Preparation of nanocrystalline TiO2-coated coal fly ash and effect of iron oxides in coal fly ash on photocatalytic activity

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Powder Technology 14

Preparation of nanocrystalline TiO2-coated coal fly ash and effect of iron

oxides in coal fly ash on photocatalytic activity

Yeon-tae Yu

Division of Advanced Materials Engineering, College of Engineering, Chonbuk National Univ., Jeonju, 561-756, Korea

Received 18 February 2003; received in revised form 6 June 2004; accepted 29 June 2004

Available online 21 September 2004

Abstract

In order to more easily separate TiO2 photocatalyst from the treated wastewater, TiO2 photocatalyst is immobilized on coal fly ash by a

precipitation method. The titanium hydroxide precipitated on coal fly ash by neutralization of titanium chloride is transformed into titanium

dioxide by heat treatment in the temperature range of 300–700 8C. The crystalline structure of the titanium dioxide shows anatase type in all

range of heat treatment temperature. The crystallite size of anatase increases with increasing heat treatment temperature, with the draw back

that the removal ability of NO gas will decrease. When the coal fly ash coated with 10 wt.% of TiO2 is calcined at the temperatures of 300

and 400 8C for 2 h, the crystallite sizes of anatase appeared about 9 nm, and the removal rate of NO gas shows 63% and 67.5%, respectively.

The major iron oxide, existing in coal fly ash as an impurity, is magnetite (Fe3O4). The phase transformation into hematite (Fe2O3) by heat

treatment improves the removal rate of NO gas for TiO2-coated coal fly ash.

D 2004 Published by Elsevier B.V.

Keywords: Photocatalyst; Titanium dioxide; Coal fly ash; Iron oxide

1. Introduction

The photocatalytic detoxification of organic and inor-

ganic compounds is very promising for the purification of

polluted air and industrial wastewater and thus has attracted

extensive attention during these 20 years. Due to its stability,

nontoxicity and low cost, TiO2 has been the most inves-

tigated photocatalyst. The band gap of this semiconductor is

3.2 eV, which corresponds to radiation wavelength of

around 380 nm; therefore, a UV light with wavelength

shorter than 380 nm is needed to excite electrons in valence

band to conduction band. The electron-hole pairs thus

generated serve as the oxidizing and reducing agents [1–3].

A photocatalyst is generally suspended in wastewater for

the degradation of pollutants and therefore requires an

additional separation step to remove the catalyst from the

treated water. Removing the photocatalyst from large

volumes of water is very difficult because of their small

0032-5910/$ - see front matter D 2004 Published by Elsevier B.V.

doi:10.1016/j.powtec.2004.06.006

E-mail address: [email protected].

particle size [4 5]. This represents a major hindrance to the

application of the photocatalytic process for treating waste-

water. To minimize this problem, research has been carried

out by immobilizing titania onto various porous substrates,

such as active carbon, silica or zeolite [6–11]. While this

approach provides a solution to the solid–liquid separation

problem, the porous materials are not cheap because they

are artificially synthesized. To treat the huge amount of

industrial wastewater by utilizing photocatalyst, a cheaper

photocatalyst should be developed.

Fly ash is generated in dry form in large quantities as a

by-product of thermal power generation plants and thus is a

major source for environment pollution. Currently, large

quantities of fly ash are landfilled. Research is in progress to

find out the various ways to utilize this by-product to

prevent any environmental problems as well as effectively

use them. Fly ashes are highly dispersive powders. They

consist mainly of silica-aluminate, ferriferous glass (about

60–80 wt.%), quartz and unburned carbon (about 2–5

wt.%). Their particle shape is spherical and their average

particle size is about 20 Am. Their phase and composition

6 (2004) 154–159

Y. Yu / Powder Technology 146 (2004) 154–159 155

are quite stable in air and water if they are not mixed with

alkali materials. They are very cheap. Thus, fly ash can be

useful as a substrate for TiO2 photocatalyst for purifying

pollutants in air and wastewater.

In the present study, TiO2 was supported on a coal fly ash

to provide a cheaper titania-immobilized photocatalyst, and

the feasibility of coal fly ash as a supporter of TiO2

photocatalyst was investigated. The effect of iron oxide

existing in a coal fly ash as an impurity on the photocatalytic

activity of TiO2 was discussed.

Fig. 1. Schematic diagram of the apparatus for testing NO removal rate. (a

UV lamp, (b) reaction vessel, (c) TiO2-coated coal fly ash, (d) metal filter

(e) circulation pump, (f) rubber bag, (g) flow meter, (h) gas inlet valve, (i

quartz plate.

