Langmuir 1993,9,3026-3032
3026
Characterization of Sorption Sites on Hydrous Titanium and Thorium Oxides by Water Sorption B.Venkataramani Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India Received May 11,1993. I n Final Form: August 2,199P Sorption sites present on variously prepared amorphous hydrous titanium and thorium oxides (HTiO and HThO, respectively) have been characterized by analyzing the water sorption isotherms of these hydrous oxides using the D’Arcy and Watt equation. Two types of OH groups have been identified as sorption sites on HTiO: one very strong, doubly bonded to Ti atoms (Ti-OH-Ti) and capable of taking part in ion exchange and the other, singly bonded to Ti (Ti-OH), acting only as a water sorption site. Cation exchange (with Na+,for example) with OH groups on HTiO can take place up to a maximum Na/Ti ratio of 0.5. The Na+present in HTiO hydrates to the same extent as Na+present in organic ion exchange resins. On HThO, only one type of OH group, acting as a water sorption site, exists. Maxima of about 1.2 sites/Ti on HTiO and 1.5 sites/Th on HThO are strong sorption sites. The interaction of OH groups with sorbed water is nearly the same for both acidic (HTiO) and basic (HThO) hydrous oxides.
Introduction Surface hydroxyl groups on oxides and hydrous oxides are responsible for their sorption These groups are characterized usually by spectroscopic and thermal techniques, such as IR, NMR, TGA, DTA, TPD, and others.621 In these techniques, the surface hydroxyl groups are progressively eliminated by heating the oxides to higher temperatures and re-formingthe hydroxyl groups again by exposure to water vapor. Such procedures cause changes in the surface characteristics and morphology of the oxide^.^ This is well documented,22especially in the case of Si02.23 Abstract published in Advance ACS Abstracts, October 1,1993. (1) Day, R. E. Prog. Org. Coatings 1973/74,2, 169. (2) Parfitt, G. D. Prog. Surf. Membrane Sci. 1976, 11, 181. (3) Wiseman, T. J. In Characterisation of Powder Surfaces; Parfitt, G. D., Sing, K. S. W., Eds.; Academic Press: London, 1976; Chapter 4, p 159. (4) Augustynski, J. Struct. Bonding 1988, 69, 1. (5) Pechenyuk, S. I. Russ. Chem. Rev. 1992,61, 385. (6) Boehm, H. P.; Knoezinger, K. Catal. Sci. Technol. 1983,4, 39. (7) Parkyns, N. D. In Chemisorption and Catalysis; Hepple, P., Ed.; Institute of Petroleum: London, 1970; p 150. (8)Fuller, E. L., Jr.; Holmes, J. S.; Gammage, R. B. J.Colloid Interface Sci. 1970, 33, 623. (9) Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chen. 1971, 75, 1216. (10) Tsyganenko, A. A.; Filimonov, V. N. J.Mol. Struct. 1973,19,579. (11) Sockart, P. 0.;Rouxhet, P. G. J. Colloid Interface Sci. 1982,86, 96. (12) Morishige, K.; Kanno, F.; Ogawara, S.; Sasaki, S. J.Phys. Chem. 1985,89,4404. (13) Montagne, X.; Lynch, J.; Freund, E.; Lamotte, J.; Lavelley, J. C. J . Chem. SOC., Faraday Trans. I 1987,83,1417. (14) van Veen, J. A. R. Z. Phys. Chem. (Munich) 1989,162, 215. (15) Fuller, E. L., Jr.; Holmes, H. F.; Secoy, C. H.; Stuckey, J. E. J. Phys. Chem. 1968, 72,573. (16) Omori, T.; Imai, J.; Nagao, M.; Morimoto, T. Bull. Chem. SOC. Jpn. 1969,42,2198. (17) Siriwardane,R. V.; Wightman, J. P. J.Colloidlnterface Sci. 1983, 94, 502. (18) Brey, W. S., Jr.; Lawson, K. D. J. Phys. Chem. 1964, 68, 1474. (19) Doremieux-Morin, C.; Enriquez, M. A.; Sanz, J.; Fraissard, J. J. Colloid Interface Sci. 1983,95, 502. (20) Boehm, H. P. Discuss. Faraday SOC. 1971,52, 264. (21) Parfitt, G. D.; Rochester, C. H. In Characterisation of Powder Surfaces; Parftt, G. D., Sing, K. S. W., Eds.; Academic Press: London, 1976; Chapter 2, p 57. (22) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San Diego, CA, 1990. (23) Barby, 0.In Characterisation of Powder Surfaces; Parfitt, G. D., Sing, K. S. W., Eds.; Academic Press: London, 1976; Chapter 8, p 353.
