Photoreduction of dioxygen to hydrogen peroxide at porphyrins and phthalocyanines adsorbed Nafion membrane

Article abstract of DOI:10.1016/S1381-1169(98)00308-2

The metal porphyrins (MP) and metal phthalocyanines (MPC) adsorbed Nafion membranes are prepared (Nf/MP and Nf/MPC) and used as photocatalysts for the photoreduction of dioxygen (O₂). Photoinduced electron transfer reactions carried out using the MP and MPC adsorbed Nafion membrane. The back electron transfer reaction between h⁺ with O₂·^- is prevented by the reaction of the hole with a electron donor triethanolamine (TEA). Hydrogen peroxide (H₂O₂) is produced by the reaction of the semiconducting nature metal porphyrins and phthalocyanines with dissolved dioxygen. The photosensitized reaction mechanism of the photocatalytic reduction of O₂ to H₂O₂ formation is explained. The turnover number of the photocatalysts are high in the presence of sacrificial electron donor. The surface morphology of the modified photocatalysts are analysed by scanning electron microscope. The detailed mechanism of the dioxygen reduction in the modified systems are presented.

Full text of DOI:10.1016/S1381-1169(98)00308-2

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Journal of Molecular Catalysis A: Chemical 142 1999 153–162
Photoreduction of dioxygen to hydrogen peroxide at porphyrins
and phthalocyanines adsorbed Nafion membrane
)
J. Premkumar, R. Ramaraj
School of Chemistry, Madurai Kamaraj UniÕersity, Madurai 625 021, India
Received 17 September 1997; accepted 20 August 1998
Abstract
Ž
.
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.
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The metal porphyrins MP and metal phthalocyanines MPC adsorbed Nafion membranes are prepared NfrMP and
.
Ž
.
NfrMPC and used as photocatalysts for the photoreduction of dioxygen O2 . Photoinduced electron transfer reactions
carried out using the MP and MPC adsorbed Nafion membrane. The back electron transfer reaction between hq with O2.
y
Ž
.
Ž
.
is prevented by the reaction of the hole with an electron donor triethanolamine TEA . Hydrogen peroxide H2O2 is
produced by the reaction of the semiconducting nature metal porphyrins and phthalocyanines with dissolved dioxygen. The
photosensitized reaction mechanism of the photocatalytic reduction of O2 to H2O2 formation is explained. The turnover
number of the photocatalysts are high in the presence of sacrificial electron donor. The surface morphology of the modified
photocatalysts are analysed by scanning electron microscope. The detailed mechanism of the dioxygen reduction in the
modified systems are presented. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Photocatalysis; Porphyrins; Phthalocyanines; Oxygen reduction; Nafion
1. Introduction
Interest in the photocatalytic reduction of dioxygen to useful products such as hydrogen peroxide
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has been increasing in recent years 1–5 . In this context solar energy conversion by artificial
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photosynthesis is becoming an increasingly important research subject 5–7 . The difficulty in
creating non-biological model system for the photoinduced multistep one-electron process lies in the
requirement to couple the multielectron reduction of O2 with the one-electron chemistry of the redox
molecules. The light-induced reduction of O2 in an aqueous solution using semiconductor and metal
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complex photocatalysts has been attempted 1–5,8–10 . The electrocatalytic reduction of O2 using
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.
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.
metal complexes, metal porphyrins MP and metal phthalocyanines MPC have been studied.
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11–13 Two- and four-electron reduction products such as hydrogen peroxide and water have been
reported.
Metalloporphyrins, which include important natural complexes such as chlorophyll and heam and
related complexes. A large number of biological system are known in which metalloporphyrins act as
)
Corresponding author. Fax: q91-452-859139
1381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
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PII: S1381-1169 98 00308-2

