Photocatalytic hydrogen evolution using 9-phenyl-10-methyl-acridinium ion derivatives as efficient electron mediators and Ru-based catalysts

Article abstract of DOI:10.1071/CH12294

Photocatalytic hydrogen evolution has been performed by photoirradiation (λ>420nm) of a mixed solution of a phthalate buffer and acetonitrile (MeCN) (1:1 (v/v)) containing EDTA disodium salt (EDTA), [Ru^II(bpy) ₃]²⁺ (bpy=2,

Full text of DOI:10.1071/CH12294

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2012, 65, 1573–1581
Full Paper
http://dx.doi.org/10.1071/CH12294
Photocatalytic Hydrogen Evolution Using 9-Phenyl-10-
methyl-acridinium Ion Derivatives as Efficient Electron
Mediators and Ru-Based Catalysts
A
A
, ,
A B C
Yusuke Yamada, Kentaro Yano, and Shunichi Fukuzumi
A
Department of Material and Life Science, Graduate School of Engineering,
Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita,
Osaka 565-0871, Japan.
B
Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea.
C
Corresponding author. Email: fukuzumi@chem.eng.osaka-u.ac.jp
Photocatalytic hydrogen evolution has been performed by photoirradiation (l . 420 nm) of a mixed solution of a phthalate
II
2þ
0
buffer and acetonitrile (MeCN) (1 : 1 (v/v)) containing EDTA disodium salt (EDTA), [Ru (bpy) ] (bpy ¼ 2,2 -
3
þ
bipyiridine), 9-phenyl-10-methylacridinium ion (Ph–Acr –Me), and Pt nanoparticles (PtNPs) as a sacrificial electron
donor, a photosensitiser, an electron mediator, and a hydrogen-evolution catalyst, respectively. The hydrogen-evolution
rate of the reaction system employing Ph–Acr –Me as an electron mediator was more than 10 times higher than that
þ
employing a conventional electron mediator of methyl viologen. In this reaction system, ruthenium nanoparticles (RuNPs)
also act as a hydrogen-evolution catalyst as well as the PtNPs. The immobilization of the efficient electron mediator on the
surface of a hydrogen-evolution catalyst is expected to enhance the hydrogen-evolution rate. The methyl group of
þ
þ
þ
Ph–Acr –Me was chemically modified with a carboxy group (Ph–Acr –CH COOH) to interact with metal oxide
2
surfaces. In the photocatalytic hydrogen-evolution system using Ph–Acr –CH COOH and Pt-loaded ruthenium oxide
2
nanoparticles (Pt/RuO NPs) as electron donor and hydrogen-evolution catalyst, respectively, the hydrogen-evolution rate
2
þ
was 1.5–2 times faster than the reaction system using Ph–Acr –Me as an electron mediator. On the other hand, no
þ
enhancement in the hydrogen-evolution rate was observed in the reaction system using Ph–Acr –CH COOH with PtNPs.
2
þ
Thus, the enhancement of hydrogen-evolution rate originated from the favourable interaction between Ph–Acr –
þ
CH COOH and RuO NPs. These results suggest that the use of Ph–Acr –Me as an electron mediator enables the
2
2
photocatalytic hydrogen evolution using PtNPs and RuNPs as hydrogen-evolution catalysts, and the chemical modifica-
þ
tion of Ph–Acr –Me with a carboxy group paves the way to utilise a supporting catalyst, Pt loaded on a metal oxide, as a
hydrogen-evolution catalyst.
Manuscript received: 19 June 2012.
Manuscript accepted: 1 August 2012.
Published online: 13 September 2012.
Introduction
an electron mediator is generally crucial to achieve high efficien-
cy for the photocatalytic hydrogen evolution. The electron
mediator oxidatively quenches the photoexcited sensitiser, there-
by creating the charge separation. Although MV and a variety
of quaternary bipyridines (viologens) have been examined as
Hydrogen has been a promising candidate as a clean fuel of the
next generation to reduce consumption of fossil fuels, because
completely harmless water is the only product after hydrogen
2þ
[1–4]
burns.
However, most hydrogen supplied for industry is
[
20,21]
currently produced from natural gas, which is composed of light
alkanes, by a steam reforming reaction at high temperature,
followed by high-temperature and low-temperature water–gas-
shift reactions with emission of carbon dioxide that is regarded
as a typical greenhouse gas. Photocatalytic hydrogen pro-
duction seems to be the most environmentally benign method to
produce hydrogen without emission of green house gases.
Photocatalytic hydrogen production by using a sacrificial
electron donor has been extensively studied since the late
electron mediators,
during photocatalytic hydrogen evolution has been problematic
the instability of these compounds
[
22]
to improve.
In addition, the electron-transfer rate from the
2þ
photoexcited state of [Ru(bpy)3] to viologens highly depends
on the reduction potentials of viologens, however, no simple
correlation was found between the hydrogen-evolution rates and
the redox potentials of viologens.
[
electron mediators such as cobalt
plexes as well as graphene oxide
[
5]
[
4,6–8]
[20,21]
Thus, various types of
[26–28]
23–25]
and rhodium
com-
have been investigated as
[
29]
[
9–18]
1
components; EDTA disodium salt (EDTA), [Ru (bpy) ]
(
970s.
A typical reaction system is composed of several
II
electron mediators. An ideal electron mediator should possess
properties suchas efficient quenching ofa photoexcitedsensitiser
and slow back electron transfer to the sensitiser.
