Effects of Zr/Ti molar ratio in SO42-/ZrO2-TiO2 calcined at different temperatures on its surface pr

Available online at www.sciencedirect.comJOURULFScienceDirectNATURALGASCHEMISTRYEL SEVIERJournal of Natural Gas Chermistry 21(2012)138 -147www.elsevier.com/locate/jngcEffects of Zt/Ti molar ratio in SO2- /ZrO2-TiO2 calcined atdifferent temperatures on its surface properties andglucose reactivity in near-critical methanolLincai Peng'，Junping Zhuangl*，Lu Lin1,2*1. State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China;2. School of Energy Research, Xiamen Universiy,Xiamen3611005, Fujian, China[Manuscript received July 17, 2011; revised September 11, 2011]Effects 0f Zr/Ti molar ratio in so2- /ZrO2-TiO2 solid acid catalyst calcined at different temperatures on its surface properties and catalyticactivity were thoroughly investigated in this paper. The physicochemical characteristics of prepared samples were determined by N2 adsorption-desorption, XRD, NH3-TPD and XPS techniques, respectively. It was found that the crysallization temperature of the samples increased afterthe combination of ZrO2 and TiO2; and phase transformations from the anatase to the ruile of TiO2 species and the tetragonal to the monoclinicof Zr02 species were efcively suppressed at higher temperature. The sample with a Zzr/Ti molar ratio of 3/1 calcined at 450 °C showed thehighest surface area and the most acid sites among all the tested samples. The acid site densities of samples were relatively closed to each otherif they were calcined at the same temperature, however, decreased with the calcination temperature. The result indicates that the sulfur contentin samples is a cucial factor to control the acid site density. Calcining the sample at 650。C and higher temperatures resulted in a sigificantdesorption of sulfate ion on the samples. The synthesized samples were evaluated as a potential catalyst for glucose conversion under the near-critical methanol conditions (200°C/4 MPa). The results suggested that the relatively weaker acid sites of the catalyst were more favorable forthe accumulation of methyl glucosides, while the moderate acid sites were responsible for the formation of methyl levulinate. The catalyticactivity for methyl levulinate production almost increases linearly with the catalyst acid sice density. The catalyst deactivation is due to the lossof sulfate ion and the two catalysts with Zr/Ti molar ratios of 3/1 and 1/3 could efectively alviate the deactivation caused by sulfate solutionin the reaction medium and can be reused after calcination with the reuse rate of over 90% in terms of the methyl levulinate selectivity.Key wordsso2- /ZrO2-TiO2; surface properties; catalysis; glucose reactivity; methyl levulinate1. IntroductionSince then, several improved procedures using sugars and cel-lulosic biomass as starting materials in the presence of variousacid catalysts have been reported. Levulinate esters are bi-Cellulose, a natural polymer consisting of glucose units,functional compounds bearing a keto group and an ester bond,is the principal structural component of biomass, and is abun-therefore, are preferred intermediates for chemical conversiondantly available on earth. A magnificent variety of high value-to many other useful products. On the other hand, levulinateadded chemicals and fuels can be accessible via the chem-esters can either be used in the flavouring and fragrance in-ical or biological conversion of cellulose [1- 3), which hasdustry or as additives for diesel transportation fuel [5].attracted more and more concerms due to the worldwide re-For future industrial application, an economically feasiblequirement to reduce our dependence on fossil fuels. It hasand highly selective process for the production of levulinatebeen well known that levulinate esters can be generated byesters from carbohydrate is strongly in demand. To developthe direct acid-catalyzed