R. I. RAZOUK AND R. S. MIKHAIL
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SURFACE PROPERTIES OF MAGNESIUM OXIDE. 111 BY R. I. RAZOUK AND R. SH. MIKHAIL Contribution f r o m the Department of Chemistry, Faculty of Science, Ain S h a m s Univewity, Cairo, E g y p t , UAR Received October 23,1968
I n continuat,ion t o an earlier communication, work has been extended to study the effect of the temperature of preparation and of the duration of heating on the activity of magnesium oxide obtained by the thermal decomposition in vacuo of two specimens of precipitated magnesium hydroxide,.precipitated magnesium carbonate and native magnesite. MgO pre ared from precipitated Mg(OH)2 or MgC03 behaves like that obtained from brucite, in that the maximum activity is deveyoped on decomposition at 350’, whereas the oxide prepared from magnesite a t 650’ shows maximum activity. Sintering develops a t higher temperatures and its extent is greater the larger is the surface area of the oxide and the higher the temperature. It is suggested that in the dehydration of Mg(OH)2, the development of the surface area is a direct consequence of the reaction, whereas in the decomposition of MgC03 a process of activation occurs as well, so that the surface area of the product increases far more than corresponds to the fraction decomposed. The results are interpreted as a consequence of the interaction of three rate processes, namely, decomposition, recrystallization and sintering. It is concluded that the development of the activity of the product obtained by the th2rmal decomposition of different parent materials varies according to the mechanism associated with ita decomposition.
The surface properties of magnesium oxide prepared by the dehydration of brucite were the subject of an earlier investigation.’ I n view of the controversial results obtained by various authors regarding the conditions of maximum activity of oxides produced by thermal decomposition2 and the variation of activity with heat treatment, the work has been extended to the study of oxides obtained by the dehydration of two specimens of precipitated magnesium hydroxide and the decomposition of precipitated and native magnesium carbonate. Experiments have been confined to oxides prepared in vacuo, since the presence of air has been found to influence the surface properties in a complicated manner.’ The effect of the temperature of decomposition and of the duration of heating a t fixed temperatures on the activity of magnesium oxide was studied, the activity being arbitrarily measured by the specific surface area as determined from the monolayer capacity of adsorbed cyclohexane in the manner described earlier.
*
the molecular area of cyclohexane as 30 It has been shown’ that estimates of the surface area based on the adsorption isotherm of cyclohexane on magnesium oxide agree with the values obtained from the low-temperature nitrogen adsorption using the BET method. Table I shows a summary of the surface area values of the various oxides prepared by the decomposition in vacuo of precipitated magnesium hydroxide and carbonate and of magnesite at various temperatures. It should be noted that the decomposition was complete in all cases except with precipitated MgC03 C a t 350°, when the decomposition was 1.2y0short of completion. TABLEI SPECIFIC SURFACE AREAOF MAGNESIUM OXIDEPREPARED AT VARIOUS TEMPERATURES
’
Experimental The apparatus, technique and the method of preparation of cyclohexane were described earlier . l Four groups of oxide were prepared by thermal decomposition, in vacuo, of recipitated Mg(OH)2 A and Mg(OH)2 B, precipitated MgC& C and native magnesite R4gC03 M. Details of the methods of preparation are described elsewhere.3 Decomposition temperatures varied between 350 and 1250°, and duration of heating between 10 min. and 20 hr. The oxide content of each product is given in the tcxt.
Results I. Effect of Temperature of Preparation.-The adsorption isotherms of cyclohexane on the various decomposition product,s obtained from the hydroxide and carbonate of magnesium by heating in vacuo for 5 hr. are all Type I1 of the Brunauer clas~ification.~The adsorption seems to be purely physical in nature, and outgassing a t room temperature brings about almost complete removal of the adsorbate. The surface area of the various products has been estimated from the B-point of the isotherms, taking ( I ) Part I, R . I . Razouk and 886 (1957).