With regard to hydrogen-evolution catalysts PtNPs are most
frequently used, because Pt has a low overpotential for proton
[30]
reduction to evolve hydrogen. However, the use of Pt metals
2þ
3
0
2þ
bpy¼ 2,2 -bipyridine), methyl viologen (MV ), and colloidal
platinum particles (PtNPs) as a sacrificial electron donor, a
photosensitiser, an electron mediator, and a hydrogen-evolution
[
9g,19]
catalyst, respectively.
In this reaction system, the choice of
Journal compilation Ó CSIRO 2012
www.publish.csiro.au/journals/ajc

1
574
Y. Yamada, K. Yano, and S. Fukuzumi
is strongly desired to be avoided because of its high price and
scarce stocks. Development of hydrogen-evolution catalysts
mainly composed of inexpensive and abundant metals has
as an efficient electron mediator together with EDTA, [Ru
2
þ
(bpy)3] , and PtNPs as a sacrificial electron donor, a photo-
sensitiser, and a hydrogen-evolution catalyst, respectively
[
31–34]
þ
merited considerable interest.
A popular method to reduce
(Scheme 1). This is the first report using Ph–Acr –Mes as an
electron mediator in a photocatalytic hydrogen-evolution
the use of Pt metals is supporting small Pt particles on a metal
oxide. Metal-oxide surfaces of a catalyst support are beneficial
compared with metal surfaces to be functionalized by organic
molecules possessing an acid group such as a carboxylate group.
ꢀ
system. The 9-phenyl-10-methylacridinyl radical (Ph–Acr –
Mes), which is formed by the quenching of photoexcited
2
þ
þ
*[Ru(bpy)3] by Ph–Acr –Mes, has been reported to act as a
0
However, Pt/TiO has so far been the only supporting catalyst
2
strong reductant with the oxidation potential of E ¼ ꢁ0.55 V
[
35]
[41]
versus SCE in acetonitrile. This strong reducing ability of the
examined in photocatalytic hydrogen evolution.
This is
ꢀ
probably because oxygen atoms interacting with Pt particles
withdraw electrons from the Pt particles to decrease their
electron density, resulting in deactivation of Pt particles in
proton reduction.
Acr moiety allows us to use not only ruthenium metal nano-
particles (RuNPs) but also ruthenium oxide nanoparticles sup-
porting Pt particles (Pt/RuO NPs) as hydrogen-evolution
2
catalysts. In order to improve the hydrogen-evolution rate of
þ
An efficient electron mediator suitable for fast hydrogen
evolution may enable utilisation of a metal oxide to support Pt
particles as a hydrogen-evolution catalyst. In addition, such an
electron mediator could also allow the use of non-Pt metal
particles such as ruthenium, nickel, or iron particles as
the reaction system using Ru-based catalysts, Ph–Acr –Mes
was chemically modified and immobilized on the surface of
RuNPs and Pt/RuO NPs.
2
Results and Discussion
1
Photocatalytic Hydrogen Evolution Using Ph–Acr –Me
as an Electron Mediator
[
36–40]
hydrogen-evolution catalysts.
However, such a combina-
tion of an ideal electron mediator and a metal-oxide catalyst
supporting Pt particles or non-platinum hydrogen-evolution
catalysts has yet to be reported.
Photocatalytic hydrogen evolution was performed by photo-
irradiation of a mixed solution (2.0 mL) of a phthalate buffer (pH
4.5) and MeCN (1 : 1 (v/v)) containing EDTA (1.0 mM),
We report herein a photocatalytic hydrogen-evolution system
þ
[41]
using 9-phenyl-10-methyl-acridinium ion (Ph–Acr –Mes)
II
2þ þ
Ru (bpy) ] (0.20 mM), Ph–Acr –Me (0.30 mM), and PtNPs
3
[
(
ꢁ1
12.5 mg L ) as a sacrificial electron donor, a photosensitiser,
ꢀ
EDTA
2[Ru(bpy)3]
an electron mediator, and a hydrogen-evolution catalyst,
respectively. Fig. 1a compares the time courses of hydrogen
ꢀ
ꢀ
2
Ph–Acr –X
2H
þ
ꢀ∗
evolution in the reaction system using Ph–Acr –Me or
2þ
2[Ru(bpy) ]2
EDTAox
3
Cat.
methylviologen (MV ) as an electron mediator. No hydrogen
evolution was confirmed in the absence of an electron mediator
•
hν
2Ph–Acr –X
H2
[Ru(bpy) ]2
ꢀ
2þ
2
or photoirradiation in advance. When MV was used as an
electron mediator, the initial hydrogen-evolution rate (60 min)
3
(X ꢁ Me, CH CO H)
2
2
(
Cat. ꢁ Pt, Ru, Pt/RuO2)
ꢁ
1
ꢁ1
was 7.1 mmol s
g
, whereas the initial hydrogen-evolution
ꢁ1
Pt
ꢁ1
þ
rate (10 min) was as high as 92 mmol s
was employed as an electron mediator. Thus, Ph–Acr –Me acts
as a much more efficient electron mediator than MV . The
final amount of evolved hydrogen (2.0 mmol) with Ph–Acr –Me
g
Pt
when Ph–Acr –Me
þ
Scheme 1. The overall catalytic cycle for the photocatalytic hydrogen
2
evolution with EDTA, [Ru(bpy)
þ
3
]
, metal or metal oxide nanoparticles
þ
2þ
þ
(
PtNPs, RuNPs or Pt/RuO NPs), and an electron mediator (Ph–Acr –Me or
2
þ
Ph–Acr –CH
2
CO
2
H).