conversion of cellulose and sugarsthis process, the research and development of a proper cata-containing hexose in alcohol medium under heating condition.lyst is of importance and has become one of the hotspot of theThe first detailed study was performed by Garves in 1988 [4].research. Several kinds of acid catalysts, such as sulfuric acid●Crresponding authors. Te: +6-22236719; Fax: +86 20 26719;7 E-mail: zhuangjp@sc中国煤化工(L.Lin)The work was suppored by the National Key Basic Research Program (2010CB732201) froCNMH(-hnology of China, theNatural Science Foundation of China (U0733001, 50776035) and the Basic Research Foundation fromMHPersities (2010121077).CopyrightO2012, Dalian Instioute of Chemical Physics, Chincse Academy of Sciences. AlI rights reserved.doi:10.1016/S 1003-9953(11)60346-0.Joumal of Natural Gas Chemistry Vol. 21 No.2 2012139[4-6], p-toluenesulfonic acid [4], mixed-acid (for example,to adjust the pH value between 9 and 10 via magnetic stir-a combination of In(OTf)3 and PTSA [7]) and sulfonic acid-ring for 1 h, and then the mixed solution was aged at roomfunctionalized ionic liquids [8] were employed in this study,temperature for 24 h. The obtained precipitate was filteredand they have exhibited good catalytic performances. Un-and washed thoroughly with de-ionized water until no chlo-fortunately, these catalysts suffer from some of the unavoid-ride ion was left and then was dried at 60 °C for 48 h. The :able disadvantages, such as equipment corrosion, side reactiondried precipitate was powdered below 80 mesh, and then wasfrom the inter -molecular dehydration of alcohol which wasimpregnated with a 0.5 mol/L H2SO4 solution with a solu-used as reaction medium, separation and recycling problems,tion/solid ratio of 15 mUg and stirred at 500 rpm for 1h. Theenvironmental pollution, and/or high cost. In light of theseresulting precipitated solid was filtered, subsequently dried atproblems, the development of novel and environmentally be-110°C for 12 h and calcined at different temperatures rangingnign catalysts with high activity is expected.from 450 to 750 °C for 3 h in static air. The so2- /ZrO2 andSulfated metal oxides have been proven to be a type ofso4- /TiO2 catalysts were labeled as sZ and ST, respectively.promising solid acid catalysts for many acid-catalyzed re-For preparing so4 /ZrO2-TiO2 catalysts with different Zr/Tiactions in environmental friendly chemical processes [9, 10].molar ratios, the solutions of containing Zr and Ti were mixedSpecially, binary metal oxides, such as ZrO2-TiO2 and sup-in desired ratios at first and then followed the same proceduresported ZrO2-TiO2 mixed oxides, have attracted considerableas mentioned above. The samples thus prepared were labeledattention as modified catalysts for various applications due toas SZT-1, SZT-2, and SZT-3 according to the Zr/Ti molar ra-their profound surface acid-base properties and high thermaltios of 3/1, 1/1, and 1/3, respectively.stability. Many studies have also revealed that ZrO2-TiO2 andsupported ZrO2-TiO2 mixed oxides are very active catalysts2.2. Catalyst characterizationfor a variety of reactions, such as vapor-phase Beckmann re-BET surface area of the samples was measured by N2arrangement of cyclohexanone oxime to ε caprolactam [11],esterification of lactic acid [12], the coupling of hydrolysisadsorption-desorption at liquid nitrogen temperature on a Mi-and dehydration reactions to produce HMF and furfural fromcromeritics Autochem I 2920 instrument. Prior to analysis,biomass [13], naphthalene hydrogenation [14] and photocat-the sample was pre-treated at 300°C for 1 h to remove ad-alytic degradation of NOx [15]. However, there are few re-sorbed species on the surface.Crystal phase of the samples was examined by X-rayports on the use of the catalyst containing ZrO2-TiO2 mixeddiffraction (XRD) on a BRUKER D8 Advance X-rayoxides for the catalytic conversion of carbohydrate in