Immobilization of a molecular cobaloxime catalyst for hydrogen evolution on a mesoporous metal oxide electrode

Article abstract of DOI:10.1002/anie.201207448

Energizing Cobalt on a 3D Electrode: An optically transparent mesoporous ITO film was derivatized with the novel proton reducing cobalt catalyst [Co]. The hybrid electrode showed high current densities, and spectroelectrochemical studies and extensive surface characterization demonstrate that the immobilized molecular catalyst remained intact on the electrode when applying a low potential. Copyright

Full text of DOI:10.1002/anie.201207448

Angewandte
Chemie
DOI: 10.1002/anie.201207448
Hybrid Materials
Immobilization of a Molecular Cobaloxime Catalyst for Hydrogen
Evolution on a Mesoporous Metal Oxide Electrode**
Nicoleta M. Muresan, Janina Willkomm, Dirk Mersch, Yana Vaynzof, and Erwin Reisner*
The sustainable production of H2 through water splitting faces
several fundamental challenges, such as the requirement of
a renewable energy input, as well as fast and inexpensive
catalysts to minimize energy losses during fuel formation.[1]
Finely tuned proton-coupled multi-electron chemistry allows
ꢀ
=
for the formation of H H and O O bonds in and from water,
which is otherwise inherently slow or requires high electro-
chemical potentials in the absence of a catalyst.[2] A devel-
oping approach in water splitting is the utilization of
molecular catalysts integrated onto electrodes to lower the
over-potential requirement to drive water oxidation[3] and
proton reduction.[4] The heterogenization of molecular cata-
lysts resulted in stimulating discussions about the identity of
the true catalyst in the solid-state material. In principle, the
immobilized catalyst can either be an active molecular
electrocatalyst or simply serve as the precursor for a deposited
electroactive metal or metal oxide species.[5]
Figure 1. Chemical structures of [CoIII(dpg)3(BF)2](BF4),[8a] [CoIII-
(dmgBF2)2(MeCN)2],[6a,8c] [CoIIICl2{(DO)(DOH)pn}],[6c] and the novel
cobaloxime [Co] with two phosphonic acid anchors for immobilization
on a metal oxide surface (this work). dpgH2 =diphenylglyoxime,
dmgH2 =dimethylglyoxime, (DO)(DOH)pn=N2,N2’-propanediyl-
bis(2,3-butanedione-2-imine-3-oxime).
Cobalt tetraimine complexes[6] are a well-known inex-
pensive alternative to precious metal catalysts,[7] and there-
fore receive much attention as inexpensive H2 evolution
catalysts. Cobalt complexes such as [CoIII(dpg)3(BF)2](BF4)],
[CoIIICl2{(DO)(DOH)pn}] and [CoIII(dmgBF2)2(MeCN)2]
(Figure 1) were recently used for the preparation of electro-
active materials for the reduction of aqueous protons.[8]
However, the former two complexes served solely as molec-
ular precursors for the deposition of electroactive Co0 or
CoOx species,[8a,b] and the latter complex is a precursor for an
unknown electroactive catalyst.[8c]
surfaces,[9] and the robust tetradentate ligand framework
provides the cobaloxime with high stability.[6c] Complex [Co]
was subsequently immobilized on conducting and mesopo-
rous ITO on ITO-coated glass (ITO j meso-ITO) with high
stability. The nanostructured surface allows for a high loading
of [Co] on ITO j meso-ITO (Scheme 1) and the hybrid
electrode exhibits some electrochemical reduction of aqueous
Herein, we report on the synthetic procedure for a novel
type of [CoIIIBr2{(DO)(DOH)pn}] complex ([Co]) that fea-
tures a doubly phosphonated propanediyl bridgehead in the
equatorial diimine–dioxime ligand (Figure 1). The phos-
phonic acid groups provide a strong anchor to metal oxide
Scheme 1. Illustration of the assembly of a cathode consisting of an
ITO conducting glass modified with a meso-ITO film and an integrated
[Co] catalyst (ITOjmeso-ITOj[Co]).
[*] Dr. N. M. Muresan, J. Willkomm, D. Mersch, Dr. E. Reisner
Christian Doppler Laboratory for Sustainable SynGas Chemistry,
Department of Chemistry, University of Cambridge
Cambridge CB2 1EW (UK)
E-mail: reisner@ch.cam.ac.uk
Homepage: http://www-reisner.ch.cam.ac.uk/
protons in a pH-neutral electrolyte solution at room temper-
ature. Limitations of the [Co]-modified ITO material arise
from electrodegradation of ITO at negative potentials, but
a SnO2 cathode shows improved stability. Finally, spectro-
electrochemical investigations and extensive surface charac-
terization confirm that the surface-immobilized cobaloxime
[Co] remains molecularly intact on the metal oxide electrode
after prolonged electrochemical treatment.
