9 research outputs found
Medium Effects Are as Important as Catalyst Design for Selectivity in Electrocatalytic Oxygen Reduction by IronāPorphyrin Complexes
Several
substituted ironāporphyrin complexes were evaluated
for oxygen reduction reaction (ORR) electrocatalysis in different
homogeneous and heterogeneous media. The selectivity for four-electron
reduction to H<sub>2</sub>O versus two-electron reduction to H<sub>2</sub>O<sub>2</sub> varies substantially from one medium to another
for a given catalyst. In many cases, the influence of the medium in
which the catalyst is evaluated has a larger effect on the observed
selectivity than the factors attributable to chemical modification
of the catalyst. For instance, introduction of potential proton relays
has variable effects depending on the catalyst medium. Thus, comparisons
of selectivity results from supported and soluble molecular ORR electrocatalysts
must be interpreted with caution, as selectivity is a property not
only of the catalyst, but also of the larger mesoscale environment
beyond the catalyst. Still, in all the direct pairwise comparisons
in the same medium, the catalysts with potential proton relays have
similar or better selectivity for the preferred 4<i>e</i><sup>ā</sup> path
Molecular Cobalt Catalysts for O<sub>2</sub> Reduction: Low-Overpotential Production of H<sub>2</sub>O<sub>2</sub> and Comparison with Iron-Based Catalysts
A series of mononuclear pseudomacrocyclic
cobalt complexes have
been investigated as catalysts for O<sub>2</sub> reduction. Each of
these complexes, with Co<sup>III/II</sup> reduction potentials that
span nearly 400 mV, mediate highly selective two-electron reduction
of O<sub>2</sub> to H<sub>2</sub>O<sub>2</sub> (93ā99%) using
decamethylferrocene (Fc*) as the reductant and acetic acid as the
proton source. Kinetic studies reveal that the rate exhibits a first-order
dependence on [Co] and [AcOH], but no dependence on [O<sub>2</sub>] or [Fc*]. A linear correlation is observed between logĀ(TOF) vs <i>E</i><sub>1/2</sub>(Co<sup>III/II</sup>) for the different cobalt
complexes (TOF = turnover frequency). The thermodynamic potential
for O<sub>2</sub> reduction to H<sub>2</sub>O<sub>2</sub> was estimated
by measuring the H<sup>+</sup>/H<sub>2</sub> open-circuit potential
under the reaction conditions. This value provides the basis for direct
assessment of the thermodynamic efficiency of the different catalysts
and shows that H<sub>2</sub>O<sub>2</sub> is formed with overpotentials
as low as 90 mV. These results are compared with a recently reported
series of Fe-porphyrin complexes, which catalyze four-electron reduction
of O<sub>2</sub> to H<sub>2</sub>O. The data show that the TOFs of
the Co complexes exhibit a shallower dependence on <i>E</i><sub>1/2</sub>(M<sup>III/II</sup>) than the Fe complexes. This behavior,
which underlies the low overpotential, is rationalized on the basis
of the catalytic rate law
Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e<sup>ā</sup>/4H<sup>+</sup> O<sub>2</sub> Reduction: Activation of OāO Bond by 2nd Sphere Hydrogen Bonding
Facile and selective
4e<sup>ā</sup>/4H<sup>+</sup> electrochemical
reduction of O<sub>2</sub> to H<sub>2</sub>O in aqueous medium has
been a sought-after goal for several decades. Elegant but synthetically
demanding cytochrome c oxidase mimics have demonstrated selective
4e<sup>ā</sup>/4H<sup>+</sup> electrochemical O<sub>2</sub> reduction to H<sub>2</sub>O is possible with rate constants as fast
as 10<sup>5</sup> M<sup>ā1</sup> s<sup>ā1</sup> under
heterogeneous conditions in aqueous media. Over the past few years,
in situ mechanistic investigations on iron porphyrin complexes adsorbed
on electrodes have revealed that the rate and selectivity of this
multielectron and multiproton process is governed by the reactivity
of a ferric hydroperoxide intermediate. The barrier of OīøO
bond cleavage determines the overall rate of O<sub>2</sub> reduction
and the site of protonation determines the selectivity. In this report,
a series of mononuclear iron porphyrin complexes are rationally designed
to achieve efficient OīøO bond activation and site-selective
proton transfer to effect facile and selective electrochemical reduction
