Synthesis, characterization, and catalytic activityof FeTiO3/TiO2 for photodegradation of organicpollutants with visible light

Marıa E. Zarazua-Morın1 • Leticia M. Torres-Martınez1 •

Edgar Moctezuma2 • Isaıas Juarez-Ramırez1 •

Brenda B. Zermeno2

Received: 24 January 2015 / Accepted: 20 April 2015

� Springer Science+Business Media Dordrecht 2015

Abstract High-purity ilmenite, FeTiO3, was prepared by the sol–gel method and

calcination under nitrogen atmosphere. Several FeTiO3/TiO2 catalysts were pre-

pared by the impregnation method using ilmenite and titania, both synthesized by

the sol–gel method with ethanol and acetic acid. FeTiO3/TiO2 photocatalysts ex-

hibited significant absorption in the ultraviolet (UV) region with an energy bandgap

between 2.9 and 3.1 eV. These materials are more active than titania (Degussa P25)

for degradation of Orange G and 4-chlorophenol under illumination from a visible-

light lamp. This effect may be attributed to the formation of a heterojunction at the

point of contact between FeTiO3 and TiO2 particles. However, 4-chlorophenol is

mineralized via formation of hydroquinone and benzoquinone, indicating that when

illuminated with a solar emulator lamp, the materials did not generate enough HO�radicals to promote formation of benzenetriol and catechol.

Keywords Ilmenite � Environmental � TiO2 � Mixed oxides � Photodegradation

Introduction

Natural ilmenite (FeTiO3) is an iron and titania mineral originally formed in

magma. It contains a wide range of impurities such as hematite (Fe2O3), wustite

(FeO), geikielite (MgTiO3), and pyrophanite (MnTiO3). Ilmenite is used as a raw

material for production of TiO2, which is one of the main ingredients in paints,

& Leticia M. Torres-Martınez

lettorresg@yahoo.com

1 Departamento de Ecomateriales y Energıa, Facultad de Ingenierıa Civil, Universidad Autonoma

de Nuevo Leon, Ciudad Universitaria, San Nicolas de los Garza, N.L. C.P. 66455, Mexico

2 Facultad de Ciencias Quımicas, Universidad Autonoma de San Luis Potosı, Av. Manuel Nava

# 6, San Luis Potosı, S.L.P. C.P. 78290, Mexico

123

Res Chem Intermed

DOI 10.1007/s11164-015-2071-9

plastics, and paper [1–3]. Pure ilmenite (FeTiO3) has rhombohedral crystal structure

in space group R-3H with hexagonal packing of the oxygen atoms occupying 2/3 of

the octahedral positions, where Fe and Ti occupy alternating layers [1, 2, 4–6].

FeTiO3 is a wide-bandgap (2.5–2.7 eV) antiferromagnetic semiconductor.

High-purity ilmenite (FeTiO3) can be prepared by solid–state reaction,

hydrothermal emulsion, and mechanochemical milling. In the first synthesis

method, the reactants must be heated up to 1200 �C under oxygen atmosphere to

prepare an iron titanate [3, 7, 8] that is not completely homogeneous. Hydrothermal

synthesis requires use of unstable alkoxides under special reaction conditions that

are difficult to maintain [9, 10]. FeTiO3 can also be prepared by grinding TiO2 in a

high-speed steel ball mill under oxygen atmosphere [11]. Fe2O3 is also formed, and

the ilmenite has low crystallinity. FeTiO3 ceramics can also be prepared by a sol–

gel process combined with a surfactant-assisted template method [12].

This material (FeTiO3) is of great interest in the fields of materials science and

engineering, having potential applications in electronic circuits, solar cells, gas

sensors, heterogeneous catalysis, and photocatalysis [3, 6, 13–17]. One research

paper has already reported use of iron titanates as heterogeneous catalysts for

reduction of NOx [17]. Gao and collaborators [18] found that commercial FeTiO3

shows strong absorption of visible light. They also reported that commercial

ilmenite mixed with TiO2 and illuminated with visible light exhibits photocatalytic

activity for degradation of 4-chlorophenol in liquid phase and 2-propanol in gas

phase. Kim et al. [10] also reported that a heterojunction catalyst of FeTiO3

prepared by the hydrothermal method and TiO2 (Degussa P25) showed good

photocatalytic activity for mineralization of 2-propanol in gas phase under visible-

light irradiation (k C 420 nm). Nanosized FeTiO3/TiO2 catalysts synthesized by a

hydrothermal method exhibited good photocatalytic activity for transformation of

CO2 to methanol under both visible and UV light irradiation [19].