Compound [Co] was prepared in six steps, as summarized
in Scheme 2, and all compounds were characterized by
1H NMR spectroscopy, ESI-MS, and elemental analysis (see
the Supporting Information). The nitrile derivative 2 was
Dr. Y. Vaynzof
Cavendish Laboratory, University of Cambridge
Cambridge, CB3 0HE (UK)
[**] This work was supported by the U.K. Engineering and Physical
Sciences Research Council (EP/H00338X/2), the Austrian Christian
Doppler Research Association (Federal Ministry of Economy, Family
and Youth, and the National Foundation for Research, Technology
and Development) and the OMV Group. We are grateful to Dr. John
Davies for his help with X-ray crystallography.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207448.
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Single crystals of [(Et)-Co]·2MeOH suitable for X-ray
analysis were obtained by slow evaporation of a concentrated
MeOH solution of the complex, and the crystal structure of
the complex is depicted in Scheme 2B. In this structure,
complex [(Et)-Co] consists of the expected octahedral envi-
ronment around the cobalt center and two bromido ligands
occupying the axial sites [Br1-Co2-Br2, 175.26(3)8]. The two
oxime units of the tetradentate ligand join together in the
equatorial plane through an intramolecular hydrogen bond
[O1···O2, 2.492(6) ꢁ]. The average Co–N (1.91 ꢁ) and Co–Br
(2.36 ꢁ) distances are in agreement with the respective bond
lengths in related cobaloxime complexes.[6c,11]
Electrochemical measurements of [Co] were first
recorded on a glassy carbon electrode in DMF with
[N(nBu)4]BF4 (0.1m). Cyclic voltammograms (CVs) display
the typical[6c,8c] electrochemical response for cobaloximes and
show two one-electron reduction waves, which were assigned
to the CoIII/CoII and CoII/CoI processes at III/IIEp = 0.01 V and
II/IE1/2 = ꢀ0.37 V vs. NHE (Supporting Information, Fig-
ure S1). We note that diimines are potentially redox non-
innocent and the two single-electron reductions of [Co] could
therefore also result in a ligand-based reduction product
(CoIILCꢀ) instead of CoI.[12] In the presence of trifluoroacetic
acid (TFA; pKa = 6 in DMF),[13] an electrocatalytic proton
reduction wave is observed at a potential close to the CoII/CoI
couple. The observed overpotential of approximately 250 mV
(Figure S1) is in agreement with previous reports for Co–
tetraimine complexes.[14] Thus, the catalytic core of [Co] is
fully active upon functionalization of the (DO)(DOH)pn
ligand.
The presence of the phosphonic acid moiety in [Co]
provides the complex with high solubility in water. The CV of
[Co] in an aqueous TEOA/Na2SO4 (TEOA = triethanol-
amine; 0.1m each) solution at pH 7 and 258C at a planar
(2D) ITO electrode displays two one-electron reduction
waves, which were assigned to CoIII/CoII and CoII/CoI
processes at III/IIEp = ꢀ0.20 V and II/IEp = ꢀ0.53 V vs. NHE at
a scan rate of 100 mVsꢀ1 (Figure S2). The catalytic onset
reduction potential is observed at Ecat of approximately
ꢀ0.72 V vs. NHE; at an overpotential of approximately 0.3 V
at pH 7 (E0’H+/H2 = ꢀ0.42 vs. NHE).
Scheme 2. A) a) Malononitrile, NaBH4, EtOH, RT, 2 h;[10] b) 4-Bromo-
benzyl bromide, K2CO3, acetone, RT, 12 h; c) HPO(OEt)2, NEt3, Pd-
(PPh3)4, PPh3, THF, reflux, 48 h; d) BH3·THF, THF, RT, 12 h; e) 2,3-
butanedione monoxime, MeOH, CoBr2·6H2O, RT, 5 d; f) TMSBr,
CH2Cl2, MeOH, RT, 2 d. B) X-ray crystal structure of [(Et)-Co]·2MeOH
with ellipsoids set at the 50% probability level; solvent molecules and
hydrogen atoms are omitted for clarity. [(Et)-Co]·2MeOH crystallizes in
ꢀ
space group P1, with the following unit cell parameters: a=8.0275(3),
b=14.6652(5), c=19.8143(7) ꢀ and a=74.307(2), b=82.998(2),
g=80.199(2)8; V=2205.83(14) ꢀ3. R1 =0.049. More crystallographic
details can be found in the Supporting Information, Tables S1–S3.
