Electrocatalytic Water Oxidation by a Copper(II) Complex of an Oxidation-Resistant Ligand

The copper­(II) complex Cu­(pyalk)₂ (pyalk = 2-pyridyl-2-propanoate) is a robust homogeneous water-oxidation electrocatalyst under basic conditions (pH > 10.4). Water oxidation occurs at a relatively low overpotential for copper of 520–580 mV with a turnover frequency of ∼0.7 s^–1. Controlled potential electrolysis experiments over 12 h at 1.1 V vs NHE resulted in the formation of >30 catalytic turnovers of O₂ with only ∼20% catalyst degradation. The robustness of the catalyst under fairly harsh conditions and the low overpotential further highlight the oxidation resistance and strong donor character of pyalk

Electrocatalytic Water Oxidation by a Copper(II) Complex of an Oxidation-Resistant Ligand

The copper­(II) complex Cu­(pyalk)₂ (pyalk = 2-pyridyl-2-propanoate) is a robust homogeneous water-oxidation electrocatalyst under basic conditions (pH > 10.4). Water oxidation occurs at a relatively low overpotential for copper of 520–580 mV with a turnover frequency of ∼0.7 s^–1. Controlled potential electrolysis experiments over 12 h at 1.1 V vs NHE resulted in the formation of >30 catalytic turnovers of O₂ with only ∼20% catalyst degradation. The robustness of the catalyst under fairly harsh conditions and the low overpotential further highlight the oxidation resistance and strong donor character of pyalk

Electrocatalytic Water Oxidation by a Copper(II) Complex of an Oxidation-Resistant Ligand

The copper­(II) complex Cu­(pyalk)₂ (pyalk = 2-pyridyl-2-propanoate) is a robust homogeneous water-oxidation electrocatalyst under basic conditions (pH > 10.4). Water oxidation occurs at a relatively low overpotential for copper of 520–580 mV with a turnover frequency of ∼0.7 s^–1. Controlled potential electrolysis experiments over 12 h at 1.1 V vs NHE resulted in the formation of >30 catalytic turnovers of O₂ with only ∼20% catalyst degradation. The robustness of the catalyst under fairly harsh conditions and the low overpotential further highlight the oxidation resistance and strong donor character of pyalk

Electrochemical CO₂ Reduction to Hydrocarbons on a Heterogeneous Molecular Cu Catalyst in Aqueous Solution

Exploration of heterogeneous molecular catalysts combining the atomic-level tunability of molecular structures and the practical handling advantages of heterogeneous catalysts represents an attractive approach to developing high-performance catalysts for important and challenging chemical reactions such as electrochemical carbon dioxide reduction which holds the promise for converting emissions back to fuels utilizing renewable energy. Thus, far, efficient and selective electroreduction of CO₂ to deeply reduced products such as hydrocarbons remains a big challenge. Here, we report a molecular copper-porphyrin complex (copper­(II)-5,10,15,20-tetrakis­(2,6-dihydroxyphenyl)­porphyrin) that can be used as a heterogeneous electrocatalyst with high activity and selectivity for reducing CO₂ to hydrocarbons in aqueous media. At −0.976 V vs the reversible hydrogen electrode, the catalyst is able to drive partial current densities of 13.2 and 8.4 mA cm^–2 for methane and ethylene production from CO₂ reduction, corresponding to turnover frequencies of 4.3 and 1.8 molecules·site^–1·s^–1 for methane and ethylene, respectively. This represents the highest catalytic activity to date for hydrocarbon production over a molecular CO₂ reduction electrocatalyst. The unprecedented catalytic performance is attributed to the built-in hydroxyl groups in the porphyrin structure and the reactivity of the copper­(I) metal center

Photodriven Oxidation of Surface-Bound Iridium-Based Molecular Water-Oxidation Catalysts on Perylene-3,4-dicarboximide-Sensitized TiO₂ Electrodes Protected by an Al₂O₃ Layer

Improving stability and slowing charge recombination are some of the greatest challenges in the development of dye-sensitized photoelectrochemical cells (DSPECs) for solar fuels production. We have investigated the effect of encasing dye molecules in varying thicknesses of Al₂O₃ deposited by atomic layer deposition (ALD) before catalyst loading on both the stability and the charge transfer dynamics in organic dye-sensitized TiO₂ photoanodes containing iridium-based molecular water-oxidation catalysts. In the TiO₂|dye|Al₂O₃|catalyst electrodes, a sufficiently thick ALD layer protects the perylene-3,4-dicarboximide (PMI) chromophores from degradation over several weeks of exposure to light. The insulating capacity of the layer allows a higher photocurrent in the presence of ALD while initial charge injection is slowed by only 1.6 times, as observed by femtosecond transient absorption spectroscopy. Rapid picosecond-scale catalyst oxidation is observed in the presence of a dinuclear catalyst, IrIr, but is slowed to tens of picoseconds for a mononuclear catalyst, IrSil, that incorporates a long linker. Photoelectrochemical experiments demonstrate higher photocurrents with IrSil compared to IrIr, which show that recombination is slower for IrSil, while higher photocurrents with IrIr upon addition of ALD layers confirm that ALD successfully slows charge recombination. These findings demonstrate that, beyond stability improvements, ALD can contribute to tuning charge transfer dynamics in photoanodes for solar fuels production and may be particularly useful for slowing charge recombination and accounting for varying charge transfer rates based on the molecular structures of incorporated catalysts