On the other hand, the high concentrations of organic pollutants in domestic and

industrial wastewater, e.g., synthetic dyes, active pharmaceutical ingredients,

personal care products, disinfectants, plasticizers, solvents, and many other complex

molecules that are chemically stable and resistant to microbiological oxidation,

cannot be completed removed by conventional wastewater treatment technologies

[20–24]. It has been reported that recalcitrant organic contaminants can be

effectively mineralized by heterogeneous photocatalysis, a special class of advanced

oxidation process [22, 25–27]. Recently, three research groups [18, 28, 29] reported

that organic pollutants can be oxidized by the action of heterojunction photo-

catalysts illuminated with UV or visible light.

This research paper presents a synthesis of pure FeTiO3 by the sol–gel method

using titanium butoxide and iron nitrate as starting materials with calcination of the

gel under nitrogen atmosphere. It also presents the preparation of FeTiO3/TiO2

catalysts by the impregnation method using ilmenite and titania, both synthesized by

the sol–gel method with ethanol and acetic acid. All materials were characterized by

several analytical techniques. Their photocatalytic activity under illumination by

visible light (k C 400 nm) was determined based on the degradation of two model

organic pollutants: Orange G and 4-chlorophenol. The photocatalytic results are

M. E. Zarazua-Morın et al.

123

compared with those of the widely used commercial Degussa P25 TiO2 powder,

which is considered to be the benchmark for degradation efficiency.

Experimental

Synthesis of pure FeTiO3 and TiO2

FeTiO3 was synthesized by the sol–gel method using titanium butoxide, Fe(NO)3�9H2O, and ethanol as starting materials. First, 26.57 g Fe(NO)3�9H2O was perfectly

dissolved in 20 mL deionized water. Then, 28 mL titanium butoxide was placed in a

three-necked round flask and perfectly mixed with 40 mL ethanol under nitrogen

atmosphere to form a homogeneous solution. The iron nitrate solution was added

dropwise to the latter solution. This reaction mixture was vigorously stirred for 3 h

under nitrogen atmosphere. After the hydrolysis step, the fresh gel was dried at

100 �C for 24 h. Finally, the resulting dried powder was calcined at 700 �C for 10 h

under N2 atmosphere.

TiO2 was synthesized by the sol–gel method using titanium butoxide and ethanol

as raw materials. First, 29 mL ethanol was placed in a three-neck round flask and

perfectly mixed with 19.3 mL deionized water. Then, 42.5 mL titanium butoxide

was added dropwise to the solution. The reaction mixture was kept at 80 �C under

reflux for 48 h. The fresh gel was dried at 80 �C for 12 h, then the powder was

calcined at 400 �C for 4 h.

Synthesis of FeTiO3/TiO2 mixed oxides

FeTiO3/TiO2 photocatalysts were prepared by the impregnation method using fresh

FeTiO3and TiO2, both synthesized by the sol–gel method. Appropriate amounts of

FeTiO3 and TiO2 were mixed with a solution containing 20 mL ethanol and 2.5 mL

acetic acid to prepare FeTiO3/TiO2 materials with different compositions (95, 90,

80, and 60 wt.% TiO2). The slurry was maintained under vigorous stirring for 2 h

using a magnetic mixer operated at 300 RPM to homogenize it. The resulting slurry

was dried at 120 �C for 24 h, then the powder was calcined at 230 �C for 3 h [10].

Materials characterization

Each catalyst was characterized by different analytical techniques. Crystal structure

was determined by X-ray diffraction (XRD) analysis using a Bruker D8 Advance

diffractometer with Cu Ka radiation (k = 1.5406 A). Particle shape and size were

determined by scanning electron microscopy (SEM) using a JEOL 6490 LV device.

