The document describes the synthesis and characterization of novel photocatalytic materials prepared by incorporating TiO2, transition metals, and heteropolyacids into zeolite structures. Specifically, it details the preparation of Zeo-Y/TiO2/Co2+/HPA and its evaluation for photocatalytic reduction of methyl orange under visible light irradiation. Characterization using XRD, UV-vis spectroscopy, and elemental analysis showed the materials retained zeolite crystallinity and contained incorporated cobalt and molybdenum as intended. Testing demonstrated Zeo-Y/TiO2/Co2+/HPA effectively photoreduced methyl orange, with better activity than related materials, indicating the synergistic roles of transition metals and

1. Photocatalytic properties of zeolite-based materials
for the photoreduction of methyl orange
Nidhi Dubey, Sadhana S. Rayalu *, Nitin. K. Labhsetwar, Rashmi R. Naidu,
Ravikrishna V. Chatti, Sukumar Devotta
Environmental Materials Unit, National Environmental Engineering Research Institute (NEERI), Nagpur 440020, India
Received 26 August 2005; received in revised form 1 January 2006; accepted 4 January 2006
Available online 20 March 2006
Abstract
Novel photocatalytic materials have been prepared by incorporation of TiO2, a transition metal and, heteropolyacid (HPA) in the zeolite
structure. These materials have been characterized using XRD, UV–vis diffuse reflectance spectroscopy and elemental analysis. The photocatalytic
activity of the materials in visible light has been evaluated for photoreduction of methyl orange solution in the presence of a sacrificial electron
donor 1:40 ethanol–water mixture. The material Zeo-Y/TiO2/Co2+
/HPA photoreduces methyl orange effectively to the extent of about 4.11 mg/g
TiO2 and shows better photocatalytic activity as compared to Zeo-Y/TiO2/HPA, indicating the role of transition metal ions. The improved
photocatalytic properties in the visible region could be due to the combined effect of transition metal ions and HPA, while these constituents along
with the zeolite framework are also likely to contribute towards delay in charge recombination.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Photocatalysis; Zeolite; Photoreduction; TiO2; Transition metals; Heteropolyacid
1. Introduction
Recent advancements in semiconductor photocatalysis,
especially related to enhanced activity in the visible light
region, have made it one of the most active interdisciplinary
research areas, attracting efforts from photochemists, photo-
physicists and environmental scientists in related fields.
Semiconductor photocatalysts are usually inexpensive and
non-toxic. A semiconductor is commonly characterized by the
energy gap between its electronically populated valence band
and its largely vacant conduction band [1]. This band gap
determines the wavelength required for excitation of an
electron from the valence band to the conduction band. The
efficiency of the electron transfer reactions governs a
semiconductor’s ability to serve as a photocatalyst. The
valence band serves as the site for oxidation, whereas the
conduction band promotes reduction reactions. Hence for an
efficient reduction reaction, the potential of the electron
acceptor should be more positive than the conduction band
potential of the semiconductor. The efficiency of a semi-
conductor-mediated photocatalytic reaction is generally deter-
mined by a number of factors, including properties of
semiconductor, type of substrate, amount of competition from
the solvent and also the experimental set-up [2].
Zeolites offer high surface area, unique nanoscaled porous
structure and ion exchange properties for utilization in the
design of efficient photocatalytic systems. The pore structure of
zeolite-Y consists of 13 A˚ super-cages connected through 7 A˚
windows [3]. Aluminosilicate zeolites have shown considerable
promise for promoting stabilization of photochemically
generated redox species as well [4]. Some very unique
photocatalytic properties, which cannot be realized in normal
catalytic systems, have been observed recently in such modified
spaces [5–10]. The arrangement of cages and channels in these
crystalline zeolites allow for placement of molecules in well-
defined and unique spatial arrangement [3], while they can be
used as constrained systems for the preparation of semicon-
ductors (TiO2) with controlled particle size and shape. Zeolites
are reported to provide specific photo physical properties such
as the control of charge transfer and electron transfer processes
[11–14]. Zeolite-Y with uniform pore size and enormous
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Applied Catalysis A: General 303 (2006) 152–157
* Corresponding author. Tel.: +91 712 2247828; fax: +91 712 2249900.
