Electrocatalysis for proton reduction by polypyridyl platinumcomplexes dispersed in a polymer membrane

Toshiyuki Abe a,*, Kazuhiro Hirano a, Yukihide Shiraishi b, Naoki Toshima b,Masao Kaneko a,1

a Department of Chemistry, Ibaraki University, Mito, Ibaraki 310-8512, Japanb Department of Materials Science and Engineering, Science University of Tokyo in Yamaguchi, Onoda-shi, Yamaguchi 756-0884, Japan

Received 5 January 2000; received in revised form 20 June 2000; accepted 18 August 2000

Abstract

Catalysis for H� reduction by polypyridyl platinum complexes (e.g., bis(2,20-bipyridine) platinum(II) nitrate

�Pt�bpy�2�2 � and chloro(2,20:60,200-terpyridine) platinum(II) chloride (Pt(terpy)Cl�)) dispersed in a Na®on membrane

was studied by using a polymer-modi®ed electrode system. These Pt complexes incorporated in the polymer membrane

worked as active catalyst for H� reduction to produce H2. The cathodic current due to the H� reduction increased with

reaction time indicating that the electrocatalytic reaction does not take place by a simple catalysis of the original Pt

complex. The valence state of the Pt complexes after the reduction was investigated by both UV±Vis spectroscopy and

X-ray photoelectron spectroscopy (XPS) showing the presence Pt0 species. Photochemical H2 formation was investi-

gated in a photochemical H� reduction system composed of Pt�bpy�2�2 (or Pt(terpy)Cl�), Ru�bpy�2�3 sensitizer, methyl

viologen (MV2�) acceptor, and sacri®cial donor (EDTA). An induction period for the H2 evolution was observed,

which was ascribed to the formation of active Pt species by MV:� reductant produced by the photochemical reaction. It

was concluded that in both the two systems the Pt complexes at ®rst change to active species (zero-valent Pt), and then

work as the H� reduction catalyst. Ó 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Electrocatalysis; Proton reduction; Dihydrogen formation; Polymer-coated electrode; Macromolecule±metal complex

1. Introduction

Water photolysis to obtain O2 and H2 called an ar-

ti®cial photosynthetic model has been attracting great

attention to create new energy resource instead of con-

ventional fossil fuels [1±3]. However, such photosyn-

thetic systems have not been established. Toward this

goal, it is important to develop active catalysts for water

oxidation and proton reduction, which should be cou-

pled later with photoexcitation center. We have been

studying e�cient catalyst sites composed of a polymer

matrix and a transition metal complex [4±6].

It is of great interest to construct a molecule-based

photosynthetic model not only in view of basic science [7]

but also from a practical point. In a H� reduction site,

active catalyst has not been found except elemental Pt

(colloids, particles, etc.) [8±10]. However, we have re-

cently developed molecule-based catalysts for H� re-

duction by using polynuclear iron±cyanide complex [11]

and metallo-porphine incorporated in a polymer ®lm

[12,13]. It was noteworthy that the catalytic activity re-

markably exceeds that of a conventional Pt catalyst.

Platinum complex also must be an interesting material as

a H� reduction catalyst. It has been well-documented

that polychloroplatinates (e.g., PtCl2ÿ4 and PtCl2ÿ

6 ) work

European Polymer Journal 37 (2001) 753±761

* Corresponding author. Address: Department of Materials

Science and Technology, Hirosaki University, 3 Bunkyo-cho,

Hirosaki, Aomori 036-8561, Japan. Fax: +81-172-39-3580.

E-mail address: tabe@cc.hirosaki-u.ac.jp (T. Abe).1 Visiting senior researcher of The Institute of Physical and

Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama

351-0198, Japan.

0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.

PII: S0 01 4 -3 05 7 (00 )0 0 17 9 -8

as active catalysts after being reduced to the zero-valent

state [14±16]. Polypyridyl Pt complexes have been used as

catalyst in photochemical H� reduction systems, where

they are used with an organic/inorganic semiconductor

or a Ru�bpy�2�3 sensitizer [17±19]. However, the details

of the catalytic activity of the polypyridyl complexes

have not been investigated. Structural and voltammetric

studies on electroreduction of polypyridyl Pt complexes

have been attempted by using spectroscopic methods

[20±23], but the electrocatalysis for H� reduction by the

polypyridyl Pt complexes have not been studied.

