低温超临界干燥制备TiO2气凝胶

Titania aerogel prepared by low temperature supercritical drying

V.S

ˇtengl *,S.Bakardjieva,J.S ˇubrt,L.Szatmary Institute of Inorganic Chemistry,Academy of Czech Science (AS CR),25068R ˇez ˇ,Czech Republic Received 31March 2005;received in revised form 28September 2005;accepted 31October 2005

Available online 27December 2005

Abstract

Mesoporous titania aerogel with high surfaces area (1086m 2g à1)were prepared by two steps:heterogeneous precipitation of aqueous solution containing TiOSO 4and ammonia,followed by continuous supercritical extraction with CO 2after replacement of the water with n -propanol/benzene mixture.The total pore volume of as obtained aerogel is 3.44cm à3g à1and the main pore diameter is 20–80nm.The structure evolution of titania aerogel during heating was studied by DTA and high temperature XRD analyses.At 450°C titania aerogel transforms to polycrystalline anatase.The morphology and microstructure characteristics of both titania aerogel and anatase samples were obtained by SEM,HRTEM,BET and BJH methods.Anatase reveals high photocatalytic activity during the degradation of 4-chlo-rophenol in aqueous suspension.

ó2005Elsevier Inc.All rights reserved.

Keywords:Aerogels;Titanium oxide;Supercritical drying;Carbon dioxide

1.Introduction

Nanophase materials,with their gram sizes or phase dimensions in the nanometer size regime,are now being produced by a wide variety of synthesis and processing methods.The interest in these new ultra?ne-grained mate-rials results primarily from the special nature of their various physical,chemical,and mechanical properties and the possibilities of controlling these properties during the synthesis and subsequent processing procedures.Since it is now becoming increasingly apparent that their proper-ties can be engineered e?ectively during synthesis and processing,and that they can also be produced in quantity,nanophase materials should have considerable potential for technological development in a variety of applications.Among these materials,one of the very important,is nanocrystalline anatase (TiO 2),widely used now for photocatalytic air and water puri?cation and many other purposes based on photocatalytic oxidation and decompo-

sition of organic pollutants [1–4].The material can be also

used for solar energy storage and conversion [5,6]organic syntheses [7],etc.Titanium dioxide is one of the most pop-ular and promising materials for these purposes,because of its stability,commercial availability and ecological safety.According to the literature [8–10],the photocatalytic activ-ity of suspended TiO 2in solution strongly depends on the physical properties of TiO 2(e.g.,crystal structure,surface area,surface hydroxyls,and particle size).In this respect,attempts have been made to prepare TiO 2particles with high surface area,suitable porosity and a distinct shape (?lms,spheres,rods,etc.)in order to suit this material to the demands of its application [11,12].To achieve good photocatalytic activity,the material should contain as low as possible amount of amorphous material [13].Su?-cient amount of surface OH àgroups is also needed in order to stabilize the active ion–hole pairs in the form of surface OH ?and O à2radicals on the surface of the TiO 2photocatalyst [14].Current attention has been focused on the preparation of nanometer TiO 2particles using super-critical ?uid drying.According to Matson et al.[15,16]supercritical ?uid state is a state of material that its temper-ature and pressure are higher than its critical temperature

1387-1811/$-see front matter ó2005Elsevier Inc.All rights reserved.doi:10.1016/j.micromeso.2005.10.046

*

Corresponding author.Tel.:+420266173534;fax:+420220940157.

E-mail address:stengl@iic.cas.cz (V.S

ˇtengl).

https://www.360docs.net/doc/735115631.html,/locate/micromeso

Microporous and Mesoporous Materials 91(2006)

1–6

and critical pressure.In supercritical?uid drying of nano-meter particle preparation,solvent can be removed without surface tension e?ect and the coagulation of particles,the-oretically can be avoided.Recently,Hu et al.[17]prepared nanometer titanium oxide with n-butanol supercritical drying.The technique mainly consisted of sol preparation, replacement of water in precipitate using n-butanol and supercritical drying.Anatase and rutile mixture with a particle size of10–20nm was obtained using the prepara-tion technique.The present state of supercritical drying ca be divided into the high-temperature and low tempera-ture methods,usually based on alcoholic solvents and CO2,respectively[18].

The aim of our research was to obtain nanosized TiO2in pure anatase form with high surface area.We try to develop relatively cheap n-propanol/benzene supercritical ?uid drying technique—the water of the primary precipi-tate obtained from aqueous solution containing TiOSO4 and ammonia was replaced with n-propanol/benzene mix-ture followed by continuous supercritical extraction with CO2.

