Photocatalytic degradation of paraquat using nano-sized Cu-TiO2/SBA-15 under UV and visible light

JOURNAL OFENVIRONMENTALSCIENCESISSN 1001-0742

CN 11-2629/X

www.jesc.ac.cn

Available online at www.sciencedirect.com

Journal of Environmental Sciences 2012, 24(6) 1125–1132

Photocatalytic degradation of paraquat using nano-sized Cu-TiO2/SBA-15under UV and visible light

Maurice G. Sorolla II1, Maria Lourdes Dalida1, Pongtanawat Khemthong2, Nurak Grisdanurak3,∗

1. Department of Chemical Engineering, College of Engineering, University of the Philippines, Diliman 1101, Quezon City, Philippines.E-mail: [email protected]

2. National Nanotechnology Center, National Science and Technology Development Agency, Klong Luang, Pathumthani 12120, Thailand3. Department of Chemical Engineering, NCE for Environmental and Hazardous Waste Management, Faculty of Engineering,

Thammasat University, Pathumthani 12120, Thailand

Received 01 July 2011; revised 12 November 2011; accepted 29 November 2011

AbstractPhotocatalytic degradation of paraquat using mesoporous-assembled Cu-TiO2/SBA15 under UV and visible light was investigated.

The catalyst was synthesized by impregnation of Cu-TiO2 colloids onto SBA-15. The colloids of Cu-TiO2 were prepared via sol-

gel method while the mesoporous support was prepared using hydrothermal technique. The catalyst was characterized using X-ray

diffraction, nitrogen adsorption-desorption, transmission electron microscopy, UV diffuse reflectance spectroscopy, Zeta potential and

X-ray adsorption spectroscopy. Results from characterizations showed that Cu doped TiO2 had a small crystalline size and was well-

dispersed on SBA-15. The inclusion of SBA-15 significantly enhanced the photocatalytic activity of the catalyst. Among the three types

of undoped catalyst in this study (P25, TiO2, TiO2/SBA-15), TiO2/SBA-15 yielded the highest degradation of paraquat for all pH under

UV illumination. Meanwhile 2 wt.% Cu-TiO2/SBA-15 yielded the highest activity under visible light.

Key words: paraquat; photocatalysis; titania; SBA-15; copper-doped

DOI: 10.1016/S1001-0742(11)60874-7

Introduction

Herbicides, such as paraquat, are one of the sources

of chemical pollutants released into the water stream,

soil as well as groundwater. Paraquat [1,1′-dimethyl-4,4′-bipyridinium dichloride] is known to display some harmful

effects on humans such as damage to the digestive system,

kidneys and lungs (Megarbane, 2003). Although it is

prohibited by European Union (Court of first instance in

Case T-229/04 Sweden 2007), paraquat is still used in

developing countries in Southeast Asia such as Philippines

and Thailand. Because of its known toxicity, degradation

and removal of paraquat in wastewater have been a matter

of paramount importance.

Photocatalytic degradation of paraquat using titania

(TiO2) under UV light has been studied extensively (Lee

et al., 2002; Moctezuma et al., 1999; Florencio et al.,

2004; Tennakone and Kottegod, 1996). However, there is

no study yet about paraquat degradation using surface-

modified TiO2 to markedly enhance its surface area.

Commercial ultra fine powders of TiO2, such as P25, show

a low adsorption capacity due to their low surface area.

To overcome this drawback, incorporation of mesoporous

support such as SBA-15 was implemented in this research.

* Corresponding author. E-mail: [email protected]

In addition, photocatalytic degradation of paraquat has not

yet been investigated under visible light. For this study,

copper was used as the metal dopant to shift the photo-

catalytic activity towards the visible range. Furthermore,

persulfate was also used to enhance the photocatalytic

activity of the catalyst under the visible region.

