Synthesis of micro–mesoporous TiO2 materials assembled via cationic surfactants: Morphology, thermal stability and surface acidity characteristics

Microporous and Mesoporous Materials 103 (2007) 174–183 www.elsevier.com/locate/micromeso

Synthesis of micro–mesoporous TiO2 materials assembled via cationic surfactants: Morphology, thermal stability and surface acidity characteristics Mohamed Mokhtar Mohamed *, W.A. Bayoumy, M. Khairy, M.A. Mousa Chemistry Department, Faculty of Science, Benha University, Benha, Egypt Received 7 November 2006; received in revised form 28 January 2007; accepted 31 January 2007 Available online 8 February 2007

Abstract Nano-structured titanium dioxide materials were synthesized hydrothermally (TiCl4, 353 K, 5 days) via assembling through cationic surfactants in particular cetyltrimethylammonium bromide (CTAB) and cetylpyridinum bromide (CPB). The bulk chemical and phase compositions, crystalline structure, particle morphology, thermal stability and surface texturing were determined by means of X-ray powder diffractometry (XRD), Infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermal analyses (DTA/TGA) and N2 sorptiometry. The acidity of synthesized materials was studied by FTIR spectroscopy of adsorbed pyridine as a probe molecule. The results revealed that the crystallites size of all materials lie in the range of 10.1–18.2 nm and organized in a morphological structure that change from nano-sized spheres to cotton fibrils shape. Surfaces thereon exposed were found to assume high specific areas (240– ˚ . Rutile phase was only produced for all TiO2 mate418 m2 g1) and micro–mesoporous surfaces with pore size in the range 23.2–43.7 A rials assembled by cationic surfactants following heating up to 623 K. The transformation of rutile to anatase TiO2 was coincided with the developed interaction between vanadia and titania assembled by CPB template. The V in the resulting vanado-titanate was entirely incorporated in TiO2 structure. The as-synthesized phases of either rutile or anatase were maintained after calcining at 973 K exhibiting a significant thermal stability. Pyridine adsorption at RT indicated the involvement of acid–base site pair on all TiO2 assembled by cationic templates where that prepared by conventional method only exposed Lewis and Bro¨nsted acid sites with a higher tendency to the latter comparatively.  2007 Published by Elsevier Inc. Keywords: Nano-structured TiO2; V/TiO2; Cationic surfactants (CTAB,CPB); Micro–mesoporous structure; Characterization

1. Introduction Nano-crystalline titania is used to show wide range of applications in gas sensors, photovoltaic cells and photocatalysis [1,2]. However, One of the main reasons postpone its application is the low surface area. Hence, many efforts have been done to increase the surface area by making titania mesoporous [3]. However, synthesizing mesoporous titania is not simple when compared to mesoporous silica because of higher reactivity of the former towards hydroly-

*

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

1387-1811/$ - see front matter  2007 Published by Elsevier Inc. doi:10.1016/j.micromeso.2007.01.052

sis and condensation than those of the latter [4]. If this high reactivity is not moderated, the synthesis will lead to an illdefined and poorly ordered structure instead of gaining unique textural and structural characteristics [5]. Controlling the reactivity of mesoporous titania was accomplished by different methods including pH adjustment, using complexated titania-precursors, working in non-aqueous solvents as well as adding a controlled amount of H2O [6–8]. Mesoporous TiO2 with a large surface area was first synthesized by a modified sol–gel method with phosphorous surfactant as templates by Antonelli and Ying [9], that opened a new outlook in the synthesis of mesoporous titania. From their on, many studies have been carried out in this field. Several techniques have been adopted to prepare

