Journal of Sol-Gel Science and Technology 29, 31–40, 2004c© 2004 Kluwer Academic Publishers. Manufactured in The United States.

Sulfate Ions in Titania Polymorphs

X. BOKHIMI AND A. MORALESInstitute of Physics, The National University of Mexico (UNAM), A.P. 20-364, 01000 Mexico D.F., Mexico

E. ORTIZ, T. LOPEZ∗ AND R. GOMEZDepartment of Chemistry, Universidad Autonoma Metropolitana-Iztapalapa, A.P. 55-534,

9340 Mexico D.F., Mexicotesy@xanum.uam.mx

J. NAVARRETEInstituto Mexicano del Petroleo, Programa de Ingenierıa Molecular, Eje central 152,

A.P. 14-805, 07730 Mexico D.F., Mexico

Received May 7, 2003; Accepted November 4, 2003

Abstract. Sol-gel titania was sulfated by using sulfuric acid as hydrolysis catalyst, or by impregnating withammonium sulfate fresh samples prepared with nitric acid or ammonium hydroxide as hydrolysis catalyst. Sampleswere characterized with X-ray powder diffraction, infrared spectroscopy, thermogravimetry and atomic absorptionspectroscopy. Sulfate ions were found anchored to brookite and anatase phases, because they have short O O atomicbond lengths slightly larger than the largest O O bond length of sulfate ion. Since the shortest O O atomic bond inanatase is smaller than the one in brookite, the sulfate ions are then less deformed when they are anchored to anatase,and consequently more stable. Therefore when the sample temperature is raised, the brookite with sulfate ions wastransformed mainly to anatase and not into rutile, which is the most probably transformation when these ions are notinvolved. Sulfate ions also hindered anatase and brookite crystallite growing and stabilized the crystalline structure ofanatase. When the sulfate ions are lost the crystalline anatase phase is transformed into rutile, leaving a large numberof vacancies that favored atom diffusion and consequently the growing of rutile crystallites. The crystalline evolutionof the samples as a function of the annealing temperature is almost independent of the sulfating method.

Keywords: sulfated titania, brookite atomic bond lengths, anatase atomic bond lengths, brookite transformation,anatase stabilization, rutile crystallite growing

Introduction

The growing requirements for high octane unleadedgasoline and for the environmental substitution of aro-matic additives, renews the interest on the alkylation ofn-butene with isobutane to produce alkylates with highoctane numbers. Nowadays, the acid catalysts used inthis process, HF and H2SO4, cause enormous safety and

∗To whom all correspondence should be addressed.

environmental problems [1]; therefore, new acidic cat-alysts have been investigated. For example: acidic ze-olites [2–4], macroreticular acidic ion exchange resinsin the presence of BF3 [4], and a number of solid su-peracids like sulfated zirconia [2, 3, 6–12]. The generalproblem of these solid catalysts is to obtain a strongsurface acidity without the loss of selectivity.

The solid superacidic catalysts could also substi-tute the liquid superacid catalysts, such as HF + SbF5

and HCl + AlCl3, which contaminate and corrode the

32 Bokhimi et al.

environment. The substitution of homogeneous liquidacids by heterogeneous solid superacids as catalysts isexpected to easy their separation from reacting mix-ture, allowing continuous operation, as well as regen-eration and neutralization of the catalyst. Furthermore,the heterogeneous solid catalysts will have additionaladvantages: the reaction is not corrosive, and the dis-posing of the catalysts will not cause environmentalproblems, as with liquid catalysts.

The acidity of these solid superacids can be strongerthan that of 100% sulfuric acid. Solid superacids can beobtained by sulfating metal oxides; some of them areeasily sulfated than others. From those zirconia andtitania were included, but silica and consequently itsnovel mesoporous structures like MCM 41, are notsulfated easily; to get this, it is necessary to dope it withzirconia or titania [13]. Although alumina can also besulfated, its acidity properties will be improved if it isdoped with zirconia before sulfating [14]. This gives abetter catalytic activity for n-butane isomerization thanbulk sulfated zirconia, mainly because of the higheractivity surface of the mixed oxide catalyst. Superacidscan also be obtained by doping metal oxides with WO3

or MoO3 [15–17].Sulfated metal oxides have been tested as catalysts

for isomerization of paraffins [18–20], acylation of aro-matics [21], and dehydration of alcohols [22, 23]. Someof them are highly stable in severe environmental re-actions [24–28].

