1735
Communicationsto the Editor C.R. Guerra and J. H. Schulman, Surface Sci., 7, 229 (1967). M. Kobayashi and T. Shirasaki, J. Catal., 28, 289 (1973).
References and Notes
R. A. Dalla Betta, J. Phys. Chem., 79, 2519 (1975). M. L. Unland, J. Catal., 31, 349 (1973). J. A. Groenewegen and W. M. H. Sachtler, J. Cafal., 27, 369 (1972). L. H. Little, "Infrared Spectra of Adsorbed Species," Academic Press, New York, N.Y., 1966, D 200. Reference 13, p 69. E.Kikuchl, P. C. Flynn, and S. E. Wanke, J. Catal., 34, 132 (1974). K. S. Kim and N. Winograd, J. Cafal., 35,136 (1974). M. F. L. Johnson, J. Catel., 39, 487 (1975). D. J. Darensbourgand R. P. Eischens, Proc. Int. Congr. Catab, 5th, 7972, I,21-371 (1972).
(1) M. Shelef and H. S . Gandhi, Ind. Eng. Chem., Prod. Res. Dev., 11, 393 (1972). (2) R. L. Klimisch and K. C. Taylor, Environ. Scl. Techno/.,7, 127 (1973). (3) K. C. Taylor and R. L. Klimisch, J. Catal., 30, 478 (1973). (4) T. P. Kobylinskl and 6. W. Taylor, J. Catal., 33, 376 (1974). (5) M. A. Vannice, J. Catal., 37, 449 (1975). (6) K. C. Taylor, R. M. Sinkevitch, and R. L. Klimisch, J. CafaL, 35, 34 (1974). (7) L. Lynds, Spectrochim.Acta, 20, 1389 (1964).
COMMUNICATIONSTO THE EDITOR
Effect of Doping on the Sublimation of Ammonium Perchlorate
T
260
0
270
280
290
I°C) 300
310
320
330
3bO
350
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,
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370 I
/
Publication costs assisted by the Indian Institute of Science
Sir: The sublimation of solids under nonequilibrium conditions has been classified in two categories,l i.e., (1)congruent and (2) noncongruent. The main difference between the above two classes is the fact that in congruent sublimation, the vaporizing substance retains nearly a constant composition during sublimation. Solids such as NHdC104, NHdCl, CdS, etc., fall in this category. Our interest in the present investigation is in this class of solids and particularly in ammonium perchlorate (AP). Interest in A P also arises because in this solid, decomposition, sublimation, or both can occur depending on experimental conditions. It is widely used as an oxidizer in solid-composite propellants. Powdered AP rather than single crystal was chosen because it is in the powdered form that AP is used as an oxidizer in the solid propellants. It is desirable to control the sublimation of A P in the propellant system. A wealth of information is available on the decomposition mechanism of AP but very little is known about the sublimation process in detail particularly from the mechanistic point of view. The following important information regarding sublimation of AP is available in the literature: (1)Inami et aL2 have experimentally determined the latent heat of sublimation and found it to be 58 f 2 kcal mol-l which agrees well with the theoretically calculated value of 58 kcal mol-l; (2) the activation energy ( E )for sublimation has been the subject of study by several worker^.^-^ Pai Verneker et aL3 also deduced E theoretically and found it to be in good agreement with experimental value (18 f 2 kcal mol-l). Since the effect of doping on the thermal decomposition of AP is known," it is, therefore, of interest to study the effect of doping on the sublimation of AP. It is worthwhile to mention at this stage that CdS, which incidentally falls in the congruent category, has been studied in detail by S ~ m o r j a iIn . ~this case they have shown that, in case of Cu2+doped crystals and the crystals heat treated with Cd and sulfur, the evaporation rate is reduced compared to the untreated pure crystal. They have explained this behavior on the basis of the rate-limiting step
360
Figure 1. Sublimation endotherm of Yo) AP.
