Kagóme Cobalt(II)-Organic Layers as Robust Scaffolds for Highly Efficient Photocatalytic Oxygen Evolution

Article abstract of DOI:10.1002/cssc.201600101

Two Kagóme cobalt(II)-organic layers of [Co₃(μ₃-OH)₂(bdc)₂]_n (1) and [Co₃(μ₃-OH)₂(chdc)₂]_n (2) (bdc=o-benzenedicarboxylate and chdc=1,2-cyclohexanedicarboxylate) that bear bridging OH^- ligands were explored as water oxidation catalysts (WOCs) for photocatalytic O₂ production. The activities of 1 and 2 towards H₂O oxidation were assessed by monitoring the in situ O₂ concentration versus time in the reaction medium by utilizing a Clark-type oxygen electrode under photochemical conditions. The oxygen evolution rate (RO2) was 24.3 μmol s^-1 g^-1 for 1 and 48.8 μmol s^-1 g^-1 for 2 at pH 8.0. Photocatalytic reaction studies show that 1 and 2 exhibit enhanced activities toward the oxidation of water compared to commercial nanosized Co₃O₄. In scaled-up photoreactions, the pH value of the reaction medium decreased from 8.0 to around 7.0 after 20 min and the O₂ production ceased. Based on the amounts of the sacrificial oxidant (K₂S₂O₈) used, the yield of O₂ produced is 49.6 % for 2 and 29.8 % for 1. However, the catalyst can be recycled without a significant loss of catalytic activity. Spectroscopic studies suggest that the structure and composition of recycled 1 and 2 are maintained. In isotope-labeling H₂¹⁸O (97 % enriched) experiments, the distribution of ¹⁶O¹⁶O/¹⁶O¹⁸O/¹⁸O¹⁸O detected was 0:7.55:92.45, which is comparable to the theoretical values of 0.09:5.82:94.09. This work not only provides new catalysts that resemble ligand-protected cobalt oxide materials but also establishes the significance of the existence of OH^- (or H₂O) binding sites at the metal center in WOCs. Water splitting: Two assembled cobalt(II)-carboxylate layers that bear bridging OH^- ligands are explored as water oxidation catalysts for photocatalytic O₂ production. Their activities towards H₂O oxidation are assessed by monitoring the in situ O₂ concentration versus time in the reaction medium by utilizing a Clark-type oxygen electrode under photochemical conditions.

Full text of DOI:10.1002/cssc.201600101

DOI: 10.1002/cssc.201600101
Full Papers
Kagóme Cobalt(II)-Organic Layers as Robust Scaffolds for
Highly Efficient Photocatalytic Oxygen Evolution
Jiaheng Xu,[a] Zhi Wang,[a] Wenguang Yu,[b] Di Sun,*[a] Qing Zhang,[b] Chen-Ho Tung,[a] and
Wenguang Wang*[a]
Two Kagóme cobalt(II)-organic layers of [Co3(m3-OH)2(bdc)2]n (1)
and [Co3(m3-OH)2(chdc)2]n (2) (bdc=o-benzenedicarboxylate
and chdc=1,2-cyclohexanedicarboxylate) that bear bridging
OHÀ ligands were explored as water oxidation catalysts (WOCs)
for photocatalytic O2 production. The activities of 1 and 2 to-
wards H2O oxidation were assessed by monitoring the in situ
O2 concentration versus time in the reaction medium by utiliz-
ing a Clark-type oxygen electrode under photochemical condi-
and the O2 production ceased. Based on the amounts of the
sacrificial oxidant (K2S2O8) used, the yield of O2 produced is
49.6% for 2 and 29.8% for 1. However, the catalyst can be re-
cycled without a significant loss of catalytic activity. Spectro-
scopic studies suggest that the structure and composition of
recycled 1 and 2 are maintained. In isotope-labeling H218O
(97% enriched) experiments, the distribution of 16O16O/16O18O/
18O18O detected was 0:7.55:92.45, which is comparable to the
theoretical values of 0.09:5.82:94.09. This work not only pro-
vides new catalysts that resemble ligand-protected cobalt
oxide materials but also establishes the significance of the exis-
tence of OHÀ (or H2O) binding sites at the metal center in
WOCs.