(
a)
(b)
100
1
st cycle
2nd cycle
2
1
1
0
.0
.5
.0
.5
0
8
6
4
2
0
0
0
0
0
ꢀ
Ph–Acr –Me
MV
0
10
20
30
40
50
60
0
0.2
0.4
0.6
0.8
1.0
ꢀ
Time [min]
Concentration of Ph–Acr –Me [mM]
Fig. 1. (a) Time courses of hydrogen evolution by photoirradiation of a mixed solution of a phthalate buffer solution (pH 4.5) and MeCN
þ
2
þ
ꢁ1
(
1 : 1 (v/v)) containing [Ru(bpy)
3
]
(0.20 mM), EDTA (1.0 mM), PtNPs (12.5 mg L ), and Ph–Acr –Me (0.30 mM, red circles) or methylviolo-
þ
gen (0.30 mM, blue squares). An additional amount of EDTA (1.0 mM) was added to the solution containing Ph–Acr –Me after H
þ
2
evolution
ceased. (b) Concentration effect of Ph–Acr –Me on hydrogen-evolution rate.

Hydrogen Evolution with Efficient Electron Mediators
1575
[
36a]
[38–40]
is the same as the amount of EDTA (2.0 mmol) in the reaction
solution, indicating that EDTA acts as a two-electron donor. An
additional amount of EDTA (1.0 mM) was added to the solution
catalyst, RuNPs
and RuO NPs
2
have been reported to
act as an efficient hydrogen-evolution catalysts in the photo-
catalytic hydrogen-evolution system. Thus, both RuNPs and
RuO NPs were examined instead of PtNPs in the photocatalytic
hydrogen-evolution system using Ph–Acr –Me as an electron
mediator.
þ
containing Ph–Acr –Me after hydrogen evolution ceased. The
2
þ
time course of hydrogen evolution from the solution is indicated
as the second cycle in Fig. 1a. A similar hydrogen-evolution rate
at the second cycle to that of the first cycle assures that
RuNPs were prepared by the reduction of RuCl with NaBH4
3
þ
Ph–Acr –Me acts as a robust electron mediator. The concen-
þ
in an aqueous solution (see Experimental section). The obtained
RuNPs were characterised by transmission electron microscopy
(TEM) as shown in Fig. 2a. The size of the primary particles of
RuNPs was around 4 nm. A powder X-ray diffraction pattern of
the RuNPs is displayed in Fig. 2c. The strong peak around 428
tration effect of Ph–Acr –Me on the hydrogen-evolution rate
was examined by changing the concentration within a range
from 0.10 to 1.0 mM. As indicated in Fig. 1b, the hydrogen-
evolution rate increases with increasing the concentration to
[
42a]
0
.3 mM. However, the hydrogen-evolution rate decreased by
þ
assignable to metallic Ru is clearly observed.
RuO NPs were prepared by calcination of the RuNPs at
increasing the concentration of Ph–Acr –Me to 1.0 mM. The
decrease in the hydrogen-evolution rate at the higher
2
6008C for 4 h in air. The size of the RuO NPs was confirmed by
2
þ
concentration of Ph–Acr –Me is ascribed to light absorption
þ
TEM observation to be ,20 nm ꢂ ,7 nm. Formation of RuO in
2
due to Ph–Acr –Me, which disturbs the photoexcitation
the rutile-type structure was confirmed by the powder X-ray
diffraction pattern as indicated in Fig. 2d. The peaks at 298, 358,
408, 418, and 548 are indexed to (110), (101), (200), (111), and
2
þ
þ
of [Ru(bpy) ]
(UV-vis spectra of Ph–Acr –Me and
þ
are shown in Fig. A1 in the Supplementary
3
2
[Ru(bpy)3]
2
þ
[42b]
Material). Thus, the concentrations of [Ru(bpy) ]
3
and
Ph–Acr derivatives were fixed as 0.2 and 0.3 mM, respec-
tively, for further investigation.
(211) planes, respectively.
RuO NPs assure high dispersity in the reaction solution by
The small sizes of the RuNPs and
þ
2
ultrasonication.
1
Photocatalytic Hydrogen Evolution with Ph–Acr –Me
Preparation of Ru-Based Catalysts
and RuNPs or RuO NPs
Platinum nanoparticles are known as an efficient catalyst for
hydrogen evolution. However, the high cost and scarce stocks
limit their use in any industrial applications. As an alternative
2
The photocatalytic hydrogen evolution was conducted using
ꢁ
1
RuNPs (0.10 g L ) or RuO NPs (0.25 g L ) as hydrogen-
ꢁ1
2
(
a)
(b)
20 nm
20 nm
(
c)
(d)
20
30
40
50
60
70 20
30
40
50
60
70
2θ [ꢃ]
2θ [ꢃ]
Fig. 2. Transmission electron microscopy images of (a) RuNPs and (b) RuO
2
NPs. X-ray diffraction patterns of (c) RuNPs and (d) RuO
2
NPs.