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J. Premkumar, R. RamarajrJournal of Molecular Catalysis A: Chemical 142 1999 153–162
initiators of a particular biological process. Most investigations have involved chlorophyll, which
being part of the protein–lipid system initiates photosynthesis in green plants and in photosynthetic
bacteria, that is it initiates the stage of accumulation and transfer of the energy of a light quantum, the
stage of electron transfer reaction through the complex photosynthetic system all the way from the
reducing to the oxidising agents. In artificial photosynthesis the development of more efficient
photocatalyst in solidrmembrane is a promising approach for the photoreduction of O2. The catalytic
molecules immobilized in a Nafion membrane are particularly advantageous due to their separation
from the reaction medium by microheterogeneous environment and a small amount of catalyst
material necessary for reaction. In an attempt to develop a photocatalytic system in solid phase, we
have utilized the MP and MPC adsorbed Nafion membranes for the photocatalytic reduction of O2.
The utility of immobilized catalysts is drastically evident since they are not leached from the
membrane where the immobilized metal phthalocyanines are quite active and stable.
Ž
.
Transition metal complexes porphyrins and phthalocyanines are a class of electrocatalyst for the
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reduction of O2 that have received extensive study 11,12 . Mechanistic interpretations of the catalytic
activity of a variety of complexes have been offered but uncertainties remain, especially about the
relationship between the formal potentials of the adsorbed catalyst in the absence of O2 and the
potential where the catalysed reduction of dioxygen proceed. The aim of this research was to examine
this point and to assess the effects of major substitution on the phthalocyanine ring on the stability and
activity of catalyst-coated electrodes. In this paper, we present the results of the photocatalytic
reduction of O2 using MPs and MPCs adsorbed Nafion membrane.
2. Experimental
Ž
Ž
.
Ž
..
The porphyrins protoporphyrin PP , hematoporphyrin HP and the phthalocyanines
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Ž
Ž
.
. Ž
Ž .
Ž
.
Ž .
Ž
..
iron II phthalocyanine FePC , cobalt II phthalocyanine CoPC and zinc II phthalocyanine ZnPC
and Nafion membrane Nf type 125, equiv.wt. 1100, thickness 0.13 mm were purchased from
.
Aldrich Chemical. Tetraphenylporphyrine was purchased from Tokyo Kasei Kogyo, Japan.
Ž .
Ž
.
Ž .
Ž
.
Cobalt II tetraphenylporphyrin CoTPP and cobalt II porphyrin CoP were prepared by reported
w
x
Ž
.
Ž
.
Ž
.
procedures 14 . Triethanolamine TEA , dimethyl formamide DMF and perchloric acid HClO4
were purchased from Merck. The Nf membrane was pretreated prior to use by boiling in concentrated
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x
nitric acid for about 10–20 min and it became clear and transparent 15 . The MP and MPC adsorbed
2
Ž
.
Nf membrane was prepared by dipping the Nf membrane 1 cm in a known concentration of MP or
MPC in DMF at various time intervals. The resulting MP or MPC adsorbed Nf membrane
Ž
.
represented NfrPP, NfrHP, NfrTPP, NfrCoTPP, NfrCoP, NfrFePC, NfrCoPC and NfrZnPC
washed with distilled water and dried at room temperature under dark. The amounts of adsorbed MP
and MPC in the Nf membrane were determined by measuring the absorbance of the MP and MPC
solutions before and after dipping. The reported molar extinction coefficient values of the MPs and
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x
MPCs 16 were used to calculate the concentrations of the complexes. The absorption spectra of MP
and MPC in DMF and adsorbed in Nf membrane were recorded using JASCO 7800 spectrophotom-
eter. The MP or MPC adsorbed Nf membrane was dipped in a O2 saturated solution containing TEA
Ž
.
and HClO4 using a glass cell with optical window volume of the solution was 15 ml and the
membrane was illuminated with visible light using a 500 W tungsten–halogen lamp with pyrex glass
and water filters to cut-off ultra-violet and infra-red radiations. The distance from the centre of the
halogen lamp to the cell was 45 cm. A procedure typical of kinetic experiment for the formation of
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H2O2 was employed using titrimetry and spectrophotometry methods 17,18 . To a 10 ml of the