In the present work, both electrochemical and

photochemical H� reduction by the polypyridyl Pt

complexes such as bis-(2,20-bipyridine)platinum(II) ni-

trate (Pt�bpy�2�2 ), chloro(2,20:60,200-terpyridine)platinum(II)

chloride (Pt(terpy)Cl�) was investigated to elucidate the

catalysis by the polypyridyl Pt complexes. In case of the

electrochemical process, the electrocatalysis was studied

by preparing a modi®ed electrode coated with a Na®on

(Nf) membrane incorporating a Pt complex. The use of Nf

membrane incorporating redox molecules at electrode

surfaces has been studied [24±26]. An Nf membrane is

an anionic and acidic polymer due to sulfonate groups

(SOÿ3 ) on the side chain. Many metal complexes are

cationic, so that they can be adsorbed easily into an Nf

membrane by cation exchange. We have shown that such

heterogeneous catalyst systems using polymer membrane

can often bring about high activity by cooperative in-

teraction between the catalysts, concentration e�ect of

the substrate in the matrix, etc [4±6,12,13]. Such a het-

erogeneous catalyst/Nf system is also of importance for

fuel cells [27]. In the photochemical H� reduction, the

system was composed of a photosensitizer, an electron

relay, a sacri®cial electron donor and a Pt complex.

2. Experimental

2.1. Material

Tris-(2,20-bipyridine)ruthenium(II) chloride (Ru-

�bpy�2�3 ) as a photosensitizer and platinum complexes

(Pt�bpy�2�2 , Pt(terpy)Cl�) as a catalyst to reduce H�

were synthesized according to previous procedures [17±

19,28±30]. A 5 wt.% Nf alcoholic solution was pur-

chased from Aldrich Chemical Co. Ltd. All the purest

grade reagents were used in the present work. A basal-

plane pyrolytic graphite (BPG) plate and an indium±tin

oxide (ITO) plate were purchased from Union Carbide

Co. Ltd. and Kinoene Kogaku Co. Ltd., respectively.

2.2. Instruments

UV/Vis absorption spectrum was measured by a

Hitachi U-2000 spectrophotometer. X-ray photoelec-

tron spectrum was obtained with AXIS-HS (KRATOS)

using MgKa radiation as the excitation source. The

hydrogen produced was analyzed by a gas chromato-

graph (Shimadzu, GC-4CPT) with a molecular sieve 5 �Acolumn and argon carrier gas.

2.3. Preparation of modi®ed electrode and electrochemical

study

A methanol solution containing 1 wt.% Nf was at

®rst prepared. The density of this solution is 0.81 g cmÿ3.

A Nf-coated BPG was at ®rst prepared by casting a 5 ll

alcoholic Nf solution onto a BPG electrode (e�ective

area, 0.21 cm2) and evaporating the solvent under room

temperature. The ®lm thickness can be estimated as �1

lm by considering the density of the Nf ®lm (2 g cmÿ3)

[26]. The Nf-coated BPG was dipped in a pure water for

30 min. Introduction of the Pt complex into the mem-

brane was carried out by a cation exchange method. The

Nf-coated BPG was immersed into an aqueous solution

containing a known concentration of the complex. The

adsorbed amount of the complex in the membrane was

estimated by the UV absorption spectral change before

and after the adsorption. For example, the complex

concentration in the membrane was calculated to be

0.75 mol dmÿ3 by considering the membrane volume

of 2:1� 10ÿ8 dmÿ3 (the membrane thickness, 1 lm)

when the adsorbed amount of the complex was 1:58�10ÿ8 mol (Fig. 1). The ITO electrode modi®ed by

Nf�Pt�bpy�2�2 � was prepared by the same method as the

Fig. 1. Cyclic voltammograms at BPG/Nf�Pt�bpy�2�2 � with a

repeated scanning. The redox couple at around ÿ0:8 V de-

creased gradually in the repeated scanning, and then the growth

of cathodic currents began to take place below ÿ0:9 V. After

the disappearance of the peak, the cathodic currents increased

slightly at around ÿ0:8 V, scan rate, 500 mV sÿ1; pH, 5.9.