Prior to the analysis, the powder was stuck onto carbon tape attached to an

aluminum sample holder that was placed into the SEM chamber. Chemical

microanalysis was performed using the energy-dispersive X-ray spectroscopy

(EDS) detector of the SEM. Transmission electron microscopy (TEM) images were

obtained using a Philips CM-200 device operated at 200 kV. The energy bandgap

(Eg) values were determined from the UV–Vis spectra using a PerkinElmer

Synthesis, characterization, and catalytic activity of FeTiO3/TiO2…

123

Lambda 35 spectrophotometer coupled with an integrating sphere [30]. Addition-

ally, the specific surface area (SBET) was measured by N2 physisorption through the

Brunauer–Emmett–Teller (BET) method using Quantachrome NOVA 2000e equip-

ment. Thermogravimetric analysis (TGA) and differential scanning calorimetry

(DSC) of precursors were performed simultaneously using a TA SDTQ600

instrument. Each sample was heated from room temperature to 900 �C under a flow

of 10 mL/min N2; the rate of temperature increase was 10 �C/min.

Photocatalytic degradation experiments

The photocatalytic activity of the FeTiO3/TiO2 materials was determined based on

the degradation of an aqueous solution of Orange G under irradiation by a solar

light emulator lamp. The experiments were carried out in a photochemical reactor

system that has a Pyrex glass tube reactor (500 mL) equipped with an external water

jacket to cool down the reaction mixture to room temperature. The system was

illuminated with a Philips Xe lamp (35 W, 3200 lumens, 6000 K). The emission

spectrum of the lamp is presented in Fig. 1. The fraction of radiation with

wavelength below 300 nm can be filtered with 5 M solution of sodium nitrite. For

each experiment, 200 mL Orange G standard solution (30 ppm) was placed inside

the glass reactor and mixed with 200 mg solid catalyst. The slurry was mixed with a

magnetic stirrer. Samples were taken from time to time to monitor the progress of

the reaction by UV–Vis spectroscopy and total organic carbon (TOC) analysis.

Fig. 1 Xe lamp emission spectrum: a without cutoff filter and b with a cutoff filter of aqueous solution ofsodium nitrite

M. E. Zarazua-Morın et al.

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These chemical analyses were carried out using a PerkinElmer Lambda 35 UV–Vis

spectrometer and a Shimadzu 5000A TOC analyzer. Before analysis, each sample

was filtered with a Millipore GV membrane (0.22 lm pore diameter). The most

active FeTiO3/TiO2 photocatalysts for degradation of Orange G were also tested for

degradation of aqueous solution of 4-chlorophenol. These experiments were carried

out in exactly the same manner, but the reaction samples were also analyzed by

high-performance liquid chromatography (HPLC) using Thermo Scientific appara-

tus equipped with a UV–Vis detector and a Nova-Pack phenyl column. The mobile

phase was a mixture of 40 % ethanol and 60 % aqueous solution of citric acid and

ethylenediaminetetraacetic acid (EDTA). The mobile phase was delivered at a rate

of 1.0 mL/min, and the detection wavelength was set at 280 nm.

Results and discussion

Characterization of FeTiO3 and TiO2 prepared by the sol–gel method

The differential thermal analysis (DTA) and TGA curves of the ilmenite precursor

gel heated under nitrogen flow are presented in Fig. 2. The first part of the TGA

curve shows a weight loss below 300 �C that can be associated with loss of

occluded water in the starting material and combustion of organic material.

Furthermore, DTA shows two exothermic peaks at 226 and 268 �C, which can be

attributed to combustion of organic material and crystallization of TiO2. The broad

exothermic peak between 500 and 800 �C may be related to crystallization of iron

titanate (FeTiO3) [30].