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J. Premkumar, R. RamarajrJournal of Molecular Catalysis A: Chemical 142 1999 153–162
155
sample solution, 5 ml of 0.2 M sulfuric acid and 5 ml of 5% potassium iodide were added and the
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x
liberated iodine was titrated against 1 mM of sodium thiosulfate solution 17 and the H2O2
concentration was determined. A blank titration was also carried out. The amount of H2O2 was also
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x
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.
determined by spectrophotometric method 18 . The sample solution 1.5 ml was taken in a 5 ml
standard measuring flask to which 1.5 ml of 0.2 M sulfuric acid was added. Then 5% potassium
iodide solution was added into the 5 ml standard measuring flask upto the mark and the absorbance
y
value was quantitatively monitored at 351 nm26 absorption maximums351 nm for I3 and molar
Ž
extinction coefficient at 351 nms26400 My cmy1 . The reference cell was filled with the blank
1
.
oxygen saturated solution treated with acidified potassium iodide solution in a similar way. From the
absorbance value, the amount of H2O2 produced was calculated. Pure grade nitrogen and oxygen
gases were used for deaeration and O2 saturation purpose. Continuous bubbling of O2 gas was
maintained in the experimental solution throughout the experiment. The surface morphology of the
Ž
MP and MPC adsorbed Nf membranes was examined by scanning electron microscope Hitachi
.
S-450 . The porphyrins and phthalocyanines adsorbed Nf membranes were stable for weeks and
reused. Experiments were repeated several times and reproducible results were obtained.
3. Results and discussion
The MPs and MPCs were irreversibly adsorbed in the Nf membrane. The MPs and MPCs were
adsorbed by dipping the Nf membrane in different concentrations of MP or MPC in DMF. The
Ž .
Ž .
Ž .
Fig. 1. Absorption spectra of MP and MPC in DMF solution and incorporated into Nf membranes. a HP, b FePC, c CoPC in DMF and
Ž . Ž . Ž .
d NfrHP, e NfrFePC, f NfrCoPC vs. Nf membrane.

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J. Premkumar, R. RamarajrJournal of Molecular Catalysis A: Chemical 142 1999 153–162
desorption of MP and MPC in the NfrMP and NfrMPC membranes was not observed in any
rigorous conditions, in acids and in neutral solutions. This has been checked by recording the
absorption spectra of the solutions before and after dipping the NfrMP and NfrMPC membranes in
solutions. The absorption spectra of the MP and MPC adsorbed Nf membranes are recorded and
shown in Fig. 1. The absorption spectra observed for the MP and MPC adsorbed Nf membrane are
Ž
.
almost similar to the spectra observed in DMF solution Fig. 1 . The Nf membrane consists of
hydrophobic fluorocarbon region, hydrophilic–SOy3 ionic cluster region and interfacial region formed
w
x
between these two 19 . The MP and MPC molecules mostly will occupy the interfacial and
hydrophobic regions of the Nf membrane due to the hydrophobic interaction.
The scanning electron micrographs of the MP and MPC adsorbed Nf membranes are shown in Fig.
2. The SEMs show dark spots on the surface of the MP and MPC adsorbed Nf membrane when
Ž .
Ž .
Ž .
Fig. 2. Scanning electron micrographs of a Nf, b NfrCoTPP and c NfrCoPC membranes.

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J. Premkumar, R. RamarajrJournal of Molecular Catalysis A: Chemical 142 1999 153–162
157
compared to plain Nf membrane. The MP and MPC adsorbed in the Nf membranes agglomerate into
larger deposits and are randomly distributed in the Nf membrane. The MPs and MPCs interact with
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.
the structural units of the Nf membrane hydrophobic fluorocarbon and interfacial regions as well as
interact with each other in forming a three dimensional disordered network of chemically reactive
molecule.
Photocatalytic O2 reduction was carried out using the MP and MPC adsorbed Nf membranes. The
NfrMP or NfrMPC membrane was dipped into a photolysis cell containing O2 saturated 0.1 M
HClO4 and 0.1 M TEA and then irradiated. After 10–60 min irradiation, the oxygen reduction
product hydrogen peroxide was analysed. When the experiments were carried out in dark or under
illumination using MP or MPC solution filter to cut off the light in the MP or MPC absorption band
region, hydrogen peroxide was not detected with any one of the MP or MPC membrane.
The yield of hydrogen peroxide formed at different light irradiation time using NfrMP and
Ž
.
NfrMPC membranes were determined and the corresponding turnover numbers TONs of MP and
Ž
MPC are shown in Fig. 3. The TONs of MP and MPC were obtained from the relation 2 mole of
hydrogen peroxide r mole of MP or MPC adsorbed . In the NfrMP and NfrMPC membrane
. Ž
.
Ž
.
Ž .
Fig. 3. Yields of H2O2 and turnover number TON in an oxygen saturated solution containing 0.1 M HClO4 and 0.1 M TEA of a NfrPP,
y7
y8
Ž .
Ž .
Ž .
Ž .
a
Ž .
mole of HP, b 2.386=10
b NfrTPP, c NfrCoPC and NfrZnPC membranes at different light irradiation time.
1.738=10
y8
y8
Ž .
mole of CoPC and d 1.054=10
mole of TPP, c 2.046=10
mole of ZnPC in Nafion membrane.

