improvement of catalytic activity in styrene oxidation of carbon

Improvement of catalytic activity in styrene oxidation of carbon-coated titaniaby formation of porous carbon layer

Surya Lubis a,b, Leny Yuliati a, Siew Ling Lee a, Imam Sumpono c, Hadi Nur a,⇑a Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysiab Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Syiah Kuala, Darussalam, Banda Aceh 23111, Indonesiac Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Negeri Semarang, Kampus Sekaran, Gunungpati, Semarang 50221, Indonesia

h i g h l i g h t s

" Carbon-coated titania was prepared by pyrolysis of polystyrene-coated titania." Polystyrene-coated titania was produced by in situ polymerization of styrene by using aqueous hydrogen peroxide." Porous carbon layer has been formed by treating the carbon-coated titania with KOH solution." The porous carbon-coated titania obtained exhibits a high catalytic activity for styrene oxidation.

a r t i c l e i n f o

Article history:Received 25 April 2012Received in revised form 29 July 2012Accepted 14 August 2012Available online 21 August 2012

Keywords:Polystyrene-coated titaniaCarbon-coated titaniaPorous carbon-coated titaniaKOH treatmentOxidation of styrene

a b s t r a c t

Porous carbon layer has been formed by treating the carbon-coated titania (C@TiO2) with KOH solution.Carbon-coated titania (C@TiO2) was obtained by pyrolysis of polystyrene-coated titania (PS@TiO2), whichwas produced by in situ polymerization of styrene by using aqueous hydrogen peroxide. The presence ofpolystyrene and carbon on the surface of titania were confirmed by X-ray photoelectron spectroscopy(XPS) and Fourier transform infrared (FTIR) spectroscopy techniques. Carbon content was about2.2 wt.% with thickness of carbon layer ca. 5 nm. After treating with KOH solution, PC@TiO2 with the poresize of ca. 5 nm, total pore volume of 0.05 cm2 g�1 and Brunauer–Emmett–Teller (BET) specific surfacearea of 46 m2 g�1 has been obtained. Catalytic activity results showed that PC@TiO2 gave a higher activityin styrene oxidation compared to bare TiO2, and C@TiO2. The highest catalytic activity was obtained byusing PC@TiO2 that obtained after treating C@TiO2 with 1.0 M KOH solution with benzaldehyde andphenylacetaldehyde as the main reaction products. At the higher concentration of KOH solution, thecatalytic activity decreased when crystallinity of TiO2 decreased.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Titanium dioxide or titania, with chemical formula TiO2, is anubiquitously used material in everyday life. It has been reportedthat titanium dioxide is used as white pigment [1], sunscreenand UV absorber [2], electronic data storage medium [3], photocat-alyst [4,5], a material in the memristor [6] and Bragg-stack styledielectric mirrors [7]. In addition to this application, titanium diox-ide can also be used as a heterogeneous catalyst for synthesis ofuseful organic compound [8].

Many attempts have been made to improve the catalytic activ-ity of titanium dioxide. One of the strategies is by coating thesurface of titania by carbon layer. Carbon-coated titania particlescan stabilize anatase structure and increase adsorption amount

of organic molecules on the catalyst surface [9]. Several methodshave been applied to coat the surface of TiO2 with carbon layer.Preparation of carbon-coated TiO2 by heating the mixture of poly-vinyl alcohol (PVA) and commercial TiO2 (P-25) with the ratio of10/90 under argon atmosphere at 800 �C gave a carbon layer withthe proper thickness (2–3 nm). Compared to the original P-25 TiO2

catalyst, carbon-coated TiO2 have much higher adsorption capacitybut lower photocatalytic activity. The performance of their photo-decomposition was lower than P-25 TiO2 because of the lower dis-persion of carbon-coated TiO2 in water and less penetration of UVlight by the coated carbon layer reduced the photocatalytic activitybecause the light reaching the surface of TiO2 particles decreased[10–12]. It also has been reported that the carbon coating on thesurface of TiO2 gave a beneficial effect to the catalytic activity. Car-bon-coated TiO2 prepared by mixing TiO2 powders and polyethyl-ene glycol (PEG) exhibits higher photocatalytic activity in benzenedegradation than that of the pristine TiO2 because of the adsorp-tion enrichment of benzene by carbon around the TiO2 particles

