1-s2.0-S0926337305003735-main

Carbon-modi?ed TiO2photocatalyst by ethanol carbonisation M.Janus a,M.Inagaki b,B.Tryba a,M.Toyoda c,A.W.Morawski a,*

a Szczecin University of Technology,Department of Water Technology and Environment Engineering,

70-322Szczecin,ul.Pu?askiego10,Poland

b Faculty of Engineering,Aichi Institute of Technology,Yakusa,Toyota470-0392,Japan

c Faculty of Engineering,Oita University,Dannoharu700,Oita870-1192,Japan

Received12July2005;received in revised form29September2005;accepted3October2005

Available online21November2005

Abstract

The new photocatalysts based on commercially available titanium dioxide powders:Tytanpol A11(Police,Poland),pure anatase and P-25 (Degussa,Germany)containing about20%rutile were modi?ed by carbon via ethanol carbonisation.Titanium dioxides were heated at different temperature from150to4008C for1h in an atmosphere of ethanol vapour.The photocatalytic activity of carbon-modi?ed TiO2was studied by oxidation of phenol in water under UVand arti?cial solar light irradiation.With increasing of carbon content in TiO2photocatalysts the activity for phenol decomposition under UV light was decreasing but that under visible light was stable.Turbidity of the slurry solution decreased with increasing of carbon content for all prepared photocatalysts because of the change of their surface character from hydrophilic to hydrophobic. #2005Elsevier B.V.All rights reserved.

Keywords:Carbon-modi?ed TiO2;Photocatalysis;Solar light

1.Introduction

Different processes of TiO2modi?cation have been investigated in order to expand their absorption spectra to the main part of the solar spectrum(l>380nm),in other words,to make them visible light sensitive.One of the approaches was doping of the foreign atoms to TiO2,such as S, N,etc.[1,2].Carbon was reported to be effective to improve the activity of TiO2photocatalyst.In carbon-coated TiO2samples, not only sintering of the aggregated particles but also crystal growth in each particles were depressed,what resulted in the suppression of the phase transformation from anatase to rutile at high temperatures of heat treatment.Moreover,carbon layer coated on TiO2particles could adsorb organic compounds from its aqueous solution and transfer it through the pores to the surface of TiO2[3–6].We developed a new preparation process for carbon-modi?ed TiO2,which worked under solar light[7]. Modi?cation of TiO2by n-hexane carbonisation markedly improved turbidity of the solution after catalyst sedimentation,what increased the possibilities of practical application of TiO2 as a photocatalyst in water treatment system[8].

Another way of improving the activity of titanium dioxide under UV and solar light is using the mixture of TiO2with activated carbon(AC)to achieve the synergistic effect between them.The synergistic effect is explained by an important adsorption of the pollutants on activated carbon surface followed by a mass transfer to the photoactive TiO2through a common interface between AC and TiO2 [9–13].

In this paper the photocatalytic activity of carbon-modi?ed TiO2photocatalysts which were prepared via ethanol carbo-nisation was presented under UV light and the arti?cial solar light irradiation.

2.Experimental

2.1.Materials

Titanium dioxides,Tytanpol A11(Police,Poland)and P-25 (Degussa,Germany),were used as the photocatalysts.Tytanpol A11has a speci?c surface area of11.4m2/g and consists mainly of anatase crystalline phase(minimum98.5%)with particle size0.2–0.4m m.

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Applied Catalysis B:Environmental63(2006)272–276

*Corresponding author.Tel.:+48914494474;fax:+48914494686.

E-mail address:amor@ps.pl(A.W.Morawski).

doi:10.1016/j.apcatb.2005.10.005

Degussa P-25has a speci?c surface area of50m2/g and it is composed from80%anatase and20%of rutile.The average sizes of the anatase and rutile elementary particles are25and 85nm,respectively[14].