R. Sh. Mikhail, THISJOURNAL, 61,
(2) S. J. Gregg, “The Surface Chemistry of Solids,” London, 1951. (3) R. I. Razouk and R. Sh. Mikhail, THISJOURNAL, 62, 920 (1958). (4) S. Brunauer, “Physical Adsorption sf Gasea and Vgpors,”
Oxford Press, New Ymk, N. Y,, 1945,
25 (undec.) 350 500 650 800 950 1100 1250
1 34 25 18 13
7
89 310 280 190 121 77 13
8 426 156 73 42 28 17
6
285 550 480 392 197 54
The results in Table I are shown in Fig. 1, in which are plotted also the areas of the dehydration products of brucite for the sake of comparison. It is evident that the oxides prepared from brucite and precipitated hydroxide or carbonate behave in a similar way. Thus the maximum activity is developed on decomposition a t 350°, and sintering soon develops at higher temperatures, its extent being the greater the larger is the surface area of the produced oxide. However, the oxide prepared from magnesite shows a striking difference; for here the maximum activity develops when the oxide is formed a t 650°, and the temperature a t which it becomes “deadburnt” and thus possesses a very low surface area, is distinctly above that for the other parent materials. It seems therefore that (5) N. Smith, C. Pierce and H. CQFdeS, J , A m , Chem. Xoc., l a ! 5595
(1950),
-
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below 650" there is an activation process while above this temperature the ordinary sintering process occurs. It is worth mentioning that although the surface area of Mg(OH), A is very close to that of brucite (about 1 m.z/g,l), yet the maximum surface area of the oxide obtained from the latter is about four times as much as the value obtained for the oxide prepared from the former. Comparison of the results obtained with the hydroxides Mg(OH)2A and Mg(OH)2 B and their dehydration products shows in a striking manner how differences in the details of preparation lead to large changes in the surface activity. Thus the surface area of the oxide prepared from Mg(0H)z B at temperatures between 350 and 800" is about ten times as big as that of the product obtained from Mg(OH)z A. Even if the surface area of the original hydroxide is subtracted from the area of the product, the ratio of the two areas still remains about 7 : l . It is interesting to note that whereas the various oxides experimented with possess surface areas which may vary up to 20-fold) variations of the water uptake a t saturation do not generally exceed 30%, so that one cannot escape the conclusion already arrivedgat by the author^^,^ that the sorption of water by magnesium oxide is not merely a surface phenomenon but mainly a bulk effect. 11. Effect of Time of Heating.-The effect of the duration of heat treatment on the surface area of magnesium oxide has been studied using Mg(OH), B, MgC03 M and MgC03 C as starting materials. Figure 2 shows the variation of the specific surface area of MgO B with duration of heating for various temperatures of dehydration, the time of heating representing the total time commencing with the parent material. The surface areafallssensiblywith increase of time of heating for the oxides prepared a t 1100 and 950", but a limiting area is obtained on heating the hydroxide for 10 hr. This area is characteristic of the temperature of dehydration, and longer heat-treatment does not produce any effect, in agreement with the results obtained with dehydrated brucite.' However, Gregg, Packer and Wheatleyl found that the surface area of magnesium oxide prepared by the calcination of the precipitated hydroxide diminishes continuously with increase of time of heating over a period of more than 300 hr. This difference may be due to the varying conditions of preparation of the parent material and of its heat-treatment especially with respect to the absence or presence of air during the decomposition of the hydroxide. Magnesium oxide prepared a t 800 and 650" behaves in a similar way, though to a much less extent, and the surface area falls slightly with increase of time of heating, reaching a limiting value after 10 hr. also. On the other hand, when Mg(OH)2 B is dehydrated in vacuo a t 500", the surface area of the product increases slightly with time but soon tends to a limiting value. Thus the area of the product (6) R . I. Raaouk and R . S h . Mikhail, THISJOURNAL, 59,636 (1955)' (7) S. J. Gregg, P. K. Packer and K. H. Wheatley, J . Chem. Soc., 46 (1955).
