Ind. Eng. Chem. Res. 1999, 38, 1065-1068
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Effect of Oxide Acidity upon the Heats of Decomposition of Metal Oxysalts Jorge G. Ibanez,* M. Andrea Silva, and Mario A. Bravo Departamento Ciencias Basicas, Universidad Iberoamericana Prol. Reforma 880, 01210 Mexico, D. F. Mexico
A large number of solid-state reactions involve acid/base equilibria of oxides. Among the chemical factors that may influence, determine, or modify such reactions and their rate-determining steps, the relative stabilities of metal oxysalts can be crucial. Thus, simple ways for predicting and/or associating chemical and thermodynamic properties are of considerable interest. A correlation between the differences in acidity of constituent oxides and the heats of decomposition of their corresponding metal oxysalts has been found. In addition, the concept of difference in oxide acidity is successfully correlated with the concept of polarizability, given by the ratio r1/2/Z* (where r ) cation radius, Z* ) effective nuclear charge). Introduction The production and decomposition of many metal oxysalts and mixed-metal oxides from/into their constituent oxides is the key step in several processes. More applications are continuously under investigation. Some examples include the following: Calcium, magnesium, iron, and aluminum oxides are produced from the thermal decomposition of their carbonates or some other minerals (Swaddle, 1990; Diefallah et al., 1996; Pacewska et al., 1996). Solid superacid catalysts are produced by reacting metal salts that yield mixed-metal oxides with zirconium oxide (Bi et al., 1996). Mixed-metal oxides are prepared by reaction of their constituent oxides or compounds that produce them (Fauteux et al., 1977; Jordanovska et al., 1996). BiSCCO superconductors can be prepared by a solid-state reaction of the oxides and carbonates Bi2O3, SrCO3, CaCO3, and CuO (Knaepen et al., 1996). Metal oxides can react with gaseous, liquid, or solid oxides to yield desired compounds or to remove undesirable compounds from a given matrix. Basic oxides (e.g., CaO, MgO) are capable of reacting with acid sulfur oxides to remove them by forming the corresponding oxysalts (Fuertes and Fernandez, 1996). Such reactions can be detrimental for other applications (Sawada et al., 1996). Another obvious example is the increase in acidity in natural water sources, whereby sulfur oxides are reactively dissolved in water (Fergusson, 1985). The thermal decomposition of some oxysalts [e.g., potassium perchlorate (Zhang et al., 1996), magnesium iodate (Ito et al., 1984)] has been shown to be catalyzed by different metal oxides (Cr2O3, Cu2O, and BaO for the former and Cr2O3, Fe2O3, and Fe3O4 for the latter). The use of reversible, uncatalyzed thermal decomposition reactions for energy storage (e.g., solar energy) has been proposed and thoroughly studied. Here, a compound AB is decomposed endothermally to A + B, which are then stored and recombined at will to regenerate AB, releasing the stored heat. Many metal oxide reactions are involved here (Ibanez et al., 1984a,b; Wentworth et al., 1978; Tmar et al., 1981). The thermal * To whom correspondence should be addressed. Tel.: +52(5)267 4074. Fax: +52(5)267 4279. E-mail:
[email protected].