1
576
Y. Yamada, K. Yano, and S. Fukuzumi
ꢁ
1
ꢁ1
g , which is much larger than that of
Pt
evolution catalysts by photoirradiation (l . 420 nm) of a mixed
solution of a phthalate buffer (pH 4.5) and MeCN (1 : 1, (v/v))
as 92 mmol s
4.2 mmol s
ꢁ
1
ꢁ1
g of the reaction system using RuNPs. However,
Ru
2
þ
containing [Ru(bpy) ]
3
(0.20 mM), EDTA (1.0 mM), and
when the hydrogen-evolution rates of the reaction systems using
RuNPs (4.2 mmol s
system using PtNPs and MV (7.1 mmol s
typical photocatalytic hydrogen-evolution system, the
hydrogen-evolution rate of the former is ,60 % of the latter as
shown in Fig. 3. The specific hydrogen-evolution rate of the
reaction system using RuO NPs was 0.22 mmol s
þ
ꢁ1 ꢁ1
g ) is compared with that of the reaction
Ru
Acr –Ph (0.30 mM) as a photosensitiser, a sacrificial electron
donor, and an electron mediator, respectively. Hydrogen
evolution was observed in both the reaction systems using
2þ
ꢁ1 ꢁ1
g ), which is a
Pt
RuNPs and RuO NPs as hydrogen-evolution catalysts (Fig. A2
2
in the Supplementary Material). The hydrogen-evolution rate of
þ
ꢁ1 ꢁ1
,
RuO2
the reaction system using PtNPs and Ph–Acr –Me as hydrogen-
evolution catalyst and an electron mediator is as high
g
2
which is very low, even compared with that of the conventional
2
reaction system using PtNPs and MV . These results suggest
þ
that RuNPs and RuO NPs act as hydrogen-evolution catalysts
2
by using Ph–Acr –Me as an electron mediator although further
improvement is required in the catalytic reactivity.
8
þ
Immobilization of an Electron Mediator on RuNPs
6
4
2
8
The immobilization of an electron mediator on the surfaces of
RuNPs was examined by direct coupling of the carboxy-termi-
þ
nated 9-phenyl-10-methylcarboxyacridinium ion (Ph–Acr –
CH COOH) with 4-mercaptophenol-functionalized RuNPs
2
(HO–C H S–RuNPs), which is prepared by mixing
6
4
þ
þ
4
-mercaptophenol and RuNPs in MeCN. Ph–Acr –CH COOH
2
acts as an electron mediator the same as Ph–Acr –Me in the
photocatalytic hydrogen-evolution system (Fig. A3 in the Sup-
plementary Material). The coupling reaction was performed in
the presence of N,N -diisopropylcarbodiimide and 4-(N,N-
dimethylamino)-pyridinium-4-toluene-sulfonate as the cou-
0
[
43]
pling agents, as shown in Scheme 2. The immobilization of
þ
Ph–Acr –CH COOH was qualitatively confirmed by the UV-
2
vis measurement of the particles after the coupling reactions.
þ
PtNPs
MV
RuNPs
Ph–Acr –Me
RuO NPs
Ph–Acr –Me
2
Fig. 4a shows the characteristic bands of Ph–Acr –CH COOH
2
2
ꢀ
ꢀ
ꢀ
around 346, 362, and 430 nm and the characteristic band of
þ
Hydrogen-evolution catalyst and electron mediator
HOC H SH around 290 nm. The amount of Ph–Acr –
6
4
CH COOH immobilized on the surfaces of the RuNPs was
2
Fig. 3. Hydrogen-evolution rates normalized by the weights of hydrogen-
estimated by thermal gravimetric (TG) measurements under
atmospheric conditions (Fig. 4b). TG measurements for Ph–
evolution catalysts. The reaction was performed by photoirradiation
(
(
l . 420 nm) of a mixed solution of an aqueous buffer (pH 4.5) and MeCN
þ
2þ
Acr –CH COOC H S–RuNPs and HOC H S–RuNPs were
6 4
1 : 1 (v/v)) containing [Ru(bpy)
þ
3
]
(0.20 mM), EDTA (1.0 mM) using
ꢁ1
2
6
4
ꢁ1
performed by increasing the temperature from room tempera-
ꢁ1
Ph–Acr –Me (0.30 mM) and RuNPs (0.10 g L ) or RuO
2þ
2
NPs (0.25 g L )
ꢁ1
or using PtNPs (0.025 g L ) and MV (0.30 mM) as an electron mediator
and a hydrogen-evolution catalyst.
ture to 6008C with a ramp rate of 108C min . In the TG mea-
surement for Ph–Acr –CH COOC H S–RuNPs, a sudden
1
2
6 4
ꢀ
N
OH
Coupling
Ru
O
OH
OH
S
S
S
HO
Ru
OH
ꢀ
ꢂ
ꢁ
Ph–Acr –CH COOC H S
2
6 4
S
S
S
HO
HO
þ
Scheme 2. Preparation scheme for the Ph–Acr –CH
2
COOC
6
H
4
S–RuNPs.

Hydrogen Evolution with Efficient Electron Mediators
1577
weight loss of 49 % was observed around 2908C, and successive
weight loss was observed up to 5008C with a decrease of 16 % of
total weight. Thus, the total weight loss observed for Ph–Acr –
Loading of Pt on RuO NPs and Immobilization
2
of an Electron Mediator on Pt/RuO NPs
2
þ
Pt particles supported on RuO NPs (Pt/RuO NPs) were
2
2
CH COOC H S–RuNPs was 65 %. In the TG measurement for
4
þ
2
6
examined as a hydrogen-evolution catalyst with Ph–Acr –
þ
HO–C H S–RuNPs, the smaller weight loss of 57 % was
6
4
CH COOH instead of Ph–Acr –Me as an electron mediator,
2
observed by heating to 6008C with a steep weight loss of ,40 %
around 2508C. Based on these weight-loss values, at least 17 %
of the HOC H S moieties were reacted with Ph–Acr –
because the carboxy group can be expected to interact with the
metal oxide surface. Before catalysis measurements, the inter-
action between Ph–Acr –COOH and RuO NP surfaces was
2
ꢁ
þ
þ
6
4
CH COOH (see the Supplementary Material for calculation
2
procedures).