The most studied sulfated oxides are ZrO2 [6–13]and TiO2 [29–35]. Sulfated titania has been foundto be efficient for isomerization [29], alkylation [30],Friedel-Crafts acylation [31], esterification [32], pho-tocatalytic oxidation [33], and reduction of NOx [34].

There is a discussion about the role of the precursorsfor sulfation [24–28, 36, 37]. Since the initial study ofsulfated oxides [38], it was shown that the surface ofthe active oxides is modified by sulfate ions [39]. Thereare different model proposing how sulfur and oxygenions are linked to the oxide atoms; for example, it isproposed that sulfur is connected though tricoordinatedoxygen atoms [40], or through a four-membered cyclicstructure [39]. When titania is supported in silica beforesulfating, it is proposed that sulfate ions are bridgingto O Ti O Si bonds because Ti O vibrations wereincreased after impregnating with sulfate [41].

From the above analysis, however, it is not clear whyzirconia and titania are more easily sulfated than silicaor alumina. Although some models propose how sul-fate ions are anchored to the titania and zirconia surface,

none of them relate titania or zirconia phases crystal-lography to the sulfate ions geometry.

In order to initiate a discussion why titania can eas-ily sulfated, we studied titania prepared by the sol-geltechnique and sulfated by using two different ways.First, by using sulfuric acid as hydrolysis catalyst; sec-ond, by impregnating with ammonium sulfate freshsol-gel powders prepared with nitric acid or ammo-nium hydroxide as hydrolysis catalyst. Samples werecharacterized with X-ray powder diffraction, infraredspectroscopy, thermogravimetry and atomic absorp-tion spectroscopy; the crystalline structure of titaniapolymorphs was refined with the Rietveld method.

Experimental

Sample Preparation

TiO2-H2SO4. Deionized water (200 ml) was mixedwith 200 ml of tert-butanol (Baker, 99% purity) in arefllux glass system. The mixture was heated at 70◦C;at this temperature concentrated sulfuric acid (Baker98%) was dropped under constant stirring until get-ting pH 3 (0.5 ml). Thereafter, 84.5 ml of titaniumn-butoxide was added to the mixture, refluxing the so-lution for 24 h.

TiO2-HNO3. The titania samples prepared at pH 3with nitric acid as hydrolysis catalyst were synthe-sized following the same procedure described above,but changing the sulfuric by nitric acid (Baker, 98%).

TiO2-pH9-NH4OH. A mixture of 200 ml of deion-ized water and 200 ml of tert-butanol (Baker, 99.9%)was refluxed at 70◦C in a glass system. Then, its pHwas adjusted to 9 by dropping in it ammonium hydrox-ide (Baker, 36%) under continuous stirring. After that,84.5 ml of titanium n-butoxide was dropped into themixture, and refluxed for 24 h.

TiO2-HNO3-(NH4)2SO4. The dried sample, pre-pared with nitric acid, was impregnated with an aque-ous 1 N ammonium sulfate solution (5 ml of solutionper gram of sample).

TiO2-NH4OH-(NH4)2SO4. The sulfating of the sam-ple prepared at pH 9 was performed by following thesame procedure described above for sulfating the oneprepared with nitric acid.

Sulfate Ions in Titania Polymorphs 33

All the samples were dried in static air at room tem-perature; thereafter, they were annealed at differenttemperatures in flowing air.