2. Experimental procedures

2.1. Preparation of TiO2-coated coal fly ash

The TiO2 photocatalyst was coated on a coal fly ash by

precipitation technique. TiCl4 (98.0%, Kanto Chemical)

was used as a start material for coating TiO2, and HCl

(36.5%, Kanto Chemical) was added into water before

diluting TiCl4 solution to prevent the formation of

orthotitanic acid; Ti(OH)4 was generated when TiCl4 was

diluted in water. The mole ratio of TiCl4 to HCl was 1:2.5.

To prepare the coal fly ash coated with 10 wt.% of TiO2,

36 g of coal fly ash was suspended in the 0.2 M TiCl4aqueous solution of 250 ml, and then 163 ml of 2 M

NH4HCO3 aqueous solution was added drop-wise as a

precipitant to the TiCl4 solution under vigorous stirring. In

this reaction of neutralization, Ti(OH)4 was deposited on

the surface of coal fly ash. The product was then washed

several times with distilled water and then filtered off and

vacuum-dried. The final product was calcined at the

temperature range of 300–700 8C for 2 h to dehydrate

Ti(OH)4. The coal fly ash was used without any pretreat-

ment in this experiment and its average particle size was

20 Am. The chemical content of coal fly ash is shown in

Table 1.

2.2. Characterization of products

The morphology of TiO2-coated coal fly ash was

observed by a scanning electron microscope (SEM, JEOL

5410). The chemical composition of coal fly ash was

analyzed by an inductively coupled plasma (ICP, OTSUTA

Electronics ELS-8000). X-ray powder diffraction patterns

of the samples were obtained using a Phillips diffracto-

meter and monochromated high-intensity CuKa1 radiation

(k=1.5405 2). The crystallite size of TiO2 coated on coal

fly ash was calculated from the peak width by using the

Scherrer equation, D=kk/(bcos h), where D is the

Table 1

Chemical compositions of coal fly ash (wt.%)

SiO2 Al2O3 Fe3O4 CaO MgO K2O Na2O TiO2 MnO2 P2O5 Loss on ignition (LOI

66.71 19.08 8.49 2.00 0.39 0.73 0.01 0.99 0.06 0.25 3.6

)

,

)

crystallite size, k is a shape factor (a value of 0.9 was used

in this study), k is the X-ray radiation wavelength (1.5405

2 for CuKa1) and b was determined from the experimental

integral width by applying standard corrections for the

effects of Ka1–Ka2 separation and instrumental broadening.

2.3. Measurement of photocatalytic activity

For the photocatalytic degradation experiment, the

reactor for the removal rate test of NO gas was used in

the study as shown in Fig. 1, in which an 8-W UV lamp

(Vilber Lourmat, VL-4LC, 254 nm) was positioned upon

the quartz glass (denoted as i) of the reactor. The 10 wt.%

TiO2-coated coal fly ash of 0.1 g was spread on the filter

paper (ID 65 mm) which was supplied upon a metal filter

(denoted as d). The standard gas of NO was introduced into

the reactor via a three-way valve (denoted as h), and then

air was supplied until two rubber bags for sampling gas

were full. The initial concentration of NO gas in the reactor

was controlled at 4–5 ppm. Prior to photoreaction, the

mixed gas was circulated in a dark condition for 2 h by a

diaphragm air pump (denoted as e) to establish an

adsorption/desorption equilibrium condition and the homo-

geneous mixing of NO gas in the reactor. The content of

NO gas was analyzed by the gas detector tube (GASTEC,

NO.11 L) of nitrogen oxides (NO+NO2) after 28 min from

irradiating UV to the TiO2-coated fly ash for photoreaction.

Nitrogen oxide is easily oxidized to nitrogen dioxide.

Nitrogen dioxide reacts with diphenylamine in the detector

tube and then produces p-nitroso-diphenylamine in which

the color is yellowish orange. The detector tube indicates

the total content of NO and NO2 with color change. The

measuring range of the detector tube is 0.04–16.5 ppm and

the detecting limit is 0.01 ppm.

)

Fig. 3. X-ray diffraction patterns of TiO2-coated coal fly ash according to

heat treatment temperature.

Y. Yu / Powder Technology 146 (2004) 154–159156

3. Results and discussion

3.1. Preparation of TiO2-coated coal fly ash

The SEM image of TiO2-coated coal fly ash and EDX

results at various spots on fly ash are illustrated in Fig. 2.

