2502
R. B. FAHIM AND G. A. KOLTA
Thermal Decomposition of Hydrated Cadmium Oxide by R. B. Fahim Department of Chemistry, Faculty of Science, University of Assiut, Assiut, United Arab Republic
and G. A. Kolta National Research Centre, Dokki, Cairo, United Arab Republic
(Received August 26, 1969)
The work comprises a comparative study of the thermal decomposition of hydrated cadmium oxide and that of precipitated cadmium hydroxide. In both cases, four processes are involved: (a) dehydration, Le., removal of excess (combined) water, (b) crystallization of the dehydration product, (c) decomposition to the oxide, and (d) deoxygenation of the formed oxide. The energy of activation of decomposition for both materials is the same, while that for dehydration process is higher in case of the hydroxide. On hydration of oxygenated cadmium oxide (ie., prepared at 300" in an oxygen atmosphere of 760 Torr), no combined water is taken up by the formed hydroxide. This fact is confirmed by thermogravimetry and infrared analysis.
Introduction I n an earlier work,l it was shown that cadmium oxide takes up water from the vapor phase almost stoichiometrically to give the hydroxide. This phenomenon was noticed primarily by the color change of the oxide, attending hydration, from dark brown to pale yellow, and it was confirmed by X-ray investigation. A mechanism was suggested for the hydration process. Similar behavior was noticed with magnesium oxide. Studying the isothermal decomposition, it was found that the rate of decomposition of the hydrated magnesium oxide is much higher than that of the original hydroxide. This result was attributed to the fact that water is bound much less strongly to the oxide as indicated from the low energy of the decomposition process. A similar conclusion was recorded for the decomposition of calcite and carbonated calcium oxideaa I n general very limited work has been devoted to the study of the isobaric dehydration of cadmium hydroxidea4,s Low and KameP studied the thermal decomposition of cadmium hydroxide and they showed that dehydration is not, in reality, continuous but occurs in steps accompanied by considerable textural changes of the solid. The present investigation deals with a comparative study of the thermal decomposition of hydrated cadmium oxide and that of the corresponding hydroxide. This was investigated by thermogravimetry, differential thermal analysis, X-ray diffraction techniques, and infrared spectroscopy. Furthermore, experiments were extended for the study of the isothermal decomposition of the parent materials. Experimental Section Apparatus and Techniques. Thermogravimetry was performed with an automatically recording thermobalance (Gebruder Hetzsch-Selb, West Germany) as described by Gordon and Campbell.' The weight change was recorded on a 641-1. chart over a range of 240 The Journal of Physical Chemistry, Vol. 74, No. 1.3, 1070
mg, simultaneously with temperatures up to 600" a t a rate of 5"/min. The isothermal decomposition was also followed with the thermobalance incorporating a device to keep the temperature constant arbitrarily throughout the experiment. The temperature of the furnace was first raised to the required temperature and the specimen was then shock-heated by lowering the furnace to surround it. Differential thermal analysis was carried out using an automatically recording Linseis apparatus, Type L 160 KS (West Germany); the rate of heating was 5"/min. For tga and dta, the technique adopCed was according to the recommendations of R!tcAdie.* X-Ray diffraction patterns were obtained with the aid of a Philips unit, Type PW 1010, applying a copper tube, and using a 114.83-mm Debye-Scherrer powder camera, Type PW 1024. The diffraction patterns were matched with ASTM cards.9 Infrared absorption spectra were obtained with the aid of a Perkin-Elmer grating infrared spectrophotometer, Model 337, and adopting the KBr technique. Materials. The preparation of cadmium hydroxide was described earlier.' Cadmium oxide was prepared by the thermal decomposition of the corresponding hydroxide a t 200, 250, or 300" for 5 hr in vacuo. Hydration was effected by directly exposing this oxide (1) R . B. Fahim and K. M . Abd El-Salaam, J . Catal., 9, 63 (1967). (2) R. I. Raaouk and R . Sh. Mikhail, J . Phys. Chem., 62, 920 (1958). (3) H . 9. Britton, S. J . Gregg, and G. M. Windsor, Trans. Faraday Soc., 48, 63 (1952). (4) G. F . Htittig and R . Mytyzek, 2. Anorg. A&. Chem., 190, 353 (1930). (5) G. F. Hattig, "Hydroxide und Oxyhydrate," R . Fricke and G . F. Htittig, Ed., Akademische Verlagsgesellschaft m.b.H., Leipzig, 1937, p 413 ff. (6) M. J. D. Low and A. M. Kamel, J . Phys. Chem., 69, 450 (1965). (7) S. Gordon and C. Campbell, Anal. Chem. Annu. Rev., 32, 287R (19 60). (8) H . G. McAdie, Anal. Chem., 37, 643 (1967). (9) J. V. Smith, Ed., "X-Ray Powder Data File," American Society for Testing AMaterials,Philadelphia, Pa., 1960.