alco-diffractometer with Cu Ka radiation. Data was collectedhol medium. As for this kind of catalyst, it is welll knownin the 20 range from 10° to 80° at a scanning rate ofthat ZrTi molar ratio and calcination temperature have enor-3%/min. All the phases were identified using the powermous influences on its surface properties and reaction activity.diffraction file (PDF) database (JCPDS, Intemational CentreIn this work, a series of so4- /ZrO2-TiO2 solid acid catalystsfor Diffraction Data). Crystallite size of tetragonal ZrO2 andwith different Zr/Ti molar ratios calcined at different temper-anatase TiO2 was determined from their characteristic peaks,atures were prepared by (Co-) precipitation and impregnationi.e.. at 20 = 30.180 (111) and 25.30° (101) for ZrO2 and TiO2,method with an aqueous solution of sulfuric acid. And theserespectively, using Scherrer equation [9,17].samples were characterized by many techniques to understandTemperature-programmed desorption of ammonia (NH3-the interactions in sulfated ZrO2-TiO2 mixed oxides. TheseTPD) was carried out on Micromeritics Autochem II 2920catalyst samples also were used for the glucose conversioninstrument to study the acidity of the catalysts. Typically,under the conditions of near -critical methanol (200 °C/4 MPa)about 0.2g of sample was first heated at a heating rate ofto understand the catalyst activity for biomass conversion and15 °C/min up to 600°C and kept for 0.5h ina flow of Heto obtain a correlation between the acidic properties and cat-gas (20 ml /min) to remove adsorbed species on the surface.alytic activity. Furthermore, the stability and reusability of theThe sample was cooled down to 100 °C in a flow of He gas,catalysts with different Zr/Ti molar ratios were also examined.and then followed by NH3 adsorption in 10% NH3 gas flow(balanced by He, 20 mL/min) for 1h. After the sample was2. Experimentalflushed with He (20 ml /min) for 1 h to remove the physicallyadsorbed NH3, the TPD data was recorded from 100 to 600 °Cwith a ramp of 15 °C/min. The peak areas were integrated to2.1. Catalyst preparationestimate the amount of acid sites of the catalyst.X-ray photoelectron spectroscopy (XPS) was done withA series of solid acid catalysts, so2-几ZrO2, so2- /TIO2,a Kratos Ultra system employing an Al K。radiation source.and So2 1/ZrO2-TiO2 with ZrTi molar ratios of 3/1, 1/1, andSamples were outgassed in a vacuum oven overnight before1/3 calcined at different temperatures (450, 550, 650, andXPS measurenptra of all elements750 °C) were prepared by (Co-) precipitation method and imin the catalyst中国煤化Lergy of 40eV andpregnation method with an aqueous solution of sulfuric acid resolution of CYHC N M H Gtive measurements[16]. ZrOCl2.8H2O (Shanghai Aladdin Reagent) and/or TiCl4of binding energy and atomic concenuraton. Charge effect(Tianjin Kermel Chemical Reagent Co.) were/was dissolvedwas corrected by adjusting the binding energy ofC 18 peak ofin de-ionized water. Concentrated NH4OH was added into itadventitious carbon to 284.6 eV..Lincai Peng et al./ Joural of Narural Gas Chermisty Vol. 21 No. 220122.3. Catalytic evaluation of the catalystumn with conductivity detector was employed to detect theso4- and a sodium hydroxide solution (30 mmol/L) wasThe catalytic evaluation for glucose conversion was car-used as the eluent with a volumetric flow rate of 1.5 mL/minried out in a 100 mL cylindrical stainless steel autoclave madeat 30°C. The amount of methyl levulinate (Alfa Aesar,by Parr instrument Co. In a typical reaction procedure, 2.5 g99% purity for calibration) was determined by GC (Agilentof glucose (Shanghai Bio Sci- Tech Co.), 1.25 g of solid acid6890 Instrument) equipped with an HP-5 capillary columncatalyst, and 50 mL of methanol (Guangdong Guanghua Sci-(30.0 mx320 μmx0.25 μm) and a flame ionization detectorTech Co.) were mixed together and poured into the reac-(FID) operating at 270°C. The following temperature