Hydrous oxides used for ion-exchange studies are prepared by precipitating the respective salts using an alkali and are generally amorphous in nature.2426 Hydroxyl groups present in these hydrous oxides (which are responsible for ion-exchange and/or sorption properties)2~s~24 cannot be easily characterized by conventional spectroscopic and thermal techniques for two reasons: 1. Water present in these hydrous oxides cannot be distinguished from the large number of hydroxyl groups, because there is a continous loss of water on heating (see for example refs 16 and 23). 2. Heat treatment modifies the surface and morphological characteristics of the hydrous oxides and, hence, one cannot Characterize the as-prepared amorphous hydrous oxide. The primary objective of the present study was to characterize the hydroxyl groups or the sorption sites present on amorphous hydrous oxides without modifying the surface and morphological characteristics. The present approachcomprises analyzingthe water sorptionisotherms of variouslyprepared hydrous titanium and thorium oxides (HTiO and HThO, respectively) over the entire range of water activity using the DArcy and Watt equati~n.~’ This approach has been used earlier to study the state of water present in organic ion exchange resins in various ionic forms28v29and to a limited extent to characterize the hydroxyl groups present on hydrous oxides.30
Experimental Section Preparation of Hydrous Oxides. Hydrous Titanium
Oxide. Titanium sponge was dissolved in 5 mol dm-9 HC1 or H2SOd at 370 K and the resulting Ti(II1) salt was oxidized to Ti(IV) salt by controlled addition of HzOz in hot conditions. By this method 0.45 mol dm-9Ti(1V)solution could be prepared. To a 1dm3solution of 0.2 mol dm3Ti(1V)salt in a beaker, an excess of 2 mol dma NaOH was added under constant stirring at 370 (24) Fuller, M. J. Chromatogr. Rev. 1971,14,45. (25) Inorganic Ion Exchange Materials; Clearfield, A., Ed.: CRC Press: Bo& Raton, FL, 1982. (26) Sugh, M.; Tsuji, M.; Abe, M. Bull. Chem. Soc. Jpn. 1990,63,659. (27) D’Arcy, R. L., Watt, I. C. Tram. Faraday SOC. 1970, 66, 1236. (28) GupG, A. R. Indian J. Chem. 1985,244,-368. (29) Nandan, D.; Venkataramani, B.; Gupta, A. R. Langmuir 1993,9, 1786. (30) Venkataramani, B.; Gupta, A. R. Indian J. Chem. 1988,27A, 290.