2503
THERMAL DECOMPOSITION OF HYDRATED CADMIUM OXIDE
: Temperature, 'C.
3
-
200
400
Temperature.
600 OC.
Figure 1. Tga curves for (a) precipitated cadmium hydroxide and (b) hydrated cadmium oxide (rate of heating 5'/min).
Figure 2. Dta curves for (a) precipitated cadmium hydroxide and (b) hydrated cadmium oxide (rate of heating 5'/min).
to saturated water vapor at 35" for enough time. The sample was then degassed thoroughly. Details of the hydration process are described elsewhere.'
410 and 350" in case of the hydroxide and the hydrated oxide, respectively; the same trend is also noticed in tga curves, where the decomposition of the former material starts at a higher temperature. An endothermic peak is finally noticed a t about 650" for the hydroxide sample, corresponding to the deoxygenation of the oxide.l0l1l A similar peak but a smaller one is also noticed at about 510" in case of the hydrated oxide which may be considered to be already partially deoxygenated having been subjected, before hydration, to thermal treatment at 300" under vacuum. ( C ) Isothermal Dehydration. This series of experiments was performed on the hydroxide and the hydration product of oxide which had been prepared a t 300". Two ranges of temperatures were used, viz., 160-240" for dehydration and 300-370" for decomposition to the oxide. The set of results on the rate of dehydration of the hydroxide is shown, as an example, in Figure 3. The mechanism of dehydration may be explained in terms of general hypothesis that the reaction consists of the formation of nuclei a t certain localized spots in the reactant, followed by relative rapid growth of these nuclei. The present results are in good agreement with Mampel's t h e ~ r y . ~By ~ J ~plotting ( ~ / w ~ ) against ~ ' ~ t, where wo is the initial mass of reactant and w is the mass at time t, a straight line is obtained from which IC the velocity constant is obtained at various temperatures for both materials. A plot of log IC against the reciprocal of the absolute temperature ( T )gives a satis-
Results ( A ) Thermogravimetry. The results of thermogravimetric experiments on the dehydration of cadmium hydroxide and the hydration product of cadmium oxide which had been prepared at 300" are shown in Figure 1. It is evident that the behaviors of the two samples are essentially the same, despite the slight difference in the characteristic temperature range. It is clear that dehydration takes place in two stages. The first stage, which can be ascribed to the removal of excess (combined) water, starts in the two materials at about 200", showing a similar loss in weight ( 7 4 % ) ; it can be noticed that the rate of removal of combined water in the hydroxide is slower than that in the hydrated oxide. The second decomposition stage, which is preceded by a well-defined plateau, starts in the two samples a t 300" and shows a great similarity in behavior both in the amount and in the shape of the curve. ( B ) Diferential Thermal Analysis. The dta curves for the precipitated cadmium hydroxide and the hydration product of oxide which had been prepared a t 300" are shown in Figure 2. The endothermic peak noticed a t 250" in both materials corresponds to the region of decrease in weight in tga curves that starts at about 200". However, this peak is followed by an exothermic peak at about 290", that can be attributed to a crystallization or rearrangement type process such as from amorphous to crystalline. It is probable that removal of excess water leaves the material in a finely divided poorly crystallized or amorphous state. The decomposition of the hydroxide to the oxide takes place a t
(IO) E. F. Lamb and F. C. Tompkins, Trans. Faraday SOC.,58,
1424 (1962). (11) 8 . A. Abd El-Hadl, I. F. Hewaidy, F. A. Khadra, and M. S. Farag, J . Phys. UAR, in press. (12) K. L. Mampel, Z . Phys. Chem., Abt. A , 187, 43, 257 (1940). (13) S. J. Gregg and R. I. Razouk, J . Chem. Soc., 36 (1949). The Journal of Physical Chemistry, Vol. 74, No. 13, 19ro
2504
R. B. FAHIM AND G. A. KOLTA
1.5
1.7
1.9
I/T x 1
I
I
39
60
I
I
90 120 Time, min.
