Mohammed Khairy Abdel â€“Fattah Omran_paper_03

7/29/2019 Mohammed Khairy Abdel Fattah Omran_paper_03

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Structural features and photocatalytic behavior of titaniaand titania supported vanadia synthesized

by polyol functionalized materials

Mohamed Mokhtar Mohamed *, W.A. Bayoumy, M. Khairy, M.A. Mousa

Chemistry Department, Faculty of Science, Benha University, Benha, Egypt

Received 31 March 2007; received in revised form 28 May 2007; accepted 29 May 2007Available online 13 June 2007

Abstract

The micro-mesoporous TiO2 materials obtained from hydrothermal crystallization of polyols (xylitol, glucose and sorbitol)/TiCl 4/acetylacetone/H2O systems were characterized; and compared with VOx/TiO2, by N2-sorption, TG/DTA, X-ray diffraction, IR andscanning electron microscope. Through comparison of the results, it was found that glucose assembled TiO2 (TG) calcinations at623 K exhibited the formation of rutileanatase nanoparticles (15 nm) as irregular shapes with disordered holes on particles surfaces,gyroidal structure (25 nm) following calcining at 773 K and cotton fibrils structure (10 nm) following vanadium incorporation (TG/V).Increasing the concentration of xylitol (TXY/C) significantly enhanced microporosites but decreases the specific surface area (309 m

2/g) and wall thickness (0.609 nm); as compared to T XY (356 m

2/g, 0.929 nm). TG presented remarkable increase in wall thickness(2.142 nm) that was principally responsible for enhancing the durability as well as the activity of this material towards photocatalyticreduction of Hg2+ (giving 100% reduction activity at 80 min reaction time) in aqueous medium. Additionally, this material offeredlow acid content compared with other polyols assembled TiO2 but in contrast confirmed high accessibility of their basic sites. 2007 Elsevier Inc. All rights reserved.

Keywords: Micromeso TiO2; Polyols templates; V2O5/TiO2; Characterization; Catalytic applications

1. Introduction

Porous materials have been investigated for decades dueto a variety of potential applications [15]. Recently, impor-tant progress has been made in the synthesis of ordered mes-oporous structures [69] to fabricate custom-made materials

for specific uses including adsorption and catalysis, separa-tion processes, drug delivery, optics and electrochemistry[1015]. For most of these applications high surface areaand interconnectivity among the mesoporous network aredesirable in order to optimize the activity of the surfaceand to provide diffusion, charge or light transfer or reactant

access into the cavities [16,17]. Due to their controlled poresize and a very narrow pore size distribution, the orderedmesoporous materials have significant potential as catalyticsupports in fine chemistry, pharmaceutical industry, as wellas for the production of special polymer materials [1820].Unfortunately, the actual use of the materials has been

severely hampered by their poor stability especially hydro-thermal and mechanical are below practical levels to synthe-size industrial application. Therefore, there is a strong needfor hydrothermal and mechanical stable structures.

On the other hand, materials with combined micro- andmesoporosity can offer significant supplementary benefits,such as an enhanced diffusion rate for transport in catalyticprocesses (faster reactions); better hydrothermal stability;multifunctionality to process a large variety of feedstocks;capabilities of encapsulated waste in the micropores; con-trolled leaching rates for a constant and gradual release

1387-1811/$ - see front matter 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.05.055

* Corresponding author. Present address: Umm Al-Qura University,Faculty of Applied Science, Chemistry Department, Saudi Arabia.Tel.: +966 206073049.

E-mail address: [email protected] (M.M. Mohamed).

www.elsevier.com/locate/micromeso

Available online at www.sciencedirect.com

Microporous and Mesoporous Materials 109 (2008) 445457
mailto:[email protected]:[email protected]


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of an active component, etc. [2123]. Therefore, greatefforts has been made to prepare ordered porous materialswith controlled mesoporosity and microporosity. Micro-mesoporous metal oxides such as TiO2; is of great interestbecause of its optic and photocatalytic properties, havebeen prepared in one-step synthesis using a single surfac-

tant as template under hydrothermal conditions [24,25].Different synthetic routes including solgel reactions andtemplating methods have been used to prepare titania-based materials with improved textural properties [26,27].The solgel method is frequently used to prepare materialswith specific textural properties because hydrolysis andcondensation rates determine the synthesis pathway ofthe experimental conditions.

TiO2-based photocatalysts have been most extensivelystudied despite its limited use in the UV region. In contrast,it was found that vanadates show photocatalytic activities inthe visible range for H2O molecule decomposition and par-ticularly BiVO4 and InVO4 represent typical examples

[28,29]. This motivated us to synthesize vanado-titanate;an important class of catalytic materials which are applica-ble in selective oxidation of aliphatic or aromatic hydrocar-bons and SO2 in the production of sulphuric acid, etc., andcompare with TiO2 either assembled by polyols functional-ized compounds or conventionally synthesized using ammo-nia solution aiming at a comprehensive view of polyolsassembled titania besides vanado-titanate catalysts family.

In this work ordered microporousmesoporous titaniahas been prepared by using polyol compounds includingxylitol, glucose and sorbitol as templates and titanium tet-rachloride as titania precursor in the solgel reactions. The

micro-mesoporous structures have been characterized byscanning electron microscopy and by gas adsorption at77 K. Bulk properties, morphologies and thermal stabilitiesin addition to acidities were investigated using XRD, IR,TG/DTA and pyridine-FT-IR techniques. Special atten-tion is given to the photocatalytic reduction of Hg 2+ overall synthesized materials with emphasis to their structuralcharacteristics.

