Facile synthesis of thermally stable anatase titania with a high-surface area and tailored pore sizes

Both affordability and stability are important for commercial-scale production and industrial applications of TiO2. In addition, the ability to tailor nanostructure and physicochemical properties can provide advantages for future applications. Herein a facile sol‒gel process was investigated by using titanyl sulfate as an inexpensive feedstock reacting with water in the media of acetic acid and isopropanol. An anatase phase was readily produced at 65 °C, followed by drying at 80 °C. The anatase was stable up to 800 °C due to the residual sulfate and nitrogen, where sulfate and ammonium slowly decomposed when heating beyond 400 °C. The monolithic TiO2 xerogels were composed of agglomerated TiO2 spherical particles with diameters of ca. 50 or 100 nm. The TiO2 spherical particles were built by anatase crystallites with a diameter of ca. 5 nm. As a result, the TiO2 exhibited both bimodal mesopores and macropores: Large mesopores (10‒30 nm) were present due to the void spaces between the TiO2 spherical particles, while the smaller mesopores (ca. 3 nm) were due to the void spaces between the anatase crystallites within each TiO2 particle. There were also larger macropores (a few micrometers), which were caused by gas bubbles generated during the sol‒gel reactions. From a mass transfer viewpoint, these large pores within TiO2 xerogels could have advantages in their potential applications for catalysis and/or filtration processes. A thermal stable anatase TiO2 with large mesopores synthesized via sol‒gel reactions from TiOSO4. A thermal stable anatase TiO2 with large mesopores synthesized via sol‒gel reactions from TiOSO4. Sol‒gel synthesis of TiO2 using inexpensive industrial feedstock. The anatase phase of TiO2 is thermal stable up to 800 °C. The bimodal mesopores are potentially beneficial for mass transfer. Sol‒gel synthesis of TiO2 using inexpensive industrial feedstock. The anatase phase of TiO2 is thermal stable up to 800 °C. The bimodal mesopores are potentially beneficial for mass transfer.

The anatase phase of TiO 2 is thermal stable up to 800°C.
• The bimodal mesopores are potentially beneficial for mass transfer.

IPA
isopropanol alcohol IC Ion chromatography BET Brunauer Emmett Teller HAc acetic acid XRD X-ray diffraction PXRD powder X-ray diffraction XPS X-ray photoelectron spectroscopy SEM scanning electron microscopy IR infrared TGA thermogravimetric analysis IC ion chromatography

