A homogeneous molecular system for the photogeneration of hydrogen from water based on a [RuII(bpy)3]2+ photosensitizer and a phthalycyanine cobalt catalyst

Article abstract of DOI:10.1016/j.jallcom.2017.05.040

In this contribution, we report an efficient homogeneous system for the visible light-driven H₂ production from aqueous protons. This comprises a macrocyclic phthalycyanine cobalt complex [Co^IIPc(-2)] as a hydrogen-evolving catalyst (HEC), [Ru^II(bpy)₃]Cl₂ (bpy?=?2,2′-bipyridine) as a photosensitizer (PS) and triethylamine (TEA) as a sacrificial electron donor (SED). The system gives up to 2400 TON versus the catalyst with an initial TOF as high as 680 TON h^?1. Phthalocyanine cobalt and its derivatives are a promising new direction for molecular catalysts readily accessible in a large scale and low-cost achieving the photogeneration of H₂.

Full text of DOI:10.1016/j.jallcom.2017.05.040

Journal of Alloys and Compounds 717 (2017) 226e231
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
A homogeneous molecular system for the photogeneration of
hydrogen from water based on a [RuII(bpy)3]2þ photosensitizer and a
phthalycyanine cobalt catalyst
,
c
,
,
,
, **
An Xie a *, Xiao-Lei Liu a c, Yu-Chen Xiang a c, Geng-Geng Luo b
a School of Materials Science and Engineering, Xiamen University of Technology, Xiamen, 361024, China
b College of Materials Science and Engineering, Huaqiao University, Xiamen, 361021, China
c Key Laboratory of Functional Materials and Applications of Fujian Province, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this contribution, we report an efficient homogeneous system for the visible light-driven H2 pro-
duction from aqueous protons. This comprises a macrocyclic phthalycyanine cobalt complex [CoIIPc(-2)]
as a hydrogen-evolving catalyst (HEC), [RuII(bpy)3]Cl2 (bpy ¼ 2,20-bipyridine) as a photosensitizer (PS)
and triethylamine (TEA) as a sacrificial electron donor (SED). The system gives up to 2400 TON versus the
catalyst with an initial TOF as high as 680 TON hꢀ1. Phthalocyanine cobalt and its derivatives are a
promising new direction for molecular catalysts readily accessible in a large scale and low-cost achieving
the photogeneration of H2.
Received 8 January 2017
Received in revised form
30 April 2017
Accepted 5 May 2017
Available online 8 May 2017
Keywords:
© 2017 Elsevier B.V. All rights reserved.
Macrocyclic cobalt complex
Homogeneous catalysis
Photochemistry
Hydrogen evolution
Solar light-driven water splitting into molecular dihydrogen
(H2) is one of the most promising scenarios for sustainable energy
supply. An efficient photocatalytic molecular system for proton
reduction to H2 generally combined a light-absorbing photosensi-
tizer (PS), a hydrogen-evolving catalyst (HEC) and a sacrificial
electron donor (SED) [1]. The development of highly efficient,
inexpensive noble-metal-free molecular HECs for the reduction of
protons to hydrogen is a prerequisite, but faces many difficult
challenges [2]. So far, many macrocyclic cobalt complexes have
been explored in the context of photocatalytic H2 evolution as
mechanistic models and functional catalysts, with cobaloximes
playing by far the main role in the field [3].
Recently, as one of important cobalt macrocycles, porphyrin
cobalt was used as a homogeneous HEC for the photoreduction of
water to H2 in the presence of [RuII(bpy)3]Cl2 (bpy ¼ 2,20-bipyr-
idine) as the PS and ascorbic acid (AA) as the SED, achieving TON
(versus the catalyst) up to 725 [4]. The system, however, is limited
by its poor stability due to the depletion of the catalyst, which does
not exceed 3 h. We are curious to know whether phthalocyanine
cobalt structurally related to porphyrin cobalt also catalyzes pho-
togeneration of H2. Our considerations are highly emboldened from
the earlier reports that phthalocyanine cobalt functioned as elec-
trocatalysts for proton reduction [5].
As part of our efforts to construct and examine systems con-
taining noble-metal-free components [6], in this contribution, we
report that a phthalycyanine cobalt ([CoIIPc(-2)]) can serve as a
novel low-cost and efficient visible light-driven catalyst in an
artificial photocatalytic system for H2 evolution in combination
with [RuII(bpy)3]Cl2 as the PS and triethylamine (TEA) as the SED
(Scheme 1). The system gives up to 2400 TON (TON ¼ amount of
produced hydrogen/amount of catalyst) with respect to the HEC
phthalocyanine cobalt with an initial TOF as high as 680 TON hꢀ1
.
In most of previous systems using [RuII(bpy)3]Cl2 as the PS for
the photogeneration of H2, ascorbic acid (AA) was often employed
as the SED [4,7]. However, when the HEC [CoIIPc(-2)] was initially
paired with [RuII(bpy)3]Cl2 as the PS and AA as the SED, no
hydrogen was detected by GC analysis after 10 h of irradiation with
a pH range of 2.0e7.0. Despite the negative results regarding H2
generation, the reports that phthalocyanine cobalt complexes
exhibited efficiently electrocatalytic proton reduction encourage us
* Corresponding author. School of Materials Science and Engineering, Xiamen
University of Technology, Xiamen, 361024, China.
** Corresponding author. College of Materials Science and Engineering, Huaqiao
University, Xiamen, 361021, China.
E-mail addresses: nbaxa@126.com (A. Xie), ggluo@hqu.edu.cn (G.-G. Luo).
http://dx.doi.org/10.1016/j.jallcom.2017.05.040
0925-8388/© 2017 Elsevier B.V. All rights reserved.