New Ir Bis-Carbonyl Precursor for Water Oxidation Catalysis

This paper introduces Ir^I(CO)₂(pyalc) (pyalc = (2-pyridyl)-2-propanoate) as an atom-efficient precursor for Ir-based homogeneous oxidation catalysis. This compound was chosen to simplify analysis of the water oxidation catalyst species formed by the previously reported Cp*Ir^III(pyalc)­OH water oxidation precatalyst. Here, we present a comparative study on the chemical and catalytic properties of these two precursors. Previous studies show that oxidative activation of Cp*Ir-based precursors with NaIO₄ results in formation of a blue Ir^IV species. This activation is concomitant with the loss of the placeholder Cp* ligand which oxidatively degrades to form acetic acid, iodate, and other obligatory byproducts. The activation process requires substantial amounts of primary oxidant, and the degradation products complicate analysis of the resulting Ir^IV species. The species formed from oxidation of the Ir­(CO)₂(pyalc) precursor, on the other hand, lacks these degradation products (the CO ligands are easily lost upon oxidation) which allows for more detailed examination of the resulting Ir­(pyalc) active species both catalytically and spectroscopically, although complete structural analysis is still elusive. Once Ir­(CO)₂(pyalc) is activated, the system requires acetic acid or acetate to prevent the formation of nanoparticles. Investigation of the activated bis-carbonyl complex also suggests several Ir­(pyalc) isomers may exist in solution. By ¹H NMR, activated Ir­(CO)₂(pyalc) has fewer isomers than activated Cp*Ir complexes, allowing for advanced characterization. Future research in this direction is expected to contribute to a better structural understanding of the active species. A diol crystallization agent was needed for the structure determination of <b>3</b>

High-Potential Porphyrins Supported on SnO₂ and TiO₂ Surfaces for Photoelectrochemical Applications

We report CF₃-substituted porphyrins and evaluate their use as photosensitizers in water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) by characterizing interfacial electron transfer on metal oxide surfaces. By using (CF₃)₂C₆H₃ instead of C₆F₅ substituents at the meso positions, we obtain the desired high potentials while avoiding the sensitivity of C₆F₅ substituents to nucleophilic substitution, a process that limits the types of synthetic reactions that can be used. Both the number of CF₃ groups and the central metal tune the ground and excited-state potentials. A pair of porphyrins bearing carboxylic acids as anchoring groups were deposited on SnO₂ and TiO₂ surfaces, and the interfacial charge-injection and charge-recombination kinetics were characterized by using a combination of computational modeling, terahertz measurements, and transient absorption spectroscopy. We find that both free-base and metalated porphyrins inject into SnO₂ and that recombination is slower for the latter case. These findings demonstrate that (CF₃)₂C₆H₃-substituted porphyrins are promising photosensitizers for use in WS-DSPECs

End-On Bound Iridium Dinuclear Heterogeneous Catalysts on WO₃ for Solar Water Oxidation

Heterogeneous catalysts with atomically defined active centers hold great promise for high-performance applications. Among them, catalysts featuring active moieties with more than one metal atom are important for chemical reactions that require synergistic effects but are rarer than single atom catalysts (SACs). The difficulty in synthesizing such catalysts has been a key challenge. Recent progress in preparing dinuclear heterogeneous catalysts (DHCs) from homogeneous molecular precursors has provided an effective route to address this challenge. Nevertheless, only side-on bound DHCs, where both metal atoms are affixed to the supporting substrate, have been reported. The competing end-on binding mode, where only one metal atom is attached to the substrate and the other metal atom is dangling, has been missing. Here, we report the first observation that end-on binding is indeed possible for Ir DHCs supported on WO₃. Unambiguous evidence supporting the binding mode was obtained by <i>in situ</i> diffuse reflectance infrared Fourier transform spectroscopy and high-angle annular dark-field scanning transmission electron microscopy. Density functional theory calculations provide additional support for the binding mode, as well as insights into how end-on bound DHCs may be beneficial for solar water oxidation reactions. The results have important implications for future studies of highly effective heterogeneous catalysts for complex chemical reactions