X-RAY STUDIES ON T H E HYDROUS OXIDES. I V
TITANIUM DIOXIDE HARRY B. WEISER
W. 0. MILLIGAN Department of Chemistry, T h e Rice Institute, Houston, T e x a s AND
Received October SO, 1933
Tetravalent titanium, like tin, is said to form two distinct acids: orthoor alpha titanic acid and meta- or beta titanic acid. Since the work of several investigators has shown that the stannic acids (13) are in reality hydrous stannic oxides differing essentially in the size of the primary particles, it seems not unlikely that the so-called titanic acids are also hydrous oxides whose properties are determined by the size and physical character of the particles. The product commonly called orthotitanic acid, or alpha titanic acid, is a white gelatinous precipitate formed by the addition of ammonia or alkali hydroxide to a solution of a tetravalent titanium salt. The highly hydrous gel is readily soluble in dilute acids and is easily peptized by dilute alkalies and suitable salts to give stable sols. On the other hand, the product referred to as meta titanic acid, or beta titanic acid, is a granular, difficultly soluble, and but slightly peptizable substance obtained (a) by the interaction of nitric acid and titanium, (b) by the aging under water of the precipitated gel of the so-called orthotitanic acid slowly in the cold and more rapidly in the hot, (c) by precipitation in the hot, and (d) by hydrolysis of solutions of tetravalent titanium salts. The composition of the preparations referred to above varies with the method of precipitation and with the subsequent treatment. The only evidence for the existence of definite hydrates corresponding to the formulas of the acids that earlier investigators (2, 5,, 9, 10, 11, 12) have assumed, is the analysis of the products formed and dried under arbitrary conditions, Carnelly and Walker (1) obtained a smooth temperature-composition isobar for a precipitated gel and hence concluded that no definite hydrates were formed, unless it is assumed that a large number exist, each stable over a very narrow temperature range. Nforley and Wood (8) prefer to assume that the change from the so-called alpha to beta form consists of a condensation to complex salt-like compounds, but there is no experimental justification for this point of view. More recently Gutbier and coworkers (3) obtained smooth composition-temperature isobars for several preparations, indicating the absence of definite hydrates of titania. 513
514
HARRY B. W E I S E R AND W. 0. MILLIGAN
No systematic study of the constitution of the alleged titanic acids has been made by the x-ray diffraction method. Hedvall (4) states that “titanium hydroxide” precipitated at some unspecified temperature is crystalline, since it gives a good interference pattern both when moist or when dried a t room temperature. On the other hand Gutbier and coworkers (3) report that the freshly precipitated products give no interference pattern, but the gels thrown down with ammonia at 20°C. to 100°C. and subsequently ignited give the pattern of the mineral rutile or a mixture of rutile and anatase, and the product obtained by oxidizing Tiz03.xHzO, followed by heating to 490”C., gives the x-ray pattern of anatase. In the following section will be given the dehydration isobars of two typical gels of titanium dioxide and the results of the x-ray examination of precipitated oxides obtained under widely varying conditions. EXPERIMENTAL
I . Preparation of samples
Precipitation with ammonia. A solution of 17 g. of titanium tetrachloride in 75 cc. of 6 N hydrochloric acid was treated with ammonia at room temperature and the resulting precipitate washed thoroughly by decantation by the aid of the centrifuge until only a trace of chloride remained. This sample, which will be designated as No. 1, was treated in different ways, as given in table 2. A second sample, designated as No. 6, was prepared by treating a boiling solution of the tetrachloride with a small excess of ammonia, boiling the mixture for 15 minutes, and finally washing and drying like sample No. 1. Oxidation of Tiz03.xHz0. A solution of 8 g. of titanium trichloride in 100 cc. of water was treated with 15 cc. of 15 N ammonia, the mixture diluted to 400 cc. and allowed to oxidize in the air. The original black precipitate changed rapidly to a gray color which changed slowly to white on conducting a current of air into the suspension. This precipitate (No. 2) was thoroughly washed and air-dried a t room temperature. Hydrolysis. A dilute solution of titanium tetrachloride in hydrochloric acid was hydrolyzed by boiling for four hours, using a reflux condenser. The resulting granular precipitate was washed with hot water and air-dried a t room temperature. Similar preparations were made by hydrolysis of the nitrate and sulfate. 2. Isobaric dehydration Temperature-composition isobars for sample No. 1 precipitated at room temperature and sample No. 2 made by oxidation of Ti203.xH20 were obtained in the following way. The samples in glass-stoppered weighing bottles were placed in a thermostated electric oven through which dry air was circulated at the rate of 72 liters per hour. The air was dried by
515
X-RAY STUDIES O N HYDROUS OXIDES. I V
passing it through a large calcium chloride tower, a sulfuric acid bottle, and finally through a tower filled with glass wool and phosphorus pentoxide. TABLE 1 Thermal dehydration isobars of hydrous t i t a n i u m dioxides COMPOSITION MOLES
TEMPERATl?RE
HgO/TiOa
No. 1 Pptd. at room
temperature
No 2 Oxidation of TizOs xHrO
0.970 0.562 0.425 0.415 0.362 0.308 0.295
1.030 0.590 0.454 0.402 0.361 0.306 0.296
1
degrees C .
