A comprehensive assessment of supported titania photocatalysts in a fluidized bed photoreactor: Photocatalytic activity and adherence stability

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Applied Catalysis B: Environmental 71 (2007) 151–162

A comprehensive assessment of supported titania photocatalysts

in a fluidized bed photoreactor: Photocatalytic

activity and adherence stability

Wei Qiu, Ying Zheng *

Department of Chemical Engineering, University of New Brunswick, 15 Dineen Dr., Fredericton,

New Brunswick E3B 5A3, Canada

Received 14 April 2006; received in revised form 20 August 2006; accepted 29 August 2006

Available online 10 October 2006

Abstract

Titania (TiO2) was immobilized onto hydroxylated glass beads (HGB) via the thermal bonding and sol–gel coating methods. The photocatalytic

activity and adherence stability of the prepared supported photocatalysts were studied in a fluidized bed photoreactor. P25 thermally bonded HGB

was found to be more active than sol–gel coated HGB prepared with the same immobilization conditions, while both of them exhibited poor

adherence stability, i.e., large amounts of immobilized TiO2 detached from HGB during the degradation. The adherence stability was improved

with limited extents by increasing the calcination temperature or reducing the coverage of TiO2 on HGB, but either of these approaches resulted in

lower activity. The poor adherence stability was ascribed to the fluid shear force and particle friction in fluidized bed, as well as the insufficient

bonding between TiO2 and HGB in terms of the bonding mechanism.

Hydroxylated quartz sands (HQS) and silica gel beads (SGB) were further studied and used as supports. Results have shown that the adherence

stability was significantly improved with SGB but only slightly improved with HQS. Characterizations results showed that a coarser surface and

more surface Si–OH groups could improve the adherence stability of supported TiO2 photocatalysts.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Titania; Supported photocatalyst; Activity; Adherence stability

1. Introduction

As a semiconductor photocatalyst, TiO2 has been extensively

studied in the last two decades due to its good photocatalytic

activity, chemical inertness, and low cost [1]. So far, two modes

of TiO2 have been investigated for photocatalytic degradation

processes: suspended fine TiO2 particles and substrate-supported

TiO2. Although the suspended TiO2 (slurry-type) was reported to

be more efficient in photocatalytic degradations [2–4], the

supported TiO2 is generally considered more practical for its

easier separation from treated water. A number of publications on

the supported TiO2 photocatalysts have emerged in the past few

years [2–15]. A variety of substrates, such as quartz sands [2–4],

glass beads [5–7], glass tubes [8], glass walls [9], ceramic

particles [10,11], and stainless steel [12–15], have been studied

* Corresponding author. Tel.: +1 506 447 3329; fax: +1 506 453 3591.

E-mail address: [email protected] (Y. Zheng).

0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2006.08.021

as supports. In most cases, the thermal bonding [2–4,6,7,10] and

sol–gel coating methods [6,7,9,11,16] are applied to immobilize

TiO2 onto the supports. With minor variances from case to case,

the thermal attachment method usually immobilizes a previously

prepared TiO2 (e.g. Degussa P25) onto supports via wet-

impregnation and calcination. In a typical sol–gel coating

method, TiO2 is immobilized on supports by incorporating

supports in the hydrolysis of Ti precursors. Other methods such

as CVD [5,17] and PVD [18] were also reported, but these

methods generally involve costly instrument and operation.

Different types of photoreactors were developed to accom-

modate the supported TiO2 photocatalysts. Generally, the

photoreactors can be classified as fluidized bed photoreactors

[2–4,10,11,16,17] and fixed bed photoreactors [8,9,19]. Com-

pared to fixed bed photoreactors, fluidized bed photoreactors can

offer superior mass transfer efficiency and light transmission. It

has been reported that the annular-type fluidized bed config-

uration could enable a most efficient use of the light source in

photochemical reactions [20]. Pozzo et al. [3,4,17] have

mailto:[email protected]

http://dx.doi.org/10.1016/j.apcatb.2006.08.021

W. Qiu, Y. Zheng / Applied Catalysis B: Environmental 71 (2007) 151–162152

systematically investigated the performance of supported TiO2

photocatalysts for the degradation of organics in annular-type

fluidized bed photoreactor. Chiovetta et al. [21] further studied

the radiation field and reaction kinetics in the fluidized bed

photoreactor and proposed the mathematical models.