This research aims to degrade paraquat under UV and

visible light using Cu-TiO2/SBA-15 photocatalyst. The

catalyst was synthesized by subsequent addition of Cu-

TiO2 colloids onto SBA-15. The colloids of Cu-TiO2

were prepared via sol-gel method while the mesoporous

support was prepared using hydrothermal technique. Char-

acterization was done using X-ray diffraction, nitrogen

adsorption-desorption, transmission electron microscopy,

UV diffuse reflectance spectroscopy, Zeta potential and X-

ray adsorption spectroscopy. Finally, the influence of initial

solution pH and dopant loading were investigated.

1 Materials and methods

1.1 Synthesis

SBA-15 was synthesized hydrothermally according

to Meynen (2009) using triblock copolymer P123

(EO20PO70EO20, Sigma-Aldrich, USA) as structure

directing agent and tetraethyl orthosilicate (TEOS, 98%,

1126 Journal of Environmental Sciences 2012, 24(6) 1125–1132 /Maurice G. Sorolla II et al. Vol. 24

Sigma-Aldrich, USA) as silica source. In accordance

with the procedure, 4 g of P123 was dissolved in 130 mL

H2O and 20 mL HCl. Subsequently, 9.1 mL of TEOS

was added and the suspension was stirred for 7.5 hr at

45°C and then aged in a Teflon-lined autoclave for 15.5 hr

at 80°C. Thereafter, the white solid obtained was filtered

and washed with 25 mL distilled water three times and

dried. The dried product was then calcined at 550°C for 6

hr using a heating rate of 1°C/min.

Cu-TiO2/SBA-15 catalyst was prepared via sol-gel hy-

drolysis and condensation of an isopropanol (i-PrOH)

solution of titanium tetraisopropoxide (TTIP, 98+%, Acros

Organics) according to van Grieken (van Grieken et al.,

2002) with some modifications to incorporate the dopant.

In a typical procedure, a predetermined amount of SBA-15

was added into a solution of TTIP in i-PrOH (TTIP:i-PrOH

weight ratio of 1:8) maintaining 50 wt.% TiO2 loading in

Cu-TiO2/SBA-15. After stirring the mixture for 45 min,

distilled water was added until a molar ratio of 160 of

H2O:TTIP was obtained. The stirring was then continued

for 1 hr before adding the dopant with the desired loading

(0.1, 0.5, 2.0 and 5.0 wt.% Cu in Cu-TiO2/SBA-15).

Copper(II) acetate (99%, Strem Chemicals) was used as

the Cu source. Thereafter, the mixture was stirred for

another 1 hr. The mixture was then dried at 60°C to remove

excess water and subsequently at 100°C. The dried powder

was then calcined at 500°C for 3 hr, using a heating rate of

2°C/min. For comparison, unsupported catalyst was also

prepared using the same procedure without the addition of

SBA-15.

Throughout the subsequent discussion, TiO2 refers to

the synthesized titania while P25 to that of commercial

titania.

1.2 Characterization

The crystalline structures and phases of the catalyst were

analyzed using powder X-ray diffraction (XRD; Bruker

AXS D5005, Germany), equipped with a Cu Kα radiation

source. All of the powder samples were run at Bragg angles

(2θ) ranging in 0.7–3◦ and 20–80◦ for low and wide angles,

respectively. The crystallite size (D) was estimated using

Scherrer’s equation:

D =Kλ

Bcosθ(1)

where, B is the full width at half the maximum of the XRD

peak, K is a constant taken at 0.94 (particle size factor), θ is

the diffraction angle, and λ is a constant taken at 1.5405 Å

(the X-ray wavelength corresponding to Cu Kα radiation).

Textural properties of the catalyst were examined using

N2 sorption isotherms at –196°C obtained with Quan-

tachrome Instruments Autosorb (USA). This apparatus is

equipped with solving tools which was used to determine

the BET (Brunauer-Emmett-Teller) specific surface area,

BJH (Barrett-Joyner-Halenda) pore size distribution and

total pore volume. The BJH pore size distribution was

taken from the adsorption isotherm branch.

