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J. Phys. Chem. C 2009, 113, 13750–13757
Surface Area, Pore Size, and Particle Size Engineering of Titania with Seeding Technique and Phosphate Modification K. Joseph Antony Raj, A. V. Ramaswamy, and B. Viswanathan* National Centre for Catalysis Research, Indian Institute of Technology-Madras, Chennai 600036 ReceiVed: March 19, 2009; ReVised Manuscript ReceiVed: May 6, 2009
The environmentally benign route of the organic free process was evolved in precipitating hydrated titania from titanium oxysulphate solution at 90 °C, using crystal nuclei with the addition of ammonia. The synthesized hydrated titania was found to be mesoporous and reactive, with a BET surface area of 277 m2/g. The hydrated titania on calcination in air at 500 °C for 2 h produced anatase mesoporous titania with a surface area of 100 m2/g. The as-synthesized and calcined mesoporous titania materials were characterized by X-ray diffraction, scanning electron microscopy-energy dispersive X-ray spectrometry, ultraviolet-visible/diffuse reflectance spectroscopy, inductively coupled plasma mass spectrometry, thermogravimetry differential thermal analysis, diffuse reflectance infrared Fourier transform spectrometry, and nitrogen adsorption-desorption measurements. The incorporation of phosphorus in the titania framework was found to inhibit crystallization and particle growth and was found to enhance thermal stability due to the formation of titanyl phosphate. The phosphatemodified hydrated titania exhibited mesopores with a high surface area (172 m2/g) after calcining at 500 °C. The esterification reaction performed on P-TiO2 samples showed the significance of an optimum quantity of anatase phase and phosphate content for catalytic activity. Introduction 1
Since the report of Fujishima and Honda in 1972, TiO2 has become the most widely used photocatalyst, and research aimed at enhancing the efficiency of this material has intensified over the years.2 In particular, the ability of the (001) surface of anatase to dissociatively adsorb water makes anatase a promising photo catalyst for the splitting of water.3 Mesoporous TiO24-8 was first synthesized through modified sol-gel routes in the presence alkyl phosphate,9,10 amine,11-16 block polymer,17-22 or nonionic surfactants as templates.8 Various organic precursors such as titanium isopropoxide23,24 and titanium butoxide25,26 have been used to prepare titania. Inorganic sources such as P-2526 and titanium tetrachloride27,6 were also used for the preparation of modified titania. With titanium tetrachloride, there is enhancement of rutile formation, which is detrimental to many catalytic applications. The method to date adopted for the preparation of TiO2 and P-TiO2 is the sol-gel procedure,23 using titanium isopropoxide as the titanium source and 2-propanol as the solvent. Several methods have been reported to improve the photo catalytic efficiency of TiO2, which include increasing the surface area of TiO2, generation of defect structures to induce the space-charge region, and modification of TiO2 with metals or nonmetals. Yanqin Wang et al.4 reported a novel synthesis of mesoporous titania with high surface area by employing ultrasonic waves and the sonochemical method. Recently, surface modification of TiO2 by anions such as F-,28-31 PO43-,32-37 SO42-,38-40 and trifluoroacetic acid41 has attracted attention because of their ability to improve the photocatalytic activity of TiO2. Phosphate anions are known to adsorb strongly on the surface of TiO2, which can greatly influence the interfacial and surface chemistry of TiO2.42 Earlier studies on the modification of TiO2 by phosphate and sulfate focused primarily on the * To whom correspondence should be addressed. E-mail: bvnathan@ iitm.ac.in.