Fig. 2 DTA and TGA curves of ilmenite precursor gel heated under nitrogen atmosphere

Synthesis, characterization, and catalytic activity of FeTiO3/TiO2…

123

Figure 3 shows the XRD patterns of FeTiO3 synthesized by the sol–gel method

and calcined at different temperatures under nitrogen atmosphere. The XRD pattern

of the sample treated at 650 �C for 5 h indicates formation of ilmenite with

rhombohedral crystal structure according to Joint Committee on Powder Diffraction

Standards (JCPDS) file 075-1208. The diffraction pattern also shows the presence of

traces of rutile. The second line in Fig. 3 shows that increase of the calcination

temperature to 700 �C improved the crystallinity of the catalyst. However, the

temperature increment also promoted formation of magnetite (Fe3O4). As the

calcination time was increased, the iron oxide was reduced to metallic iron.

Residual carbon from organic reagents may be transformed to CO, which in turn

could act as a reducing agent to transform Fe3? to Fe2? and then to Fe. The upper

line in Fig. 3 shows that pure crystalline ilmenite (FeTiO3) with major reflections at

23.97�, 33.04�, 35.67�, 40.90�, 49.32�, 53.76�, 57.09�, 62.36�, and 64.20� was

obtained after calcination at 700 �C for 10 h. Miller indices of this catalyst are also

indicated on the diffractogram. The crystal structure parameters of pure ilmenite

(FeTiO3) were determined by Rietveld refinement using Topaz software. These

results confirm that pure ilmenite has rhombohedral crystal structure with space

group R-3H.

The SEM micrograph in Fig. 4a shows irregular FeTiO3 particles of different

sizes. Big particles larger than 10 nm may be formed by sinterization during the

calcination step. The results of elemental analysis (Fig. 4b; Table 1) of this catalyst

by EDS clearly indicate that the chemical composition corresponds to iron titanate.

Fig. 3 X-ray diffraction patterns of FeTiO3 catalysts prepared at different temperatures: a calcined at650 �C for 5 h, b calcined at 700 �C for 1 h, c calcined at 700 �C for 5 h, and d calcined at 700 �C for10 h

M. E. Zarazua-Morın et al.

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Table 1 also presents the results of the chemical analysis of FeTiO3 by atomic

absorption spectroscopy, indicating an atomic relationship between Fe and Ti close

to 1. Both chemical analyses confirm that the stoichiometry of the material

corresponds to ilmenite (FeTiO3) and corroborate the X-ray diffraction results.

FeTiO3 was also characterized by TEM. The representative micrograph in

Fig. 4c shows hexagonal FeTiO3 particles with size of around 100 nm. Experimen-

tal and theoretical electron diffraction patterns of FeTiO3 are shown in Fig. 4d and

e, respectively, indicating a well-defined spot pattern obtained from the [001] axis.

These results indicate formation of phase-pure crystals of FeTiO3 with (110), (300),

and (220) planes.

Fig. 4 SEM microphotograph (a) and selected-area elemental chemical analysis (b), TEM image (c) andexperimental (d) and theoretical (e) 001 axis electron diffraction pattern of FeTiO3 calcined at 700 �C for10 h

Table 1 Elemental analysis of

FeTiO3 prepared by the sol–gel

method

AA atomic absorption

Element Oxygen Titanium Iron Total

wt.% (EDS) 32 32 36 100

at.% (EDS) 60.39 20.17 19.43 100

Atomic ratio (EDS) 3.11 1.04 1.0

wt.% (AA) – 32.0 34.0 –

Atomic ratio (AA) – 1.01 1.0 –

Synthesis, characterization, and catalytic activity of FeTiO3/TiO2…

123

The UV–Vis spectrum of ilmenite shows an absorption line in the range of

200–800 nm. The inflection point of the spectrum may be located between 400 and

470 nm. Then, the energy bandgap of FeTiO3 may have a value between 2.63 and

3.1 eV. Similar results have been reported previously [31].

The bottom line in Fig. 5 shows the X-ray diffraction pattern of TiO2 synthesized

by the sol–gel method and calcined at 400 �C. It shows main reflections at 25.28�,38.47�, and 48.37�, corresponding to TiO2 anatase phase according to JCPDS file

021-1272. The SEM and TEM micrograph presented in Fig. 6a and b, respectively,

show well-defined TiO2 nanoparticles with particle size smaller than 10 nm, while

Fig. 6c also shows the EDS chemical composition of TiO2. The bandgap energy and

specific surface area of anatase TiO2 obtained by the sol–gel method were 3.2 eV

and 137 m2/g, respectively.

