Synthesis and Characterization of High Surface Area Nano Titanium Dioxide

TiO2 and TiO2-Al2O3 nanoparticles were synthesized via sol-gel method using hydrolysis of Titanium tetraisopropoxide (TTIP) with ethanol and water mixture as titania source. TiO2-Al2O3 Nano-composite was successfully synthesized using the sol-gel technique. Tetraisopropoxide and aluminium isopropoxide were used to prepare TiO2-Al2O3. All prepared samples calcination were conducted at different temperature (400 to 700) o C. The synthesized TiO2 and TiO2-Al2O3 nanocomposites were then characterized by XRD, AFM, BET surface area, SEM, XRF. XRD, the analysis showed that the presence of alumina (Al2O3) in the TiO2 has an effect on crystal size, particles size, surface area, and crystal phases; The XRD result revealed that the prepared TiO2 nanoparticles were anatase phase at 400oC, and 500oC, and transformed to rutile from 600oC to 700oC, but after addition of alumina TiO2 was of anatase phase, without any rutile at all calcination temperatures, also, the addition of alumina leads to a significant decrease in the crystal size, particles size, especially at high temperatures while the surface area of pure titanium was increased, and this corresponds to the results of the AFM and SEM. The best-obtained surface area was 355.18 m2/ gm. with 34.98 nm of average particle size at 500oC in comparison with pure nano titanium dioxide.