synthesized in 90% yield from commercially available
malononitrile, 4-bromobenzylaldehyde (1), and sodium bor-
ohydride.[10] Deprotonation of the methyne proton in 2 with
K2CO3 allowed for the introduction of a second alkyl chain
with 4-bromobenzyl bromide in acetone to give 3 in 85%
yield. Compound 3 reacts with diethylphosphite and Pd-
(PPh3)4/PPh3/NEt3 in refluxing THF to afford the cross-
coupled phosphonate ester 4 in 86% yield. Reduction of
nitrile 4 with excess BH3 at room temperature gives 5 in 90%
yield.
Complex [(Et)-Co] was prepared by stirring the diamine 5
with two equivalents of 2,3-butanedione monoxime in MeOH
for five days at room temperature, followed by complexation
with CoBr2·6H2O and exposure to air for five min. [(Et)-Co]
was isolated as a green microcrystalline solid and was
hydrolyzed to the cobalt phosphonic acid complex [Co] in
92% yield by reaction with trimethylsilyl bromide in CH2Cl2
at room temperature. Complex [Co] was isolated as a green
powder and was characterized by 1H, 13C, and 31P NMR
spectroscopy, ESI-MS, and elemental analysis. The diamag-
netic complexes [Co] and [(Et)-Co] showed well-defined
NMR spectra with a characteristic signal at 19.0 ppm in the
1H NMR spectrum, which was assigned to the equatorial
Subsequently, we prepared ITO j meso-ITO electrodes as
substrates for the immobilization of [Co]. The electrodes
were prepared by spreading a suspension of ITO nano-
particles (< 40 nm diameter) on ITO-coated glass slides
followed by annealing at 3508C (Scheme 1).[3a,e] The geo-
metrical meso-ITO area was 0.25 cm2, with a thickness of
13 mm (Figure S3). The ITO j meso-ITO working electrodes
were then modified with the catalyst by exposing the films to
a 6 mm solution of [Co] in DMF or an aqueous TEOA/Na2SO4
solution (pH 7). The attachment of [Co] to meso-ITO is not
electrochemically assisted, but the adsorption process can be
monitored by the increase in current of the CoIII/CoII redox
couple by running continuous voltammetric cycles (Fig-
ure 2A, inset). Immobilization was completed within two
hours in DMF and ten hours in aqueous TEOA solution. CVs
and differential pulse voltammograms (DPVs) of an ITO j
meso-ITO j [Co] electrode in TEOA/Na2SO4 solution at pH 7
show two one-electron irreversible reduction waves at
ꢀ
O···H O bridge protons. The resonance of the bridge
methylene protons at 3.0 ppm in 5 is shifted downfield to
3.9 ppm upon complexation to form [(Et)-Co] or [Co]. The
UV/Vis spectrum of [Co] in dimethylformamide (DMF)
shows one intense absorption at 295 nm (e = 1.9 ꢀ
104 mꢀ1 cmꢀ1) and two weak features at 596 nm and 670 nm
(e = 33 and 31mꢀ1 cmꢀ1, respectively). This high-yield syn-
thetic procedure gives access to a range of (DO)(DOH)pn-
(CH2-R)2 type ligands with different functional groups at the
equatorial bridge position.
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meso-ITO j [Co] in TEOA buffer at pH 7, at an applied
potential of ꢀ0.7 V vs. NHE. However, a disadvantage of ITO
is its degradation at negative potentials,[15] and we therefore
determined the Faraday efficiency and turnover numbers for
electrocatalytic H2 evolution using a more robust electrode
material. Mesoporous SnO2 on FTO-coated glass (FTO j
meso-SnO2; FTO = fluoride-doped tin oxide) was prepared
from SnO2 nanoparticles (< 100 nm diameter; Figure S3) in
analogy to ITO j meso-ITO electrodes (see the Supporting
Information). FTO j mesoSnO2 is a low-cost alternative to
ITO-based electrodes and displays enhanced stability at
negative potentials.