protor. The reaction mixture was then heated to 200°C by ex-gram was used in the analysis: 40 °C (4 min)- -30 °C/min-termal heating and stirred at 500 rpm. After certain reaction250 °C(1 min). The product yield (namely, methyl glucosidestime, the reaction mixture was cooled and filtered to ob-and methyl levulinate) on a molar base according to reactiontain the catalyst, and then the liquid samples was collectedstoichiometry was calculated by the following equation [18]:Cix Mo x 100for analysis. .Yield (%)=(1)The quantitative analysis of methyl glucosides (Bei-Cox Mi .jing Yangcun Chemical Co., 98% purity for calibration)where, Co denotes the initial concentration of glucose; Ciand so2- in certain liquid samples was performed on anis the concentration of products obtained from the acid-ion chromatograph (DIONEX ICS -3000). A CarboPac PA1catalyzed glucose conversion in methanol medium; The(2 mmx250 mm) analytical column with an electrochemi-terms Mo and M; represent the molecular weight of glucosecal detector was employed to detect the methyl glucosides(180 g/mol) and products (194 g/mol for methyl glucosidesand a sodium hydroxide solution (80 mmol/L) was used asand 130 g/mol for methyl levulinae), respectively.the eluent with a volumetic flow rate of 0.35 mL/min atBased on the yield of methyl levulinate at 200°C for 1h,30 °C. An IonPac AS1 1-HC (4 mmx 250 mm) analytical col-the turnover number (TON) was calculated as follows [19]:Tumover oumber (molo.g-.- 1) = Moles of methyl leliane fomed(2)Mass of catalyst X reaction time3. Results and discussionsamples have no radically effect on their surface areas underthe relatively higher temperatures. The surface area variations3.1. BET surface areaof these samples are roughly proportional to the Zr/Ti molarratio changes based on the surface areas of SZ and ST.BET surface areas of SZ, ST and SZT catalysts withvarious Zr/Ti molar ratios calcined at different temperaturesTable 1. Physicochemical properties of sz, sT and SZTare presented in Table 1. In the case of sulfated single 0X-catalysts calcined at diferent temperatureside catalysts, sz and ST calcined at 450。C had nearly theCalcination BET surface Crysallite Amount of Density ofsame surface areas (93.3 and 92.8 m-/g, respectively). How-Catalyst temperatureareasize* acid siteb acid siteever, the surface areas of sulfated dual oxide catalysts after(°C)(m2/g)_(nm)(umoVg) (umo/m2)the combination of ZrO2 and TiO2 calcined at 450。C variedSZ45093.3417.34.5significantly. The largest surface area of 143.1 m2/g can beSZT-150143.1582.4.1obtained on SZT-1 sample and the lowest surface area wasSZT-287.73363.831.9 m-/g on SZT-3 sample. For the SZT-2 sample calcinedSZT-331.912142.2.5sT92.88.456.3.9at 450 °C, its BET surface area was about 87.7 m2/g which87.811.3354.74.0was slightly lower than those of sZ and ST. With the increas-78.7ing of calcination temperature, the surface area of samples de-73.8264.03.6creased linearly except SZT-3. This trend was more obviousSZT363.6252.3for the ST and SZT-1 samples; the surface area decreased fromST67.815.3278.4.179.914.4261.03.392.8 to 18.5 m2/g for the former and from 143.1 to 52.4 m2/g74.717.5246.6.3for the latter when the calcination temperature increased from450 to 750 °C. Similar effect of calcination temperature on66.016.4226.03.48.1158.9the surface area was also reported in Refs. [13,20]. How-40.368.6104.82.6ever, it is distinctive that the surface area of SZT-3 increased58.418.8159.02.7in the calcination temperature range of 450- 550°C, and then2.4130.82.5decreased if the temperature higher than 550°C. When theSZT.2中国煤化工127;83.2S2ZT-373.6calcination temperatures were 650 and 750 °C, respectively,TTYHC NMH G_ 34.2.8the surface areas gradually reduced with the Zr/Ti molar ra-ion; b esti-tios changing from 1/0 (SZ) to 0/1 (ST) via 3/1 (SZT-1), 1/1. Dermined from XRD alyis using Scerer cquation; b esti-(SZT-2), and 1/3 (SZT-3). It seems that