0743-7463/93/2409-3026$04.00/0 0 1993 American Chemical Society
Sorption Sites on HTiO and HThO
Langmuir, Vol. 9,No. 11,1993 3027
Table I. Characteristics of Hydrous Titanium Oxides ~
sample HTO 1 HTO 2 HTO 3 HTO 4 HTO 5 HTO 6 HTO 7 HTO 8 HTO 9 HTO 10 HTO 11 HTO 12 HTO 13 HTO 14 HTO 15 HTO 16 HTO 17 HTO 18
preparation conditions Ti(IV) salt precipitant aging time HTO 4 heated to 573 K for 4 h sulfate NaOH 15 days" 4h sulfate NaOH HTO 12 acid treated sulfate NaOH 4h NaOH 1 day sulfate 7 days NaOH sulfate 1day sulfate NaOH sulfate urea (hmg)f 2 days 6 months chloride NaOH 3 days NaOH sulfate 2 days sulfate NaOH commercial sample chloridee NaOH 2 days HTO 13 acid treated chloridee NaOH 2 days chloride NaOH 6 months chloride 6 months NaOH
chemical &alysis (mmol/g dry obde) Ti Na
~
~~~
chemical form& (dry oxide)
nature TiOl.se(OH)o.26(0Na)o.osSemicrystalline
sorption site per Tib (OH + ONa)/Ti 0.28
12.04
0.36
11.59 10.75 11.70
1.85 4.19 0.47
amorphous amorphous amorphous
0.32 0.51 0.52
11.24 11.27 10.42 10.16 10.53 11.18 10.64 9.91 10.96
1.69 1.47 3.96 2.14 0.74c 0.36 1.60 2.97
d
amorphous amorphous amorphous amorphous amorphous amorphous amorphous amorphous amorphous
0.64 0.66 0.86 1.02 1.06 1.12 1.20 1.60 1.27
10.83 10.54
d d
semicrystalline amorphous
1.37 1.66
10.52 9.75 10.03
d
semicrystalline amorphous amorphous
1.68 2.00 2.72
2.05 0.01
Acid treated sample. * Based on chemical analysis. SO4 content. Na content below 50 ppm. e Partially hydrolyzed. f Homogeneously precipitated. Table 11. Characteristics of Hydrous Thorium Oxides.
sample THO 1 THO 2 THO3 THO4 THO5 THO6 THO 7 THO8 THO 9
chemical analysis preparation conditions (mmol/gdry of oxide) precipitant aging Th NO3 Na 3.78 THO 5 heated to 573 K for 4 h urea (hmg)' 2 days 3.66 2 days 3.64 NI-LOH 0.29 NaOH 2 days 3.62 0.17 0.01 NaOH 2 days 3.49 NaOH 2 days 3.48 3.41 0.10 THO 8 aged in 1 mol dm-l NH4OH for 15 days 0.14 NH4OH 2 days 3.33 oxalate heated at
chemicalformula(dryoxide)
nature
sorption site per Thb (OH + ONa + NOs)/Th 0.16
1.01 1.16 1.19 2.22 2.56 3.02 3.90
1273 K a
For all samples Th(1V) nitrate was solution used except for THO 10. Based on chemical analysis. Homogeneously precipitated.
K. The precipitate was digested for 2 h a t 370 K, and the precipitate was allowed to cool and age at room temperature
(298 f 2 K). The precipitate waa filtered, washed free of alkali, dried in air (298 f 2 K), rewashed, and finally stored aa air-dried samples. HTiO was also prepared by homogeneousprecipitation. For this, titanium(IV) sulfate solution, containing 40 g of urea per g of Ti, was initially neutralized with NaOH. The precipitation was effected by heating the mixture to near boiling. The fiial pH waa -7. The precipitate was allowed to age in the mother liquor for 2 days at room temperature and processed aa described earlier. While titanium sponge dissolved in HCl, sometimes partially hydrolyzed titanium(1V) chloride solutions were obtained. Two samples of HTiO were prepared from partially hydrolyzed titanium(IV) chloride solution. Commercial HTiO (OXTI from Applied Research Sprl, Belgium) was also used. Many HTiO samples had Na+ incorporated in them. The equilibrium pH of the water in contact with oxides was >9. They were treated with diluted HClOb to remove Na+ and bring the equilibrium pH of water in contact with the oxide to -7. Asprepared and acid-treated HTiO samples were used for water sorption experiments. Sulfate ion WBB present in homogeneously precipitated HTiOS1 and was used as such without further (31) Venkataramani,B.;Gupta,A. R.ProceedingsoftheZntenational Meeting on the Recouery of Uranium from Sea Water; IMRUS-1983; Atomic Energy Society of Japan: 19W, p 313.