150
Figure 3. Weight changes accompanying the dehydration of precipitated cadmium hydroxide.
factory straight line from which it is possible to evaluate the energy of activation from the Arrhenius equation d In lc/d(l/T)
=
-E/R
The energy of activation for the dehydration of the hydroxide and the hydrated oxide is then determined. The same mechanism is considered also to apply for the decomposition of both materials, after removal of excess water, to the corresponding oxide. The activation energies of the two processes, dehydration and decomposition, that can be evaluated from Figure 4, are shown below Activation energy, koa1 mol-' Hydrated Hydroxide oxide
Dehydration (-excess water) Decomposition to the oxide
21.90 13.77
17.52 13.24
It can be noticed that while the energy of activation of decomposition for both materials is almost the same, that for the dehydration process is higher in the case of the hydroxide. (D)X-Ray Analysis. X-Ray diffraction patterns of the hydroxide, the hydrated oxide, and the products of dehydration of both materials are illustrated in Figure 5. Although the results of X-ray are only qualitative, they can throw much light on the mechanism of hydration of the oxide as well as dehydration process. The diagrams of the hydroxide and the hydrated oxide do not give the characteristic d spacings and relative intensities of the hydroxide given in the ASTM cards. However, the partial dehydration of both compounds, to expel?.the excess water, can give products having the original pattern of the hydroxide given in the ASTM cards. The main d spacings of the products of partial The Journal of Physical Chemistry, Vol. 74, hTo.18, I970
IO-^
2.1
2.3
I
Figure 4. Plot of log k against 1/T for: (a) dehydration of precipitated cadmium hydroxide, (b) dehydration of hydrated cadmium oxide prepared at 300°, (c) decomposition of the hydroxide, and (d) decomposition of the hydrated oxide.
dehydration of both materials are slightly shifted. It is not possible, however, to detect the presence of the principal lines of the oxide in the products of partial dehydration, i.e., Cd(OH)2. Finally, the products of complete dehydration, formed a t 300°, showed only the pattern of cadmium oxide. Despite of the fact that there is a difference in the color of the hydroxide (white) and the hydrated oxide (pale yellow), the X-ray analysis of both materials is the same. ( E )IT Analysis. The ir spectra of four original samples and some of their products were investigated and the results are shown in Figures 6 and 7, the curves being displaced to prevent overlapping. The spectrum of the hydroxide (Figure 6a) shows characteristic bands : a sharp one at 3600 cm-l which is probably due to the 0-H stretching14-16 of the hydroxides, two distinct bands at 3400 and 1700 cm-' that can be referred to the water molecules adsorbed by the solid" and three other sharp bands a t 1450, 860, and 720 cm-l. The latter three bands may be related to the water molecules combined with the hydroxide. Similar bands are noticed in some inorganic compounds containing combined water.'B To confirm this view, the ir spectrum of a hydroxide specimen which had been freshly dehydrated a t 200" was determined and found to have a diminished band a t 1400 cm-l (Figure 7 4 . This indicates the presence of traces of combined water. The other two bands a t 860 and 720 em-' have completely disappeared. The bands corresponding to the adsorbed water have also disappeared. (14) B. A.Philips and W. A. Busing, J . Phys. Chem., 61,502 (1957). (15) R. T. Mara and G. B. B. M. Sutherland, J. Opt. SOC.Amer., 46, 464 (1956). (16) R.M. Hexter, ibid., 48, 770 (1958). (17) R. S, McDonald, J. Amer. Chem. SOC., 79, 850 (1957). (18) M. Y. Karmarrec, C. R . Acad. Sci., 258, 6836 (1964).