Introduction
As an emerging semiconductor for solar energy harvest [1][2][3], the photocatalyst for hydrogen production [4], industrial catalysts [5,6], and an environmental remedy [7], titania (TiO 2 ) nanomaterials have been extensively studied. For example, TiO 2 is catalytically more active than Al 2 O 3 in the hydrolysis reaction of CS 2 due to its oxygen deficiency (Eq. (1)) [6,8]; however, the activity is also highly related to its porosity, surface area, and potential dopants. When capillary condensation of certain species occurs (such as water and elemental sulfur) in a certain process, there will be a higher resistance to mass transfer, and reactant/product diffusion will be limited to access/leave the active sites. In this circumstance, a larger pore size will be essential for maintaining the catalytic activity [9,10]. In addition, large pore size is also important for the deposition of secondary and tertiary species, especially with a high loading, when TiO 2 is used as catalyst support [11,12].
Anatase and rutile are two dominant crystalline phases among TiO 2 polymorphs, which also include brookite and TiO 2 (B) [13][14][15]. While anatase plays a significant role in its application as photocatalysts and photovoltaic devices, rutile is used more for pigment, sunlight-blocking products, and optical communication electronics [16]. Anatase is a more desirable phase (if kinetically stable), while rutile is the more thermodynamically stable crystalline phase, thus, the phase transition from anatase to rutile is irreversible. From a chemistry viewpoint, this solid-solid phase transition to rutile requires breaking some Ti-O bonds, where literature reports a wide range of activation energies (20-837 kJ mol −1 , depending on particle size, impurity, etc.) [16]. The transition inevitably involves the growth of rutile crystallites and consequent collapse of micro-and mesopore structure, in addition to a decrease in surface area and an increase in the bandgap. Therefore, it is essential to know the kinetic crystalline stability when TiO 2 is desired for a catalyst, catalyst support, and photocatalyst, especially at an elevated temperature and in the presence of steam [17].
The bench scale synthesis methods include sol-gel [7], hydrothermal [18], self-assembly [19], templating [20], electrospinning [21], and chemical vapor deposition [22]. In these laboratory synthesis routes, titanium alkoxides or titanium chloride (TiCl 4 ) are commonly used as starting materials. As an important industrial product for pigment and catalysis applications, TiO 2 has been produced from the ilmenite and rutile minerals. The ore extraction strategies are limited to either the chloride or sulfate route, where a soluble intermediate of TiCl 4 or TiOSO 4 , respectively, is formed [23,24]. The oxidation processing of TiCl 4 has been used to produce benchmark TiO 2 nanoparticles, Degussa P25, which are extensively studied for their photocatalysis applications. There is an interest to use TiOSO 4 to make TiO 2 with desired nanostructures because TiOSO 4 is significantly less expensive than titanium alkoxides [24].
Sol-gel reactions of titanium alkoxides or salts have been extensively studied in the past decades, which include either aqueous or non-aqueous processes [25,26]. The direct TiO 2 products are often amorphous with some exceptions [27], but they can change into the anatase phase after calcination at 400°C. The anatase phase normally starts to transform to rutile at 500°C [28]. Therefore, a variety of research has been conducted to make anatase that is kinetically stable up to very high temperatures, e.g., 1000°C [29][30][31].
In our previous research, anatase TiO 2 was synthesized in different reaction media to control the morphology and pore structure for different purposes [32,33]. However, the starting materials have been titanium alkoxides, which are expensive for commercial-scale production. The inexpensive precursor, TiOSO 4 has been explored previously for making TiO 2 [34][35][36][37][38][39], but very few studies have been reported on the crystalline stability and pore structure control of the final products [40,41]. Herein, we explored a new sol-gel procedure for making stable anatase with a large pore size by using TiOSO 4 as the starting material, which is an inexpensive feedstock used for making non-solgel TiO 2 . The thermal stability of the resulting material was compared with the anatase prepared from titanium isopropoxide. The resulting materials have been of interest for both a Claus catalyst and catalyst support for hydrogenation desulfurization reactions.

Synthesis of TiO 2
The sol-gel synthesis was carried out using the experimental setup that has been described previously [42]. For the typical synthesis of TiO 2 from TiOSO 4 , 72.17 g IPA, 72.12 g HAc, and 47.54 g TiOSO 4 were added to a 2 L 3-neck flask while stirring. The slurry was heated to 65°C and then 72 mL of deionized water was added. The solid was dissolved within 2 h and a clear solution was obtained (pH was measured as 1.6). Into this acidic solution, NH 4 OH was added dropwise till the pH reached the target level. Both gas and precipitate appeared upon and after the addition of NH 4 OH. The obtained material was aged for 2 days at 65°C, followed by filtration and drying at 80°C under vacuum. Following this procedure, a series of TiO 2 samples was synthesized as shown in Table 1. In the labels of TiO 2 samples, the first letter "s" was for sulfate from the precursor TiOSO 4 , and the number represented the pH after the addition of NH 4 OH.
To study the solvent's effect, the hydrolysis of TiOSO 4 was conducted in isopropanol and water (without acetic acid) while other synthesis parameters were kept the same as for making TiO 2 -s-4.3, and the resulting material was called TiO 2 -i-4.3 (where "i" stands for isopropanol). Similarly, when acetic acid and water were used as the solvents (without isopropanol), the resulting material was called TiO 2 -a-4.3 (where "a" stands for acetic acid). In addition, TiO 2 was also prepared without acetic acid and isopropanol, and the resulting material was called TiO 2 -w-4.3 (where "w" stands for water). The molar ratios for making each material are summarized in Table 1.
For comparison, TiO 2 was also synthesized from Ti(O i Pr) 4 at room temperature (~22°C). 200.00 g of Ti(O i Pr) 4 was dissolved in 694 mL IPA, and then 104 mL of 1 M HNO 3 was added dropwise. At this point, the pH was measured at 2.2. The cloudy material was kept at room temperature for two days, followed by filtration and drying at 80°C under a vacuum. The resulting material was labeled as TiO 2 -p-2.2-ap, where the first suffix "p" stood for isopropoxide from the precursor Ti(O i Pr) 4 , and the second suffix represented the pH, and the third suffix "-ap" was for as -prepared sample. To identify the calcined samples, a suffix was added to indicate the calcination temperature, such as -400 for calcination at 400°C. In a separate experiment, TiO 2 was synthesized following the procedure for making TiO 2 -p-2.2, but NH 4 OH was used to adjust the pH from 2.2 to 7.0. This new sample was labeled as TiO 2 -p-7.0, as summarized in Table 2.

