Nanoporous TiO(2) were synthesized via hydrolysis of titanium tetraisopropoxide (TTIP) in HCl acidic conditions using sodium dodecylsulfate (SDS) as a templating agent. The order of the nanostructure of samples before calcination was improved with increasing the SDS/TTIP molar ratio. The calcined samples presented uniform mesopore structure with the average diameter of 4-5 nm and considerably large surface areas over 200m(2)/g, although the periodicity of the nanostructure was exceedingly reduced by the removal of the template. The prepared TiO(2)
exhibited exceedingly higher activities on the catalytic oxidation of benzene compared to the commercial TiO(2), probably due to the difference of their pore suructures

Nagamine, Shinsuke

Sasaoka, Eiji

Takahashi, Shinsuke

Okayama University Digital Information Repository

Synthesis and Catalytic Activity of Surfactant-Templated Nanoporous TiO(2)

Okayama University Scientific Achievement Repository

Journal of the Faculty of Enviromental Science and Technology,Okayama UniversityVo1.8, No.1, pp.93·96, March 2003Synthesis and Catalytic Activity of Surfactant-TemplatedNanoporous Ti02Shinsuke NAGAMINE*, Shinsuke TAKAHASHI** and Eiji SASAOKA**(Received October 23, 2002)Nanoporous Ti02 were synthesized via hydrolysis of titanium tetraisopropoxide (TrIP) in HCI acidicconditions using sodium dodecylsulfate (SDS) as a templating agent. The order of the nanostructure ofsamples before calcination was improved with increasing the SDSmIP molar ratio. The calcinedsamples presented uniform mesopore structure with the average diameter of 4 - 5 om and considerablylarge surface areas over 200 m2jg, although the periodicity of the nanostructure was exceedingly reducedby the removal of the template. The prepared Ti02 exhibited exceedingly higher activities on thecatalytic oxidation of benzene compared to the commercial Ti02, probably due to the difference of theirpore structures.Key words: template, titania, lamellar nanostructure, nanopore structure, catalyst1 INTRODUCTIONOn the design of a catalyst or a catalyst support, thecontrol of its pore structure is one of the most importantproblems. Since the reaction is caused on the surface ofcatalyst, a large surface area accessible for reactantmolecules is required. In this viewpoint, the presence ofmesopores where molecules can smoothly diffuse isadvantageous for catalytic reactions. Recently,nanoporous solids with controlled pore sizes and highsurface areas have been synthesized using the surfactantmicellar structures as structure-directors (Kresge, et al.,1992, Ogawa, 1996, Tanev and Pinnavaia, 1996).In this study, the authors have attempted to preparenanoporous Ti02, which is widely used as aphotocatalyst or a catalyst support, with the help ofsurfactant as a template. The nanoporous Ti02 werepreviously synthesized using the ligand-assistedhydrolysis of titanium alkoxide templated byalkylphosphate (Antonelli and Ying, 1995, Putnam, et al,1997) or using nonionic alkylamine (Antonelli, 1999).Here we consider that anionic surfactants are suitable for• Graduate School of Natural Science and technology, OkayamaUniversity.•• Department of Environmental Chemistry and Materials, Faculty ofEnvironmental Science and technology, Okayama University.93the template to give a nanostructure to Ti02 because ofits tendency to be positively charged in acidic conditions.In this study, nanostructured Ti02 templated by ananionic surfactant sodium dodecylsulfate (SDS) weresynthesized via the hydrolysis of titaniumtetraisopropoxide (TrIP). The dependence ofnanostructure on the molar ratio of SDS to TrIP wasinvestigated. The prepared Ti02 were applied to thecatalytic oxidation of benzene, the emission of which toatmosphere is restrained recently. The activities of thesamples were compared with that of a commercial Ti02and their dependence on the pore structures werediscussed.2 EXPERIMENTAL SECTIONSDS, TrIP, and HCI aqueous solution were providedby Wako Chemical Co. Ltd., Japan. Thesurfactant-templated Ti02 were prepared by thefollowing procedure. An adjusted amount of SDS wasfully dissolved into 20 ml of HCI aqueous solution (1.0mol/l). 