While most studies concerned the activity of supported

TiO2 photocatalysts in fluidized bed photoreactors [2–

7,10,11,16], the adherence stability of immobilized TiO2

on supports has not yet caught wide attention except for a few

examinations reported by Kanki et al. [11] and Keshmiri et al.

[22]. Actually, stability is as important as activity for any kind

of catalysts under realistic operations [23]. In our case, if the

adherence is not stable, the immobilized TiO2 can easily

detach from supports and eventually act as suspended TiO2

particles, which will make the immobilization procedure

meaningless.

In this study, we investigated the photocatalytic activity and,

moreover, the adherence stability of supported TiO2 photo-

catalysts. The adherence stability was quantifiably evaluated by

the term ‘‘particle loss percentage’’ of TiO2. The particle loss

percentage of TiO2 was determined by the turbidity-SPC

(suspended particle concentration) method, which is conve-

nient and widely applied in hydrology studies. Soda-lime glass

beads were used as support for their good UV-transmission,

chemical inertness, and higher surface Si–OH density than

other commercial glasses [28]. Before used as supports, the

glass beads were etched by NaOH to roughen their surfaces and

increase their surface Si–OH density [29,30]. As the most

prevalent methods, the thermal bonding and sol–gel coating

methods were employed to prepare the supported TiO2

photocatalysts. Effects of immobilization methods, calcination

temperatures, nominal surface coverages (NSC) were inves-

tigated for their influences on the activity and adherence

stability of supported TiO2. Quartz sands and silica gel beads

were also employed as supports to investigate the effect of

Table 1

Actual coating amount of P25 thermally HGB photocatalysts

50% NSC 100

P25-350 P25-450 P25-550 P25

Calcination temperature (8C) 350 450 550 350

Desired coating amount (g) 0.175 0.175 0.175 0.3

Actual coating amount (g) 0.146 0.159 0.167 0.3

TiO2 concentration (g/l) 0.017 0.018 0.019 0.0

Table 2

Actual coating amount of sol–gel coated HGB photocatalysts

50% NSC 10

SG-350 SG-450 SG-550 SG

Calcination temperature (8C) 350 450 550 35

Desired coating amount (g) 0.175 0.175 0.175 0.3

Actual coating amount (g) 0.162 0.155 0.153 0.3

TiO2 concentration (g/l) 0.019 0.018 0.018 0.0

surface texture and the surface Si–OH groups on the adherence

stability of supported TiO2.

2. Experimental

2.1. Photocatalyst preparation

Soda-lime glass beads (50–70 mesh, Flex–O–Lite) were

washed by acetone and etched in a 5 M NaOH solution at 95 8Cfor 30 min with stirring. The surface hydroxylated glass beads

(HGB) were washed and dried at 80 8C.

The HGB supported TiO2 photocatalysts were prepared by

thermal bonding and sol–gel coating methods with three NSC

(NSC = 50%, 100% and 400%) and three calcination

temperatures (350, 450 and 550 8C). For a monolayer of

TiO2 (100% NSC), the weight ratio of 1 mg TiO2/g HGB was

adopted from literature [3]. Actual coating amounts of TiO2

were determined by UV–vis spectroscopy at 410 nm by

dissolving the photocatalysts in a boiling mixture of

concentrated H2SO4 (95–98%, Fisher) and (NH4)2SO4 (ACS

grade, Fisher; weight ratio: H2SO4/(NH4)2SO4 = 4) and adding

drops of H2O2 [6]. The results of actual coating amounts of

TiO2 on photocatalysts are shown in Tables 1 and 2. The

prepared photocatalysts were designated by immobilization

method, calcination temperature, and NSC, e.g., SG-450-

100% is prepared by sol–gel coating method and calcined at

450 8C with 100% NSC of TiO2.

The P25 thermally bonded HGB was prepared by the

methods reported in literature [2–4,6,7,10]. A calculated

amount of P25 particles was dispersed in 120 ml deionized

water by sonicating on a 500 W sonic dismembrator (Model

500, Fisher) for 1 min. The P25 suspension was added to 350 g

HGB with stirring, allowed to stand for 2 h, dried at 80 8C, and

calcined for 12 h. The obtained solids were flushed with

deionized water to remove the excess TiO2 and dried at 80 8C.