2. Experimental

2.1. Materials

The materials used in our preparations are: titanium(IV)chloride (TiCl4) (99%) provided from BDH, ammoniummeta-vanadate (99%) and acetyl acetone (98%) fromSigma, ammonium hydroxide (28%) provided from Adwic.During preparation the following polyol compounds wereused: xylitol (XY) {HOCH2[CH(OH)]3CH2OH} (98%)from Aldrich, glucose (G) {O[CH(OH)]4CHCH2OH}(99%) and sorbitol (S) {HOCH2[CH(OH)]4CH2OH}(99%) from Adwic.

2.2. Preparation methods

The titanium dioxide was prepared by two methods:

2.2.1. Conventional method

TiO2 was prepared by the hydrolysis of TiCl4 withammonium hydroxide as follows: 150 ml of TiCl4 wasadded to 20 ml of distilled water contained in an ice bath,then ammonia solution (28%) was added dropwise withvigorous stirring till pH value of 8. The white precipitate

was filtered and washed with distilled water till free fromchloride ions (AgNO3 test). The precipitate was then driedat 383 K for 12 h, and then calcined at 773 K for 6 h. Thissample is denoted as T.

2.2.2. Polyols as template for TiO2 synthesis under

hydrothermal conditions

The different samples of TiO2 were prepared in presenceof xylitol, glucose and sorbitol.

A definite weight (12.60 g) of polyol compound eitherxylitol, glucose or sorbitol was dissolved in 100 ml of dis-tilled water. This solution (25 ml) along with 43.90 mmolof TiCl4 were added simultaneously to a beaker containing

43.90 mmol of acetylacetone with vigorous stirring for 4 hin an ice bath. The resultant gel was then sealed in auto-clave and heated at 353 K for 5 days. Afterwards, theresulted precipitate was filtered, washed several times withdistilled water and then dried at 393 K for 12 h. Finally, thetemperature was gradually raised from 393 K to 623 K in2 h and then calcined at 623 K for 4 h. Portions of thismaterial were further calcined at 773, 873 and 973 K. Thetitanium oxide materials derived from xylitol, glucose andsorbitol are denoted, respectively as TXY, TG and TS. Sim-ilarly, other TiO2 samples were prepared using xylitol withdifferent concentrations (molar ratio 1:4 of TiO2:xylitol)

and calcination temperatures (623 K, 773 K). Such solidsare denoted as TXY/C(623 K)where subscript C signifies highxylitol concentration and 623 K represents the final dryingtemperature of the sample.

2.2.3. Synthesis of vanadiatitania using polyol as template

under hydrothermal conditions

The V2O5/TiO2 material with a 6 wt% V2O5 was synthe-sized by the polyol-template method as discussed above.The required quantity of ammonium meta-vanadate(Fluka, AR grade) was dissolved in an aqueous oxalic acidsolution (1 M). To this clear solution, the template andTiCl4 were added simultaneously under vigorous stirringthen transferred to an autoclave and heated. These materi-als are denoted as TG/V and TXY/V.

2.2.4. Physicochemical characterization of materials

Powdered X-ray diffraction (XRD) patterns wasrecorded on a diifractometer (made by Diano Corporation,USA) using co-filtered CoKa radiation (k 1.79 A). Theparticle shape and the particle size distribution of the pre-cipitate were obtained by a JSM-5200 scanning electronmicroscope (JEOL) using conductive carbon paint.

FT-IR spectra of the samples were recorded in a Bruker(vector 22) spectrophotometer, in the range of 4000

400 cm

1, using KBr pellets.

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The surface properties namely BET surface area, totalpore volume (Vp), mean pore radius (r

) and other param-eters were determined from N2 adsorption isotherms mea-sured at 77 K using conventional volumetric apparatus.The total pore volume was taken from the desorptionbranch of the isotherm at p/p0 = 0.95, assuming complete

pore saturation.The thermogravimetric (TG) analysis and differentialthermal analysis (DTA) were carried out on a TG/DTAapparatus (Shimadzu 50, at a heating rate of 10 C/minin flowing high purity N2 gas with 10 ml/min).

The surface acidity was investigated using the irrevers-ible adsorption of pyridine on the solid surfaces. The sam-ples were activated for 3 h under vacuum (105 Torr) at473 K prior to admission of 8 Torr of pyridine at roomtemperature on the samples. IR-spectra of pyridine-adsorbed on samples were depicted by subtracting spectrabefore and after exposure to pyridine.

2.3. Photoreactor and experimental procedure

The UV source is a 12 W Hg lamp (output mainly at330 nm, Toshiba SHLS-1002A) that was placed 25 cmabove the batch photoreactor. It should be mentioned thatan appropriate cut-off filter is placed to completely removethe radiations that were shorter than 330 nm. The lightintensity was estimated to be approximately 15 W/m2 fromthe light source incident to the top of the Pyrex beaker withthe total volume of 500 ml. The UV-irradiated photoreac-tor was placed on a magnetic stirrer to ensure homoge-neous mixing during irradiation. The catalytic activity

was studied by measuring the photocatalytic reduction ofthe HgCl2 on the prepared catalysts using the followingmethod. The catalyst (0.1 g) was suspended in a fresh aque-ous HgCl2 (95%) provided by Adwic in distilled water solu-tion (Co = 50 ppm, 100 ml). The suspension was stirred inthe dark for 35 min to ensure the establishment of adsorp-tiondesorption equilibrium of Hg2+ ions. Prior to expos-ing the suspension to UV irradiation, sorption kineticsexperiments were determined. The samples were withdrawnfrom a sample point at certain time intervals and analyzedfor Hg2+ reduction. UVvis Shimadzu 2101-PC Spectro-photometer was used to follow up the reduction of Hg2+

as mercury(II) dithiazonate [30]. A pH-meter (Hach) wasused to the adjustment and record of pH variation duringthe process. The pH of the dye solutions was adjusted priorto irradiation by NaOH or H2SO4 at pH 6.