the sulfated dual oxidemated from NH3-TPD measurement by integrating the areas of thepeaks; “the ratio of the amount of acid sites to BET surface area.Joural of Natural Gas Chemisty VoL. 21 No. 220121413.2. Crystalline structurecination at 450 °C. This could be attributed to that sulfate pro-moted ZrO2 increases crystallization temperature. It was alsoThe XRD patterms of sZ, ST, and SZT catalysts withproposed in several earlier literatures that the presence of sul-different Zr/Ti molar ratios as a function of calcination tem-phate, phosphate or tungstate can hinder the sintering process,peratures from 450 to 750°C are shown in Figure 1, and an therefor increase cystallization temperature and stabilizlethe corresponding results of the crystallite size estimated bysurface area of ZrO2 and ZrO2-TiO2 [23- 26]. When the cal-Scherrer equation are listed in Table 1. The formation ofcination temperature is set at 650 °C, both SZT-1 and SZT-2anatase phase could be detected on ST and SZT-3 samplessamples form crystalline solids. Reddy et al. reported that thecalcined at 450°C. The crstallile size of SZT-3 (12.9 nm) TiO2-Zr02 mixed oxide transfrms from an amorphous to awas slightly larger than that of ST (8.8 nm) when calcinationcrystalline compound if the calcinations temperature is higherat 450 °C, and they were about the same when calcination atthan 600 °C [16]. The formation of orthorhombic phase of550 °C. These results suggest that low Zr/Ti molar ratio hasZrTiO4 was observed for SZT samples, and its XRD peak in-little effect on the formation of anatase phase and crystallinetensity increased with the calcination temperature, indicatingparticles when the samples were calcined between 450 andcrystal growth of ZrTiO4. Several researchers also reportedSs0°C. On the other hand, the samples calcined at 550°C the formation of the ZrTiO4 compound at calcination temper-with high ZrTi molar ratios, i.e, SZT-1 and SZT-2 samples atures higher than 600°C for ZzrO-TiO2 126], NiSO4/ZrO2-were in amorphous form. Some researchers [15,21,22] alsoTO2 catalysts [27] and sO2 /ZrO2-TiO2 [16]. And our XRDreported the similar phenomenon in the ZrO2-TiO2 mixed ox-observations were in good agreement with those. In addition,ides. The SZ sample calcined at 450 °C exhibited amorphousSZT-2 sample shows an exactly stoichiometric compositionor poorly crystalline phase, and the formation of tetragonalphase of TiZr04; both SZT-1 and SZT-3 samples also ex-ZrO2 phase was observed when calcination at higher thanhibit the other diffraction peaks for the tetragonal and anatase550°C. However, Zou and Lin [221 reported that tetragonal phase structures, respectively. These resuls are similar to thephase structure can clearly be observed on pure ZrO2 after cal-previous study on ZrO2-TiO2 oxides after calcined at 800°C,▲Anatase TiO2■Tetragonal ZrO,(a(b)心jie会SZT-3| S2T-2L SZT-3SZT-1S21.2。sZ1-1SZ人i203(40 50 6(030507020/(° )’ Orthorhombic ZrTiO，▲ Anatase TiO2(cRutile TiO2(dMonoclinie ZrO,STsTSZT-3Orthorhombic ZrTiO,。. Tetragonal 2ZrO2SZT-2__ !口Monoclinic ZrO2Anatase TiO2SZ_sz中国煤化工10 204050 60 701020FYHCNMHG70 8020/(°)2010 )Figure 1. XRD pttems of the sz, sT and SZT catalys calcined at (a) 450 °C, (b) 550°C, (C) 650°C and (d) 750°C.142Lincai Peng et al/ Jourmal of Natural Gas Chemitry Vol. 21 No. 22012but only the tetragonal phase of ZrO2 was observed in oursponding results of the amount of acid site estimated fromstudy instead of the monoclinic phase due to relatively lowintegrating the peak areas are listed in Table 1. The tem-calcination temperature [22]. Furthermore, Reddy et al.perature at which the adsorbed NH3 desorbs is an indication[16] reported that a few extra lines due to the formation ofof the strength of the corresponding acid sites. And the acidTi2(SO4)3 and Zr(SO4)2 compounds can be detected for sul-sites existed on the tested samples were artificially classifiedfate ion impregnated ZrO2-TiO2 