A. Xie et al. / Journal of Alloys and Compounds 717 (2017) 226e231
227
to explore the system in the basic solutions by employment of
different SED components. Triethanolamine (TEOA) and triethyl-
amine (TEA) are two kinds of SEDs that have been previously uti-
lized in the basic solutions for the photocatalytic water reduction
systems [2]. We evaluated the use of these two SEDs, and the ki-
netic traces in Fig. 1a show the aliphatic amine TEA gives a two-fold
increase in total hydrogen production compared to TEOA. Thus, all
subsequent studies were conducted using TEA as the SED. It should
be noteworthy that when employing TEA as the SED, the net re-
action being driven photochemically can be expressed by the
equation (Eq. 1: NEt3 þ H2O / H2 þ HNEt2 þ CH3CHO). Thermo-
chemical data indicate that the overall reaction is a thermody-
namically unfavorable reaction and must be driven by light energy
[8].
[10]. The amount of H2 only produced from [RuII(bpy)3]Cl2, which
cannot be neglected, was subtracted from the total H2 produced, to
calculate the corrected TON.
An induction period with no detectable H2 evolution in the first
half an hour from the reaction profile is observed. Generally, the
appearance of induction phase may be ascribed to the formation of
CoI species or cobalt colloids produced from the photodecomposi-
tion of molecular Co catalysts. Consequently, mercury-poisoning
experiments are carried out to determine if colloidal cobalt is
possibly formed and responsible the activity of H2 evolution. The
addition of a large excess of mercury has no significant effect on the
catalytic activity of [CoIIPc(-2)] (Fig. S2, in the Supporting Infor-
mation), excluding the formation of metal colloids during the
photocatalytic reaction. Experiments with D2O in place of water
while keeping other conditions identical are conducted. The large
deuterium incorporation into the gaseous hydrogen products pro-
vides strong evidence that the only hydrogen source in the pro-
duced hydrogen gas is water (Fig. S3, in the Supporting
Information).
The influence of solvents on the H2-evolving reaction was
explored with the system of [CoIIPc(-2)] (2 ꢁ 10ꢀ5 M), [RuII(bpy)3]
Cl2 (2 ꢁ 10ꢀ4 M) and TEA (0.36 M) at pH 9.0 within 10 h of visible-
light irradiation (l > 420 nm). It is well known that solvents have an
apparent influence on the photoinduced H2 production [6,9]. As
shown in Fig. 1b, when the mixture of CH3CH2OH/H2O (4:1, v/v)
was used as solvent, more than 300 mmol H2 was produced, cor-
To investigate the catalytic activity of the complex [CoIIPc(-2)],
we studied the effect of the relative concentration of [CoIIPc(-2)]
and [RuII(bpy)3]Cl2 on the H2 production. The catalyst is most active
at low concentrations, and hydrogen evolution is observed using
[CoIIPc(-2)] as low as 2 ꢁ 10ꢀ6 M. Fig. 2a shows the effect of varying