. These metal oxides are generally used as active metals (Mo and W) support in hydrodesulfurization, like TiO 2 , ZrO 2 showed prominent activity, but some disadvantages are prevented their commercial exploitation like limited thermal stability, inappropriate mechanical properties and reduced surface area and low thermal stability. Many methods are used to overcome these disadvantages such as; mixed oxides to get the benefit of preferred features for both systems like mixed with g-Al 2 O 3 , also some systems research to get understand the function of support in the reactions of hydrotreating such as mixed with SiO 2 , MgO, ZrO 2 , and B 2 O 3 [8].
Titanium dioxide (TiO 2 ) has many properties such as chemical stability, low cost, easy handling, non-toxicity, and environment-friendly, this made it of great importance in the field of technology applications. Therefore, a lot is being driven to perform researches varied from the applications of energy to environmental ones that are dealing with catalytic features in these wide fields. TiO 2 is a semiconductor with a wide bandgap; it is exhibiting resistance to erosion in its two types of chemical and photochemical. These features make TiO 2 a substance widely used in chemical sensors, solar cells, hydrogen gas production, also as pigments, self-cleaning surfaces, and application of environmental purification [9].
Sulfur compounds are one of the most common impurities in crude oil which is widely used as transportation fuels such as gasoline, diesel and jet fuels [10].
Residual sulfur species in fuels will lead to the emission of sulfur oxide gases naturally, these gases react with water in the atmosphere to form sulfates and acid rain which can damage buildings, destroy automotive paint finishes, acidifies soil and ultimately lead to the destruction of forest and various other ecosystems [11]. Automobiles are also adversely affected by the presence of sulfur species in the fuels as the sulfur species have a profound effect on the efficacy of catalytic converters [12]. There are many processes applied to reduce sulfur content in refined petroleum liquid hydrocarbons [13]. The hydrodesulfurization method (HDS) is one of the most important and efficient methods of removing sulfur compounds from a petroleum stream [14], [15]. The γ-Al 2 O 3 supported with Mo Oxide catalysts and promoted with Co or Ni is still used extensively in the process of sulfur compounds removal in the refining industry, γ-Al 2 O 3 is very important among other phases because of its structure that possesses a high surface which gains the focus for many chemical and petrochemical separation processes and catalysis [16].
In recent years, many stringent environmental regulations have been issued, and this has led to an increased need for hydrodesulfurization (HDS) catalyst that is more active, effective, and different from the standard sulfide catalyst. Titanium oxide (TiO 2 ) is good new support for hydrotreating (HDT) catalysts [17], it improves the performance of catalyst for many reactions, such as dehydrogenation [18], [19], hydrodesulfurization [20], water-gas shift, and thermal catalytic decomposition [21]. But TiO 2 has the disadvantages of presenting poor thermal stability, a low value of a surface area, and bad mechanical properties.
There are several methods followed by some authors to overcome these disadvantages such as the grafting of TiO 2 with g-Al 2 O 3 , SiO 2 , etc. TiO 2 with other mixed oxides to be formed such as SiO 2 , Al 2 O 3 , and ZrO 2 . TiO 2 -Al 2 O 3 are among these mixed oxides systems that have received maximum attention while many researchers also studied TiO 2 -ZrO 2 and TiO 2 -SiO 2 is relatively less studied [22]., and an important requirement for improving the TiO 2 catalytic activity is to increase its specific surface area, this property is increased considerably through the high surface-to-volume ratio of the nanoparticles. (Nano-object with all three external dimensions in the Nanoscale or nanoscale, the size range for approximately 1nm to 100 nm) as compared to that of microparticles [23].
Authors in [24] were proposed TiO 2 -Al 2 O 3 mixed oxides as catalyst support. TiO 2 -Al 2 O 3 mixed oxides were prepared by sol-gel route using titanium and aluminium isopropoxides, chelated with acetylacetone to promote a similar hydrolysis ratio for both the alkoxides.
The surface area was superior to 200 m 2 /gm. and mean pore size of about 1nm. These characteristics of porous texture are preserved after firing at 673 K. The diffraction patterns of sample fired above 973K showed only the presence of anatase crystalline phase. The crystalline structure of the support remained unaltered after molybdenum adsorption, but the surface area and the micropore volume were drastically reduced.
photocatalytic activity than those of the as-synthesized sample and the commercial nano TiO 2 powders (P-25, JRC-01, JRC-03). Also, this synthesis method provided a simple route to fabricate nanostructures TiO2 with high photocatalytic activity. Mehdi Karimi et al. (2013) [26] are synthesized High purity titanium dioxide (TiO 2 ) nanoparticles via the sol-gel method with surface area relatively 75.64m2/g, using titanium tetra-isopropoxide (TTIP), and synthesized material was used as a photocatalyst for the removal of mercaptans from gasoil. The XRD analysis showed that the product was TiO 2 nanoparticles in anatase form. The crystalline phase product was composed of fine particles with dimensions between 17 and 20 nm. The result showed that up to 78% of the mercaptans in gas-oil was removed using the synthesized TiO 2 photocatalyst.
The aim of this study is the preparation of TiO 2 , and TiO 2 -Al 2 O 3 nanoparticles via a sol-gel technique, also this work focused on the effect of alumina addition on the crystal size, particles size, phase transformation, and surface area, as well as the effect of temperature on the properties of pure titanium and its comparison with TiO 2 -Al 2 O 3 mixed oxides. Where many uses and applications of nano titanium oxide are based on these properties.
In the meantime, a little volume of concentrated HCl (0.5 ml) was mixed with (20 ml) of anhydrous ethanol in addition to (2.5 ml) of water for preparing the solution (B). Solution A homogeneous mixture solution was obtained after vigorous stirring for a period of 2 hours, where a sol was generated. After 24 hours, finally, the sol was converted into a gel to obtain nanoparticles, centrifugation process was used to separate the product at (5000 rpm for 15 min) which was finally dried at 100 o C for 24 hours to ensure removing residual organic material and water. The dried gel was annealed at 400 o C for 4 hours (heating rate=3 o C/min), to acquire the desired TiO 2 NPs.

Preparation of nano TiO 2 -Al 2 O 3
The TiO 2 -γAl 2 O 3 was prepared through the sol-gel process previously reported by Wenjie After stirring vigorously for 4 hours a homogeneous solution was obtained, and a sol was formed. After 48 hours of ageing time, the sol was finally transformed into a gel.
Centrifugations at (5000 rpm for 15 min) were used to separate the product and obtain the nanoparticles. Then the product was subjected to drying at 100 o C for 24 hours to ensure removing the water and residual organic material. The dried gel was annealed at 500 o C for 3 hours (heating rate=3 o C/min), to acquire the desired TiO 2 -Al 2 O 3 nanoparticles.