2.Experimental

2.1.Preparation of aerogel of titanium oxide

A wet gel of titanium(IV)hydroxide was prepared by mixing aqueous solutions of ammonia(25%)and0.2M solution of TiOSO4under stirring at temperature0°C. The wet gel(denoted as SAMPLE1)was separated by ?ltration,washed few time with distilled water by decanta-tion,and loaded into the?ask of an apparatus for solvent replacement by distillation,as described in[17].The?ask was?lled with a1:2mixture of propanol and benzene. The distillate,composed of a mixture of water and organic phase,was collected in a separating funnel,the organic material was recycled in to?ask and the process was con-tinued until all water had been removed from the gel. After solvent replacement,the gel was?ltered of and loaded in to?ask with acetone on24h.The wet gel from acetone was converted to an aerogel(denoted as SAM-PLE2)by low temperature supercritical drying with CO2 in supercritical extraction system(model CPD7501Quo-rum Technologies)at pressure80bars,drying times24h and temperatureà12°C,?ow rate10mL minà1at tem-perature33°C.

2.2.Heat treatment of aerogel of titanium oxide

On the basis of high temperature XRD results(Fig.1) the aerogel of titanium oxide obtained from the low tem-perature supercritical drying was heated under dynamic vacuum(80mbar)at temperature450and550°C for2h by using furnace controlled by the PID controller in the stainless steel tube.The temperature increase rate was 1°C/min.After the heat treatment,the sample was allowed to cool in room temperature.2.3.Characterization methods

Speci?c surface area of the samples was determined from nitrogen adsorption–desorption isotherms at liquid nitrogen temperature using a Coulter SA3100instrument with outgas15min at120°C.Speci?c surface area was cal-culated by the BET[19]method,while the pore size distri-bution(pore diameter and pore volume)was determined by the BJH[20]method.

Transmission electron micrographs were obtained using a JEOL JEM3010microscope(LaB6cathode),operating at accelerating voltage of300kV and equipped with CCD GATAN MULTISCAN(model794)and EDS spec-trometer(INCA x-stream module OXFORD).The sam-ples for electron microscopy were prepared by grinding in an agate mortar and subsequent dispersing the powder in ethanol.The suspension was treated in ultrasonic bath for about10min.A drop of very dilute suspension was applied on carbon-coated grid.

SEM studies were performed using a Philips XL30CP microscope equipped with EDX,Robinson,SE and BSE detectors.The sample was placed on an adhesive C slice and coated with Au–Pd alloy10nm thick layer.

X-ray powder di?raction patterns were obtained on a Siemens D5005instrument using Cu K a radiation(40kV, 30mA)and di?racted beam monochromator.Qualitative analysis was performed with the Eva Application and the Xpert HighScore using the JCPDS PDF-2database.Crys-tallite size of the heated samples was calculated from the Scherrer[21]equation using the X-ray di?raction peak at 2H=25,29°.

DTA–TG–MS measurements were carried out by using a simultaneous Netzsch Instrument STA409coupled to Quadrupol Mass Spectrometer Balzers QMS-420under dynamic conditions in the air(?ow rate75mL minà1). The samples were heated at the rate of10K minà1.

Photocatalytic activity characterization.The mineraliza-tion of4-chlorophenol(4-CP)in an aerated aqueous slur-ries was measured and the kinetics of the reaction was assessed by?tting.Seventy milligrams of the investigated photocatalyst sample were quantitatively transferred,using 63mL of distilled water,into the inner tube of the photo-reactor and kept in dark for about1h.Then,7mL of 0.001M solution of4-chlorophenol(4-CP)were added that completed the total volume up to70mL.The prepared sus-pension was then magnetically stirred for about1h in dark to achieve the adsorption equilibrium.The solution amount of1mL was taken as a blank probe before irradi-ation and,after?ltration of the catalyst submitted to the HPLC analysis.After that,the inner tube was put into the photoreactor and irradiation started.At appropriate irradiation times,1mL probes of the irradiated suspension were taken and analyzed by HPLC in the same way as the blank probe.To complete a set of HPLC measurements for a sample,the amount of0.0001M4-CP solution in water was analyzed under the same conditions.By this way we estimated the amount of4-CP initially adsorbed on TiO2

2V.Sˇtengl et al./Microporous and Mesoporous Materials91(2006)1–6

particles,which was removed together with the powder in the?ltration process.After?nishing the HPLC analysis of each sample the integrated areas or heights of the peaks were plotted as a function of the irradiation time.