UV-Visible absorbance spectra with Hitachi UV-3501

(Japan), using a wavelength scan range of 300–600 nm

and BaSO4 as the standard, was used to determine the

light absorption of the catalysts. The surface charge of

the catalyst was examined using Zetasizer Malvern ZS90

(UK).

The local environment of Cu-TiO2 and Cu-TiO2/SBA-

15 were studied by XANES at beamline 8 of the

Synchrotron Light Research Institute (Public Organiza-

tion), Thailand. A double Ge (220) crystal monochromator

was used for selection of photon energy. The XANES and

EXAFS spectra of transition metals were obtained at room

temperature in fluorescent mode using a 13-element Ge

detector. Incident photon intensity was monitored by an

ion chamber filled with Ar gas. The photon energy was

calibrated by a metal foil at K-edge. The spectra were

normalized in absorbance by fitting the spectral region

from –30 to 80 eV regard to edge energy using a Victoreen

function and subtracting this as background absorption.

Throughout the subsequent discussion on catalyst char-

acterization, Cu-TiO2 and Cu-TiO2/SBA-15 refers to 2

wt.% Cu-TiO2 and 2 wt.% Cu-TiO2/SBA-15, respectively.

1.3 Photocatalytic evaluation

Photocatalytic runs were carried in a continuously-stirred

cylindrical Pyrex batch reactor using 200 mL paraquat

solution as the effective sample volume containing 0.5 g/L

catalyst. The UV-source was an 18-W Toshiba flourescent

backlight with a UV (A+B) intensity of 0.5 mW/cm2,

measured using Solarimeter Model 5.0 digital UV meter,

taken at the topmost surface of the paraquat solution.

Prior to UV illumination, the solution was kept under

dark conditions with continuous agitation for 1 hr to allow

adsorption to equilibrate. The photocatalytic degradation

of the 10 ppm paraquat was performed using its natural pH

(ca. 6.5) as well as at pH 3 and 9. About 0.1 mol/L HNO3

and 0.1 mol/L NaOH were used to adjust the initial pH of

the solution.

For the reaction under visible light, 500-W xenon lamp

(Shanghai DianGuang device, China) was used as the

light source. The intensity of the visible light at the outer

wall surface of the batch reactor was about 400 W/cm2

measured using KIMO SL100 digital meter (France). The

batch reactor was immersed in 1 mol/L NaNO2 solution

bath, maintained at 28°C, to filter light with less than 400

nm wavelength. About 5 mmol/L dipotassium peroxysul-

phate was used as an additive for the degradation of 40

ppm paraquat solution.

Samples were collected during the photoreaction and

were analyzed for paraquat content using JASCO V-630

UV-Vis spectrophotometer (USA). Each sample were fil-

tered using a nylon membrane (0.45 μm, VertiPure PVDF).

2 Results and discussion

2.1 Characterization of catalyst

Figure 1 displays the wide-angle (2θ: 20–80◦) XRD

patterns of selected catalysts. Based on the position of

the diffraction peaks, only the anatase phase TiO2 was

observed in all measurements. For both the supported and

No. 6 Photocatalytic degradation of paraquat using nano-sized Cu-TiO2/SBA-15 under UV and visible light 1127

20 30 40 50 60 70 80

2θ (degree)

A

A A

Fig. 1 High angle XRD patterns of selected catalysts. A: anatase.

unsupported catalyst, the introduction of Cu dopant did

not produce any major shifts or distortions to the main

characteristic peaks of titania. Moreover, no copper species

(Cu, CuO, Cu2O) were detected. The above observations

may suggest that Cu was well-dispersed on the surface

of titania. Meanwhile the inclusion of SBA-15 as support

resulted in broadening of peaks, indicative of the meso-

porous nature of the catalyst. As such, the crystallite sizes

of supported catalyst were relatively lower than those of

the unsupported catalyst. Table 1 lists the crystallite sizes

estimated using Scherrer’s equation. As can be seen from

the table, the doping process had no considerable effect on

the crystallite size. Unlike SBA-15, the addition of Cu after

the 1 hr hydrolysis did not effectively suppress the growth

of titania particles.