X-Ray Diffraction (XRD)
The identity of the titanium dioxide (TiO 2 ), TiO 2 -Al 2 O 3 , nanoparticles prepared at different temperature (400,500,600, and 700) o C that was determined by the X-ray diffraction technique.
The XRD pattern of the TiO 2 is observed to be in good agreement with the well-known reference pattern (JCPDS 21-1272) of TiO 2 [29]. Some researchers used the Scherrer equation to calculate particle size where the equation measures crystallite size [30]. It should be noted that only anatase TiO 2 [29], and there is no peak present assigned to the rutile phase (2θ=27.360 o ) [31]. The peaks of the sample that was sintered at 500 o C become much sharper revealing the formation of larger crystallites of anatase and the enhancement of crystallization.
It can be concluded from the calcination of 600°C Figure (  Crystallite sizes of anatase and rutile phases are listed in Table (1). It is seen that the crystallite size of pure TiO 2 of the anatase phase increases with increasing calcination temperatures from 400 o C to 700 o C [35]. That is because the particles' growth is induced by an increased temperature [32].In single-phase anatase, the size of crystals increases from 9.24 nm to 12.03 nm when calcination temperatures are raised from 400 o C to 500 o C. The anatase phase is transformed to the rutile phase and rutile phase peaks appear at temperatures of between 600 o C and 700 o C and the crystallite size of more than 30 nm [34,35].

Table (1) Crystallite sizes of anatase and rutile phases
The XRD patterns of TiO 2 -Al 2 O 3 (63% TiO 2 , 32% Al 2 O 3 ) mixed oxide prepared by solgel method calcined at (400, 500, 600, and 700) o C are shown in Figure 2(a-d).  Through the XRD of TiO 2 -Al 2 O 3 and its comparison with pure titanium, it was observed that the peaks in titanium were less wide, this refers that TiO 2 is a highly crystalline product, also has a low surface area [36].    [38]. At the same time, the peak intensities of anatase TiO 2 are diminished obviously with the loading γ-alumina of (32 wt. %). This refers that there is a considerable modification effect on TiO 2 support with the presence of γalumina, which is obviously in line with the surface area results [38]. Table (2) Average crystal size, particles size, and surface area of the prepared particles

Scanning Electron Microscope (SEM)
The precursor TiO 2 nanoparticles morphologies and particle size distribution at different calcination temperatures (400, 500, 600, and 700) o C obtained by the sol-gel method is indicated in the SEM micrographs as shown in Figure (4). The obtained SEM images revealed that the particles of synthesized TiO 2 samples were in the range of nano size, at temperature 400 o C particle-sized TiO 2 samples consist of (32.5) nm spherical particles as shown in figure 4a. When the temperature increases, the sizes become bigger and the agglomeration becomes significant.
It was observed that the large particle size is obtained at higher calcination temperatures, the particle size (58.4) nm is for the samples calcined at 700 o C [32]. Figure (4d). The obtained results are in good agreement with XRD results showing that the particle size of the rutile phase is greater than that of the anatase phase due to the aggregation of nanoparticles with increasing calcination temperature.   [39]. For all calcination temperatures (400,500,600,700) o C the crystal, and particle size for composite TiO 2 -Al 2 O 3 decreases when compared with pure TiO 2 , at the same time they are increased with an increase in temperature [36], [39] as shown in Table (2), that may be due to the addition of alumina that can inhibit the aggregation of TiO 2 particles [37]. Surface morphology is very important for the activity of the materials, and some properties can provide more active centers such as rough and structured surfaces with high surface area [40]. The chemical composition of the samples was analyzed by XRF the results indicated the chemical composition was 32% Al 2 O 3 , 64% TiO 2 .