E-mail addresses: s_rayalu@neeri.res.in, emu_neeri@yahoo.com
(S.S. Rayalu).
0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2006.01.043

2. surface area serves as the support where molecules like
heteropolyacid (HPA) can be stabilized by supporting them on
the zeolite structure [15–19]. Another possible advantage of
zeolites in photocatalysis is their ion exchange property, which
can be utilized for incorporating transition metal ions which
show important photocatalytic properties due to the presence of
vacant d-orbitals. Zeolites have amphoteric properties and the
existence of acid and basic sites are well known. The three
coordinated aluminium sites on the framework and non-
framework Al sites are normally considered to be Lewis acid
sites. Additionally charge compensating cations present in the
pores of zeolite act as Lewis acids, while the framework oxygen
represents a base. In particular, the oxygen atoms adjacent to Al
(Si–O–Al oxygen) are more basic because of a larger negative
charge on the oxygen. The Lewis acidity is connected to an
electron-accepting property and the Lewis basicity to the
electron donating property [20].
In the present work, we attempted to combine the above
properties of zeolites in an appropriate manner to prepare novel
photocatalytic materials. This involves incorporation of TiO2
into zeolite-Yand further incorporation of HPA as well as Co++
ions, with the aim to observe photoinduced interfacial electron
transfer from TiO2 to the incorporated HPA. This appears to
have resulted in synergistic enhancement of the photocatalytic
activity under visible irradiation for the photoreduction of
methyl orange, analogous to the Z-scheme mechanism
followed by plant photo system for water splitting [21].
2. Experimental
2.1. Materials
The materials used are NaY zeolite (Tricat Germany),
titanium isoproproxide (Acros Organics) phosphomolybdic
acid, cobalt chloride, and methyl orange (all E-Merck grade).
All other chemicals were the purest research grade available.
2.2. Preparation of the photocatalytic materials
Zeolite-Y (SiO2/Al2O3 = 2.5) was used as the support
material for the preparation of photocatalysts. This involves the
following steps:
(a) Incorporation of TiO2: 5 g zeolite-Yand 1.779 g of titanium
isopropoxide corresponding to 10% w/w loading of TiO2 on
zeolites, were mixed thoroughly resulting into a homo-
geneous solid mass. This mixture was calcined in air at
500 8C for 1 h followed by cooling and grinding. This
material is designated as Zeo-Y/TiO2.
(b) Incorporation of HPA: 0.5 g of phosphomolybdic acid
(HPA) was dissolved in 10 ml of doubly distilled water.
Five grams of Zeo-Y/TiO2 was added to this solution. The
slurry was stirred with a glass rod and dried at 70–80 8C on
a hot plate. This was then ground to obtain a homogeneous
mixture. This material is designated as Zeo-Y/TiO2/HPA.
(c) Incorporation of Co2+
: Alternatively, Zeo-Y/TiO2 was
exchanged with Co2+
ion prior to incorporation of HPA.
Five grams of the synthesized Zeo-Y/TiO2 was dispersed in
100 ml of doubly distilled water. The pH of this dispersion
was maintained at 6.5–7.0. A solution of CoCl2Á6H2O was
prepared by dissolving 0.2319 g of salt in 250 ml of doubly
distilled water. Only 5% cation exchange capacity of
zeolite-Y was used to exchange Co2+
ions. The pH of this
solution was found to be 5.8. This solution was then mixed
with the dispersion of Zeo-Y/TiO2 in doubly distilled water
and subjected to stirring for 40 min, followed by filtration
and drying at 60 8C. This Zeo-Y/TiO2/Co2+
was then
subjected to incorporation of heteropolyacid to enhance its
photocatalytic activity in the visible range. One-half a gram
of phosphomolybdic acid (HPA), which corresponds to
10% w/w on Zeo-Y/TiO2, was dissolved in 10 ml of doubly
distilled water. To this solution was added Zeo-Y/TiO2/
Co2+
, resulting in formation of a slurry. The slurry was dried
at 70–80 8C on a hot plate with constant stirring, followed
by grinding of the dried mass. This material is designated as
Zeo-Y/TiO2/Co2+
/HPA.
