Langmuir 1987,3, 173-179 Scheme 11. Active Sites of Rearrangement Reactions in Zeolites-"Gas Phase" Carbocations Entrapped in the Zeolite Cages by the Negatively Charged Cage Walls
endo-TCD
z e o l i t e cavity
0 C,oH,,
-
exo-TCD + Adamantane
ference of the La ions with the hydrocarbon rearrangement process. Conclusion The active sites of Y zeolite catalyzed rearrangements of polycyclic hydrocarbons are highly acidic Brernsted sites related to the presence of framework aluminum (synergistic sites). The rearrangement reaction is not occurring on the surface of the zeolite but in the "gas phase" of the zeolite cage. H/D exchange experiments provide evidence for the formation of carbonium ions as reactive intermediates in the rearrangement process as well as in the effective exchange with Dz carrier gas. Intermolecular proton transfer may also contribute to the efficient intermolecular hydrogen scrambling. We propose that the first hydrocarbons which enter the zeolite are protonated and, upon loss of hydrogen, remain as carbenium ions in the zeolite cage. There may be more than one carbenium ion present per zeolite cage, however, considering the size of the supercages and that of ada-
173
mantane, only one adamantyl cation would fit in the supercage as a "free" gas-phase-type cation. Two or more such carbenium ions per cage would be accompanied by steric hindrance and an increase of negative charging of the zeolite cage, which should result in a close surface contact of the cations, disfavoring a second ionization site per cage. In addition, the presence of carbocations in all or many cages in the zeolite crystal will result in a considerable charging effect of the crystal, affecting the acidity of remaining Brernsted sites. The acidity of Brernsted sites in the zeolites is therefore not only a function of the exact structure of the acidic site but also a function of the extent of ionization the zeolite crystal has already encountered. The cations are kept in the cages by the Coulomb interaction and represent the actual carriers of catalytic activity in the zeolites which transfer the cationic charge to incoming hydrocarbons by hydride abstractions (Scheme 11). This supports that there is an optimal number of Brtansted sites as suggested by previous s t u d i e ~ . ~The "~~ presence of more than optimal Brtansted sites may be related to the extent of coking. Acknowledgment. This work was supported by the Director, Office of Basic Energy Science, Materials Science Division of the US. Department of Energy under Contract DE-AC03-76SF00098. We thank Dr. Rollmann and Dr. Heinemann for valuable discussions and Union 'Carbide and Mobil Development Co. for a generous supply of zeolite samples. Registry No. endo-TCD, 2825-83-4;exo-TCD, 2825-82-3;Na, 7440-23-5;NH4+,14798-03-9;La, 7439-91-0; adamantane, 281-23-2; 1-methyladamantane, 768-91-2; tricyclo[6.2.1.02~']undecane, 28691-42-1.
Effect of High-Temperature Treatments on the Surface Properties of Rutile and Anatase Pigmentary Powders F. Garbassi,* E. MelloCeresa, E. Occhiello, and L. Pozzi Istituto Guido Donegani S.p.A., Centro Ricerche Novara, 28100 Novara, Italy
M. Visca and D. M. Lenti Montefluos S.p.A., Colloid Laboratory, 15047 Spinetta Marengo, Italy Received June 4, 1986. I n Final Form: October 9, 1986 TiOz rutile and anatase pigments have been studied by using various techniques, XRD, TEM, XPS, and leaching. Particular attention has been devoted to understand the behavior of doping ions as a function of temperature during calcination processes. It has been shown that sulfate impurities, introduced during manufacturing, are segregated and eliminated as volatile sulfur oxides by calcination. Additives, such as potassium phosphate, aluminum, and zinc ions, behave differently,according to the dimensions of the ion. Potassium and phosphate ions, quite bulky, are segregated by both rutile and anatase; in the latter case the segregation is less complete due to the lower density of anatase crystalline structure. Zinc hastens the anatase-rutile transition and is partially segregated at the final calcination temperature. It forms a zinc titanate crystalline phase and seems to reduce the potassium segregation, perhaps forming a mixed phase. Aluminum is a very small ion, in rutile it is partially segregated during calcination, while the rest probably occupies substitutional sites. In anatase, the segregation is nearly absent owing to the presence in the crystal lattice of accessible interstitial sites. Introduction Titanium dioxide, owing to its outstanding physicochemical properties, is the most commonly used white p i p e n t in both rutile and m a h e crystalline f O m s . 1 The rutile structure shows a higher refractive index and this
property seems to be related to its more compact crystal structure.' The dispersion behavior of the particles and other important properties like the photoreactivity of the pigment are strongly dependent upon the presence of impurities,
(1) The Handbook; Titanium Pigment Corporation: New York, 1955.