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600 800 1000 1200 Temp., "C. Fig. 1.-Effect of temperature of preparation on the surface area of magnesium oxide obtained from brucite, Mg(0H)z A, Mg(0H)z B, MgC03 C and MgCOa M. 400
2 3 4 5 6 Time, hr. Fig. 2.-Effect of duration of heating on the surface area of magnesium oxide prepared from Mg(0H)g B a t various temperatures ( 0 , original hydroxide). 0
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of dehydration obtained after heating the hydroxide for 0.5 hr. is 264 m.2/g., while that of the oxide obtained after 5 hr. is 280 m.2/g. However, it has been found that shock heating a t this temperature for 0.5 hr. is not sufficient to bring about complete dehydration but results in a loss of 28.7y0 as compared with the theoretical value of 30.9%. Now if the area is corrected for incomplete dehydration it rises to 278 m.2/g. which is very close to the value of the completely dehydrated specimen obtained after 5 hr. of heating. Similar results are obtained with the product prepared a t 350". Thus when the hydroxide was heated for 0.5 hr. a t this temperature, it lost 22.4% of its weight, and the specific surface area of the product was found to be 249 m.2/g. of the dehydration product, whereas when the area is referred to the weight of magnesium oxide actually present, it becomes 309 m.2/g. as compared with the value of 310 m.2/g. obtained with the oxide prepared by dehydration for 5 hr. a t the same temperature. It may thus be concluded that in the case of MgO prepared from precipitated Mg(OH), B, the development of the surface is a direct consequence of the
R. I. RAZOUK AND R. S. MIKHAIL
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100 200 300 Time, min. Fig. 3.-Effect of duration of heating on the surface area of magnesium oxide prepared from MgC03 M at various temperatures.
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40 60 80 100 Decomposition, %. Fig. 4.-The surface area of magnesium oxide pre ared from brucite, Mg(0H)s A, Mg(OH)2 B, MgC03 8 a n d MgC03 M as a function of the fraction decomposed. 0
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decomposition of the hydroxide, and that the process of sintering becomes sensible only above 650". The effect of the duration of heating on the surface area of the product of decomposition of native magnesite, MgC03 M, is shown in Fig. 3. It is evident that a t temperatures above 650°, the behavior is essentially the same as that of the dehydration products of magnesium hydroxide, namely, that the surface area decreases continuously with the increase of the time of heating until a limiting value is obtained with the oxide prepared by heating magnesite for 5 hr. However, the oxide prepared from magnesite a t 650" represents a special and critical case; for the duration of heating has apparently no effect on the surface area of specimens heated for periods varying between 0.5 and 5 hr. and, under these conditions, the oxide possesses the maximum surface area of 550 m.2/g. It is to be noted that heating for 0.5
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hr. a t this temperature brings about complete decomposition of magnesite. The results of experiments on the products prepared by decomposition a t 500" are more interesting. Specimens heated for 0.25, 0.5, 1, 2, 3.5 and 5 hr. a t this temperature and having lost 9.6, 17.8, 26.2, 49.3, 52.07 and 52.20% of their weight as carbon dioxide, possess surface areas of 17, 26, 39, 116, 237 and 285 m.2/g. of the product, respectively. These values show that the area increases far more than corresponds to decomposition, indicating that in the case of magnesite the development of the surface is not a direct consequence of decomposition, but proceeds either with a slower rate than decarbonation, or develops even after it has ceased. A similar increase in the ratio of area to percentage decomposition also has been observed by Gregg, Packer and Wheatley7 using MgO from Mg(OH)* and by Glasson8 using CaO from Ca(OH)2. Precipitated MgC03C showed the same behavior. Thus the products prepared by decomposition in vucuo a t 350" for intervals of 10 min., 1, 1.5, 3 and 5 hr. and having lost 9.2, 27, 39, 47.5 and 51.6% of their weight possess surface areas of 31, 76, 147,310 and 426 m.2/g. of the product of decomposition, respectively; the increase in area far exceeding the extent of decomposition. Figure 4 summarizes the results of the measurements of the surface area of the products of decomposition of brucite,l precipitated Mg(OH)2B, magnesite MgC03 M and precipitated MgC03 C as a function of the percentage of decomposition. The curves show clearly that in the case of natural and precipitated hydroxide, the surface area of the product is a direct consequence of decomposition, whereas in the case of natural and precipitated magnesium carbonate a process of activation takes place in the mean time a t the lower temperatures of preparation.
Discussion The surface properties of magnesium oxide produced by thermal decomposition have been the subject of many investigations. According to the general theory of Gregg for the preparation of active solid^,^ the formation of magnesium oxide by thermal decomposition may proceed in one of two ways. Thus during the decomposition of the parent material, one nucleus of the oxide may first form within each crystallite, and then the lattice rearranges by growth of this nucleus, a process which is a function of both temperature and time. Under these conditions, one would expect limited activation of the oxide, the development of much strain and slight variation of surface area.l0 But none of these effects has been observed. Indeed, the present investigation reveals that the effect of heat treatment is a pronounced variation in the surface area, while Thomas and Bakerll could not detect the presence of lattice strain in their X-ray studies, and even suggested that the line broaden(8) D. R. Glasson, ibid., 1506 (1956). (9) S. J. Gregg, ibid., 3940 (1953). (10) D. T. Livey, B. M. Wanklyn, M. Hewitt and P. Murray, Trans. Brit. Ceram. Soc., 66, 217 (1957). (11) D. Thomas and T. W. Baker, "X-Ray Diffraction Studies of Active Magnesium Oxide," quoted in ref. 10.
