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

www.elsevier.com/locate/apcata

Applied Catalysis A: General 303 (2006) 152–157

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

* 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

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

N. Dubey et al. / Applied Catalysis A: General 303 (2006) 152–157 153

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-Y and 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) I

ncorporation 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) I

ncorporation 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) I

ncorporation 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 CoCl2�6H2O 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.

� C

o–P25. This composite is synthesized by impregnation of

CoCl2�6H2O on P25 TiO2.

� H

PA–P25. This composite is synthesized by impregnation of

phosphomolybdic acid on P25 TiO2.

� C

o–HPA–P25. This composite is synthesized by impregna-

tion of CoCl2�6H2O 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 cm�1.

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) 152–157154

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

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) 152–157 155

Fig. 2. (a–e) UV–vis diffuse reflectance spectra.

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-

Fig. 3. (a) FTIR spectra of zeolite-Y. (b) F

rated zeolite sample. This can explain the improved photo-

catalytic activity of HPA and Co� 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 cm�1 (P–O) and at 1120 cm�1 (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

TIR spectra of Zeo-Y/TiO2/Co2+/HPA.

N. Dubey et al. / Applied Catalysis A: General 303 (2006) 152–157156

Fig. 4. Variation of photoreduction with illumination time.

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.

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-

Fig. 5. Effect of methyl orange concentration on photoreduction.

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

�0.52 Vand that of Co2+/Co is�0.29 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) 152–157 157

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. T

iO2 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. C

o2+ 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. T

he heteropoly anion is not adsorbed in the pores but is

present on the external surface.

4. T

he 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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