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J. Premkumar, R. RamarajrJournal of Molecular Catalysis A: Chemical 142 1999 153–162
Ž
.
systems Fig. 3 the yield of hydrogen peroxide reached maximum at longer irradiation time. This
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x
may due to the decomposition of hydrogen peroxide by the holes 20 of the semiconducting property
of MP or MPC molecules. When the same NfrMP and NfrMPC membranes were washed and
reused in the O2 reduction experiment similar results Fig. 3 were obtained. This clearly shows that
the membranes are highly stable and active for O2 reduction. The photocatalytic reactions occurring
Ž
.
Ž . Ž .
at MP and MPC adsorbed Nf membranes can be considered as given in Eqs. 1 – 3 .
hn
Catalyst MP or MPC ™ Catalyst) ey qhq . . .
1
Ž .
Ž
.
Ž
.
cb
vb
oxidation: 2 TEAq2 hqvb ™ 2 TEA
. . .
2
Ž .
Ž
.
ox
reduction: O2 q2eycb q2Hq ™ H2O2 . . .
3
Ž .
where eycb and hqvb denote an electron in the conduction band and a positive hole in the valance band
of the MP or MPC film, respectively. The TEA acts as a sacrificial electron donor. Porphyrin and
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x
phthalocyanine films were relatively well behaved as p-type semiconductors 21,22 . Band edges and
interfacial states have been mapped and the nature of the interfacial charge transfer has been discussed
in terms of Gerischer’s model and modified by inclusion of surface states in the band gap region.
Recently, the properties of ZnPC and H2 PC have been illuminated in detail by studying the rectifying
w
x
properties and photovoltaic effects using phthalocyaninermetal junctions 23 . These materials are
p-type semiconductors with a high density of intermediate energy levels. The electrochemical
properties of ZnPC and H2 PC electrodes in solution containing different redox couples are therefore
predictable, provided the electrode processes are not complicated by additional reactions such as
chemisorption.
The p-type character of many MPC films have been taken for granted by several investigators, but
the origins of such behavior have not been fully explored. Some reports of the O2 sensitivity of MPC
conductivities and photoconductivities have appeared, as well as the sensitivity of these parameters to
the presence of other chemisorbed dopants. The presence of radical oxygen and MPC films have been
postulated as the result of the O2 doping process. For the trivalent-metal MPC’s this is confirmed in
w
x
the studies reported 24 . MPC thin films produced for photoelectrochemical studies as well as for the
6
formation of solid-state photovoltaics are generally deposited in vacuum that are no better than 10y
w
x
Torr 24 . It is clear from these studies that these conditions can give rise to extensive O2
Ž
.
incorporation into the MPC film if substrate temperatures are high enough , which may explain in
part the p-type character attributed to these materials.
Photoelectrochemical cells in the presence of various redox couples in aqueous solution have been
investigated with thin films of metal-free and metal containing phthalocyanines obtained by vapour
w
x
deposition and spin or drop coating techniques 22 . The interface between the phthalocyanine as
molecular semiconductor and the electrolyte forms a junction which is active in photoconversion.
Casted film consisting of metal-free phthalocyanine dispersed in a polymer binder exhibits the
maximum short-circuit photocurrent with redox couples, whose redox levels are located within the
bandgap of the semiconductor. The acceptor O2 is the most important dopant incorporated into the
w
x
phthalocyanine films 23 . The presence of the so-formed defect sites is a fundamental prerequisite for
a good performance of the phthalocyanines photovoltaicrphotoelectrochemical cells and for their
Ž
electrical conductivity as well as photoconductivity. In the charge carrier generation process in the
.
bulk of the film , O2 plays an active role as a dopant of the p-type porphyrin or phthalocyanine film.
For MPs and MPCs the width of the band gap in a slip-stack orientation of adjacent molecules is
U
U
w
x
around 2.0 eV 22 . Illumination results in a S0 ™S1 transition, i.e., formation of MP and MPC