To highlight the role of zeolite in photocatalysis and also as a
support for stabilising different molecular species, we also
prepared the following composites: Co–P25, HPA–P25, and
Co–HPA–P25.
Co–P25. This composite is synthesized by impregnation of
CoCl2Á6H2O on P25 TiO2.
HPA–P25. This composite is synthesized by impregnation of
phosphomolybdic acid on P25 TiO2.
Co–HPA–P25. This composite is synthesized by impregna-
tion of CoCl2Á6H2O and phosphomolybdic acid on P25 TiO2.
2.3. Characterization of the photocatalysts
The photocatalysts thus synthesized were thoroughly
characterized using XRD, UV–vis diffuse reflectance and
elemental analysis. Powder X-ray diffraction studies were
carried out using a Philips Analytical Xpert diffractometer with
monochromated Cu Ka radiation (l = 1.54 A˚´
). The samples
were analyzed in a 2u range of 108–608 to identify the
crystalline phase and also to assess the structural integrity of
zeolite samples during the course of photocatalyst preparation.
Elemental analysis of the photocatalytic materials was
conducted using a Perkin-Elmer ICP-OES, Optima 4100 BV
to assess the content of cobalt and molybdenum present.
Diffuse reflectance UV–vis spectra of the samples were
recorded using a JASCO Spectrometer equipped with an
integrating sphere. BaSO4 was used as a reference material. IR
spectra of the samples were recorded using a Perkin-Elmer
FTIR spectrometer with KBr pellets. The samples were
analyzed in the wavenumber range of 4000–400 cmÀ1
.
2.4. Photocatalytic reduction of methyl orange
Photocatalytic reduction studies were carried out in a
borosilicate glass reactor. The light sources used were two
tungsten filament Philips lamps of 200 W each. In order to
N. Dubey et al. / Applied Catalysis A: General 303 (2006) 152–157 153

3. check any evaporation losses of reaction solution due to the
heating effect of the light source, a closed water condenser
was also attached to the open end of the cylindrical glass
reactor. A measured amount (0.075 g) of the photocatalyst
was suspended in 10 ml of 5 mg/l methyl orange solution
prepared in an ethanol:water mixture (1:40) [13]. Ethanol was
used as a sacrificial electron donor to improve the rate of
photocatalytic reduction. The solution was stirred on a
magnetic stirrer and exposed to irradiation for 4 h. After the
irradiation the suspension was filtered using 0.45 mm
cellulose nitrate filters. Progress of the reaction was measured
spectrophotometrically using a Perkin-Elmer Lambda
900 UV/Vis/NIR spectrophotometer. The concentration
change was calculated from the linear calibration plot of
methyl orange at a wavelength of 464 nm. The change in
concentration was reported taking into account different
factors which may influence the experiments, like filtration,
bleaching effect and adsorption of methyl orange on zeolite-
based photocatalyst.
The composites Co–P25, HPA–P25, and Co–HPA–P25 were
also evaluated in the same manner. However, due to absence of
zeolite matrix, the pH of methyl orange solution shifts to the
acidic side; this resulted in a shift in its lmax. This is particularly
observed in case of composites HPA–P25 and Co–HPA–P25. In
samples Co–P25 and Co–HPA–P25, cobalt is not present in the
form of Co2+
but as salt impregnated on P25.