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mesoporous TiO2 such as sol–gel technique [10], hydrothermal process [11] and ultrasonic irradiation process [12]. The use of templating materials as pore-directing agents has also been demonstrated for the synthesis of highly active mesoporous TiO2 photocatalyst [13]. More recently, the preparation of mesoporous titania by templating with polymer and surfactant has also been reported [5]. Other alternative for obtaining materials with enhanced surface areas and pore volume is by drying of aqua gels with supercritical carbon dioxide, thus suppressing any liquid-vapor interface inside the sol–gel product [14]. Unfortunately, the actual use of such mesoporous materials has been severely hampered by their poor hydrothermal and mechanical stability [15]. Therefore, there is a strong need for hydrothermally and mechanically stable structures. Moreover, materials with combined microand mesoporosity will offer significant supplementary advantages, such as an improved diffusion rate for transport in catalytic processes (faster reactions); better hydrothermal stability [16] multifunctionality to process a large variety of feedstocks; capabilities of encapsulated waste in the micropores; controlled leaching rates for a constant and gradual release of an active component [17]. Cationic surfactants are used for the structuring of negatively charged inorganic species, and vice versa when using anionic ones such as sodium deodecyl sulphate as template [13]. These materials still suffering several drawbacks including low and weak acidity and low ion exchange capacity [18]. Switching onto developing their characteristics to be alike-zeolite structures has attracted a lot of attention to be used as a superior candidate for specific adsorption and catalysis applications. Synthesizing TiO2 particles in the nano-range with well defined physical and chemical properties is another motivating step in nano-technology area. However, the next most important aspect is how to arrange them into well defined assemblies/structures or porous aggregates possessing unique properties like pore size and its distribution as well as thermal stability of structures. If the particles become small, they may indeed have structures that can not be derived from the bulk. Thus, this work is designed to improve the efficiency and stability of (photo)-catalysts TiO2 by offering a new route for the preparation of molecularly ordered micro–mesoporous titanate and vanado-titanate materials through assembling with cationic surfactants namely CTAB and CPB. The structural characteristics of TiO2 and V2O5/ TiO2 materials were investigated using XRD, SEM, thermal analyses (DTA/TGA), N2 adsorption and pyridineFTIR techniques. 2. Experimental 2.1. Materials The materials used in our preparations are TiCl4 (99%) provided from BDH, ammonium metavanadate (99%) and acetyl acetone (98%) provided from Sigma, ammonium

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hydroxide (33%) provided from Adwic. The cationic surfactants used cetyltrimethylammonium bromide (CTAB) {CH3(CH2)15N(CH3)3Br} (98%) and cetylpyridinium bromide monohydrate (CPB) {CH3 (CH2)15NC5H5Br Æ H2O} (98%) provided from Aldrich Company. 2.2. Preparation methods The titanium dioxide was prepared by two different methods. 2.2.1. Conventional method TiO2 was prepared by the hydrolysis of TiCl4 with ammonium hydroxide as follows; 150 ml of TiCl4 was added to 20 ml of distilled water in an ice path, then ammonia solution (28%) was added dropwise with continuous vigorous stirring till pH value of 8. The obtained white precipitate was filtered and washed with distilled water. The precipitate was then washed several times by distilled water till freeing from chloride ions that was tested by AgNO3 solution. The precipitate was then dried at 383 K for 17 h, and hence calcined at 773 K for 6 h. This sample is denoted as T. 2.2.2. Micelle-template method A definite weight (12.63 gm) of the cationic surfactant either CTAB or CPB was dissolved in 100 ml of distilled water. 25 ml of this solution along with 43.92 mmol of TiCl4 were added simultaneously to a beaker containing 43.92 mmol of acetyl acetone with continuous vigorous stirring for 4 h in an ice path. The resulting gel was then sealed in autoclave and heated at 353 K for 5 days. Afterwards, the resulted precipitate was filtered, washed several times by distilled water and then dried at 393 K for 12 h. Finely, the temperature was gradually raised from 393 K to 623 K in 2 h and then left at 623 K for 4 h. Portions of this material were further calcined at 773, 873 and 973 K. The titanium oxide materials derived from CTAB and CPB are denoted as TCTAB and TCPB, respectively. 2.2.3. Synthesis of vanadia–titania using Micelle-template method The V2O5/TiO2 material with a nominal 6 wt% V2O5 was synthesized by the Micelle-template method declared above. The required quantity of ammonium meta-vanadate (Fluka, AR grade) was dissolved in an aqueous oxalic acid solution (1 M). To this clear solution, the template and TiCl4 were added simultaneously under vigorous stirring then transferred to an autoclave for hydrothermal treatment in a typical procedure to that mentioned above. These materials are denoted as TCTAB/V and TCPB/V. 2.2.4. Physicochemical characterization of materials Powdered X-ray diffraction (XRD) patterns was performed on a Diano (made by Diano Corporation, USA) ˚ ). The partiusing Co-filtered Co Ka radiation (k = 1.79 A cle shape and the particle size distribution of the precipitate