2.2.Preparation of carbon-modi?ed TiO2photocatalysts and characterization of samples

Carbon-modi?ed TiO2photocatalysts were obtained by exposure of TiO2to the vapour of ethanol for1h at temperatures between150and4008C.In Fig.1,the scheme of installation used is presented.A porcelain boat with about 1g of sample was put into a pipe furnace type R40/250/12-C40 (Nabertherm,Germany)where the vapour of ethanol was supplied by bubbling of the pure argon through the ethanol liquid at room temperature.The samples were heated up to the demanded temperature for40min,and then were kept for1h, after that time the furnace was slowly cooled down to the room temperature.

The content of carbon in carbon-modi?ed TiO2photo-catalysts was determined by burning off the carbon during heating to10008C in an air?ow(25ml/min)followed by using a thermogravimetric system(STA449of Netzsch Company, Germany).

The photocatalysts were characterized by UV–vis/DR spectra measured in Specord M40spectrophotometer(Carl Zeiss,Jena,Germany)equipped with an integrating sphere accessory for diffuse re?ectance spectra(BaSO4was used as a reference).Both,spectra and band gap energy calculation(E G) were performed by Jasco procedure described elsewhere[15].

Diffuse re?ectance FTIR spectra were recorded by using FTIR spectrometer(Jasco,Japan)equipped with DR accessory of Harrick Company(USA).

2.3.Experimental procedures and techniques

The photocatalytic decomposition of phenol was carried out in the glass beakers.The sample powder of0.1g was dispersed into500ml of aqueous phenol solution with a concentration of 0.1g/l.The solution was mixed with a magnetic stirrer during reaction.Firstly,the solution mixture was stirred for15min without irradiation in order to get the equilibrium of phenol adsorption.Irradiation of the solution was performed under two conditions:for24h under arti?cial solar light irradiation (Philips100W)with radiation intensity of about354W/m2 visible and0.09W/m2UV and for5h under UV light(six lamps with power of20W,Phillips)with radiation intensity of about100W/m2visible and154W/m2UV.The radiation intensity was measured with using the radiation intensity meter LB901with CM3and PD204AB Cos sensors.To determine the concentration of phenol,the reaction mixture was?ltrated with a Millipore disk(0.45m m)and solution was loaded in a UV–vis spectrometer(Jasco).The concentration of phenol in the samples was calculated by a computer program based on the calibration curve.

2.4.Turbidity

Turbidity of TiO2slurry after either24h irradiation under arti?cial solar light or5h irradiation of UV light followed by 10min sedimentation was measured by using turbidimeter (Hach,Model2100N IS).

3.Results

The results of carbon content,photocatalytic activities and turbidity are summarized in Table1with the preparation conditions of carbon-modi?ed TiO2.

3.1.Photocatalytic activity

At the highest temperature of modi?cation the carbon content in A11and P-25photocatalysts were quantitatively different.It was not possible to obtain the high amount of carbon content in A11(1.66mass%)as it was in case of P-25 (6.65mass%).At the same temperature of heat-treatment for A11and P-25the different amounts of incorporated carbon were obtained.For example,at the temperature of3508C the amount of carbon content in P-25was six times higher than in

M.Janus et al./Applied Catalysis B:Environmental63(2006)272–276

273

Fig.1.The installation setup for deposition of carbon on TiO2by carbonisation

of ethanol.(1)Gas cylinder with argon,(2)controller,(3)washer with ethanol,

(4)pipe furnace,(5)sample and(6)scrubber.

Table1

Experimental results

Photocatalyst C(%)Turbidity(NTU)Phenol

decomposition

(%)

UV Visible UV Visible

A1102628320041.516.7

A11at3008C0.172278169029.310.1

A11at3258C0.69818117335.110.0

A11at3508C 1.0455534534.48.3

A11at4008C 1.66616115 2.212.2

P-25068161545.88.1

P-25at1508C0.0738440342.210.4

P-25at2008C0.1736042045.59.4

P-25at3008C 3.7431321226.710.3

P-25at3258C 5.6925220012.49.3

P-25at3508C 6.65207297 4.88.5

A11.Since P-25has the speci?c surface area of about ?ve times higher than A11,P-25could incorporate higher amount of carbon.