Fig. 5 compares the hydrogen-evolution rates normalized by
catalyst weight determined for reaction systems using Ph–
5
4
þ
ꢁ1
Acr –CH COO–C H S–RuNPs (0.10 g L
Ru
for Ph–Acr moiety) with that using Ph–Acr –CH COOH
and ,0.18 mM
2
6
4
þ
þ
2
ꢁ
L ). In both
Ru
1
(
0.30 mM) and HO–C H S–RuNPs (0.10 g
6
4
cases, the hydrogen evolution rates were significantly lower
compared with that of the reaction system using RuNPs
ꢁ þ
1
(0.10 g L ) and Ph–Acr –Mes (0.30 mM). In particular, only
a small amount of hydrogen was evolved from the solution using
þ
Ph–Acr –CH COO–C H S–RuNPs. Thus, the coverage of
2
3
2
1
6 4
þ
RuNP surfaces by sulfur and the immobilization of the Ph–Acr
ꢁ
moiety by –COO–C H S decelerates the hydrogen evolution.
4
6
þ
When Ph–Acr –CH COOH was added to the reaction system
2
þ
using Ph–Acr –CH COO–C H S–RuNPs, the hydrogen-
2
6 4
evolution rate increased to the same level of that of the reaction
þ
system using RuNPs and Ph–Acr –Mes. This result indicates
ꢁ
that the linker of –CH COO–C H S is not suitable for immo-
2
6 4
þ
bilization of the Ph–Acr moiety. In a previous report, the
2þ
0
0
1
immobilized on Pt clusters acts as an efficient electron mediator
-(1-hexyl-6-thiol)-1 -methyl-4,4 -bipyridinium ion (MVA
)
[
44]
for photocatalytic hydrogen evolution. From the comparison
of the chemical structure of the electron mediators for immobi-
lization (shown in Fig. A4 in the Supplementary Material), the
distance between the acridinium and thiolate moieties of
0
ꢀ
RuNP
HO–C
H
S–RuNPs Ph–Acr –CH CO –
6
4
2
2
ꢀ
ꢀ
ꢀ
C H S–RuNPs
6
4
ꢀ
Ph–Acr –CO H
Ph–Acr –CH CO H
2
2
2
þ
ꢁ
Ph–Acr –CH COO–C H S is comparable to that between
2
6 4
2
þ
Fig. 5. Comparisonof specifichydrogen-evolution rates normalized by the
the pyridinium ion and thiolate of MVA . Thus, the rigid
ꢁ
catalyst weight. Hydrogen evolutions were performed by photoirradiation
structure of the linker part (–CH COO–C H S ) may disturb
2
6 4
(
l . 420 nm) of the mixed solution (2.0 mL) of a phthalate buffer (pH 4.5)
and MeCN (1 : 1 (v/v)) containing [Ru(bpy)
efficient electron transfer from the acridinyl radical to RuNPs.
These results suggest that improvement of metallic RuNPs by
surface modification with organic molecules is not straightfor-
ward because of the less interactive nature of the metal surfaces
with organic molecules.
2þ
3
]
(0.20 mM), EDTA
ꢁ1
(
1.0 mM), and Ph–Acr–COOH (0.30 mM) and RuNPs (0.10 g-Ru
L
),
ꢁ1
HO–C H S–RuNPs (0.10 g
-Ru
6
4
L ) and Ph–Acr–COOH (0.30 mM), or
ꢁ1
þ
þ
2
6
4
Ru
Ph–Acr –CH COO–C H S–RuNPs (0.10 g L
, 0.17 mM of Ph–Acr
moiety).
10
(a)
(b)
0
ꢀ
Ph–Acr –CH COOC H S–RuNPs
ꢂ10
ꢂ20
ꢂ30
2
6
4
HOC H S–RuNPs
6
4
ꢂ40
ꢂ50
HOC H SH
ꢀ
6
4
Ph–Acr –CH COOH
2
ꢂ60
ꢂ70
ꢀ
Ph–Acr –CH COOC H S–RuNPs
2
6
4
250
300
350
400
450
500
0
100
200
300
400
500
600
Wavelength [nm]
Temperature [ꢃC]
þ
þ
Fig. 4. (a) UV-vis spectra of Ph–Acr –CH
2
þ
COOC
6
H
4
S–RuNPs (black solid), Ph–Acr –CH
2
COOH (red dashed), and HOC
6 4
H SH (black dotted) in MeCN.
(
b) Thermal gravimetric curves of Ph–Acr –CH
1
from room temperature to 6008C with a ramp rate of 108C min under air.
2
COOC
6
H
4
S–RuNPs (black solid) and HOC
6
H
4
S–RuNPs (black dashed). The temperature was increased
ꢁ

1
578
Y. Yamada, K. Yano, and S. Fukuzumi
(
a) 1.0
(b) 1.0
0.8
Before
After
Before
After
0
0
0
0
.8
.6
.4
.2
0
0.6
0.4
0.2
0
3
00
350
400
450
500
550
300
350
400
450
500
550
Wavelength [nm]
Wavelength [nm]
þ
þ
Fig. 6. (a) UV-vis spectra of Ph–Acr –COOH in MeCN before and after addition of RuO
2
NPs and (b) UV-vis spectra of Ph–Acr –Me in MeCN before and
2
after addition of RuO NPs.
1
.0
þ
þ
procedure was repeated with Ph–Acr –Me instead of Ph–Acr –
CH COOH, no difference was observed in the UV-vis spectra
Electron mediator:
2
ꢀ
Ph–Acr –Mes
before and after addition of RuO NPs. From the decrease in the
2
þ
absorbance, the amount of adsorbed Ph–Acr –CH COOH was
2
0
0
0
.8
.6
.4
ꢀ
Ph–Acr –CH COOH
2
ꢁ5
higher than 2.0 ꢂ 10 mmol, which corresponds to 280 mole-
cules attached to one RuO NP under the assumption of a cyl-
2
inder structure with a 10 nm diameter as the base and 20 nm
in height.