3. Results and discussion
3.1. Characterization of the photocatalysts
The X-ray diffraction results shown in Fig. 1(a–c) indicate
that the crystallinity of the zeolite remains unaltered in zeolite-
Y/TiO2, zeolite-Y/TiO2/HPA and Zeo-Y/TiO2/Co2+
/HPA sam-
ples. This rules out any structural damage to the zeolite due to
the incorporation of various components. Also, the TiO2
particles formed on zeolite using organic precursor are too
small, amorphous and well-dispersed to be detected by XRD
[22]. As seen from Table 1, elemental analysis of the prepared
photocatalysts shows the presence of cobalt and molybdenum
in the material. The respective loadings (mg/g) of these
elements on the photocatalysts agree well with the theoretically
calculated values.
N. Dubey et al. / Applied Catalysis A: General 303 (2006) 152–157154
Fig. 1. (a) X-ray diffractogram of zeolite-Y/TiO2. (b) X-ray diffractogram of zeolite-Y/TiO2/HPA. (c) X-ray diffractogram of zeolite-Y/TiO2/Co2+
/HPA.
Table 1
Elemental analysis results for various materials
Sample Cobalt (mg/g) Molybdenum (mg/g)
Zeo-Y 0.00 0.00
Zeo-Y/TiO2 0.00 0.00
Zeo-Y/TiO2/HPA 0.00 48.28
Zeo-Y/TiO2/Co2+
/HPA 11.875 47.31

4. The UV–vis diffuse reflectance spectra for Zeo-Y/TiO2
(Fig. 2(a–e)) show a characteristic peak of TiO2 at wavelength
of 413 nm. There is considerable red shift in the absorption
band of TiO2 with incorporation of HPA, which promotes its
activity in the visible range, as seen from the absorption
spectrum of Zeo-Y/TiO2/HPA (Fig. 2(a–e)). The sample Zeo-
Y/TiO2/Co2+
/HPA, apart from showing a predominant red shift
in the absorption band of TiO2, also shows absorbance in the
visible range at around 668 nm. The wavelengths correspond-
ing to absorbance values were obtained by extrapolating the
curve on the abscissa [23]. In this way, the diffused reflectance
studies clearly indicate the red shift in the HPA and Co2+
-
incorporated samples, as compared to that for TiO2-incorpo-
rated zeolite sample. This can explain the improved photo-
catalytic activity of HPA and CoÀ
incorporated samples under
visible region. The Co2+
ions have been introduced in zeolites
by an exchange process and therefore expected to be well-
dispersed in the system. The IR spectra of the samples were
recorded and are presented in Fig. 3(a and b) for zeolite-Y and
Zeo-Y/TiO2/Co2+
/HPA, respectively. This illustrates that the
Keggin structure of HPA is retained in the photocatalyst sample
(Zeo-Y/TiO2/Co2+
/HPA). The major peaks are identified for
HPA at 783.2 cmÀ1
(P–O) and at 1120 cmÀ1
(Mo–Oe–Mo).
The photocatalytic materials were subjected to UV
radiation; it was observed that the colour of the photocatalytic
materials changed to a distinct blue colour, which substantiated
the possibility of its usage in visible solar spectrum.
3.2. Photocatalytic reduction of methyl orange
The photocatalytic reduction of methyl orange solution of
fixed concentration (5 mg/l) and at a fixed catalyst dose of
0.075 g shows that the rate of reduction increased linearly with
increase in irradiation time (Fig. 4). A similar study for
photoreduction of methyl orange was carried out for various
catalyst doses for a fixed concentration of methyl orange and a
constant illumination exposure. It is inferred from this study
that the photoreduction rate increases with increase in catalyst
amount, obviously due to the higher number of photocataly-
tically active sites for photoreduction. Effects of different
concentrations of methyl orange on photoreduction efficiency
N. Dubey et al. / Applied Catalysis A: General 303 (2006) 152–157 155
Fig. 2. (a–e) UV–vis diffuse reflectance spectra.