1385-8947/$ - see front matter � 2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2012.08.041

⇑ Corresponding author. Tel.: +60 7 5536162; fax: +60 7 5536080.E-mail address: [email protected] (H. Nur).URL: http://www.hadinur.com (H. Nur).

Chemical Engineering Journal 209 (2012) 486–493

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and of the effective charge separation due to the electronic conduc-tion of carbon [13].

As generally known, the first step in heterogeneous catalysis isthe adsorption of the molecules of the reactants on the surface ofTiO2 catalyst. It is believed that the carbon layer improved theadsorption of substrate on the surface of catalyst [14]. First, thesubstrate molecule have to be adsorbed into carbon layer and thento diffuse to the surface of catalyst particle to be reacted. However,this cannot be achieved when the carbon layer is too thick and pre-venting the substrates to adsorb on the surface of titania. To over-come this problem, one of the strategies is to make the porouscarbon layer on the surface of titania particles [12,15,16]. Thus,the substrate may be channelled through the pore system of por-ous carbon layer to titania surface.

In this work, we herein report a simple method for synthesis ofporous carbon-coated TiO2 by treating the carbon-coated titaniawith KOH solution. Carbon-coated titania (C@TiO2) was obtainedby pyrolysis of polystyrene-coated titania (PS@TiO2) that producedby in situ polymerization of styrene using aqueous hydrogen per-oxide. It has been reported that the graphitization of carbon wasinfluenced by KOH treatment [17]. One expects that the carbonlayer become porous after this treatment. Here, we demonstrateda method to synthesize porous coated titania. The presence of por-ous carbon in the surface of titania gave the beneficial in liquidphase oxidation of styrene with aqueous hydrogen peroxide.

The oxidation of styrene by using aqueous hydrogen peroxide isof considerable interest for academic research and utilization in

the industry for the synthesis of important products such as benz-aldehyde, styrene oxide and phenylacetaldehyde. The use of theaqueous hydrogen peroxide for oxidizing organic substrates has re-ceived much attention because of its environmental implications,water being the only chemical by-product of oxidation reactions.It is known that the styrene oxidation by hydrogen peroxide atthe side chain can lead to various reaction products, dependingon the catalyst and reaction conditions. Two major reactions takeplace during styrene oxidation depending on the nature of the cat-alyst and the reaction conditions. They are the oxidative C@Ccleavage into benzaldehyde and epoxidation followed by isomeri-sation into phenylacetaldehyde [18].

2. Experimental

2.1. Preparation of polystyrene-coated TiO2 and carbon-coated TiO2

Polystyrene-coated TiO2 (PS@TiO2) was prepared by using TiO2

(supplied by Aldrich with over 99.9% purity), styrene and aqueoushydrogen peroxide. Typically, 30% aqueous hydrogen peroxide(1 ml) was added drop wise to 1 g of TiO2 powder and then styrenewas added to the mixture with the molar ratio of styrene to TiO2

was 3:1. The mixture was heated under stirring at 100 �C for 8 h.The product was then washed with ethanol, sonificated and cen-trifugated three times and dried at 100 �C overnight. The polysty-rene-coated TiO2 obtained labeled as PS@TiO2. The PS@TiO2

TiO2

H2O2

adsorption

In-situ polymerization of

styrene

(a)

(c)

Pyrolysis at 450 °C

H2O2

Polystyrene

CarbonKOH treatment at

450 °C

(d) Porous carbon

(b)

PS@TiO2

TiO2

TiO2

TiO2

TiO2

C@TiO2PC@TiO2

TiO2

100 nm 5 nm 100 nm 5 nm

100 nm 5 nm 100 nm 5 nm

FESEM TEM FESEM TEM

FESEM TEM FESEM TEM

(a) (a) (b) (b)

(c) (c) (d) (d)

Fig. 1. Schematic diagram of the preparation of PS@TiO2, C@TiO2 and PC@TiO2 particles and their FESEM and TEM photographs.