catalyst concentration on the rate and overall yield of hydrogen
generation under continuous irradiation. Increasing the concen-
tration of [CoIIPc(-2)] increases the overall rate of H2 generation and
the total amount of hydrogen evolved for the system. The initial
rates for hydrogen evolution are obtained from the linear portion of
each curve and indicate a first order dependence on catalyst con-
centration for this system. When the catalyst concentration is fixed
at 2 ꢁ 10ꢀ6 M and the PS [RuII(bpy)3]Cl2 concentration increases
from 2 ꢁ 10ꢀ4 to 6 ꢁ 10ꢀ4 M, TONcat between 1300 and 2400 are
obtained, corresponding to a TOFcat of 310 and 680 hꢀ1, respectively
(Fig. 2b). The apparent quantum efficiency of the H2-evolving sys-
tem at 420 nm can be calculated to be 4.20% [11]. In the previous
report, porphyrin cobalt was used as molecular photocatalyst to
reduce protons into H2 in the presence of [RuII(bpy)3]Cl2
(bpy ¼ 2,20-bipyridine) as the PS and ascorbic acid (AA) as the SED.
Though the system achieves TON (versus the catalyst) up to 725,
the catalytic activity quickly decomposes after only 2 h of irradia-
tion [4]. For comparison, the longevity of the hydrogen-generating
system with the complex phthalycyanine cobalt can be increased.
Furthermore, re-addition of the same quantity of fresh HEC
responding to 150 TON with respect to the catalyst. In contrast, 130
and 80 TON were obtained, respectively, in the mixed solvents of
CH3OH/H2O (4:1) and CH3CN/H2O (4:1), while DMF/H2O (4:1) or
THF/H2O (4:1) resulted in no hydrogen production. The solvent
dependence for hydrogen production probably may be ascribed to
many factors including solvent polarity, stabilization of reduction
intermediates and Co(II/I) reduction potential necessary for
hydrogen generation [6a,9].
The pH value significantly affected the interaction of PS and HEC,
the state of PS and TEA, and the subsequent driving force for
photocatalytic H2 evolution in the photocatalytic system. The
[RuII(bpy)3]Cl2/[CoIIPc(-2)]/TEA photocatalytic system functioned
between pH 8.0 and pH 11.0, with a maximum catalytic activity at
pH 10.0 (Fig. S1, in the Supporting Information). It sharply
decreased in a more acidic or more basic medium. The great
decrease in TONs at lower pH values was probably because of the
protonation of TEA, which resulted in poor electron-donating
ability. In higher pH values, the decreased concentrations of pro-
ton will lead to a deceleration of efficiency of photogenerated
hydrogen. Control experiments showed that the absence of PS or
TEA led to no significant H2 production. However, weak H2 pro-
duction was observed from solutions containing only the PS
[RuII(bpy)3]Cl2, which is in agreement with previous observations
Scheme 1. Photocatalytic H2-production system using [CoIIPc(-2)] as HEC, [RuII(bpy)3]Cl2 as PS and TEA as SED.