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were performed by a JSM-5200 Scanning Electron Microscope (JEOL) using conductive carbon paint. FTIR spectra of the samples were recorded using KBr pellets in the range of 4000–400 cm1, using spectrophotometer of a Bruker (vector 22), made in Germany. The surface properties namely BET surface area, total pore volume (Vp), mean pore radius (r) and other parameters were determined from N2 adsorption isotherms measured at 77 K using conventional volumetric apparatus. The total pore volume was taken from the desorption branch of the isotherm at p/p = 0.95, assuming complete pore saturation. The thermogravimetric (TG) analysis and differential thermal analysis (DTA) were carried out on a TG/DTA apparatus (Shimadzu 50, at a heating rate of 10 C/min in flowing high purity N2 gas with 10 ml/min). The surface acidity was investigated using the irreversible adsorption of pyridine on the solid surfaces. The samples were activated for 3 h under vacuum (105 Torr) at 473 K prior to admission of 8 Torr of pyridine at room temperature on the samples. IR-spectra of pyridineadsorbed on samples were depicted by subtracting spectra before and after exposure to pyridine. 3. Results and discussion 3.1. X-ray diffraction, stability and morphology of materials The XRD patterns of T, TCTAB, TCPB and vanadia loaded samples (TCTAB/V and TCPB/V) are shown in Fig. 1. The surfactant free TiO2 sample (T) shows two crystalline phases of anatase and rutile with a crystallinity ratio close to 3:1, depending on the intensities of (1 0 1) and (1 1 0) planes, respectively. For TCTAB sample, the XRD pattern exhibits only peaks for rutile phase with no evidence of any anatase one. The rutile phase is essentially formed at higher calcination temperatures (>723 K or above) [19], however, its persistence and stability at such low temperature (623 K) proposes the important role played by CTAB surfactant in initiating the crystallization of the rutile phase right from the beginning. Incorporating vanadium ions during the synthesis of TiO2 using CTAB (TCTAB/V) shows typical rutile peaks nevertheless with lower intensities and broadness. It is well known that vanadia can initiate the anatase-to-rutile phase transformation [20]. Thus, the persistence of the rutile phase following vanadium incorporation implies its stability even following vanadium incorporation. No diffraction lines for vanadium oxide species (V2O5) were revealed reflecting either their absence or high dispersion in TiO2. The X-ray pattern of the T sample derived from CPB (Fig. 1d) shows the same lines as that revealed for TCTAB, in evolving only rutile phase. On the contrary, vanadium incorporation into TCPB exhibits an anatase phase. The particle size calculated from XRD by Scherrer equation [21] is found to decrease slightly after vanadium incorporation from 18.2 nm (in T) to 15.2 nm (in TCTAB/V)

Fig. 1. XRD diffraction patterns of (a) T, (b) TCTAB, (c) TCTAB/V, (d) TCPB, (e) TCPB/V.

(Table 1). This was however more significant in TCPB/V sample (10.1 nm) reflecting the strong interaction of vanadium ions with TiO2; derived from CPB. The formation of anatase at crystallites size of 10 nm stabilizes remarkably the anatase phase without allowing its transformation to the more stable phase rutile. This result agrees with that revealed by Banfield et al. [22]. A synchronized effect between CPB and vanadia moieties could be responsible for decreasing the crystallites size of anatase produced. The wall thickness of the tetragonal TiO2 (the lattice constants a and c calculated from XRD patterns correspond well to those of the tetragonal phase reported in literatures [13–21]) was found to change with varying sur-

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Table 1 XRD data of the investigated TiO2 and V2O5/TiO2 materials Samples

D (nm)a

Lattice parameters A

b

c

T TCTAB TCTAB/V TCPB TCPB/V

18.2 18.2 15.2 18.2 10.1

4.626 4.556 4.6 4.612 3.801

4.556 4.634 4.609 4.581

2.955 2.965 2.955 2.957 9.536

Cell volume (nm3)

Phase

Crystallinity R%

A%

6.228 6.2599 6.265 6.2474

Mixed Rutile Rutile Rutile Anatase

34 76 66 46

66

WT (nm)

30

0.87 1.58 0.34 2.3 –

R% – ratio of rutile with respect to that present in T sample; A% – ratio of anatase phase with respect to that present in T sample; D – particle size of crystallites; WT – wall thickness = unit cell  pore size. a From most prominent XRD lines.