(
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J. Premkumar, R. RamarajrJournal of Molecular Catalysis A: Chemical 142 1999 153–162
159
q
q
located yabout 0.2 eV below the conduction band edge. The ionized intermediate MP . or MPC .
and O2. are formed by electron transfer. In a semiconductor catalyst, the photoexcited electrons in
Ž
.
the more negative conduction band have the greater ability to reduce O2 in solution Scheme 1 .
Ž
.
The TON of ZnPC is higher than the other metal phthalocyanines Fig. 3 . This may due to the
greater interaction of ZnPC with O2 than the interaction of other MPCs with O2. In dioxygen
Ž
.
reduction experiment, the ZnPC film coated on ITO Indium Tin Oxide electrode showed higher
w
x
photocurrent for dioxygen reduction than other MPCs in a photoelectrochemical cell 22 . This
observation is understood in terms of the axial coordinating capability of ZnPC. The axial coordinat-
ing capability of other metals is smaller when compared to Zn. Therefore, the phthalocyanine with Zn
as central metal turned out to be the most suitable for the photoreduction of O2. The formation of
Ž
.
H2O2 depends on the intensity of the incident light Io . The yields of H2O2 obtained at NfrCoTPP
and NfrZnPC systems with O2 saturated solution of 0.1 M HClO4 and 0.1 M TEA using different
light intensities at a light irradiation time of 15 min are shown in Fig. 4 and linear plot was obtained.
w
x
In photoelectrochemical experiments 22 , the metal-free phthalocyanine shows higher dark cur-
rents which are slightly enhanced under illumination. The FePC and CoPC are well-known as
electrocatalysts for the dioxygen reduction in fuel cell reactions. Their high dark current cannot be
Ž
.
improved under illumination. For zinc complex MP, MPC yielded a high photocurrent and a high
ratio of the current under illumination to that in the dark. This indicates that the influence of the
central metal seems to be similar for the studied porphyrin compounds and that the central metal plays
an important role in the charge carrier generation andror in the surface kinetics. Compared to the
other ring systems the phthalocyanines exhibited the highest photocurrents.
Nafion membrane provides good regions to the p-conducting porphyrin. The active part for the
cathodic photoreduction of O2 is the porphyrin film in contact with the electrolyte. The strong
influence of the central metal on the efficiency of the O2 photoreduction is explained by the
importance of the central metal for the photoelectrical properties in the capability of a porphyrin to
O2. The reduction of O2 to H2O2 involves two electrons and the redox potential has to be regarded as
an average value of two one-step potentials. The O2 reduction potential is located in the negative
region and at higher energy than the MP and MPC valance band edge. No charge transfer seems to be
necessary from the MP and MPC films to the solvated species in the bulk solution, but the O2
reduction product at the Nf membrane would diffuse into the electrolyte solution and will be replaced
Scheme 1. Schematic illustration of photocatalytic reduction of dioxygen at MP or MPC adsorbed Nafion membrane. TEAstriethanolamine,
cbsconduction band, vbs valance band and Ef sfermi level.

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160
J. Premkumar, R. RamarajrJournal of Molecular Catalysis A: Chemical 142 1999 153–162
Ž
.
Ž .
Fig. 4. Yields of H2O2 at different light intensity Io in an oxygen saturated solution containing 0.1 M HClO4 and 0.1 M TEA. a
y8
NfrCoTPP and b NfrZnPC. Light irradiation times15 min. Adsorbed amount of CoTPPs5.631=10 mole and ZnPCs1.824=10y9
Ž .
mole.
by O2 molecules from the solution. The surface states are populated by electrons only under
illumination so that the described mechanism works only under illumination of thqe membraneq.
The results are best interpreted by a mechanism in which the reaction of MP . or MPC . with
the electron donor TEA occurs efficiently. Under the conditions employed, the concentration of TEA
q
q
is sufficiently large and scavenge the holes of MP . and MPC . efficiently. The main features of
Ž . Ž
.
the reaction mechanism of the photocatalysed reduction of O2 are represented by Eqs. 4 – 13 .
hn
M
O2 Hq TEA s ™ M)
O2 Hq TEA
n
s
4
Ä
4
Ä
4
Ž .
a
m
n
a
m
1
1
M)
M)
M
O2 Hq TEA s ™ M O2 Hq TEA s qhnX
5
Ž .
Ä
Ä
4
Ä
4
a
a
m
n
a
a
m
n
1
1
q
y
O2 Hq TEA s ™ M
M .
O2
O2.
Hq TEA
6
Ž .
4
Ä
n 4s
m
n
y1
m
1
1y1
q
y
M .
M .
O2
O2
O2.
Hq TEA s ™ M O2 Hq TEA
7
Ž .
aÄ
n4s
Ä
Ä
n 4
a
y1
m
m
1y1
1
q
y
M
O2
O2.
Hq TEA
™
n 4s
a
y1
m
1y1
y
M
O2.
Hq TEA
TEA
8
Ž .
Ä
ox14s
a
m
n
y1
1y1