3. Results and discussion

3.1. Morphology and thermal stability

The synthesis of TiO2 assembled by polyols as well asvanadium incorporated titania during synthesis was fol-lowed by thermal analyses (DTA/TG). The TG result ofthe T sample showed a weight loss of 13.9% at tempera-

tures below 713 K (Table 1) beyond it no further loss in

weight was detected. DTA, on the other hand, shows threepeaks: one endothermic at 353 K and two exothermic at553 K and 653 K. The peak at 353 K is due to desorptionof water (6.7%) and the exothermic peak at 553 K isassigned to the crystallization of amorphous TiO2 to ana-tase structure as confirmed by the work of Bekkermann

et al. [31]. Accordingly, the peak at 653 K is attributed toa phase transition from anatase to rutile that was accompa-nied by no weight loss. No further phase change isobserved up to 1273 K. On the other hand, XRD patternof this sample presents both rutile and anatase structures(Fig. 1 and Table 2).

TG/DTA data of TXY, TXY/C and TXY/V samples aresummarized in Table 1 and they all indicate anatase struc-ture (Fig. 1) with no evidence of V2O5 species in TXY/V.These samples exhibited small reduction in intensities ofthe anatase peaks when compared with the correspond-ing one in T. TXY sample showed a weight loss of ca. 37.5%ascribed to oxidative thermolysis of xylitol in the sample.

From DTA results, two endothermic peaks are observedat 493 K and 773 K. The lower peak is possibly associatedwith the decomposition of xylitol in wide pores or thatremained on the surface. While the higher peak is due tothe decomposition of xylitol in narrow pores. It can benoted that the nanocrystal formed for TXY with particlessize of 15.1 nm (as depicted from XRD data Table 2)showed stability against temperature till 1121 K reflectingthe stability of the synthesized anatase from xylitol. TheDTA/TG of TXY/C showed higher total weight loss(47.6%) than TXY along with displaying three endothermicpeaks in the DTA curve instead of two: the first peak at

$353 K is attributed to water desorption, the last twopeaks at 543 K and 873 K are assigned to the decomposi-tion of xylitol molecules interacting with titania in differentporosities. Exceeding the last endothermic peak in TXY/Cinto 873 K when compared with TXY (773 K) during struc-tural reorganization while forming TiO2 may give a hintabout increasing the microporosity of the former compar-atively as confirmed by surface texturing data that will beelaborated later. The calculated pore wall thickness (Table2) designated for some of the samples was in the order:TXY > T > TXY/C. The increased weight loss in TXY/V com-pared to T, TXY/C and TXY shows greater incorporation oftemplate molecules (Table 1 and Fig. 2). The DTA thermo-gram of TXY/V shows one endothermic peak at 353 K, sim-ilar to that seen in TXY, and two exothermic peaks at 543 Kand 753 K (unlike those of endothermicity in TXY at 493and 773 K), ascribed to crystallization of amorphous phaseto anatase and oxidative decomposition of both xylitoltemplate and vanadate, respectively. Other exothermicpeak positioned at 973 K; that was accompanied by noweight loss, is likely ascribed to phase transition of anataseto rutile that was markedly lower than that denoted forTXY (1133 K), signifying the lower thermal stability ofthe produced anatase of TXY/V.

The thermal analysis of TS shows a higher weight loss

than that for TXY (Fig. 3 and Table 1) and DTA curves

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of different shapes. The DTA shows one endothermic peakat 353 K and two exothermic peaks at 553 K and 1073 K.The loss of weight 13.4% in the range of 498613 K, asso-ciated with the exothermic DTA peak is attributed to theloss of sorbitol. The other exothermic peak at 1073 K,accompanied with no weight loss is ascribed to transforma-tion of anatase to rutile. This shows that anatase synthe-sized from sorbitol is less thermally stable than thatdevoted from xylitol.

The XRD patterns of these samples (TXY, TXY/C, TXY/Vand TS) were comparable to tetragonal structure of moreor less similar lattice parameters to reference data ofTiO2 (taken from international Tables for crystallogra-phers; JCPDS-No. 21-1272). The particles size were ofthe range 1115 nm (Table 2). The smaller size of thesematerials when compared with T sample (25 nm) suggestshindering of TiO2 crystal growth in the presence of xylitoland sorbitol. Interestingly, these samples; except TXY/V,showed a peak in the low-angle XRD patterns at about2h = 2, displaying the presence of some ordered mesopor-ous structures of anatase phase [32].

The image of the T sample characterized by SEM shows

spherulitic homogenous grains of varying dimensions

(Fig. 4). This shows a particle size distribution around25 nm. The morphology of TiO2 samples produced whenusing xylitol shows irregular structure consisting of parti-cles of different shapes and sizes with lower average sizes(15 nm) than those of T sample. The TXY/C sample showsan aggregation of small particles of cotton fibrils (11 nm)structure; illustrative of the dependence of TiO2 particlessize on xylitol concentration. Similarly, TXY/V possesseduniform particles size of cotton like structures with highdispersity (13 nm). These results were in good agreementwith those calculated from XRD lines using Scherrerequation.