Characterization

X-ray Diffraction. X-ray diffraction patterns of thesamples packed in a glass holder were recorded atroom temperature with Cu Kα radiation in a BrukerAdvance D-8 diffractometer that had theta-theta con-figuration and a graphite secondary-beam monochro-mator. Diffraction intensity was measured in the 2θ

range between 18 and 110◦, with a 2θ step of 0.02◦

for 8 s per point. Crystalline structures were refinedwith the Rietveld technique by using DBWS-9411 code[42]; peak profiles modeled with pseudo-Voigt func-tions [43] contained average crystallite size as oneof its characteristic parameters [44]. Standard devia-tions, which give the last figure variation of a number,are given in parentheses; when they correspond to re-fined parameters, their values are not estimates of theprobable error in the analysis as a whole, but only ofthe minimum possible probable errors based on theirnormal distribution [45].

Thermoanalysis. Weight loss was determined bythermogravimetry with a Perkin-Elmer TG-7 apparatusfrom room temperature up to 800◦C at 10◦C/min.

Infrared Spectroscopy. FTIR spectra were measuredin a Nicolet-710 equipment, which had a resolution of4 cm−1. Sample powder was mixed with KBr (2%) andpressured into thin disks to be measured in a Pyrex cellthat had CsI windows.

Chemical Composition. The chemical determinationof sulfur in the sample annealed at 600◦C was deter-mined by atomic absorption spectroscopy with a PerkinElmer 2380 apparatus.

Results and Discussion

Samples dehydrated below 100◦C (Fig. 1). This gaverise to a weight loss, which together with the one gen-erated by the evaporation of residual ter-butanol pro-duced a total weight loss between 6 and 10 wt% of theinitial sample weight. Above this temperature samplescontinued loosing weight because of their dehydrox-ylation, which for the non-impregnated samples, oc-curred mainly below 300◦C (Figs. 2 and 3), but for the

0 200 400 600 80010

%

650

550

750 600

500 300

100

TiO2-HNO

3

TiO2-(NH

4)OH-(NH

4)

2SO

4

TiO2-(NH

4)OH

Wei

ght (

%)

TiO2-H

2SO

4

TiO2-HNO

3-(NH

4)

2SO

4

Temperature (oC)

Figure 1. Thermogravimetric curves of the fresh samples as afunction of temperature.

4000 3000 2000 1000

Wavenumber (cm-1)

3200

NH4OH-(NH

4)

2SO

4

TiO2-HNO

3

TiO2-H

2SO

4

TiO2-NH

4OH

TiO2-HNO

3-(NH

4)2SO

4

TiO2-NH

4OH-(NH

4)

2SO

4

1624

1386

1111

Abs

orba

nce

(a.u

)

25oC

Figure 2. FTIR spectra of the samples at room temperature.

impregnated ones it occurred above this temperatureand below 500◦C (Figs. 1, 3 and 4). The phase analysisperformed with X-ray powder diffraction did not showany phase transformation in this temperature interval.

34 Bokhimi et al.

4000 3000 2000 1000

3200

TiO2-H

2SO

4

TiO2-HNO

3

TiO2-NH

4OH

TiO2-NH

4OH-(NH

4)

2SO

4

TiO2-HNO

3-(NH

4)2SO

4

1386

1111

Abs

orba

nce

(a.u

)

300oC

Wavenumber (cm-1)

Figure 3. FTIR spectra of the samples at 300◦C.

The sample synthesized with sulfuric acid had an ad-ditional weight loss occurring between 600 and 750◦C(Fig. 1), produced by the leaving of sulfate ions fromthe sample bulk, which also gave rise to the transfor-mation of anatase into rutile (Fig. 5). At 500◦C thecharacteristic absorption band of the stretching S Ovibration at 1111 cm−1 was still present (Fig. 4). Thisband was more intense in the impregnated samples thanin the one synthesized with sulfuric acid, because thenumber of sulfate ions on the crystallites after impreg-nation was larger. The amount of sulfate ions from thesulfuric acid was small, because it was only in a smallquantity to catalyze the hydrolysis.