2505
THERMAL DECOMPOSITION OF HYDRATED CADMIUM OXIDE 1
10
~
20 30 40
'
1
50
60 70
~
80 90
1
'
'
'
4000
~
3500
1
I
3000
2500
MOO
1500
500
1000 I
I
V
a ?r-
-
s : .c
-..-2! c
0
2
Frequency, crn-'.
10 20
30 40 50 60 70 80 90 Spacing 28 Cu k,rad.
Figure 6. Infrared absorption spectra of: (a) precipitated cadmium hydroxide, (b) hydrated cadmium oxide prepared at 200", (c) hydrated oxide prepared a t 250", and (d) hydrated oxide prepared at 300".
1 :
4000
Figure 5. X-Ray diffraction patterns of: (a) precipitated cadmium hydroxide, (b) dehydrated cadmium hydroxide, (c) ASTM pattern for Cd(OH)2, (d) cadmium oxide prepared from (a), (e) ASTM pattern for CdO, ( f ) hydrated cadmium oxide prepared at 300", ( 9 ) dehydrated sample of (f), and (h) cadmium oxide prepared from (f).
On the other hand, it is noticed from Figure 6 that the three bands related to the combined water show decreased absorbance in the hydrated oxides, which had been prepared at 200 and 250°, compared to that of the hydroxide. The lower the temperature of treatment of the oxide before hydration, the bigger the relative absorbance of the three bands. Hence, the ir spectrum of the hydrated oxide, previously prepared a t 300", does not show the latter two bands but a broad band is given at 1400 cm-', with small absorbance value (Figure 6d). Furthermore the hydrated oxide, which had been prepared a t 300", gives a spectrum (Figure 7b) which is similar to Figure 7a of the hydroxide when both materials are treated at 200" in air thus removing the combined water. The spectrum of the oxiae (Figure 7d) does not show any characteristic band in accord with the observation of J'IiIler and Wilkinslgthat metal oxides generally have no sharply defined infrared absorption between 2 and 16 p o Figure 7c which is similar to Figure 6a of the original hydroxide is the ir spectrum for the sample prepared by heating the hydroxide in air a t 300" and then exposing it to moisture for 24 hr during which time it turned white. Cadmium oxide, particularly when prepared in uucuo, is known to be nonstoichiometric with deficient oxygen in the lattice.'O In order to find out whether or not this nonstoichiometry of the oxide has an effect on the
t
3500
3000
2500
2000
1500
1000
0
+/ d
I
Figure 7. Infrared absorption spectra of: (a) dehydrated precipitated cadmium hydroxide, (b) dehydrated sample of hydrated cadmium oxide prepared at 300", (c) moistened cadmium oxide, (d) cadmium oxide, and (e) hydrated oxygenated cadmium oxide.
processes of hydration and dehydration, an ir spectrum (Figure 7e) was investigated for an oxide prepared a t 300" in an oxygen atmosphere of 760 Torr thus preventing deoxygenation; then the oxide was subjected after cooling and careful evacuation to saturated water vapor a t 35". From Figure 7e, it is clear that the three bands of the hydroxide (Figure 6a) a t 1450, 860, and 720 cm-' have completely disappeared indicating the absence of combined water. This fact was confirmed by a tga experiment for this sample. The results show that only one weight loss is recorded in the temperature range 300-370°, corresponding to the decomposition of the hydrated oxide (hydroxide) to the oxide, while the (19) F. A . Miller and C. H. Wilkins, Anal. Chem., 5 9 , 638 (1952).
The Journal of Physical Chemistry, Vol. '74, N o . 12, 1970
2506
R. B. FAHIM AND G. A. KOLTA
Table I : Composition of Starting Materials and Their Decomposition Products Temp, Process
oc
(Starting material) Dehydration Crystallization Decomposition Deoxygenation
35 200 290 360 600
---
Precipitated hydroxide-Empirical % Cd formula
70.91 76.89 77.01 88.02 88.71
weight loss representing the removal of excess water from the sample is not detected. ( F ) Product Compositions. I n order to support the interpretations already given for the processes involved in the thermal decomposition of the starting materials, the composition of the various products was determined by chemical analysis. Cadmium was estimated by ethylenediaminetetraacetic acid (EDTA) according to Welcher.20 The results of analysis (% Cd) and the empirical formulas of the analyzed samples are cited in Table I. The empirical formulas were calculated6 on the assumption that the samples consisted only of cadmium and oxygen and that the difference between them lies in the loss of water and oxygen.