Hydrogen evolution has been conducted using RuO NPs and
Ph–Acr –CH COOH as a hydrogen-evolution catalyst and an
2
electron mediator by photoirradiation (l . 420 nm). The
hydrogen-evolution rate of 0.47 mmol s gRuO2 determined
for the reaction system was more than two times larger than that
of 0.22 mmol s gRuO2 determined for a reaction system using
Ph–Acr –Me as an electron mediator as shown in Fig. 7. The
2
þ
ꢁ
1
ꢁ1
ꢁ
1
ꢁ1
þ
0.2
inefficient hydrogen evolution with RuO NPs even after the
2
improvement indicates that RuO NPs are not suitable as a
2
catalyst but a support material of Pt particles. A small amount
of Pt metal was loaded on the surfaces of RuO NPs to evaluate
0
RuO2
0.1 %Pt
0.5 %Pt
/RuO2
1.0 %Pt
/RuO2
2
/RuO2
RuO NPs as a support. The Pt metal was deposited on the
2
surfaces of RuO NPs by reducing H PtCl in ethanol, which is
2
2
6
Fig. 7. Comparison of hydrogen-evolution rate normalized by catalyst
weight in the photocatalytic hydrogen evolution using Pt/RuO as hydrogen-
COOH (blue)
is 0.1, 0.5, or
.0 %. The hydrogen evolution was performed by photoirradiation
l . 420 nm) of a mixed solution of a phthalate buffer (pH 4.5) and MeCN
both solvent and reductant. Fig. 7 compares specific hydrogen-
evolution rates normalized by weight of RuO NPs in the
2
þ
þ
2
evolution catalyst and Ph–Acr –Mes (grey) or Ph–Acr –CH
as an electron mediator. The loading amount of Pt on Pt/RuO
2
reaction systems using RuO supporting different amounts of
2
2
þ
Pt as a hydrogen-evolution catalyst and Ph–Acr –Me or
þ
1
Ph–Acr –CH COOH as an electron mediator. The improve-
2
(
2þ
þ
(
1 : 1 (v/v)) containing [Ru(bpy)
3
]
(0.20 mM), EDTA (1.0 mM), RuO
þ
2
NPs
ment in hydrogen-evolution rate by using Ph–Acr –CH COOH
2
ꢁ1
þ
loaded with Pt (0.25 gRuO2
L
), and Acr –Ph–Me (0.30 mM) or Acr –Ph–
was also observed in the reaction systems using Pt/RuO as a
2
COOH (0.30 mM). Time courses of hydrogen evolution with each catalyst
are indicated in Fig. A5 in the Supplementary Material.
hydrogen-evolution catalyst. When the loading amount of
Pt was 0.10 %, the hydrogen-evolution rates of 0.36 and
ꢁ
1
ꢁ1
0
determined for the reaction systems using Ph–Acr –Me and
.60 mmol s
g
cat
(normalized by total catalyst weight) were
þ
directly confirmed by the UV-vis spectroscopic change as
ꢁ
1
indicated in Fig. 6. RuO NPs (0.25 g L ) were added to a
þ
Ph–Acr –CH
tion rates are normalized by the Pt weight of the catalysts,
COOH, respectively. When the hydrogen-evolu-
2
2
þ
MeCN solution (2.0 mL) containing Ph–Acr –COOH
(0.05 mM) and slowly stirred for several minutes. The suspen-
ꢁ1
hydrogen-evolution rates are as high as 360 and 600 mmol s
ꢁ1
sion was then slowly evaporated to dryness under reduced
pressure and the same volume (2.0 mL) of MeCN was added to
the residues. The UV-vis spectra were measured for the original
g
Pt . As described above, the maximum hydrogen-evolution rate
ꢁ1
ꢁ1
gPt when PtNPs
were used as a hydrogen-evolution catalyst. The hydrogen-
normalized by the Pt weight is 92 mmol s
solution and the supernatant solution after adsorption of
þ
evolution rates of the reaction systems using Pt/RuO NPs
2
Ph–Acr –COOH on RuO NPs. As shown in Fig. 6a, the char-
2
increases in proportion to the loading amount of Pt. The
hydrogen-evolution rates normalized by catalyst weight of the
þ
acteristic absorption bands of Ph–Acr –CH COOH at 346, 362,
2
þ
and 430 nm decreased after the addition of RuO NPs. The
2
decreases in absorption bands were ,20 %. When the same
reaction systems using 0.5 and 1.0 % Pt/RuO
ꢁ1
2
with Ph–Acr –
gcat where the specific
ꢁ1
CH
COOH were 0.67 and 0.81 mmol s
2

Hydrogen Evolution with Efficient Electron Mediators
1579
hydrogen-evolution rates normalized by Pt weight were 133 and
were recorded by an Agilent UV8453 by using a quartz
cuvette with 1.0 cm light-path length. TG/DTA analysis
was performed with an SII nanotechnology EXSTAR 7000
under atmospheric conditions. Each sample was loaded in an
Al pan (5 mm i.d.).
ꢁ ꢁ1 þ
g . When Ph–Acr –Me was employed as an
Pt
1
8
1 mmol s
electron mediator in these reaction systems, the hydrogen-
ꢁ
1
ꢁ1
. Thus,
Pt
evolution rates were decreased to 84 and 47 mmol s
g
improvement in the specific hydrogen rates by 50–80 % was
þ
þ
observed using Ph–Acr –CH COOH instead of Ph–Acr –Me
2
as an electron mediator when RuO -based catalysts were used as
2
hydrogen-evolution catalysts.