% NSC 400% NSC

-350 P25-450 P25-550 P25-550 P25-350 P25-450

450 550 350 450 550

50 0.350 0.350 1.400 1.400 1.400

47 0.371 0.355 1.411 1.379 1.421

41 0.044 0.042 0.166 0.162 0.167

0% NSC 400% NSC

-350 SG-450 SG-550 SG-550 SG-350 SG-450

0 450 550 350 450 550

50 0.350 0.350 1.400 1.400 1.400

25 0.330 0.347 1.339 1.332 1.351

38 0.039 0.041 0.158 0.157 0.159

W. Qiu, Y. Zheng / Applied Catalysis B: Environmental 71 (2007) 151–162 153

Fig. 2. Correlations of turbidity vs. suspended particle concentration (SPC) of

the detached TiO2 particles from P25 thermally bonded HGB and sol–gel coated

HGB.

Fig. 1. Photocatalytic experimental set-up: (1) photoreactor, (2) liquid dis-

tributor, (3) valve, (4) cooling water tank, (5) submerged pump and (6) water

tank.

The method for preparing the sol–gel coated HGB was also

adopted from the literature [6,7,9,11]. A calculated amount of

titanium isopropoxide (TTIP, 98+%, Acros Organics) was

added into 120 ml ethanol (�99.5%, Aldrich). After adding

350 g HGB, the mixture was stirred and allowed to stand for

2 h. Ten milliliters of deionized water was added dropwise into

the mixture with drops of HCl acid (ACS reagent, 36.5%,

Fisher) to adjust the pH to 1–2. The mixture was irradiated by

an infrared lamp (250 W, Cole-Parmer) until a gel–solid

mixture was formed. Then the mixture was dried and calcined

for 12 h. The obtained solids were flushed and dried at 80 8C.

Quartz sands (50–70 mesh, Aldrich) and silica gel beads

(30–60 mesh, Aldrich) were also used as supports. The quartz

sands undertook the same hydroxylation treatment as the glass

beads before used as supports. The optimized immobilization

conditions (SG-450-100%) were determined from the result of

HGB supported TiO2. The TiO2 coating amounts on HQS and

SGB were 0.319 and 0.340 g, respectively.

2.2. Characterization

The average particle size of detached TiO2 was measured by

dynamic light scattering (DLS) on a Mastersizer 2000 and

Hydro 2000S system (optical unit and sampler unit, Malvern)

after sonicating for 1 min at 250 W. The crystal phase of TiO2

was examined by powder X-ray diffraction (XRD) on a Bruker

Table 3

Average particle size of TiO2 particles

Samples Source of TiO2 Calcin

P25-original P25 (as received) N/A

P25-350 Detached P25 particles from P25-350-100% 350



SG-350 Detached TiO2 particles from SG-350-100% 350



AXS D8 Advance X-ray diffractometer with Cu Ka radiation.

The HGB supported TiO2 photocatalysts were analyzed by

X-ray photoelectron spectroscopy (XPS) on a VG Microtech

Multilab ESCA 2000 system to examine the interaction

between TiO2 and HGB surfaces. Surface morphologies of the

glass beads, HGB, quartz sands, HQS, and SGB were

characterized by scanning electronic microscopy (SEM, JEOL

JSM6400) and atomic force microscopy (AFM, MultiMode,

Veeco Instruments). The supports were also analyzed by

Fourier transform infrared spectroscopy (FTIR, Nicolet Nexus

470).

2.3. Photocatalytic degradation

The scheme of the experimental set-up is shown in Fig. 1.

The annular fluidized bed photoreactor with a height of 65 cm

consists of two coaxial columns: the outer acrylic resin

column (i.d.: 102 mm; o.d.: 108 mm) and the inner quartz

tube (i.d.: 48 mm; o.d.: 50 mm). A 30 W germicidal lamp

(254 nm, Spectroline) was installed at the axis of the quartz

tube. 8.5 l of Congo Red (10 mg l�1) aqueous solution were

used as the model contaminated water. A sewage submerged

pump (0.18 kW, Mastercraft), which could tolerate solids

smaller than 2 mm, was mounted in the water tank to pump

the water into the fluidized bed. The bed was fluidized and

kept at 700% expansion during the degradation to ensure a

ation temperature (8C) Average particle size of detached TiO2 (nm)

68

321

343

454

345

438

510


good light transmission. A cooling water tank was installed

outside the water tank to avoid significant temperature rise in

the water. Treated water samples were taken from the outlet

of the photoreactor and measured on a turbidimeter (Hach

2100AN) after sonicating for 1 min at 250 W. For the

concentration measurement, samples were centrifuged at

17,000 rpm for 30 min on a refrigerated centrifuge (Sorvall

RC-5B, Dupont) and the supernatant was measured on a UV–

vis spectrometer (Spectronic 1001 Plus, Milton Roy) at

580 nm.