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)
J. Premkumar, R. RamarajrJournal of Molecular Catalysis A: Chemical 142 1999 153–162
161
y
M
M
O2
O2.
Hq TEA
TEA
ny1
™
Ä
M
ox14s
TEA
a
m
1y1
O2
HO2 Hq
TEA
9
Ž .
Ä
ox14s
a
my1
ny1
1y1
hn
O2
HO2 Hq
TEA TEA
ny1
™
Ä
ox14s
a
my1
1y1
M)
O2
HO2 Hq
TEA TEA
ny1
10
Ä
ox14s
™
ox14s
Ž
Ž
.
a
my1
1y1
M)
O2
HO2 Hq
TEA TEA
ny1
Ä
a
my1
1y1
q
y
M
M .
O2
HO2.
Hq
TEA TEA
ny1
11
.
Ä
ox14s
ay1
my1
ny2
1y1
q
y
M
M
M .
O2
HO2.
Hq
TEA
TEA
™
Ä
ox14s
a
y1
my1
ny1
1y1
y
M
O2
HO2.
Hq
TEA
TEA
12
Ž
Ž
.
Ä
ox 24s
a
m
y1
1y1
y
O2
HO2.
Hq
TEA TEA
ny2
™
Ä
M
ox 24s
a
my1
1y1
O2
Hq
TEA
TEA
s q
ox 24
Ä
H2O24s
13
Ä
.
a
my2
ny2
1y1
In these equations, the chemical species written in brackets with subscript ‘a’ represents those
Ž
.
adsorbed MP or MPC in Nf membrane and ‘s’ represents the species in solution, O2, HClO4 and
TEA. The subscript ‘l’, ‘m’ and ‘n’ denote the ratios oqf the average number of O2, HClO4 and TEA
.
U
U
U
molecule, respectqively in the solution. M, M and M
represents MP or MPC, MP or MPC and
MP . q or MPC . , respectively. TEA is the oxidised species of triethanolamine. Eqs. 4 and 10
.
w
x
Ž .
Ž
ox
Ž .
.
represent the light induced charge separation in the MP or MPC film. Eq. 5 represents the possible
y
q
Ž .
Ž
process leading to the recombination of ecb and hvb. Eqs. 6 and 11 represent the two-step
Ž .
light-induced electron transfer reactions between MP or MPC and O2 and HO2 species. Eq. 7
q
Ž .
Ž
.
represents the back electron transfer reaction. Eqs. 8 and 12 repyresent the reduction of h in the
.
Ž .
Ž
.
MP or MPC film by TEA. Eq. 9 represent the protonation of O2 species. Eq. 13 represent the
protonation of HO2. species and release of H2O2 molecules from the reactive site in the membrane.
y
The light induced reaction of O2 can also be represented as one-step two-electron reduction process.
The Nf membrane plays an important role in the photocatalytic O2 reduction system. The
hydrophobic environment around the MP and MPC imposed by the Nf membrane would allow the
molecule to function selectively as catalyst for O2 reduction and not as catalyst for Hq reduction
q
Ž
which usually takes place predominantly in water. The diffusion of the reacting species O2, H and
.
TEA from the bulk solution to the membrane is facilitated by the well solvate ionic cluster region of
the Nafion membrane. In the present catalytic system, the MPs and MPCs are highly concentrated in
the Nf membrane and may form specifically active sites where these complexes are cooperatively
involved in the reduction of O2 molecules. Thus, the photocatalytic reactions occurring at MP and
MPC adsorbed Nf membranes appear to take place through the action of the photogenerated carriers
i.e., reduction by electrons in the conduction band and oxidation by holes in the valance band. The
present work demonstrates the importance of immobilization of the photocatalytic molecules in a solid
Ž
.
matrix to realize multielectron transfer process Scheme 1 .
Acknowledgements
We acknowledge the DST and CSIR, New Delhi for providing the financial assistance. JP
acknowledges the CSIR for providing the senior research fellowship.

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J. Premkumar, R. RamarajrJournal of Molecular Catalysis A: Chemical 142 1999 153–162
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