In the samples impregnated with ammonium sulfate,the weight loss caused by the leaving of sulfate ions oc-curred between 550 and 650◦C; at lower temperaturesthan in the sample prepared sulfuric acid as hydrolysiscatalyst. This suggests that in the impregnated samplesthe sulfate ions were not in crystallite bulk but on itssurface, where the interaction with the titania octahedrawas weaker than in bulk. The amount of sulfate ions inthe impregnated samples (Figs. 2 through 4) was largerthan in those synthesized with sulfuric acid.

In the impregnated samples an additional weightloss occurred between 300 and 450◦C. Since sulfateammonium was the compound used for sulfating, this

4000 3000 2000 1000

1386

1111

TiO2-HNO

3

TiO2-H

2SO

4

TiO2-NH

4OH

TiO2-HNO

3-(NH

4)2SO

4

TiO2-NH

4OH-(NH

4)2SO

4

500oC

Abs

orba

nce

Wavenumber (cm-1)

Figure 4. FTIR spectra of the samples at 500◦C.

20 40 60 80 100

b

ra

a

r

r

r

800oC

Inte

nsity

(a.

u.)

TiO2-H

2SO

4

Fresh

400oC

600oC

Two Theta (degree)

Figure 5. X-ray diffraction patterns of the samples prepared withsulfuric acid and annealed at different temperatures. Lower tickmarks correspond to anatase; those in the middle, to brookite, andthe upper ones to rutile.

Sulfate Ions in Titania Polymorphs 35

20 40 60 80 100

b

r

r

r

r

aa

a

800oC

Inte

nsity

(a.

u.)

TiO2-HNO

3-(NH

4)

2SO

4

Fresh

400oC

600oC

Two Theta (degree)

Figure 6. X-ray diffraction patterns of the sulfated sample preparedwith nitric acid, and annealed at different temperatures. The tickmarks under the pattern of the fresh sample correspond to ammoniumsulfate; those under the pattern of the sample annealed at 400◦C, tobrookite. Uppest tick marks represent rutile, and those between thepatterns of the samples annealed 400 and 600◦C represent anatase.

additional weight loss must be associated to the ammo-nium ions adsorbed on anatase and brookite crystallitessurface. These ions produced an additional FTIR ab-sorption band near 3200 cm−1 caused by the stretch-ing vibrations of NH ions (Figs. 2 and 3). The absorp-tion band at 1386 cm−1 is associated to the speciesthat gave rise to the weight loss in this temperaturerange, because its concentration was very small whensamples were annealed at 500◦C (Fig. 4). It is worthwhile to note that the FTIR spectra of the fresh im-pregnated samples was essentially the one of pure am-monium sulfate (Fig. 2). This is because only a part ofthe ammonium sulfate reacted during this impregna-tion (Figs. 6 and 7), and the rest stayed mixed with thesulfated phases.

Annealed samples contained only titania crystallinephases: Brookite, anatase and rutile (Figs. 5 through9). In the fresh samples brookite and anatase were yetobserved; in the impregnated ones, non-reacting am-monium sulfate was also observed (Figs. 6 and 7), ex-plaining why the FTIR spectra of this samples was verysimilar to the one of pure ammonium sulfate. In or-

20 40 60 80 100

b

aa

a

r

r

r

800oC

400oC

Inte

nsity

(a.

u.)

TiO2-NH

4OH-(NH

4)

2SO

4

Fresh

600oC

Two Theta (degree)

Figure 7. X-ray diffraction patterns of the sulfated sample preparedwith ammonium hydroxide, and annealed at different temperatures.The tick marks under the pattern of the fresh sample correspond toammonium sulfate; those under the pattern of the sample annealed at400◦C, to brookite. Uppest tick marks represent rutile, and those be-tween the patterns of the samples annealed 400 and 600◦C representanatase.

der to make a quantitative analysis of the crystallinephases, the crystalline structures were refined by usingthe Rietveld method.