Synthesis of Nanoparticles (RuNPs, RuO NPs,
and Pt/RuO2)
2
Conclusions
Sodium borohydride (5.0 mmol, 0.20 g) was added to an aque-
ous solution (15 mL) containing ruthenium(III) chloride
Highly efficient hydrogen evolution has been successfully
achieved by photoirradiation (l . 420 nm) of a mixed solution
of a phthalate buffer (pH 4.5) and MeCN (1 : 1 (v/v)) containing
(0.20 mmol, 0.11 g) with vigorous stirring for 10 min. The
obtained particles were collected by filtration and washed with
pure water three times and dried under vacuum at room
temperature. The metallic phase of RuNPs was confirmed by
powder X-ray diffraction. The sizes of the obtained nano-
particles were determined by transmittance electron microscopy
(TEM).
þ
Ph–Acr –Me as an electron mediator together with EDTA,
[
2
Ru(bpy)3] , and PtNPs as a sacrificial electron donor, pho-
þ
tosensitiser, and hydrogen-evolution catalyst, respectively. The
hydrogen-evolution rate of this reaction system was more than
2
0 times higher than that of the reaction system using MV as
þ
1
an electron mediator. The use of Ph–Acr –Me as the electron
þ
The RuO2 nanoparticles (RuO NPs) were obtained by
2
calcination of RuNPs under O atmosphere at 6008C for 4 h.
2
Formation of a RuO phase was confirmed by powder X-ray
2
mediator has enabled the use of Pt/RuO NPs and RuNPs as
2
hydrogen-evolution catalysts instead of PtNPs. The electron
mediator was immobilized on the surfaces of the hydrogen-
evolution catalyst to examine the effect of immobilization on the
electron-transfer rate from the electron mediator to the
hydrogen-evolution catalyst. The introduction of a carboxylate
diffraction. The sizes of the obtained nanoparticles were
determined by TEM.
A calculated amount of hydrogen hexachloro platinate(IV)
hexahydrate was dissolved in absolute ethanol (5 mL). RuO2
was then immersed in the solution. The suspension was then
refluxed for 3 h to reduce the platinum ions. The obtained
powder was collected by filtration.
þ
þ
–
group on the methyl group of Ph–Acr –Me (Ph–Acr –COO )
allowed interaction with the RuO NP surfaces. The hydrogen-
2
þ
evolution rate of the reaction system using Ph–Acr –COO–
RuO NPs was about two times larger than that of the system
2
þ
using Ph–Acr –Me and RuO NPs. Such improvement in the
2
Synthesis of 9-Phenyl-10-carboxymethylacridinium
hydrogen-evolution catalysis was also observed for Pt/
RuO NPs in which the loading amount of Pt is lower than 0.5 %.
1
Hexafluorophosphate (Ph–Acr –CH COOH)(PF )
2
6
2
þ
The triflate of benzyl glycolate was prepared by a reported
[
Ph–Acr –COOH was successfully immobilized on the RuNPs
43]
method with slight modifications. Trifluoromethanesulfonic
anhydride (1.84 mL, 11 mmol) was added to a CH Cl solution
surfaces by coupling with 4-mercaptophenol, which coordinate
to RuNPs, however, no improvement in the hydrogen-evolution
rate was observed. This result indicates that not only the distance
between the redox centre of an electron mediator and the surface
of RuNPs but also the rigidity of the linker is important.
2
2
(
(
40 mL) of benzyl glycolate (1.66 g, 10 mmol) and pyridine
0.89 mL, 11 mmol) over a period of 5–10 min at ꢁ208C. After
complete addition, the reaction mixture was stirred for 30 min
and warmed to room temperature and stirred for an additional
3
through a short column of silica gel eluting with CH Cl . The
0 min. The reaction mixture was evaporated and rapidly passed
Experimental
2
2
All chemicals used for synthesising ruthenium nanoparticles
(RuNPs), ruthenium oxide nanoparticles (RuO NPs), and
fractions containing triflate were combined, the solvent was
evaporated, and the residue was subjected to a second rapid
chromatography (CH Cl /hexane, 1/1 (v/v)) to provide the
2
þ
9
-phenyl-10-carboxymethylacridinium
(Ph–Acr –COOH)
2
2
hexafluorophosphate were obtained from chemical companies
and used without further purification. Ruthenium(III) chloride
triflate as a pale yellow oil which solidified at 08C. This material
was suitable for direct use without any further purification. The
yellow solid (0.11 g, 0.37 mmol) was added to a CH Cl solution
(
RuCl ꢃnH O, 38 % Ru) and Pt-PVP nanoparticles (2 nm,
3
2
2
2
PVP ¼ poly(vinylpyrrolidone)) were supplied by Tanaka
Kikinzoku Kogyo. Sodium borohydride, potassium hexa-
fluorophosphate, hydrogen hexachloro platinate(IV) hexahy-
drate, pyridine, and dichloromethane were received from Wako
Pure Chemicals. 9-Phenylacridine, N-methyl-9-acridone,
EDTA disodium salt (EDTA), hydrogen bromide (30 % in acetic
acid, ,5.1 mol L ), and trifluoromethanesulfonic anhydride
were obtained from Tokyo Chemical Industry Co., Ltd. Benzyl
glycolate was purchased from Sigma–Aldrich Co. Purified
water was provided by a Millipore MilliQ water purification
system where the electronic conductance was 18.2 MO cm.
