63 research outputs found
Electrocatalytic H<sub>2</sub> Evolution by Proton-Gated Hangman Iron Porphyrins
The
ability to control proton translocation is essential for optimizing electrocatalytic
reductions in acidic solutions. We have synthesized a series of new
hangman iron porphyrins with hanging groups of differing proton-donating
abilities and evaluated their electrocatalytic hydrogen-evolving ability
using foot-of-the-wave analysis. In the presence of excess triphenylphosphine,
iron porphyrins initiate proton reduction electrocatalysis upon reduction
to Fe<sup>I</sup>. By changing the proton-donating ability of the
hanging group, we can affect the rate of catalysis by nearly 3 orders
of magnitude. The presence of an acid/base moiety in the second coordination
sphere results in a marked increase in turnover frequency when extrapolated
to zero overpotential
Chromium(IV) Siloxide
The reaction of NaÂ(OSi<sup>t</sup>Bu<sub>2</sub>Me) with
CrCl<sub>3</sub> yields solid [CrÂ(OSi<sup>t</sup>Bu<sub>2</sub>Me)<sub>3</sub>]<sub><i>n</i></sub> (<b>1</b>), which can
be crystallized in the presence of excess NaÂ(OSi<sup>t</sup>Bu<sub>2</sub>Me) to yield [NaÂ(THF)]Â[CrÂ(OSi<sup>t</sup>Bu<sub>2</sub>Me)<sub>4</sub>] (<b>2</b>). This complex is oxidized to yield CrÂ(OSi<sup>t</sup>Bu<sub>2</sub>Me)<sub>4</sub> (<b>3</b>), a crystalline
chromiumÂ(IV) siloxide complex that is air- and moisture-stable. Electronic
spectroscopic analysis of the absorption spectrum of <b>3</b> indicates a particularly weak ligand field (Î<sub>T</sub> =
7940 cm<sup>â1</sup>) and covalent CrâO bonding. <b>3</b> provides the first structural and spectroscopic characterization
of a homoleptic chromiumÂ(IV) siloxide complex and provides a benchmark
for tetrahedral chromiumÂ(IV) ions residing in solid oxide lattices
Spectroscopic Studies of Nanoparticulate Thin Films of a Cobalt-Based Oxygen Evolution Catalyst
Nanoparticle (NP) cobaltâphosphate
(Co-P<sub>i</sub>) water
oxidation catalysts are prepared as thin films by anodic electrodeposition
from solutions of Co<sup>2+</sup> dissolved in proton-accepting electrolytes.
Compositional and structural insight into the nature of the catalyst
film is provided from advanced spectroscopy. Infrared spectra demonstrate
that counteranions incorporate into the Co-P<sub>i</sub> thin films
and that the phosphate ion, among various anion electrolytes, exhibits
the highest binding affinity to the cobalt centers. Atomic force microscopy
images show a highly porous morphology of the thin film that is composed
of Co-P<sub>i</sub> NPs. Whereas conventional X-ray powder diffraction
technique shows catalyst films to be amorphous, synchrotron-based
X-ray grazing incidence diffraction reveals well-defined diffraction
patterns that are indicative of long-range ordering within the film.
Azimuthal scans imply that as-prepared films possess a highly preferred
orientation and texture on the electrode surface
Oxygen Reduction Reactions of Monometallic Rhodium Hydride Complexes
Selective reduction of oxygen is mediated by a series
of monometallic
rhodiumÂ(III) hydride complexes. Oxidative addition of HCl to <i>trans</i>-Rh<sup>I</sup>ClÂ(L)Â(PEt<sub>3</sub>)<sub>2</sub> (<b>1a</b>, L = CO; <b>1b</b>, L = 2,6-dimethylphenylisocyanide
(CNXy); <b>1c</b>, L = 1-adamantylisocyanide (CNAd)) produces
the corresponding Rh<sup>III</sup> hydride complex <i>cis</i>-<i>trans</i>-Rh<sup>III</sup>Cl<sub>2</sub>HÂ(L)Â(PEt<sub>3</sub>)<sub>2</sub> (<b>2a</b>â<b>c</b>). The
measured equilibrium constants for the HCl-addition reactions show
a pronounced dependence on the identity of the âLâ ligand.