tions. The oxygen evolution rate (RO ) was 24.3 mmolsÀ1 gÀ1 for
2
1 and 48.8 mmolsÀ1 gÀ1 for 2 at pH 8.0. Photocatalytic reaction
studies show that 1 and 2 exhibit enhanced activities toward
the oxidation of water compared to commercial nanosized
Co3O4. In scaled-up photoreactions, the pH value of the reac-
tion medium decreased from 8.0 to around 7.0 after 20 min
Introduction
The photocatalytic oxidation of water to produce O2, protons,
and electrons is an attractive reaction with which to capture
solar energy in both sustainable energy storage and transfor-
mation.[1–5] The released protons and electrons can potentially
be utilized to synthesize various chemicals, and for example,
i) to produce H2 as a fuel directly;[6] ii) to reduce CO2 to
HCOOH, CO, or CH3OH;[7] and iii) to store as a hydride
(HÀ$H++2eÀ) in reduction products such as dihydropyridine
derivatives for further transformation.[8] To achieve this multi-
step eÀ/H+ transfer reaction, a natural oxygen evolution center
uses an intact Mn4CaO5 cluster surrounded by m-O and m-OH li-
gands in photosystem II (PS II).[9] However, in an artificial cycle,
photocatalytic water oxidation to produce O2 is still a chal-
lenge.
In recent years, efforts have focused on the development of
heterogeneous[10] and homogenous[11] water oxidation catalysts
(WOC). WOCs based on complexes of Ru,[12–18] Ir,[19–24] Mn,[25–27]
Cu,[28–30] Co,[31–38] and Fe[39–44] with well-defined structures have
provided mechanistic insights into the critical steps to lead to
O=O bond formation. For example, one crucial intermediate in
the process of O=O bond formation is the high-valence metal
oxo (MÀO) species, which is generated by a multistep proton-
coupled electron transfer (PCET) oxidation of metal hydroxyl
(MÀOH) or metal aqua (MÀOH2) units on the WOCs. We are es-
pecially interested in photo-WOCs based on abundant metals,
which require systems with high “photo-robustness”.[45] Recent-
ly, Co- and Ni-based polyoxometalates (POMs) were found to
perform well for photocatalytic O2 production.[31,46–49] However,
these POMs degraded into heterogeneous metal oxides or hy-
droxyl oxides in some cases.[8b,50] In contrast to molecular
WOCs, heterogeneous catalysts such as metal oxides have high
turnover frequencies and robustness but their characterization
is challenging, which inhibits the optimization of the cata-
lyst.[51]
[a] J. Xu, Z. Wang, Dr. D. Sun, Dr. C.-H. Tung, Dr. W. Wang
Key Lab of Colloid and Interface Chemistry, Ministry of Education
School of Chemistry and Chemical Engineering
Shandong University
Jinan, 250100 (P. R. China)
Metal–organic frameworks (MOFs) provide suitable scaffolds
for photo-WOCs because of their well-organized structures,
multiple metal centers with redox activity, and high surface
area for the reaction to occur. As heterogeneous WOCs they
are photo-robust and recycled easily.[52] Specifically, MOFs that
consist of m-O and m-OH ligands are potential sites for the acti-
vation of H2O to lead to O2 evolution. To date, however, exam-
ples of MOFs-based photo-WOCs are rare, especially those
E-mail: wwg@sdu.edu.cn
dsun@sdu.edu.cn
[b] W. Yu, Dr. Q. Zhang
Dalian Institute of Chemical Physics
Chinese Academy of Sciences
457 Zhongshan Road, Dalian 116023 (China)
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cssc.201600101.
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with low-cost metals.[53] Very recently, Mao et al. re-
ported photocatalytic O2 production by layered or-
ganic cobalt phosphonates as heterogeneous cata-
lysts for H2O oxidation.[54] The oxo-bridged metal
centers in these cobalt(II) phosphonates is thought
to facilitate water oxidation. Herein, we report two
new layered cobalt(II) dicarboxylates as photo-
WOCs. The materials [Co3(m3-OH)2(bdc)2]n (1) and
[Co3(m3-OH)2(chdc)2]n (2) (bdc=o-benzenedicarboxy-
late and chdc=1,2-cyclohexanedicarboxylate) have
the advantage that they are assembled readily in
good yield and high purity. Importantly these new
materials exhibit a high activity for photocatalytic
water oxidation (Scheme 1). The catalytic activities of
Figure 1. ORTEP view of the coordination environment of CoII in the structure of 1 (left)
and polyhedral representation of the distorted Kagóme lattice of 1 (right).