Fig. 3. Degradation kinetic curves using P25 thermally bonded HGB and sol–ge

wavelength: 254 nm, UV incident intensity: 8.390 mW cm�2).

3. Method for adherence stability assessment

The term ‘‘particle loss percentage’’ was defined to

quantifiably assess the adherence stability of the supported

TiO2 photocatalysts. It equals to the ratio of TiO2 peeled off

from supports to the total amount of immobilized TiO2,

particle loss percentage ¼ SPC� VR

mcoated

(1)

l coated HGB as photocatalysts (C0 = 10 mg l�1, reaction time: 120 min, UV


Fig. 4. XRD patterns of TiO2: (a) calcined P25 (b) sol–gel synthesized TiO2.

where SPC is the suspended particle concentration in treated

water, VR is the total volume of water solution, and mcoated is the

total coating amount.

Turbidity measurement has been widely applied in

hydrology studies to estimate the suspended particle concen-

tration in water [24–27]. Our study adopts this method to

determine the amount of detached TiO2 particle in treated water

after the degradation. To prepare the standard suspensions, the

detached TiO2 particles in treated water were collected by

centrifuging and drying at 80 8C. A series of standard TiO2

suspensions with various SPC were prepared by dispersing

various amounts of detached TiO2 particles into deionized

water. Correlations of turbidity and suspended particle

concentration (SPC) were established by measuring the

turbidity of TiO2 standard suspensions with known SPC. As

shown in Fig. 2, linear relationships were observed for turbidity

and SPC, which are in agreement with literature [24,25].

Moreover, it was shown that the suspension exhibited lower

turbidity when the calcination temperature was higher. In

Table 3, DLS results showed that the TiO2 calcined at higher

temperatures had larger average particle sizes due to the more

agglomerates of TiO2 particles formed at higher temperatures.

This observation is consistent with the reported result that small

particles scatter light more efficiently and result in higher

turbidity [25].

To minimize the experimental error for the turbidity

measurement, a blank experiment by using bare HGB was

conducted. Result confirmed that no glass beads were crushed

during a continuous 12 h fluidization. In addition, we found that

the Congo Red dissolved in water and absorbed on TiO2

particles had no considerable effects on the turbidity, which was

probably due to the low concentration (10 mg l�1) of Congo

Red used in this study.

4. Results and discussion

4.1. Degradation efficiency

A photolysis test was conducted before running the

degradation experiments. It was found that, without the

presence of photocatalysts, Congo Red was decomposed by

5.2% after 2 h irradiation. The contribution of photolysis was

assumed to be constant (5.2%) in all degradation experiments,

and this fraction was deducted from the total degradation of

Congo Red. Fig. 3 shows the photocatalytic degradation results

of Congo Red. The observed first-order exponential decay of

C/C0 agreed well with the generally suggested first-order

kinetics [31,32]. For either P25 thermally bonded HGB or sol–

gel coated HGB, higher decolorization rates of Congo Red were

observed with increasing NSC and decreasing calcination

temperature. A comparison between P25 thermally bonded

HGB and sol–gel coated HGB indicates that the P25 thermally

bonded HGB generally has a higher activity than sol–gel coated

HGB with the same NSC and calcination temperature.

The higher decolorization rate by using photocatalysts with

higher NSC (up to 100% NSC) can be easily explained by the

larger surface area of TiO2 exposed under UV. When

NSC > 100%, however, the decolorization rate should not

increase much because the total TiO2-covered surface area on

HGB was approximately the same. However, this does not

agree with the results in Fig. 3 where the decolorization rate

increased significantly when NSC increased from 100% to

400%. This contradiction leads to the assumption that the TiO2

on 400% NSC photocatalysts might suffer from a serious

detachment from the support during the degradation since the

detached TiO2 small particles could greatly enhance the

reaction rate. This assumption will be further discussed in

Section 4.2.