Characterization
N 2 physisorption was conducted using a 3Flex (Micromeritics) instrument. The samples were activated in situ at 200°C until the vacuum reached 1.0 × 10 −4 torr. The reported pore size and pore size distribution were based on the desorption isotherm calculated using a BJH model. XRD was analyzed using a Rigaku Ultima IV diffractometer at a speed of 2°min −1 and a step size of 0.02°. XPS analyses were carried out with a Kratos AXIS Supra X-ray photoelectron spectrometer using a monochromatic Al K(alpha) source (15 mA, 15 kV). It probes the sample's surface to a depth of 7-10 nanometers and has detection limits ranging from 0.1 to 0.5 atomic percent depending on the element. The instrument work function was calibrated to give binding energy (BE) of 83.96 eV for the Au 4f7/2 line for metallic gold, and the spectrometer dispersion was adjusted to give a BE of 932.62 eV for the Cu 2p3/2 line of metallic copper. The Kratos charge neutralizer system was used on all specimens. Survey scan analyses were carried out with an analysis area of 300 × 700 microns and a pass energy of 160 eV. High-resolution analyses were carried out with an area of 300 × 700 microns and a pass energy of 20 eV. Spectra have been corrected to the main line of the carbon 1 s spectrum (adventitious carbon) set to 284.8 eV. Spectra were analyzed using CasaXPS software (version 2.3.14). SEM images were collected on an FEI Philips XL30. The HRTEM images were obtained using a Tacnai 20 operated at 200 kV. The thermal destruction of sulfate in TiO 2 was measured on a TA TGA 550 system. The samples were heated from 23 to 800°C with a ramp of 5°C min −1 under a flowing helium atmosphere (10 mL min −1 ). IR studies on solid samples were carried out on a Varian 7000 FTIR at a resolution of 4 cm −1 . Ion chromatography was used to estimate the concentration of sulfate on the surface of TiO 2 samples. The samples were prepared using 0.10 N NaOH to extract sulfate and sulfite from the spent catalysts using sonication. All the processes were conducted under an inert environment in an N 2 glove bag to prevent the oxidation of sulfur compounds by air. GC analysis was conducted on a Varian CP-3800 equipped with two columns (a molsieve 5A and a Restek RT-U-Bond Plot) and two TCD detectors.

Synthesis
TiOSO 4 was found to be insoluble in the mixture of IPA and HAc (with a 1:1 molar ratio) in the temperature range of 22-70°C. Unlike titanium alkoxide, titanium (IV) in TiOSO 4 was unable to react with HAc. The addition of water (from 72 to 144 mL) in the mixture at 65°C could slowly dissolve TiOSO 4 to form a clear solution, and then it became opaque within 2 h. The 2 h precipitation time at pH = 1.6 is significantly less than the precipitation time at pH = 1.4 and 1.0 (6 days and 45 days, respectively) [34]. This is because Ti salt hydrolysis depends on hydroxide concentration. Indeed, after the addition of NH 4 OH into the mixture of TiOSO 4 -IPA-HAc, precipitate and gas bubbles appeared immediately.