torted octahedral geometry coordinated by four Ocarboxylate
atoms (CoÀO, 2.011(6)–2.106(6) Š) and two m3-OH groups (CoÀ
O, 2.115(7)–2.342(7) Š). Co1 and Co2 are connected to two dif-
ferent hydroxyls groups, and Co3 and Co4 are coordinated by
the two same symmetry-related m3-OH groups (O10 for Co3
and O9 for Co4), which create two different triangles
(Co1Co2Co3 and Co1Co2Co4) in 1. Three Co atoms in these
two triangles are consolidated by two different m3-OH groups,
O10 for Co1Co2Co3 and O9 for Co1Co2Co4 (Scheme 1). The
Co···Co separation in these two triangles is in the range of
3.11–3.53 and 3.09–3.52 Š, respectively. The two triangles are
corner-shared to furnish a distorted Kagóme lattice that runs
along the bc plane (Figure 1). All the Co atoms are located in
the vertices of the Kagóme lattice. The bdc2À ligand is almost
Scheme 1. Schematic photocatalytic O2 production in a three-component
2À
system: [RuII(bpy)3]2+ (sensitizer), S2O8 (sacrificial oxidant), and catalyst
perpendicular to the cobalt(II) hydroxide layer. Benzene rings
of bdc2À act as an organic skin to provide protection up and
down the resultant layer. The layers are packed along the a di-
rection by van der Waals interactions with an interlayer dis-
tance of 10.2 Š.
(Cat.). For Cat.: ball-and-stick view of the arrangement of CoII in the synthetic
cobalt(II) dicarboxylate layer of 1 (dicarboxylate ligands are omitted for clari-
ty, details are shown in Figure 1).
these two WOCs were evaluated using a Clark-type oxygen
electrode under photochemical conditions. The difference in
O2 production performance between 1 and 2 provides an op-
portunity to correlate the structure with the activity. Our work
provides not only a new family of WOCs but also valuable in-
formation about the potential sites in such catalysts that are
activators of water oxidation.
Complex 2 has the same ratio of CoII atoms, hydroxyl
groups, and dicarboxylate groups (3:2:2) as 1 (Figure S1). The
detailed structure of 2 has been described previously.[55] Nota-
bly, not all CoII atoms are in the six-coordinated octahedral ge-
ometry. Co1 is coordinated by two O atoms from two carboxyl-
ate ligands and the O atoms from two hydroxyl groups to give
a distorted CoO4 tetrahedron, and the other two CoII atoms are
in the usual octahedral geometry. As a result of the tetrahedral
Co1 atom in 2, the 2D layer exhibits a much more distorted
Kagóme lattice in contrast to that of 1, in which the tetrahe-
Results and Discussion
dral Co1 is in
a corner-shared position and forms the
Synthesis and structures of 1 and 2
Co1Co2Co3 triangle with m3-OH groups. The interlayer distance
in 2 is increased to 13.6 Š, which is one of the most important
factors to govern its outstanding O2 evolution performance as
shown in the following section.
Compound 1 was synthesized from Co(NO3)2·6H2O, 1-aminote-
trazole, and o-benzenedicarboxylic acid in H2O/CH3OH mixed
solvent at 1808C, and 2 was prepared according to a modified
method similar to that described in the literature.[55] Com-
pounds 1 and 2 were isolated as fine pink and purple crystals,
respectively, some of which were suitable for single-crystal
XRD analysis.