The lower activity of TiO2 calcined at higher calcination

temperature has been well recognized by researchers [3,4].

Generally, the anatase (A) to rutile (R) phase transformation in

TiO2 occurs at high temperature and it leads to a lower

photocatalytic activity of TiO2. The phase ratio of anatase/rutile

(A/R) can be calculated by [33],

% transformation of rutile ¼ 1

1þ 0:8ðIA=IRÞ(2)


Fig. 5. Turbidity and TiO2 particle loss percentage after 2 h degradation: (a) P25 thermally bonded HGB and (b) sol–gel coated HGB.

where IA and IR are the intensity of the reflection peaks of

anatase (25.38) and rutile (27.68) in the XRD patterns,

respectively. Fig. 4 shows the XRD patterns of the calcined

P25 and sol–gel synthesized TiO2. It is found that the A/R

ratio of P25 was well preserved from 350 to 450 8C (A/

R = 3.3) but decreased greatly (A/R = 1.6) at 550 8C, indi-

cating that the A! R phase transformation in P25 occurred

above 450 8C. For the sol–gel synthesized TiO2, Fig. 4(b)

shows the amorphous morphology of SG-350 and a strong

rutile peak in SG-450 (A/R = 1.2), indicating that the A! R

phase transformation already began at 350–450 8C. At

550 8C, the rutile phase surpassed the anatase phase in

SG-550 (A/R = 0.5). The different A! R phase transforma-

tion temperatures in calcined P25 and sol–gel TiO2 are

ascribed to the different preparation methods [33,34].

Since a higher content of rutile leads to a lower activity

of TiO2, it is concluded that the calcined P25 had a higher

activity than sol–gel TiO2 after calcined at the same tem-

perature.

4.2. Adherence stability

The adherence evaluation results for P25 thermally bonded

HGB and sol–gel coated HGB are shown in Fig. 5(a) and (b),

respectively. TiO2 particle loss percentages were obtained

using the corresponding turbidity–SPC correlation in Fig. 2.

As shown, the immobilized TiO2 on both the P25 thermally

bonded HGB and the sol–gel coated HGB shows significant

detachment percentages after 2 h degradation. Compared to

the P25 thermally bonded HGB, the sol–gel coated HGB show

slightly more durable adherence. It is also found that

photocatalysts with a lower NSC (�100%) exhibits much

less particle loss percentages of TiO2 than the photocatalysts

with multiple coating of TiO2 (400% NSC). As well, calcining

the supported TiO2 photocatalysts at a higher temperature

could reduce the particle loss percentage, but the improvement

was limited and the increase of calcination temperature led

to a lower photocatalytic activity of TiO2, as shown in

Fig. 3.

The adherence stability between TiO2 and the support is

closely associated with the bonding mechanism in the

immobilization process. In this study, we infer that two

major factors contributed to the TiO2 attachment: (a) the

densification of TiO2 particles during the calcination, i.e.

agglomeration by van der Waals forces; (b) the formation of

Ti–O–Si bonds at the interface during the calcination.

Evidently, only these TiO2 particles that form Ti–O–Si bonds

with supports could be durably attached. Other TiO2 particles

were densified, agglomerated to clusters, and attached to the

support by van der Waals forces. These particles may detach

easily from the supports by the particle attrition and the fluid

shear force in the fluidized bed photoreactor. Since Ti–O–Si

bonds are the only durable connection between the TiO2 and

the HGB surfaces, the amount of Ti–O–Si at the P25/HGB

interface was critical for the adherence stability of photo-

catalysts.


Fig. 6. Photoelectron peaks of: (a) Ti 2p3/2 in pure TiO2, P25 thermally bonded

HGB, sol–gel coated HGB and (b) Si 2p in bare HGB, P25 thermally bonded

HGB, sol–gel coated HGB.