(2) Cromer, D. T.; Herrington, K. J . Am. Chem. SOC.1955, 77, 4708.
0743-7463/87/2403-0173$01.50/0
0 1987 American Chemical Society
174 Langmuir, Vol. 3, No. 2, 1987 Table I. Bulk Concentrations of Additives and Final Calcination Temperature in the Examined Samples final final crystal temp, form samDle % K,O % P,O, % ZnO % AL0, K rutile A 0.18 0.15 0.25 1193 1223 B 0.18 0.15 0.02 C 0.18 0.15 0.08 1233 D 0.18 0.15 0.20 1223 1243 E 0.18 0.15 1.15 anatase F 0.22 0.30 0.02 1273 G 0.22 0.30 0.04 1273 H 0.22 0.30 0.10 1273 1273 I 0.22 0.30
both in the bulk and on the surface. Some impurities (e.g., sulfates3) come from the preparation process, while other additives might be added to control the particle growth and m~rphology.'~~ In the case of the industrial "via sulfate" process, a most important step is the calcination of the hydrous titanium oxide obtained by the hydrolysis of titanyl sulfate solutions. The aim of this study is to understand the behavior of anatase and rutile pigments as a function of temperature during the calcination process. Specific surface area (SSA),crystalline form (in the case of rutile), and distribution of impurities and additives in the bulk and at the surface have been considered. All these characteristics are important from a technological point of view. The surface area and the crystallinity have been studied using BET nitrogen absorption and X-ray diffraction, while the concentration of impurities at the surface has been determined by XPS and leaching. XPS (X-ray photoelectron spectroscopy) is a surface sensitive technique, however it is not able to reveal the presence of impurities and additives at the surface in pores. The insolubility of TiOl prompted us to use various leaching procedure^.^ This chemical attack is able to leach also species present at the surface in pores. The dissolved species can be subsequently titrated. Therefore XPS and leaching procedures can provide complementary informations about the distribution of impurities and additives, even if the surface specificity of leaching should be taken with some caution.
Experimental Section Materials. A sample of hydrous titania gel, coming from the sulfate process, was kindly provided by SIBIT S.p.A., Milano, Italy. Different portions of the gel have been calcined in a laboratory rotary furnace and in the presence of different amounts of additives, the presence of which is necessary in order to control the size and crystal structure of the pigment particles. Five calcination runs have been carried out to obtain rutile particles (samples A-E): in this case, a small amount (3%) of microcrystalline hydrous rutile has been added to the gel as a seed, to favor the crystallization of the particles. In all cases, the final calcination temperature was established when complete crystallization in rutile form occurred. Small samples were taken a t intermediate temperatures (723, 993, 1113 K) in order to follow the changes in bulk and surface properties. Four samples (F-I) were calcined in the absence of rutile seeds, in order to obtain the particles in the anatase form. Samples were taken during the firing process, a t 723, 1113, and 1238 K. Different amounts of additives have been added to modify the properties of the pigments. In Table I, for each of the samples the amounts of added K20, Pz05,Alz03, and ZnO are reported. Chemical compositions after calcination have been obtained by (3) Barksdale, J. Titanium, Its Occurrence, Chemistry and Technology; Ronald: New York, 1962. (4) Garbassi, F.; Mello Ceresa, E.; Visca, M. J . Mater. Sei. 1981, 16,
1680. (5) Fisicaro, E.; Visca, M.; Garbassi, F.; Mello Ceresa, E. Colloids Surf. 1981, 3, 209.