factants type as well as following V incorporation (Table 1). It has been acknowledged that the thermal stability of the meso-structured materials is enhanced with increasing the pore wall thickness. Accordingly, TCTAB and TCPB materials propose higher stabilities than T. On the contrary, V incorporation seriously affects the stabilities of these materials (Table 1). SEM of the investigated materials is shown in Fig. 2. SEM image of T sample shows spherulitic grains of varying dimensions. The particle size distributes in a homogenous way around 25 nm. Involving CTAB in the synthesis results in a morphological shift to randomly shaped aggregated particles of decreasing dimension than those of T. This probably due to faster condensation of titania resulting in randomly shaped aggregated particles. The micrograph of TCTAB/V sample acquires cotton fibrils morphologies of nano-sized aggregated spheres of approximately 16 nm in diameter. The SEM morphology of TCPB and TCPB/V samples also show cotton fibrils morphologies of nano-crystals reflecting the structural effects of CPB surfactant on the particles morphology produced. The size of cotton like structure of uniform-sized crystals was rather controlled via V incorporation to give particles with a diameter of 11 nm with excellent porous characteristics (Table 2). The before mentioned sizes range meet relatively with XRD determined crystallites size range except those derived from T and TCTAB samples those exhibited an increase comprised of 15–25% when measured by SEM comparatively. This may be due to the limitation of the SEM technique in measuring different zones of a specimen and its incapability in measuring too smaller particles [23]. TG/DTA analysis curves obtained for the as-synthesized T, TCTAB/V and TCPB/V materials are shown in Figs. 3–5. The TG curve of T indicates a weight loss of 13.4% at temperatures below 673 K beyond it no further loss in weight is depicted. DTA, on the other hand, shows one endothermic peak at 353 K and two exothermic ones at 553 K and 653 K representing respectively, desorbed water, crystallization of the amorphous to anatase and phase transition of anatase to rutile that was accompanied with no weight loss in the TG curve. Assigning the exothermic peak at 553 K to the crystallization of anatase is confirmed by Bekkermann et al. [24] whom establishes the formation of anatase structure in the temperature range 520–600 K.

Fig. 2. Scanning electron microscope of (a) T, (b) TCTAB, (c) TCTAB/V, (d) TCPB, (e) TCPB/V.

The TG curves of TCTAB/V and TCPB/V materials show some variations: the TCPB/V sample indicates a higher weight loss (33.5%) than that derived from TCTAB/V (7.6%). In addition, the former sample showed a lower thermal stability as traced by DTA so as to an earlier decomposition of anatase phase at 823 K was depicted where the latter was highly resistant to temperature as high as 1073 K, as it indicated a rutile structure. The transformation of rutile to anatase

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Table 2 Textural properties of various TiO2 and V2O5/TiO2 materials ˚ St, m2/g Vp, ml/g r, A Samples SBET, m2/g

V mic p , ml/g

Sext, m2/g

V wide , ml/g p

Smic, m2/g

Swide, m2/g

% m.p

T TCTAB TCTAB(V) TCPB TCPB/V

0.167 0.087 0.306 0.176 0.473

167 141 238 66 91

0.293 0.203 0.347 0.099 0.257

111 73 180 190 271

197 167 203 107 147

36 30 47 64 65

308 240 383 297 418

312 194 360 278 497

0.46 0.29 0.65 0.28 0.73

37.6 29.8 42.6 23.2 43.7

SBET – total surface area by Brunauer–Emmet–Teller equation; Smic – surface area of micropores; Sext – external surface of microporous solids; St – – total pore volume of adsorbent; Vwide – volume of the wide pores; r – mean surface area derived from Vl–t plot; Swide – surface area of widepores; Vtotal p p pore radius determined using BJH Model, desorption branch; Vmic – volume of the micropores; % m.p – percent of microporosity. p

DTGA (mg/min)

TGA mg

0.10

12.0

DTGA (mg/min)

TGA mg

0.20

11.0

10.0

0.10

0.00 11.0

9.0 0.00

-0.10

8.0 10.0 -0.20

-0.30

9.0

↑

-0.10 7.0 -0.20

6.0

EXO

EXO

↑

DTA

0

200

400 600 Temp [°C]

800

DTA

1000

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

0

200

600 400 Temp [°C]

800

1000

Fig. 4. DTA and TGA (DTGA) curves of TCPB/V.

in TCPB/V indicates the non-stability of rutile structure when assessed by CPB surfactant particularly following V incorporation. This lack of stability could be due to the low degree of condensation of the inorganic walls. Although the partial positive charge resides on nitrogen containing CTAB is lower than that on N of CPB, a higher thermal stability for the former is obtained comparatively. This suggests that upon decomposition, the heterocyclic ring (pyridinium moieties) in CPB is likely to break down primarily once it compared with trimethylamine head group in CTAB [25] exhibiting lower thermal stability. This explains increasing the magnitude of the mass loss of TCPB than TCTAB disclosing in addition the hydrophobicity of CTAB

containing TiO2 samples. The existence of only endothermic peaks in DTA curves for both samples clarify the stability of maintained phases throughout the thermal treatment. In conformity, XRD runs for precalcined TCPB/V and TCTAB/V materials at 773, 873 and 973 K were measured (not shown) and they indeed indicate no changes in the phase of these samples emphasizing the stability of the corresponding phases since they are shaped. In a way of understanding the formation of rutile TiO2 during the hydrothermal treatment, an overview on the concept of the interaction of titanium dioxide with surfactant should be estimated. It has been acknowledged that pH