S. Lubis et al. / Chemical Engineering Journal 209 (2012) 486–493 487

obtained was then pyrolyzed at 450 �C for 1 h with the heating ratewas 10 �C min�1 in flow nitrogen to produce carbon-coated titania(C@TiO2). In order to produce porous-carbon coated TiO2, theC@TiO2 (0.3 g) was treated with KOH solution (2 ml) in a variousconcentrations: 0.5 M; 1.0 M; 1.5 M and 2.0 M. After heating at450 �C under a N2 flow for 1 h, the treated sample was washedwith deionized water until neutral (pH = 7), dried at 100 �C over-night and labeled as PC@TiO2.

2.2. Characterizations

The FTIR spectra of samples were taken by the following proto-col. The samples were mixed with dry KBr (1:100 ratio) in a mortarand pressed at 8 tons for 3 min to obtain the self-supporting pel-lets. All measurements were performed at an ambient temperatureover the range 400–4000 cm�1 using Perkin–Elmer Spectrum spec-trometer. Diffuse reflectance UV–Vis (DR UV–Vis) spectra were re-corded under ambient conditions on a Perkin Elmer Lamda 900 UV/VIS/NIR spectrometer. The spectra were monitored in the range ofwavenumber 200–450 nm.

The morphology and structural investigation were performedby field emission scanning electron microscope (FESEM) and trans-mission electron microscope (TEM). Prior to sample scanning, thepowder sample was attached to sample holder by using double-sided tape. The samples were then coated with platinum, and theimage was taken on a JOEL JED-2300 analysis station.

The TEM images of bare TiO2, carbon-coated TiO2 before andafter treating with KOH solution were taken with a JEOL JEM-2100 transmission electron microscope. Specimens were preparedby grinding the material into a fine powder, dispersing the powderin acetone under the ultrasonic bath for 30 min. The dispersionwas then dropped on a carbon copper grid follow by drying atroom temperature. The microscope was operated at an accelerat-ing voltage of 200 kV. The X-ray photoelectron spectroscopy(XPS) measurements were performed on a high resolution Augerelectron spectrometer AXIS ULTRA (Kratos Analytical, Shimadzu)with monochromatic Al Ka X-ray.

Thermogravimetric analysis of carbon-coated TiO2 was carriedout in TGA-SDTA 851e METTLER thermogravimetry instrument.The mass of sample was 10 mg and the reaction environmentwas flowing nitrogen with the heating rate was 10 �C min�1. Thecarbon content of sample was determined from the weight lossof sample after heating from room temperature to 600 �C in air.Carbon content was determined by calculating the weight loss ofsample.

Specific surface area of samples was determined by nitrogenadsorption instrument (Micromeritics ASAP 2010) utilizing theBrunauer–Emmett–Teller (BET) method [19]. The sample wasdegassed at 150 �C overnight prior to the measurement. Pore sizedistribution was calculated from the adsorption/desorption branchof isotherms by the Barrett–Joyner–Halenda (BJH) model [20].

The X-ray diffraction pattern of samples was collected on a Bru-ker AXS D8 Automatic Powder Diffractometer using the Cu Ka radi-ation with k = 1.5406 Å at 40 kV and 40 mA, over the range 2h = 2–90� at step 0.030 and step time 1s (scanning speed of 1.2� min�1).The particle size of the samples was calculated by using the Scher-rer’s equation based on XRD data:

L ¼ Kk=b cos h

where K is the shape factor, k is the wavelength of the X-ray used,b is the full-width at half-maximum (FWHM) and h is the anglebetween the incident and the scattered X-ray.