Garbassi et al. Table 11. XPS Binding Energies (BE) and Full Widths at Half-Maximum (fwhm) Observed Experimentally for Samales A-I fwhm, fwhm, Deak BE. eV eV Deak BE. eV eV 0 1s 529.7 f 0.2 2.2 K 2p 293.1 f 0.4 2.3 531.4 f 0.3 2.0 P 2p 133.3 f 0.3 2.1 Ti 2pSi2 458.5 f 0.1 1.8 Zn 2p3,2 1021.8 f 0.3 2.2 2.5 C 1s 284.6 f 0.1 2.2 5 2p 168.8 f 0.2 solubilization of the solid with a mixture of sulfuric acid and ammonium sulfate under heating.5 Al, Zn, and K have been determined by atomic absorption spectroscopy by using a Perkin-Elmer Model 503 spectrometer. The amount of P has been measured by the blue molybdenum colorimetric method. Also sulfate impurities are present in the sample due to the preparation process. The sulfur concentration has been determined by gravimetry after dissolution with Eschka rea~tive.~ Morphological Characterization. X-ray diffraction (XRD) diagrams have been taken by means of a Philips P W 1050 diffractometer using the Cu K a radiation. Transmission electron microscopy (TEM) observations have been made with a Philips E M 300 microscope. Specific surface areas (SSA) have been measured by the B E T method using a Carlo Erba Sorptomatic equipment a t the liquid nitrogen temperature. XPS. The surface of powder has been examined by X-ray photoelectron spectroscopy (XPS) after insertion in pure In foil. A Physical Electronics (PHI) AES-XPS spectrometer has been used for this purpose. Details on the instrument and methods have been already reportedS6 For all samples analyzed, very similar binding energy values have been measured. Average values with standard deviations are reported in Table 11, together with full widths a t half-maximum (fwhm) of the photoemission peaks. Comparisons with reference compounds' suggest that the elements are present in oxidized states. In particular TiOz, ZnO, SO:-, and phosphate are easily recognized, while potassium is almost certainly present as a positive ion. The oxygen peak is a sum of two components. The first (B.E. 529.7 f 0.2 eV) is relative to TiOz oxygens. The second (B.E. 531.4 f 0.3 eV) is initially assigned to OH groups due to surface hydration, in agreement with previous observation^.^ The intensity ratio between oxide and hydroxyl oxygen peaks is around 3 in untreated samples. After heating, it generally tends to a lower value, but some rehydration due to air exposure before analysis does not allow observation of clear behavior. Furthermore, the increasing presence a t the surface of other species (ZnO, sulfate, phosphate, etc.) causes a fair broadening of the peaks, for the presence of oxygen atoms having fairly different binding energies. The presence of C Is peaks is attributed to some hydrocarbon contamination present a t the surface. Leaching Procedure. Soluble species at the surface of samples A-I have been determined by the chemical analysis of filtered solutions after several leaching procedures: (a) washing by bidistilled water in a Soxhlet apparatus; (b) basic attack with 1 N NaOH solution; (c) acid attack with 1 N H N 0 3 solution. Quantitative determination of ions present in the filtered solutions has been made by the same methods described for the chemical analysis of solids. Details on the above procedure have been reported e l ~ e w h e r e . ~
Results Morphology. Phase composition data from XRD analysis at intermediate and final calcination temperatures are reported in Table 111. In the A-D products the anatase-rutile transformation occurs at temperatures higher than 1113 K. The behavior of the sample E is rather different, since the disappearance of the anatase peak is (6) Garbassi, F. SIA, Surf. Interface Anal. 1980, 2, 165. (7) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Corp.; Eden Prairie, 1978.
High-Temperature Treatments of Rutile and Anatase
Langmuir, Vol. 3, No. 2, 1987 175
Table 111. XRD Phase Compositions as a Function of Temperature
A
temp. K 723 993 1113 A+R(2%) A+R(2%) A+R(6%)
B
A+R(2%) A+R(2%)
A+R(5%)
C
A+R(3%) A+R(3%)
A+R(4%)
D
A+R(3%)
A+R(3%)
A+R(3%)
E
A+R(3%)
A+R(3%)
A+R (97%) + ZnTiOI (2%)
sample
F
finala A+R (99.8%) A+R (99.8%) A+R (99.9%) A+R '
n
I
993 K
(99 % ~ .4...),.
A+R (97.5%)
ZnTiO.
+
(2%)
A A A A
G
723 K
+ R (0.9%)
+ R (0.2%) + R (0.5%) + R (0.5%)
1223K
lll3K
Figure 2. Micrographies of sample D at 723,993,1113, and 1223 K.
'See Table I.
-
160-
*&:\ -* ?
..-
>
Srmplc
0
. 5.