M.M. Mohamed et al. / Microporous and Mesoporous Materials 103 (2007) 174–183

d

b

FTIR spectra of T, TCTAB, TCPB, TCTAB/V and TCPB/V materials, in two different regions namely hydroxyl groups

410

534

b

415

c

500 434

c

a a 3500

3750

3.2. FTIR spectra of materials

e

524

d

518

e

647

influences the surface state of titania and the ionization state of ionizable organic molecules [26] (surfactants). The isoelectric point (pzc) of TiO2 controls the adsorption interactions with either surfactants or vanadium at equilibrium conditions. A measured decrease in the net pH at pzc of the hydrated titanium dioxide surface is obtained as a result of involving the chloride ions, derived from TiCl4, i.e., the net pH at pzc should be lower than 5.9. Accordingly, one may expect that surface OH groups may react as Ti–OH þ Hþ $ TiOHþ 2 . Since cationic surfactants are used for the structuring of negatively charged inorganic species and vice versa, a counter charged ion has to be involved; such as Cl ions, to mediate microstructure obtained from organic–inorganic combinations of identical charged partners. Similarly, Mahanti et al. [27] reported that electrostatic charge-matching interactions between ionic surfactants and ionic inorganic precursors at the surfactant/inorganic framework interface play a major role in the mechanism of mesostructure assembly [28,29]. Thus, it is expected that the synthesis of this material is based on the compensation of the ionic charges between surfactant head groups and the inorganic species (charge matching approach).

587

Fig. 5. DTA and TGA (DTGA) curves of TCTAB/V.

615

1000

615

800

Absorbance/a.u.

600

Temp [°C]

3565

400

3565

200

3677 3650

0

3679 3650 3620

EXO

DTA

3678 3654

↑

3679 3650 3620

-0.20

3661 3633

b

8.0

3750

-0.10

3693

0.00

9.0

3750

0.10

3750

0.20

3747

10.0

(3500–3800 cm1) and low infrared (400–1000 cm1) regions, are shown in Fig. 6. Spectrum (a) that corresponds to T sample indicates two bands in the low frequency region at 500 and 434 cm1 assignable, respectively, to the stretching of Ti–O–Ti bonds in anatase and rutile, similar to those revealed previously by Beattie et al. [30]. The hydroxyl stretching region, on the other hand, shows bands at 3783 and 3693 cm1 corresponding to free hydroxyl groups of anatase and rutile [31] respectively, and 3661 and 3633 cm1 attributed, respectively, to bridging (Ti)2OH and hydrogen bonded OH groups [32]. It should be mentioned that XRD of this sample (T) indicates the presence of anatase and rutile. Therefore, these OH groups are presumed to be due to anatase and rutile; however, assigning specific band to either one of them is uncertain. Accordingly, synthesizing TiO2 with a definite structure will indeed help assigning the bands in an accurate way. The spectrum (b) of TCTAB (Fig. 6), which evidenced only a rutile structure as traced by XRD, shows bands at 3747, 3679, 3650, 3620 and 3565 cm1 as well as bands at 518 cm1 and 419 and a shoulder at 615 cm1 [31]. The stretching OH band at 3560 cm1 could be due to adsorbed water molecules [33] or originated from hydrogen bridges from OH groups bonded to each other. As it can be seen, the 3747 cm1 band that was not observed in spectrum T is most likely advocating another type of terminal Ti–OH groups. Spectrum (b) also shows a shift to lower wavenumbers for the bands at 3650 and 3620 cm1 when compared

3783

DTGA mg/min

Absorbance/a.u.

TGA mg

179

-1

Wavenumber/cm

1000

750

250 500

Wavenumber/cm-1

Fig. 6. FTIR spectra in the high frequency (4000–3500 cm1) and low frequency (1000–400 cm1) regions of (a) T, (b) TCTAB, (c) TCTAB/V, (d) TCPB, (e) TCPB/V.