improvement of the thermal stability and an increase in surface area and acid sites on the TiO2 surface.43,44 Other known ways of regulating the specific surface area and other properties of titanium dioxide catalysts comprise regulation of precipitation and calcination temperatures and other conditions in the production of titanium hydrate. It is a challenge to develop an organic free and mass production route to synthesize mesoporous titania. The change in surface chemistry by the introduction of phosphorus can be used to engineer the properties of titania for catalytic applications. It was reported that phosphated TiO2 has excellent catalytic properties when used in organic chemical transformations. Thus, it is necessary to study the effect of phosphatemodified TiO2 for various commercial applications.33,45 To date, titanium dioxide intended as a photocatalyst has mostly been produced from titanium tetrachloride or from organic titania materials. Generally, the alcohols such as 2-propanol and n-butanol are used to improve the surface area and pore volume of inorganic oxides. Hence, titanium sources such as titanium isopropoxide and titanium butoxide, when used as the source for titania synthesis, may be anticipated to give a higher surface area of titania than the inorganic titania sources because of the templating effect of alcohols. Therefore, the use of a purely inorganic starting material as a source for high surface area titania requires the addition of alcohol or amines as a template. It is reported elsewhere23 that there is higher surface area for the samples calcined at temperatures less than 450 °C. However, the complete removal of carbon from the samples synthesized using organic sources is only possible above 600 °C. In addition, the calcination temperature of 600 °C is anticipated to lower the surface area. Therefore, the exclusion of organic solvents and organic titania sources will have a significant impact on the economic and eco-friendly means of producing high surface area titania. In this paper, a green and inexpensive synthetic route is reported for the preparation of high surface area titania using
10.1021/jp902468v CCC: $40.75 2009 American Chemical Society Published on Web 07/10/2009
Engineering of Titania titanium oxysulphate as the titania source. Phosphate modification of the synthesized titania and its properties are studied. Although the phosphate modified titania may not form an ordered mesoporous material, it is anticipated that this work will lead to the formation of porous titania, which may be useful for catalytic transformation of bulkier molecules. Experimental Section Chemicals. Titanium oxysulphate (Aldrich), ortho-phosphoric acid (Qualigens), P-25 titania (Degussa), di-isopropanolamine (Qualigens), TiO2 (Fluka) and ammonia (Qualigens) were used without further purification. Double distilled water was used as a solvent. Preparation of Seed. Four grams of P-25 was put in a 100 mL SS container, and to this was added 40 mL of water and 4 mg of di-isopropanolamine. About 100 zirconia balls having a 2 mm diameter were added, and the container was firmly closed and wet milled for 1 h. The obtained crystal nuclei was used as precipitation seed, which may have a size of a few nanometers. An organic dispersing agent, di-isopropanolamine, was added to influence the size, shape, and stability of the titanium dioxide nanoparticles formed from the aqueous titanium compounds. Synthesis of Hydrated Titania. A titanium oxysulphate solution containing 12% TiO2 and 26% H2SO4 was employed as the starting material for the preparation of high surface area TiO2. To this, we slowly added 10 gpl ammonia solution until the pH was 1.8. A wet milled seed of 5 mL was added to 83 g of titanyl sulfate. Hydrolysis was performed in a constant boiling apparatus at 90 °C for 3 h. At the end of 3 h, the contents of the constant boiling apparatus were rapidly transferred to a beaker containing 800 mL of water to completely precipitate TiO2. The TiO2 in hydrated form was obtained after thoroughly washing the precipitate to remove ammonium sulfate. The precipitate obtained was dried at 100 °C for 12 h to produce hydrated titania (HT) and thereafter calcined at 500 °C in the presence of air to convert the HT into anhydrous TiO2 (HTC). The purpose of the addition of the seed is to initiate, induce, and control the hydrolysis of titanium oxysulphate. Hydrolysis of titanium oxysulphate is different from other titania sources such as titanium isopropoxide and titanium butoxide because they do not contain any acid. The acid content of about 24% H2SO4 in titanium oxysulphate requires the addition of ammonia and seed for the control of hydrolysis. Hydrolysis without seed used to take more than 5 h for the completion precipitation. The addition of seed controls hydrolysis in order to obtain smaller particles of titania. Hydrolysis can be performed by excluding the seed