Previous studies have reported the existence of Ti–O–Si at

the interface of TiO2 and SiO2 [35–39]. In P25 thermally

bonded HGB, Ti–O–Si bonds can be formed by the

dehydration of the Ti–OH groups on P25 particles and the

Fig. 7. FTIR spectra of origin

Si–OH groups on HGB:

nðSi�OHÞþ nðTi�OHÞ ! nðSi�O�TiÞþ nH2O (3)

nðTi�OHÞþ nðTi�OHÞ ! nðTi�O�TiÞþ nH2O (4)

In terms of the proposed bonding mechanism, it is implied

that only these P25 particles adjacent to HGB surfaces were

able to form durable Ti–O–Si bonds with the Si–OH groups on

HGB via dehydration (Eq. (3)). On the surface area of HGB

where there was no Ti–OH group present, P25 particles were

attached to the HGB surface by van der Waals forces. Other

than the first monolayer, the above layers of immobilized TiO2

particles may be attached either by the dehydration with the

residual Ti–OH groups on the first monolayer of TiO2 particles

(Eq. (4)), or by van der Waals forces. These P25 particles

attached to the HGB surface or the first monolayer of TiO2

particles by van der Waals forces could easily detach from

supports in the fluidized bed photoreactor due to the unstable

attachment. Fig. 5 shows the fact that the majority of particles

detached from the HGB surfaces during the degradation when

HGB were coated with multiple-layer P25 particles (400%

NSC), from which we can imply that the multilayer attachment

of TiO2 was mainly via the van der Waals forces. Moreover,

Fig. 5(a) shows that the P25 thermally bonded HGB with 50%

or 100% NSC TiO2 also showed significant particle loss

percentages, indicating there were no sufficient Ti–O–Si

bonding between TiO2 and HGB even with less than one

monolayer coating. From this result, it was implied that the

HGB might not provide sufficient surface Si–OH groups for the

formation of Ti–O–Si during the TiO2 immobilization.

In sol–gel coated HGB, Ti–O–Si bonds formed in a different

way. First, TTIP reacted with the surface OH groups on HGB

(Eq. (5)). The (Si–O–)nTi(OCH(CH3)2)4�n was produced in this

step and attached to HGB surfaces. After adding the water, the

surface attached (Si–O–)nTi(OCH(CH3)2)4�n completely hydro-

lyzed with water and transformed into the surface attached

(Si–O–)nTi(OH)4�n (Eq. (6)). Also, excess TTIP hydrolyzed and

Ti(OH)4 was produced in the bulk liquid (Eq. (8)). After drying

al glass beads and HGB.


Fig. 8. SEM images of support surfaces: (a) original glass beads, (b) HQS, (c) original quartz sands, (d) HQS and (e) silica gel beads.

and calcining, the surface attached (Si–O–)nTi(OH)4�n could

either dehydrate with themselves (Eq. (7)), or with the suspended

Ti(OH)4 in the bulk mixture (Eq. (9)). Both of these dehydrations

led to the formation of Ti–O–Si bonds between TiO2 and HGB.

On the other hand, if the Ti(OH)4 in the bulk liquid is significantly

excessive, massive TiO2 will be produced during the calcination

which eventually attached to HGB supports by van der Waals

forces. The immobilization process in sol–gel coated HGB is

described as follows:

nðSi�OHÞ þ Ti½OCHðCH3Þ2�4! ðSi�O�ÞnTiðOCHðCH3Þ2Þ4�nþ nC3H8O (5)

ðSi�O�ÞnTiðOCHðCH3Þ2Þ4�nþð4� nÞH2O

! ðSi�O�ÞnTiðOHÞ4�nþð4� nÞC3H8O (6)


Fig. 9. AFM images of support surfaces: (a) HGB, (b) HQS and (c) SGB.

Table 4

Surface roughness of the HGB, HQS, and SGB

Support Rq (nm) Ra (nm)

HGB 16.1 10.9

HQS 354 287

SGB 55.3 41.6

ðSi�O�ÞnTiðOHÞ4�n ! ðSi�O�ÞnTið�OÞ4�nþ ð4� nÞH2O

(7)

TiðOCHðCH3Þ2Þ4þ 4H2O ! TiðOHÞ4þ 4C3H8O (8)

ðSi�O�ÞnTiðOHÞ4�nþð4� nÞðTi�OHÞ! ðSi�O�ÞnTið�O�TiÞ4�nþð4� nÞH2O (9)

The proposed Ti–O–Si formation process indicated that Ti–O–

Si bonds were more likely to form in sol–gel coated HGB than

in P25 thermally bonded HGB. This is in good agreement with

our experimental results shown in Fig. 5(a and b). However, it

should be noted that only slight improvements in adherence

stability were observed. According to the bonding mechanism,

if TTIP hydrolyzed with Si–OH groups on HGB, the resulting

(Si–O–)nTi(OCH(CH3)2)4�n could eventually transform into

the surface attached TiO2 via Ti–O–Si bonds. The limited

improvement of adherence stability in sol–gel coated TiO2

reveals that the key factor for the poor adherence in sol–gel

coated HGB was the insufficient Si–OH groups on HGB. The

immobilized TiO2 cannot form sufficient Ti–O–Si bonding

with HGB surfaces, and a serious particle loss of immobilized

TiO2 was observed in the fluidized bed photoreactor.