In Fig.2,photoactivity measured by phenol decomposition is plotted against carbon content.With increasing the amount of carbon in carbon-modi?ed TiO 2photocatalysts the activity for phenol decomposition under UV irradiation is decreasing.Such behaviour is not observed when the titanium dioxide is used in the mixture with an activated carbon [9–13];even the presence of 13mass%of activated carbon in the mixture with TiO 2does not decrease the photoactivity of TiO 2under UV light irradiation,even being higher than that of pure titanium dioxide [13].It means that not amount of carbon but the way how it is incorporated to TiO 2has an in?uence on the photocatalytic activity of the photocatalyst.In case when the arti?cial solar light irradiation was used,the content of carbon in carbon-modi?ed TiO 2photocatalysts had not signi?cant impact on the grade of phenol decomposition and kept practically the same level.In Fig.3,the UV–vis/DR absorption spectra of the original and modi?ed A11samples are presented.It can be seen that in the samples contained carbon the intensity of re?ectance is reduced,those photocatalysts could absorb the visible light.The same behaviour was observed for the original and modi?ed P-25samples,although the results are not presented here.

3.2.Turbidity and FTIR/DRS spectra

The turbidity of all slurry solutions decreased with increasing of the carbon content in TiO 2photocatalysts (Fig.4).This was caused by the changing of TiO 2surface character from hydrophilic to hydrophobic by reduction the hydroxyl groups and by appearance the –CH 3groups.In Figs.5and 6,the FTIR spectra of the carbon-modi?ed A11and P-25photocatalysts are presented,respectively.With increasing the heat-treatment temperature the intensities of the band:at 3695cm à1assigned to the hydroxyl groups chemisorbed on the surface defect side of the photocatalyst,at 3300cm à1for both dissociated and molecularly adsorbed water and at 1623cm à1for a molecular water [15]were reduced.In Fig.7,the dependence of the intensity of the OH groups (measured on the FTIR/DRS spectra)from the content of carbon in the carbon-modi?ed TiO 2photocatalysts is presented.With increasing the content of carbon in carbon-modi?ed TiO 2the intensity of OH groups on the surface of the photocatalysts decreases.The in?uence of those OH groups on the phenol decomposition is presented in Fig.8.It can be noticed that the amount of OH groups present on the photocatalysts surface has not in?uence on the grade of phenol decomposition under visible light but under UV light higher decomposition of phenol has been observed for the higher amount of OH groups.

M.Janus et al./Applied Catalysis B:Environmental 63(2006)272–276

274Fig.2.The in?uence of the content of carbon in TiO 2/carbon catalysts on the phenol

decomposition.

Fig.3.The UV–vis/DR absorption spectra of a pure TiO 2and modi?ed by ethanol TiO 2

–A11.Fig.4.The in?uence of the content of carbon in TiO 2/carbon catalysts on the turbidity of

slurry.

Fig.5.The FTIR/DRS spectra of the pure P-25and modi?ed by ethanol in different temperature for 1h.

The excitation of the photocatalyst under UV irradiation is much higher than under visible light and leads to formation a lot of electron/hole pairs,and then holes could react with water adsorbed on the surface to produce OH radicals which are very active in the oxidation of the organic compounds.For that reason,the higher amount of OH groups on the surface of the photocatalyst could affect on the higher amount of OH radicals formation and as a consequence could give higher grade of phenol decomposition under UV irradiation.Excitation of TiO 2-based photocatalysts under visible light is not so effective as under UV,because of the low energy of the visible light,which is not enough to transfer electrons from the valence to conductive band.However,some electrons are able to jump to the conductive band,but they could follow the process of recombination with holes easily because they have a low energy.In that case,amount of OH groups on the photocatalysts surface has not much in?uence on the formation of OH radicals and so not on the grade of phenol decomposition.Additionally,carbon contained in carbon-modi?ed TiO 2photocatalysts can serve as an electron scavenger to protect the process of electron–hole recombination and that could be important when photocatalyst is excited under visible light,because of the formation of low amount of free carriers.Therefore,the process of separation of carriers is more ef?cient for higher amount of carbon in the carbon-modi?ed TiO 2photocatalysts,but reaction of holes with adsorbed water for the lower amount of carbon.