00-
8 :
-?;
0
-
!I 993K
72lK \ \ \ \\ '\
\\\
v1
0 ,
--
Y --a
Figure 3. Micrographiesof sample E at 723,993,1113, and 1243 K. In samples A-E the anataserutile transition begins a t the final stages of calcination. It is strongly influenced by the presence either of rutile seeds or of other additives3" The presence of additives influences the size, shape, and sintering of the particles. Surface vs. Bulk Composition. Surface atomic concentrations measured by XPS on samples calcined a t different temperatures are reported in Table IV. Although AI is present in some samples, it has not been detected by XPS, perhaps because the XPS atomic sensitivity factor of AI' is quite low. Sulfur, as sulfate ion, is introduced in the material during manufacturing. As a consequence of calcination it is likely eliminated in the form of volatile sulfur oxides. In Table V the SO3 bulk concentrations in samples A-I are reported as a function of temperature. All samples behave similarly, apart from minor discrepancies. A small fraction of sulfur is present after calcination, possibly bonded to Ti'+ ions as sulfate. Sulfur surface concentration decrease at increasing temperatures (Table IV). In order to relate the bulk and surface compositions a t different temperatures, the (S/ Ti)8/(S/Ti)bratios have been calculated, s refers to XPS surface atomic ratios and b to bulk values drawn from chemical analysis. The tendency of such ratios to be equal or higher than 1depends on whether S is homogeneously distributed in the particles or segregates a t their surfaces. (8) Latty, J.
E.J. App. Chem. 1558.8.96.
(9) Wiseman, T. J. In Choroeterization of Powder Surfaces; Parfitt, G. D., King, K. S. W., Eds.; Academic Press: New York, 1976; p 163.
176 Langmuir, Vol. 3, No. 2, 1987 Table IV. Surface Concentrations by XPS sample temp, K 0 Ti A 723 71.8 24.2 993 72.0 24.6 1113 75.0 21.1 1193 66.5 21.6 B 723 71.3 25.5 993 74.1 23.2 1113 72.1 23.8 1223 67.1 24.7 C 723 72.3 23.2 993 74.2 21.8 1113 70.2 25.6 1233 70.1 22.2 D 723 68.0 28.4 993 70.7 25.8 68.4 28.2 1113 1223 74.8 19.2 E 723 70.9 24.8 993 73.1 21.9 1113 66.7 18.3 1243 70.4 17.7 F 723 72.4 22.0 1113 71.3 24.3 1238 69.0 25.0 1273 69.8 24.2 G 723 71.4 23.1 1113 69.8 25.0 1238 69.0 23.7 1273 68.5 23.7 H 723 72.2 23.2 1113 72.5 23.5 1238 69.0 24.2 1273 70.5 23.5 I 723 72.3 23.6 1113 70.1 25.9 1238 69.0 24.2 1273 69.3 24.3
Garbassi et al.
(Atomic 70) as Measured
S
K 0.6 0.6
2.8 2.1 1.7 0.3 2.9 2.2 1.7 1.1 3.7 2.7 2.0 0.2 3.5 2.8 1.5
1.1
2.4 0.3 1.6 4.2 0.3 0.6 1.6 4.4 0.1 0.4 1.2 3.1
0.1
3.9 2.5 1.6 0.5 4.5 2.1 0.3
0.3 1.3 1.0 0.5 1.3 3.0 3.0 0.9 2.1 3.8 4.2 0.2 1.4 3.0 3.3 0.4 1.4 3.4 3.2
3.6 2.0 0.3
3.8 1.3 0.7 3.7 1.9 0.4
P 0.2 0.1 0.3 2.7 0.3 0.2 0.8 2.9 0.5 0.7 0.6 3.1
0.3 0.7 2.8 0.2 0.2 1.7 2.0 0.6
Zn 0.4 0.6 0.8 6.5
D
Sample
F
4
30
I
900
D E
d
Sample
F
3
01
*----
1
700
900
-A--A
1
1100
1303 Temperalure ( K )
Figure 5. Difference of SO3 bulk and leached solution (1 N NaOH) concentrations (wt %) for samples D-F. 0.2 2.0 10.4 8.4
1.0
3.0 3.0 1.0 1.1 3.2 3.6 0.6 1.1
3.1 2.7 0.7 3.0 3.2
I
1100 Temperature ( K )
Sample
03-
1
700
Sample
05-
Table V. SO3 Bulk Concentration (wt % ) as a Function of Temperature temp, K sample 723 993 1113 1238 final A 6.3 1.6 0.7 0.2 6.4 2.3 0.6 0.2 B C 6.5 1.9 1.2 0.3 D 0.1 6.8 2.1 0.7 E 6.4 2.1 0.3 0.2 6.5 F 0.6 0.2 0.2 G 6.6 0.7 0.1 0.1 H 0.1 6.0 0.6 0.1 I 5.9 0.6 0.2 0.2 o Sample
0 t
1300
Figure 4. Trend of the (S/Ti),/(S/Ti)b ratio vs. temperature in samples D-F.