decomposition of metal oxysalts has also been used in the preparation of catalysts, molecular sieves, ore beneficiation, metallurgical dead-roasting (Mu and Perlmutter, 1981), and thermochemical cycles for water splitting (De Beni, 1980). The thermal decomposition of solids is a powerful analytical tool in the elucidation of stoichiometries, composition, and mechanisms involving solid samples. In addition, it is well-known that thermal events can be like fingerprints for specific compounds and reactions (see, for example, Pelovski et al., 1996). Precise thermochemical data can also be obtained from them (Mishra et al., 1996). The literature on this is enormous, and no attempt will be made to encompass it. A review on this is available (Carr and Galwey, 1984). In addition, the direction and rate of some organic reactions can be modified by the deposition of appropriate materials (usually inactive) on oxide-based supports (Cseri et al., 1996). From the above discussion, it can be deduced that there is a plethora of applications for the thermal decomposition of solids and that many solid-state reactions are far from simple. A case in point is the thermal decomposition of a simple salt, Fe(SO4)‚H2O, that is postulated to be comprised of seven reactions (Pelovski et al., 1996). There are several physical or chemical factors that may influence, determine, or modify solid reactions and their rate-determining steps (see, for example, Yilmaz and Icbudak, 1996; Jacobs, 1969). It is important to emphasize that a large number of such reactions involve acid/base equilibria of different oxides (including water) and that the relative stabilities of metal oxysalts can also be crucial for different purposes (Malinowski et al., 1996). Thus, simple ways for predicting and/or correlating chemical and thermodynamic properties can be of considerable interest for the applications described. It is in this regard that we have analyzed a possible correlation between the differences in acidity of constituent oxides and the heats of decomposition of their corresponding metal oxysalts. Results and Discussion The thermal decomposition of oxysalts is affected by process parameters such as the nature of the atmosphere, compactness of the salts, their weight, foreign
10.1021/ie970877f CCC: $18.00 © 1999 American Chemical Society Published on Web 01/20/1999
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ion additives, and rate of heating. From a more fundamental point of view, it is well-known that the facility for this thermal decomposition is dependent on the ability of the cation to polarize the anion (Huheey, 1981). Such is the case for the alkaline-earth carbonate series, where the temperatures required for the general reaction
MCO3 f MO + CO2, ∆H°
(1)
follow the order Ba > Sr > Ca > Mg > Be. In this way, since the charge and anion are constant for this group, this difference can be attributed to the ability of the cation to polarize the anion (i.e., attract the negative charge cloud of the anion toward the cation center). Be has a larger polarizing power than the other alkalineearth ions (it is a harder cation, in terms of Parson’s hard and soft acid-base theory; see Huheey, 1981) and attracts better one of the oxygens in the carbonate anion, thus facilitating the detachment of CO2. This polarizing power has been found to be a function of the effective nuclear charge (Z*) (Manku, 1983) and size (r) of the cation (Huheey, 1981; Stern, 1969). A correlation between the decomposition temperature and ∆H° was then sought. Since
∆G° ) ∆H° - T∆S°
Figure 1. Heat of decomposition of sulfates vs difference in acidity parameters.
(2)
a turning (decomposition) temperature can be defined (Stern, 1969; Wentworth and Chen, 1976; Casarin and Ibanez, 1993) by setting ∆G° ) 0, and thus:
Td ) ∆H°/∆S°
(3)
Furthermore, an examination of the data on sulfate decompositions showed ∆S° to be nearly independent of the cation (Stern, 1969), which leads to the conclusion that Td is approximately proportional to ∆H°. A plot of this ∆H° of decomposition vs the ratio (r1/2/Z*) for a series of metal sulfates shows good linearity (Stern, 1969; Huheey, 1981) in agreement with the arguments discussed earlier. The above reactions of formation or decomposition of oxysalts can also be viewed in terms of the relative acidic and basic character of the corresponding oxides. Let us generalize the decomposition reaction involving binary oxides as follows (obviously, the formation reaction is the reverse reaction):
AwByOx+z ) AwOx + ByOz, ∆Hd°
(4)
It has been recognized that the stronger the acidic character in a series of acid oxides AwOx reacting with a common basic oxide, the higher the heat of the formation reaction (Smith, 1987). (Actually, this concept is the equivalent to that of heat of neutralization in liquid media.) Likewise, the higher the basic character of oxides reacting with a common acidic oxide, the larger this heat. An acidity scale has been developed using this concept, and an acidity parameter a (or b, see below) can be defined in terms of the heat of decomposition as follows:
[a(AwOx) - b(ByOz)]2 ) ∆Hd°
(5)
where a stoichiometry such that 1 mol of oxide anion is transferred from the basic oxide to the acidic oxide is
Figure 2. Heat of decomposition of carbonates vs difference in acidity parameters.