Characterization of FeTiO3/TiO2 photocatalysts

The X-ray diffraction patterns of FeTiO3/TiO2 catalysts prepared by the impreg-

nation method are also presented in Fig. 5. Since these materials were prepared with

titania synthesized by the sol–gel method, the lines corresponding to the catalysts

with low ilmenite concentrations (5 and 10 %) mainly show reflections corre-

sponding to anatase phase. The X-ray diffraction pattern of the catalyst with 20 %

Fig. 5 X-ray diffraction patterns of TiO2 and FeTiO3/TiO2 with different compositions: FT5 = 5 %FeTiO3 and 95 % TiO2, FT10 = 10 % FeTiO3 and 90 % TiO2, FT20 = 20 % FeTiO3 and 80 % TiO2,FT40 = 40 % FeTiO3 and 60 % TiO2

M. E. Zarazua-Morın et al.

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FeTiO3 shows some low-intensity signals located at 24.43�, 33.24�, 35.91�, 41.06�,and 53.81� corresponding to ilmenite phase. As the concentration of FeTiO3

increases, the presence of these reflections becomes more evident.

Fig. 6 SEM (a) and TEM (b) images, and EDS (c) analysis of TiO2, and SEM (d) and TEM (e) images,and EDS (f) analysis of 10/90 (FeTiO3/TiO2) photocatalyst

Fig. 7 UV–Vis absorption spectra of a TiO2 sol–gel, b FT5, c FT10, d FT20, e FT40, and f FeTiO3

Synthesis, characterization, and catalytic activity of FeTiO3/TiO2…

123

SEM and TEM micrographs of the 10/90 (FeTiO3/TiO2) catalyst are presented in

Fig. 6d, e, respectively. Both images provide some evidence that large FeTiO3

particles are partially covered with small titania particles. Figure 6f shows the EDS

chemical composition of the FeTiO3/TiO2 catalyst.

The UV–Vis absorption spectra of TiO2, FeTiO3, and FeTiO3/TiO2 catalysts are

presented in Fig. 7. The energy bandgap (Eg) of each catalyst (Table 2) was

calculated from the Planck equation [1], and kg was obtained from the absorption

spectrum by calculating the point where an abrupt change in slope occurs [32].

Eg ¼ hc=kg: ð1Þ

Eg values were also determined using the Kubelka–Munk method [33] by

plotting the following relationship:

Khmð Þ1=2¼ f hmð Þ; ð2Þ

K ¼ 1 � Rð Þ2=2R: ð3Þ

Since the mixed catalysts have high titania content, they exhibit significant

absorption in the UV region with bandgap energy between 2.9 and 3.2 eV.

The results of the surface area measurements are also presented in Table 2. Pure

ilmenite, TiO2 prepared by the sol–gel method, and Degussa P25 titania had specific

surface area of 10, 137, and 50 m2/g, respectively. This table shows that the FeTiO3/

TiO2 catalysts have surface area values between 170 and 190 m2/g. These results

suggest that the organic acid used to prepare the mixed catalysts modified the

surface of the precursors, increasing the porosity and specific surface area.

Photocatalytic activity

To determine the photocatalytic activity of TiO2, pure ilmenite, and FeTiO3/TiO2

mixed oxides, all the reaction samples were analyzed by UV–Vis spectroscopy. The

concentration of Orange G was calculated using the absorbance values of the band

Table 2 Energy bandgap, BET surface area, kinetic parameters, and percentage degradation and min-

eralization of Orange G aqueous solution for TiO2, FeTiO3, and FeTiO3/TiO2 photocatalysts

Photocatalyst Surface

area (m2/g)

Eg (eV) K 9 1 9 102

(min-1)

t�(min)

Degradation

(%)

Mineralization

(%)