In Figure 4 the (S/Ti)a/(S/Ti)bratios of samples D-F are plotted vs. temperature. The ratio increases up to 1113 K, then it decreases a t higher temperatures. Up to 1113 K the sulfate migration from the bulk to the surface is
Table VI. (Me/Ti),,/(Me/Ti), Ratios, Me = K, P, Zn, for Samples A-I, Me sample temp K P Zn A 723 8.1 4.7 6.5 993 7.9 2.4 9.4 1113 17.4 8.4 15.2 1193 37.0 73.5 120.4 B 723 6.6 993 4.4 5.2 1113 22.2 20.2 1223 59.9 62.4 C 723 4.1 12.1 9.0 19.2 993 1113 21.2 14.0 1233 65.4 82.5 D 723 1.1 993 5.1 6.9 1113 14.4 14.8 1223 53.5 84.9 E 723 4.3 0.7 993 4.4 5.4 7.8 1113 23.7 54.6 49.4 18.8 66.5 41.3 1243 F 723 6.1 8.5 1113 15.5 12.7 1238 31.1 33.3 1273 33.2 37.0 G 723 9.2 13.6 1113 21.3 14.6 1238 40.5 41.5 1273 45.1 46.9 H 723 2.4 7.9 1113 16.1 15.8 1238 32.1 38.2 1273 36.6 34.2 I 723 4.5 1113 14.2 8.2 1238 36.6 37.6 1273 34.7 40.0
faster than the surface removal of SO3. A t higher temperatures the latter process is dominant. In Figure 5 the leaching results for samples D-F using basic solutions are presented, again as a function of temperature. At the end of calcination nearly all the remaining sulfur is located at the surface of particles. Lower leaching efficiencies are obtained using neutral or acid solutions. (Me/Ti),/ (Me/Ti)b ratios for samples A-I, calculated for Me = K, P, and Zn, are listed in Table VI. K+ distribution is clarified by the data in Table VI. The tendency is to a segregation of K+ ions a t the surface of the material, the extent of which depends on temperature, crystallinity of the material, and presence of additives. (K/Ti)J (K/Ti), ratios behave similarly when anatase is considered. In fact, their values in samples A-D and
Langmuir, Vol. 3, No. 2, 1987 177
High-Temperature Treatments of Rutile and Anatase
0
Sample Sample b Sample
0
-s
0 045
"."".
1
Sample
o 1
0
E F
I
900
I
I
.
1100
,
1332
0 02
1 700
900
1300
1100 Temperature ( K )
Temperature ( K )
Figure 6. Difference of K 2 0 bulk and leached solution (1 N
Figure 7. Difference of
in samples F-I are similar at 1113 K. Above this temperature, the anatase-rutile transformation occurs in samples A-D. At final calcination temperatures the (K/Ti),/(K/Ti)b ratios are consistently higher when the crystalline form is rutile. It can be noted comparing samples B-D to samples F-H, where additives are qualitatively similar. Rutile samples (B-D) have (K/Ti),/ (K/Ti)b ratios which are some 50% higher than in anatase samples (F-H). The more compact crystalline structure of rutile accounts for this phenomenon. The presence of zinc significantly lowers the rutile (K/Ti),/(K/Ti)b ratio. It decreases both in sample A and E, particularly in sample E. The maximum value of this ratio is reached after the anatase-rutile transformation. We attribute the low (K/Ti)s/(K/Ti)bratio in sample E to the formation of ZnTiO, aggregates incorporating some potassium. In sample A this phase has not been detected by XRD. Perhaps a very limited amount of a surface phase containing zinc and potassium titanates is formed. The result of leaching experiments on samples D-F is shown in Figure 6. The amount of leached K+ is very high (some 80% of the original content) already at 723 K and it remains reasonably constant below 1113 K. In samples D and E (and also in samples A-C, not reported here for sake of brevity) there is a further increase of leached K+ approaching the final calcination temperature, while in sample F (and G-I, not reported) a lesser increase was recorded. Again the different crystalline forms account for these results. When rutile is formed, as in samples A-E, the complete leaching of K+ is favored. Zinc alters the XPS (K/Ti),/(K/Ti)b ratios in samples A and E but not the leaching efficiency. This event is explained remembering that also the zinc-containing compounds present on the surface of the particles are soluble in the leaching solution. In rutile the leached amount at the final calcination temperature is about 90% of the original content, while in anatase it is about 80%, confirming XPS evidence of the preferred K+ segregation in rutile. At any temperature at least 80% of K+ is leached. This fact, suggests that K+ resides preferentially at the surface of crystallites or pigmentary particles. According to L a t t ~the , ~ potassium ions, located at the surface of crystallites act as a regulator of the crystal growth process, directing the growth to particular crystallographic directions. The presence of phosphates stabilizes the anatase ~hase.,PhosDhates are leached most efficientlv bv means bf an alkaline-leaching. In Figure 7 leaching res&ts"relative to samples D-F are presented, while XPS (P/Ti),/(P/Ti), ratios are in Table VI. Up to 1113 K, apart from sample
E, (P/Ti),/(P/Ti)b ratios are similar, in samples A-D and
HN03) concentrations (wt %) for samples D-F.