754 T. Abe et al. / European Polymer Journal 37 (2001) 753±761

modi®ed BPG one. The membrane thickness of Nf was

�1 lm in every condition. An electrochemical cell was

equipped with the BPG/Nf[Pt-complex] working, a spi-

ral Pt counter and a silver/silver chloride (Ag/AgCl, in

saturated KCl electrolyte) reference electrode. Electro-

chemical study was carried out by using a potentiostat

(HOKUTO DENKO, HA-301) equipped with a func-

tion generator (HOKUTO DENKO, HB-104), a cou-

lomb meter (HOKUTO DENKO, HF-201) and a

recorder (RIKA DENKI, RW-211). All the electro-

chemical studies were run in a pH 5.9 phosphate bu�er

solution under argon atmosphere.

2.4. Photochemical study

A 500 W Xenon lamp (USHIO, UI-502Q) combined

with a cut-o� ®lter (Toshiba, L-39) for irradiation at

k > 390 nm was used as a light source. Light intensity was

1.6 W cmÿ2. All the photochemical H2 formations were

carried out in a pH 5.9 aqueous bu�er solution containing

a sacri®cial electron donor (disodium dihydrogen ethy-

lenediamine tetraacetate dihydrate, EDTA), photosen-

sitizer(tris-(2,20-bipyridine)ruthenium(II), Ru�bpy�2�3 ), an

electron relay (methylviologen, MV2�) and a Pt com-

plex catalyst to reduce H� under argon atmosphere.

3. Results and discussion

3.1. Electrochemical study

Fig. 1 shows the cyclic voltammogram (CV) at BPG/

Nf�Pt�bpy�2�2 �. Redox couple of the Pt�bpy�2�2 =Pt�bpy��2was observed at around ÿ0:8 V in the initial scanning. It

was found that the corresponding reoxidation peak

(ÿ0:9 V) is much smaller than the cathodic one. This

indicates that the electrogenerated Pt�bpy��2 is unstable

and further irreversibly changes to other Pt species at the

more negative potentials. After the repeated scanning,

the redox peaks disappeared gradually, and the cathodic

currents due to H� reduction increased remarkably

below ÿ0:9 V. This shows that the electrocatalytic H�

Fig. 2. Time-course of current in both (a) ITO/Nf�Pt�bpy�2�2 � and (b) ITO/Nf[Pt(terpy)Cl�] systems. Charging of the electric double

layer takes place initially at the ®rst spike, and then the cathodic currents in each system increases with time, corresponding to the CV

obtained in Fig. 1. pH 5.9; reaction time 10 min; �Pt�bpy�2�2 �, 0.52 M; [Pt(terpy)Cl�], 0.31 M.

T. Abe et al. / European Polymer Journal 37 (2001) 753±761 755

reduction takes place by some active Pt species trans-

formed from the Pt�bpy��2 .

The CV at BPG/Nf[Pt(terpy)Cl�] showed a similar

behavior as Fig. 1. The electrochemistry of terpyridyl

metal complexes in a solution has been studied [22];

the ®rst reduction takes place on the ligand and the

electrochemical process is usually reversible. However,

in the present heterogeneous system, similar to the

Nf�Pt�bpy�2�2 �, the peaks disappeared gradually in a re-

peated scanning, and then the cathodic current due to

the H� reduction grew up. The catalysis features of the

Pt complexes for the H� reduction will be discussed later

by using both UV±Vis and XPS spectroscopies.

Potentiostatic electrolyses by the Pt complexes in-

corporated into an Nf membrane coated on an ITO

electrode (denoted as ITO/Nf[Pt complex]) were inves-

Table 1

Results of potentiostatic electrolysis at applied potential of

ÿ0:95 V (vs. Ag/AgCl) in a pH 5.9 aqueous solution (10 min)

System H2 produced/ll TN/hÿ1

A bare ITO �0 ±

ITO/Nf �0 ±

ITO/Nf�Pt�bpy�2�2 �a 51.2 238.3

ITO/Nf[Pt(terpy)Cl�]b 7.1 55.2

a 0.52 M.b 0.31 M.