Brookite’s crystalline structure was refined with anorthorhombic unit cell having titanium and oxygenatoms in the positions given in Table 1; atoms weredistributed in unit cell according to space group Pbca.Anatase was modeled with a tetragonal unit cell havingtwo titanium and four oxygen atoms in the positionsgiven in Table 2, and distributed according to spacegroup I41/amd. The unit cell for rutile was also tetrag-onal, but its atom distribution corresponded to spacegroup P42/mnm (Table 3). Figure 10 shows a typicalRietveld refinement plot; it corresponds to the sampleprepared with nitric acid and annealed at 600◦C, whichonly contained anatase and rutile.

When samples were annealed at 400◦C or below,they contained only brookite and anatase with circa30 wt% brookite (Table 4). The lowest concentration ofbrookite corresponded to the sample synthesized withsulfuric acid, where sulfate ions were incorporated into

36 Bokhimi et al.

20 40 60 80 100

b

a

a

r

r

r

r

800oC

Inte

nsity

(a.

u.)

TiO2-HNO

3

Fresh

400oC

600oC

Two Theta (degree)

Figure 8. X-ray diffraction patterns of the samples prepared withnitric acid and annealed at different temperatures. Lower tick markscorrespond to brookite; those in the middle, to anatase, and the upperones to rutile.

20 40 60 80 100

b

r

r

r

rr

a

a

600oCInte

nsity

(a.

u.)

TiO2-NH

4OH

Fresh

400oC

800oC

Two Theta (degree)

Figure 9. X-ray diffraction patterns of the samples prepared withammonium hydroxide and annealed at different temperatures. Lowertick marks correspond to brookite; those in the middle, to anatase,and the upper ones to rutile.

Table 1. Brookite, space group Pbca (61): Atoms fractionalcoordinates.

Atom Site x y z

Ti 8c 0.129 0.100 0.863

O1 8c 0.010 0.148 0.183

O2 8c 0.232 0.112 0.537

Note. These coordinates were fixed during the refinement.

Table 2. Anatase, space group I 41/amd (141): Atoms fractionalcoordinates.

Atom Site x y z

Ti 4a 0.0 0.75 0.125

O 8e 0.0 0.25 u

Note. u varied between 0.800(1) and 0.848(8).

Table 3. Rutile, space group P42/mnm (136): Atoms fractionalcoordinates.

Atom Site x y z

Ti 2a 0.0 0.0 0.0

O 4 f v V 0.0

Note. v had values between 0.2951(5) and 0.3054(6).

the crystallite bulk, as it will be demonstrated below.Brookite and anatase concentrations after annealing thesample at 200 and 400◦C were the same (Table 4). Themain difference observed at this temperature was theincrease of crystallite size with temperature in the sam-ples without sulfate ions, being it larger for the samplesprepared with ammonium hydroxide as hydrolysis cat-alyst (Table 5).

Brookite was transformed into anatase and rutile,when samples were annealed at 600◦C. Brookite’s con-version into rutile occurred when no sulfate ions werepresent in the sample, but if the amount of sulfate ionswas large enough, brookite was totally transformed intoanatase. This result is clear when the samples were im-pregnated with ammonium sulfate (Figs. 6 and 7 andTable 4). Sulfate ions then promoted the transformationof brookite into anatase, but not into rutile. This trans-formation occurred between 400 and 600◦C, a temper-ature range in which under the absence of sulfate ionsanatase should transform into rutile [46].

To understand the effect of sulfate ions on the trans-formations of brookite into anatase and not into rutile,and on the stabilization of anatase, the O O atomic

Sulfate Ions in Titania Polymorphs 37

Table 4. Phase concentrations in wt% as a function of sampleannealing temperature.