and with the addition of bases such as ammonia or NaOH; however, the resulting product will have lower surface area (∼100 m2/g). Therefore, it is critical to add seed to obtain higher surface area and smaller particles of titania. The advantageous precipitation temperature is 90 °C. Experiments showed that a high surface area catalyst had precipitated at 90 °C. A precipitation temperature greater than 90 °C resulted in lower surface area and higher particle size. A precipitation temperature lower than 90 °C resulted in an additional 2 h for completion of precipitation. Although the addition of ammonia to titanium oxysulphate is not mandatory, it was added for the acceleration of the precipitation process. Preparation of Phosphate-Modified Titania. HT was used to prepare phosphate-modified titania (P-TiO2) of varying phosphoric acid (0.5-10 wt % as P2O5) content by the pore volume impregnation method. In the preparation of P-TiO2, 4 g of HT was put in a 50 mL glass beaker, and 20 mL of double distilled water was added. Then, the solution was thoroughly
J. Phys. Chem. C, Vol. 113, No. 31, 2009 13751 mixed using a magnetic stirrer for 1 h. To this, we added 0.27 g of 10 wt % phosphoric acid dropwise with stirring. The resulting suspension was heated to 80 °C under mild stirring, and the temperature was maintained until the slurry was just dried. Thereafter, the beaker was dried at 100 °C for 6 h and subsequently calcined in air at 500 °C for 2 h to yield P-TiO2 containing 0.5 wt% P2O5 (0.5P-TiO2). Similarly, P-TiO2 containing 2.5, 5, 7.5, and 10 wt % P2O5 was prepared. Characterization. Wide-angle X-ray diffraction (XRD) patterns for the calcined and as-synthesized materials were obtained using a Rigaku Miniflex II with Cu KR irradiation. Scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS) studies were carried out on a FEI Quanta 200 electron microscope. The bulk composition of these catalysts was analyzed using inductively coupled plasma mass spectrometry (ICP-MS) optima 4300 DV spectrometer. The ultraviolet-visible/diffuse reflectance spectroscopy (UV-vis/DRS) studies of the samples were performed on a Thermo Scientific instrument. The band gap of the as-synthesized and calcined materials was evaluated using UV-vis/ DRS. Fourier transform infrared (FT-IR) spectra for the samples were recorded using a Bruker Tensor-27 instrument. Thermal analyses of the samples were performed on PerkinElmer thermogravimetry differential thermal analyzer (TG/DTA) using alumina as the reference. The nitrogen adsorption and desorption isotherms at -196 °C were measured using a Micromeritics ASAP-2020 surface area and porosity analyzer after the samples were degassed in vacuum at 300 °C for 3 h. BET surface areas were calculated from the BET plot. Pore size distributions were calculated using the BJH model. Pore volume was measured at the single point of P/Po ) 0.99. Results and Discussion Figure 1 shows the XRD patterns of anatase (Fluka), P-25, calcined hydrated titania (HTC), and phosphate-modified hydrated titania samples. Anatase titania shows higher crystallinity than P-25 and phosphate-modified hydrated titania. Phosphatemodified titania samples showed poor crystallinity and pure anatase phase compared to P-25. The crystallite size of the samples were calculated using the Scherrer equation and are presented in Table 1. It is shown that there is a decrease in crystallite size and pore size with an increase in the phosphate content of phosphate-modified titania. Particle size was calculated using the surface area data, which showed a similar trend of decreasing particle size with increasing phosphate content. The particle size and transformation of amorphous to crystalline titania of the P-TiO2 was found to decrease with an increase in phosphate content because phosphate molecules are anticipated to break the Ti-O-Ti bond for the formation of Ti-O-P linkage. In addition, the small quantity of phosphate present in the titania hinders crystal growth as it is present on the surface of titania as a titanium phosphate coating. The XRD data revealed the modification of hydrated titania by polyvalent anions such as phosphate (P-TiO2) and sulfate (HT) resulting in anatase type titania. The titania samples synthesized in the present work are highly amorphous in nature; hence, to identify the effect of phosphate treatment in crystalline anatase TiO2, anatase was treated with phosphoric acid and calcined at 500 °C as discussed in the Experimental Section. Figure 2a shows the XRD patterns for the crystalline anatase sample obtained from Fluka (purity >99 wt %) and its 5 wt % P2O5 modified sample (5P-anatase). The phosphoric acid treatment of anatase was found to lower the crystallinity and particle size by 50%, which should be due to the formation of titanium phosphate. The discussion on the
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Figure 1. XRD patterns of titania samples.