The thermal analyses for TG and TG/V samples showed ahigher weight loss for TG (71.9%) than that for TG/V(49.6%). The DTA thermogram of TG (Fig. 5) showedtwo small exothermic peaks at 673 and 773 K with a smallundefined endothermic peak at low temperatures. The for-mer peaks are likely attributable to the oxidative thermol-ysis of glucose, respectively, in wide and narrow poresbecause these temperatures emerged in the range of 579888 K that accompanied by a weight change comprisedof 62.3%. Decreasing the weight loss in the lower tempera-

ture region (less than 373 K Table 1) suggest a decrease in

Table 1Thermal analysis data

Samples Temperature range (K) Weight loss (%) Peak temperature (K) (DTA) Type of peak Assignment

T 295398 6.7 353 Endo W. L454623 5.1 553 Exo Ph. T (a)623713 0.9 653 Exo Ph. T (b)295713 13.9

TXY 298407 2.8 493 Endo Dec. T in wide pores403605 24.6 773 Endo Dec. T in mic. pores606670 1.9670837 5.6888973 1.32981121 37.5

TXY/C 295388 11.2 353 Endo W. L481637 21. 9 543 Endo Dec. T (a)295823 47.6 873 Endo Dec. T (b)

TXY/V 295431 17.5 353 Endo W. L484632 16.8 543 Exo Ph. T (a)632770 23.3 753 Exo Dec. T (b)295770 59.6 973 Exo Ph. T (b)

TS 295448 12.0 353 Endo W. L498613 13.4 553 Exo Ph. T (a)613788 12.5 1073 Exo Ph. T (b)295788 39.5

TG 295448 1.2 673 Exo Ph. T (a)448579 8.4 773 Exo Ph. T (b)579888 62.3295888 71.9

TG/V 300423 9.3 373 Endo W. L479713 18.1 603 Exo Ph. T (a)713792 4.0 1073 Exo Ph. T (b)7921098 15.93001098 49.6

Ph. T (a): phase transition from amorphous to anatase; Ph. T (b): phase transition from anatase to rutile; W. L: water loss; Dec. T (a): decomposition oftemplate in wide pores and Dec. T (b): decomposition of template in micropores.



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the concentration of protons considerably; compared withother samples. This may favor the formation of a more

rutile phase; as conceived from XRD pattern of this sample(Fig. 6) that committed higher crystallinity favoring com-paratively the later phase subsequent heating at 623 K(Table 2). We suppose the broadness of DTA profile andDTG curve without existence of real distinctive peaks;indicative of different stages of decomposition, is relatedto the strong interaction of glucose molecules with titaniamoieties. It exhibited also the thickest wall among all sam-ples proposing highest thermal stability. This high thermalstability is probably correlated with increasing the number

Fig. 1. XRD patterns of: (a) T, (b) TXY(623 K), (c) TXY/C(623 K), (d) TXY/Vand (e) TS.

Table 2

XRD data of the investigated TiO2 and V2O5/TiO2 materials

Samples aD (nm) Lattice parameters Cell volume (nm3) Phase Cryst (%) WT

a b c A R

T 18.2 4.626 4.556 2.955 6.23 Mixed 75 25 0.886TXY(623 K) 15.1 3.809 9.44 2.48 Anatase 40 0.929TXY/C(623 K) 11.3 3.809 9.44 2.47 Anatase 43 0.609TXY/V 12.9 3.792 9.556 2.51 Anatase 62 TS(623 K) 11.3 3.799 9.504 2.50 Anatase 48 TG(623 K) 15.2 4.632 4.553 2.956 6.2 Mixed 17 83 2.142TG(773 K) 22.6 4.632 4.578 2.95 6.26 Mixed 57 43TG/V 10.1 3.8 9.444 Anatase 40

R %: Weight fraction of rutile, WR = AR/KAAA + AR, A %: weight fraction of anatase WA = KAAR/KAAA + AR; where KA is the coefficient of anataseand AA is the integrated intensity of anatase, D: particle size of crystallites and WT: wall thickness (nm) = unit cell pore size.

a

From most prominent XRD lines.

Fig. 2. DTA and TGA (DTGA) curves of TXY/V(623 K).



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of CHOH chains in glucose; exceeding those in xylitol, as

well as the potential of CHO groups. The ability of inor-ganic precursor to form polyoxoions in solution; which isable to undergo further condensation, is thought to be aprerequisite for the formation of stable structures [33].The stability of oxoanions can be correlated with evolutionof decreased crystallites size and anatase to rutile transfor-mation as emerged in our case wherein major percentagesof rutile (83%) were developed. This relevant result indi-cates that more glucose molecules aggregates around tita-nia moieties; in the initial stage, due to strongelectrostatic interaction between charged polyol (negativelycharged) and titania species (positively charged) in acidicsolution. This induced stronger binding of titania with glu-cose than xylitol and sorbitol could be correlated withhigher charge density and flexibility of the former compar-atively. Increasing the temperature to 773 K (TG(773 K))showed an increase in the percentages of anatase crystallin-ity as well as in particles size (Table 2). Interestingly,TG/V(623 K) showed only an anatase phase with the absenceof any lines ascribable either to rutile or to vanadium oxidespecies. This indicates that the presence of vanadium pro-vokes the formation of only anatase phase.