IR
All TiO 2 -s intermediates and products showed significant stretching modes for Ti-O-Ti oxo bonds at 570-660 cm −1 . There were also chemi-or physisorbed water bands at 3200-3300 cm −1 and~1630 cm −1 , and residual sulfate at 1050-1240 cm −1 (blue curve in Fig. 1) [43,44]. To our surprise, the spectra did not show carboxylic groups at ca. 1720 cm −1 . The absence of carboxylate ligands, but the presence of sulfate, suggested that the acetate ligand could not replace sulfate to form Ti-acetate complexes. The oxo bands could also be found with the TiO 2 -s made from TiOSO 4 reacting with HAc in IPA only (i.e., without adding H 2 O and NH 4 OH), even though the oxo intensity is lower (black curve in Fig. 1). This result suggested that the sol-gel process could occur under strongly acidic conditions and with a relatively small amount of water (from the starting material of TiOSO 4 , which contains 25.5% water). However, its morphology is more like the starting material TiOSO 4 than the sol-gel product TiO 2 (see the SEM image in the SEM section).
The IR spectra of the as-prepared  Given that the presence of sulfate in the products was an indication of incomplete hydrolysis, it was reasonable to conclude that acetic acid and isopropanol influenced the hydrolysis process. A slower and controlled sol-gel process is preferred for a welldefined product and carboxylic acid and alcohols are often used for this purpose [45]. This is because carboxylic and -OR groups can coordinate with Ti(IV). The spectrum of the as-prepared TiO 2 -p-7.0 in Fig. 1 (dark green) showed residual C-H bands at 1360 cm −1 , in addition to the oxo and OH bonds.

PXRD
The powder XRD patterns of TiO 2 -s, TiO 2 -a, TiO 2 -i, TiO 2w, and TiO 2 -p showed that anatase was already formed in the as-prepared samples without calcination (see the blue curves in Figs. 2 and 3 as examples), which makes this process attractive for certain potential applications. For example, it could be used for one-pot synthesis of organicinorganic hybrid composite materials, because it circumvents the calcination step that would destroy the organic components [46].
The anatase stabilities of these materials were different. The anatase in TiO 2 -p transitioned into rutile at a temperature as low as 500°C, like many other TiO 2 materials [47]. This low-temperature transition from anatase to rutile of TiO 2 -a likely followed the anatase-brookite-rutile route, given the fact that the brookite phase appeared in the as-prepared samples and after calcination at 400°C too (see the orange and dark blue curves in Fig. 3). In contrast, the anatase phase of TiO 2 -s was kinetically stable up to 800°C (Fig. 2). The stability may be ascribed to the small crystallite size of anatase [48], where higher temperatures would eventually cause sintering and loss of crystallite dispersion. At this point, it could be speculated that the high thermal stability of anatase in TiO 2 -s was due to the presence of 10    sulfate, which was relatively thermal stable and potentially decelerated the sintering process. This speculation was also supported by a report that sulfate could facilitate anatase formation and stability [49]. However, this theory was challenged by the findings of TiO 2 -w-ap. As shown in Fig. 2b, the anatase phase in TiO 2 -w also exhibited high thermal stability; however, TiO 2 -w did not contain sulfate according to our IR and XPS analysis results. It is noted that TiO 2 -i and TiO 2 -a samples are similar in kinetic stability when compared to TiO 2 -s. The anatase crystallite sizes of these samples will be described later.