Sample T (◦C) Brookite Anatase Rutile

TiO2-HNO3 200 31(1) 69(2)

400 34(3) 66(3)

600 77(2) 23(2)

800 100

TiO2-NH4OH 200 28(4) 72(4)

400 29(3) 71(3)

600 81(2) 19(2)

800 100

TiO2-H2SO4 200 24(1) 76(5)

400 23(1) 77(4)

600 92(3) 8(1)

800 100

TiO2-HNO3-(NH4)2SO4 200 31(6) 69(5)

400 31(5) 69(5)

600 100

800 100

TiO2-NH4OH-(NH4)2SO4 200 34(5) 66(5)

400 34(5) 66(5)

600 100

800 100

bond lengths in sulfate group are compared with thosein the characteristic octahedra of brookite, anatase andrutile.

The atomic bond lengths and the angle between themassociated to sulfate groups vary a little depending onthe environment where the sulfate ions are embedded.For example, in titanyl sulfate, TiOSO4, which is veryimportant because its crystalline structure if made oftitanium-oxygen octahedra similar to those forming ti-tania crystalline phases, and sulfate tetrahedra sharingvertices with three titanium-oxygen octahedra [47], theO O atomic bond lengths of the sulfate tetrahedra varybetween 0.238 and 0.243 nm, the S O atomic bondlengths between 0.145 and 0.149 nm, and the O S Oangles, between 107.51 and 112.28 degree. These val-ues have the order of magnitude found in other sulfates[48–51]. For example, in potassium hydrogen sulfate[48] the O O atomic bond lengths in the sulfate ionsvary between 0.2387 and 0.2437 nm, the S O atomicbond lengths, between 0.1428 and 0.1564 nm, and theO S O angles, between 103.12 and 116.57 degree.

Taking the examples of the brookite, anatase andrutile samples used for analyzing their typical octahe-

Table 5. Average crystallite size in nm of the phases as a functionof sample annealing temperature.

Sample T (◦C) Brookite Anatase Rutile

TiO2-HNO3 200 6.0(5) 6.4(5)

400 10(3) 10(1)

600 48(3) 59(15)

800 103(4)

TiO2-NH4OH 200 6(1) 7.0(8)

400 12(4) 10(1)

600 49(2) 63(18)

800 116(5)

TiO2-H2SO4 200 6(1) 6.6(7)

400 7(1) 8.4(9)

600 37(2) 40(22)

800 132(3)

TiO2-HNO3-(NH4)2SO4 200 7(2) 6.4(7)

400 7(2) 8(1)

600 34(2)

800 133(5)

TiO2-NH4OH-(NH4)2SO4 200 6(1) 6.9(9)

400 6(1) 6.8(8)

600 35(2)

800 124(5)

dra [52], the corresponding O O atomic bond lengthsvary as follows: For brookite they vary between 0.251and 0.319 nm; for anatase, between 0.245 and 0.304nm; for rutile, between 0.260 and 0.296 nm. The short-est atomic bond length was found in anatase, and itis a bit larger than the one reported for the sulfateions. This anatase atomic bond length corresponds tothe side shared by octahedra. This means that fromthe three titania crystalline phases brookite, anataseand rutile, the most probable phase for fixing sulfateions to its structure is anatase, whose octahedra havefour O O atomic bond lengths compatible with O Oatomic bond lengths of sulfate ion. To fit to the shortestbond length of anatase the sulfate ions should deformslightly; the corresponding O S O bond angle shouldbe larger than those found in titanyl sulfate, in whichthe largest one is of 112.28 degree. This angle enlarge-ment is possible as it is observed in other systems; forexample, in the potassium hydrogen sulfate, the largestO S O angle is 116.57 degree [48].

The shortest O O atomic bond length in brookite is0.251 nm, which is not far away from the largest dis-tance in the sulfate ion, therefore, this structure could

38 Bokhimi et al.

20 40 60 80 100-0.3

0.0

0.3

0.6

0.9

ar

a

600oCx103 C

ount

s

Two theta (degree)

TiO2-HNO

3

Figure 10. Rietveld refinement plot of the sample prepared with nitric acid and annealed at 600◦C. Upper tick marks correspond to anatase;lower, to rutile.

also have sulfate ions in or on it; their stability, how-ever, would be smaller than in anatase. In contrast, theshortest distance in rutile is much larger, implying thatsulfate ion on its crystallite would be instable. Thisexplains the weight loss observed between 600 and750◦C in the sample synthesized with sulfate acid, andbetween 550 and 650◦C in the impregnated sampleswith ammonium sulfate, where rutile structure couldnot retain the sulfate ions coming from the sulfatedanatase.