Photochemical O2 production monitored using a Clark elec-
trode
In the structure of 1, each asymmetric unit contains two
bdc2À ligands, two m3-OH groups, and four CoII atoms. Two of
the CoII atoms (Co3 and Co4) are located at the crystallograph-
ic inversion center (Figure 1). Each CoII atom is in a slightly dis-
The catalytic activity of 1 and 2 for O2 production was evaluat-
ed using a Clark-type oxygen electrode system under photo-
chemical conditions. A 2.0 mL degassed borate buffer solution
(pH 8.0) that contained crystals of 1 or 2 (2 mg), 3.0 mm
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K2S2O8, and 0.4 mm [Ru(bpy)3]2+ (bpy=bipyridine) was irradiat-
ed using a 450 nm light-emitting diode (LED) lamp. The
amount of evolved O2 dissolved in solution versus time (t) was
measured in situ using a Clark electrode. Compounds 1 and 2
The catalysts operate optimally, and the highest level of O2
was produced at pH 8.0. At pH 8.0 and with 1.0 mm
[Ru(bpy)3]2+ and 9.0 mm K2S2O8, the plateau amount of O2
reached 1.7 mmol for 1 and 2.3 mmol for 2. Compound 2 is
more active with respect to O2 production than 1, reflected by
their RO values (48.8 mmolsÀ1 gÀ1 for 2 vs. 24.3 mmolsÀ1 gÀ1 for
2
1). A decrease of the pH to 7.4 or an increase to 9.0, resulted
in a decrease of RO . The effect of pH on RO for 1 is 24.3
2
2
(pH 8.0)> 16.9 (pH 9.0)>12.5 mmolsÀ1 gÀ1 (pH 7.4), and 48.8
(pH 8.0)>21.4 (pH 9.0)>15.3 mmolsÀ1 gÀ1 (pH 7.4) for
2
(Figure 3). Compounds 1 and 2 are much more active than
commercial Co3O4 particles (ꢀ30 nm size; Figure S7 and
Table 1). Compounds 1 and 2 can be recycled and remain
active for several cycles (Figure S9). The IR spectra recorded for
the recycled samples match well with those of the fresh crys-
tals (Figure S11).
Scaled-up photocatalytic O2 production
Figure 2. O2 evolution catalyzed by 1 (red) and 2 (black) under photochemi-
cal conditions: 2 mg catalysts, 0.4 mm [Ru(bpy)3]2+, 3.0 mm K2S2O8, 0.2m
borate buffer (2.0 mL, initial pH 8.0), and with a 450 nm LED lamp light
source.
To determine the highest amount of O2 produced, we conduct-
ed a scaled-up reaction in a Pyrex tube and quantified the O2
production by GC analysis. Typically, 10.0 mL of borate solution
show the same tendency to produce O2 upon the ir-
radiation of the reaction medium (Figure 2). After ir-
radiation for ꢀ2 s, O2 evolution starts, and the
amount of O2 increases almost linearly during the
first 20 s. Then, the oxygen evolution rates (RO
2
[mmolsÀ1 gÀ1]) declined over the next 10 s, and sub-
sequently, oxygen generation ceased gradually. This
photocatalytic O2 production system recovered if
fresh K2S2O8 buffer solution was added. For instance,
the total amount of O2 in the solution of 1 increased
from 0.7 mmol (first step in Figure 2, RO
12.6 mmolsÀ1 gÀ1) to 1.4 mmol (second step, RO
=
2
=
2
9.2 mmolsÀ1 gÀ1). Although the O2 production contin-
ued, the rate of production decreased (RO
=
2
4.4 mmolsÀ1 gÀ1) after an extra equivalent of K2S2O8
was added. Compound 2 shows higher levels of O2
production, and the total amount of O2 increased
from
1.2 mmol
(first
step
in
Figure 2,
RO =27.1 mmolsÀ1 gÀ1
)
)
to 1.8 mmol (second step,
and 2.0 mmol (third step,
2
RO =12.4 mmolsÀ1 gÀ1
Figure 3. Results of O2 evolution for a) 1 and b) 2 at different pH values, and the fitting
curve of the initial linear parts the curves of c) 1 and d) 2. Conditions: 1.0 mm
[Ru(bpy)3]2+, 9.0 mm K2S2O8, 0.2m borate buffer (2.0 mL, initial pH 7.4, 8.0, 9.0), and
a 450 nm LED lamp light source.
2
RO =6.5 mmolsÀ1 gÀ1).
2
Further studies revealed that the initial RO was af-
2
fected by the concentrations of both [Ru(bpy)3]2+
and [S2O8]2À and the amount of catalyst (Figures S3–
S6). For example, if the concentration of [Ru(bpy)3]2+
was increased from 0.1 to 1.0 mm, the RO increased almost lin-
2
Table 1. O2 evolution, turnover number (TON), and turnover frequency
(TOF) of 1, 2, and Co3O4 measured using a Clark-type oxygen electrode.
early from 1.75 to 18.35 mmolsÀ1 gÀ1 (Figure S4). Several control
experiments indicated that 1 and 2 are indispensable for O2
production. In the absence of 1 or 2, under otherwise identical
conditions, no O2 was measured in the degassed solution. In
contrast to the same amount of free Co2+ provided by cobalt
acetate or commercial Co3O4 (ꢀ30 nm size) particles, 1 and 2
are much more active (Figure S7).