In Fig.9,the in?uence of the amount of OH groups present on the photocatalyst surface on the turbidity of the solution is illustrated.It can be noticed that with increasing the amount of OH groups,turbidity increases,but the effect is different on two TiO 2,A11and P-25.As shown in Figs.5and 6,the bands in a region of 3100–2800cm à1assigned to –CH 3groups were always observed on all the prepared carbon-modi?ed TiO 2photocatalysts,but bands in a region of 1580–1630cm à1(1440cm à1CO band,1378cm à1–CH 3band)appeared on P-25only.Those groups were not present on the surface of A11photocatalysts.The presence of those hydrophobic groups was supposed to be responsible for their easiness for sedimentation measured by turbidity of slurry.In our previous publication,the modi?cation of A11by n -hexane was presented,by that way the maximal amount of built-in carbon was 0.85%.In these studies,we used ethanol for carbonisation,what allowed to obtain much higher carbon content even when the lower temperature and shorter time of carbonisation were used.Although the different organic compounds were used,ethanol in the present work and n -hexane in our previous work [16]for modi?cation by carbon,the same results were obtained for the photocatalysts with the same amount of built-in carbon.The modi?cation of TiO 2(A11)by carbon in the amount of around 0.7mass%and more improved turbidity of the solution after photocatalyst sedimentation,which may increase the possibilities for the practical application of TiO 2as a photocatalyst in water treatment system under a visible light.

M.Janus et al./Applied Catalysis B:Environmental 63(2006)272–276

275

Fig.6.The FTIR/DRS spectra of the pure A11and modi?ed by ethanol in different temperature for 1

Fig.7.The in?uence of the content of carbon in TiO 2/carbon catalysts on the intensity of band at 1620cm à1in FTIR/DRS

spectra.

Fig.8.The in?uence of the intensity of peak at 1623cm à1in FTIR/DRS spectra on the phenol decomposition under visible and UV

light.

Fig.9.The in?uence of the intensity of band at 1623cm à1in FTIR/DRS spectra on the turbidity of the titanium slurry.

4.Conclusions

A new method of modi?cation of TiO2photocatalyst by carbon has been described.The photoactivities of carbon-modi?ed TiO2photocatalysts under visible light irradiation for phenol decomposition are comparable for two starting TiO2 (A11and P-25),irrespective of the carbon content but the turbidity of the slurry after photocatalyst sedimentation decreased with increasing carbon content in the photocatalyst. This was caused by the changing of TiO2surface character from hydrophilic to hydrophobic by reduction the hydroxyl groups and by appearance the–CH3groups.

P-25photocatalyst showed better abilities for the carbon deposition than A11,with using the same temperature of carbonisation it was possible to receive six times higher carbon content in P-25catalysts.The carbon-modi?ed TiO2photocatalysts prepared on the basis of P-25had lower values of turbidity of the slurry after photocatalyst sedimentation than those prepared from A11.The photo-activities of the carbon-modi?ed TiO2samples prepared from P-25were a little bit higher under UV irradiation than those prepared from A11and almost the same under visible light irradiation in comparison with carbon-doped TiO2obtained from A11.Therefore,it could be concluded that P-25 photocatalyst is better material than A11for the preparation of carbon-doped TiO2photocatalysts active under visible light irradiation.References

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