Fig. 3. (A) UV/Vis absorption spectral changes of ITO/Nf�Pt�bpy�2�2 � dipped in a pH 5.9 electrolyte solution (a) before and (b) after

electrolysis to form H2. After the electrolysis, the modi®ed ITO electrode was once polarized under anodic conditions at �0:7 V. (B)

UV/Vis absorption spectral changes of ITO/Nf[Pt(terpy)Cl�] (a) before and (b) after electrolysis to form H2 as well as an aqueous Pt

complex solution at (c) pH 0 and (d) pH 7. After the electrolysis, the modi®ed ITO electrode was once polarized at �0:7 V.

756 T. Abe et al. / European Polymer Journal 37 (2001) 753±761

tigated at the applied potential of ÿ0:95 V, and the time-

courses of the currents are shown in Fig. 2(a) and (b). As

expected from the CV, the cathodic current increased

remarkably with time. The electrolysis data for H2 for-

mation are shown in Table 1. Production of H2 did not

take place at both the bare and the Nf-coated ITOs,

while the Pt complexes dispersed in the membrane re-

sulted in much H2 formation. All these results (Fig. 2

and Table 1) show that the Pt complexes at ®rst trans-

form to active species and then the electrocatalytic H�

reduction occurs by them.

3.2. UV/Vis spectroscopy

In order to investigate the spectral features of the

polypyridyl Pt complexes before and after the electrol-

ysis (ÿ0:95 V, at pH 5.9), UV/Vis absorption spectrum

was measured at the modi®ed ITO. Fig. 3(A) shows the

spectral changes of the Nf�Pt�bpy�2�2 �. Identical spec-

trum of Pt�bpy�2�2 can be obtained in both aqueous

solution and membrane. After the electrolysis, the

modi®ed ITO was polarized at �0:7 V in order to

reoxidize any possible reversibly reduced Pt complex. By

this procedure the absorption in the region from 400 to

800 nm slightly increased with an intense decrease of the

p±p� band at 323 nm. As has been reported previously,

the redox reaction of the Pt�bpy�2�=�2 is electrochemi-

cally reversible in neutral pH conditions [21]. However,

as shown in Fig. 1(a), the 1-electron reduced Pt�bpy��2embedded in the Nf membrane changes irreversibly into

some species. This can be ascribed to much lower local

pH in the Nf membrane than in the electrolyte solution

due to strongly acidic property of the sulfonic side group

[31±34]. When considering the typical unit structure of

Nf as analyzed by the present authors [34], the concen-

tration of the dissociated sulfonic group is calculated to

be 0.9 M in Nf membrane which corresponds to local

pH < 0. It is most probable that such a highly acidic

environment induces the structural change of the

Pt�bpy��2 , and further reduction of the Pt�bpy��2 would

take place to form catalytically active species leading to

the e�cient H� reduction. Any other distinct absorption

band was not observed after the electrolysis except

Pt�bpy�2�2 and very broad band appeared in the whole

visible region (Fig. 3(A)), showing that zero-valent Pt

species are formed.

Fig. 3(B) shows the UV/Vis spectrum of the

Nf[Pt(terpy)Cl�] compared with the aqueous

�Pt�terpy�Cl�� solution. The spectrum at pH 7 solution

was in good agreement with the earlier data [35]. How-

ever, in the acidic condition (pH 0), the spectrum is

much di�erent from that at pH 7. This shows that, in the

acidic condition, a ligand substitution (acid hydrolysis)

of the Pt(terpy)Cl� takes place to form most probably

Pt(terpy)(H2O)2� by the loss of the Clÿ ion. The spec-

trum of the Nf[Pt(terpy)Cl�] is almost identical to that

in the aqueous solution at pH 0 rather than that at pH 7.

As mentioned before, the local H� concentration in

Nf membrane (local pH < 0) should be much high

in comparison with that of the electrolyte solution.