Catalyst
O2 evolved [mmol]
TON
TOF”103 [sÀ1
]
1
2
1.78
2.28
0.41
0.96
1.26
0.049
14
20
0.77
Co3O4
Conditions: 1.0 mm [Ru(bpy)3]2+
(2.0 mL, initial pH 8.0), and a 450 nm LED lamp light source.
, 9.0 mm K2S2O8, 0.2m borate buffer
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(pH 8.0) that contained 1 or 2 (1.0 mgmLÀ1), [Ru(bpy)3]2+
(1.0 mm), and K2S2O8 (9.0 mm) was photolyzed using a 450 nm
LED lamp. The produced O2 in the headspace was analyzed
using a GC equipped with a 5 Š molecular sieve column and
a thermal conductivity detector with Ar as the carrier gas and
methane as the internal standard. Contamination of the head-
space caused by air in the degassed samples was nullified by
measurement of the N2 peak using GC.
S14), which indicates that the surface compositions also sur-
vive well. Moreover, in contrast to the same amount of free
Co2+ provided by cobalt acetate or commercial Co3O4 particles
(ꢀ30 nm size), 1 and 2 are much more active, consistent with
the results obtained using the Clark-type electrode (Figure S8
and Table S1).
Electrochemistry
The O2 produced by 1 and 2 over time is shown in Figure 4.
Remarkably, both 1 and 2 are extremely active toward water
To further elucidate the catalytic properties of 1 and 2, electro-
chemical experiments were conducted. Cyclic voltammetry
(CV) and linear sweep voltammetry (LSV) of 1, 2, and Co3O4
nanoparticles (ꢀ30 nm) were performed using a three-elec-
trode arrangement in 0.2m borate buffer at pH 8.0. The CVs of
1 and 2 display an irreversible oxidation wave at around 0.90 V
[vs. the normal hydrogen electrode (NHE)] and, subsequently,
an onset catalytic wave (Figure 5a). The oxidation process is at-
tributed to CoII/CoIII in the layers, which is more negative than
the 1.12 V of that in Co3O4. The overpotential to obtain the cur-
rent density of 1 mAcmÀ2 of 1, 2, and Co3O4 is 425, 412, and
622 mV, respectively (Figure 5b). These observations are con-
sistent with the results of photocatalytic O2 production,
namely, that 1 and 2 have higher activities than Co3O4. The sta-
bility of 1 and 2 during electrocatalytic water oxidation was
tested by performing continuous CV scans (Figure S15). We
propose that a CoIV=O active species was generated, followed
by water nucleophilic attack to form CoOOH, a similar active
catalytic species exists in a water oxidation system catalyzed
by nickel porphyrin.[56] The moderate overpotential of 1 and 2
may result from their layered carboxylate framework,[57] but
further work is required to explore details of such structure–
property relationships.
Figure 4. O2 evolution results from scaled-up photocatalytic reactions for
1 and 2 at different pH conditions: 1.0 mgmLÀ1 catalysts, 1.0 mm
[Ru(bpy)3]2+, 9.0 mm K2S2O8, 0.2m borate buffer (10.0 mL, initial pH 8.0, 9.0),
with a 450 nm LED lamp light source.
oxidation, consistent with the results obtained using the Clark-
type electrode. Independent of O2 dissolved in the solution,
the highest amount of O2 after 20 min photolysis was 0.3 mL
for 1 and 0.5 mL for 2 at pH 8.0. Based on the amounts of
K2S2O8 used, the yields of O2 are 29.8% for 1 and 49.6% for 2.