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with those of T (3661 and 3633 cm1). This is due to lowering the force constant of the O–H bonds of rutile than those of anatase [34]. This let us presume that the bands at 3661 and 3633 cm1 seen in spectrum (a) are due to anatase OH groups. The spectrum (c) (Fig. 6) for TCTAB/Vshows three bands at 3750, 3678 and 3654 cm1 similar to those seen in spectrum (b) but of lower intensities. This spectrum shows disappearance those emerged at 3565 and 3620 cm1 following V incorporation besides the band at 3747 cm1 is markedly decreased in intensity. This indicates that V interacts with free hydroxyl groups represented by the 3747 cm1 band and rather interacts strongly with low density bands positioned at 3565 and 3620 cm1. This type of interaction was responsible for decreasing the intensity of the bands of TCTAB/V when compared with those of TCTAB. Consequently, an expected decrease in rutile concentration following V incorporation is anticipated as has been recognized from XRD results. No changes were depicted in the low frequency region following V incorporation except a shift to higher wavenumbers for the 518 (524) cm1 band. No evidence of V2O5 crystallites is observed [35], implying the well dispersion of vanadium in the structure of rutile TiO2. This let us presume that V ions are well incorporated on titania support. Comparing IR spectra of TiO2 samples prepared by different surfactants (TCPB and TCTAB) those evidenced only a rutile structure showed some changes. The spectrum of TCPB shows, in the low frequency region, three bands at 587, 534 and 410 cm1 with the appearance of a shoulder at 806 cm1 probably advocating the (TiO2)6 clusters [36]. The bands arising from hydroxyl groups in both samples do not change. In addition, the band at 518 cm1 in TCTAB shows a shift to higher wavenumbers into 534 cm1 in TCPB. This reflects to what extent the effect of surfactants in producing smaller distances in Ti–Ti and Ti–O bonds. Vanadium incorporation during TiO2 synthesis using CPB provokes hydroxyl bands at 3750, 3677 and 3650 cm1. This explains that vanadium preferentially interacts with definite crystal faces of TiO2 specifically those compose the OH groups at 3565 and 3620 cm1. The consumption of the two OH groups at 3565 and 3620 cm1suggests that isolated vanadium species are oxo-hydroxy vanadium bonded to titania through two oxygen bridges. Similar result was also depicted by Jonson et al. [37] during their spectroscopic studies of vanadium oxides supported on titania catalysts. In the low frequency region, the appearance of large broad band in the range 500–900 cm1 maximized at 647 cm1 is probably characteristics of Ti–O–Ti anatase structure or due to vanadate species in the interlayer spaces [38]. 3.3. Texture assessment of materials The adsorption–desorption isotherms of the T, TCTAB, TCTAB/V, TCPB and TCPB/V samples are displayed in Fig. 7. The isotherms indicate the presence of (meso)-

Fig. 7. Adsorption–desorption isotherms of N2 at 77 K on precalcined: (a) T, (b) TCTAB, (c) TCTAB/V, (d) TCPB, (e) TCPB/V materials at 773 K.

porosity with a broad size distribution except the one derived from CPB that showed only microporosity as it exhibits the absence of any hystersis loop. The shape of the isotherms in the partial pressure (P/P) below 0.8 suggests the presence of micropores (P/P < 0.2) and a broad distribution of mesopores (P/P 0.2–0.8). The porosity below P/P 0.8 is likely to be linked to the interlayer spaces in the surfactant-free sample (T). At P/P above 0.8, the isotherms are characterized by a steep adsorption into large ˚ except TCPB. A mesopores with pore size larger than 40 A possible explanation for these large mesopores is that they arise from interparticle voids [39,40]. Table 2 indicates a microporosity percentage comprised of 64% for this particular sample. In addition, this specimen (TCPB) measured lower SBET (297 m2 g1) than that of TCPB/V (418 m2 g1) probably due to decreasing the particles size of the latter comparatively. Surprisingly, the TCPB/V specimen showed the highest Vp and r among all samples, indicating the enforced location of V species in the pores of this particular sample leading to an effective pore widening and pore volume.

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3.4. Acidity of materials In order to investigate the nature as well as to estimate the number of the surface acidic sites, FTIR spectra of adsorbed pyridine (mCCN spectrum) at 300 K for all samples are investigated and presented in Fig. 9. The spectrum (a) of Py/T displays many bands in the wavenumber range 1450–1700 cm1 which attributed to the interaction of pyridine with Bro¨nsted (B) and Lewis (L) acid sites on TiO2 [41,42]. The strongest feature of the band at 1536 cm1 is an indicative of formation of Bro¨nsted pyridine species

e

400 300 200 100 0 160

d

120 80 40 0 400 Vads (ml/g)