A closed and stirred electrochemical cell containing an
aqueous TEOA/Na2SO4 solution at pH 7 with [Co] (0.18 mm)
was poised at a potential of ꢀ0.7 V vs. NHE with an FTO j
meso-SnO2 electrode (0.5 cm2, thickness of 13 mm; Figure S3)
for two hours. The passage of an average of ꢀ1.2 C of charge
resulted in 5.3 mmol headspace H2 (Figures 2 and S7), which
corresponds to a high Faraday yield of (87 ꢁ 4)% for
dissolved [Co]. No H2 was detected in the headspace after
electrolysis without [Co]. Extended controlled potential
electrolysis (ꢀ0.7 V vs. NHE) over 12 h of the same [Co]
system resulted in the passage of 7.2 C of charge and an
accumulation of approximately 32 mmol of H2 in the head-
space, which corresponds to a cobalt-based turnover number
of (12 ꢁ 2) mol H2 (per mol [Co]). We note that this number is
the lower limit, because [Co] was still active after this time
and we took into account all of the [Co] from the bulk
solution. The current density remained constant
(ca. 240 mAcmꢀ2) for several hours during electrolysis,
thereby confirming the catalytic formation of H2 at a relatively
low overpotential of approximately 0.3 V (Figure 2). Degra-
dation of FTO j mesoSnO2 j [Co] was only observed after
three days of electrolysis, when the deposition of a black layer
(presumably metallic Co)[8a] started to appear on the working
electrode.
The high [Co] loading and the optical transparency of
meso-ITO allowed us to study the identity and stability of the
molecular catalyst on the ITO j meso-ITO surface in more
detail. Scanning towards negative potentials in DMF or
TEOA/Na2SO4 with a partially [Co]-loaded ITO j meso-ITO
electrode, allowed for the observation of a color change of
adsorbed [Co] from yellowish green to red at the CoIII/CoII
redox potential, and to deep blue following the reduction of
the CoII species (Figure 3). The potential-dependent color
changes are fully reversible during multiple CV scans and
were investigated by transmission UV/Vis spectroelectro-
chemistry. Electronic absorption spectra for the CoIII, CoII,
and CoI species were recorded for ITO j meso-ITO j [Co] in
DMF containing [N(nBu)4]BF4 (0.10m) at + 0.60, ꢀ0.10, and
ꢀ0.60 V vs. NHE with absorbance maxima, lmax, at 450, 507,
and 655 nm, respectively (Figure 3B, red traces). Comparable
spectroelectrochemical absorption spectra of the CoIII, CoII,
and CoI species with ITO j meso-ITO j [Co] were recorded in
TEOA/Na2SO4 solution (Figure S8).
Figure 2. Electrochemistry of [Co] in TEOA/Na2SO4 solution (0.1m
each, pH 7) at 258C. A Ag/AgCl reference, a platinum wire or glassy
carbon rod counter, and an ITO, ITOjmeso-ITO, or FTOjmeso-SnO2
working electrode with a scan rate of 100 mVsꢀ1 were employed in all
experiments. A) CV and DPV (10 mV step potential) of the ITOjmeso-
ITOj[Co] electrode, and a control experiment for ITOjmeso-ITO in the
absence of [Co], as well as a CV with [Co] adsorbed on planar ITO
(ITOj[Co] with no [Co] in solution), for comparison. Inset: Consecutive
CVs of an ITOjmeso-ITO electrode (0.25 cm2) immersed in an aqueous
TEOA/Na2SO4 solution containing [Co] (6 mm) at 258C, showing the
increase in the loading of ITOjmeso-ITO with [Co] with time (first scan
after 1min and last scan after approximately 1h); B) and C) Controlled
potential electrolysis at ꢀ0.7 V vs. NHE with [Co] in solution
(0.18 mm) at an FTOjmeso-SnO2 working electrode for six two-hour
intervals. H2 was detected in the headspace by gas chromatography
with an airtight electrochemical cell.
III/IIEP = 0.14 V and II/IEP = ꢀ0.54 V vs. NHE at a scan rate of
100 mVsꢀ1 (Figures 2A and S4). CVs in the absence of TEOA
or Na2SO4 show an identical voltammetric response and both
components are therefore innocent in our experiments (Fig-
ure S5).
The amount of [Co] on the ITO j meso-ITO surface was
determined by measuring the absorbance difference of the
catalyst solution before and after exposure to the meso-ITO
electrode. The absorbance at 596 nm was used to quantify the
loading of the catalyst on the meso-ITO electrode (Figure S6).