chemical treatment. Table I gives details of preparation conditions and treatment used for different HTiO samples used in the present study. The samples are designated HTO 1to HTO 18. Hydrous Thorium Oxide. HThO was precipitated from a 1 dm3solution of 0.1 mol dm3 thorium(1V) nitrate in 0.5 mol dm4 HNOs using an excess of either 1mol dm3 NaOH or NHdOH at 370 K, digesting the precipitate for 2 h at 370 K, and aging in the mother liquor. One sample of HThO was prepared by homogeneous precipitation using urea (30 g of urea per g of Th) at 370 K after initially neutralizing the thorium(1V) nitrate solution with NaOH (asdescribed in the case of HTiO) (final pH N 7). Pure ThOz was prepared by calcining thorium(1V) oxalate at 1273 K. Table I1 gives details of preparation conditions used and treatment given for different HThO samples used in the present study. The samples are designated THO 1 to THO 9. One sample each of HTiO (sample HTO 1) and HThO (sample THO 1) was heated to 573 K for 4 h. Characterizationof Hydrous Oxides. The hydrous oxides were dissolved in acid and the solutions analyzed for Ti and Th gravimetrically aa their respective oxides. Na+ was estimated by atomic absorption and nitrate and sulfate contents were estimated by conventional techniques. X-ray powder diffraction was used to characterise the crystallinity of the different samples. Tables I and I1 give results of these analyses.
3028 Langmuir, Vol. 9, No. 11,1993 Water Sorption. Water sorption experiment8 were done at room temperature (298 f 2 K) in an isopiestic set up. Equilibrating the hydrous oxide samples in vacuo over concentrated HzSOd led to dehydration of the samples to the same extent as drying at 773 K. Hence, samplee driedin vacuo over concentrated HzS04 were taken as completely dry samples. Water sorption isotherms were determinedusing HzSOd solutionsof knownwater acti~ity.3~ Other experimental details are given in an earlier publication.30
Results Characterization of Hydrous Titanium and Thorium Oxides. The chemical composition of hydrous oxides is between that of pure oxide (e.g. TiO2) and pure hydroxide (e.g. Ti(OH)3 and can be represented by the emperical formulas Ti0LX(OH)bfor HTiO and Th02-x(0H)bfor HTh0.33,34The OH groups present are responsible for sorption and ion exchange properties. During ion exchange (for example, with Na+ in alkaline solution), the H+ of the OH groups is replaced by the cation and gets converted as shown:
Such ion exchange takes place when the freshly precipitated hydrous oxide is aged in the mother liquor (e.g. NaOH). Hence, the chemical formula of hydrous oxides prepared by precipitation using NaOH and aged in that solution can be written as Ti02-x(OH),(ONa)b (a + b = 2x) for HTiO. With Ti and Na contents of the dry HTiO sample known,its empericalformula can be deduced.Table I gives the chemical formulas of various HTiO samples used in the present study. When hydrolysis is incomplete, as in the case of homogeneously precipitated HTiO (sample HTO 9, in Table I),sulfate ions are present as an impuritpl and the chemical formula can be written as Ti02-,(0H),(S04)b ( a + b / 2 = 2x). HThO, being more basic than HTi0,24has more anionic impurity (NO3-1 than Na+ and the chemical formula can be represented as Th02-x(OH),(ONa)b(N03),( a + b + c = 2 x ) . Knowing the Th, Na+, and No3- contents in dry HThO samples, the corresponding chemical formula can be arrived at and Table I1 gives the chemical formulas of different HThO samples used in the present study. OH, Na+, NOa-, and s042-groups act as sorption sites on the hydrous oxides. Tables I and I1 also give the sorption sites per Ti or Th as deduced from the chemical formulas. In the present study, HTiO samples having sorption sites/Ti varying from 0.28 to 2.72 and HThO having sorption sites/Th varying from 0.15 to 3.19 have been used (Tables I and 11). The aim of the present study is to characterize these sorption sites by water sorption isotherms. The nature of the hydrous oxide samples used in the study is also given in Tables I and 11. HTiO prepared from partially hydrolyzed titanium(1V)chloride solutions (HTO 14 and HTO 16, Table I) were semicrystalline. HThO prepared homogeneouslyusing urea (THO 2, Table 11) was also semicrystalline. HTiO and HThO samples heated to 573 K for 4 h (HTO 1in Table I and THO 1, in Table 11) were also semicrystalline. Thorium oxide prepared by calcining thorium(IV)oxalate at 1273K (THO (32) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions; Butterworths: London, 1969. (33) Heitner-Wirguin, C.; Albu-Yaron, A. J . Inorg. NucI. Chem. 1966, 28, 2379. (34) Clearfield, A. Reu. Pure Appl. Chem. 1964, 14, 91.