The SBET of TCTAB exhibits the lowest value between all samples (240 m2 g1) as can be seen in Table 2. A typical mesoporous structure is revealed for this sample (70%) representing the highest mesoporosity among all samples. Interestingly, this sample presented also some ultramicropores of ca. 30% developed due to varying the specific area value obtained from BET from that depicted from t-plot (St). The construction of mesoporosity when using CTAB ˚ and Vp = 0.29 ml/g) was accompasurfactant (r = 29.8 A nied by a loss in the area implying the effect of packing densities of surfactant; in tailoring mesopores of different dimensions. Addition of vanadium, during the preparation of TiO2 assembled with CTAB; increases significantly the surface area of the sample by 37% when compared with TCTAB (Table 2). This is due to decreasing the particle size (15.2 nm) and the pore narrowing caused by vanadium and thus rendering the surface microporosity. TCTAB/V, TCTAB and T developed the highest external surface areas and thus possess the highest contribution from mesoporous surfaces. On the contrary, TCPB and TCPB/V samples show the lowest external surface areas, comparatively. The obtained Vl–t plots of various samples, comparable to those of adsorption–desorption isotherms, are given in Fig. 8. The verified agreement between the area calculated from the plot, St, and SBET are shown in Table 2. The T sample has a Vl–t curve which possesses unexpectedly downward deviation implicating the presence of micropores. The Vl–t plots, given in Fig. 8, indicate the presence of mesoporosity in all samples (indicative of the presence of capillary condensation in the pores of the adsorbent). The TCTAB/V material that starts upward deviation at ˚ presented a higher swing when compared with t = 7.0 A that of V free TCTAB proposing wider pore and volume. The TCPB/V sample exhibits mainly widepores (mesopores) besides micropores that were represented by some points lie exactly below the straight line extended to path through the ˚ presented origin. The upward deviation starts at t = 6.4 A a higher swing when compared with that of V free TCPB proposing wide pore and volume. Such type of mesoporosity is a direct response not only the surfactant chain length but also vanadium ions those control the pore texturing. The % microporosity of this sample (65%) was almost alike that evaluated for TCPB (64%) that hardly indicated an upward deviation.

181

c

300 200 100 0 180 120

b

60 0 300

a

200 100 0 0

2

6 t(Å)

10

14

Fig. 8. Vl–t plots of N2 adsorbed at 77 K on precalcined: (a) T, (b) TCTAB, (c) TCTAB/V, (d) TCPB, (e) TCPB/V materials at 773 K.

(Bpy). The display of the strong band at 1623 cm1, in the presence of some residual band at 1460 cm1, may also account for Bpy species. As has been shown, this sample exposes Ti4+ ions belonging to anatase, of major amount as evidenced by XRD data. This may explain the protonic capabilities of OH groups because anatase has more protonic features [41]. The band at 1561 cm1 and the shoulder at 1600 cm1 may be due to hydrogen bonded Py (HPY) in consistency with that depicted by Zaki et al. [42]. The mCCN spectrum exhibited by Py/TCTAB (Fig. 8b) displays bands at 1563 and 1600sh cm1, indicative of hydrogen bonded pyridine (HPY), and the bands observed at 1460, 1493 and 1622 cm1 are characteristic of Lewis bonded pyridine (LPY), whereas the bands at 1543, 1632 and 1655 cm1 are due to Bro¨nsted bonded pyridine (BPY) species [43]. In addition, the appearance of bands at 1670 and 1683 cm1 can be taken as a sign of Py conversion to a-pyridone implying that Ti surfaces derived during synthesis with CTAB surfactant contain reactive basic sites. Similar results have been reported previously by Hussein et al. [44] and recently by Martin et al. [45].

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1460 1459

1542 1521

1460

1482 1536 1514

1561

1670

a

1700

1458

1508 1490

1560 1542 1562

1623

1563 1543 1525 1514 1493 1460

1683 1670

b

Samples

B/La

Concentration Bb, Concentration L, m(B), m(L), mmol/cm2 · 102 mmol/cm2 · 103 cm1 cm1

T TCTAB TCTAB/V TCPB TCPB/V

52.2 1.24 2.48 2.52 2.00

25.4 16.1 48.5 29.2 24.3

9.20 9.3 1.5 2.1 1.0

1536 1543 1542 1542 1544

1460 1560 1459 1459 1460

a

1626

1654

c 1655 1632 1622

Absorbance/a.u.

d

1655 1637 1626

1492

1562 1543

1689

e

1654 1637 1623

Table 3 Quantitative results of various Lewis (L) and Bro¨nsted (B) acid sites assessed by Py adsorption

1600 1500 1450 Wavenumber/cm-1

Fig. 9. IR spectra of pyridine adsorbed at 300 K on (a) T, (b) TCTAB, (c) TCTAB/V, (d) TCPB, (e) TCPB/V.