TABLE 1: BET Surface Area, Particle Size (XRD Method and SBET Method), Pore Volume, and Pore Size of Titania Samples sample number
sample
SBET (m2/g)
particle size (nm) SBET method
crystallite size (nm) XRD method
pore volume (cm3/g)
pore size (nm)
1 2 3 4 5 6 7 8 9 10 11 12 13
HT HTC (500 °C) 0.5P-TiO2 (500 °C) 2.5P-TiO2 (500 °C) 5P-TiO2 (500 °C) 7.5P-TiO2 (500 °C) 10P-TiO2 (500 °C) P-25 anatase (Fluka) 5P-anatase 5P-TiO2 (650 °C) 5P-TiO2 (800 °C) 5P-TiO2 (900 °C)
277 100 110 130 154 166 172 50 12 26 126 106 18
6 15 14 12 10 9.3 8.9 30 128 59 12 15 85
6.9 14.6 13.5 9.1 8.6 8.2 7.8 21.1 48 27 9.5 12.2 32.6
0.36 0.36 0.35 0.34 0.32 0.24 -
14.5 13.4 11.3 9.0 8.0 17.2 -
percentage of crystallinity is based on the calculation of peak height intensity with standard crystalline anatase and rutile (Fluka). Most available anatase titania crystals normally form a thermodynamically stable (101) surface (>94%) and are found to be highly reactive with phosphate groups. The other facets such as anatase (004) and (200) are also found to be reactive, however, lower than anatase (101). The adsorption of phosphate on the titania surface caused a large destabilization of the (101) surface due to the formation of Ti-O-P linkage resulting in a substantial decrease of the (101) fraction in phosphate-stabilized anatase. Figure 2b shows the proposed structure for the substitution of oxygen by phosphorus in the anatase framework. The substitution of phosphate in the anatase framework leading to the formation of a titanium phosphate network and crystal structure disintegration was indicated by thin black lines in Figure 2b. Hence, crystalline anatase and amorphous titania (HT) evidently have irreversible reactivity with phosphorus. The reports suggest P-25 titania has 75% anatase and 25% rutile. However, the crystallinity data for P-25 was not reported elsewhere. Figures 1 and 2a show the XRD patterns of P-25, anatase, HTC, and P-TiO2. When comparing the crystallinity of P-25 with crystalline anatase and rutile, P-25 is only 90% crystalline, and the remaining is an amorphous form of titania. The higher activity of P-25 titania should be attributed to its amorphous content and higher surface area. The crystallinity
for HTC was calculated on the basis of the XRD peak intensity obtained at a 2θ of 25.3 and was found to have about 15% anatase. The remaining ∼85% of titania is amorphous in form. The crystallinity is lowered from about 15% to e10% with the phosphate treatment, which is observed to enhance the surface area and anticipated to improve the overall catalytic activity for the metal-supported catalysts. The particle size calculated using BET surface area and crystallite size using XRD data showed good agreement for hydrated titania and its phosphate-modified samples. However, anatase and P-25 showed disagreement. The doubling of the surface area for the 5P-anatase and the halving of particle size (Table 1) should be due to the degeneration of Ti-O-Ti linkage and formation of Ti-O-P leading to disintegration of the anatase crystal. In general, the heat treatment in air at higher temperatures leads to particle growth and crystallization; this fact is also evidenced by XRD for the samples HT and HTC (Figure 1). This typical trend is not manifested in phosphatemodified titania samples. The catalyst samples are more often than not subjected to extended heat treatment, and its thermal stability is an important application in reactions. Considering this fact, the thermal stability of 5P-TiO2 was studied using XRD. Figure 2c shows the XRD patterns for the 5P-TiO2 samples treated at various temperatures. The sample is treated at each temperature in air for 2 h. Crystallinity was found to
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J. Phys. Chem. C, Vol. 113, No. 31, 2009 13753
Figure 2. (a) XRD patterns of anatase and 5P-anatase. (b) Unit cell of 5P-anatase. (c) Effect of temperature on the crystallinity of 5P-TiO2.