Fig. 3. (a) FTIR spectra of zeolite-Y. (b) FTIR spectra of Zeo-Y/TiO2/Co2+
/HPA.

5. at a fixed catalyst dose were also studied (Fig. 5); the efficiency
of the catalyst decreases with increasing concentration of
methyl orange, due to the fact that the latter gets adsorbed on
the zeolite-based photocatalysts, which offer a high surface
area for adsorption. This adsorbed methyl orange blocks
photocatalytically active centers and prevents their interaction
with photons of the light, thus resulting in a decrease in
efficiency of photoreduction. The reaction appears to follow
first order kinetics. The photoreduction experiment with Zeo-Y/
TiO2/Co2+
/HPA was carried out at different intensities of light.
It is observed that the photoreduction increases with increase in
light intensity, which confirms the photoactive nature of the
reaction. This is very much expected for any photocatalytic
reaction, because the photocatalysis rate is directly proportional
to the number of photons.
Table 2 shows the relative activity values of different
photocatalysts for photoreduction of methyl orange. These
results show maximum photoreduction activity for Zeo-Y/
TiO2/Co2+
/HPA. The photoreduction efficiency appears to be
improved considerably with incorporation of Co2+
in the
photocatalyst. Co2+
ion is present in well-dispersed exchange-
able form and probably acts as an electron acceptor, delaying
the back electron transfer reaction which is the cause of low
quantum efficiency in most of the photocatalytic reactions.
Another reason for better efficiency of the photocatalyst with
incorporation of Co2+
may be the fact that it is a coloured ion
and thus acts as a chromophore, which absorbs light in the
visible range. This can be seen from the UV–vis-diffuse
reflectance spectra of Zeo-Y/TiO2/Co2+
/HPA where there is a
characteristic absorbance around 668 nm. The zeolite structure
also possesses electron-accepting and donating properties,
which are important for the control of photo-induced charge
transfer reactions. The zeolite framework in combination with
Co2+
can play an important role in delaying electron hole
recombination reactions, which are a common cause of inferior
photocatalytic activity in many photocatalytic reactions.
Anandan and Yoon [22] have proposed an interesting
mechanism for photoreduction of methyl orange to hydrazine.
The tentative mechanism proposed for methyl orange photo-
reduction in the present work is very much similar to that
proposed by Anandan and Yoon except for significant delay in
recombination reaction due to Co ions in the exchanged state.
In the tentative mechanism proposed here, the electron from the
conduction band (CB) of TiO2 shifts to HPA through the zeolite
framework and Co2+
by a hopping mechanism and delays the
electron hole recombination. This is expected to happen more
efficiently in the catalyst Zeo-Y/TiO2/Co2+
/HPA as compared
to that in Zeo-Y/TiO2/HPA, where the electron from the
conduction band is expected to shift through the zeolite
framework directly to HPA. The reduction potential of CB is
À0.52 Vand that of Co2+
/Co is À0.29 V, which clearly explains
the transfer of electrons from CB to Co2+
. The electron-
accepting species Co2+
and HPA work synergistically, which
can explain the better efficiency of Zeo-Y/TiO2/Co2+
/HPA in
photoreducing methyl orange as compared to Zeo-Y/TiO2/
HPA. The photoreduction proceeds to the extent of 51%
(4.11 mg/gTiO2) in Zeo-Y/TiO2/Co2+
/HPA as compared to
only 12% (0.981 mg/g TiO2) in the case of Zeo-Y/TiO2/HPA
under the same experimental conditions. The benefit of
transition metal doping is better capability of trapping electrons
to inhibit electron hole recombinations during illumination.
The different opinions about the inhibition of electron hole
recombination indicate that this field of research requires
further studies, which in consequence can lead to the significant
improvement of the splitting of water. Studies on the effects of
N. Dubey et al. / Applied Catalysis A: General 303 (2006) 152–157156
Fig. 4. Variation of photoreduction with illumination time.