5.68 g of TrIP was added to the solution,resulting in the immediate formation of whiteprecipitates. The molar composition of each componentwas TrIP: 1.0, SDS: 0 - 1.0, H20: 55.6, and HCI: 1.0.The resultant white precipitates were immersed in 40 mlof ion-exchanged water and kept at 323 K for Ih to94 J. Fac. Environ. Sci. and Tech., Okayama Univ. 8 (1) 20033 RESULTS AND DISCUSSIONThe nanostructures were characterized by XRD forsamples both before and after calcination, and typical50nmresults are presented in Fig. I for vanous SDSfITIPmolar ratios. At the SDSfITIP of 0.1, no obviousdiffraction peak but only a shoulder is observed. At theSDSfITIP over 0.2, a single diffraction peak exists at 2(}= 1.6 - 1.7 0 in the pattern before calcination, exhibitingthe presence of an ordered nanostructure with d value of4.6 - 5.2 nrn. The intensity and sharpness of thediffraction peak are enhanced with the increasedSDS/TTIP molar ratio. It is reasonable that a moreordered nanostructure is formed at a higher surfactantconcentration. The peak shifts to the higher diffractionangle with increasing the SDSfITIP ratio, indicating thedecrease in spatial periodic length of the nanostructure.This tendency probably results from the reduction ofthickness of Ti02 waIl due to the reduced amount of Ti02per SDS molecule. However, this diffraction peakdisappears after calcination, indicating that the orderednanostructure is severely damaged by calcination. Thisresult suggests the formation of the lamellarnanostructure supported by surfactant molecular bilayersbetween the discrete Ti02 sheet-like structures, which issimilar to the results as to the alkylphosphate-templatedTi02 by Putnam et al (1997). The retention of thenanostructure through the calcination is the mostsignificant problem and being studied at present.The XRD measurements in the range of 2() = 5 - 80 0showed that the samples were comprised of the anatasecrystal structure. The crystallite sizes were calculatedfrom the full-width at half maximum (FWHM) of the(101) diffraction peaks by Scherrer method. As to thecalcined samples, the crystallite size without the additionof SDS was 12.6 nrn, while the sizes in the presence ofSDS ranged in 4.5 - 5.0 nrn and decreased with theincreased SDSfITIP molar ratio. The reduction ofcrystallite size with the SDSfITIP ratio suggests thatSDS has an effect to suppress the growth of Ti02particle.The nanostructure after calcination was directlyobserved by TEM and an image is presented in Fig. 2.862After Calcination8 0642Before Calcinationoremove sodium ions. At this stage, the sample preparedat SDSfITIP = 0 turned to a transparent gel, while theothers are still white solids. The same treatment wasrepeated once again and then the solids were dried at 323K for 1 - 2 days. The dried solids were calcined at 723K in air for 3 h to remove the surfactant.The nanostructures of the prepared Ti02 werecharacterized using the powder X-ray diffraction (XRD,Shimadzu XRD-6100, Cu/Ku radiation, A = 0.154 nrn,operated at 30 kV and 30 rnA). The pore structures ofthe calcined samples were characterized by the nitrogenadsorption measurements (Micromeritics Gemini 2375)at 77 K after outgassing the samples at 573 K for 2 h innitrogen.The catalytic oxidation of benzene was carried outusing a packed-bed tubular reactor at reactiontemperatures ranging from 448 to 623 K under theatmospheric pressure. The weight and particle size ofpacked catalysts were 0.50 g and 1 mm, respectively.The inlet gas composed of benzene (50 ppm) and air(balance) was fed into the reactor at 250 cm3-STP/min.The outlet gas was analyzed by a gas chromatograph(Shimadzu GC-9A) equipped with a hydrogen flameionization detector (FID) after mixed with an adjustedamount of methane as a reference to give a standard peakon the chromatogram. The concentration of benzene inthe outlet gas was determined by the ratio of the peakareas of benzene to methane..e-';]=~-=-Fig. 1. Powder XRD patterns of samples prepared at variousSDSrrTlP molar ratios.Fig. 2. TEM image of a calcined sample prepared at theSDS / TTIP molar ratio of 1.0.S. NAGAMINE et al. / Synthesis of Nanoporous TiO, 95o8O&.-lL....f~L..L.L...a....L..&..I.....a....L...&....lU400 500 600 700Reaction Temperature [K]0.54 CONCLUSIONFig. 4. Dependence of conversion of benzene on the reactiontemperature for the oxidation of benzene catalyzed by Ti02"Nanostructured Ti02 were prepared by hydrolysis ofTTIP in Hel acidic conditions using SDS as a templatingagent and the dependence of the nanostructure on themolar ratios of SDS to TTIP was investigated. Theorder of the nanostructure increased with the increases inthe SDSmlP molar ratios. However, the orderednanostructure collapsed by the removal of SDS bycalcination at 723 K, suggesting the presence of lamellarstructure supported by SDS molecular bilayers. Thesamples had anatase crystal structure and the crystallitesize decreased with the increased SDSmlP ratio. Thecalcined samples showed high BET surface areas of 230m2/g at the largest and mesopore structures with uniformpore size depending on the SDSmIP molar ratios.1 ....._..,...,......