increase from 10-80% for the temperature range of 500-900 °C. The 5P-TiO2 treated at less than 900 °C showed purely anatase phase with crystallinty of e28%. The treatment at 900 °C for 2 h showed a steep enhancement in crystallinity to 80% with two additional peaks at a 2θ of 22.58 and 27.76. These two peaks are not due to rutile as it forms a major peak at a 2θ of 27.52. Hence the peak formed at a 2θ of 27.76 is possibly due to the formation pyrophosphate. The surface area and particle size data for the various temperature-treated 5P-TiO2 samples are presented in Table 1 (samples 5, 11, 12, and 13). The trend of decreasing surface area and enhancement of particle size were the effects observed with temperature due to transformation of amorphous to anatase phase. At 900 °C, a steep drop in surface area and an unanticipated increase in particle size show it could be an optimum temperature for amorphous to anatase conversion. In essence, phosphate-modified titania showed good thermal stability at temperatures lower than 800 °C. Figure 3 shows scanning electron micrographs of HTC and 5P-TiO2, which reveal a more uniform dispersion for P-TiO2 than for HTC. This homogeneous dispersion for P-TiO2 exhibits the presence of all the phosphorus on the surface of titania. The enhanced dispersion also accounts for the smaller particle size for P-TiO2 than for HTC. The morphology is not apparent for the samples because of the amorphous nature of HTC and P-TiO2. The energy dispersed spectra (EDS) measured for HTC
and 5P-TiO2 showed a composition of 43.8% O and 56.2% Ti and 51.1% O, 43.6% Ti, and 5.3% P; respectively. The EDS was measured in order to identify the surface composition. The decrease in titanium and the presence of phosphorus on the surface of P-TiO2 shows the effect of phosphorus to breakdown the O-Ti-O linkage for the formation of Ti-O-P. In addition, the increase in oxygen content for P-TiO2 demonstrates the existence of a higher hydroxyl concentration for P-TiO2 than for HTC. Hence, the surface composition for P-TiO2 and HTC reveals the presence of all the phosphorus on the surface of titania. Table 2 shows the bulk composition of hydrated titania (HT) and P-TiO2 samples that were obtained from ICP-MS. The purity of titania for samples 1-3 was confirmed by the zinc amalgam reduction (nakazono reductor) method. The composition of P-TiO2 samples revealed the retention of all the phosphorus in titania and its insoluble nature in water. Calcined hydrated titania (HTC) showed a purity of 99.9 wt % TiO2, and the remaining 0.1 wt % was due to the presence of silica and iron. Figure 4 shows the thermograms of titania samples. A sample size of 5 mg was employed for the analysis, and alumina was used as reference. The as-synthesized P-TiO2 samples showed a weight loss of 16.5% and was found to match that of HT. The loss of weight is attributed to the presence of moisture and hydroxyl groups in the sample. The simple structure of HT is TiO(OH)2, which on heat treatment converted to TiO2 by giving
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Figure 4. Thermograms of titania samples.
Figure 3. SEM of titania samples.