A large loading of (0.030 ꢁ 0.002) mg of [Co] on ITO j meso-
ITO (G0 = ca. 1.50 ꢀ 10ꢀ7 molcmꢀ2) was obtained from DMF
and TEOA/Na2SO4 solutions. An ideal monolayer on a 2D
surface would allow for only approximately G0 = ca. 3 ꢀ
10ꢀ10 molcmꢀ2 assuming that one molecule of [Co] occupies
6 nm2 on a surface (estimated from the X-ray molecular
structure of [(Et)-Co]). The high [Co] loading on ITO j meso-
ITO resulted in a current density of 3.7 mAcmꢀ2 at ꢀ0.7 V vs.
NHE (background subtracted) during CV scans in TEOA/
Na2SO4 (0.1m, pH 7) solution with a scan rate of 100 mVsꢀ1.
An ITO j [Co] electrode ([Co] adsorbed on planar ITO, no
[Co] in solution) showed only
a current density of
0.014 mAcmꢀ2 under the same conditions. Thus, the 3D
environment in meso-ITO enhances the [Co] loading by more
than two orders of magnitude, which is in agreement with
previously reported values for the integration of a ruthenium
complex in meso-ITO.[3a]
The increase in current density at ꢀ0.7 V vs. NHE in
Figure 2 is partially attributed to electrochemical proton
reduction in the aqueous pH 7 electrolyte solution.[6d]
A
The spectroelectrochemical response of ITO j meso-ITO j
[Co] is in agreement with electronic absorption spectra of
chemically reduced [Co] in DMF. Exposure of [Co]III to one
and two equivalents of cobaltocene (reduction potential is
closed cell allowed us to detect the formation of some H2 in
the gas headspace by gas chromatography using a thermal
conductivity detector during bulk electrolysis with ITO j
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at ꢀ0.7 V in TEOA/Na2SO4 displayed the genuine CoIII/II
redox couple at III/IIEP = 0.14 V, thus demonstrating that the
majority of [Co] is neither decomposed nor desorbed from the
electrode surface when forming the catalytically active CoI
species. The cobalt catalyst can be at least partially desorbed
from ITO j meso-ITO j [Co] when phosphate (1 mL, 25 mm) is
added to the TEOA/Na2SO4 electrolyte solution during
voltammetric experiments (Figure S15), which supports the
strong and reversible binding of molecular [Co] on ITO
through the phosphonic acid anchors.
We subsequently investigated the [Co]-modified electrode
surfaces by scanning electron microscopy (SEM), energy
dispersive X-ray (EDX) and X-ray photoemission spectros-
copy (XPS). SEM and EDX analysis before and after
controlled potential electrolysis at ꢀ0.7 V vs. NHE for 12 h
in an aqueous TEOA solution at pH 7 did not reveal
significant changes in surface morphology or elemental
composition, or the formation of CoOx or metallic Co
particles on meso-ITO or planar ITO (Figure S16 and S17).
XPS analysis on ITO j meso-ITO j [Co] before performing
electrochemical experiments in DMF shows the expected
spectra for the cobaloxime immobilized on ITO (Figure S18):
A Co2p doublet at binding energies of 781.4 eV and 796.4 eV
for CoIII,[18] a Br3d doublet at binding energies of 68.8 eV and
69.7 eV, a P2p doublet at 133.0 eV and 134.2 eV, and two N1s
Figure 3. A) The immobilized catalyst [Co] is reduced reversibly on
meso-ITO in two one-electron reduction processes from CoIII!CoII!
CoI. The pictures of ITOjmeso-ITOj[Co] electrodes were taken during
electrochemical experiments in anhydrous DMF containing [N-
(nBu)4]BF4 (0.1m) at an applied potential of 0.60 V (left), ꢀ0.10 V
(middle) and ꢀ0.60 V (right). B) The corresponding electronic absorp-
tion spectra of [Co]III, [Co]II, and [Co]I in DMF on the ITOjmeso-ITO
electrode (c) and chemically reduced [Co]III, [Co]II, and [Co]I in
solution (c). See text for details.
ꢀ0.7 vs. NHE)[16] resulted in the formation of [Co]II and [Co]I,
respectively (Figures 3B and S9). The UV/Vis spectra showed
an absorption maximum at 445 nm (e = 1.55 ꢀ 103 mꢀ1 cmꢀ1)
for [Co]III, 505 nm (e = 2.4 ꢀ 102 mꢀ1 cmꢀ1) for [Co]II, and
680 nm (e = 1.9 ꢀ 102 mꢀ1 cmꢀ1) for [Co]I. The UV/Vis spectra
are also in agreement with previous spectroscopic data in
solution for reduced cobaloxime species,[6a,17] and no particle
formation became apparent during chemical reduction. The
agreement between spectroscopic data of [Co] in solution and
on ITO j meso-ITO confirms that the majority of the coba-
loxime is intact on the electrode surface, even at low potential
and over multiple scanning cycles.
