Isothermal Decomposition of lsothiocyanatopentammine Cobalt(ll1) Perchlorate-Particle Size Effect Chieh J u Choul and Ferron A. Olson Department of Mining, Metallurgical and Fuels Engineering, Uniuersity of Utah, Salt Lake City, Utah 84112
The rather unusual dependence of the decomposition rate of isothiocyanatopentammine cobalt, ICCP, upon particle size is explained on the basis of thermogravimetric analysis, infrared spectrometry, and optical microscopic studies of the residues. The additional independent variables of temperature and the partial pressure of a product gas, NHs,were also used. Up to 9 per cent weight loss, the larger particles decomposed faster than the smaller particles, but after 9 per cent weight loss the smaller particles decomposed faster. An explanation of the particle size dependence is based upon experimental evidence and a “geometric model” of the kinetic results. Up to 9 per cent reaction, an activation enthalpy of 33 kcal/mole was independent of particle size indicating a rate dependence due to entropy of activation or number of reaction sites. After 9 per cent weight loss, the activation enthalpy varied from 21 to 28 kcal/mole as particle size increased. Evidently up to 9 per cent weight loss the larger particles had relatively more reaction sites possibly due to internal strain. After 9 per cent weight loss, build-up of the product gas NH, in the larger particles reversed the dependence of the rate upon particle size.
Table I. Particle Size Distribution of ICCP Mesh per inch -14 -28 -48 -65 -100 - 150 -200 - 270
Percentage 0.95 4.34 35.38 37.66 11.86 4.71 3.25 1.75
+ 28 + 48 + 65 + 100 + 150 + 200 + 270
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0.6
W W.
0.4
THE KINETICS AND MECHANISM of decomposition of isothiocyanatopentammine cobalt(II1) perchlorate (ICCP) were considered in previous papers (1, 2 ) . The effect of particle size on decomposition is emphasized in this paper. The sample of >99% purity had a rather wide range in particle size as seen by Table I. In the majority of the study the two 65 and - 100 150 mesh were samples of mesh size -48 used because they were significant fractions of the sample and gave a fair spread in particle size. Thermogravimetric analysis based upon the Cahn electrobalance was used (1). The environment was open air of about 15 Torr water vapor pressure unless specified otherwise. Additionally infrared analysis using the pellet technique (3) and optical evidence were used to advantage in deducing the effect variation in particle size has on the decomposition mechanism.
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t I
IO0
200
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RESULTS
Thermogravimetric Analysis. Experimental results showed that the rate of decomposition of ICCP was independent of sample size and of distribution in a quartz container if the sample size was less than 0.7 mg. For this reason most of the thermogravimetric tests were conducted using 0.5-mg samples. The effect of particle size on decomposition rate Present address, Kennecott Copper Corporation, Research Center, 151 5 Mineral Square, Salt Lake City, Utah. (1) D. E. Richardson and F. A. Olson, University of Utah, Salt Lake City. Utah, unpublished results, 1972. (2) B. D. Chun, C. J. Chou and F. A. Olson, University of Utah, Salt Lake City, Utah, unpublished results, 1972. (3) W. Brugel, “Equipment and Experimental Technique of Infrared Spectroscopy,” Part 11, An Introduction t o Infrared Spectroscopy, John Wiley and Sons. New York, N.Y., 1963.
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I
400
300
300
’
lime, minutes
Figure la. Thermogravimetric decomposition of ICCP in static air at 214 “C for four different particle sizes using 0.5-mg samples are shown in Figure l a with the initial part of Figure l a expanded in Figure lb. In the first stage of the reaction (WjW, = 0.99 to 0.91) as shown in Figure 16, the rate of weight loss of the coarse particles was faster than for the fine particles. At the end of this region, an intersection of the curves results and a region follows showing a plateau increasing in prominence as particle size increases; i.e., the finer particles show a higher initial rate in this region than the coarse particles. With further reaction, the four bisigmoidal curves rejoin at 38 per cent decomposition ( W / W o = 0.62) as shown in Figure l a and thereafter (38 to 69 per cent decomposition) particle size seems to have little effect. Further studies of thermogravimetric decomposition for 65 mesh and -100 150 mesh were particle sizes -48 studied for 0.5-mg samples at a temperature of 214 “C under static environments of different partial pressures of ammonia in helium at total pressures of one atmosphere. The results in Figures 2 and 3 show a pronounced retardation of the decomposition by NH3 in the second sigmoidal region of the bisigmoidal curves at partial pressures of NH3 >46 Torr 65 mesh size and >26.2 Torr in Figure in Figure 2 for -48 150 mesh particle size. At the limiting pres3 for -100 sures reaction of the second sigmoidal region indicated in
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
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. - 48 + 6 5 mesh . - 66 +IO0 m t c h . - 100+160 mash
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Table 11. Infrared Absorbance Data for ICCP Decomposition at 214 "C -48 65 mesh Absorbance Decom- Percent___ NCS NH4+ NHI position age of 3 time, weight (2100 (1400 (1310 (810 cm-l cm-1 cm-' cm-l min. loss
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.b
a5
IO
20
ao
40
50
60
0 5 15 20 70 90 130
70
t i m a , minutes
0 5
15 20
0 0.3 7 8.7
70
21.7
90 130
IO0
time,
Figure 2. Effect of ammonia partial pressure on ICCP (-48 65 mesh) decomposition at 214 "C (Total pressure = 644Torr)
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8 -
8 -
K W
7 -
1 50
100
time, mlnutes
Figure 3. Effect of ammonia partial pressure on ICCP (- 100 150 mesh) decompositionat 214 "C(Total pressure = 644 Torr)
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other studies as principally NH4C104 decomposition (2), was apparently completely stopped and further increase in the partial pressure of NHBpartial pressure apparently had little further effect (Figure 2). The effect of temperature, in the range 200 to 227 "C on decomposition of the three predominant particle sizes -48 65, -65 100, and -100 150 mesh using 0.5-mg samples are summarized in Figures 4, 5, and 6 and show that the finer particles had longer nucleation times consistent with the results of Figure la.
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26 35
0 0.328 0.845 0.676 0.568 0.578 0.396
0.339 0.288 0.084 0.013 0 0 0
0.167 0.119 0.041 0 0 0 0
0 0.061 0.361 0.448 0.413 0.400 0.338
0.292 0.279 0.189 0.047 0 0 0
0.138 0.113 0.089 0.013 0 0 0
+ 150 mesh
0 0 0.096 0.106 0.091 0.066 0.025
Infrared Analysis. The weight loss data represented in Figures la and l b show that the rate of weight loss of large particles was greater than that of small ones in the weight loss region, 0 < WjW, < 0.91 and corresponding time region 0 < t < 20 minutes at the decomposition temperature of 214 "C. After that, the rate of weight loss of small particles was greater. In order to explain this phenomena, some residues of two particle size fractions, -48 65 and -100 150 mesh, were decomposed at 214 "C and submitted to infrared analysis. The absorbance results of some active bands, such as NH3,NH4+,and NCS-are given in Table 11. Initially (