TGA and XPS
We carried out both TGA and XPS analyses to understand why TiOSO 4 -derived TiO 2 materials displayed better anatase stability than TiO 2 -p. According to Fig. 4a, there were three stages of significant weight loss for TiO 2 -s-2.7-ap. The first stage occurred before 350°C, and the weight loss was attributed to the removal of both physical and chemical adsorbed water. The second (350-436°C) and the third stages (436-800°C) were mainly attributed to the loss of monodentate and bidentate sulfate, respectively. The XPS results of the same sample showed the presence of sulfur in the form of sulfate (Fig. 5a), and the molar ratio of S:Ti was as high as 1:4.8. After calcination at 700°C, the S:Ti ratio dropped to 0.7:4, which agrees with the TGA weight loss in the same temperature range. It is noted that the decomposition of sulfate is a prolonged and complicated process, and a very high temperature is often required [50]. In contrast to TiO 2 -s, which showed weight loss at a temperature as high as 700°C, TiO 2 -p did not have a significant weight loss above 400°C. As shown in Fig. 4b, the first stage of weight loss at T < 125°C was due to the removal of adsorbed water, and the second weight loss in the range of 190-400°C was assigned to the elimination of the -OC 3 H 7 and hydroxyl groups. The residual hydrocarbon was detected by IR, as described earlier.
In contrast to TiO 2 -p, which showed plateaued mass loss at 400-800°C, TiO 2 -w and TiO 2 -i had a small and gradual weight loss above 400°C (Fig. 6), which could be assigned to the loss of water and/or sulfate during sintering. The weight loss of TiO 2 -a above 400°C was more pronounced, which was due to the higher concentration of sulfate in this sample (Table 3).
Nitrogen was noticeable in the TiO 2 samples from the XPS results. Interestingly, there is still nitrogen after calcination at 700 and 900°C (see TiO 2 -s-700 and TiO 2 -s-900  Table 3). The presence of nitrogen was ascribed to the added ammonium hydroxide during the synthesis when ammonium could attach to sulfate to form bidentate ≡Ti-SO 4 -NH 4 (Scheme 1). Upon calcination, ammonium cations decomposed to nitrogen atoms, and the latter was trapped in the TiO 2 matrix to form doped TiO 2 [51]. The doped nitrogen could be responsible for the higher stability of the anatase phase in these materials [52].
Sulfate anions have been found to facilitate anatase formation from TiO 6 2− octahedra and stabilize the phase [53][54][55][56]. This phenomenon is important for the application of TiO 2 as a heterogenous catalyst when a high temperature and water content are involved [57].
It is noteworthy that XPS is a surface characterization technique, but it can still detect the top layers of atoms. The TiO 2 -s-2.7-ap sample was also extracted using NaOH, and the extraction solution was analyzed by means of ion chromatography (IC). The extracted sulfate was associated with the sample surface only. It is noted that this analysis technique has been widely used to analyze the sulfate poisoning of industrial catalysts. The IC result showed that the SO 4 2− concentration was only 0.39 mole % (assuming that the sample contains TiO 2 and sulfate), which was significantly lower than the XPS result (3.4%). This indicates that there was a significant amount of sulfate underneath the TiO 2 surface.
The powder XRD data was also used to estimate the anatase lattice constants and unit cell volume, which could reveal if SO 4 2− and N exist in the bulk TiO 2 [58,59]. As shown in Table 4, TiO 2 -s-2.7-900 has the same lattice constants and cell volume as the ideal anatase, suggesting all dopants were removed at this elevated temperature. On the other hand, all other samples (either as prepared or calcined at lower temperatures) have either different a, b, c lengths or unit cell volumes, indicating the doping effect of sulfate and N in these samples.