The SEM image of the as-synthesized TG(623 K) showsparticles of irregular (15 nm) shapes with some disorderedholes on particles surfaces compared to the TS (14 nm)

sample, which presented more uniform particles of flaky

structure. TG(773 K) shows the effect of calcinations on themorphology of this material in exhibiting big particles(25 nm) of gyroidal structural obtained as a result of sinter-ing the particles by calcinations, as emphasized by XRDinvestigations. Vanadium incorporation during synthesis(TG/V(623 K)) affected the morphology by transforming the

irregular structure of titania into aggregated small particles(11 nm) of cottonfibrils structure. This points out thatassembling TiO2 using polyol groups those differ from eachother by very limited chain number can affect the morphol-ogy as well as the particle size of the synthesized TiO2.

3.2. Surface texturing

The N2 adsorptiondesorption isotherms are presentedin Fig. 7. The shape of the curve of T is typical of typeIV isotherm, while for TXY(C)623 K, TXY(C)773 K and TGthe isotherms are close to type II of the original Brunauerclassification. The results of N2-adsorption obtained for all

samples including the specific surface area, SBET; total porevolume, Vp; average pore radius, r

; micro and mesoporesvolumes and surfaces (Vmicp , V

widp , S

mic and Swid) are col-lected in Table 3. High concentration of xylitol resultedin higher microporosity (60%), larger pore radius (32 A)and microporous volume (0.238 ml/g). This can beexplained by inducing a higher number of xylitol chainswith titania whilst using high xylitol concentration in thesynthesis. At p/p0 above 0.8, the isotherms are character-ized by very steep adsorption indicative of mesopores.Generally, experimental isotherms measured on mesopor-ous solids display hysteresis loops (ads and des go along

different paths) in the mesopore region, however, such sit-uation is not achieved in TXY/C(773 K) showing that thedesorption branch did not meet the adsorption one form-ing a loop. This was due to the presence of some xylitolmolecules not completely degraded based on thermal anal-ysis data (Table 1). Given that glucose has larger chainlength than xylitol, a higher microporosity is depicted com-prised of 65%. Thus, the appearance of type II isothermdoes not signify the absence of microporosity. Using glu-cose as template produced TiO2 with thicker walls than restof materials. The external surface area (Sext) calculatedfrom the slope of the Vlt curves, was lower for TG thanfor TXY and TXY/C. This confirms that TG preserved highermicroporous characteristics (microporous volume andinternal surface) than the other two samples, TXY andTXY/C. This indeed was responsible for increasing the SBETof TG when compared with TXY. The pore volume of thesamples was found to be less sensitive to the increase inchains by CHOH units while employing the polyols. Theproportion of micropore surface area (obtained by t-plotanalysis) was the highest for TG that showed very smallhysteresis loop comparable to that obtained for TXY/C.Correspondingly, the former sample adopted Vlt plot(not shown) composed of both upward and downwarddeviations confirming the heterogeneity of the material

i.e. acquires both micro- and mesoporous character. In

Fig. 3. DTA and TGA (DTGA) curves of TS.



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contrast, the rest of samples revealed upward deviationsindicative of the presence of a capillary condensation inthe pores of the adsorbent; that is characterized of cylin-der-shaped pores, ink-bottle pores or spheroidal cavities[34]. In conformity these samples showed a peak in low-angle XRD curves displaying the presence of some orderedmesoporous structure unlike the case of TG that did notexhibit any peak in low-angle region. Despite the apparentabsence of ordered mesoporosity in TG, the data in Table 3demonstrate that this sample retains 35% mesoporosity.

3.3. FT-IR spectra

The FT-IR spectra of T, TXY and TXY/V are presented inFig. 8. Spectrum (a) that corresponds to T sample indicatestwo bands in the low frequency region at 499 and 434 cm1

assignable, respectively, to the stretching of TiOTi bonds

in anatase and rutile, similar to those revealed previouslyby Mohamed et al. [24,25]. The hydroxyl stretching region,on the other hand, shows bands at 3783 and 3693 cm1

corresponding to free hydroxyl groups of anatase and rutile[35] respectively, and 3661 and 3633 cm1 attributed,respectively, to bridging (Ti)2OH and hydrogen bondedOH groups [29]. It should be mentioned that XRD of thissample (T) indicates the presence of anatase and rutile.

The spectrum of TXY shows bands at 3758, 3683 and3655 cm1 in the hydroxyl groups region, as well as a bandat 552 cm1, in the low frequency region, all indicative ofanatase phase of TiO2. On the other hand, the spectrumof TXY/V shows bands at 3784, 3694, 3661, 3633 and3578 cm1 in the hydroxyl stretching region differs fromthose seen in TXY. A shift to higher frequency is depictedfor the bands at 3683 (3694) and 3655 (3661 cm1) after

vanadium incorporation as well as developing bands at

Fig. 4. (a) Scanning electron micrographs of: (a) T, (b) TS, (c) TG(623 K), (d) TG(773 K) and (e) TG/V(623 K). (b) Scanning electron micrographs of: (a) T, (b)TXY, (c) TXY/C and (d) TXY/V.