Therefore, the ligand (Clÿ ion) substitution of the

Pt(terpy)Cl� would take place after the adsorption of

the complex (Pt(terpy)Cl�) in the membrane (adsorption

of the Pt(terpy)Cl� into Nf membrane was carried out in

a neutral pH solution containing the complex). As

shown in Fig. 3(B), the spectrum of this Pt complex in

Nf after the electrolysis was not recovered even after the

anodic polarization prior to the spectral measurement.

Although it has been reported that the redox reaction of

the Pt(terpy)Cl�=0 couple is reversible [22], the formation

of the Pt(terpy)(H2O)2� must be irreversible, which then

would induce formation of catalytically active species

for e�cient H� reduction.

3.3. X-ray photoelectron spectroscopy study

An XPS study was carried out to investigate the va-

lence state of the Pt complexes after the electrolysis. Fig.

4 shows the XPS spectrum of the Nf[Pt�bpy�2�2 ] after the

potentiostatic electrolysis at the applied potential of

ÿ0:95 V. Shoulder peaks of both 76.2 eV (Pt 4f5=2) and

Fig. 4. XPS spectrum of Nf�Pt�bpy�2�2 � after electrolysis for H�

reduction.

T. Abe et al. / European Polymer Journal 37 (2001) 753±761 757

72.9 eV (Pt 4f7=2) can be assigned to the Pt�bpy��2 , which

is almost identical to the crystal state of the one-electron

reduced complex in the previous work [21]. In addition,

peaks assignable to the zero-valent Pt were observed at

both 74.5 eV (Pt 4f5=2) and 71.2 eV (Pt 4f7=2) similar to

the Pt foil [36]. Molecular catalysis by the Pt�bpy��2could be excluded in the present H� reduction because

the CV shows that the e�cient H� reduction takes place

by some transformed species rather than by the

Pt�bpy��2 . Therefore, it is most probable that the elect-

rocatalytic H� reduction is induced by the zero-valent

Pt. In addition, intense peaks having higher binding

energy than the Pt�bpy��2 appeared after the electrolysis.

Although the details cannot be determined, these peaks

could be ascribed to some species induced from the

Pt�bpy��2 . This species might also be associated with the

present H� reduction.

Similar XPS analysis for the Nf[Pt(terpy)Cl�] was

also carried out. Fig. 5(a) shows a typical XPS spectrum

of the Nf[Pt(terpy)Cl�]. Intense doublet peaks attribut-

able to the [Pt(terpy)(H2O)]2� were found at both 77.9

eV (Pt 4f5=2) and 74.6 eV (Pt 4f7=2). In addition, shoulder

peaks due to the Pt(terpy)Cl� were observed. Binding

energy of the Pt ion of [Pt(terpy)Cl]� shifted to higher

energy regions than that of [Pt(terpy)(H2O)]2�. The

peaks by Pt(terpy)Cl� remains, showing that part of the

Pt(terpy)Cl� is not hydrolyzed even in the acidic mem-

brane. This might be due to the presence of the Pt

complex in an intermediate or hydrophobic region of Nf

where the acid hydrolysis of the Pt complex can be

suppressed. The XPS spectrum obtained after the elec-

trolysis is extremely complicated as shown in Fig. 5(b),

showing at least several kinds of Pt species. The elec-

troreduction of the Pt complexes produces compounds

with lower binding energy than the starting Pt complex.

Although the complicated spectrum makes it di�cult to

assign each peak, the zero-valent Pt was found evidently

at both 74.5 eV (Pt 4f5=2) and 71.2 eV (Pt 4f7=2). Con-

sidering the electrochemical behavior of the terpyridyl Pt

complex similar to the Pt�bpy�2�2 , the zero-valent Pt

should be the catalytically active species for the H� re-

duction also for the terpyridyl complex.

3.4. Photochemical H� reduction by polypyridyl Pt

complexes

The Pt complexes were used in a photochemical H�

reduction system composed of Ru�bpy�2�3 sensitizer,

methyl viologen (MV2�) acceptor, sacri®cial donor

(EDTA) and the Pt complex catalyst [37]. The time-

course of the photochemical H2 formation catalyzed by

Fig. 5. XPS spectrum of Nf[Pt(terpy)Cl�] (a) before and (b) after electrolysis.