35.1 71.3 87.2 92.6 100.0 105.4 110.0
i
-
TEMPERATURE
COMPOSITION MOLES
X20/Ti02
No 1 Pptd. at room temperature
No. 2 Oxidation of Ti2Os'xHzO
0.288 0,218 0.184 0.114 0.081 0.054 0,046
0.291 0,217 0.154 0.101 0.084 0.061 0.053
degrees C
'
122.5 150 184 205 224 255 270
\
0, I cr)
TEMPERATURE, DEGREES C FIG.1. DEHYDRATION ISOBARS OF HYDROUS Ti02 2. Made by oxidation of TisOa.xHzOin
1. Precipitated a t room temperature.
air.
The rate was measured with a calibrated oil-filled flow meter left permanently in the line. The air was filtered through cotton just before passing :nto the oven. The air was preheated by passing through a &foot coil of
516
HARRY B. WEISER AND W. 0. MILLIGAN
glass tubing inside the oven and was then conducted into a small enameled pan with a loose fitting cover, which held the samples. This arrangement is similar to that of L. H. Milligan (6). The temperatures were measured with standardized thermometers and a standardized thermocouple. The temperature was kept constant to within 1 to 2°C. To attain equilibrium conditions at a given temperature, each sample was heated a t least four days and in some cases for as long as thirty days. In general, less time was required to attain equilibrium at higher than at lower temperatures. The results are given in table 1 and figure 1. In agreement with the results of Gutbier, the smooth isobars for both samples indicate the absence of hydrates of the oxide corresponding to the alleged acids. TABLE 2
SAM PLE
X - r a y examination of hydrous t i t a n i u m dioxides MBTHOD OF P R E P A R A T I O N
NO. -
1 Precipitated a t room temperature 3a No. 1aged a t room temperature 14 weeks 3 No. 1aged a t room temperature 30weeks 5 No. 1 aged 6 hours a t 100°C. 4 No. 1 heated to 184°C. 11 No. 1 heated to 1000°C. 6 Precipitated a t 100°C. 2 Oxidation of Ti203.xHzO 14 No. 2 heated to 184°C. 12 No. 2 heated to 1000°C. 9 Hydrolysis of Tic14 9a Hydrolysis of Ti(N03)d 7 Hydrolysis of Ti(SO4)z 10 Mineral rutile, TiOz 8 Mineral anatase, Ti02 13 Mineral brookite, Ti02
-
I
RESULTS
No visible lines or bands No visible lines or bands Anatase pattern, very broad bands Anatase pattern, broad bands Anatase pattern, broad bands Rutile pattern (and trace of anatase?) Anatase pattern, broad bands No lines or bands visible No lines or bands visible Rutile pattern (and trace of anatase?) Rutile pattern, broad bands Rutile pattern, broad bands Anatase pattern, broad bands Rutile pattern, sharp lines Anatase pattern, sharp lines Brookite pattern, sharp lines
3. X-ray analysis
The specimens listed in table 2 were examined as described in preceding papers of this series. The diffraction data are given in table 3 and are shown in chart form in figure 2 for purposes of comparison. Densitometer curves of the films from samples 1, 4, 6, and 8 are given in figure 3. These curves were obtained with a simplified and inexpensive recording photodensitometer that has already been described (7). From these data it is apparent that the samples precipitated in the cold give little or no x-ray diffraction pattern, whereas both the aged oxide and the one thrown down at 100°C. give the anatase pattern. The precipitate freshly
517
X-RAY STUDIES ON HYDROUS OXIDES. I V
formed in the cold consists either of amorphous TiOz with adsorbed water or, what is more likely, of anatase crystals so minute that little or no x-ray diffraction effect is observed. As already noted, Hedvall obtained an x-ray diffraction pattern with a precipitated titania. Since both Gutbier and ourselves failed to get a pattern with the cold precipitated oxide, it is probable that Hedvall's TABLE 3 X-ray diffraction data SAMPLE NO, 4
(Heated at 184'C. ) D
__
3.50 2.37 1.89C 1.66C 1,475 1,26(
1.16(
I
SAMPLE NO. 5
SAMPLE NO. 9
(Aged at 100OC.)
100°C.)