assumed. By using available thermodynamic data, such an acidity scale has been established (Smith, 1987). We shall use here the term a to represent the acidity parameter of the acidic oxide and b to represent that of the basic oxide. (This acid-base character can also be influenced by parameters such as crystal structure, surface area, texture of the salts, preparation, chemical composition, etc., that are beyond the scope of this work.) It was described earlier that a plot of r1/2/Z* vs ∆Hd° was found to yield a satisfactory linear fit (Stern, 1969; Huheey, 1981). Since the difference in acidity parameters (a - b) is likewise related to ∆Hd° (see its definition above), a plot of a - b vs ∆Hd° should also show a similar behavior. We have found this to be the case, as shown in Figure 1 for the series of sulfates and in Figure 2 for the series of carbonates. A similar trend in ∆Hd° was observed earlier for group IA and IIA metals, and a more elaborate ionic model that does not involve the acidity parameter was developed (Wentworth et al., 1978). Thermochemical data used for our calculations (Stern, 1969; Huheey, 1981; Smith, 1987; Cotton, 1976; Perry and Green, 1984) are shown in Table 1.
Ind. Eng. Chem. Res., Vol. 38, No. 3, 1999 1067 Table 1. Thermochemical Data, Acidic Parameters, Ionic Radii, and Nuclear Charges -∆Hf (kcal/mol)
∆Hd (kcal/mol)
cation or compound
M2O/MO
M2SO4/MSO4
M2CO3/MCO3
sulfate
carbonate
a or b
dev. a (or b)
r (Å)
Z+
Z*
Li Na K Cs Be Mg Ca Sr Ba Ni Fe Cd Mn Zn SO3 CO2
142.30 99.45 86.20 82.10 145.30 143.84 151.70 140.80 133.00 58.40 64.62 62.35 92.04 83.36 94.39 94.05
340.23 330.50 342.65 344.86 281.00 304.94 338.73 345.30 350.20 216.00 221.30 222.23 254.18 233.40
289.70 269.46 274.01 271.88 N.D. 261.70 289.50 290.90 284.20 N.D. 172.40 178.20 211.00 192.90
103.54 136.66 162.06 168.37 41.31 66.71 92.64 110.11 122.81 63.21 62.29 65.49 67.75 55.65
53.35 75.96 93.76 95.73 N.D. 23.81 43.75 56.05 57.15 N.D. 13.73 21.80 24.91 15.49
-9.20 -12.50 -14.60 -15.20 -2.20 -4.50 -7.50 -9.40 -10.80 -2.40 -3.40 -4.40 -4.80 -3.20 5.50 10.50
0.50 0.40 0.50 0.70 0.00 1.00 0.40 0.40 0.40 0.50 0.00 0.20 0.40 0.30 0.40 0.30
0.60 0.96 1.33 1.69 0.31 0.65 0.99 1.13 1.35 0.69 0.75 0.97 0.80 0.74 1.03 0.77
1.00 1.00 1.00 1.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 6.00 4.00
1.85 1.85 1.85 1.85 2.50 2.50 2.50 2.50 2.50 3.70 3.40 4.00 3.25 4.00 5.10 2.90
an oxide ion) and the greater the easiness for breaking the oxysalt. Then, a plot of the function r1/2/Z* vs a b should show such a correlation. This has indeed been found to be the case, as can be seen in the highly linear plots shown in Figures 3 (sulfates) and 4 (carbonates). Conclusions There exists a plethora of reactions involving the thermal decomposition or formation of compounds containing binary oxides. The polarizing power of the corresponding cation can influence decisively the course of the reaction and the nature of the products, as well as the thermodynamic parameters involved. We have shown here that the heat of decomposition of a series of sulfates and carbonates correlates rather well with the difference in acidity parameters of such oxides. Likewise, this difference correlates equally well with the parameter r1/2/Z*, which is taken as a measure of the polarizing power of the cations. Figure 3. Radius/effective charge ratio vs difference in acidic parameters for sulfates.