(b)

(a)

(c)

(d)

Inte

nsity

/ a.

u.

Wavenumber / cm–1

–CH aromatic C=C

–CH aromatic out of

plane

3200 2400 1800 1400 1000 6004000

Fig. 2. FTIR spectra of: (a) TiO2, (b) polystyrene, (c) PS@TiO2 and (d) PC@TiO2.

Wavelength / nm

K -

M /

a.u.

200 240 280 360320 400 440

(a)

(b)

(c)

(d)

Fig. 3. DR UV–Vis spectra of: (a) TiO2, (b) polystyrene, (c) PS@TiO2 and (d) C@TiO2.

488 S. Lubis et al. / Chemical Engineering Journal 209 (2012) 486–493

2.3. Catalytic activity in styrene oxidation

Catalytic activity of samples obtained were evaluated in the oxi-dation of styrene using 30% aqueous H2O2 as an oxidizing agent.The reaction mixture containing styrene (5 mmol), catalyst(50 mg), aqueous H2O2 (8 mmol) and acetonitrile (5 ml) as solventwere put in a round-bottom flask equipped with condenser. Thereaction was carried out in oil bath under stirring at 80 �C for8 h. The reactant and products of the reaction were analyzed bya Gas Chromatography (HP Agilent 6890N gas chromatography)with a capillary column (Ultra-2, l 25 m, Ø 0.32 mm � 0.52 lm)using a flame ionization detector. A column flow of 3 ml min�1

with He as carrier gas was used. The GC column rate program used:80 �C for 1 min followed by heating to 150 �C at 10 �C min�1. Thefinal temperature (150 �C) was then kept for an additional

0.5 min. The product yield and styrene conversion were calculatedaccording to the way of previous report [21].

3. Results and discussion

3.1. Physical properties

Fig. 1 shows schematic diagram of the preparation of PS@TiO2,C@TiO2 and PC@TiO2 particles and their FESEM and TEM photo-graphs. Hydrogen peroxide can be easily adsorbed on the TiO2 sur-face due to hydrophilicity of TiO2, allowing the formation ofpolystyrene on the surface of TiO2. The polystyrene layer was thenconverted into the carbon layer by pyrolysis. The PC@TiO2 is pre-pared after treating C@TiO2 with KOH solution. Fig. 1a shows thatTiO2 particles had irregular shape and are in the form of aggrega-tion. As shown in FESEM images in Fig. 1a, c and d, there is nochange in the shape of PS@TiO2, C@TiO2 and PC@TiO2 particlesafter modification. The transmission electron micrograph of bareTiO2, carbon-coated titanium dioxide before and after treating withKOH is given in the Fig. 1. The C@TiO2 image (Fig. 1b) shows a darkpart that are reasonably supposed to be TiO2 particles, which weresurrounded by a thin layer with faint contrast, which are believedto be a carbon layer. Carbon content of sample is about 2.2 wt.%with thickness of carbon layer that covered titanium dioxide sur-face is about ca. 5 nm. Fig. 1d shows FESEM and TEM images ofPC@TiO2 after treated with 1.0 M KOH solution. It can be seen thatthe carbon layer has been changed into a porous form.

The existence of polystyrene on the TiO2 surface was also veri-fied by FTIR spectrometer. FTIR spectrum of the PS@TiO2 is shownin Fig. 2c. The characteristic bands of polystyrene were clearly ob-served in polystyrene-coated titanium dioxide sample: CAH aro-matic stretching vibration at 3058 and 3024 cm�1, CAHstretching vibration at 2920 and 2847 cm�1, aromatic CAC stretch-ing vibration around 1451–1492 cm�1, CAH out of plane bend at757 cm�1 and aromatic CAC out of plane band at 697 cm�1. Aro-matic overtones were visible in the range 1703–1943 cm�1. Thesebands almost disappeared in the sample after pyrolysis at 450 �Cfor 1 h (Fig. 2d), showing that most of the polystyrene was re-moved and converted to carbon.