758 T. Abe et al. / European Polymer Journal 37 (2001) 753±761

the polypyridyl Pt complex is shown in Fig. 6. The inset

shows the time-course of the H2 formation in the initial

term. Photochemical H2 formation took place by both

the Pt�bpy�2�2 (d) and Pt(terpy)Cl�(m). It was found

that there is an induction period for the H2 evolution.

These results suggest that the photochemical H2 for-

mations begin to occur by some Pt species transformed

from the starting Pt complexes. It was also noted that

much higher amount of H2 was observed by Pt�bpy�2�2than by Pt(terpy)Cl� similar to the electrocatalytic H�

reduction as mentioned above. In a separate experiment

using emission spectroscopy, it was con®rmed that

quenching of the excited Ru�bpy�2��3 does not occur by

the polypyridyl Pt complex. Therefore, direct reduction

of the Pt complex by the excited Ru�bpy�2��3 can be

excluded so that it must be reduced to the catalytically

active species by MV�� which is formed by the photo-

chemical reaction between Ru�bpy�2�3 and MV2�. In the

present system, viologen radical (MV��) was observed

steadily during the photochemical H2 formation. For-

mal potential of the MV2�/MV��(EM) is ÿ0:64 V (vs.

Ag/AgCl) [38] which is close to that of the H�/H2 (EH,

ÿ0:54 V (vs. Ag/AgCl) in pH 5.9). If the Pt complexes

work as a H� reduction catalyst in their original mo-

lecular states, the redox potential of the Pt complexes

should be between EM and EH. However, the redox

potentials of Pt�bpy�2�=�2 (ÿ0:8 V) and Pt(terpy)Cl�=0

(ÿ0:85 V) show that this is not the case. The photo-

chemical H2 formation could be brought about only by

the transformed species. It appears that the potential at

which the Pt complexes are changed to active species is

slightly lower than that of MV2�/MV��. The induction

period for H2 formation is associated with the formation

of active Pt species from the Pt complexes by the MV��

reductant.

In conclusion, the catalysis for the H� reduction

by the polypyridyl Pt complexes was studied by using

electrochemical and photochemical processes. In any

case, the Pt complexes at ®rst change to catalytically

active species, and then work as the H� reduction cat-

alyst. Although the details of the unknown reduced

species of the polypyridyl Pt complexes have not been

made clear, it was proposed that the reduced polypyridyl

Pt complex, most probably the zero-valent Pt, is the

active species for the H� reduction.

Acknowledgements

The authors acknowledge the grant-in-aid (no. 475/

10650862) from Ministry of Education, Science, Sports

and Culture. T.A. has been granted JSPS research fel-

lowships for young scientists.

References

[1] Kalyanasundaram K, Gr�atzel M, editors. Photosensitiza-

tion and photocatalysis using inorganic and organometallic

compounds. Netherlands: Kluwer Academic Publishers,

1993.

[2] Kalyanasundaram K, editor. Photochemistry of polypyri-

dine and porphyrin complexes. San Diego: Academic

Press, 1992.

Fig. 6. Time-course of photochemical H2 formation ((d), Pt�bpy�2�2 ; (m), Pt(terpy)Cl�) in the presence of a photosensitizer

(Ru�bpy�2�3 ), an electron donor (EDTA), an electron relay and a Pt complex catalyst in a pH 5.9 aqueous solution. Inset shows the

time-course of the H2 formation in the initial term. �Ru�bpy�2�3 �, 100 lM; [EDTA], 50 mM; [MV2�], 10 mM; [Pt complex], 100 lM.

T. Abe et al. / European Polymer Journal 37 (2001) 753±761 759

[3] Norris Jr JR, Meisel D, editors. Photochemical energy

conversion. USA: Elsevier, 1989.

[4] Abe T, Yoshida T, Tokita S, Taguchi F, Imaya H, Kaneko

M. Factors a�ecting selective electrocatalytic CO2 reduc-

tion with cobalt phthalocyanine incorporated in a polyvi-

nylpyridine membrane coated on a graphite electrode.