The hydride complexes effect the reduction of O<sub>2</sub> to water
in the presence of HCl, generating <i>trans</i>-Rh<sup>III</sup>Cl<sub>3</sub>(L)Â(PEt<sub>3</sub>)<sub>2</sub> (<b>3a</b>â<b>c</b>) as the metal-containing product. In the case of <b>2a</b>, smooth conversion to <b>3a</b> proceeds without spectroscopic
evidence for an intermediate species. For <b>2b/c</b>, an aqua
intermediate, <i>cis</i>-<i>trans</i>-[Rh<sup>III</sup>(OH<sub>2</sub>)ÂCl<sub>2</sub>(L)Â(PEt<sub>3</sub>)<sub>2</sub>]Cl (<b>5b/c</b>), forms along the pathway to producing <b>3b/c</b> as the final products. The aqua complexes were independently
prepared by treating peroxo complexes <i>trans</i>-Rh<sup>III</sup>ClÂ(L)Â(Ρ<sup>2</sup>-O<sub>2</sub>)Â(PEt<sub>3</sub>)<sub>2</sub> (<b>4b/c</b>) with HCl to rapidly produce a mixture
of <b>5b/c</b> and <b>3b/c</b>. The reactivity of the
peroxo species demonstrates that they are plausible intermediates
in the O<sub>2</sub>-reduction chemistry of hydride complexes <b>2a</b>â<b>c</b>. These results together show that
monometallic rhodium hydride complexes are capable of promoting selective
reduction of oxygen to water and that this reaction may be controlled
with systematic alteration of the ancillary ligand set
Chromium(IV) Siloxide
The reaction of NaÂ(OSi<sup>t</sup>Bu<sub>2</sub>Me) with
CrCl<sub>3</sub> yields solid [CrÂ(OSi<sup>t</sup>Bu<sub>2</sub>Me)<sub>3</sub>]<sub><i>n</i></sub> (<b>1</b>), which can
be crystallized in the presence of excess NaÂ(OSi<sup>t</sup>Bu<sub>2</sub>Me) to yield [NaÂ(THF)]Â[CrÂ(OSi<sup>t</sup>Bu<sub>2</sub>Me)<sub>4</sub>] (<b>2</b>). This complex is oxidized to yield CrÂ(OSi<sup>t</sup>Bu<sub>2</sub>Me)<sub>4</sub> (<b>3</b>), a crystalline
chromiumÂ(IV) siloxide complex that is air- and moisture-stable. Electronic
spectroscopic analysis of the absorption spectrum of <b>3</b> indicates a particularly weak ligand field (Î<sub>T</sub> =
7940 cm<sup>â1</sup>) and covalent CrâO bonding. <b>3</b> provides the first structural and spectroscopic characterization
of a homoleptic chromiumÂ(IV) siloxide complex and provides a benchmark
for tetrahedral chromiumÂ(IV) ions residing in solid oxide lattices
Oxygen Reduction Reactions of Monometallic Rhodium Hydride Complexes
Selective reduction of oxygen is mediated by a series
of monometallic
rhodiumÂ(III) hydride complexes. Oxidative addition of HCl to <i>trans</i>-Rh<sup>I</sup>ClÂ(L)Â(PEt<sub>3</sub>)<sub>2</sub> (<b>1a</b>, L = CO; <b>1b</b>, L = 2,6-dimethylphenylisocyanide
(CNXy); <b>1c</b>, L = 1-adamantylisocyanide (CNAd)) produces
the corresponding Rh<sup>III</sup> hydride complex <i>cis</i>-<i>trans</i>-Rh<sup>III</sup>Cl<sub>2</sub>HÂ(L)Â(PEt<sub>3</sub>)<sub>2</sub> (<b>2a</b>â<b>c</b>). The
measured equilibrium constants for the HCl-addition reactions show
a pronounced dependence on the identity of the âLâ ligand.
The hydride complexes effect the reduction of O<sub>2</sub> to water
in the presence of HCl, generating <i>trans</i>-Rh<sup>III</sup>Cl<sub>3</sub>(L)Â(PEt<sub>3</sub>)<sub>2</sub> (<b>3a</b>â<b>c</b>) as the metal-containing product. In the case of <b>2a</b>, smooth conversion to <b>3a</b> proceeds without spectroscopic
evidence for an intermediate species. For <b>2b/c</b>, an aqua
intermediate, <i>cis</i>-<i>trans</i>-[Rh<sup>III</sup>(OH<sub>2</sub>)ÂCl<sub>2</sub>(L)Â(PEt<sub>3</sub>)<sub>2</sub>]Cl (<b>5b/c</b>), forms along the pathway to producing <b>3b/c</b> as the final products. The aqua complexes were independently
prepared by treating peroxo complexes <i>trans</i>-Rh<sup>III</sup>ClÂ(L)Â(Ρ<sup>2</sup>-O<sub>2</sub>)Â(PEt<sub>3</sub>)<sub>2</sub> (<b>4b/c</b>) with HCl to rapidly produce a mixture
of <b>5b/c</b> and <b>3b/c</b>. The reactivity of the
peroxo species demonstrates that they are plausible intermediates
in the O<sub>2</sub>-reduction chemistry of hydride complexes <b>2a</b>â<b>c</b>. These results together show that
monometallic rhodium hydride complexes are capable of promoting selective
reduction of oxygen to water and that this reaction may be controlled
with systematic alteration of the ancillary ligand set
A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid
First-row
metals have been a target for the development of oxygen
evolution reaction (OER) catalysts because they comprise noncritical
elements. We now report a comprehensive electrochemical characterization
of manganese oxide (MnOx) over a wide pH range, and establish MnOx
as a functionally stable OER catalyst owing to self-healing, is derived
from MnOx redeposition that offsets catalyst dissolution during turnover.