TiO2 P25 50 3.2 0.297 233.4 49.5 22

FeTiO3 10 2.63–3.1 N/a N/a 8 0

TiO2 SG 137 3.2 0.277 250.2 47 16

FeTiO3/TiO2 (5/95) 191 3.1 1.056 65.6 90.5 30

FeTiO3/TiO2 (10/90) 185 3.0 3.900 17.8 100 40

FeTiO3/TiO2 (20/80) 170 3.0 1.574 44.0 100 35

FeTiO3/TiO2 (40/60) 52 2.9 0.272 254.6 48 18

SG sol–gel, N/a not applicable

M. E. Zarazua-Morın et al.

123

associated with the color of the azo group. One of the upper lines in Fig. 8a clearly

indicates that Orange G is not degraded by a pure photochemical effect for light

with wavelength longer than 350 nm. Figure 8a also shows that pure ilmenite does

not have any photocatalytic activity. It is very well known that Degussa P25 titania

is the most active semiconductor catalyst. However, it can only be activated by UV

light with wavelength shorter than 385 nm [22, 25, 27, 33]. Since the reaction

mixture was illuminated with a small fraction of radiation with wavelength between

350 and 385 nm, Degussa P25 titania could only degrade 45 % of the organic dye in

3 h of reaction. FeTiO3/TiO2 catalysts with low ilmenite content (5/95, 10/90, and

20/80) showed higher photocatalytic activity than TiO2 P25 or TiO2 prepared by the

sol–gel method. Figure 8b shows the UV–Vis spectra of the samples in the

photocatalytic degradation of aqueous solution of Orange G experiment with 10/90

(FeTiO3/TiO2) material illuminated by visible light. This figure clearly indicates

that the absorbance of the band located at 478 nm decreases with reaction time,

indicating loss of the N=N bond of the azo group of the organic molecules.

Figure 8b also shows that the absorbance of the bands attributed to p–p* transitions

of the naphthalenic ring and SO3 groups located at 328 and 410 nm also decrease

with reaction time. These results suggest that the aromatic part of Orange G

molecules is degraded by HO� radicals generated on the surface of the semicon-

ductor catalysts. Total degradation of Orange G, indicated by total loss of color, was

reached in 2 h using 10/90 (FeTiO3/TiO2) photocatalyst.

This effect suggests that there is synergy between the FeTiO3 and TiO2. Gao

et al. [18] and Kim et al. [10] reported that FeTiO3 and TiO2 form a heterojunction

semiconductor where visible light promotes an electron of the valence band (VB) of

iron titanate, leaving behind a positively charged hole that can be occupied by an

electron transferred from the VB of TiO2. As a consequence, h? holes generated in

the VB of TiO2 have sufficient lifetime to react with adsorbed water and produce

HO� radicals to oxidize organic molecules. As the load of ilmenite is increased, the

photocatalytic activity decreases. The 20/80 (FeTiO3/TiO2) catalyst required 3 h of

Fig. 8 a Relative concentration of Orange G as a function of time during photocatalytic degradationwith TiO2, FeTiO3, and several FeTiO3/TiO2 catalysts. b Photocatalytic degradation of aqueous solutionof Orange G with 10/90 (FeTiO3/TiO2) catalyst monitored by UV–Vis spectroscopy

Synthesis, characterization, and catalytic activity of FeTiO3/TiO2…

123

reaction to degrade 100 % of the organic dye. The 40/60 (FeTiO3/TiO2) catalyst

showed almost the same activity as Degussa P25 TiO2 and TiO2 prepared by the

sol–gel method. This negative effect suggests that most of the FeTiO3 particles

cover the surface of the large TiO2 particles and do not form active heterojunctions.

Even though the concentration of Orange G was almost a linear function of

reaction time for the degradation experiments carried out with TiO2 P25, TiO2 sol–

gel, and the 40/60 FeTiO3/TiO2 materials, the specific reaction rate constant for all

the experiments was calculated assuming that the photocatalytic degradation

reactions followed a first-order reaction rate equation. Therefore, the reaction rate

constant was calculated by linear regression of a plot of the natural logarithm of

Orange G concentration as a function of reaction time. The numerical values of the

reaction rate constant and the half-life time are also presented in Table 2.

In addition, the final sample of each of the photocatalytic degradation

experiments was analyzed by TOC. The results (Table 2) confirm that the 10/90

(FeTiO3/TiO2) catalyst was the most active semiconductor for degradation and

mineralization of Orange G under illumination by a visible light lamp.