Discussion It was shown' that cadmium oxide, prepared by the thermal decomposition of the corresponding hydroxide in vacuo, retains water when exposed to saturation water vapor pressure at 35". The plot of water retained against percentage decomposition of the hydroxide is a straight line which on extrapolation gives a value of 0.154 g of HzO/g of hydrated material at 100% decomposition as compared to 0.123 g of HzO/g of Cd(OH)2. This fact shows that the oxide prepared at 300" on hydration gives the corresponding hydrous hydroxide. However, in case of magnesium oxide,21the water uptake by samples prepared a t temperatures below 650" is stoichiometric giving an anhydrous hydroxide. From tga experiments on cadmium hydroxide and the hydrated oxide, it became clear that both materials behave similarly and that water is removed in two stages, vix. , dehydration and decomposition. The mechanism of dehydration in both materials is the same, where the initial stages of the dehydration spreads from sites on the external surface across the external faces and simultaneously down the subgrain boundary network. This mechanism reflects a process leading the establishment of the reactant-product (hydrous-anhydrous) interface which penetrates the crystal at a constant rate. This is preferred to a true contracting envelope which is suggested by Mampel.12 The fact that the energy of activation for dehydration process is, on the whole, high indicates that excess water attached to both materials, hydroxide and hydrated oxide, is not only adsorbed but mainly diffused in the The Journal of Physical Chemistry, Vol. 7 4 , N o . 12, 1970
CdO. 1.67H20 CdO -0.99HzO CdO- 0.98Hz0 CdO Cd0o.m
_--_% Cd
71.19 77.13 77.61 88.38 88.68
Hydrated oxide----Empirical formula
CdO 1.64H20 CdO -0.96HzO Cd0.0.92Hz0 CdOo.sa Cd0o.m
crystal lattice. That dehydration is slower in case of the precipitated hydroxide may be attributed to the difference in chemical bonding between excess water and the hydroxide materials, as indicated by ir spectra. Hence, when hydration of the oxide is effected by moistening, we get an ir spectrum very similar to that of the precipitated hydroxide, While in case of hydration of oxides under reduced pressure, ir absorption indicated that combined water decreased the higher the temperature of the oxide preparation showing a gradual reduction in the bond strength of excess water compared to that of the formed hydroxide. Moreover, we can assume that the hydrated oxide, which was subjected to thermal treatment before hydration, already contains channels for diffusion of water which permit dehydration to proceed faster. In dta, the endothermic peak a t about 250" corresponding to the dehydration process is followed by an exothermic peak at about 290" which may be attributed to a crystallization process. It is probable that removal of excess water causes disruption of the crystal structure and leaves the anhydrous material in a very fine or amorphous state. Then at slightly higher temperatures the material crystallizes releasing some heat of crystallization. The contracting envelope suggested by Mampel as a mechanism for dehydration seems to be applicable also to the decomposition process. The isothermal decomposition of the anhydrous hydroxide and hydrated oxide is found to be of the same order of magnitude, with almost similar energy of activation, indicating the presence of similar bonds attaching the hydroxyl groups to the metal in both materials. However, the hydroxide shows a delayed decomposition. Hence, while the reaction is complete in case of the hydroxide a t 380" after 15 min, in case of the hydrated oxide it is achieved a t only 360" after 7 min. This fact may be interpreted in terms of the absence of the intermolecular OH COordination bond in the hydrated oxide as indicated from its relatively decreased absorbance in the ir spectra a t 3600 cm-l, when compared with that of the hydroxide." (20) F. J. Welcher, "The Analytical Uses of Ethylenediamine Tetraacetic Acid," D. Van Nostrand Co., Inc., Princeton, N. J. 1958, p 161. (21) R. I. Razouk and R. Sh. Mikhail, J. Phy8. Chem., 59, 638 (1955).