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3633 and 3578 cm1 those do not show up in TXY. This

suggests that vanadium incorporation assists the evolutionof hydroxyl groups, most probably of isolated ones,attached to different crystal defects, corners and/or edges;associated with the concomitant increase in anatase pro-portions (62%) when compared with TXY that acquiredlower amounts (43%). This comprehensive increase inintensities of OH groups following V incorporation as wellas developing some others may be related to destruction ofTiOTi linkages and the formation of TiOH and VOHon the surface of TiO2 crystallites. The disappearance ofthe 3758 cm1 band; in TXY, and the existence of the strongone at 3784 cm1; in TXY/V following vanadium incorpora-tion indicates the participation of vanadium in promotingthe latter OH groups. The low IR region of TXY/V sample,shows bands at 488, 536, 612sh and 715sh cm

1 thatappeared to be made up by contribution of anatase TiO2,in accordance with XRD results. However, the exhibitedband at 715 cm1 might be due to the formation of V2O5species [36]. Consequently, the modes at 488, 612sh and715sh cm

1 upon vanadium incorporation when comparedwith V free TiO2 sample; indicates only a band at552 cm1, implies that vanadia assist not only exposingother TiO2 anatase faces but also produces highly dispersedcrystalline V2O5 species; cannot detected by XRD compre-hending that its lower than 2 nm. Accordingly, the cell vol-

ume of this particular sample showed expansion;

contrasting to TXY, emphasizing the confinement of theV2O5 species inside titania lattice. On the other hand,matching up the spectrum of TXY/C (not shown) to thatof TXY, signifies no changes in intensity or in wavenumbersof all bands, except exhibiting a shoulder at 644 cm1 forTXY/C. This is mostly ascribed to stretching vibrations ofun-dissociated water [37]; taking into consideration thatanatase values in both samples are alike. In addition,increasing the xylitol concentration indicates a shift ofTiO stretching vibration, in the low IR region, from 552to 544 cm1 reflecting a decrease in the bond strength ofTiO linkages synthesized at high concentrations.

The FT-IR spectra of TG(623 K) and TG/V in comparisonwith T are given in Fig. 9. The spectrum of TG(623 K) indi-cates bands at 3750, 3678, 3650, 3620; in hydroxyl groupsregion, in addition to small bands at 2925 and 2853 cm1,in the CH stretching vibrations. The latter bands impliedthe presence of residual glucose molecules inside TiO2pores. Upon calcination (TG(773 K)) (not shown) a markeddecrease in intensity of all the bands was attained particu-larly in OH groups region; due to dehydroxylation, in addi-tion to vanishing those in the CH stretching vibrations

confirming complete thermolysis of glucose molecules.

Fig. 5. DTA and TGA (DTGA) curves of TG.

Fig. 6. X-ray diffraction of: (a) T, (b) TG(623 K) and (c) TG(773 K).



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Vanadium incorporation during TiO2 synthesis using glu-cose induces a noticeable increase in intensities of hydroxylgroups at 3750, 3677 and 3650 cm1 seen in TG where thatat 3620 cm1 was almost vanished. This explains that vana-dium preferentially interacts with specific faces of TiO 2 spe-cifically those constitute the latter OH groups. Accordingly,intense bands due to alkyl chain (CH)n vibrations (2962,

2924 and 2853 cm1

) were developed following vanadiumincorporation pointing that the interaction between Ti

and V species was at the expense of that with glucose. Inaddition, the absence of V2O5 species; as depicted forTXY/V at 715 cm

1, proposes the formation of VxTi1xO2phase responsible for the transformation into anatasephase. A similar interaction has been proposed betweenvanadium oxide and titania in V2O5/TiO2-SiO2 forming aVxTi1xO2 solid solution [38]. The disappearance of theOH groups at 3620 cm1 following vanadium incorpora-tion during synthesis of TiO2 using glucose suggests thatisolated vanadium species are oxohydroxy vanadiumbonded to titania through two oxygen bridges. Similar

result was also depicted [39], during the spectroscopic stud-ies of vanadium oxides supported on titania catalysts.

20

40

60

80

100

120

140

160

180

200

220

240

260

280

Vadd(ml/g)

120

80

40

0

0

50

100

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250

300

00.20.40.60.81

100

0

020406080

100120140160180200220240260280

v(cc

50

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150

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120

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240

0

50

100

150

200

250 AdsorptionDesorption

a

b

c

d

P/po0.2 10.80.60.40

Fig. 7. Adsorptiondesorption isotherms of nitrogen at 77 K on: (a) T, (b)TXY/C(623 K), (c) TXY/C(773 K) and (d) TG(773 K).

Table 3Textural properties of some synthesized TiO2 materials

Samples SBET (m2/g) St (m

2/g) Vp (ml/g) r (A) Vmicp (ml/g) Sext (m

2/g) Vwidp (ml/g) Smic (m2/g) Swid (m

2/g) % m.p.

T 308 312 0.46 37.6 0.167 167 0.293 111 197 36TXY(623 K) 356 354 0.41 28.8 0.217 133 0.193 188 52 53TXY/C(773 K) 309 268 0.40 32 0.238 108 0.157 186 123 60TG(773 K) 351 350 0.350 24.9 0.226 84 0.124 227 124 65

SBET: total surface area by BrunauerEmmetTeller equation; Smic: surface area of micropores; Sext: external surface of microporous solids; St: surfacearea derived from Vlt plot; Swid: surface area of widepores; V

totalp : total pore volume of adsorbent; V

widp : volume of the wide pores; r: mean pore radius was

determined using the equation: r

= 2.5VP 104

/SBET; Vmicp : volume of the micropores and m.p. % (percent of microporosity) = V

micp =V

totalp 100.

Fig. 8. FT-IR spectra of: (a) T, (b) TXY and (c) TXY/V.