sample with Zr/Ti molar ra-as weak (100- -250 °C), medium (250 -450°C), and strongtio of 1/1 calcined at 800 °C, however, no distinct independent(450- 600°C) acid sites [28].lines due to these phases were observed for all samples in ourIt is clear that all the catalysts displayed a very broad des-present study. After calcined at 650-750°C, the sZ sampleorption profiles in the temperature range from 100 to 600°C,was in tetragonal phase with a small amount of monoclinicwhich is indicative of heterogeneous acid sites. The distribu-ZrO2. A large amount of anatase TiO2 in the ST sample wastion of the primary acid strength seems to be similar, thoughtransformed to the rutile phase after calcined at 750 °C andthe amount of acid site was different for the samples withthe crystallite size was markedly enhanced when the calcina-different Zr/Ti molar ratios calcined at different temperatures.tion temperature increased from 550 to 750 °C. It can also beTwo distinct desorption peaks were observed at temperatureseen that the crystallite size of all samples increased graduallybetween 100 and 200。C as well as 450 and 600 °C, attributingwith the calcination temperature. It seems that SZT samplesto weak and strong acid sties, respectively. The SZT-1 sam-with the combination of ZrO2 and TiO2 can effectively inhibitple exhibited the highest peak temperature of NH3 desorptionthe phase transformations from the meta- stable anatase to theappeared at 568 °C and the ST sample had the lowest peakthermally stable rutile of TiO2 and the tetragonal to mono-temperature with the strongest peak appeared at 510 °C afterclinic of ZrO2 as well as the formation of crystalline particles,calcination at 450°C. The peak corresponding to weak acidthereby improving the thermal stability of the samples.site was observed around 170°C for all the tested samples,and in most cases, the intensity of these peaks decreased with3.3. NH-TPD profilesthe calcination temperature. After calcination at 450°C, theamount of acid site on SZT-1 sample was the highest, whileNH3-TPD profiles of various catalysts calcined atthe SZT-3 catalyst had the lowest acid site amount among alldifferent temperatures are shown in Figure 2 and the corre-the tested catalysts. These were in well agreement with the(a(b)STSZT-3SZT-22SZT-1_S002003010020600Temperature (C)(c)d)S2中国煤化工-YHCNMHG500- 6o0Temperature(C)Figure 2. NH3-TPD profiles of the sz, ST and SZT catalysts calcined at (a) 450 °C, (b) 550 °C (c) 650。C and (d) 750°C.Joumal of Natural Gas Chemistry Vvol. 21 No. 22012143results of the surface area measurements. The highest valueTable 2. Zr/TI molar ratio and S content of variousof acid site on SZT-1 sample may be due to its largest surfacecatalysts measured from XPS analysisarea, and vice versa, for the SZT3. With the increasing of cal-CalcinationMolar ratio of ZzrTiS contentCatalystcination temperature, the amount of acid site decreased for thetemperature (°C) controlled measured(wt%)SZ, SZT-1, SZT-2 and ST samples, presumably due to the losssz4503.32of sulfate groups on the catalyst surface. It was reported thatSZT-13/3.18/13.52surface acid properties were strongly dependent on the pre-SZT-2 .1/1.09/1.53SZT-25501.16/13.37treatment temperature [18,29]. However, the amount of acidsite on SZT-3 sample increased first, and then decreased after6501/11 .02/11.557500.99/11.06the calcination temperature reached 550°C. It probably muchSZT-31.05/33.89depends on the textural properties at the lower calcinationS2.83temperature. The variations of acid site amount of ST sarmplewere the most noticeable among all the catalysts. The amountof acid site decreased from 456.3 to 34.2 umolg when thestrongly affect its catalytic activity [30,31]. Our experimen-calcination temperature increased from 450 to 750 °C. Com-tal data in Table 2 shows that the S content in SZT samplesbined with the results of XRD, it might be related to the for-was slightly higher