228
A. Xie et al. / Journal of Alloys and Compounds 717 (2017) 226e231
Fig. 1. (a) Photoinduced H2 evolution vs. time using TEOA or TEA as SED with [CoIIPc(-2)] (2 ꢁ 10ꢀ5 M) and [RuII(bpy)3]Cl2 (2 ꢁ 10ꢀ4 M) in CH3CN/H2O (4:1 v/v) at pH 9.0. (b)
Comparison of H2 production using different solvent:water (4:1, v/v) with 2 ꢁ 10ꢀ5 M [CoIIPc(-2)], 2 ꢁ 10ꢀ4 M [RuII(bpy)3]Cl2, 0.36 M TEA at pH 9.0.
Fig. 2. Photocatalytic H2 production with 0.36 M TEA in 4:1 CH3CH2OH/H2O solution at pH 10.0 as a function of time under irradiation (l > 420 nm) in the presence of (a)
[RuII(bpy)3]Cl2 (2 ꢁ 10ꢀ4 M) and [CoIIPc(-2)] at various concentrations, and (b) [CoIIPc(-2)] (2 ꢁ 10ꢀ6 M) and [RuII(bpy)3]Cl2 at various concentrations. Left inset: the initial rate of
hydrogen production.
[CoIIPc(-2)] partly resumed the production of H2, suggesting the
present three-component system was thought to be largely limited
by the stability of the HEC and PS.
The first light-driven step of the photocatalytic mechanism with
the system [RuII(bpy)3]Cl2/[CoIIPc(-2)]/TEA (or TEOA) can be a
reductive quenching of *[RuII(bpy)3]Cl2 by TEA (or TEOA) leading to
the reduced form of [RuII(bpy)2(bpyꢂ-)]þ (Eq. 2: *[RuII(bpy)3]
Cl2 þ TEA / [RuII(bpy)2(bpyꢂ-)]þ þ TEAꢂ-), or an oxidative
quenching of *[RuII(bpy)3]Cl2 by [CoIIPc(-2)] yielding the oxidized
[RuII(bpy)3]Cl2/TEA system should proceed by oxidative quenching
of *[RuII(bpy)3]Cl2 by [CoIIPc(-2)] to generate the reduced form
[CoIPc(-2)]- species, because the reductive quenching of
*[RuII(bpy)3]Cl2 by TEOA or TEA is not exergonic.
In order to check for these possibilities, the interaction between
*[RuII(bpy)3]Cl2 and [CoIIPc(-2)] was investigated through emission
quenching experiments. As shown in Fig. 3a, the PS [RuII(bpy)3]Cl2
gave
a strong emission band as expected in the range of
500e800 nm with a maximum peak at 610 nm upon excitation at
460 nm in CH3CH2OH/H2O (4:1, v/v) at room temperature. The
luminescence intensity of [RuII(bpy)3]Cl2 gradually is decreased
upon increasing the concentration of [CoIIPc(-2)] in the [RuII(bpy)3]
Cl2 solution. After carefully correcting for inner filter effects, on the
basis of the Stern-Volmer plot (Fig. 3b) [13], the quenching constant
kq, calculated using the equation I0/I ¼ 1 þ Ksv[Q] ¼ 1 þ kqt0[Q], was
1.6 ꢁ 1010 Mꢀ1 sꢀ1 for [CoIIPc(-2)], which was at the diffusion-
controlled limit. Interestingly, as shown in Fig. 3c or 3d, it was
found that the emission quenching of [RuII(bpy)3]Cl2 in CH3CH2OH/
H2O (4:1, v/v) was not observed when an excess of TEOA or TEA was
added to the photocatalytic system. It is notable that neither TEOA
form of [RuIII(bpy)3]3þ (Eq. 3: [RuII(bpy)3]Cl*2
þ
[CoIIPc(-
2)] / [RuIII(bpy)3]3þ þ [CoIPc(-2)]-). According to the electro-
chemical
and
photophysical
¼
data
(E*1/2[RuII(bpy)3]Cl2/
[RuII(bpy)2(bpyꢂ-)]þ
0.67 V, E1/2[RuIII(bpy)3]3þ/*[RuII(bpy)3]
Cl2
¼
ꢀ0.76 V, E1/2[CoIIPc(-2)]/[CoIPc(-2)]-
¼
ꢀ0.48 V,
Eox(TEOA) ¼ 0.90 V and Eox(TEA) ¼ 0.73 V, respectively, vs. SCE)
[5,12,2c], the driving force of the reductive quenching of
*[RuII(bpy)3]Cl2 by TEOA (or TEA) and of the oxidative quenching of
[RuII(bpy)3]Cl*2 by [CoIIPc(-2)] were estimated to be
D
G0 ¼ 0.23 (or
0.06 eV) and ꢀ0.25 eV, respectively. The calculated free-energy G0
suggested that the [CoIIPc(-2)]/[RuII(bpy)3]Cl2/TEOA or [CoIIPc(-2)]/
D