Quartz water-jacketed laboratory photoreactor,mag-netically stirred and continuously irradiated with one ‘‘black light’’lamp(k=365nm,I0=5.3·10à5Ein-stein dmà3sà1)was used.A self-constructed photoreactor consists of two coaxial quartz tubes placed in the middle of a steel cylinder with an aluminum foil covering its inner wall.Inner quartz tube(diameter24mm,length300mm) was?lled with the investigated suspension(70mL)and magnetically stirred.Cooling water was circulating between the inner and the outer quartz tube to keep con-stant temperature of20°C.The HPLC measurements were carried out by using a Merck device with L-6200Intelligent Pump,L-3000Photo Diode Array Detector and D-2500 Chromato-Integrator.Mobile phase methanol/water(2:3; v/v)and a Merck column LiChroCART125-4?lled with LiChrosphere100RP-18(5l m)were used,injection loop was20l L,?ow rate1mL minà1and detection wavelength 280nm were applied.The solid sample necessarily together with the adsorbed portions of the dissolved molecular spe-cies was removed before HPLC,using a?ltration by a Millipore syringe adapter(diameter13mm)with?lter 408(porosity0.45l m).Detailed experimental procedures can be found elsewhere[22].

3.Results and discussion

3.1.Thermal analysis of the titanium gel

Fig.1show a typical thermal analysis of the titanium gel SAMPLE1used in the synthesis.DTA curve shows,that water started evolves at low temperature(endothermic peak start at temperature155°C)and achieves maximum at250°C,which correspond fracturing of–OH groups. Carbon dioxide was detected in the gas phase at tempera-ture$150°and peaked at331°C.The exothermic maxi-mum at temperature430°C correspond with transition to anatase in high temperature XRD.The limit of the calcina-tions temperature to anatase used in the synthesis was 450°C.At these conditions we obtained the?nal solids product consisting of anatase nanoparticles without any traces of precursors.

3.2.Adsorptions isotherms and speci?c surface area

Surface area,pore volume and pore size distribution of the materials were obtained from N2adsorption–desorp-tion isotherms(BET and BJH methods)at temperature of liquid nitrogen on Coulter SA3100instrument.Prior to measurements the Ti-aerogels,directly after CO2extrac-tion,were degassed at temperature150°C for30min.A typical nitrogen adsorption–desorption isotherms obtained for all samples is shown in Fig.2.The isotherms are closed hysteresis loops type A and have classi?cation for cylindri-cal pores open at both ends[23].The speci?c surface area and total pore volume of samples are shown in Table1. Pore size distribution shift to mesoporous using low tem-perature supercritical drying(see Table2).

3.3.X-ray di?raction(XRD)

Fig.3shows the X-ray di?raction patterns of both,an aerogel SAMPLE2prepared by low temperature supercrit-ical drying and SAMPLE3prepared by thermal treatment of titania aerogel at450°C.They are all similar and

exhibit Fig.1.DTA curves of the

SAMPLE1.Fig.2.Nitrogen adsorption–desorption isotherm of the SAMPLE2.

V.Sˇtengl et al./Microporous and Mesoporous Materials91(2006)1–63

characteristic peaks for TiO2(anatase modi?cation,PDF 21-1272).

The XRD spectrum of the Ti-aerogel(SAMPLE2)syn-thesized by low temperature supercritical drying with CO2

indicate a di?use pattern with relatively broad peaks and weak intensity and con?rms X-ray amorphous sample. Energy-dispersive X-ray microanalysis was performed on the Ti-aerogel due to acquire semiquantitative elemental information.EDX analysis con?rmed the empirical com-position of58.47wt%Ti and41.53wt%O.This value were looked to be in accordance with the theoretical composi-tions of TiO2.The EDX information was believed to be correct to about±3%.

The XRD spectrum labeled SAMPLE3belongs to the heated Ti-aerogel at450°C.Calcination in oxygen at 450°C using a heating rate of1°C minà1was an e?ective heat treatment procedure for obtaining pure polycrystalline anatase TiO2(ICDD PDF21-1272).

High temperature XRD di?ractogram of SAMPLE2is in Fig.4.The transition to anatase modi?cation is at tem-perature420°C,and transition anatase–rutile at tempera-ture920°C.The temperature of transition to anatase correspond with the results of DTA–TG(see Fig.1).

3.4.Electron microscopy(SEM and HRTEM)

Fig.5shows the SEM micrographs of Ti-aerogel(SAM-PLE2)obtained by continuous supercritical extraction with

Table1

Speci?c surface areas,S BET and total pore volume,V p,of Ti-aerogels

Sample Temperature of heating(°C)S BET(m2gà1)V p(cc gà1)Phase identi?ed by XRD L25,29(2H)(nm) SAMPLE1–785.20.78Amorphous–

SAMPLE2–1085.2 3.44Amorphous–

SAMPLE3450364.3 2.23Anatase 4.2

SAMPLE4550253.1 1.74Anatase 4.4

Table2

Desorption BJH pore size distribution of prepared samples

SAMPLE1SAMPLE2SAMPLE3SAMPLE4

Pore diameter (nm)Pore

volume

(cc/g)

%Pore diameter

(nm)

Pore volume

(cc/g)

(%)Pore diameter

(nm)

Pore volume

(cc/g)

(%)Pore diameter

(nm)