J Electroanal Chem 1996;412:125±32.

[5] Kinoshita K, Yagi M, Kaneko M. Enhancing e�ect of

amino acid residue model compounds on catalytic activity

of an arti®cial oxygen evolving center. Macromolecules

1998;31:6042±5.

[6] Yagi M, Nagoshi K, Kaneko M. Cooperative catalysis and

critical decomposition distances between molecular water

oxidation catalysts incorporated in a polymer membrane.

J Phys Chem B 1997;101:5143±6.

[7] Kaneko M, Yamada A. Solar energy conversion by

functional polymers. Adv Polym Sci 1984;55:1±47.

[8] Kiwi J, Gr�atzel M. Protection, size factors, and reaction

dynamics of colloidal redox catalysts mediating light

induced hydrogen evolution from water. J Am Chem Soc

1979;101:7214±7.

[9] Kalyanasundaram K, Kiwi J, Gr�atzel M. Hydrogen

evolution from water by visible light, a homogeneous

three component test system for redox catalysis. Helv Chim

Acta 1978;61:2720±30.

[10] Toshima N, Takahashi T, Hirai H. Colloidal platinum

catalyst protected by nonionic monomeric and polymer-

ized micelle for photochemical hydrogen generation from

water. Chem Lett 1987:1031±1034.

[11] Abe T, Taguchi F, Tokita S, Kaneko M. Prussian white

as a highly active molecular catalyst for proton reduction.

J Mol Catal A: Chem 1997;126:L89±92.

[12] Abe T, Taguchi F, Imaya H, Zhao F, Zhang J, Kaneko M.

Highly active electrocatalysis by cobalt tetraphenylporphy-

rin incorporated in a Na®on membrane for proton

reduction. Polym Adv Technol 1998;9:559±62.

[13] Taguchi F, Abe T, Kaneko M. New molecule-based

catalyst to produce H2 by metallo-porphine dispersed

into polymer membrane. J Mol Catal A: Chem 1999;

140:41±6.

[14] Kraeutler B, Bard AJ. Heterogeneous photolytic prepara-

tion of supported catalysts. Photoreduction of platinum on

TiO2 powder and other substrates. J Am Chem Soc

1978;100:4317±8.

[15] Hosono H, Kaneko M. E�ects of con®guration of violo-

gen-linked porphyrin on photocurrent generation and on

photoinduced hydrogen evolution. J Chem Soc Faraday

Trans 1997;93(7):1313±9.

[16] Lehn JM, Sauvage JP. Chemical storage of light energy.

Catalytic generation of hydrogen by visible light or

sunlight irradiation of neutral aqueous solutions. Nouv J

Chim 1977;1:449±51.

[17] Palmans R, Frank AJ. A molecular water-reduction

catalyst: Surface derivatization of TiO2 colloids and

suspensions with a platinum complex. J Phys Chem

1991;95:9438±43.

[18] Maruyama T, Yamamoto T. E�ective photocatalytic

system based on chelating p-conjugated poly(2,20-bipyri-

dine-5,50-diyl) and platinum of H2 from aqueous media

and spectroscopic analysis of the catalyst. J Phys Chem B

1997;101:3806±10.

[19] Suzuki M, Kobayashi S, Kimura M, Hanabusa K, Shirai

H. Hydrogen generation using water-insoluble polymer-

bound ruthenium(II) complexes. J Chem Soc Chem Com-

mun 1997:227±8.

[20] Brown AR, Guo Z, Mosselmans FWJ, Parmons S,

Schr�oder M, Yellowlees JJ. Structural voltammetric studies

on the reduction of the bis(2,20-bipyridyl)platinum(II) ca-

tion in aprotic media. J Am Chem Soc 1998;120:8805±11.

[21] Palmans R, MacQueen DB, Pierpont CG, Frank AJ.

Synthesis and characterization of bis(2,20-bipyridyl)plati-

num(I): a model microtubular linear-chain complex. J Am

Chem Soc 1996;118:12647±53.