The interaction of TiO2 and HGB at the interface was

examined by XPS. As seen from Fig. 6(a), both the P25

thermally bonded HGB and sol–gel coated HGB exhibited

shifts of the maximum position in the photoelectron spectra of

the Ti 2p3/2 electron. Binding energies of Ti 2p3/2 electron in

P25 thermally bonded HGB and sol–gel coated HGB are 0.1

and 0.3 eV higher than that of pure TiO2, respectively. In terms

of previous studies [35,36], this shift was the result of the four-

fold coordination of Ti in the strongly interacting Ti–O–Si.

However, the small shift (0.1 eV) in P25 thermally bonded

HGB may not be considered as an experimental evidence for

the existence of Ti–O–Si due to the possible measurement error.

Therefore, the photoelectron peaks of the Si 2p electron were

further investigated. Fig. 6(b) shows that the binding energy of

the Si 2p electron in P25 thermally bonded HGB and sol–gel

coated HGB are 0.5 eV and 0.9 eV higher than that of bare

HGB, respectively. These shifts of binding energies of the Si 2p

electron confirmed the existence of Ti–O–Si on both

photocatalysts. Moreover, the larger shift of sol–gel coated

HGB indicated that more Ti–O–Si bonds were formed on the

sol–gel coated HGB than that on the P25 thermally bonded P25.

FTIR spectra of the original glass beads and the HGB are

shown in Fig. 7. To compare the density of surface OH groups,

the IR spectra were normalized based on the Si–O stretching

vibration peak at 1041 cm�1.The weak peak at 3447 cm�1 was

attributed to the surface OH groups, which consist of a majority

of Si–OH with a minor amount of adsorbed water [28]. The

existence of surface adsorbed water is unavoidable at the

ambient condition, and it can be identified by the broadness of

the peak. A slightly stronger OH peak was observed in the FTIR

spectrum of HGB, indicating that the density of surface OH

groups was slightly increased after the hydroxylation treatment

[29,30]. However, in general, the density of Si–OH groups on

HGB was very low. These results support our assumption in the

bonding mechanism section that the HGB supports do not have

sufficient surface Si–OH groups to form the Ti–O–Si bonding

with TiO2.


Fig. 10. Comparison of the FTIR spectra of HGB, HQS and SGB.

Fig. 11. Comparison of the performance of TiO2 supported on HGB, HQS and

SGB: (a) degradation results and (b) particle loss percentages of TiO2 (sol–gel

coating, 100% NSC, 450 8C calcination, C0 = 10 mg l�1, reaction time:

120 min, UV wavelength: 254 nm, UV incident intensity: 8.390 mW cm�2).

4.3. TiO2 supported on other substrates

Two other supports, i.e., hydroxylated quartz sands (HQS)

and silica gel beads (SGB) were used as the supports for TiO2

immobilization. The surface morphology of different supports,

including the HGB, were examined by SEM and AFM. The

SEM images of quartz sands, glass beads and silica gel beads

are shown in Fig. 8. By comparing Fig. 8(a) with (b), it was

found that the hydroxylation treatment etched the surface of

glass beads. Fig. 8(c) and (d) show that, although smoothed a bit

after surface hydroxylation, quartz sands have coarser surfaces

than HGB, which is favorable for the TiO2 attachment. Fig. 8(e)

shows that the surface of SGB is smoother than HQS but

coarser than HGB.

Similar conclusions were obtained from the AFM results

shown in Fig. 9. The surface morphologies indicate that HQS

has the coarsest surface, while HGB shows the smoothest and

SGB shows a medium surface roughness. The surface

roughness is calculated by the built-in program and the data

is present in Table 4. It was shown that the roughness results are

consistent with the conclusions from SEM characterizations.