__............~....__~The Ti02 prepared at two different SDSmIP molarratios of 0.1 and 0.2 were applied to the catalyticoxidation of benzene. The conversions of benzene areshown as the function of reaction temperatures in Fig. 4.For comparison, a series of results from a commercialTi02 is also presented. The conversions of benzeneincrease with the reaction temperature and exceed 90 %over 598 K for both prepared Ti02 and the activity isalmost independent of the SDSmIP molar ratio. TheTi02 prepared in this work exhibit much higher activitiescompared to that of commercial Ti02 at all the examinedtemperatures. The molecular size of benzene is ca. 0.6nm in width and therefore surfaces of nanometer-sizedpores accessible for the benzene molecules are expectedto efficiently contribute to the oxidation of benzene.The prepared Ti02 possess larger surface areasoriginating from the nanometer-sized pores than thecommercial one, resulting in the higher catalyticactivities on the oxidation of benzene.SDSffTIP = 0.2,....,I-0 ~=0 0.5 ~p/Po [-] N=0.15 ~=""'e0.1=e.-~0.05 ~~=eU5 10 15Dp [om]Partly, a few sheet-like structures are aligned in order,which are the traces of the SDS-templated nanostructureexisting before calcination. However, most part of thesample is formed by aggregated nano-sized particles.This result suggests the collapse of the orderednanostructure by calcination and is in accordance withthe disappearance of the diffraction peak in the XRi:>pattern.The pore structures of the calcined samples wereinvestigated by nitrogen adsorption measurements.Examples of nitrogen adsorption / desorption isothermsat the SDSmlP molar ratios of 0 and 0.2 are shown inFig. 3. The desorption isotherms show the steep changein the adsorbed volume at a certain p/Po depending on theSDSmIP ratio. These results indicate the presence ofuniform-sized mesopores, in spite of the reduction of theorder of nanostructure. Note that even the samplewithout SDS (SDSmIP = 0) shows a peak at thediameter of ca: 8 nm in the pore size distribution. Thisuniform mesopore structure, which is formed withoutany templating structure, is probably derived from thespaces between the nano-particles as shown in Fig. 2.The pore size distributions calculated from thedesorption isotherms by Dollimore-Heal method areshown together. The peak diameters are 4 - 5 nm anddecrease with the increased SDSmlP molar ratio up to0.2. The BET surface area reaches to the maximumvalue of 230 m2/g at SDSmlP = 0.2 and considerablylarger compared to the surface area of 60 m2/g evaluatedfor the sample prepared without SDS.SDSffTIP=O100~ SBET==- 61 m1/gEo-fIJI50..,e~...";;;...-0.5p/Po [-].-.0.15ec::l~ 0.1e~Iloo~0.05:\~~ 00 5 10 15Dp [om]Fig. 3. Nitrogen adsorption (open circles) / desorption (filledcircles) isotherms and Dollimore-Heal pore size distributionsof calcined samples.96 J. Fac. Environ. Sci. and Tech., Okayama Univ. 8 (1) 2003These mesopore structures, which appeared even withoutSDS, are supposed to originate from the spaces betweennano-sized particles attracted around the SDS molecularassemblies. The retention of the nanostructure throughthe removal of template is the most important problem tobe solved.The catalytic activity of the prepared Ti02 wasrevealed to be much higher than that of commercial Ti02when they were applied to the oxidation of benzene.This result was probably due to the large surface area ofnanometer-sized pores which allowed the easy access ofthe benzene molecules.REFERENCESKresge, C. T., Leonowicz, M. E., Roth, W. J., Vartuli, J. C., &Beck, J. S. (1992): Ordered mesoporous molecular sievessynthesized by a liquid-crystal template mechanism, Nature,359, pp. 710-712.Tanev, P. T., & Pinnavaia, T. J. (1996): Biomimetic templatingof porous lamellar silicas by vesicular surfactant assemblies,Science, 271, pp. 1267-1269.Ogawa, M. (1996): A simple sol-gel route for the preparationof silica-surfactant mesostructured materials, ChemicalCommunication, pp. 1149-1150.Antonelli, D. M., & Ying, J. Y. (1995): Synthesis ofhexagonally packed mesoporous Ti02 by a modified sol-gelmethod, Angewandte Chemie International Edition in English,34, pp. 2014-2017.Putnam, R. L., Nakagawa, N., McGrath, K. M., Yao, N., Aksay,1. A., Gruner, S. M., & Navrotsky, A. (1997): Titaniumdioxide-surfactant mesophases and Ti-TMS1, Chemistry ofMaterials, 9, pp. 2690-2693.Antonelli, D. M. (1999): Synthesis of phosphorus-freemesoporous titania via templating with amine surfactants,Microporous and Mesoporous Materials, 30, pp. 315-319.

Synthesis and Catalytic Activity of Surfactant-Templated Nanoporous TiO(2)

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