[22] Hill MG, Bailey JA, Miskowski VM, Gray HB. Spectro-

electrochemistry and dimerization equilibria of chloro

(terpyridine)platinum (II). Nature of the reduced complex-

es. Inorg Chem 1996;35:4585±90.

[23] Bailey JA, Hill MG, Marsh RE, Miskowski VM, Schaefer

WP, Gray HB. Electronic spectroscopy of chloro (terpyri-

dine)platinum (II). Inorg Chem 1995;34:4591±9.

[24] Rubinstein I, Bard AJ. Polymer ®lms on electrode. Na®on-

coated electrodes. 4. and electrogenerated chemilumines-

cence of surface-attached Ru(bpy)32�. J Am Chem Soc

1980;102:6641±2.

[25] Henning TP, White HS, Bard AJ. Polymer ®lms on

electrode. 6. Biconductive polymers produced by incorpo-

ration of tetrathisfulvalenium in polyelectrolyte (Na®on)

matrix. J Am Chem Soc 1981;103:3937±8.

[26] White HS, Leddy J, Bard AJ. Polymer ®lms on electrode.

8. Investigation of charge-transport mechanism in Na®on

polymer modi®ed electrode. J Am Chem Soc 1982;

104:4811±7.

[27] Uchida M, Aoyama Y, Eda N, Ohta A. New preparation

method for polymer-electrolyte fuel cells. J Electrochem

Soc 1995;142:463±8.

[28] Kaneko M, Yamada A. Solid phase photoreduction of

methylviologen on cellulose sensitized by tris(2,20-bipyri-

dyl) ruthenium complex. J Photochem Photobio 1981;

33:793±8.

[29] Morgan GT, Burstall FH. Researches on residual a�nity

and co-ordination. Part XXXIV. 2,:20-dipyridyl platinum

salts. J Chem Soc 1934:965±971.

[30] Howegrant M, Lippard S. Inorg Synth 1980;20:101.

[31] Yoshida T, Tsutsumida K, Teratani S, Yasufuku K,

Kaneko M. Electrocatalytic reduction of CO2 in water by

[Re(bpy)(CO)3Br] and [Re(terpy)(CO)3Br] complexes in-

corporated into coated Na®on membrane. J Chem Soc

Chem Commun 1993:631±3.

[32] Lowry SR, Mauritz KA. An investigation of ionic hydra-

tion e�ects in per¯uorosulfonate ionomers by Fourier

transform infrared spectroscopy. J Am Chem Soc 1980;

102:4665±7.

[33] Komoroski RA, Mauritz KA. A sodium23 nuclear mag-

netic resonance study of ionic mobility and contact ion

pairing in a per¯uorosulfonate ionomer. J Am Chem Soc

1978;100:7487±90.

[34] Yagi M, Nagai K, Kira A, Kaneko M. Charge transfer

distance between tris(2,20-bipyridyl) ruthenium(II) redox

centers incorporated in Na®on membrane. J Electroanal

Chem 1995;394:169±75.

[35] Ratilla EMA, Brothers II HM, Kostic NM. A transition-

metal chromophore as a new selective spectroscopic tag

760 T. Abe et al. / European Polymer Journal 37 (2001) 753±761

for proteins selective covalent labeling of histidine residues

in cytochromes c with chloro(2,20:6,200-terpyridine)plati-

num(II)chloride. J Am Chem Soc 1987;109:4592±9.

[36] Cahen D, Laster JE. Mixed and partial oxidation states.

Photoelectron spectroscopic evidence. Chem Phys Lett

1973;18:108±11.

[37] Amouyal E, Grand D, Moradpour A, Keller P. Photo-

chemical model system for hydrogen production from

water: the e�ciency of colloidal platinum catalyst associ-

ated with viologen electron relay. Nouv J Chim 1982;

6:241±4.

[38] Rafalo� R, Haruvy Y, Binenboym J, Baruch G, Rajben-

bach LA. Radiolytic method of preparation of colloidal

redox catalysts and their application in light-induced

hydrogen generation from water. J Mol Catal 1983;

22:219±33.

T. Abe et al. / European Polymer Journal 37 (2001) 753±761 761