Atomic Force Microscopy (AFM)
The atomic force microscopy (AFM) technique was used to find the distribution of average particle size, and shape of the surface. The AFM images in three dimensions and distribution of average particle view of TiO 2 nanoparticles at different temperatures (400, 500, 600, and 700) o C, are shown in Figure (6, a -d) respectively. AFM analysis of the nano composed TiO 2 -Al 2 O 3 was performed to get a better understanding of the calcination temperature effect. All experiments showed an average particle size of the prepared nano titania in the nanoscale. It was observed that increasing calcination temperature from 400 o C to 700 o C leads to an increase in the particle size ranged from 41.52 nm to 47.78 nm. At higher annealing temperatures bigger clusters come to appear and granular structure has occurred with the effect of better crystallization [43]. The particles size at a temperature of 700 o C is greater than the other (47.78) nm, which is maybe due to the amorphous phase of TiO2, and Al 2 O 3 [43]., it is observed that the result obtained through AFM is close to what was obtained in the XRD. In figures 7(a to d) respectively it can be seen that there is a change in morphology as the annealing temperature increases, although this change was gradually and slightly, the roughness indicates that the effect of better crystallization [43].

Fourier Transforms Infrared Spectroscopy (FT-IR)
FT-IR spectrum in the range between (4000-400) cm -1 was used to characterize the prepared samples, the results showed the existence of hydroxyl groups in samples. The broad peaks centered 3361 cm -1 and 3429cm-1 at calcination temperatures (400, and 500) o C respectively are referred to the stretching vibrations of the O−H group, which is attributed to the significant amount of H 2 O molecules in the interlayer space and surface [44,45], and also due to the reaction between the O-H groups of TiO 2 [46]. Another peak of around (1620 -1650) cm -1 is attributed to the H-O-H bending mode at the same temperatures above [47]. The Ti-OH vibration band becomes much weaker when calcination temperature has been increased [48]. So TiO 2 calcined at (600, and 700) ᵒC  [46].
Two bands at 2854 cm -1 , and 2916 cm -1 , and band at around 1514 cm -1 , are assigned to the asymmetric stretching, and symmetric stretching and bending C-H vibrations respectively at 400 o C, can be attributed to the organic residues, also this band appear at 500 o C in range 2869cm -1 , 2927cm -1 and 1515cm -1 which remain in TiO 2 even after low calcination temperatures [48], [49]. It is observed that with increasing calcination temperature (600-700) o C the bands of links stretch will decrease the organic part is degraded with increasing temperature [34]. The broadband from about 400 cm -1 , till 1000 cm-1 was assigned to Ti-O [44], [34] the peaks appear at range about 1000 cm -1 to 1200 cm -1 , was assigned Ti−O−Ti skeletal frequency region [50]- [52]. The results of the FTIR spectrum for the TiO 2 -Al 2 O 3 nano composited at calcination temperatures (400, 500, 600, and 700) o C are shown in Figure (9, a to d) respectively.
The pronounced band around (3400, 3427, 3460, and 3490) cm -1 at calcination temperatures (400, 500, 600, and 700) o C respectively, are attributed to the stretching modes for the surface adsorbed water and hydroxyl groups [53], [54], also beak has appeared (1620-1649) cm -1 were assigned to bending modes of O-H from the mixed oxides and surface adsorbed water [55]. The −OH absorption peaks become weaker with increasing calcination temperature.
In addition, absorption bands have appeared at a range (1666-1683) cm -1 and (1415-1494) indicated the formation of γ-alumina, the intensity of these bands decreased gradually as increasing calcination temperatures, which was attributed to the rapid growth of the crystalline nanoparticles [37].  [56]. Some weak peak appears around (2717-2935.4)cm -1 stretching C-H vibrations, which can be attributed to the organic residues, that remain in the sample after low calcination temperature [57], organic part is degraded with increasing temperature, so these peaks become weaker [34].   [41].

Effect of Al 2 O 3 addition, and calcination temperature
The addition of Al 2 O 3 also affects the phase transformation of TiO 2 , Pure TiO 2 is anatase at (400,500) o C, and transforms to rutile at 600 o C, which was in TiO 2 -Al 2 O 3 mixed oxide the only anatase appeared at all calcined temperatures, this means no phase transformation had occurred [37]. The prepared mixed oxides: titanium dioxide cemented with gamma-alumina having a relatively high surface area could be a favorable candidate as catalyst support with high stability for different petroleum refining processes.