From these, the small and big spheres are coal fly ashes, and

TiO2 is found on the surface of these spheres. The coating

layer of TiO2 was not uniform as can be seen in the

photograph. It was found that the amount of TiO2 found at a,

b and c is much more than at d.

Fig. 3 is the XRD patterns of TiO2-supported coal fly

ash according to different temperatures of heat treatment.

The crystalline structure of TiO2 coated on coal fly ash

showed a typical anatase and was well retained in the

temperature range of 300–700 8C. Many studies have

reported that pure anatase converted to rutile above about

500 8C at a slow rate. In general, anatase-based material

transformed to rutile depending on the temperature and

time. However, the presence of different dopants may

strongly affect the kinetics of this process. Additives such

as CuO, MnO, CoO, NiO and Sb2O3 accelerate the

transformation of anatase to rutile [12,13]. Conversely,

Cr2O3, WO3 and rare-earth oxides stabilize the anatase

[14,15]. Fly ash has a lot of impurities. Therefore, it is

Fig. 2. SEM photograph of TiO2-coated coal fly

possible that these impurities stabilize anatase even above

500 8C. However, further study is needed for understanding

what element retards the anatase transformation to rutile in

fly ash system.

The peak corresponding to the 101 plane of anatase

TiO2 appeared at 2h=25.48, and the peaks of mullite of

ash and EDX results at a, b, c and d spot.

Fig. 6. X-ray diffraction patterns of raw fly ash (a), nonmagnetic fraction of

the fly ash (b) and magnetic fraction of the fly ash (c). E, mullite; x,quartz; ., Fe3O4.

Fig. 4. The change of crystallite size of TiO2 on coal fly ash according to

heat treatment temperature.


coal fly ash appeared at 2h=268 and 26.58. The peak of

anatase was broader at the lower temperature of heat

treatment, and this intensity grew with increasing the

temperature of heat treatment. This result implies that the

crystallinity and the crystallite size of the anatase TiO2

supported on fly ash increase as the temperature of heat

treatment rises. The crystallite sizes of anatase calculated

from Scherrer’s equation are plotted in Fig. 4 as a

function of heat treatment temperature. The crystallite

size of TiO2 was about 9 nm in the temperature range of

300–400 8C and started to grow rapidly at 600 8C.The photocatalytic activity of TiO2-coated coal fly ash

was evaluated by the removal ability of NO gas. Fig. 5

shows the change of NO gas removal rate as a function

of the crystal size of TiO2 coated on the surface of fly

ash when the samples were irradiated for 28 min. The

removal rate of NO gas decreased with increasing the

crystal size. This is probably due to the lower surface

area of the TiO2 coated on fly ash. The removal rate

showed a maximum value of 67.5% when the crystallite

size was 9 nm.

Fig. 5. The NO gas removal rate of TiO2-coated coal fly ash according to

crystal size.

3.2. Effect of iron oxide on photocatalytic activity

The effect of iron oxide in fly ash on the photocatalytic

activity of TiO2 was studied by partially removing it with

magnetic separation and HCl leaching. First, coal fly ash

was divided into two parts, a nonmagnetic fraction and a

magnetic fraction. The nonmagnetic fraction of the fly ash

was leached in 1 M HCl solution at 70 8C for 2 h to

eliminate a partial iron oxide left on the surface of coal fly

ash. With this process, the content of iron oxide in the

nonmagnetic fraction of fly ash was reduced from 8.5 to

4.6 wt.%.

Fig. 6 shows the X-ray diffraction patterns of a raw fly

ash (with 8.5 wt.% Fe3O4), a nonmagnetic fraction fly ash

(with 4.6 wt.% Fe3O4) and a magnetic fraction fly ash (rich

with Fe3O4). The crystallite structure of the raw fly ash is

mainly composed of quartz and mullite. The X-ray

diffraction pattern of the nonmagnetic fraction fly ash is

not much different when compared to that of the raw fly ash.

This is due to the iron oxide left in fly ash. The X-ray

Fig. 7. X-ray diffraction patterns of nonmagnetic fraction fly ash according

to heat treatment temperature. 5; mullite, .; Fe3O4, o; Fe2O3.

Fig. 8. The NO gas removal rate of TiO2-coated coal fly ash according to

the preheating temperature for transforming magnetite to hematite.