D
D
I
-___ 10 3.50 10 3.50 5 2.37 6 2.37 8 1.89( 8 1.89C 5 1.69! 5 1.694 5 1.67( 5 1.67: 4 1.48( 5 1.471 3 1.36( 2 1.35E 1.34( 2 1.26C 1.261 3 1.16C 1.165 1 1 1.04
I
(Hydrolysis of TiClr)
D
I
D
I
- ~ - _ _ - _ _ 10 3.49 10 3.24 10 5 2.37 8 5 2.48 3 8 1.883 8 2.18 6 1.695 6 2.045 2 6 1.655 6 1.685 10 5 1.477 5 1.616 3 1 1.358 2 1.480 2 4 1.334 2 1.360 3 1.261 4 1.3371 1.161 3 1,090 1 1.044 2 1.038 1 1.015 2 0.951 1 0.913 1 0.892 1 0.876 1 0.843 1 0.824 1 0.806 1 0.795 1 0.740 1 0.702 1 0.668' 1
~
SAMPLE NO, 10
(Rutile)
1 1 SAMPLE
SAMPLE
(Heated at
NO. 12 (Heated at lOOO"C,)
l",",o:j
D
I
D
I
D
1
3.24 IO 3.51 2.48 9 3.24 2.29 1 2.48 4 2.29 2.18 2.04 2 2.18 1.68t 10 2.05 1.61; 3 1.895 1.47: 2 1.685 1.445 2 1.623 1.351 8 1.481 1.24( 1 1.450 1 1.359 1 1.242 1 1.169 1 1.145 1.092 1.039
1 10 8 1
5 2
1 10 3 1 1 8 1 1 1 1 1
3.23 10 3.00 1 1 2.84 2.47 8 2.29 1 2.18 3 2.11 1 1 2.04 1.89 1 1.78: 1 1.681 10 1.61' 4 1.478 1 1.44, 1 1.42; 1 1.35' 4 1.34, 4 1.241 1 1.16 1 1 1.14 1.09, 2 2
-
precipitate was obtained in the hot and that the pattern showed the lines Df anhydrous TiOz (anatase modification) as we observed. The samples ignited at about 1000°C. consist for the most part of rutile. [nspection of table 3 shows that sample No. 11 gives in addition to the "utile pattern very weak lines at 3.51 A.U. and 1.895 A.U. It will be ibserved that the two most intense anatase lines are 3.49 A.U. and 1.883 1.U. It seems likely that a small amount of unchanged anatase is present n this ignited sample. Sample No. 12 also consists of rutile with the
PRECIPITATED AT ROOM TEMPERA ~,03XH,0 OXIDIZE0 IN THE AIF
PRECIPITATED AT ROOM TEMPE1
No I HEATED TO 184' PRECIPITATED AT 100'
MINERAL ANATASE. TIO,
X-RAY STUDIES ON HYDROUS O X I D E S , IV
519
SUNNARY
The following is a brief summary of the results of this investigation: 1. X-ray diffraction examination of hydrous titanium dioxide freshly precipitated at room temperature or freshly prepared by the oxidation in air of Tiz03.xHz0gives no lines or bands. Thermal dehydration isobars for these two samples show the waterto be given off in a continuous manner. The dehydration experiments of Gutbier and coworkers for their similar samples are confirmed. There is no indication of the existence of any definite hydrate. 2. Aging under water a t room temperature for fourteen weeks of hydrous titanium dioxide precipitated at room temperature produces no change detectable by x-ray diffraction methods. Further aging of this sample for thirty weeks a t room temperature or for 6 hours at 100OC. under water, gives minute anatase crystals; heating of the freshly formed material to 184°C. gives anatase. 3. Hydrous titanium dioxide precipitated at 100°C. shows the x-radiogram of anatase. 4. Hydrous titanium dioxide prepared by the hydrolysis at the boiling point of solutions of titanium tetrachloride and titanium nitrate, shows the x-radiogram of rutile, whereas hydrolysis of titanium sulfate under similar conditions gives anatase. 5. The product obtained by ignition at approximately 1000°C. of the hydrous oxide precipitated in the cold, shows the x-radiogram of rutile with indications of the presence of a little unchanged anatase. 6. X-ray analysis and thermal dehydration isobars indicate that no definite hydrates of titanium dioxide have been prepared. 7. Precipitated titanium dioxide is a hydrous oxide consisting, especially when precipitated or aged in the hot, essentially of anhydrous TiOz (anatase modification) and adsorbed water. REFERENCES (1) CARNELLY AND WALKER: J. Chem. SOC.63, 81 (1888). Jahresber., p. 271 (1849); Ann. 72, 259 (1849). (2) DEMOLY: (3) GUTBIER, OTTENSTEIN,LEUTHEVSSER, LOSSEN,A N D ALLAM. 2. anorg. allgem. Chem. 162, 87 (1927). (4) HEDVALL: Z. anorg. allgem. Chem. 120, 322 (1922). (5) MERZ:J. prakt. Chem. 99, 157 (1866). (6) MILLICTAN, L. H.: J. Phys. Chem. 26, 247 (1922). (7) MILLICAN, W. 0 . : Rev. Sei. Instruments 4, 496 (1933). (8) MORLEY ASD WOOD:J. SOC.Dyers Colourists 39, 100 (1923); J. Chem. SOC.126, 1626 (1924). (9) ROSE:Ann. 63, 267 (1845). (IO) TUTTSCHEW: Ann. 141, 111 (1867). (11) WAGNER:Ber. 21, 960 (1888). (12) WEBER:Pogg. Ann. 120, 287 (1863). (13) Cf. WEISER A N D MILLIGAN: J. Phys. Chem. 36, 3030 (1932).