Fig. 10 shows a comparison of the FTIR spectra of HQS and

SGB with HGB. Spectra have been normalized based on the Si–

O stretching vibration peak at 1079 cm�1. It was shown that the

HQS has a similarly weak surface OH peak as HGB, while the

SGB has a much stronger�OH peak than HQS and HGB. Since

the fundamental bands of H2O and Si–OH groups cannot be

separated and interpreted quantitatively in Fig. 10, the amount

of surface adsorbed water was unknown and thus a precise

comparison of the surface Si–OH cannot be achieved. However,

two general conclusions can be obtained from Fig. 10: (1) the

HGB and HQS did not have sufficient Si–OH groups on their

surfaces, which was well supported by the FTIR spectra in

Fig. 10 that the total amount of surface OH (including the Si–

OH and the surface absorbed water) was quite limited on HGB

and HQS; (2) SGB has a higher density of surface Si–OH than

HGB and HQS, which is clear in Fig. 10 that the silica gel beads


showed a much stronger surface OH peak in the FTIR spectrum

than HGB and HQS. These conclusions also agree well with the

results reported in the literature. Takeda et al. [28] reported that

the soda-lime glass beads showed a higher surface OH density

than pure silica (quartz) glass. An extensive study of silica gel

found that silica gels generally had very high surface Si–OH

densities (�5.0 OH nm�2 [40]), which is much higher than

glass beads (�1.8 OH nm�2 [41]). de Farias and Airoldi [42]

applied a thermogravimetry technique and determined the

surface Si–OH density on the silica gel was 4.3–6.7 OH nm�2 s

covered on the surface. Since the silica gel has a high surface

Si–OH density, improved adherence stability was expected

with the SGB-supported TiO2.

The degradation results of HQS and SGB supported TiO2

(SG-450-100%) were shown in Fig. 11(a). The HGB-supported

TiO2 photocatalyst (SG-450-100%) was used as the reference

photocatalyst. The C/C0 was 48.5% after 2 h degradation for

the HGB-supported TiO2. Compared to the HGB supported

TiO2 prepared at the same conditions, both photocatalysts

exhibited lower decolorization percentage, i.e., a C/C0 of 53.9%

and 67.1% was obtained for the HQS supported TiO2 and the

SGB supported TiO2, respectively. As for the adherence

stability, a particle loss percentage of 22.7% was observed in

HQS supported TiO2. This is slightly lower than that of HGB

supported TiO2 (25.3%). A remarkably lower particle loss

percentage (11.1%) was observed in SGB supported TiO2.

From the particle loss percentage results, we can conclude that

the higher decolorization percentage observed in HGB

supported TiO2 was due to its particle loss percentage during

the degradation because more detached TiO2 particles

suspended in water will enhance the degradation. From the

result in Fig. 11(b), we can conclude that: (a) a rougher surface

texture of quartz sands was favorable for the TiO2 immobiliza-

tion, as it could provide a larger surface area for the attachment

of TiO2; (b) surface OH groups play an important role in the

immobilization of TiO2 on supports, as they may provide more

reactive Si–OH groups on the support surfaces which could

form the firm Ti–O–Si bonds with the immobilized TiO2 during

the calcination.

5. Conclusion

This study shows that the supported TiO2 photocatalysts,

prepared by immobilizing TiO2 onto hydroxylated glass beads

(HGB) via thermal bonding and sol–gel coating methods, had a

generally good activity but poor adherence stability in a

fluidized bed photoreactor. It was also found that increasing the

calcination temperature or reducing the coverage of TiO2 could

improve the adherence stability, but the improvement is rather

limited and either of these approaches lowered the activity of

photocatalysts. Slightly improved adherence stability was

observed with HQS supports, while the adherence stability was

significantly improved by using silica gel beads (SGB) as

supports. The Ti–O–Si bonds at the interface of HGB and TiO2

played an important role in the adherence stability of supported

TiO2 in terms of the bonding mechanism. Our study shows that

the adherence stability of the supported TiO2 photocatalysts,

prepared by the conventional thermal bonding and sol–gel

coating methods, should be examined and improved as much as

possible before using in the fluidized bed photoreactors.

Acknowledgements

The authors gratefully acknowledge the financial assistance

from Natural Sciences and Engineering Research Council of

Canada and New Brunswick Innovation Foundation.

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Documents

A comprehensive assessment of supported titania photocatalysts in a fluidized bed photoreactor: Photocatalytic activity and adherence stability