TABLE 2: Chemical Composition of Titania Samples sample number
sample
TiO2 (wt %)
P2O5 (wt %)
1 2 3 4 5 6
HTC 0.5P-TiO2 2.5P-TiO2 5P-TiO2 7.5P-TiO2 10P-TiO2
99.9 99.3 97.3 94.7 92.3 89.3
0.6 2.6 5.2 7.6 10.2
out water. In case of phosphate modification, the surface hydroxyls are replaced with phosphate groups, thereby enhancing its thermal stability. The HTC showed a total weight loss of 4.8%, and P-25 showed a weight loss of 2.4%. The HT showed a lower conversion temperature for the transition of HT to TiO2 than that of the P-TiO2 samples. The data illustrate the increase in conversion temperature from 600 to 616 °C for an increase in P2O5 content of 0.5-10 wt %. P2O5 is essentially a fluxing agent, and hence, when present in titania as P2O5, it is expected to lower the crystallization temperature. However, the addition of various quantities of P2O5 increased the conversion temperature of HT to TiO2, suggesting the formation of titanium phosphate rather than the discrete presence of P2O5. The hydrophilic nature of titania is a established fact and thereby found to lower the hydrothermal stability of unmodified titania samples. However, titanyl phosphate is insoluble in water and is anticipated to enhance the thermal and hydrothermal stability of the P-TiO2 samples. Figure 5 shows the differential thermograms obtained for the titania samples. The weight loss pattern
Figure 5. DTG of titania samples.
TABLE 3: Weight Loss of Titania Samples sample number
sample
temperature (°C)
loss of wt %
1 2 3 4
HT 0.5P-TiO2 2.5P-TiO2 5P-TiO2
610 614 606 616
7.8 7.3 5.3 4.3
is found to be different for the samples. Hydrated titania showed a weight loss at 100 and 610 °C. The 0.5P-TiO2 sample showed a weight loss at 109 and 614 °C. The 2.5P-TiO2 sample showed a weight loss at 280, 450, and 606 °C. The 5P-TiO2 sample showed a weight loss at 616 °C. The 5P-TiO2 sample was not observed to show any significant weight loss peaks at lower temperature. Although all of the samples showed a total weight loss of 16.5%, the weight loss for 5P-TiO2 is uniform through out and significant at 616 °C. These data show that a large amount of the hydroxyl groups in HT were replaced by
Engineering of Titania
Figure 6. UV-vis/DRS of (a) anatase, (b) HTC,(c) 5P-TiO2, (d) HT, and (e) P-25.
phosphate groups in 5P-TiO2. The order of hydroxyl groups replaced with phosphate groups follows the order of 0.5P-TiO2 < 2.5P-TiO2 < 5P-TiO2. Table 3 shows the quantity of weight loss for the samples at 600 °C. The weight loss of 4.3-7.8% at 600 °C is attributed to the conversion of hydrated titania to titania. The decrease in weight loss at 600 °C with the increase in phosphate content shows a higher thermal stability for the samples containing higher phosphate content. Figure 6 shows the diffuse reflectance spectra for hydrated titania and P-TiO2 samples. The band gap is calculated by the tangent method. The hydrated titania and P-TiO2 samples modified with 2.5-10 wt % P2O5 showed the same band gap of 3.19 eV irrespective of the phosphate content. The DRS measured for the anatase TiO2 showed a band gap of 3.19, which match the P-TiO2 samples loaded with various P2O5 content. However, the higher surface area and smaller particle size for P-TiO2 samples rather than crystalline anatase will have the advantage for P-TiO2 samples over anatase for the use as a catalyst or catalyst support. The absorbance for anatase shows a steep decrease compared to that of P-25 and HT. This is essentially due to its crystalline nature. P-25 shows a flat absorbance pattern compared to that of the other samples because it contains a mixture of anatase, rutile, and amorphous samples. The HT shows a flat absorbance similar to that of P-25 because it is amorphous. In addition, HT contains 1.8 wt % of sulfur in the form of Ti-O-S, and this can promote higher photo catalytic activity than other titania material without any further modification. HTC and 5P-TiO2 show almost the same pattern of absorbance; however, 5P-TiO2 shows more flat absorbance than that of HTC and suggests 5PTiO2 is more amorphous than HTC. This fact is also confirmed by XRD and surface area data. The BET surface area, crystallite size (XRD method), particle size (SBET method), pore volume, and pore size of the samples are presented in Table 1. The surface area of HT was 277 m2/ g, which is about five times higher than that of the commercial P-25 sample. The pore volume of HT and its phosphate-modified analogs are higher than that of P-25. In general, alcohols such as 2-propanol and n-butanol are used to improve the surface area and pore volume of inorganic oxides. Hence, titanium sources such as titanium isopropoxide, titanium butoxide, etc., when used as the source for titania synthesis, may be anticipated to produce a higher surface area of titania than the inorganic titania sources such as TiCl4. This is attributed to the templating