The FTIR spectra of carbon-coated titanium dioxide samples(Fig. 2d) show absorption peaks at about 3400 and 1630 cm�1 wereassociated with the stretching vibrations of surface water mole-cules. Peaks around 3000 cm�1 were often assigned to molecularlychemisorbed water and the existence of hydroxyl groups. Mean-while, the peaks corresponding to the stretching vibrations ofwater and hydroxyl groups were broader and stronger in the car-bon-coated titanium dioxide (C@TiO2) than that of the bare TiO2,indicating that the samples have different hydrophilicity.

Diffuse reflectance UV–Vis (DR UV–Vis) spectra of bare TiO2,polystyrene, PS@TiO2 and C@TiO2 are shown in Fig 3. It can be seenthat there are two absorbance peaks at 250–260 and 260–330 nm.The absorbance peak at 250–260 nm is attributed to a chargetransfer of the tetrahedral titanium site between O2� and the cen-tral Ti(IV) atom, while octahedral Ti was reported appeared around260–330 [8]. The intensity of band at 250–260 nm on polystyrene-coated titania was increased due to the polystyrene (Fig. 3c). Afterpyrolysis at 450 �C for 1 h, DR UV–Vis spectrum of C@TiO2 sample(Fig. 3d) showed the intensity of absorbance peak at 250–260 nmwhich is corresponded to peak of polystyrene, was reduced dueto conversion of polystyrene to carbon on the surface of TiO2.

The XPS measurements were used to study the chemical state ofthe polystyrene-coated TiO2 (PS@TiO2), carbon-coated TiO2

(C@TiO2) and carbon-coated TiO2 after treating with 1.0 M KOHsolution (PC@TiO2). The XPS peaks show the characteristic bindingenergy of elements. It revealed that the C (1s) spectrum of PS@TiO2

Ti (2p)

O(1s)

0 100 200 300 400 500 600 700 800

C (1s)

Binding Energy / eV

872092292 284 282 280286288

284.5 eV

283.4 eV285.7 eV

286.2 eV

284.5 eV

292.4 eV

291.4 eV

286.2 eV

290.7 eV

284.5 eV

296 294 278288 286 280292 092 284 282

292 290 278284 282 280286288

(a)

(b)

(c)

(d)

Fig. 4. XPS spectra of: (a) PS@TiO2, (b) C@TiO2, (c) PC@TiO2 and (d) C@TiO2 aftertreatment with 1.0 M KOH solution.


(Fig. 4a) showed two components centered at 284.5 eV (CAC andCAH bonds), and at 291.4 eV which correspond to a shake-up sa-tellite of p–p� of the aromatic ring. The two peaks were fitted tofour components centered at 284.5 eV and 286.2 eV (CAC andCAH bond) while peaks centered at 290.7 eV and 292.4 eV corre-spond to C@C aromatics. These peaks confirm the existence ofpolystyrene on the sample. These results are in agreements withprevious report [22].

The C (1s) peak of carbon-coated titanium dioxide (C@TiO2) asshown in Fig. 4b displays one peak centered at 284.5 eV which cor-respond to the graphitic thin carbon layer [23]. This C (1s) peakwas fitted to two peaks centered at 284.5 eV and 286.2 eV. Basedon the above results, one concludes that the binding energy ofpolystyrene and graphitic is different. The C (1s) peak centered at291.4 eV which correspond to a shake-up satellite of p–p� of thearomatic ring which appeared at PS@TiO2 XPS spectrum was notobserved in C@TiO2. This result proved that polystyrene layer hasbeen successfully converted into the carbon layer on the surfaceof TiO2.