Table 1 Crystallite sizes derived from Scherrer’s equation

Catalyst Crystallite size (nm)

TiO2 16.4

2 wt.% Cu-TiO2 14.2

TiO2/SBA-15 4.8

2 wt.% Cu-TiO2/SBA-15 4.7

The low-angle (2θ: 0.7–3◦) XRD patterns are illustrated

in Fig. 2. After the addition of titania on SBA-15, the two

minor peaks indexed at (110) and (200) disappeared as a

consequence of broadening of the main peak indexed at

(100). Furthermore the main peak, which is associated with

the ordered hexagonal mesoporous framework of SBA-15

(Zhao et al., 1998), shifted to higher angle. This may imply

distortion, but not collapse, of the mesoporous structure.

Meanwhile, loading of the dopant did not impose any

significant changes in the XRD patterns of the supported

catalyst.

The N2 adsorption-desorption isotherms are shown in

Fig. 3, indicating that the materials displayed type IV

isotherms, which is associated with capillary condensation

taking place in mesoporous solids, as well as H1 hysteresis

loops according to IUPAC classification. Distorted H1

hysteresis loops suggest changes in the meso-structures

of SBA-15 upon incorporation of TiO2. The results (pre-

sented in Table 2) show the BET surface areas of SBA-15

110

110 200

Cu-TiO2/SBA-15

TiO2/SBA-15

SBA-15

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Fig. 2 Low angle XRD patterns of selected catalysts.

SBA-15

TiO2-SBA-15

Cu-TiO2/SBA-15

TiO2

Cu-TiO2

0 0.2 0.4 0.6 0.8 1.0

Relative pressure (P/P0)

Volu

me

adso

rpti

on a

t S

TP

(cm

3/g

) (a

.u.)

Fig. 3 Nitrogen sorption isotherms of SBA15 and selected catalysts.

(ca. 900 m2/g), TiO2/SBA-15 (426 m2/g) and 2 wt.% Cu-

TiO2/SBA-15 (406 m2/g). They are all higher than that

of TiO2 itself. The surface area of the prepared catalyst

decreases due to their blocking of the pore entrances on the

surface of the SBA-15 substrate, as supported by total pore

volume data. The total pore volume of the original SBA-15

is 1.23 cm3/g, while the total pore volume of catalysts are

(0.22–0.56 cm3/g) smaller than that of the original SBA-

15, indicating the TiO2 particles take part in blocking the

pore entrances. However, the pore blockage does not take

place over the entire material, resulting in the appearances

of hysteresis loops in all samples.

Figure 4 depicts the pore size distributions while Table 2

lists the total pore volume and pore diameters of the

catalysts and the SBA-15. As it can be observed, the

introduction of TiO2 onto SBA-15 resulted in noticeably

lower pore volume as well as in pore diameter. Pore size

uniformity, however, was maintained. This further suggests

that the TiO2 was well-disperse onto SBA-15 and may

indicate surface modifications in the meso-structures of

SBA-15 consistent with the results of low angle XRD.

The addition of support effectively enhanced the total pore

volume of titania. Furthermore, this led to smaller pore


0 8 16 24 0 8 16 24 0 8 16 24 32

Pore diameter (nm) Pore diameter (nm) Pore diameter (nm)

Pore

volu

me

(cm

3/(

nm

. g))

(a.

u.)

SBA-15 TiO2/SBA-15

Cu-TiO2/SBA-15 Cu-TiO2

TiO2

Fig. 4 BJH pore size distribution of SBA-15 and selected catalysts.