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SURFACE PROPERTIES OF MAGNESIUM OXIDE
ing given by some specimens of oxide is rather due to changes in the particle size. Alternatively, each crystallite of the parent material, immediately on decomposition, may give rise to a large number of nuclei, and the original lattice collapses as a result of decomposition. The development of some lattice strain cannot, however, be overlooked, especially with specimens prepared at the lower temperatures, when the oxide is formed a t first in the form of a pseudo-lattice, with the ions of Mg++ and 0-- still occupying the same positions that they held in the original lattice. Thus Fricke and Luke12found that the active oxide possessed an energy content 67 cal./g. higher than the sintered oxide. But the pseudo-lattice has probably a short-life existence, since it has not been detected by X-ray e~amination,’~ and this form soon collapses to yield a great number of magnesium oxide crystallites corresponding to the number of nuclei formed in the parent material. This process, which is associated with the disappearance of the original structure of the parent material and the formation of the new lattice of the oxide, may be designated “recrystallization,” and accounts for the activation and the development of large surface areas of the products. Both decomposition and recrystallization are rate processes, and the variation of the surface area with percentage decomposition is dependent on the rate-determining step. When the decomposition of the parent material is rate-determining, the area of the product will be a linear function of the fraction decomposed, as in the production of magnesium oxide from precipitated and natural hydroxide. On the other hand, if recrystallization is the rate-determining process, then further activation of the product may occur, resulting in an increase of the surface area far more than corresponds to the fraction decomposed, and this is the case with precipitated and native carbonate. Thus the development of activity of the product obtained by the thermal decomposition of different starting materials, varies according to the mechanism associated with its decomposition. Accompanying the process of recrystallization, a third rate process is of frequent occurrence, namely, the growth of the formed magnesium oxide crystal(12) R . Fricke and I. Ltike, Z. Eleklrochem., 41, 174 (1935). (13) R . Sh. Mikhail, Ph.D. Thesis, A h Shams University, 1957.
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lites which is usually called sintering. Increase in the grain size of the particles by sintering has been illustrated in a striking manner by a set of electron micrographs taken by Birks and Friedman14 using magnesium oxide prepared from commercial magnesite. Raising the temperature invariably increases sintering until a temperature is reached when the oxide becomes “deadburnt.” The effect of time is also to increase sintering. The activity generally diminishes exponentially with time, but it has been found in the present investigation that a limiting value of the surface area always has been obtained for each temperature. It is interesting to note that the oxides which possess higher surface areas sinter more rapidly. This may be understood in view of the higher surface energy associated with finer powders, facilitating thus the process of surface adhesion and surface diffusion which are so important in the process of sintering.I5 Furthermore, two other effects may contribute to the easier sintering of finer particles, namely, the presence of edge and corner energies16and the considerable rise in the surface energy due to the screening effect of the cations and the consequent approach of the whole structure to a disordered lattice as predicted by Weyl.17 The net result of the interaction of the three above-mentioned processes, vix., decomposition, recrystallization and grain growth of the product may lead under certain conditions to the appearance of a maximum in the surface area-temperature curves. Before the maximum is reached the first two processes are predominating, whereas beyond it sintering becomes more significant. Finally, the conclusion could be drawn that temperature or duration of heating can no longer be arbitrary in the production of active magnesium oxide by thermal decomposition. Heating must be conducted a t temperatures and for periods ensuring complete decomposition as well as maximum activation and avoiding any sintering effects. The use of parent materials possessing large surface areas will generally favor sintering a t comparatively lower temperatures of preparation. (14) L. S. Birks and H. Friedman, J . A p p l . Phys., 17, 687 (1946). (15) G. F. Huttig, Kolloid Z., 98, 263 (1942); 99, 262 (1942). (16) B. Weissenbrtch, Radez-Rundschau, 6 , 257 (1951). (17) W. A. Weyl, “Structure and Properties of Solid Surfaces,” Ed. R. Gomer and C. S. Smith, Univ. of Chicago Press, Chapter I V 1952.