Since phenol and halogenated phenols are used as model compounds to study

photocatalytic degradation of organic industrial pollutants [18, 31, 34], the most

active FeTiO3/TiO2 catalysts were also tested for degradation of aqueous solution of

4-chlorophenol.

Figure 9a shows the UV–Vis spectra of the reaction samples for photocatalytic

degradation of 4-chlorophenol with the 10/90 (FeTiO3/TiO2) catalyst. As expected,

the absorbance of the bands associated to the aromatic ring located at 225 and

280 nm decreased with reaction time. However, the aromatic ring of 4-chlorophenol

was not completely transformed to other nonaromatic compounds. According to

these experimental results, only 40 % of the aromatic compounds was degraded in

Fig. 9 a Photocatalytic degradation of aqueous solution of 4-chlorophenol with 10/90 FeTiO3/TiO2

catalyst monitored by UV–Vis spectroscopy. b Degradation of reactant and subsequent formation anddisappearance of intermediate organic reaction products as a function of time during photocatalyticdegradation of 4-chlorophenol with 10/90 FeTiO3/TiO2 catalyst. Samples were analyzed by HPLC

M. E. Zarazua-Morın et al.

123

3 h of reaction. Furthermore, the samples of the degradation of 4-chlorophenol with

the 10/90 FeTiO3/TiO2 catalyst were also analyzed by HPLC. Figure 9b shows that

almost 40 % of 4-chlorophenol was degraded to other chemical species. It also

shows the presence of small amounts of phenol, benzoquinone, and hydroquinone.

Previous studies have demonstrated that 4-chlorophenol is transformed to phenol

and hydroquinone by abstraction and substitution of the chlorine atom [31, 34, 35].

Then, these aromatic products can be converted to benzoquinone and benzenetriol,

which are transformed to other nonaromatic organic molecules before being

mineralized to CO2 and water. Benzenetriol was not detected by HPLC analysis of

the reaction samples, indicating that it was consumed immediately after it was

formed. Figure 10 confirms that the FeTiO3/TiO2 catalysts with low ilmenite load

were more active than Degussa P25 TiO2 for photocatalytic degradation of

4-chlorophenol under illumination by a visible light lamp.

Conclusions

High-purity ilmenite, FeTiO3, prepared by the sol–gel method and calcined at high

temperature in nitrogen atmosphere showed rhombohedral crystal structure in space

group R-3H. Chemical analysis by EDS and atomic absorption spectroscopy clearly

Fig. 10 Photocatalytic degradation of aqueous solution of 4-chlorophenol with TiO2 and two FeTiO3/TiO2 catalysts. Samples were analyzed by UV–Vis spectroscopy

Synthesis, characterization, and catalytic activity of FeTiO3/TiO2…

123

indicated that the stoichiometry of the material corresponds to ilmenite. Dark-black

ilmenite shows an absorption line in the range of 200–800 nm and has a bandgap

located between 2.6 and 3.2 eV.

X-ray diffraction patterns of FeTiO3/TiO2 catalysts prepared by the impregnation

method showed some low-intensity signals of the ilmenite phase and the signals of

the anatase phase. SEM and TEM micrographs provided some evidence that large

FeTiO3 particles are partially covered with small titania particles, forming a

heterojunction semiconductor photocatalyst.

FeTiO3/TiO2 mixed oxide with low ilmenite content showed higher photocat-

alytic activity for degradation of Orange G and 4-chlorophenol than Degussa P25

TiO2 or TiO2 prepared by the sol–gel method. This effect is attributed to the

formation of a heterojunction at the point of contact of the two semiconductor

photocatalysts.

Acknowledgments The authors would like to thank CONACYT for financial support through projects

FOINS/75/2012 Fotosıntesis Artificial, CNPq Mexico-Brasil Nanotecnologıas 2011-174247, CB-168730,

ProyectoRedes 2012 clave 194451, CB-2008-01-103532 and Ph.D. Scholarship No. 87101. We also thank

the Universidad Autonoma de Nuevo Leon for its support through projects PAICYT-UANL-2011–2012.

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