Pz05bulk and leached solution (1 N NaOH) concentrations (wt %) for samples D-F. F-I. A t final calcination temperatures rutile samples (A-E) show a phosphate content more than 50% higher than in anatase (F-I) samples. This behavior has been already observed for K+ ions and is caused by the denser crystalliie structure of rutile. In samples F-I, (P/Ti),/(P/Ti)b ratios are analogous to (K/Ti),/(K/Ti)b ones. The bulk atomic concentrations are also nearly equal, thus the segregation of K+ and phosphates is similar. In samples A-E the bulk atomic concentration of K+ is about 1.7 times that of phosphorus. In samples B-D the surface K / P ratios at final calcination temperature are always lower than 1.7 while (P/Ti)s/(P/Ti)bratios are higher than (K/Ti)s/(K/Ti)bones. An inclusion of a limited amount of K4P207in rutile crystals'OJ' would account for these facts. Zinc-containing samples (A and E) have somewhat lower (P-Ti),/(P/Ti)b ratios than non-zinc-containing ones (BD), but the difference is not eminent. The presence of ZnTiO, does not influence significantly the extent of phosphate segregation while it does in the case of K+. Leaching of phosphate is easily done already at low temperatures (723 K), when phosphorus is located and very weakly bound at the surface of anatase crystallites in the crystallite aggregates. With increasing temperatures, a minimum of leaching efficiency is observed in samples A-E. A possible explanation is that phosphorus, at this point, remains trapped into the crystallites, where it acts as a retarding agent for the anatase-rutile rearrangement. A further increase of calcination temperature (>1113 K) gives rise to phosphorus segregation at the surface of the sintered particles, which assume the rutile structure. Also in anatase samples (F-I) there is an increase in leaching efficiency after 1113 K, when the primary particles start to grow. In final products, both with anatase and rutile crystalline structure, phosphorus is mostly (80-90% of the original content) located at the surface of particles. Zinc ions are added to decrease the pigment photoactivity, to prevent TiOa reduction during calcination? and to lower the anatase-rutile transition temperature.8 The different amounts of Zn2+ions in samples A and E explain the different relationship between the (Zn/Ti),/(Zn/Ti)b ratio and the calcination temperature (Table VI). Sample E reaches its final structural and morphological state at a lower temperature (about 1113 K).. A further heating of already formed pigmentary particles causes a (10) Rechmann, H. Ber. Bunsenges. Phys. Chem. 1967, 71, 277. (11) Belyaev, I. N.; Sigida, N. P.2.Neorg. Khim. 1967, 3, 428.
178 Langmuir, Vol. 3, No. 2, 1987
Garbassi et al.
Figure 8. Difference of ZnO bulk and leached solution (1 N HNO,) concentrations (wt %) for samples A and E. Figure 10. Illustration of anatase structural arrangement and of the possibility to allocate Ala+ions in interstitial voids.