N 2 physisorption
All TiO 2 produced in this project exhibited Type IV isotherms, indicating that they were all mesoporous materials. Two types of isotherms are shown in Fig. 7: one for TiO 2 -s synthesized from TiOSO 4 (black curve in Fig. 7a) and the     Fig. 7a). Both curves showed either H1 and/or H2 hysteresis loops in the relative pressure range of 0.4-0.6, even though the black loop was less significant than the orange counterpart. In the high relative pressure range of 0.8-0.95, the TiO 2s sample showed a significant secondary hysteresis loop. The interconnected pores generated these H2 loops, and the resulting pores may be attributed to the void space between the packed (or slightly fused) pseudo-spherical particles [60,61]. The BJH pore size distributions of these materials are shown in Fig. 7b. While TiO 2 -a sample had only one maximum in the pore size distribution, TiO 2 -s showed two maxima (bimodal). The maxima at 3.3 and 3.6 nm from TiO 2s-2.7-400 and TiO 2 -a-2.2-400, respectively, were related to their hysteresis loops at 0.4-0.6 relative pressure. These smaller mesopores were caused by the void space between the anatase crystallites, which were in the range of 7-8 nm (see Table 5). The maximum at 37 nm (BJH desorption) corresponded to the void space between larger particles, about 100 nm (as shown in the SEM section), which were aggregated anatase crystallites. For H2 type hysteresis loops, the adsorption curve is normally used to estimate pore size, and the desorption curve is related to the pore neck size [62]. But for H1 type loops, the BJH desorption isotherm is preferred for the calculation of mesopore size distribution [63]. These large mesopores were related to the hysteresis loop in the 0.8-0.95 relative pressure range. It is noted that the macropores contributed to the loops beyond 0.95 relative pressure, but the large pore sizes (>100 nm) cannot be accurately determined using the N 2 physisorption technique. According to the N 2 physisorption results, as shown in Table 5, the BET surface areas of TiO 2 -s-4.3 decreased from 203 to 141 and 100 m 2 g −1 after heating to 400 and 500°C, respectively. The surface area of this sample was still 10 m 2 g −1 at 800°C, which was comparable to the value of 11 m 2 g −1 of TiO 2 nanofibers calcined at 800°C [64]. On the other hand, the surface areas of TiO 2 -p-2.2 dropped more dramatically after thermal treatment: its values were 209, 96, and 0.6 m 2 g −1 at 80, 400, and 500°C, respectively. The thermal stability of anatase crystallites can explain the significant difference between these calcined samples. The anatase crystallites of TiO 2 -s-4.3 did not transform but slowly grew from 6 to 37 nm when the calcination temperature increased from 80 to 800°C (see Table 5 and Fig. 8a). The increment of crystallite size would decrease surface area (see the orange line in Fig. 8b) and cause smaller pores to collapse but generate larger pores (see the blue line in Fig. 8b). In contrast, the anatase crystals of TiO 2 -p-2.2 grew quickly from 8 (400°C) to 30 nm (500°C) and started to transform to rutile (Fig. 3). The phase change caused the surface area to drop significantly from 96 to 0.6 m 2 g −1 when the temperature rose by 100°C. Interestingly, the rutile crystallites of TiO 2 -p-2.2 did not grow, and their size remained at 41 nm when the temperature rose from 500 to 900°C. It is also worth mentioning that the anatase phase of TiO 2 -s-2.7 was stable even though it grew up to 37 nm, which seemed to contradict the prediction that the anatase would convert to rutile when its crystallite size is over 11 nm [65]. As mentioned earlier, we believe that both doped nitrogen and sulfate would stabilize the anatase phase.
The N 2 physisorption and powder XRD analysis results of the TiO 2 -i, TiO 2 -a, and TiO 2 -w series are summarized in Table 6. The BET surface area, pore size, and pore volume were similar among these samples, and they were also close to the values of the TiO 2 -s series. However, the pore size distributions of the TiO 2 -i, TiO 2a, and TiO 2 -w series are monomodal, in contrast to the bimodal TiO 2 -s series. This suggests that the solvents played a role in the microstructure of the sol-gel product. At this point, however, it is unclear how acetic acid and isopropanol affected the aggregation of the anatase crystallites. These two solvents were added to chelate Ti(VI) to manipulate the product pore structure, but our IR results did not show any Ti-acetate bands in either the reaction intermediates or products.