The above analysis about the compatibility of sul-fate geometry and the one of titania phases explainsthe transformation of brookite into anatase when sul-fate ions were present. As the temperature of the sam-ple was increased, the brookite octahedra with fixedsulfate octahedra were transformed into a more stablestructure: the one of anatase with sulfate ions fixed toits crystalline structure. In the sample prepared withsulfuric acid (Fig. 5 and Table 4), not all brookite oc-tahedra had fixed sulfate ions, because the number ofthese ions was not large enough; then, when the sam-ple temperature was increased most of these octahedrawere transformed into rutile, and the rest into anatase.

The stability of anatase at 600◦C was promoted bythe presence of sulfate ions in its environment (Table 4).This stabilization was weaker than in the samples pre-pared with acetic acid [46]; the influence of nitrate andammonium ions on anatase stability is not discussed inthe present work, they are, however, also important for

it. The presence of sulfur in the samples synthesizedwith sulfuric acid and in those impregnated with am-monium sulfate at this temperature was corroboratedby atomic absorption. As it was discussed above, be-cause of the compatibility of the shortest O O atomicbond length in anatase with the largest O O atomicbond lengths in sulfate ion, they can coexist in thecrystalline structure of anatase. From the structure ofanatase, it is observed that one Ti O octahedron can besubstituted by two S O tetrahedra of sulfate ions, eachanatase octahedron shares four edges with its neighbor-ing octahedra. This causes an excess on oxygen atomsin the crystalline structure, and gives rise to an negativecharge that can be compensated by replacing the nonshared oxygen atoms by OH ions or H2O molecules, asit happens in the titanyl sulfate [47], or in many otherwater complexes; for example, AlCl3·6H2O [53].

When anatase and rutile coexisted, their averagecrystallite size was of the same order of magnitude.Eventually, rutile’s average crystallite size grew rapidlyby annealing samples at 800◦C, which was favored bythe large amount of vacancies produced when sulfateions left the crystallites, which induced the atom diffu-sion necessary for crystallite growing.

Another effect of sulfate ions was to hinder crystal-lite growing (Table 5). In the samples with sulfate ionsin its environment, at temperatures of 400◦C the av-erage crystallite size was equal to the one observed atlower temperatures (Table 5); although it grew when

Sulfate Ions in Titania Polymorphs 39

samples were annealed at 600◦C, the increase wassmaller in the samples having sulfate ions. In the sul-fated sample the rutile phase, however, had larger crys-tallites, because of the vacancies produced when sulfateions left them.

Conclusions

Sol-gel titania can be sulfated by using sulfuric acidas hydrolysis catalyst, or by impregnating with am-monium sulfate the fresh synthesized sample preparednitric acid or ammonium hydroxide as hydrolysis cat-alysts. The crystalline evolution as a function of theannealing temperature is almost independent of the sul-fating method. Sulfate ions can be anchored to brookiteand anatase, because they have short O O atomicbond lengths that are slightly larger than the largestO O bond length of the sulfate ion. Since the shortestO O atomic bond in anatase is smaller than the one inbrookite, the sulfate ions are less deformed when theyare attached to anatase, and consequently more stable.Therefore when the sample temperature is raised, thebrookite with sulfate ions was transformed into anataseand not into rutile, which is the most probably trans-formation when these ions are not involved. Sulfateions also hinder anatase and brookite crystallite grow-ing and stabilized the crystalline structure of anatase,which lost the sulfate ions from his structure when itwas transformed into rutile, leaving a large number ofvacancies that favored atom diffusion and consequentlythe growing of rutile crystallites.

Acknowledgments

We would like to thank Eng. M. Aguilar for his tech-nical assistance.

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