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3.4. Acidic properties

The influence of polyol templates on the surface acidity(Bronsted and Lewis) of the produced TiO2 examined byFT-IR spectra of pyridine (Py) adsorption is calculatedand presented in Table 4. It can be observed that T pre-pared with NH4OH displays high amounts of Bronstedacidity exhibiting the largest B/L ratio. The XRD resultof this sample exposes Ti4+ ions belonging to anatase, ofmajor amount (75%). This may explain the photonic capa-bilities of OH groups since anatase constitutes the highestpercentages in the sample [40].

It seems also that polyol compounds stimulated the cre-ation of high concentrations of Bronsted acidities (Table 4)as for TG/V623 K (24.3), TXY (26.4), TXY/C (27.3), TXY/V(28.2) and TS (31.1) with higher strengths (1542 cm

1) thanthose in T (1536 cm1), exhibiting wider acidity distribu-tion for TiO2 synthesized with polyols. These results werehighly correlated with exceeding the amounts of water; thatobtained by comparing thermal analysis data of these sam-ples, that proposed growing the content of OH groups onthe surface of these samples as a result of water activation.In addition, samples containing vanadate species renderedBronsted acidity probably for the strong interaction exhib-ited with titania suggesting the formation of TiOH and V

OH moieties. Interestingly, these samples expose only ana-

tase structure. Vanadium containing samples showed moreOH groups with higher intensities (Figs. 8 and 9), whichseem to be beneficial to the activation of the anatase phase.Decreasing the acidic sites of TG/V623 K than TXY/V623 K ispresumably correlated with the highly dispersed vanadiaphase (vanado-titanate) on the former sample compara-tively; as conceded from thermal analyses and IR results.

Acid sites with intermediate populations were depictedfor TG(623 K), TG(623 K) evacuated at 413 K and TG(773 K)samples those possess anataserutile structures. DisplayingLewis and Bronsted acid sites at the maximum limit of theircorresponding range at 1460 and 1543 cm1, respectively,indicate that either site is bound to strong species in titania.When glucose used as template, the formation of moreLewis acid sites with greater tendency of forming rutile(83%) is attained. Evacuation of same sample at 413 Kshowed the highest Lewis acidity among all investigatedsamples. On the contrary, the calcined TG sample showedthat the population of acid sites particularly Bronsted onesare substantially decreased giving the lowest B/L ratio(1.58). These results indicate that it is possible to controlthe surface acidity of TiO2 formed.

From the experimental results, it was shown that TiO2synthesized using glucose as structure directing agentexposes reactive basic O2 species, on titania surfaces.

Doubtless, this is associated with the peripheral oxygen

Fig. 9. FT-IR spectra of: (a) T, (b) TG and (c) TG/V.



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in glucose structure opposite to xylitol that possesses aCH2OH group. However, such species at 300 K discloses

the reactivity of the nucleophilic (basic) OH groups in theTiO2 sample assembled by glucose. These basic sites indeedaffect the geometric arrangement of OH or/and O2ligands on TiO2 surfaces. The acidity values indicate thatTG indicates less acidity since it shows a basic character.

3.5. Photocatalytic activity

The sorption (dark experiment) and photoreduction(experiment under UV-illumination) of Hg2+ on variousTiO2 samples synthesized by using different polyols areshown in Figs. 10 and 11. Marked sorption of Hg2+ on var-

ious samples is achieved in the dark to a considerableextent varying from 18% to 30%. Under illumination, thephotoreduction of Hg2+ is maintained at the same level;for some samples, as observed in the dark as for TXYand TXY/V where a little enhancement was observed forTXY/C and TS reaching 40% following illumination. Wesuppose that the adsorption in the first 35 min is stronglyinfluenced by the microporosity of the samples as well asthe particles morphology (shape and size) those possessdriving forces for allowing fast diffusion of Hg2+ insidethe pores. By contrast, TG(623 K) that showed anisotropicparticles gives lower access for Hg2+ sorption, but how-ever, revealed the maximum photoreduction activityachieving 84% after 70 min illumination time (Fig. 11).The calculated percentages of photoreduced Hg2+ (pro-duced by measuring the absorbance of Hg2+ as mercuricdithizonate spectrophotometrically) on various TiO2 sam-ples are presented in Table 5. Inspecting the above men-tioned activity together with samples characterization in afirm way, one can reveal the followings: (i) Enhancing thephotoreduction activity of Hg2+ on TG(623 K) can be dueto the synergetic effect between anatase and rutile struc-tures despite anatase constitutes small percentages (17%)compared with rutile (83%). Accordingly, rutile could actas antenna, and therefore, the charge pairs can be gener-

ated in this phase, pumping electron to lower energy ana-

Fig. 10. Sorption (dark) and photoreduction of Hg(II) (50 ppm) onvarious TiO2: (a) T, (b) TXY, (c) TXY(C), (d) TXY/V and (e) TS.Experimental conditions: the illumination time 70 min, the photocatalyst(4 g per dm3 of solution), pH 5.5, at 298 K.

Fig. 11. Sorption (dark) and photoreduction of Hg(II) (50 ppm) on: (a) T(b) TG(623 K) (c) TG(773 K) (d) TG/V; experimental conditions: the illumi-

nation time 70 min, (4 g per dm3

of solution), pH 5.5 and at 298 K.