SEM
The sol-gel products of TiO 2 -s and TiO 2 -a exhibited monolithic structures and their dimensions were on the order of centimeters. The SEM images revealed their ubiquitous macropores on the surface of broken monoliths (Fig. 9a). The eruption of gas bubbles caused these macropores to form during the sol-gel reactions. Both NH 3 and propylene could have bubbled through the gel structure when the reaction temperature was at 65°C. C 3 H 6 was produced through the dehydration reaction of isopropanol (with Ti (IV) as a catalyst [66]), and it was believed to be more likely to generate the macropores [33]. These macropores are favorable for mass transfer for heterogeneous catalysis [67]. Higher magnification SEM images revealed the spherical microstructures of TiO 2 . The diameters of these structures in  Adsorption average pore diameter (4 V/A by BET) c Anatase crystallite size calculated from PXRD data by using Scherrer's equation (the numbers in the bracket with " † " are rutile crystallite sizes). The maximum errors for N 2 physisorption results are within 5% *The amount of water was doubled TiO 2 -s-2.7-ap and TiO 2 -s-4.3-ap were about 100 and 40 nm, respectively. Because their anatase crystallite sizes were as small as 5.8 nm for TiO 2 -s-2.7-ap and 5.7 nm for TiO 2 -s-4.3-ap (see Table 5), each spherical particle in Fig. 9b, c should contain hundreds of anatase crystallites.
According to our IR observation, TiOSO 4 reacted with HAc and IPA to a certain extent without adding H 2 O and NH 4 OH (see the black curve in Fig. 1). The SEM image, however, showed otherwise. Figure 9d shows the product of TiOSO 4 reacting with HAc and IPA for 3 days at 65°C. The rod-like morphology was identical to the raw material TiOSO 4 . It is noted that the starting material (67.3% TiOSO 4 ) contained 25.5% water; apparently, this amount of water was not enough to complete the sol-gel conversion.
The ubiquitous presence of the anatase phase in the TiO 2 samples was confirmed by using HRTEM. In Fig. 10, the dominant (101) planes and some (004) planes of the anatase phase in the TiO 2 -s-2.7-400 sample can be observed.

Summary
Thermally stable anatase (kinetically stable) was synthesized by a low-temperature sol-gel reaction of TiOSO 4 with water in HAc and IPA. The anatase phase Adsorption average pore diameter (4 V/A by BET); The anatase crystallite size calculated from PXRD data by using Scherrer's equation. The maximum errors for N 2 physisorption results are within 5% was readily produced without calcination and was stable at an elevated temperature as high as 800°C, even though the crystallite size increased from~5 to 37 nm. The improved thermal stability of anatase was attributed to a small amount of nitrogen and sulfate trapped in the TiO 2 matrix. According to our TGA and XPS data, the nitrogen and sulfate contents in the anatase decreased with calcination temperature. The TiO 2 -s materials exhibited bimodal mesopores. The smaller mesopores (ca. 3 nm) were attributed to the void space between the anatase crystallites, and the large mesopores (10-37 nm) were due to the void space between the TiO 2 spherical particles that were an agglomeration of anatase crystallites. These TiO 2 spherical particles were about 40 and 100 nm in diameter when the solution's pH was 4.3 and 2.7, respectively. The TiO 2 spherical particles were the building blocks of TiO 2 xerogel monoliths in the order of centimeters. The SEM images revealed the presence of macropores with a dimension of a few micrometers. These macropores were generated by escaping gas bubbles of propylene and ammonia during the sol-gel process. Our experimental results show that HAc and IPA played a role in the formation of large mesopores, even though the mechanism was not clear.
Because of the high thermal stability and large pore size of the resulting materials derived from TiOSO 4 , the anatase TiO 2 would be useful for a variety of industrial applications. Our future work will be testing this material for both Claus catalysts for lower-temperature CS 2 conversion and catalyst support for tail gas treatment. Our previous data showed that only excess sulfate would reduce the catalytic activities of Claus catalysts [68]. In the follow-up research, the effect of sulfate and N on the catalytic performance will be further studied.
Acknowledgements The authors are grateful to NSERC for a Discovery Grant and the supporting member companies of ASRL. We thank Dr. Mark Biesinger of Surface Science Western, Western University for the XPS analysis. We also thank Drs. Christopher Debuhr, Yongliang Wang, and Robert Marr of the University of Calgary for their SEM, TEM, and powder XRD work.
Author contributions The manuscript was written with the contributions of all authors. All authors have approved the final version of the manuscript.
Funding This research has been funded through the Natural Science and Engineering Research Council of Canada (NSERC) and Alberta Sulfur Research Ltd. (ASRL) Industrial Research Chair program in Applied Sulfur Chemistry.

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