The TCTAB/V sample, that acquired only a rutile structure as evidenced by XRD results, shows a small ratio of B/L, Table 3. The appearance of the mBPY mode at 1626 cm1 in spectrum (c), as a broad band, while the disappearance of 1632 cm1 band, in comparison with that in spectrum (b), may account of prevailing the Lewis acid sites in TCTAB/V sample. The disappearance of the 1482 cm1 band; that represents the combined L and B sites, reflects the lower absorbability of the rutile structure to Pyridine molecules comparatively. The conversion of LPY species into a-pyridone species, is evidenced by emergence of the C@O band near 1680 cm1. Accordingly, revealing basic Ti–OH groups on TCTAB/V is achieved and hence, acid–base pair sites, is exhibited. The mCCN spectrum exhibited by py/TCPB at 300 K (Fig. 8d) displays bands referring to LPY (at 1458 and 1626 cm1), bands due to BPY (at 1542, 1637 and 1655 cm1) and bands at 1560 and 1678 cm1 indicative of HPY [37,38]. The mCCN spectrum exhibited by py/

B/L ratio: Bro¨nsted to Lewis acid sites concentration that obtained by dividing, respectively the integrated area of the bands in 1536–1545 cm1 over 1450–1460 cm1 regions. b The concentration of Py attached to either B or L sites calculated using the Lambart–Beer’s law: A = ecl, where e is the extinction coefficient (e of B = 0.059 lmol1, e of L = 0.84 lmol1 for the peaks at 1550 and 1445 of B and L, respectively), c is the concentration (mmol/cm3), l is the path length (cm), and A is the absorbance.

TCPB/V at 300 K (Fig. 8e) displays bands indicative of HPY (1600sh cm1), bands due to LPY at 1460, 1492, 1623 cm1, bands at 1543, 1637, 1654 cm1 characteristic of BPY and a band at 1562 cm1 attributed to Py oxidation species. This seems to have been preceded by conversion into a-pyridone species through evolution of a band at 1686 cm1 [46]. Thus, one can conclude that TiO2 incorporated by V during synthesis using CPB exposes reactive basic O 2 species higher than that devoted from V free CPB template. The formation of such pyridinum oxide species must be formed at the expense of fading the LPY. However, provoking such species at very low temperature (300 K) discloses the reactivity of the nucleophilic (basic) OH groups in the sample derived from CPB, comparatively. On the other hand, this could be correlated with the exposed anatase phase on this particular sample. It can be inferred that differences in reactivity of basic sites are related to the employed surfactant that affect the geometric arrangement of surface OH or/and O 2 ligands. Noticeably, the symmetric band due to LPY at 1460 cm1 bound to strongly acidic type of cus Ti4+ sites, observed in the TCPB/Vsample, is of higher intensity (high density) than that in the TCTAB/V sample. This explains the reactivity of basic sites of the TCPB/V material due to the major contribution of LPY species comparatively. Py adsorption data, Table 3, shows the following B/L acid site ratio: TCTAB < TCPB/V < TCTAB/V < TCPB < T. TCTAB has negligible Lewis acidity, which almost does not agree with previous investigations revealed for some pure titania materials [47]. 4. Conclusion The hydrothermally synthesized titania samples using cationic surfactants (CTAB and CPB) as templates, showed high potentialities in the formation of pure rutile phase following heating at 623 K; except the material assembled by CPB when supported vanadia that showed the formation of pure anatase phase. All the prepared TiO2 and V2O5/TiO2 materials showed markedly high

M.M. Mohamed et al. / Microporous and Mesoporous Materials 103 (2007) 174–183

surface areas and exposed micro–mesoporous surfaces with ˚ with nanodifferent pore sizes ranging from 23.2 to 43.7 A particles size ranging from 10.1 to 18.2 nm. Combining the results obtained from XRD and IR investigations facilitate assigning specific bands to either anatase or rutile structure. The micro–mesoporous structures of as-prepared rutile and anatase TiO2 materials were maintained after calcining at 973 K exhibiting appreciably thermal stability. Applications of TiO2 materials synthesized by cationic surfactants in surface driven reactions are worth attempting due to the substantial surface area devoted for these materials (240–418 m2/g). The detection of pyridinium oxide species on cationic surfactants assembled TiO2 samples is indicative of the availability of acid–base pair sites. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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