Table 4Quantitative results of various Lewis (L) and Bronsted (B) acid sites assessed by Py adsorption for all materials under study

Samples B/La Concentration Bb (mmol/cm2 102) Concentration L (mmol/cm2 103) m (B) (cm1) m (L) (cm1)

T 52.2 25.4 9.2 1536 1460TG(623 K) 2.58 11.7 2.7 1542 1460TG(623 K) at 413 K 1.82 14.4 5.1 1543 1460TG(773 K) 1.58 8.3 3.3 1543 1460

TG/V(623 K) 2.0 24.3 1.0 1542 1458TXY(623 K) 2.25 26.4 1.3 1542 1458TXY/C(623 K) 2.11 27.3 1.3 1542 1458TXY/V(623 K) 2.28 28.2 1.3 1542 1458TS(623 K) 2.46 31.1 1.5 1542 1458

a B/L ratio; Bronsted to Lewis acid sites concentration that obtained by dividing, the integrated area of the bands in 15361545 cm1 region over 14501460 cm1 one.

b The concentration of Py attached to either B or L sites calculated using the LambartBeers law: A = ecl, where e is the extinction coefficient (e ofB = 0.059 lmol1, e of L = 0.84 lmol1 for the peaks at 1550 and 1445 cm1 of B and L, respectively), c is the concentration (mmol/cm3), l is the pathlength (cm), and A is the absorbance.



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tase centers leading to a more stable charge separation, ashas been reported in the description of TiO2 Degussaphoto-induced electron process [41]. Enhancing the photo-reduction efficiency on TG(623 K) was in part due to thedecreased particles size of the produced TiO2 species(15 nm). Calcining TG at 773 K indicates lower activity(66%) although of the markedly higher anatase percentage

(57%). This can be due to increased particles size that hin-der the activity as a result of enlarging charges separationbesides, lowering Bronsted and Lewis acidic sites thus pro-viding lower adsorption centers for Hg2+ions. Exceedinganatase phase might lead to charge separation, however,the fact that anatase particles are covered by amorphouslayer (Fig. 4a) hindering the well adsorption of Hg2+ ionsby the surface so proposing the non-accessibility of anatase[35]. (ii) The presence of vanadium ions on TiO2 samplessynthesized using glucose and xylitol indicated a markeddecrease in their sorption and photoreduction efficienciescompared with V free TiO2 ones, pointing to a decrease

in the active sites available for sorption/photoreductionprocesses following V incorporation, in spite of exposingpure anatase phase.

The stoichiometry of the photocatalysed reduction ofHg2+ using TiO2 may be summarized in the following [42]:

2hm TiO2 ! 2e 2h 1

Hg2aq: Hg2ads: 2

Hg2ads: 2e ! Hg0ads 3

Ti4 e ! Ti3 4

Ti3 h ! Ti4ads: 5

2H2O 2H

2OH

62OH 2h ! 2H2O 1=2O2 7

The net reaction is :

Hg2aq: H2O *hm

TiO2Hg0ads 2H

1=2O2 8

3.6. Catalyst durability

The durability of the TG(623 K) catalyst was investigatedthrough recycling three times under the same experimentalconditions via adding fresh molar amounts of Hg2+ ions

for each run. As can be seen in Fig. 12, the photoreduction

activity of the catalyst was neither decreased nor influencedon moving from the first cycle to the third one, i.e. the per-formance of the catalyst was almost unchanged in all threeruns achieving complete photoreduction of Hg2+ in about80 min illumination time. This indicates that the successiveillumination by UV on TG(623 K) during the three runs doesnot influence the TiO2 surface properties and thus retainthe photoreduction efficiency of the sample following eachrun. One can indeed relate the higher photoreduction rate

of Hg2+ on nanosized TiO2, assembled by glucose at623 K, to the remarkable stability of micro-mesoporousstructure; envisaged from increased wall thickness, prevent-ing the photocatalyst deactivation.

4. Conclusion

We have characterized polyols-free titanates andvanado-titanates and shown that it is possible to obtainmicro-mesoporous materials with wide structural ordering.N2-sorption, XRD, IR, SEM, TG/DTA and FT-IR of pyr-idine-adsorbed samples confirmed the structural andmolecular ordering as well as stability of the materialsand acidity. TiO2 prepared by hydrothermal crystallizationvia glucose/TiCl4/AcA/H2O at 353 K for 5 days followedby template removal at 623773 K was found to the mosteffective photocatalyst in reducing Hg2+ into Hg0 uponUV irradiation. This material generates low acid contentcompared to titania produced from other polyols (TXYand TS) but on the other hand presented unique texturalproperties (high SBET; 351 m

2/g and high microporosities),reduced particles size (15 nm) and high accessibility ofbasic sites that take part in the photocatalytic performanceof the material. This material offered remarkable thermal

stability as observed from increased wall thickness of the

Fig. 12. Durability of the TG catalyst during Sorption (dark) andphotoreduction of Hg(II) (50 ppm) using 0.1 g amount of the catalystfor the three run cycles: (a) first run, (b) second run and (c) third runExperimental conditions: the illumination time 80 min, pH 5.5 and at298 K.

Table 5Photoreduction data of Hg2+ on various TiO2 samples

Sample Photoreduced percentages of Hg2+

T 40TXY(623 K) 20TXY/C(623 K) 50TXY/V(623 K) 11

TS(623 K) 46TG(623 K) 84TG(773 K) 66TG/V(623 K) 18



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catalysts. Highly dispersed vanadium oxide may possiblyinitiate the formation of VxTi1xO2 in TG and accordinglyinfluence rigorously the catalyst performance.

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