Pore volume

(cc/g)

(%)

Under60.2955.03Under60.15 4.42Under60.020.98Under60.05 3.18 6–80.0917.966–80.09 2.716–80.07 3.006–80.03 1.99 8–100.047.428–100.06 2.008–100.08 3.638–100.02 1.42 10–120.02 5.2810–120.09 2.7510–120.15 6.2110–120.03 1.90 12–160.01 3.5612–160.15 4.5712–160.187.5612–160.05 2.93 16–200.01 2.3416–200.257.3116–200.197.7016–200.08 4.44 20–800.03 6.1120–80 2.2363.9520–80 1.3956.3120–800.9349.44

Over800.01 2.32Over800.4212.29Over800.3614.61Over800.65

34.72

Fig.3.XRD patterns of the specimens SAMPLE2and

SAMPLE3.

Fig.4.High temperature XRD of the SAMPLE2.

4V.Sˇtengl et al./Microporous and Mesoporous Materials91(2006)1–6

CO 2after replacement of the water with n -propanol/ben-zene mixture.It can be seen that titania aerogel consists

of large micron-scale agglomerates.This agglomerates are not accurate spheres in shape and not very uniform in size.We suppose that the morphology of a SAMPLE2depend on the weak coagulation among particles,which may be resulted from the in?uence of H 2O and solvent mixture (n -propanol:benzene =1:2).

Fig.6(a)and (b)are the morphologies of the TiO 2,ana-tase modi?cation (ICDD PDF 21-1272),obtained by heat-ing of titania aerogel (SAMPLE2)at 450°C.The spherical particles consist of primary 4–5nm anatase nanocrystals can be observed.The result indicates that there existed no marked the sintering between particles at this temperature.Fig.6(c)and (d)reveal polycrystalline anatase with high quality of nanocrystallites without fraction of amorphous substance.The ?ne fringe spacing is 0.37nm corresponding to the (103)plane of anatase.This fact seems to be the main reason for the highest catalytic activity of SAMPLE3dur-ing the degradation of 4-chlorophenol in H 2O solution.Selected area di?raction (SAD)patterns (inserted in Fig.6)include a series of continuous Debye–Scherrer rings that correspond to (1)0.36,(2)0.24,(3)0.19nm in the d -spacing,denoting anatase crystal lattice planes with Miler indices (010),(004),(200),respectively.The ring patterns con?rmed the polycrystalline anatase specimen (tetragonal,space group I4/amd )consisting of many nano-crystals (4–5nm)with a random orientation.3.5.Photocatalytic activity

It followed from Fig.7that the prepared specimen SAMPLE3and SAMPLE4show good

photocatalytic

Fig.5.SEM image of the

SAMPLE2.

Fig.6.HRTEM image of the SAMPLE3.

V.S

ˇtengl et al./Microporous and Mesoporous Materials 91(2006)1–65

properties for the decomposition of 4-chlorophenol in an aqueous slurry under UV radiation.The time dependences of the 4-CP decomposition were ?tted by using Eq.(1)for the ?rst-order kinetics reaction [24].It is obvious from Fig.7that the ?rst-order kinetics curves (plotted as lines)?tted to all experimental points.d ?CP

d t

?k ea 0à?CP Te1T

where [CP]is concentration of 4-CP,a 0is initial concentra-tion of 4-CP and k is rate constant.

The rate constant k of the SAMPLE3heated at temper-ature 450°C is 0.0291and SAMPLE4heated at tempera-ture 550°C is 0.135.It is evident that the photoactivity of SAMPLE3is higher to the standard photocatalyst P25Degussa (rate constant 0.0203).Lower photoactivity of SAMPLE4depend on decreasing of surface area (see Table 1).

4.Conclusion

Amorphous TiO 2?n H 2O (SAMPLE1,surface

area =785m 2g à1

)were prepared reaction of titanium sul-phate with aqueous solution of ammonium hydroxide at temperature 0°C.The wet gel was converted to titania aerogel (SAMPLE2,surface area =1085m 2g à1)by method low temperature supercritical drying with CO 2.The titania aerogel was converted to anatase (SAMPLE3,surface area =364m 2g à1,SAMPLE4,surface area =

253m 2g à1),by heating at temperature 450and 550°C.

The conversion of modi?cation was determined by high temperature XRD.The anatase SAMPLE3prepare by heating of titanium aerogel at temperature 450°C have higher photocatalytic activity at decomposition 4-chloro-phenol then Degussa P25.Acknowledgement

This work was supported by the Ministry of Education,Youth and Sports of the Czech Republic (Project No.1M4531477201).References

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Fig.7.Photocatalytic activity of the SAMPLE3,SAMPLE4and Degussa P25.

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ˇtengl et al./Microporous and Mesoporous Materials 91(2006)1–6