Acknowledgment We thank one of the reviewers for pointing out the influence of several parameters on the decomposition reactions. This paper is dedicated to Margarita Watty (U. Iberoamericana) on the occasion of her 70th birthday. Nomenclature a(AwOx) ) acidity parameter for the acidic oxide, AwOx b(ByOz) ) acidity parameter for the basic oxide, ByOz r ) nuclear radius, Å T ) absolute temperature, K Z* ) effective nuclear charge ∆G° ) standard free energy change, kJ/(g mol) ∆H° ) standard heat of reaction, kJ/(g mol) ∆S° ) standard entropy change, kJ/(g mol K)
Literature Cited
Figure 4. Radius/effective charge ratio vs difference in acidic parameters for carbonates.
The concept of difference in acidity can then be correlated with the concept of polarizability described above as follows: the better the polarizing ability of the cation in ByOz, the stronger its oxide retention (i.e., the weaker the base and the greater the difficulty to release
Bi, M.; Li, H.; Pan, W. P.; Lloyd, W. G.; Davis, B. H. Thermal Studies of (NH4)2Cr2O7, (NH4)2WO4, and (NH4)6Mo7O24‚H2O Deposited on ZrO2. Thermochim. Acta 1996, 284, 153. Carr, N. J.; Galwey, A. K. Decomposition Reactions of Solids (an Experiment in Reviewing). Thermochim. Acta 1984, 79, 323. Casarin, C.; Ibanez, J. G. Experimental Demonstration of the Principles of Thermal Energy Storage and Chemical Heat Pumps. J. Chem. Educ. 1993, 70, 158. Cotton, F. A.; Wilkinson, G. Basic Inorganic Chemistry; John Wiley: New York, 1976. Cseri, T.; Bekassy, S.; Kenessey, G.; Liptai, G.; Figueras, F. Characterization of Metal Nitrates and Clay Supported Metal Nitrates by Thermal Analysis. Thermochim. Acta 1996, 288, 137.
1068
Ind. Eng. Chem. Res., Vol. 38, No. 3, 1999
De Beni, G. Thermochemical Processes for Hydrogen Production by Water Splitting: From Theory to Practice. Extended Abstracts of the 158th Meeting of The Electrochemical Society, The Electrochemical Society: Pennington, NJ, 1980; Vol. 80-2, pp 1469-1470. Diefallah, El-H. M.; Basahel, S. N.; El-Bellihi, A. A. Thermal Decomposition of Ammonium Trioxalatoferrate(III) in Air. Thermochim. Acta 1996, 290, 123. Fauteux, D. G.; Massucco, A.; Shi, J.; Lampe-Onnerud, C. Flexible Synthesis of Metal Oxides Illustrated for LiMn2O4 and LiCoO2. J. Appl. Electrochem. 1997, 27, 543. Fergusson, J. E. Inorganic Chemistry and the Earth: Chemical Resources, Their Extraction, Use and Environmental Impact; Pergamon Press: Oxford, 1985. Fuertes, A. B.; Fernandez, M. J. The Effect Of Metallic Salt Additives On Direct Sulfation of Calcium Carbonate and on Decomposition of Sulfated Samples. Thermochim. Acta 1996, 276, 257. Huheey, J. E. Quı´mica Inorga´ nica: Principios de Estructura y Reactividad, 2nd Ed.; Harla: Me´xico, 1981; Chapters 3 and 7. Ibanez, J. G.; Wentworth, W. E.; Batten, C. F.; Chen, E. C. M. Study of the Reaction of NH4HSO4 with ZnO as Part of an Energy Storage Cycle. Rev. Int. Hautes Temp. Refract. 1984a, 21, 125. Ibanez, J. G.; Wentworth, W. E.; Batten, C. F.; Chen, E. C. M. Kinetics of the Thermal Decomposition of Zinc Sulfate. Rev. Int. Hautes Temp. Refract. 