J. Phys. Chem. C, Vol. 113, No. 31, 2009 13755 effect of alcohols. Hence, the use of a purely inorganic starting material as a source for high surface area titania requires the addition of alcohol or amines as a template. Herein, titanium oxysulphate has been used as the starting material for the synthesis of anatase type titania because titanium tetrachloride leads to the enhancement of rutile formation, which is detrimental for many catalytic applications. Also, the use of titanium tetrachloride leads to handling problems, corrosion of down stream equipment, and need for repeated maintenance. Organic titania sources such as titanium isopropoxide, titanium butoxide, etc., are expensive and cause problems related to effluent treatment. It is for these reasons titanium oxysulphate is preferred over the other titania sources. Figure 7 shows the adsorption and desorption isotherm for HTC and P-TiO2 samples. The adsorption isotherm is classified as type IV with a H4 hysteresis loop and a pore size of 8-14.5 nm. Pore size distribution is broad for the samples, which are shown in the inset of Figure 7. However, the pore size is found to fall in the mesoporous range. Pore size, pore volume, and surface area of the various P-TiO2 samples are presented in Table 1. The data show a decrease in pore size from 14.5 to 8 nm and pore volume from 0.36 to 0.3 cm3/g with an increase in phosphate content of the P-TiO2 samples. The lowering of pore volume and particle size in the P-TiO2 samples is attributed to the presence of small pores, which is furthermore indicated by the nitrogen adsorption-desorption curves shown in Figure 7. A steep adsorption-desorption isotherm in the 0.4-0.6 relative pressure region is generally attributed to small pores. The P-TiO2 samples modified with higher phosphate content show a higher hysteresis effect in the 0.4-0.6 relative pressure region, which is characteristic of the small-pore texture with high surface area. The surface area was found to increase from 100 to 172 m2/g with an increase in phosphate content of the P-TiO2 samples. The decrease in mesopore size and increase of surface area show the possibility for the formation of a titanyl phosphate network, which is more amorphous in nature as evidenced by XRD (Figure 1). The P-TiO2 samples on calcination in air at 500 °C showed the reciprocal dependence of surface area, particle size, pore volume, and thermal stability with phosphate content. It is noticeable from the data that the surface area, pore volume, particle size, and thermal stability of the titania samples are influenced by the phosphate content up to 800 °C. Figure 8 shows the FT-IR spectra of the titania samples measured in DRIFT mode. Korosi et al.23 reported a new absorption peak in the range of 980-1200 cm-1 for the phosphate-modified titania samples. In addition, the report compared the characteristic stretching vibrations and their intensities, which are proportional to the phosphate content. Phosphate significantly affects the surface properties and is strongly bound to titania; however, the measurement of DRIFTIR with and without KBr did not show any significant change in spectra for the HTC and various calcined P-TiO2 samples. Nevertheless, 10% P-TiO2 showed a peak at 980 cm-1 with the characteristic stretching vibrations of phosphate groups. Hence, it is assumed that FT-IR is noneffective for the determination of phosphate content in P-TiO2 samples. The pH measurement for the 10 wt % solution of HTC and P-TiO2 samples showed a pH of 5.8-6. The pH values showed that prepared samples are acidic in nature. However, the nearly same pH values for P-TiO2 samples containing varying phosphate content showed the absence of free phosphate or P2O5. The carbon free hydrated titania is a good candidate for producing transition metal-modified catalysts, for example
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Figure 7. Adsorption and desorption isotherm of titania samples.