(8.0 mL) of 9-phenylacridine (0.086 g, 0.34 mmol) over a period
of 5–10 min at ꢁ158C. After complete addition, the reaction
mixture was stirred for 30 min, warmed to room temperature,
and stirred for an additional 24 h. Dried diethyl ether was added
to the reaction mixture and the precipitated crystals were filtered
to yield 10-benzyloxycarbonylmethyl-9-phenylacridinium
triflate. The acridinium ester protected by a benzyl group
(0.15 g, 0.32 mmol) was suspended in a 30 % solution of
hydrogen bromide/acetic acid (3.5 mL, 18 mmol) and heated for
30 min at 508C, and the solvent was removed under reduced
pressure. The resulting solid was dissolved in 35 mL of distilled
water and this was added to an aqueous solution (34 mL) of
potassium hexafluorophosphate (3.0 g, 16 mmol). The resulting
solution was stirred for 24 h at room temperature and
precipitated crystals were filtered off to yield 9-phenyl-10-
carboxymethylacridinium hexafluorophosphate.
ꢁ1
9
was synthesised by following the reported methods.
-Phenyl-10-carboxymethylacridinium hexafluorophosphate
[
41]
TEM images of nanoparticles, which were mounted on a
copper microgrid coated with elastic carbon, were observed
by a JEOL JEM 2100 operating at 200 keV. UV-vis spectra

1
580
Y. Yamada, K. Yano, and S. Fukuzumi
Photocatalytic Hydrogen Evolution
(c) N. Sutin, C. Creutz, E. Fujita, Comment. Inorg. Chem. 1997, 19, 67.
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PAC200072010317
Typical reaction procedures are as follows; RuO catalyst
2
ꢁ1
(
0.25 g L ) was added to a mixed solution (2.0 mL) of a
phthalate buffer (50 mM, pH 4.5) and MeCN (1 : 1 (v/v))
containing EDTA (1 mM), [Ru(bpy) ](ClO ) (0.2 mM), and
(
b) N. Toshima, K. Hirakawa, Polym. J. 1999, 31, 1127. doi:10.1295/
POLYMJ.31.1127
c) N. Toshima, M. Kuriyama, Y. Yamada, H. Hirai, Chem. Lett. 1981,
3
4 2
þ
Ph–Acr –COOH (0.3 mM) in a reaction vial (3.0 mL) flushed
with N gas. The solution was then irradiated with a xenon lamp
(
2
10, 793. doi:10.1246/CL.1981.793
(
Ushio Optical, Model X SX-UID 500X AMQ) through a colour
(d) N. Toshima, T. Yonezawa, Makromol. Chem. Macromol. Symp.
1992, 59, 281. doi:10.1002/MASY.19920590123
filter glass (Asahi Techno Glass) transmitting l . 420 nm at
room temperature. Evolved hydrogen gas in the headspace was
quantified by a Shimadzu GC-14B gas chromatograph (detector,
TCD; column temperature, 508C; column, active carbon with
(
e) N. Toshima, T. Yonezawa, New J. Chem. 1998, 22, 1179.
doi:10.1039/A805753B
[
13] (a) T. Yonezawa, N. Toshima, J. Mol. Catal. 1993, 83, 167.
doi:10.1016/0304-5102(93)87017-3
6
0–80 mesh particles size; carrier gas, N2).
(
0
(
3
b) I. Okura, N. Kimthuan, J. Mol. Catal. 1979, 6, 227. doi:10.1016/
304-5102(79)85004-X
c) I. Okura, M. Takeuchi, N. Kimthuan, Photochem. Photobiol. 1981,
3, 413. doi:10.1111/J.1751-1097.1981.TB05439.X
d) I. Okura, S. Aono, S. Kusunoki, Inorg. Chim. Acta 1983, 71, 77.
doi:10.1016/S0020-1693(00)83641-5
Supplementary Material
þ
2þ
UV-vis spectra of Ph–Acr –Me and [Ru(bpy) ] (Fig. A1),
3
time courses of hydrogen evolutions (Figs A2, A3 and A5),
chemical structures of linkers (Fig. A4), and calculation
(
ꢁ
[14] (a) L. Persaud, A. J. Bard, A. Campion, M. A. Fox, T. E. Mallouk, S. E.
Webber, J. M. White, J. Am. Chem. Soc. 1987, 109, 7309. doi:10.1021/
JA00258A011
procedure to estimate the ratio of reacted HO–C H -S are
6
available as Supplementary Material on the Journal’s website.
4
(
b) D. L. Jiang, C. K. Choi, K. Honda, W. S. Li, T. Yuzawa, T. Aida,
Acknowledgements
J. Am. Chem. Soc. 2004, 126, 12084. doi:10.1021/JA048912E
[15] (a) Y. Amao, ChemCatChem 2011, 3, 458. doi:10.1002/CCTC.
201000293
This work was supported by a Grant-in-Aid (Nos. 20108010 to SF and
2
4350069 to YY) from the Ministry of Education, Culture, Sports, Science
and Technology, Japan, NRF/MEST of Korea through WCU (R312008-000-
0010-0) and GRL (2010-00353) Programs (to SF). The authors acknowl-
(b) N. Himeshima, Y. Amao, Energy Fuels 2003, 17, 1641.
doi:10.1021/EF034006W
1
edge the Research Center for Ultra-Precision Science & Technology, Osaka
University for TEM measurements and Prof. Norimitsu Tohnai for powder
X-ray diffraction measurements.
[16] (a) S. Rau, B. Schafer, D. Gleich, E. Anders, M. Rudolph, M. Friedrich,
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