The stability of the ITO j meso-ITO j [Co] electrode was
studied by spectroelectrochemistry in DMF and aqueous
TEOA/Na2SO4 solution. The electrode was immersed in
a fresh DMF solution and no spectroscopic or voltammetric
change was observed over a period of six hours (Figure S10).
The stability of the electrochemically reduced [Co]II (at
ꢀ0.10 V vs. NHE) and [Co]I (at ꢀ0.60 V vs. NHE) on an ITO j
meso-ITO electrode in dry DMF were also monitored
(Figure S11). Electronic absorption spectra show a high
stability on the surface with negligible or small leaching into
the bulk solution within one hour for the CoII and CoI species,
respectively. Analogous experiments were performed with
ITO j meso-ITO j [Co] in an aqueous TEOA/Na2SO4 solution
and no desorption was observed for [Co] (no potential
applied) after six hours (Figure S12). Electrochemically
generated [Co]II (ꢀ0.10 V vs. NHE) disappeared over
a period of three hours from the ITO j meso-ITO j [Co]
electrode with a concomitant coloration of the solution with
an absorption maximum at 505 nm, which was assigned to
intact, but desorbed [Co]II (Figure S13B). The absorption
band at 655 nm of the electrochemically generated [Co]I on
ITO j meso-ITO disappeared after several seconds (Fig-
ure S14), presumably because of catalytic turnover and H2
formation. Scanning towards positive potential after reversal
ꢀ
singlet peaks at 399.9 eV and 401.4 eV in a 1:1 ratio for N C
ꢀ
and N O, respectively. The C1s spectrum consists of two peaks
ꢀ
ꢀ
at 284.9 eV (C C) and 285.9 eV (C N). In3d, Sn3d, and O1s
peaks from meso-ITO were also observed. XPS gave almost
identical results for ITO j meso-ITO j [Co] in aqueous TEOA
solution, but the bromides were not observed in the Br3d
spectrum, presumably owing to their replacement by aqua
ligands (Figure S19).
XPS measurements on ITO j meso-ITO j [Co] after four
hours of continuous redox cycling (from ꢀ0.8 V to 0.7 V vs.
NHE; Figure S20) and after 12 h of controlled potential
electrolysis at ꢀ0.7 V vs. NHE (Figure S21) in TEOA solution
gave comparable results. The P2p and C1s signals in [Co] and
In3d and Sn3d for the ITO substrate remained unchanged
(ꢁ0.2 eV) compared to XPS spectra recorded before electro-
chemical treatment. The N1s spectrum shows an unchanged
ꢀ
peak for N C at 399.9 eV, but a small shift to lower binding
ꢀ
energies (400.7 eV) was observed for N O after CV and
chronoamperometry. The shift could be due to partial
protonation of the oxygen atom. The Co2p doublet peaks at
781.3 eV and 796.8 eV after electrochemical treatment are
accompanied by an additional strong satellite feature at
786.1 eV. The satellite and increased multiplet splitting are
indicative of a CoII oxidation state.[18]
XPS analysis of ITO j meso-ITO j [Co] therefore further
supports that a molecular compound is present on the metal
oxide and no indication for the formation of CoOx or metallic
Co particles on meso-ITO was obtained after several hours of
redox cycling or electrolysis. Our results are in agreement
with a recent report, where no cobalt-containing nanoparti-
cles were formed from [CoCl2{(DO)(DOH)pn}] on FTO at
ꢀ0.7 V vs. NHE.[8b] XPS analysis of FTO j meso-SnO2 j [Co]
was not possible, owing to the low conductivity of SnO2
relative to ITO. However, XPS analysis of a planar FTO
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Ashford, A. Vannucci, Z. Chen, J. J. Concepcion, P. L. Holland,
electrode modified with [Co] (and [Co] in solution) after 12 h
of controlled potential electrolysis at ꢀ0.7 V vs. NHE in
TEOA solution gave comparable results to ITO j meso-ITO j
[Co] and showed the presence of the molecular cobalt catalyst
on FTO.
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Received: September 14, 2012
Published online: November 21, 2012
Keywords: cobalt · heterogeneous catalysis · hybrid materials ·
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proton reduction · water splitting
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