TRANSPORT PROCESSES IN MOLTEN BINARY ACETATESYSTEMS This interpretation may also explain the contradicting results obtained for the decomposition process in tga and dta, where the same weight loss is recorded in this process for both the hydroxide and the hydrated oxide, while the endothermic peak in case of the hydroxide is longer and broader than that of the hydrated oxide. Despite this difference in structure, the anhydrous hydroxide and hydrated oxide (being heated a t 200") give the same X-ray pattern as that of Cd(0H)Z. Deoxygenation takes place a t about 650 and 510" for the hydroxide and the hydrated oxide, respectively. This process takes place earlier with the hydrated oxide because this material had already suffered partial deoxygenation while being previously heated a t 300" under vacuum before rehydration. An interesting experimental fact has revealed that when nonstoichiometric cadmium oxide, being deficient
2507
in oxygen, is oxygenated and rehydrated, no combined water is taken up by formed hydroxide. This fact is confirmed by thermogravimetry and infrared analysis. A similar observation was made by Glemser, et a1.,22in studying the crystal lattice of some MnOz varieties. They came to the conclusion that in the nonstoichiometric MnOz, water replaces the oxygen deficiency in the lattice; hence, the stoichiometric oxide does not contain water. The behavior of hydrated oxygenated cadmium oxide can be explained on the same basis. Acknowledgments. Thanks are due to Mr. K. M. Abd El-Salaam for preparing the starting materials and also to Dr. F. El-Milligy for his help in discussing the results of ir analysis. (22) V. 0.Glemser and E. Hartert, 2.Anorg. AlZg. Chem., 283, 111 (1956).
Transport Processes in Molten Binary Acetate Systems
by Roger F. Bartholomew Research and Development Laboratories, Corning Glass works, Corning, New York 1@SO
(Received January 16, 1970)
Equivalent conductance and glass-transition temperature measurements were made on two melts in the b h r y system sodium acetate-lithium acetate. The conductance exhibited non-Arrhenius behavior but could be described by a three-parameter equation of the form A = A O T - ~exp[ / ~ -ka/(T - To)].The ideal glass-transition temperature To,determined by a computer best fit of the data to the above equation, was found to increase with an increase in the mean cationic potential term, ZNiZJri. Experimental glass-transition temperatures T,,obtained from d t a data, showed similar behavior. The ratio T,/To was found to be approximately unity for acetate glass-forming melts.
Introduction The theory of Adam and Gibbs1 for relaxation rates at low temperatures has been applied in the past few years to the transport properties of low-melting glassforming ionic liquids. A recent review by Angel1 and Moynihan2 covers the work in this field up to the end of 1968. Considerable effort has been directed toward the study of the known nitrate glass-forming melts as well as ZnClz. However, glass formation from molten acetate mixtures has recently been reported in the litera t ~ r e . ~ These -~ systems are very extensive in their nature' for quenching lithium acetate results in glass formation. Because Of this ease of glass formation and the similarity between the nitrate anion and the acetate anion,transport propOf two glass-forming acetate melts in the binary system CH3C00Na-CH3COOLi were determined.
The purpose of the present paper is to show that the equivalent conductance of these glass-forming melts can be by the low-temperature approximation to the Akn-Gibbs theory. This theory relates the equivalent conductance A to the absolute temperature T by the three-parameter equation A
=
n , ~ - ' / ' e xTp-( ATo)
(1)
(1) G . Adam and J. H. Gibbs, J . Chem. Phys., 43, 139 (1965). (2) C.A. Angel1 and C. T. Moynihan in "Molten Salts. Charaoterization and Analvsis." G. Mamantov.. Ed... Marcel Dekker Inc.. New yorkp N. l9k99 pp 315-376Ceram. s0C.v 52, 224 (3) J, A. Duffy and M. D. Ingram, J . (1969). (4) R. F. Bartholomew and H. J. Holland, ibid., 52, 402 (1969). (5) R. F. Bartholomew and 5 . S. Lewek, ibid., in press. y.l"
The Journal of Phusical Chemistry, Vol. 74, N o . 12, 1970