Venkataramani
U
m LT
F 30.1 LL
0 c
z
3
0 6
0
Figure 1. Typical water sorption isotherms of HTiO 1, HTO 16; 2, HTO 12; 3, HTO 17; 4,HTO 13; 5, HTO 2.
i
/
a
! W -
.01
5 U
0 Iz
3
0 d
aw
Figure 2. Typical water sorption isotherms of HThO: 1, THO 9; 2, THO 1; 3, THO 2; 4,THO 5. 9, Table 11)was pure crystalline ThO2. All other samples were amorphous in nature. Water Sorption Ieotherms. Typical water sorption isotherms of HTiO and HThO are shown in Figures 1and 2. In general, amorphous HTiO and HThO samples exhibited Langmuir-type water sorption isotherms. The slopes of the initial portion of the curves vary from sample to sample (Figures 1and 2). Some of the samples (curve 5, for HTO 2, Figure 1)did not showplateau but continuous increase in water sorption with increase in water activity. Semicrystalline (curve 1,for HTO 16, Figure 1, and curve 3, for TH02, Figure 2) and crystalline (curve 1for THO 9, Figure 2) samples showed multilayer sorption isotherm, the monolayer capacity decreasing with increase in crystallinity (curves 1, and 3, in Figure 2) of the samples. Analysis of Water Sorption Isotherms. Langmuir and BET and related equation^^^*^^ have been used to analyze the water sorption data. It has been shown that the D'Arcy and Watt equationz7yields more information about the ion-water interaction in ion exchangemmTBThe D'Arcy aftd Watt equation2' is versatile in the sense that it can be used to analyze not only Langmuir (monolayer) or multilayer-type isotherm but can also distinguish between different types of sorption sites. The general form of the D'Arcy and Watt equation is: ~~
(35) Zettlemoyer, A. C.; Micale, F. J.; Klier, K. In WaterComprehensiue Treatise; Franks, F., Ed.: Pleunum Press: New York, 1976; Vol. 5, p 249. (36) Yamanaka, S.;Malla, P. B.; Komarneni, S. Zeolites 1989,9, 18.
1
Sorption Sites on HTiO and HThO
Langmuir, Vol. 0, No.11, 1993 3029
Table 111. Parameters of the D’Arcy and Watt Equation Obtained for Hydrous Titanium Oxides by T w o Methods of Analysis method 36
method 1’ sample HTO 1 HTO 2 HTO 3 HT04 HTO 5 HTO 6 HT07 HT08 HT09 HTO10 HTO11 HTO12 HTO13 HTO14 HTO15 HTO16 HTO17 HTO18 4
error sorption Bum site Wm wc WPl WP2 WP k squarec perTid (mmol/g) K (mmol/g) (mm0Wg) Kl Kz (mmoWg) (mm0Wg) 2.24 2.04 0.20 6.17 10.51 0.30 0.0033 1.73 0.30 0.10 0.28 2.64 142.10 4.80 0.01 0.81 0.00014 3.84 30.80 0.52 6.06 0.32 8.26 0.10 0.0025 3.47 78.90 0.30 4.56 54.00 3.09 5.04 0.51 2.16 o.oO069 8.49 78.90 1.10 6.49 6.60 4.26 0.52 4.52 4.20 0.99 0.0016 0.03 7.62 4.00 4.10 12.10 0.64 5.95 4.70 3.90 11.47 5.54 4.30 0.00057 0.66 0.84 o.ooOo2 14.30 0.05 8.65 14.30 8.66 0.86 0.0004 3.50 12.11 3.50 12.11 1.02 0.21 43.40 3.02 0.87 0.00017 0.80 38.70 0.40 3.99 3.96 1.06 500.00 1.47 12.02 0.57 0.0116 1.48 749.80 1.12 0.47 0.0024 0.94 41.90 0.10 6.02 2.87 2.84 40.00 11.46 1.20 0.0076 6.25 121.90 1.00 3.00 20.40 4.42 1.60 5.08 0.0014 11.34 198.30 2.30 2.50 14.16 5.00 0.06 1.27 0.01 11.70 0.10 0.99 O.ooOo3 1.38 1.38 11.60 0.32 0.52 1.37 499.60 1.57 14.58 0.47 0.0021 1.56 475.40 1.66 0.98 O.ooOo2 0.01 19.00 0.02 1.20 0.61 19.20 2.09 1.19 1.68 8.43 232.00 1.80 3.78 0.001 16.70 4.39 8.04 2.00 7.83 395.80 0.10 1.96 295.80 8.31 13.96 0.23 0.023 2.00 2.72
Method 1: w = wp
+ wc + w m (eq 3).