Table 2 Derived properties from nitrogen sorption isotherms

Catalyst Surface Total pore Pore

area volume diameter*

(m2/g) (cm3/g) (nm)

SBA-15 921 1.23 5–9

TiO2/SBA-15 426 0.56 3–7

2 wt.% Cu-TiO2/SBA-15 406 0.53 3–7

TiO2 66 0.23 13–28

2 wt.% Cu-TiO2 54 0.22 13–30

* 80% range.

diameters as well as more uniform pore size distribution

compared with unsupported titania. Meanwhile, loading of

dopant decreased the pore volume of the supported and

unsupported catalyst.

The TEM images of SBA-15, Cu-TiO2/SBA-15 and

TiO2/SBA-15 are shown in Fig. 5. TEM investigation of

the SBA-15 material revealed a clearly hexagonal pore

structure and the curved nature of the pores. It consists

of thick microporous silica pore with a uniform pore size.

The characteristic of s parallel channel of SBA-15 was also

present on Cu-TiO2/SBA-15 and TiO2/SBA-15, indicating

that some Cu-TiO2 and TiO2 nanoparticles were located

inside the mesopore channels of the SBA-15 matrix. In

addition, the presence of large crystallites on the outter-

most surface of the mesoporous matrix could block pores,

resulting lower BET surface area. These obtained results

were consistent to the N2 adsorption-desorption isotherms.

Figure 6 shows the diffuse reflectance UV-Vis Spec-

tra (UV-DRS) of TiO2 and TiO2/SBA-15 with different

Cu loadings. Noticeable shifts of the optical absorption

shoulders toward the visible light regions of the solar

spectrum were observed as Cu loading was increased for

both supported and supported catalyst. Compared with

unsupported TiO2, absorption band edge of the TiO2/SBA-

15 was noticeably blue shifted. This may imply smaller

anatase crystallite sizes for the supported catalyst consis-

tent with the results of XRD (Table 1), which suggests

that the addition of SBA-15 can effectively suppress the

growth of TiO2 clusters. Moreover, under the same basis

of catalyst weight, titania content on each catalyst would

be different. As observed in Fig. 6, the optical absorption

could be ranked as follows: Cu-TiO2/SBA-15 > TiO2 >TiO2/SBA-15. The low optical absorption for TiO2/SBA-

15 compared to that of TiO2 may be due to the content of

titania.

The undoped catalysts were also characterized using

Zeta potential. As shown in Fig. 7, the zero point charge

(zpc) of TiO2 was at pH 3.6. At pH 1.65, the zpc of

TiO2/SBA-15 was still negative. The inclusion of SBA-

15, effectively shifted the zpc towards a very low pH. The

surface charge of catalyst becomes negative upon increase

of pH beyond the zpc and vice-versa. For comparison, the

zpc of P25 is pH 6 (Wantala et al., 2009).

To understand the chemical state of Ti species in metal

doped TiO2 samples, their XANES have been measured

because the pre-edge and XANES regions supply electron-

SBA-15

Ordered hexagonal mesopores

Cu-TiO2/SBA-15

Ordered

hexagonal

mesopores

TiO2/SBA-15

Ordered

hexagonal

mesopores

Fig. 5 TEM images of SBA-15, TiO2/SBA-15 and Cu-TiO2/SBA-15.


1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0300 400 500 600 700 800

Wavelength (nm)

Abso

rban

ce

TiO2

TiO2/SBA-15

Cu-TiO2

Cu-TiO2/SBA-15

Fig. 6 UVDRS of selected catalysts.

TiO2

TiO2/SBA-15

15

0

-15

-30

Zet

a pote

nti

al (

mV

)

1.5 3.0 4.5 6.0 7.5 9.0

pH

Fig. 7 Zeta potential of selected catalysts.

ic information of the absorbing atom which in principle can

be translated into geometrical information. The pre-edge of

Ti atom at around 4970 eV is related to the transition in 3d

bands possibly from direct transition 1s to 3d quadrupole

and dipole transition to 4p character hybridized with the 3d

band. The electronic state of the absorber Ti atom reflects

the chemical bond with oxygen atoms. Although the Ti

atoms have the same coordination and valence electrons,

the oxygen atoms around Ti have different electronic state

and orbitals. Thus, the XANES spectra show the different

electronic state of the Ti–O bonds in each compound. The

transition of Ti pre-edge and K-edge in rutile, anatase and

modified TiO2 compounds are expressed in Fig. 8. The

XANES spectra showed the characteristic observed pre-

A1

A2 A3

TiO2 (rutile)