Notably, the pH value decreases from 8.0 to 7.1 after the pho-
toreaction. Although the O2 evolution ceased, the samples can
be recycled and remained active for several further cycles (Fig-
ure S10). We found that recycled 1 and 2 essentially retain
their activities. IR spectra and powder XRD patterns of recycled
1 and 2 confirm that the layered frameworks are maintained
after the reaction (Figures S11 and S12). The X-ray photoelec-
tron spectroscopy (XPS) spectra of the recycled samples are
similar to those of the as-prepared samples (Figures S13 and
Isotope-labeled H218O oxidation
To prove that the evolved O2 resulted from the photocatalyzed
oxidation of water, an isotope-labeling experiment was per-
formed using H218O (97% enriched) and 2 as the catalyst. The
resulting gas samples were analyzed by MS. The results of the
isotopic oxygen distribution after the elimination of 32O2 con-
tamination of the headspace caused by air in the degassed
samples are shown in Figure 6. The distribution of 16O16O/
16O18O/18O18O observed in the experiments was 0:7.55:92.45,
which matches the theoretical value of 0.09:5.82:94.09. These
Figure 5. a) CV and b) LSV curves of 1, 2, and Co3O4 in 0.2m borate buffer (pH 8.0). Conditions: scan rate: 20 mVsÀ1; glassy carbon working electrode (catalyst
loading: 1.12 mgcmÀ2), Ag/AgCl (3.0m KCl) reference electrode, and Pt wire counter electrode.
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tablishes the significance of the existence of OHÀ (or H2O)
binding sites at the metal center in water oxidation catalysts.
Experimental Section
Materials and methods
Co(NO3)2·6H2O, o-benzenedicarboxylic acid, 1,2-cyclohexanedicar-
boxylic acid, [Ru(bpy)3]Cl2·6H2O, K2S2O8, H3BO3, and Na2B4O7 were
purchased from Acros. Nafion perfluorinated resin solution (5 wt%)
was purchased from Sigma–Aldrich. 18O-enriched normalized water
(H218O, 97%) was purchased from Shanghai Research Institute of
Chemical Industry. Compound 2 was prepared according to a pub-
lished procedure.[55] The amount of evolved O2 dissolved in solu-
tion was measured in situ using a standard Clark-type oxygen elec-
trode (Hansatech Instruments). The total amount of evolved O2
was quantified using a Techcomp 7890 II GC equipped with a 5 Š
molecular sieve column with Ar as the carrier gas and thermal con-
ductivity detector. FTIR spectra were obtained using a PerkinElmer
Figure 6. Comparison of the theoretical and observed ratio of 32O2, 34O2, and
36O2 evolved from the isotope-labeling water oxidation experiment using
H218O (97% enriched).
FTIR Spectrometer Spectrum Two in the range of 4000–450 cmÀ1
.
observations indicate that the O2 evolved was from H2O
caused by photocatalyzed water oxidation.
Powder XRD patterns were obtained using a Rigaku D/Max 2200PC
diffractometer equipped with a graphite monochromator and CuKa
radiation (l=1.5418 Š) in the range of 2<2q<508. XPS was per-
formed using an Omicron (ESCA+) spectrometer, using an AlKa X-
ray source (1486.6 eV) equipped with a flood gun. Binding energies
(BEs) were calibrated using the C1s band (BE=284.7 eV). Electro-
chemical measurements (CV and LSV) were performed using a CHI
760e electrochemical workstation (Shanghai Chen Hua Instrument
Co., Ltd) with a conventional three-electrode arrangement. The iso-
tope-labeling experiment was performed using a mass spectrome-
ter (MID, model; Omnistar/quadrupole mass analyzer; Pfeiffer
Vacuum) using He as the carrier gas (30 mLminÀ1 flow).
The high yield of 34O2 implies that m-OH binding sites in the
cobalt(II) carboxylate layers are responsible for the catalytic O2
evolution. In the cases of Co-Pi[32,58] (Pi=phosphate) and
Co4O4(OAc)4Py4 (OAc=acetate, Py=pyridine),[59] the CoIV(O)
active species was generated by PCET oxidation of CoIIIÀOH
units in the precursors. We propose that a similar CoIV(16O) spe-
cies is generated in our photocatalytic reactions. The oxidative
2À
quenching of excited *[RuII(bpy)3]2+ by S2O8
affords
[RuIII(bpy)3]3+ as a strong oxidant (Ered =1.05 V vs. Ag/AgCl) and
the SO4 radical (Ered =2.1 V).[60,61] The successive oxidation of
ÀC
CoIIÀOH in 1 or 2 produces the CoIV(16O) species and achieves
CoIV(16OÀ OH) generation, which under basic conditions re-
18
Synthesis of 1
leases 34O2. These steps require the highly oxidative SO4 radi-
ÀC
A mixture of Co(NO3)2·6H2O (0.1 mmol, 29.0 mg), o-benzenedicar-
boxylic acid (0.1 mmol, 16.6 mg), 1-aminotetrazole (0.1 mmol,
8.5 mg), and CH3OH/H2O (5 mL, 1:1 v/v) was heated to 1808C for
10 h in a 25 mL Teflon-lined reaction vessel, kept at 1808C for 50 h,
and then cooled slowly to 308C in 13 h. Pink needle-like crystals of
1 were isolated by filtration, washed with EtOH, and dried in air. IR
(KBr pellets): n˜ =3605(w), 1573(m), 1473(m), 1373(s), 1143(m),
796(s), 766(s), 727(s), 635(s) cmÀ1; elemental analysis calcd (%) for
Co3C16O10H10: C 35.65, H 1.87; found: C 35.55, H, 2.02.