of O<sub>2</sub> to water. Indeed, these crystallographically characterized
complexes accomplish facile and selective reduction of O<sub>2</sub> with rate constants >10<sup>7</sup> M<sup>ā1</sup> s<sup>ā1</sup> while retaining >95% selectivity when adsorbed
on
electrode surfaces (EPG) in water. These oxygen reduction reaction
rate constants are 2 orders of magnitude faster than all known heme/Cu
complexes and these complexes retain >90% selectivity even under
rate
determining electron transfer conditions that generally can only be
achieved by installing additional redox active groups in the catalyst
Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e<sup>ā</sup>/4H<sup>+</sup> O<sub>2</sub> Reduction: Activation of OāO Bond by 2nd Sphere Hydrogen Bonding
Facile and selective
4e<sup>ā</sup>/4H<sup>+</sup> electrochemical
reduction of O<sub>2</sub> to H<sub>2</sub>O in aqueous medium has
been a sought-after goal for several decades. Elegant but synthetically
demanding cytochrome c oxidase mimics have demonstrated selective
4e<sup>ā</sup>/4H<sup>+</sup> electrochemical O<sub>2</sub> reduction to H<sub>2</sub>O is possible with rate constants as fast
as 10<sup>5</sup> M<sup>ā1</sup> s<sup>ā1</sup> under
heterogeneous conditions in aqueous media. Over the past few years,
in situ mechanistic investigations on iron porphyrin complexes adsorbed
on electrodes have revealed that the rate and selectivity of this
multielectron and multiproton process is governed by the reactivity
of a ferric hydroperoxide intermediate. The barrier of OīøO
bond cleavage determines the overall rate of O<sub>2</sub> reduction
and the site of protonation determines the selectivity. In this report,
a series of mononuclear iron porphyrin complexes are rationally designed
to achieve efficient OīøO bond activation and site-selective
proton transfer to effect facile and selective electrochemical reduction
of O<sub>2</sub> to water. Indeed, these crystallographically characterized
complexes accomplish facile and selective reduction of O<sub>2</sub> with rate constants >10<sup>7</sup> M<sup>ā1</sup> s<sup>ā1</sup> while retaining >95% selectivity when adsorbed
on
electrode surfaces (EPG) in water. These oxygen reduction reaction
rate constants are 2 orders of magnitude faster than all known heme/Cu
complexes and these complexes retain >90% selectivity even under
rate
determining electron transfer conditions that generally can only be
achieved by installing additional redox active groups in the catalyst
Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e<sup>ā</sup>/4H<sup>+</sup> O<sub>2</sub> Reduction: Activation of OāO Bond by 2nd Sphere Hydrogen Bonding
Facile and selective
4e<sup>ā</sup>/4H<sup>+</sup> electrochemical
reduction of O<sub>2</sub> to H<sub>2</sub>O in aqueous medium has
been a sought-after goal for several decades. Elegant but synthetically
demanding cytochrome c oxidase mimics have demonstrated selective
4e<sup>ā</sup>/4H<sup>+</sup> electrochemical O<sub>2</sub> reduction to H<sub>2</sub>O is possible with rate constants as fast
as 10<sup>5</sup> M<sup>ā1</sup> s<sup>ā1</sup> under
heterogeneous conditions in aqueous media. Over the past few years,
in situ mechanistic investigations on iron porphyrin complexes adsorbed
on electrodes have revealed that the rate and selectivity of this
multielectron and multiproton process is governed by the reactivity
of a ferric hydroperoxide intermediate. The barrier of OīøO
bond cleavage determines the overall rate of O<sub>2</sub> reduction
and the site of protonation determines the selectivity. In this report,
a series of mononuclear iron porphyrin complexes are rationally designed
to achieve efficient OīøO bond activation and site-selective
proton transfer to effect facile and selective electrochemical reduction
of O<sub>2</sub> to water. Indeed, these crystallographically characterized
complexes accomplish facile and selective reduction of O<sub>2</sub> with rate constants >10<sup>7</sup> M<sup>ā1</sup> s<sup>ā1</sup> while retaining >95% selectivity when adsorbed
on
electrode surfaces (EPG) in water. These oxygen reduction reaction
rate constants are 2 orders of magnitude faster than all known heme/Cu
complexes and these complexes retain >90% selectivity even under