mo
903
11w
Tempcralur~I n )
13W
Figure 9. Difference of A1,0, hulk and leached solution (1N HNO,) concentrations (wt %) for samples D and H. weak diffusion from the surface to the bulk, reducing the surface concentration. On the contrary, in sample A the phase transition occurs near 1193 K and the segregation prevails on the surface to bulk diffusion. The leaching behavior of zinc is pe~uliar.4.~ Basic leaching solubilizes ZnTi03, but Zn2+ions are not leached out because they can reprecipitate as hydroxides at the surface of particles? In Figure 8 the acid leaching results have been displayed. The behavior of sample A is different from that of sample E. Up to 1113 K (anatase crystalline phase) some 60% of zinc is confined in the bulk. After the anataserutile transition, about 30% of zinc is in the bulk, perhaps as a dispersed zinc and potassium-containing phase. In sample E, up to 993 K (anatase crystalline phase) ahout 40% of the original zinc is in the bulk. With the anataserutile transition the leachable amount shows some decrease. At final calcination temperature some 50% of zinc is leached by acid attack; the remainder is distributed into the rutile network, probably as a structural ZnTiO, phase. The seemingly contrasting behavior of leaching and XPS concentrations is again ascribed to the fact that up to the proximity of the transition temperature the material is quite porous. So there is an "inner" surface which cannot be observed by XPS but is reached by the leaching solution. Aluminum has not been observed by XPS. Beside the weak sensitivity of the technique toward this element, its detection is also interfered by inelastic Ti 3s electrons. In the case of aluminum, acid and basic leachings show similar efficiency, as expected since A120, is amphoteric. In Figure 9 acid leaching data are displayed for samples D and H. With increasing calcination temperature, in samples B-D AI3+ ions become more accessible to the chemical
treatment, i.e., more a t the surface. Between 1113 and 1233 K the anataserutile transition favors the segregation of aluminum. In any case the leached amount is always less than 50% of the original concentration, showing that AP+ has a marked preference for the bulk of the material.'2J3 Actually AP+ is quite a small cation, its ionic radius being 0.053 nm: its lower charge with respect to Ti4+could be useful for neutralizing n-sites in the TiOz crystal lattice, due to the presence of Nb5+or to oxygen vacancies. Low and Offenba~her'~ reported the possibility for AP+ to be placed in substitutional sites. In samples F-H, only sample H, which has the highest original concentration of A1203 shows some sensibility to leaching, some 20% of the original content is leached. This inertness can be ascribed to the presence of nearly all the AP+ ions in the hulk. In fact, in anatase interstitial sites, which can easily accommodate A13+ions, they are present. Discussion Rutile and anatase have a different tendency to segregate or allocate impurities in their lattice. Both rutile and anatase are tetragonal, but rutile structure is more efficiently packed. In anatase there are interstitial voids, for example, at the crystallographic ('/2,'/z,0) location. These sites have ellypsoidal shape, where it is possible to allocate AP+ cations (Figure 10). In rutile the packing is quite denser. In ref 15 the presence of a preferential interstitial channel of diffusion has been reported. Such a channel, having a radius of 0.77 A, favors the diffusion of impurities by an interstitial mechanism rather than substitutional exchanges.'6 The diffusion of metal ions in rutile has already been studied and it has been shown that metal ions are preferentially placed in substitutional sites. Diffusion constants and concentration levels a t different temperatures have been measured for several ~pecies.'~J' It is also known that the rutile TiO, structure allows the presence of substitutional cations having a charge different from 4+. For example the tetragonal symmetry is maintained by substitution of Ti" with divalent ions, suggesting that the charge compensation is either long range or fully rand~m.'~.'~ (12) Hurlen, T.Acto Chem. Scond. 1959,13,365. (13)Slepetys, R. A. Dim. Abstr. 1967,2E, 636. (14)Low. W.:Offenbacher. E. L. In Solid State Phvsics: Seitz. F.. Tukbull, 0.. Eds.: Academie'Press: New York. 1965:~Vvo1.'11. D i35. (15)Wittke, J. P.J. Electrochem. Soc. 1966,113, 193. (16)Huntington,H. B.;Sullivan, G. A. Phys. Re". Lett. 1965,24,177. (17)Yan, M.F.;Rhodes, W. W. J. Appl. Phys. 1982,53,8809.