cal. If K2S2O8 was replaced by other sacrificial oxidants such as
AgNO3 or FcBF4, no O2 was produced as monitored using the
Clark-type oxygen electrode. Compared with the catalytic pro-
cess of Co3O4,[62] the structural characteristics of 1 and 2 seem
to facilitate water oxidation, but further work is required to
probe the details of such structure–property relationships.
Conclusions
We have developed two Kagóme cobalt(II) metal hydroxo car-
boxylate layered compounds that function as water oxidation
photocatalysts for the conversion of water into O2. These re-
sults were verified by in situ monitoring using a Clark-type
oxygen electrode as well as GC analysis. Isotope-labeling H218O
(97% enriched) experiments are consistent with a direct role of
the m-OH sites in the O2 evolution pathway. The catalytic activi-
ty of the layers was enhanced by changing the o-benzenedi-
carboxylate ligand to trans-1,2-cyclohexanedicarboxylate. This
change of the organic ligand increases the interlayer distance
by 3.4 Š. Photocatalytic reaction studies show that 1 and 2
possess enhanced activities toward the oxidation of water
compared to commercial nanosized Co3O4. Further work is re-
quired to further probe the details of such structure–property
relationships. This work not only provides new catalysts that
resemble ligand-protected cobalt oxide materials but also es-
Photocatalytic oxygen evolution measured using a Clark-
type oxygen electrode
The amount of evolved O2 dissolved in solution was measured
in situ using a standard Clark-type oxygen electrode (Hansatech In-
struments), the electrode was calibrated following a standard pro-
cedure before the measurements. Typically, each component was
mixed in 2.0 mL buffer solution, and the solution was degassed
with Ar for 30 min before measurement. The degassed solution
was then irradiated using a 450 nm LED lamp, and the amount of
O2 evolved in solution was measured in situ.
Photocatalytic oxygen evolution measured using GC
The total amount of evolved O2 was quantified using GC using
a Techcomp 7890 II GC with Ar as the carrier gas and a 5 Š molecu-
lar sieve column to separate O2, N2, and CH4. The gases were de-
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tected using a thermal conductivity detector. Typically, a Pyrex
tube was filled with borate buffer solution (10.0 mL, 0.2m) that
contained the catalyst (1.0 mgmLÀ1), [Ru(bpy)3]2+ (1.0 mm), and
K2S2O8 (9.0 mm). Then the tube was sealed with a rubber plug, and
the junction between the tube and the rubber plug was sealed
with wax. Subsequently, the tube was flushed with Ar for 60 min
to remove the residual air. After degassing, CH4 (200 mL) was inject-
ed into the tube as the internal standard. The tube was photolyzed
using a 450 nm LED lamp. After photolysis, 200 mL of the gas in
the headspace was sampled using a Hamilton (1750 SL) gas-tight
microliter syringe and then analyzed by GC. Contamination of the
headspace caused by air in the degassed sample was nullified by
measurement of the N2 peak.
Keywords: heterogeneous
frameworks · oxygen · photochemistry · water splitting
catalysis
·
metal–organic
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Isotope-labeling experiment
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Acknowledgements
We gratefully acknowledge the Natural Science Foundation of
China and Shandong Province (21402107, 91427303, 21571115
ZR2014BM011), fundamental research and subject construction
funds, and Young Scholars Program of Shandong University
(2015WLJH23).
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Received: January 25, 2016
Revised: March 6, 2016
Published online on April 21, 2016
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