than that in SZ and ST if these catalystsmation of rutile TiO2 phase.were all calcined at 450 °C. Among these tested catalysts,Generally speaking, the effects of Zr/Ti molar ratio andSZT-3 catalyst had the largest S content of 3.89 wt%; how-calcination temperature on the amount of acid site were sim-ever, its amount of acid site was the lowest. This findingilar to those on the surface area. Therefore, the distributionsuggests that even if the S content of a catalyst is high, itsof acid site on the catalyst surface (namely the density of acidacid site amount may be not high. A comprehensive anal-site) was also evaluated, and the resuts are listed in Table 1. ysis found that the acid site amount of the catalyst mightLike the amount of acid site, the acid site density was alsoalso relate to its textural properties. When the calcinationconsidered an important indicator to determine the catalysttemperature increased from 550 to 650°C, the S content inactivity [12,13]. It is clear that the acid site density of all theSZT-2 sample declined dramatically from 3.37 to 1.55 wt%，catalysts was relatively closed to each other under the sameindicating there were serious desorption and decompositioncalcination temperature, which declined gradually as the cal-of sulfate ions. Similar phenomenon was also observed incination temperature increasing from 450 to 750 °C. The acidprevious study, where the asymmetric stretching band inten-site density of ST sample was the highest after calcination atsity of the S= 0 bonds was determined by FT-IR [26]. The450 °C (4.9 pumol/m2); however, it was also the fastest-alling0 ls profile of the so4- /ZrO2-TiO2 catalyst is more com-with the calcination temperature. This could be caused by itsplicated because of the overlapping contribution of variousoxygen species from zirconia and titania as well as sulfatecrystalline structure that a large amount of TiO2 in anatase[16]. It can be seen from Figure 3(b) that the SZT-2 cata-phase was transformed to the rutile phase after calcination at lyst produced a broad range of the 0 Is photoelectron profile.750 °C (see Figure 1d).Two distinct peaks were observed at 531.4 and 529.9 eV, re-spectively, and the relative peak intensity at 531.4eV was3.4. XPS analysisslightly higher than that at 529.9 eV after calcination at 450and 550 °C. However, when the calcination temperature in-To obtain information regarding to chemical environmentcreased to 650°C, the results were opposite, and the peakand elemental concentration presented in various samples inintensity at 529.9 eV increased significantly. At higher cal-detail, XPS experiments were carried out for the sz, STcination temperature such as 750 °C, only one 01s peak wasand SZT catalysts with various ZrTi molar ratios calcinedobviously observed at 529.9 eV. This observation was similarat 450 °C. Moreover, the SZT-2 sample calcined at differentto that reported by Reddy et al. [16], where an 0 Is peak wastemperatures was investigated. The XPS spectra revealed thatobserved at 531.3 eV if the sample was calcined at 800°C.all the SZT samples consist of O, Z, T and small amountsThe variations of peak intensity and binding energy can beof S. The XPS analysis indicated that there were only smallattributed to different chemical environments at various cal-cination temperatures. On the basis of the above mentioneddifferences between the measured and nominal ZzrTi molaranalysis, one can conjecture that it might be related to theratios in SZT samples calcined at 450°C, and even in SZT-2formation of crystalline ZrTiO4 phase and the partial loss ofsamples calcined at different temperatures, as shown in Ta-sulfate groups in SZT-2 sample. Figure 4(a) shows the bind-ble 2. The XPS spectra ofS 2p, 0 Is, Zr 3d andTi 2p recordeding energy of the Zr 3d photoelectron peaks at ca.184.7eVunder high-resolution scans are shown in Figures 3 and4,re- and 182.4 eV for Zr 3dxn and Zr 3d