Method 3 w = wpl
+ wp2 + w c+ w m (eq 7). e Equation 8.
error wc (-OW@
3.90 1.56
where W is the amount of water sorbed (g/g of dry oxide) and a, is the water activity. The first term in eq 2 refers to the Langmuir-type sorption sites, the second term to the sorption sites which can be approximated to a linear isotherm, and the third term to multilayer formation. In this equation (eq 2))Ki’ = mni/N,where m is the molecular weight of water, ni is the number of sites of the ith type, N is Avogadro’s number and 1 is the number of different types of primary sites. Ki is the interaction parameter related to the heat of sorption of water on to the ith site.27 Cis a constant assigned for the linear form of the sorption isotherm.27 k = mD/N,where D is the number of sites for multilayer formation and k is a parameter related to the heat of sorption of sorbate in the multilayer formation.27 When the water sorption isotherm is analyzed by setting i = 1, in eq 2, the total amount of water sorbed on the hydrous oxide at a, =e 1 (w, mmol/g) (which represents a fully hydrated state) can be given in terms of the water associated with the strong primary sites, w p (mmol/g, contribution from the first term in eq 2)) water associated with weak sites, w, (mmol/g, contribution from second term in eq 21, and that present in the multilayer, W m (mmoV g, contribution from third term in eq 2). Thus, at a, = 1 (and using eq 21, we have
w = w p + w,+ w,
(3)
Water sorption isotherms of hydrous oxides which did not show plateau (for example, curve 5, Figure 1)gave, on analysis, small contributions to w, and wm. A contribution from the second term in eqs 2 or 3led to a broad conclusion about the existence of weak sorption sites,3obut no specific informationregarding the number and strength of binding of sites could be obtained. In order to get information about the weak sorption sites, the water sorption data were analyzed by a modified D’Arcy and Watt equation in two ways. 1. Setting C = 0 and i = 2, by which the Langmuir-type sites are split into strong (wPd and weak (wPd sorption sites. This modification results in the following equation
sum
k
square
(mm0Wg) 9.92 0.33 0.0034 0.00011 0.00023 0.00055 0.03 0.99 0.0016 0.00057 0.03 0.90 0.00017 O.OOO4
2.08 3.67 0.31
0.22 12.17 11.11
2.07
2.52 14.55 0.61
1.10
10.49
0.88 0.00017 0.57 0.012 0.53 0.0025 0.00068 0.0010 0.99 O.ooOo3 0.47 0.0021 0.98 O.ooOo2 o.Oo049 0.28 0.0023
Based on chemical analysis.
KIKl‘a, K&2/aw W = 1 Klaw 1 + K2a,
+
+
Wm
+-1kk’a, - ka,
(4)
and at a, = 1,the distribution of sorbed water is given by (modification 1) w = W P l + wp2 + wm (5) 2. Setting C # 0, and i = 2, by which the presence of two types of Langmuir-type sites and weak sites can be analyzed and estimated. This leads to the modified equation:
kk’a, K&iQw (6) 1 + K2a, +Caw+- 1 - ka, and the distribution of sorbed water (in mmol/g) at a, = 1is given by W=
KlK