Y. Yu / Powder Technology 146 (2004) 154–159158

diffraction pattern of the magnetic fraction fly ash composed

of the peaks of coal fly ash and iron oxide of magnetite

phase. This result indicates that the crystallite structure of

iron oxide is mainly a magnetite. The nonmagnetic fraction

fly ash was coated with 10 wt.% TiO2 and calcined at 300

8C for 2 h, and the photocatalytic activity was estimated.

However, the photocatalytic activity was not improved as

compared with that before reducing the amount of iron

oxide in fly ash.

To improve the photocatalytic activity of TiO2 coating,

the nonmagnetic fraction fly ash was calcined to transform

magnetite to hematite before coating TiO2 on fly ash. This

preheating treatment of fly ash was done in the temperature

range of 400–700 8C, and the samples were heated for 2 h at

each 100 8C temperature range. The X-ray diffraction

patterns of the samples are shown in Fig. 7. The diffraction

peaks of magnetite and hematite doubled compared with

those of mullite in coal fly ash except for a few peaks. The

independent peaks of magnetite appear at 2h=248 and 648,and the independent peaks of hematite occur at 2h=30.58and 57.28. The peaks corresponding to hematite appeared to

a small extent in the X-ray diffraction profile of the sample

calcined at 400 8C. The intensity of these peaks grew with

increasing temperature, whereas the peak intensity of

magnetite decreased with increasing temperature. However,

the magnetite was not perfectly transformed to the hematite,

although the temperature of heat treatment increased up to

700 8C.The coal fly ashes calcined at various temperatures

were coated with Ti(OH)4 corresponding to 10 wt.% of

TiO2 by the same coating method mentioned in Section

2.1, and the Ti(OH)4 deposited on fly ashes was calcined at

300 8C for 2 h again for dehydration. Over these samples,

the photocatalytic activity was estimated by measuring the

removal rate of NO gas. The rate increases as the preheating

temperature of coal fly ash increases, as shown in Fig. 8.

The maximum removal rate was found to be 95% at the

temperature range of 600–700 8C when the samples were

irradiated by UV lamp for 28 min. These results indicate

that the transformation of magnetite in fly ash to hematite

contributes to the increase of photocatalytic activity of TiO2-

coated fly ash. In fact, the iron oxide can be present as

magnetite, maghematite and hematite or as a mixture of

these phases, depending on the calcination conditions. The

band gap values are different for each of the oxides,

magnetite (0.1 eV), maghematite (2.3 eV) and hematite (2.2

eV). Taking into account of the narrower band gap of

magnetite, this is believed to lead to an increase in the

incidence of electron-hole recombination [16–19].

However, the removal rate of NO gas did not increase

above the preheating temperature of 600 8C (Fig. 8). This is

due to the fact that TiO2 photocatalyst is present on only the

surface of fly ash, and all iron oxides existing on the surface

were transformed into hematite by heat treatment of 600 8C.Thus, in Fig. 7, it was shown that the peaks of magnetite

existing on the X-ray diffraction profile of the sample

calcined at 700 8C referred to the iron oxide of magnetite

phase being present inside of coal fly ash.

The commercial photocatalyst, P-25, showed a removal

rate of 100% when the irradiation time of UV lamp was

28 min. However, although the photocatalytic activity of

TiO2 on fly ash is a little lower than that of P-25, it was

found that the TiO2-coated coal fly ash could be used as a

photocatalyst for removing various pollutants in wastewater

and air.

4. Conclusions

TiO2-coated coal fly ash could be prepared by precip-

itation from TiCl4 aqueous solution in this study. The

crystalline structure of TiO2 supported on coal fly ash was

present in anatase phase, and its crystallite size was about 9

nm in the heat treatment temperature range of 300–400 8C.The removal rate of NO gas for the TiO2-coated coal fly ash

was 63–67.5% in the same temperature range when UV was

irradiated for 28 min for photoreaction.

The crystalline structure of iron oxide in coal fly ash

showed magnetite phase, which was partially transformed to

hematite by heat treatment in the temperature range of 400–

700 8C. This phase transformation contributed to the

improvement of photocatalytic activity of TiO2-coated coal

fly ash. The removal rate of NO gas for the TiO2-coated coal

fly ash, for which iron oxide was transformed into hematite,

increased up to 95%.

From the results above, it was found that the coal fly ash

could be a good supporter for a TiO2 photocatalyst.

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Preparation of nanocrystalline TiO2-coated coal fly ash and effect of iron oxides in coal fly ash on photocatalytic activity