rate
determining electron transfer conditions that generally can only be
achieved by installing additional redox active groups in the catalyst
Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e<sup>ā</sup>/4H<sup>+</sup> O<sub>2</sub> Reduction: Activation of OāO Bond by 2nd Sphere Hydrogen Bonding
Facile and selective
4e<sup>ā</sup>/4H<sup>+</sup> electrochemical
reduction of O<sub>2</sub> to H<sub>2</sub>O in aqueous medium has
been a sought-after goal for several decades. Elegant but synthetically
demanding cytochrome c oxidase mimics have demonstrated selective
4e<sup>ā</sup>/4H<sup>+</sup> electrochemical O<sub>2</sub> reduction to H<sub>2</sub>O is possible with rate constants as fast
as 10<sup>5</sup> M<sup>ā1</sup> s<sup>ā1</sup> under
heterogeneous conditions in aqueous media. Over the past few years,
in situ mechanistic investigations on iron porphyrin complexes adsorbed
on electrodes have revealed that the rate and selectivity of this
multielectron and multiproton process is governed by the reactivity
of a ferric hydroperoxide intermediate. The barrier of OīøO
bond cleavage determines the overall rate of O<sub>2</sub> reduction
and the site of protonation determines the selectivity. In this report,
a series of mononuclear iron porphyrin complexes are rationally designed
to achieve efficient OīøO bond activation and site-selective
proton transfer to effect facile and selective electrochemical reduction
of O<sub>2</sub> to water. Indeed, these crystallographically characterized
complexes accomplish facile and selective reduction of O<sub>2</sub> with rate constants >10<sup>7</sup> M<sup>ā1</sup> s<sup>ā1</sup> while retaining >95% selectivity when adsorbed
on
electrode surfaces (EPG) in water. These oxygen reduction reaction
rate constants are 2 orders of magnitude faster than all known heme/Cu
complexes and these complexes retain >90% selectivity even under
rate
determining electron transfer conditions that generally can only be
achieved by installing additional redox active groups in the catalyst
Standard Reduction Potentials for Oxygen and Carbon Dioxide Couples in Acetonitrile and <i>N</i>,<i>N</i>āDimethylformamide
A variety
of next-generation energy processes utilize the electrochemical interconversions
of dioxygen and water as the oxygen reduction reaction (ORR) and the
oxygen evolution reaction (OER). Reported here are the first estimates
of the standard reduction potential of the O<sub>2</sub> + 4<i>e</i><sup>ā</sup> + 4H<sup>+</sup> ā 2H<sub>2</sub>O couple in organic solvents. The values are +1.21 V in acetonitrile
(MeCN) and +0.60 V in <i>N</i>,<i>N</i>-dimethylformamide
(DMF), each versus the ferrocenium/ferrocene couple (Fc<sup>+/0</sup>) in the respective solvent (as are all of the potentials reported
here). The potentials have been determined using a thermochemical
cycle that combines the free energy for transferring water from aqueous
solution to organic solvent, ā0.43 kcal mol<sup>ā1</sup> for MeCN and ā1.47 kcal mol<sup>ā1</sup> for DMF,
and the potential of the H<sup>+</sup>/H<sub>2</sub> couple, ā
0.028 V in MeCN and ā0.662 V in DMF. The H<sup>+</sup>/H<sub>2</sub> couple in DMF has been directly measured electrochemically
using the previously reported procedure for the MeCN value. The thermochemical
approach used for the O<sub>2</sub>/H<sub>2</sub>O couple has been
extended to the CO<sub>2</sub>/CO and CO<sub>2</sub>/CH<sub>4</sub> couples to give values of ā0.12 and +0.15 V in MeCN and ā0.73
and ā0.48 V in DMF, respectively. Extensions to other reduction
potentials are discussed. Additionally, the free energy for transfer
of protons from water to organic solvent is estimated as +14 kcal
mol<sup>ā1</sup> for acetonitrile and +0.6 kcal mol<sup>ā1</sup> for DMF
Synthesis and Reactivity of Tripodal Complexes Containing Pendant Bases
The
synthesis of a new tripodal ligand family that contains tertiary amine
groups in the second-coordination sphere is reported. The ligands
are trisĀ(amido)Āamine derivatives, with the pendant amines attached
via a peptide coupling strategy. They were designed to function as
new molecular catalysts for the oxygen reduction reaction (ORR), in
which the pendant acid/base group could improve the catalyst performance.
