ~10 compared to TiO2.