The C (1s) peak of carbon-coated titanium dioxide after treat-ment with 1.0 M KOH solution (PC@TiO2) is given in Fig. 4c. Thispeak centered at 284.5 eV and was fitted to three peaks centeredat 283.4 eV, 284.5 eV and 285.7 eV. Fig. 4d shows the wide spectraof C@TiO2 after treatment with 1.0 M KOH solution. It can be seenthat there is no peak at binding energy at 292–296 eV which cor-respond to K (2p) was observed. It can explain the absence ofKOH in C@TiO2.

The effect of polystyrene, carbon layer and porous carbon coat-ings on TiO2 surface on hydrophylicity of titania was determinedby sinkability test by using water. The solid sample was added to5 ml of water into a tube. Titania was a hydrophilic particle that wet-ted rapidly and immersed in water (Fig. 5a). The hydrophobic char-acter of polystyrene makes the PS@TiO2 particles were floating onwater (Fig. 5b). After pyrolysis at 450 �C for 1 h, the hydrophobiccharacter of the C@TiO2 particles was still maintained and madethe sample float above the water’s surface (Fig. 5c). It confirmed thatthe carbon layer of C@TiO2 and PC@TiO2 are hydrophobic and hydro-philic, respectively. As shown in Fig. 5d, the PC@TiO2 particles settledown to the bottom of the tube. This result indicates that water can

be easily adsorbed on the PC@TiO2 particles due to the porous struc-ture of its carbon layer.

The nitrogen adsorption–desorption isotherm of C@TiO2 andPC@TiO2 samples are shown in Fig. 6. Specific surface area ofC@TiO2 is 13 m2 g�1 and after treating with KOH solution 1.0 Mthe specific surface area is increased to 46 m2 g�1, while the initialBET specific surface area of TiO2 is 11 m2 g�1. The increment spe-cific surface area of C@TiO2 and PC@TiO2 is almost a factor of 3.5,which is attributed to the contribution of porous structure of thecarbon. It can be seen from Fig. 6 that the isotherms can be catego-rized as type IV with hysteresis loops at relative pressure (P/P0) be-tween 0.4 and 1.0, confirming its mesoporous feature. Pore sizedistribution calculated by the Barrett–Joyner–Halenda (BJH) meth-od indicated that the C@TiO2 was nonporous (inset of Fig. 6a) withthe total pore volume 0.02 cm3 g�1 and 0.03 cm3 g�1 by theadsorption and desorption branch respectively. From the calcula-tion of pore size distribution there are indications of pore in sizeof 6.06 nm, with very small pore volume. This lack of porositywas also confirmed by TEM (see Fig. 1c). The pore size distributionof PC@TiO2 shows a narrow peak centered at �5.0 nm (in the insetof Fig. 6b.) indicates the existence of mesopores (2–5 nm in size).The average pore diameter was ca. 5 nm calculated from theadsorption branch of the N2 sorption isotherm. The total pore vol-ume of PC@TiO2 was 0.05 cm3 g�1 and 0.095 cm3 g�1 given by theadsorption and desorption branch of the N2 isotherm.

Based on the above discussion, it is clearly demonstrated thatPS@TiO2 can be prepared by in situ polymerization of styrene byadsorbed aqueous hydrogen peroxide on the surface of titaniumdioxide particles. It is also observed that after pyrolysis of PS@TiO2at450 �C for 1 h, the polystyrene converted to carbon layer to produceC@TiO2. When C@TiO2 obtained was treated with KOH solution, theporous carbon layer was formed on the surface of TiO2.

3.2. Catalytic activity

The oxidation of styrene with aqueous H2O2 as an oxidizingagent for 8 h at 80 �C was used to evaluate the catalytic activityof the catalysts. Fig. 7 presents the results of oxidation of styreneover bare TiO2, carbon-coated TiO2 before and after treating with

(a) (b) (c) (d)

TiO2 PS@TiO2 C@TiO2 PC@TiO2

water

catalyst

Fig. 5. The photographs of: (a) TiO2, (b) PS@TiO2, (c) C@TiO2 and (d) PC@TiO2 particles in water.