Langmuir 1987,3, 179-183
In rutile, A13+ions are probably situated in substitutional sites, as suggested in ref 17. In anatase, on the contrary, there are suitable interstitial voids, and A13+very likely should be allocated in these sites. An experimental support to these hypotheses come from the fact that in rutile samples (B-D) the leaching of A13+ ions is effective, while in anatase (samples F-H) it is null or very low. Zinc ions are quite bigger than Ti4+,with a ionic radius of 0.75 A. Their possibility to enter in substitutional sites is then questionable. In rutile they could be partially allocated in interstital sites, but the formation of a zinc titanate phase seems to provide a better opportunity to allocate zinc ions. Potassium and phosphate ions are both bulky and cannot be allocated either in substitutional or interstitial sites in rutile. An inclusion of low amounts of K4P2O7can be postulated. In anatase, potassium ions (the ionic radius is 1.33 A) could be present in interstitial voids. An alternative explanation is the inclusion of some potassium phosphate inside the anatase particles. Pigmentary properties of the calcined particles are known to be strongly influenced by the amounts of additive~.~ Particularly it is interesting to notice the different amounts of K20 and P205added in the A-E and F-I series
179
of samples (Table I). Lower levels of P205in series A-E, together with the addition of rutile seeds, favor the formation of particles showing rutile structure. On the contrary, the absence of rutile seeds and a higher content of P205in series F-I favor the formation of anatase particles. In the latter case, a higher potassium concentration is necessary in order to control the final particle size. ZnO and A1203are known to increase the photostability of the pigment: however, they should act with different mechanisms. For ZnO, it has been observed18 that zinc ions enhance the reoxidation of photoreduced Ti02. This action has been connected to the presence of a zinc titanate layer on the surface of particles. The lower A1203concentration required to photostabilize Ti0219should in turn be dependent upon the residual amount of AP+ remaining within the Ti02 crystat lattice, acting as neutralizer of the n-sites, either intrinsic or generated by substitutional Nb5+ ions. Registry No. Ti02, 13463-67-7. (18) Morterra, C., private communication. (19).Balducci, L.; Gallia, F.; Gambino, F.; Visca, M.; Burlamacchi,L.; Pedulh, G. Congr. FATIPEC 1984,16th.
Isotherm Studies of Benzene and Aniline on Chemically Modified Silica Surfaces Joseph Gorse 111, M. F. Burke, and G. K. Vemulapalli* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received March 18, 1986. I n Final Form: October 28, 1986
Adsorption of benzene and aniline on C18 modified silica columns is investigated as a function of solute concentration and solvent composition. Results indicate that aggregation of solute on the stationary material is much more significant than has been assumed. A theoretical equation that takes aggregation into account is developed and is shown to represent the experimental data accurately. Variations of equilibrium constants and adsorption capacity with solvent composition and with the nature of the stationary phase are explained by the theoretical model.
Introduction The advent of chemically modified silica absorbents, commonly referred to as bonded phases, has greatly increased the power of chromatographic separation systems. Reverse-phase chromatography-where the unsolvated stationary phase consists of hydrocarbon chains bonded to silica particles and the mobile phase, a mixture of polar solvents-has emerged as the most frequently used method of separation. Hence understanding the composition and structure of the effective stationary phase in reverse-phase chromatography has become important for both practical and theoretical reasons. Previous studies of the nature of the bonded surface have utilized NMR' and UV absorption,2 fluore~cence,~ and IR4 techniques, in addition to a variety of chromatographic methods?P6 Current understanding of the effective (1) Gilpin, R. K.; Gangoda, M . E. J. Chromatogr. Sci. 1983,21,352. (2) Elhassan, A.; Blevins, D. D.; Burke, M. F. Anal. Chem., in press. (3) Lochmuller, C. H.; Marshall, D. B.; Harris,J. M . Anal. Chim. Acta 1981,131,263-269. (4) Sander, L. C.; Callis, J. B.; Field, L. R. Anal. Chem. 1983,55,1068. (5) Yonker,C. R.; Zwier, T. A.; Burke, M. F. J. Chromatogr. 1982,241, 257-268. (6) Yonker, C. R.; Zwier, T.A.; Burke, M. F. J. Chromatogr. 1982,241, 269-280.
stationary phase is that it consists of a heteroenergetic surface whose interaction with solutes depends on the specific properties of solute species. The work reported here will introduce yet another technique for the exploration of the stationary phase in reverse-phase systems. The experimental work consists of the measurement of the quantity of solute retained by the interface as a function of the concentration of t h e , solute in the mobile phase. We will show that experimental studies of absorption and desorption over a wide concentration range and a mathematical expression derived from a multiple-step equilibrium model provide a promising method for exploring the nature of the stationary phase. This approach also provides insight into the active roles played by both the solvent and the solute species in the separation process. Experimental Section Chromatograph. The precolumn equilibration technique developed by Cantwell and co-workers7 is used to determine the isotherms. Figure 1 represents the column switching setup with two valves. VI is a Rheodyne valve (Model 7125, Rheodyne, Inc., Cotati, CA) and V2 is a Chromatronix valve (Model 413-8600, (7) Cantwell, F. F.; Puow, S. Anal. Chem. 1979, 51, 623.
0743-7463/87/2403-0179$01.50/0 0 1987 American Chemical Society