A. Xie et al. / Journal of Alloys and Compounds 717 (2017) 226e231
229
Fig. 3. (a) Fluorescence quenching of [RuII(bpy)3]Cl2 solution in 8:2 CH3CH2OH/H2O in pH 10.0 after the sequential additions of [CoIIPc(-2)], inner filter correction applied. (b) Stern-
Volmer analysis of the integrated intensity yields a bimolecular quenching rate constant of kq ¼ 1.6 ꢁ 1010 Mꢀ1 sꢀ1. Photoluminescence spectra of solution containing [RuII(bpy)3]Cl2
after addition of different aliquots of TEOA (c) or TEA (d).
Fig. 4. (a) Absorption spectra of systems containing [RuII(bpy)3]Cl2 and TEA (5%, v/v) in EtOH/H2O (4:1) at pH 10.0 before and after 0.5 h irradiation with [CoIIPc(-2)]. (b) UVevis
spectra of [CoIIPc(-2)], and CoPcꢀ generated after the addition of NaBH4 to[CoIIPc(-2)].
nor TEA can quench [RuII(bpy)3]Cl2, which is in accordance with the
published work [14].
The above excited quenching experiments suggested that
intermolecular quenching was at nearly diffusion-controlled and

230
A. Xie et al. / Journal of Alloys and Compounds 717 (2017) 226e231
Fig. 5. (a) CV of [CoIIPc(-2)] in Ar-saturated DMSO solution containing 0.1 M TBAPF6 as the supporting electrolyte at the scan rates 0.6 V sꢀ1. (b) CV of [CoIIPc(-2)] after addition of
TFA.
the initial reaction is an oxidative (not a reductive) quenching of the
excited PS by the formation of [RuIII(bpy)3]3þ and [CoIPc(-2)]-
through a photo-induced electron transfer from *[RuII(bpy)3]Cl2 to
[CoIIPc(-2)]. The formation of [CoIPc(-2)]- can be observed by
following the UVevis absorption spectra for the [CoIIPc(-2)]/
[RuII(bpy)3]Cl2/TEA photosystem under H2-producing conditions.
As shown in Fig. 4a, before irradiation, the absorption spectra are
simply the sums of the individual components. After 0.5 h illumi-
Science Foundation of China (21641011 and 11204258), the Natural
Science Foundation of Fujian Province (2015J01053), Outstanding
Young Scientific Research Personnel Training Plan in Colleges and
Universities of Fujian Province (JA13229), the Program for New
Century Excellent Talents in University of Fujian Province
(ZA14228), Project of science and technology plan of Fujian Prov-
ince (2016H0037), and Science and Technology Plan of Xiamen City
(3502Z20151236 and 3502Z20153018).
nation (l > 420 nm), a significant bathochromic shift of the Q band
for phthalocyanine cobalt from 667 to 707 nm is observed. The
absorbance well matches the reported spectrum [15] of [CoIPc(-2)]-
and also matches the spectrum of [CoIPc(-2)]- generated from CoPc
and NaBH4 (Fig. 4b) [16].
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jallcom.2017.05.040.
Electrochemical studies were further carried out to get insight
into the possible catalytic protonation mechanism. As shown in
Fig. 5a, cyclic voltammograms (CVs) of [CoIIPc(-2)] in DMSO con-
taining 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6)
showed two reduction waves from 0 to ꢀ1.80 V vs. SCE in the
absence of available protons. The first reversible phthalocyanine
ring reduction was observed at E1/2 ¼ ꢀ0.48 V. The second
reversible redox wave at E1/2 ¼ ꢀ1.37 V was ascribed to CoII/CoI.
When trifluoroacetic acid (TFA) is added to a solution of [CoIIPc(-2)],
the first wave of redox process remains practically unchanged
while the onset of a catalytic irreversible wave is observed at ca.
-0.9 V (Fig. 5b). The results indicate that the formed CoI species
further reacts with a proton to produce a postulated CoIII-hydride,
which releases molecular hydrogen via a homo- or heterolytic
pathway. Further studies including theoretical calculations to
identify the exact mechanisms for H2 evolution catalyzed by cobalt
phthalocyanine are in progress.
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This work was financially supported by the National Natural

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