0.00025

0.0000

0.0012

0.0014

0.0004

0.0002

0.0010

0.0008

0

Vol

ume

adso

rbed

/ cm

3g–1

ST

P

(a)

Relative pressure / P/Po

Vol

ume

adso

rbed

/ cm

3g–1

ST

P

Relative pressure / P/Po

35

30

25

20

15

Desorption Adsorption

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

5

10

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

60

50

40

10

20

30

Desorption

Adsorption

0

0.00040

0.00035

0.00030

0.00020

0.00015

0.00010

0.00005

0.00000

Pore diameter / nm

Por

e vo

lum

e / c

m3

g–1

10 105 500

(b)

Por

e vo

lum

e / c

m3 g

–1

Pore diameter / nm

0.0006

10 100 5 50

Fig. 6. N2 adsorption/desorption isotherm of: (a) C@TiO2 and (b) PC@TiO2. Theinsets are pore size distribution calculated by BJH method.

Yie

ld /

μmol

300

200

a b c d e f g

50

100

0

250

150

No catalyst

TiO2 C@TiO2

PC@TiO2

Phenylacetaldehyde Benzaldehyde

Fig. 7. The yield of products in oxidation of styrene by using aqueous hydrogenperoxide in the system containing: (a) no catalyst, (b) TiO2, (c) C@TiO2 and PC@TiO2

after treated with KOH solution (d) 0.5 M, (e) 1.0 M (f) 1.5 M and (g) 2.0 M.

No catalyst

TiO2 C@TiO2

PC@TiO2

Con

vers

ion

/ %

30

25

20

15

a b c d e f g

5

0

10

Fig. 8. The conversion of styrene in the styrene oxidation by using aqueoushydrogen peroxide in the system containing: (a) no catalyst, (b) TiO2, (c) C@TiO2

and PC@TiO2 which is obtained after C@TiO2 treated with KOH solution (d) 0.5 M,(e) 1.0 M (f) 1.5 M and (g) 2.0 M.

(e)

(f)

(d)

(c)

(b)

(a)

Inte

nsity

/ a.

u.

2θ / o

10 20 30 40 50 60 70 80 90

101

103

004

112

200

105

211

204

116

220

215

301

224

Fig. 9. The X-ray diffractograms of: (a) TiO2, (b) C@TiO2 and PC@TiO2 which isobtained after C@TiO2 treated with KOH solution (c) 0.5 M, (d) 1.0 M (e) 1.5 M and(f) 2.0 M.


KOH solution. Gas chromatography analyses indicated that benzal-dehyde and phenylacetaldehyde were the main products, andstyrene oxide as minor product. All of these products have beenreconfirmed by GC–MS analyzed. As shown in Fig. 7, the presentresult indicated that the selectivity towards the formation of benz-aldehyde is much higher than that of phenylacetaldehyde. This re-sult is different from those previously reported that styreneoxidation using TS-1 gave phenylacetaldehyde as a main product[24–28].

Fig. 7 shows that the yield of styrene oxidation products with-out catalyst, with TiO2 and C@TiO2 is almost similar. C@TiO2 gaveslightly decreased in the formation of products because nonporouscarbon layer that covered TiO2 reduced the amount of styrenereaching the TiO2 active site. After treating with 0.5 M KOH solu-tion, carbon layer on C@TiO2 turns into the porous structure to pro-duce PC@TiO2. This allowed more of styrene molecules which wereadsorbed on PC@TiO2 carbon layer to reach the TiO2 active sitesand gave higher catalytic activity (192 lmol of benzaldehyde and35 lmol of phenylacetaldehyde). The highest product was ob-tained by using PC@TiO2 (260 lmol of benzaldehyde and 41 lmolof phenylacetaldehyde). As shown in Fig. 8, the highest conversionof styrene, ca. 27%, was obtained when PC@TiO2, C@TiO2 aftertreating with 1.0 M KOH, was used as catalyst.