Two members of the ligand family were each metalated with cobaltĀ(II)
and zincĀ(II) to afford trigonal-monopyramidal complexes. The reaction
of the cobalt complexes <b>[CoĀ(L)]</b><sup><b>ā</b></sup> with dioxygen reversibly generates a small amount of a cobaltĀ(III)
superoxo species, which was characterized by electron paramagnetic
resonance (EPR) spectroscopy. Protonation of the zinc complex ZnĀ[NĀ{CH<sub>2</sub>CH<sub>2</sub>NCĀ(O)ĀCH<sub>2</sub>NĀ(CH<sub>2</sub>Ph)<sub>2</sub>}<sub>3</sub>)]<sup>ā</sup> (<b>[ZnĀ(TN</b><sup><b>Bn</b></sup><b>)]</b><sup><b>ā</b></sup>) with
1 equiv of acid occurs at a primary-coordination-sphere amide moiety
rather than at a pendant basic site. The addition of excess acid to
any of the complexes <b>[MĀ(L)]</b><sup><b>ā</b></sup> results in complete proteolysis and formation of the ligands <b>H</b><sub><b>3</b></sub><b>L</b>. These undesired
reactions limit the use of these complexes as catalysts for the ORR.
An alternative ligand with two pyridyl arms was also prepared but
could not be metalated. These studies highlight the importance of
the stability of the primary-coordination sphere of ORR electrocatalysts
to both oxidative <i>and</i> acidic conditions
Highly Active NiO Photocathodes for H<sub>2</sub>O<sub>2</sub> Production Enabled via Outer-Sphere Electron Transfer
Tandem dye-sensitized
photoelectrosynthesis cells are promising
architectures for the production of solar fuels and commodity chemicals.
A key bottleneck in the development of these architectures is the
low efficiency of the photocathodes, leading to small current densities.
Herein, we report a new design principle for highly active photocathodes
that relies on the outer-sphere reduction of a substrate from the
dye, generating an unstable radical that proceeds to the desired product.
We show that the direct reduction of dioxygen from dye-sensitized
nickel oxide (NiO) leads to the production of H<sub>2</sub>O<sub>2</sub>. In the presence of oxygen and visible light, NiO photocathodes
sensitized with commercially available porphyrin, coumarin, and ruthenium
dyes exhibit large photocurrents (up to 400 Ī¼A/cm<sup>2</sup>) near the thermodynamic potential for O<sub>2</sub>/H<sub>2</sub>O<sub>2</sub> in near-neutral water. Bulk photoelectrolysis of porphyrin-sensitized
NiO over 24 h results in millimolar concentrations of H<sub>2</sub>O<sub>2</sub> with essentially 100% faradaic efficiency. To our knowledge,
these are among the most active NiO photocathodes reported for multiproton/multielectron
transformations. The photoelectrosynthesis proceeds by initial formation
of superoxide, which disproportionates to H<sub>2</sub>O<sub>2</sub>. This disproportionation-driven charge separation circumvents the
inherent challenges in separating electronāhole pairs for photocathodes
tethered to inner sphere electrocatalysts and enables new applications
for photoelectrosynthesis cells