TABLE 4: Effect of Crystallinity and Surface Area on Esterification of Propanoic Acid with n-Butanol
Figure 8. DRIFT spectra of (a) P-25, (b) HT, (c) HTC, (d) 0.5PTiO2, (e) 2.5P-TiO2, (f) 5P-TiO2, (g) 7.5P-TiO2, and (h) 10P-TiO2.
vanadia-modified titania. The thermally stable mesoporous P-TiO2 can also find application as a photocatalyst and should be useful to alkylation of bulky molecules, dehydration, and oxidation reactions as it is more stable than the ordered mesoporous counterparts such as SBAs and MCMs. Catalytic Activity. The esterification of n-butanol with propanoic acid was studied on P-25 and P-TiO2 samples. The reaction was performed at 60 and 75 °C for 24 h with 0.1 g of catalyst and 10 g of reactants containing an equimolar ratio of n-butanol and propanoic acid. The effect of crystallinity and surface area on esterification is presented in Table 4. The esterification reaction led to a sole product of n-butyl propanoate. The P-TiO2 samples showed higher conversion of propanoic acid than P-25. Among the P-TiO2 samples, the lowest conversion was obtained on 0.5P-TiO2. Comparison of the structural characteristics of samples 5P-TiO2 and 0.5P-TiO2 revealed that the specific surface area of 5P-TiO2 is 28% larger, whereas its crystallinity is 65% lower than that of 0.5P-TiO2. The anatase (Fluka) and P-25 showed lower conversion of propanoic acid than the P-TiO2 samples and may be attributed to their higher crystallinity and lower surface area. The conversion of propanoic acid was found to be lowered for 7.5P-TiO2 and 10P-TiO2
sample number
sample
1 2 3 4 5 6 7
anatase (Fluka) P-25 0.5P-TiO2 2.5P-TiO2 5P-TiO2 7.5P-TiO2 10P-TiO2
conversion conversion of propanoic of propanoic % SBET acid at 60 °C acid at 75 °C crystallinity (m2/g) (wt %) (wt %) 100 90 20 10 7 2 amorphous
12 50 10 130 154 166 172
nil 0.2 0.9 2.8 3.7 3.6 3.4
nil 0.3 1.1 3.3 4.1 3.9 3.7
despite an increase in surface area that may be due to the lowering of anatase phase and/or complete conversion of anatase to amorphous titania. Although the data showed impeded conversion of propanoic acid with crystallinity up to 5P-TiO2, the effect of the optimum quantity of anatase phase and phosphate content seems to be more pronounced. Conclusion In summary, a simple organic free synthetic process has been evolved to prepare mesoporous titania using titanium oxysulphate as the titania source. The incorporation of phosphorus into the titania framework was found to inhibit particle growth and crystallization resulting in a significant increase in surface area. XRD, thermal studies, BET surface area, and nitrogen adsorption-desorption isotherm results show that mesoporous P-TiO2 possesses a higher surface area (>100 m2/g) and pore volume (>0.3 cm3/ml) than that of P-25. The SEM-EDS characterization of HT and P-TiO2 samples showed the formation of a titanyl phosphate coating as a layer on the surface of titania. Phosphate-modified titania exhibited enhanced thermal stability up to 800 °C, which prevented the phase transformation from anatase to rutile. The esterification reaction performed on P-25 and P-TiO2 samples showed the significance of an optimum quantity of anatase phase and phosphate content for catalytic activity.
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