It has been reported that crystallinity and crystallite size of TiO2

was effect the catalytic activity of TiO2 [29,30]. One suggests thatthe concentration of KOH used to create the porosity of the carbonlayer affects the crystallinity and crystallite size of TiO2. Fig. 9shows the X-ray diffractogram of TiO2, C@TiO2 and PC@TiO2 aftertreated with different concentrations of KOH. All samples haveshown similar peaks with the highest peak at 25.68� which wasindicated as (101) plane of the crystal phase of anatase. In general,

the intensity of the X-ray peaks decreases with increasing the KOHconcentration suggesting that the crystallinity of the TiO2 affect thecatalytic activity in styrene oxidation. This is the reason why thecatalytic activity of PC@TiO2 treated with 1.5 and 2.0 M KOH islower compared than that of PC@TiO2 treated with 1.0 M KOHsolution. It is also suggested that the optimum concentration ofKOH solution for producing high porosity carbon layer and rela-tively high crystallinity of TiO2 was 1.0 M. The distribution of crys-tallite sizes of TiO2, C@TiO2, PC@TiO2 and after treated with KOHcalculated by Scherrer’s formula was in the range of 30.6–33.5 nm. It can be seen that there is no significant differences ofaverage particle size of the samples.

In the styrene oxidation reaction, the free-radical mechanisminvolves the interaction of hydrogen peroxide with the TiO2 toform a hydroperoxy species. This species being unstable, under-goes one-electron reduction with a loss of water molecule andrearranges to give the Ti peroxo species (Ti(IV)–O� radical). Theperoxo species interacts with styrene to form a p-bonded transientspecies. The presence of the polar water molecules in the reactionsystem catalyzed the cleavage of the C@C bond of styrene to pro-duce benzaldehyde through radical transformation. While the phe-nylacetaldehyde obtained from styrene oxide isomerisation [31].Based on these considerations, a model of the possible mechanismof the oxidation of styrene with aqueous H2O2 as an oxidationagent over PC@TiO2 is proposed (see Fig. 10).

4. Conclusion

Porous carbon coating of titania particle was possible preparedby in situ polymerization of styrene on the TiO2 surface followed bypyrolysis of polystyrene formed into carbon and treatment with

Phenylacetaldehyde

Benzaldehyde

isomerization

H2O2H2O

Ti

O•

Ti

H

O OH

Ti

O

O

Styrene oxide

H2C CH2

Ti

O

H2C CH2

Ti

O•

H2C CH2

O

Fig. 10. Proposed reaction mechanism for the formation of benzaldehyde and phenyl acetaldehyde over PC@TiO2 catalyst.


KOH solution under N2 gas flow. The existence of polystyrene andcarbon were confirmed by FTIR, DR UV–Vis, TEM and XPS. Carboncontent was about 2.2 wt.% with thickness of carbon layer ca. 5 nm.Catalytic results showed that the carbon layer is thick enough thathinder the substrate to reach the TiO2 active site. Treatment ofKOH solution on carbon-coated TiO2 caused the carbon layer turnsinto porous carbon which allowed the substrate to reach the TiO2

active site and gave a higher catalytic activity in styrene oxidation.The highest catalytic activity was obtained by using carbon-coatedtitania treated with 1.0 M KOH solution with benzaldehyde andphenylacetaldehyde as main products. At the higher concentrationof KOH solution, the catalytic activity was decreased due to the de-crease in the crystallinity of TiO2.

Acknowledgments

The authors gratefully acknowledge the Ministry of Higher Edu-cation (MOHE), Malaysia through Fundamental Research GrantScheme, Universiti Teknologi Malaysia through Research Univer-sity Grant, and Directorate General of Higher Education, the Minis-try of National Education Indonesia for financial support.

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improvement of catalytic activity in styrene oxidation of carbon

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