4 research outputs found
Probing the Viability of Oxo-Coupling Pathways in Iridium-Catalyzed Oxygen Evolution
A series
of Cp*Ir<sup>III</sup> dimers have been synthesized to elucidate the
mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed
O<sub>2</sub> evolution. The oxidative stability of the precursors
toward nanoparticle formation and their oxygen evolution activity
have been investigated and compared to suitable monomeric analogues.
We found that precursors bearing monodentate NHC ligands degraded
to form nanoparticles (NPs), and accordingly their O<sub>2</sub> evolution
rates were not significantly influenced by their nuclearity or distance
between the two metals in the dimeric precursors. A doubly chelating
bis-pyridineâpyrazolide ligand provided an oxidation-resistant
ligand framework that allowed a more meaningful comparison of catalytic
performance of dimers with their corresponding monomers. With sodium
periodate (NaIO<sub>4</sub>) as the oxidant, the dimers provided significantly
lower O<sub>2</sub> evolution rates per [Ir] than the monomer, suggesting
a negative interaction instead of cooperativity in the catalytic cycle.
Electrochemical analysis of the dimers further substantiates the notion
that no radical oxyl-coupling pathways are accessible. We thus conclude
that the alternative path, nucleophilic attack of water on high-valent
Ir-oxo species, may be the preferred mechanistic pathway of water
oxidation with these catalysts, and bimolecular oxo-coupling is not
a valid mechanistic alternative as in the related ruthenium chemistry,
at least in the present system
New Ir Bis-Carbonyl Precursor for Water Oxidation Catalysis
This paper introduces Ir<sup>I</sup>(CO)<sub>2</sub>(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<sup>III</sup>(pyalc)ÂOH 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 NaIO<sub>4</sub> results in formation of a blue Ir<sup>IV</sup> 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<sup>IV</sup> species. The species formed
from oxidation of the IrÂ(CO)<sub>2</sub>(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 IrÂ(pyalc) active species both catalytically and spectroscopically,
although complete structural analysis is still elusive. Once IrÂ(CO)<sub>2</sub>(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 IrÂ(pyalc) isomers may exist
in solution. By <sup>1</sup>H NMR, activated IrÂ(CO)<sub>2</sub>(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>
Photodriven Oxidation of Surface-Bound Iridium-Based Molecular Water-Oxidation Catalysts on Perylene-3,4-dicarboximide-Sensitized TiO<sub>2</sub> Electrodes Protected by an Al<sub>2</sub>O<sub>3</sub> 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 Al<sub>2</sub>O<sub>3</sub> deposited by atomic layer deposition (ALD) before
catalyst loading on both the stability and the charge transfer dynamics
in organic dye-sensitized TiO<sub>2</sub> photoanodes containing iridium-based
molecular water-oxidation catalysts. In the TiO<sub>2</sub>|dye|Al<sub>2</sub>O<sub>3</sub>|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
Solution Structures of Highly Active Molecular Ir Water-Oxidation Catalysts from Density Functional Theory Combined with High-Energy Xâray Scattering and EXAFS Spectroscopy
The
solution structures of highly active Ir water-oxidation catalysts
are elucidated by combining density functional theory, high-energy
X-ray scattering (HEXS), and extended X-ray absorption fine structure
(EXAFS) spectroscopy. We find that the catalysts are Ir dimers with
mono-Ό-O cores and terminal anionic ligands, generated in situ
through partial oxidation of a common catalyst precursor. The proposed
structures are supported by <sup>1</sup>H and <sup>17</sup>O NMR,
EPR, resonance Raman and UVâvis spectra, electrophoresis, etc.
Our findings are particularly valuable to understand the mechanism
of water oxidation by highly reactive Ir catalysts. Importantly, our
DFT-EXAFS-HEXS methodology provides a new in situ technique for characterization
of active species in catalytic systems