TiO2 (anatase)

Cu-TiO2/SBA-15

TiO2/SBA-15

Cu-TiO2

TiO2

4940 4960 4980 5000 5020 5040

Photoelectron energy (eV)

Norm

aliz

ed a

bso

rpti

on (

a.u.)

Fig. 8 XANES spectra of Ti K-edge of catalyst samples compared with

standard compounds.

edge features which were found to vary in both positions

as a function of Ti coordinated with oxygen neighbors

of 4, 5, and 6 regarded to A1, A2, and A3, respectively

(Farges et al., 1997). The threshold energy of all samples

was the same as in the crystalline TiO2, indicating that

most of the Ti ion were Ti (IV). The shape of pre-edge and

whiteline of all samples were similar to anatase TiO2 with

the characteristic triplet structure of 1s→3d4p core level

excitations suggested that our samples were anatase form.

This result is in agreement with the XRD result.

To investigate the grain boundaries and the position

of Cu atoms in nanocrystalline oxides, only the EXAFS

result of Cu-TiO2 and Cu-TiO2/SBA-15 were employed

in this study. The typical Ti K-edge EXAFS spectra and

the Fourier transforms of the data for Cu-TiO2 and Cu-

TiO2 /SBA-15 are shown in Fig. 9. The best fit parameters

for the EXAFS are given in Table 3. Based on the best fit

curve, three major peaks were identified corresponding to

the shell of O and Ti atoms, and mixed shells of O and

Ti atoms. From EXAFS result, no contribution of Cu in

TiO2 could be noticed. This is possibly due to a weak

interaction signal of Cu atoms when compared with Ti

atoms. A large distance and low coordination number of Ti

atom supported on SBA-15 indicated that TiO2 had small

crystallite size.

2.2 Photocatalytic activity

Photocatalytic activity under UV irradiation over various

types of catalyst was investigated using the natural pH

Table 3 Best-fit paprameter to Ti K-edge EAXFS

Sample Component CN R (Å) σ2 (Å2) E0 (eV) Weight factor

Cu-TiO2/SBA-15 Ti–O 3.44 ± 0.21 2.00 ± 0.003 0.004 ± 0.0006 13.15 33.40%

Ti–Ti 1.88 ± 0.35 2.71 ± 0.005 0.007 ± 0.0017

Ti–O 3.54 ± 1.12 3.55 ± 0.009 0.005 ± 0.0022

Ti–Ti 1.85 ± 1.12 3.47 ± 0.003 0.022 ± 0.0054

Cu-TiO2 Ti–O 4.12 ± 0.22 1.98 ± 0.003 0.005 ± 0.0006 11.75 28.66%

Ti–Ti 1.23 ± 0.24 2.72 ± 0.005 0.003 ± 0.0015

Ti–O 5.23 ± 1.36 3.54 ± 0.014 0.009 ± 0.0032

Ti–Ti 3.38 ± 1.12 3.50 ± 0.016 0.018 ± 0.0044

CN: coordination number; R: the nearest-neighbor distance; σ2: Debye-waller factor; E0: energy shift.