1984b, 21, 113. Ito, N.; Obata, K.; Shindo, Y.; Hakuta, T.; Yoshitome, H. Catalytic Thermal Decomposition of Magnesium Iodate in the Presence of Metal Oxides or Carbon. Thermochim. Acta 1984, 73, 33. Jacobs, P. W. M. Kinetics of the Thermal Decomposition of Solids. In Materials Science Research, Vol. 4: Kinetics of Reactions in Ionic Systems; Gray, T. J., Frechette, V. D., Eds.; Plenum Press: New York, 1969. Jordanovska, V.; Trojko, R.; Boyanov, B. Synthesis, Characterization And Thermal Decomposition of Double Sulfates of Trivalent Al and Fe with Di- or Triethanolammonium Monovalent Cations. Thermochim. Acta 1996, 275, 301. Knaepen, E.; Mullens, J.; Yperman, J.; Van Poucke, L. C. Preparation And Thermal Decomposition of Various Forms of Strontium Oxalate. Thermochim. Acta 1996, 284, 213. Malinowski, C.; Malinowska, K.; Malecki, S. Analysis of the Chemical Processes Occurring in the System PbSO4-ZnS. Thermochim. Acta 1996, 275, 117. Manku, G. S. Principios de Quı´mica Inorga´ nica; McGraw-Hill: Me´xico, 1983.
Mishra, R.; Samant, M. S.; Kerkar, A. S.; Dharwadkar, S. R. A Thermoanalytical Study of Solid State Reactions between Tellurium Oxide and the Oxides of Zirconium and Hafnium. Thermochim. Acta 1996, 273, 85. Mu, J.; Perlmutter, D. D. The Thermal Decomposition Of Inorganic Sulfates and Their Hydrates. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 640. Pacewska, B.; Zmijewsaki, T.; Mioduska, M. Properties of the Products of Basic Aluminum-Ammonium Sulphate Decomposition in Hydrogen Atmosphere and of the Aluminum Oxides Obtained by Their Calcination. Thermochim. Acta 1996, 273, 145. Pelovski, Y.; Petkova, V.; Nikolov, S. Study of the Mechanism of the Thermochemical Decomposition of Ferrous Sulphate Monohydrate. Thermochim. Acta 1996, 274, 273. Perry, R. H.; Green, D. W. Perry’s Chemical Engineer’s Handbook, 6th ed.; McGraw-Hill: Singapore, 1988. Sawada, Y.; Murakami, M.; Nishide, T. Thermal Analysis Of Basic Zinc Carbonate. Part 1. Carbonation Process of Zinc Oxide Powders at 8 and 13 °C. Thermochim. Acta 1996, 273, 95. Smith, D. W. An Acidity Scale for Binary Oxides. J. Chem. Educ. 1987, 64, 480. Stern, K. H. The Effect of Cations on the Thermal Decomposition of Salts with Oxyanions. A Semiempirical Correlation. J. Chem. Educ. 1969, 46, 645. Swaddle, T. W. Applied Inorganic Chemistry; University of Calgary Press: Calgary, Canada, 1990; Chapter 5. Tmar, M.; Bernard, C.; Ducarroir, M. Local Storage of Solar Energy by Reversible Reactions with Sulfates. Sol. Energy 1981, 26, 529. Wentworth, W. E.; Chen, E. C. M. Sol. Energy 1976, 18, 205. Wentworth, W. E.; Hildebrandt, A. F.; Batten, C. F.; Corbett, G. E.; Chen, E. C. M. Rev. Int. Hautes Temp. Refract. 1978, 15, 231. Yilmaz, V. T.; Icbudak, H. Thermal Decomposition Characteristics of Ammonium Hexachlorometalate(IV) Complex Salts of Platinum Metals. Thermochim. Acta 1996, 276, 115. Zhang, Y.; Kshirsagar, G.; Ellison, J. E.; Cannon, J. C. Catalytic Effects of Metal Oxides on the Decomposition of Potassium Perchlorate. Thermochim. Acta 1996, 278, 119.
Received for review December 1, 1997 Accepted August 30, 1998 IE970877F