To study this process in detail, the oxygen evolution mechanism of
MnOx was investigated electrokinetically over a pH range spanning
acidic, neutral, and alkaline conditions. In the alkaline pH regime,
a âź60 mV/decade Tafel slope and inverse first-order dependence
on proton concentration were observed, whereas the OER acidic pH regime
exhibited a quasi-infinite Tafel slope and zeroth-order dependence
on proton concentration. The results reflect two competing mechanisms:
a one-electron one-proton PCET pathway that is dominant under alkaline
conditions and a Mn<sup>3+</sup> disproportionation process, which
predominates under acidic conditions. Reconciling the rate laws of
these two OER pathways with that of MnOx electrodeposition elucidates
the self-healing characteristics of these catalyst films. The intersection
of the kinetic profile of deposition and that of water oxidation as
a function of pH defines the region of kinetic stability for MnOx
and importantly establishes that a non-noble metal oxide OER catalyst
may be operated in acid by exploiting a self-healing process
Cobalt in a Bis-β-diketiminate Environment
The reaction of Co<sub>2</sub>(mesityl)<sub>4</sub> with
acetonitrile
leads to the formation of a planar, low spin, bis-β-diketiminate
cobaltÂ(II) complex, (1-mesitylbutane-1,3-diimine)<sub>2</sub>Co (<b>1</b>). EPR spectroscopy, magnetic studies, and DFT calculations
reveal the CoÂ(II) ion to reside in a tetragonal ligand field with
a <sup>2</sup>B<sub>2</sub>(d<sub><i>yz</i></sub>)<sup>1</sup> ground state electronic configuration. Oxidation of <b>1</b> with ferrocenium hexafluorophosphate furnishes (1-mesitylbutane-1,3-diimine)<sub>2</sub>CoÂ(THF)<sub>2</sub>PF<sub>6</sub> (<b>2</b>). The absence
of significant changes in the metalâligand bond metrics of
the X-ray crystal structures of <b>1</b> and <b>2</b> supports
ligand participation in the oxidation event. Moreover, no significant
changes in CâC or CâN bond lengths are observed by X-ray
crystallography upon oxidation of a β-diketiminate ligand, in
contrast to typical redox noninnocent ligand platforms
Mechanistic Studies of the Oxygen Evolution Reaction Mediated by a NickelâBorate Thin Film Electrocatalyst
A critical determinant
of solar-driven water splitting efficiency
is the kinetic profile of the O<sub>2</sub> evolving catalyst (OEC).
We now report the kinetic profiles of water splitting by a self-assembled
nickelâborate (NiB<sub>i</sub>) OEC. Mechanistic studies of
anodized films of NiB<sub>i</sub> exhibit the low Tafel slope of 2.3
Ă <i>RT</i>/2<i>F</i> (30 mV/decade at 25
°C). This Tafel slope together with an inverse third order rate
dependence on H<sup>+</sup> activity establishes NiB<sub>i</sub> as
an ideal catalyst to be used in the construction of photoelectrochemical
devices for water splitting. In contrast, nonanodized NiB<sub>i</sub> films display significantly poorer activity relative to their anodized
congeners that we attribute to a more sluggish electron transfer from
the catalyst resting state. Borate is shown to play two ostensibly
antagonistic roles in OEC activity: as a promulgator of catalyst activity
by enabling proton-coupled electron transfer (PCET) and as an inhibitor
in its role as an adsorbate of active sites. By defining the nature
of the PCET pre-equilibrium that occurs during turnover, trends in
catalyst activity may be completely reversed at intermediate pH as
compared to those at pH extremes. These results highlight the critical
role of PCET pre-equilibria in catalyst self-assembly and turnover,
and accordingly suggest a reassessment in how OEC activities of different
catalysts are compared and rationalized
Oxidative Degradation of Multi-Carbon Substrates by an Oxidic Cobalt Phosphate Catalyst
The
development of heterogeneous catalysts to affect the activation of recalcitrant biomolecules
has applications for biomass processing, biomass fuel cells, and wastewater
remediation. We demonstrate that a cobalt oxygen evolution catalyst
(Co-OEC) can catalyze the oxidation of carbon feedstocks completely
to CO<sub>2</sub>. A quantitative analysis of the product distribution
from the oxidative degradation of the C<sub>2</sub> compound, ethylene
glycol, is elaborated and a reaction sequence is proposed. The Co-OEC
is also found to be competent for oxidatively degrading C<sub>2+</sub> compounds, including glucose and lignin, to carbon dioxide at consequential
Faradaic efficiencies
- âŚ