Cu-TiO2 Cu-TiO2

0.08

0.04

0.00

-0.04

-0.08

3 4 5 6 7 8 9 10

3 4 5 6 7 8 9 10

k (Å-1)

k (Å-1)

2.0

1.6

1.2

0.8

0.4

0.0

FT

mag

nit

ude

FT

mag

nit

ude

0 1 2 3 4 5 6

Radial distance (Å)

0.08

0.04

0.00

-0.04

-0.08

2.0

1.5

1.0

0.5

0.00 1 2 3 4 5 6

Radial distance (Å)

k3 χ

(k)

k3 χ

(k)

a

a

b

bCu-TiO2/SBA-15 Cu-TiO2/SBA-15

Fig. 9 Ti K-edge EXAFS spectra (k3-weighted) and corresponding Fourier transforms for Cu-TiO2 and Cu-TiO2/SBA-15. (a) normalized EXAFS; (b)

the corresponding Fourier transform corrected with the phase shift of the first shell. The solid line is experimental data and the dotted line is fit result.

of paraquat solution (ca. 6.5). Using 0.5 mW/cm2 UV

(A + B) irradiation intensity, no degradation of paraquat

was observed without the presence of any catalyst. After

mixing the catalyst in the paraquat solution for 1 hr in

the dark, the concentration-time course of the pollutant

was followed. Figure 10 presents the relative concentration

profiles of paraquat using various undoped catalysts. As it

can be observed, the TiO2/SBA-15 catalyst yielded by far

the greatest adsorption of the herbicide. After 1 hr mixing,

the percentages of paraquat absorbed using P25, TiO2 and

TiO2/SBA-15 were 4%, 6%, and 27%, respectively. This

can be attributed to the significant increase of specific sur-

face area of titania upon incorporation of the mesoporous

support. In addition, the supported catalyst produced the

highest removal of paraquat after 8 hr of UV illumination.

Figure 11 shows that the photocatalytic degradation

corresponds to a pseudo first-order reaction kinetics since

it followed the following relationship:

ln(C/C0) = kt (2)

where, C is the concentration of paraquat, C0 is the initial

concentration of paraquat, k is the apparent rate constant

and t is the reaction time. Half life (t1/2) can then be

calculated by using the following equation:

t1/2 =ln2

k(3)

As summarized in Table 4, the unsupported catalyst

yielded the highest activity among the catalyst. This can

be ascribed to the increase in adsorption capacity of the

catalyst upon addition of SBA-15. It can be seen that the

supported catalyst had higher paraquat removal after 8 hr

of illumination compared to the unsupported catalyst.

Photocatalytic activity was also investigated under

acidic and alkaline environment. As shown in Fig. 10

(pH 3 and 9), the adsorption capacities as well as the

degradation rates were generally better toward alkaline

conditions in agreement with previous studies (Lee et al.,

2002; Moctezuma et al., 1999). Furthermore for the same

pH, TiO2/SBA-15 produced the greatest photocatalytic

activity. At pH 3, in particular, the supported catalyst

yielded by far the highest paraquat removal. In contrast,

the unsupported catalyst had insignificant effect on the

removal of the herbicide. Aside from the increased in

specific surface area of titania due to the inclusion of

the support, the above observations can be attributed to

Table 4 Kinetic values for the photodegradation of paraquat under UV

irradiation

Catalyst Apparent rate R2 Half-life,

constant, k (hr−1) t1/2 (hr)

P25 0.33 0.94 2.1

TiO2 0.15 0.98 4.7

TiO2-SBA15 0.48 0.98 1.4


1.0

0.8

0.6

0.4

0.2

0.010 2 3 10 2 3 10 2 3 4

Illumination time (hr) Illumination time (hr) Illumination time (hr)

C/C

0

pH = 3 pH = 6.5 pH = 9

P25 TiO2 TiO2/SBA-15

Fig. 10 Photocatalytic activity at pH 3, 6.5 and 9 under UV irradiation.

TiO2/SBA-15

TiO2

P254

3

2

1

0

-ln

(C/C

0)

0 1 2 3 4 5 6 7 8

Illumination time (hr)

Fig. 11 First order kinetics for paraquat degradation.

the increased in hydroxyl ions at higher pH. Hydroxyl

ions can act as hole-scavengers which reduce hole-electron

recombination. In addition, solution pH affects the surface

charges of the catalyst as previously shown using the Zeta

potential diagram (Fig. 7). At pH 9, the surface charges

of the catalyst were negative favoring the adsorption of

cations like paraquat. However at pH 3, TiO2 as well as

P25 were cationic in contrast with TiO2/SBA-15. Thus

paraquat treatment using TiO2/SBA-15, which is negative-

ly charged even at very low pH, is more advantageous since

it does not require the adjustment of solution pH for the

efficient degradation.