Fig. 5. Effect of methyl orange concentration on photoreduction.
Table 2
Photocatalytic evaluation results
S. no. Catalyst composition Methyl orange photoreduced
(mg) per TiO2 (g)
1 Commercial zeolite-Y 0
2 P25 TiO2 0.508
2 Zeolite-Y/TiO2 0.308
3 Zeolite-Y/TiO2/HPA 0.981
4 Zeolite-Y/TiO2/Co2+
/HPA 4.111
Initial concentration of methyl orange solution: 5 mg/l; catalyst dose: 0.075 g/
10 ml; illumination time: 4 h; source of illumination: 400 W tungsten filament
lamp.

6. various transition metal cations on photocatalytic properties of
such materials are in progress. Also we are working on water
splitting for hydrogen production using similar photocatalytic
materials. The new photocatalytic material based on cobalt has
not been reported so far, to the best of our knowledge. The
salient features of the new photocatalyst can be summarised as
follows:
1. TiO2 has been incorporated in zeolite by using Ti-
isopropoxide, which has a kinetic diameter greater than
the pore size of zeolite-Yand therefore does not enter into the
pores [24]. On calcination, Ti-isopropoxide gets converted to
TiO2 on the surface.
2. Co2+
is present in the pores in well-dispersed form as it is
incorporated by an ion exchange process and therefore
aggregation of Co2+
is not envisaged. As already explained,
by virtue of differences of reduction potential, the transfer of
electron from CB of TiO2 to Co2+
is possible.
3. The heteropoly anion is not adsorbed in the pores but is
present on the external surface.
4. The Keggin structure in the composite catalyst is definitely
retained on the surface; this is substantiated by the fact that
IR gives spectral peaks identified exclusively for HPA
structure (the IR pattern has been included as Fig. 3b).
The proposed mechanism can be summarised as follows.
Zeolites being amphoteric in nature function as electron donors
and acceptors due to the presence of Lewis acids and bases.
TiO2 on illumination results in formation of electron-rich
centers and holes. The zeolite framework donates electrons to
the holes and facilitates separation of the charge. Similarly, the
electron from conduction band (CB) of TiO2 is transferred to
the electron acceptor that is coordinated aluminium in zeolite.
(Zeolite-Y, having an enriched aluminium content, therefore
facilitates this reaction to a greater extent.) The electrons from
these aluminium sites are then transferred to the Co2+
ions in
the pores, which results in delay in the recombination reaction.
The zeolite matrices are thus contributing to the delay in
recombination reaction by a hopping mechanism of electrons in
the framework as reported elsewhere [25]. In addition to
delaying electron hole recombination reaction, zeolite serves to
support TiO2 and HPA, which increase its surface area. Also,
Co2+
is supported in well-dispersed form in the matrices.
Further studies pertaining to electron transfer mechanism are in
progress.
4. Conclusion
The zeolite-based photocatalysts having Co2+
in combina-
tion with TiO2 and HPA are found to show high efficiency for
photo reduction of methyl orange in the visible light range.
Zeolite plays an important role as it not only provides a high
surface area and ion exchange properties for incorporation of
TiO2, HPA and Co2+
, but also serves as an electron acceptor
which delays the back electron transfer reaction and promotes
photoreduction of methyl orange. The present work is a
preliminary investigation mainly to study the role of transition
metal in exchanged form on zeolite matrix. This study will help
to improve the photocatalytic properties of zeolite-supported
photocatalytic systems.
Acknowledgements
This work was carried out under the MITSUI Environmental
Engineering Trust (MEET) sponsored project No. G-5-1148
and CSIR Network Project No. CORE-08 (1.1). The authors are
thankful to Director, NEERI for providing the research
facilities. Thanks are also due to NCL Pune, JNARDDC
Nagpur, and NIMS Tsukuba, Japan, for help in various
evaluation and characterization studies.
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