Finally, the photocatalytic activity under visible light

was studied (Table 5). Removal of paraquat via photolysis

was not observed without the addition of persulfate. Since

photocatalytic activites of all catalyst were insignificant

without any additive under visible light, 5 mmol/L of

persulfate was required. Persulfate accelerated paraquat

degradation by acting both as a hole-electron scavenger

and a strong oxidant. The relationship of paraquat removal

among the catalyst was as follows: 2 wt.% Cu-TiO2/SBA-

15 > 5 wt.% Cu-TiO2/SBA-15 >> P25 > 0.5 wt.%

Cu-TiO2 0.5 wt.% Cu-TiO2/SBA-15 > 2 wt.% Cu-TiO2

TiO2/SBA-15 TiO2 > 5 wt.% Cu-TiO2. For both supported

and unsupported catalyst, it appeared that the photocat-

alytic activity increased with Cu loading up to certain

levels. As previously shown in the UVDRS, increasing

the metal dopant loading resulted in red shift favoring the

optical absorption towards the visible region. The optimum

degradation was obtained using 2 wt.% and 0.5 wt.%

Cu loading for the supported and unsupported catalyst,

respectively. The results are consistent with Yang et al.

(2009). Small amount of Cu doped on TiO2 exhibits good

photocatalytic efficiency due to that Cu2+ acts as electron

and hole trappers to reduce the photogenerated hole-

electron recombination rate. However, the surplus doped

Cu2+ ions (5 wt.% Cu) could serve as the recombination

sites between photoinduced electrons and holes by promot-

ing charge-carrier recombination with electron trapping

at CuO+ centers or with hole trapping at Cu2+ impurity

centers. This leads to an abrupt decrease in quantum

efficiency of the photocatalysis, reported by Chen and Mao

(2007).

Table 5 Photocatalytic activity under visible light

Paraquat removal (%)

Catalyst 4 hr 8 hr

Photolysis without persulfate 0 0

Photolysis with persulfate 8 18

P25 27 44

TiO2 15 30

0.5 wt.% Cu-TiO2 22 38

2 wt.% Cu-TiO2 17 30

5 wt.% Cu-TiO2 13 25

TiO2/SBA-15 15 30

0.5 wt.% Cu-TiO2/SBA-15 21 36

2 wt.% Cu-TiO2/SBA-15 55 71

5 wt.% Cu-TiO2/SBA-15 43 67

3 Conclusions

Paraquat has been degraded using Cu-TiO2/SBA-15 photo-

catalyst under UV and visible light. Among the three types

of undoped catalyst in this study (P25, TiO2, TiO2/SBA-

15), TiO2/SBA-15 yielded the highest degradation of

paraquat for all pH under UV illumination. At pH 3, in

particular, the supported catalyst resulted in by far the


greatest removal of paraquat. This could be attributed

to the incorporation of the mesoporous SBA-15 support

which markedly increased the surface area of titania there-

by enhancing adsorption of paraquat. Another reason is

the shift of zero point charge of TiO2 upon introduction of

SBA-15 to very low pH level. Thus paraquat treatment us-

ing TiO2/SBA-15, which is negatively charged even at very

low pH, is more advantageous since it does not require the

adjustment of solution pH for the efficient degradation. Cu-

leaching from the catalyst should be investigated further.

Acknowledgments

The authors would like to thank University of the Philip-

pines and Thammasat University. The authors would also

like to express their gratitude to Synchrotron Light Re-

search Institute (Public Organization), Thailand for XAS

measurement. This project was funded by the National

Research University Project of Thailand Office of Higher

Education Commission.

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