pure and Ca2+ doped
mol
which is the diffusion of Cd and S from the bulk to the surface.7 Ca2+ doping in NaCl crystals has been shown to desensitize the sublimation rate.8r9 Most of the studies concerning the effect of doping on the sublimation process have been done with systems such as NaCI, KCI, etc., and mostly cation dopants have been studied so far. Kinetics of the sublimation process could be studied either by a conventional microbalance or by mass spectrometry. Because sublimation is an endothermic process, such a study can also be carried out by the DTA technique. Pai Verneker et aL3 have shown that above 20 Torr in air, AP decomposes and gives rise to two exotherms following the phase-transformation endotherm at 240 "C. Below 20 Torr, the predominent process is the sublimation which gives rise to a broad endotherm following the phase-transformation endotherm. By comparing the amount sublimed at a given temperature (provided the particle size and heating rate are kept constant), it is then possible to arrive at comparative rates of sublimation. Alternatively, one can compare the temperature for 50% sublimation. The sublimation studies were carried out on a home-made DTA assembly as described elsewhere.1° Additional arrangement was provided by which the sample could be sublimed in vaccuo. The pressure inside the assembly was 40 f 5 p. The samples were run at heating rate of 11.7 "C min-1. The amount of sample in each run was taken to be 50 mg. Recrystallized AP was used for doping. A P and the dopant (calcium perchlorate, ammonium sulfate, and ammonium The Journal of Physical Chemistry?Vol. 80, No. 15, 1976
Communications to the Editor
1736
TABLE I: Sublimation behavior of Pure and Doped AP
Fraction sublimed Doped AP Temp,a "C Pure AP ( 320 335 350
0.383 0.685 0.973
Ca2+ S042c1mol %) ( mol %) (10-2 mol %) 0.294 0.555 0.868
0.304 0.546 0.875
0.341 0.631 0.957
%
sublimed 50
'Temp ("C) for 50%sublimation 326
332
332
329
This is the temperature at which the fraction sublimed ( a ) has been calculated. a
chloride) were taken in an aqueous solution in definite proportions and the coprecipitation was done by cooling the saturated solution at 70 "C to room temperature. The particle size of doped and undoped AP was kept constant. The exact amount of the dopant in the AP crystal was not analyzed and therefore the amount to which we are referring is in the solution. so42-doped samples were subjected to usual conduction measurements and the enchancement in conduction is indicative of the incorporation of S042- in the crystal lattice resulting in vacancies.11 Figure 1represents typical sublimation endotherms of pure and doped ammonium perchlorate. The fractional areas at different temperatures and the total area of the endotherm were measured with a planimeter. The fraction sublimed ( a ) at different temperatures could then be calculated from the ratio of the respective areas. This yielded a plot of a vs. temperature which was then used to calculate (i) the fraction decomposed at a particular temperature and (ii) the temperature for 50% sublimation. The results are given in Table I. It is evident from the data in Table I that the sublimation rate is desensitized by Ca2+,S042-, and C1- doping. The behavior of the Ca2+and S042-dopants (in the low concentration range) in the thermal decomposition of AP has been explained on the basis of an ionic diffusion mechanism.6 In thermal decomposition, the Ca2+doping desensitizes and the S042- doping sensitizes the process. This shows that the mechanism of sublimation is different from that of the decomposition of AP. Considering the proton-transfer process on the surface to be the rate-controlling step, the sublimation mechanism can be explained for S042- and C1- doped AP in terms of a proton trap.12 A similar explanation based on Herrington-Stavely's (HS) molecular defects13 can also be given, according to which the defects which are formed via sublimation reactions will dominate near the surface. Although the above explanation can very well explain the sublimation mechanism of S042-and C1- doped AP, it cannot explain the desensitization observed in case of Ca2+doped AP (Table I). This shows that, although the proton-transfer basically remains the rate-controlling process, the overall mechanistic path through which the sublimation occurs may be more than one. Sublimation mechanism can also be explained in the following ways: (I) Higher bond strength of the dopant ion (i.e., Ca2+ and S042-) with counterion on the surface compared to the bond strength between NH4+ and The Journal of Physical Chemistry, Vol. 80, No. 15, 1976
c104- ions. (11)The presence of the dopant ion makes the surface non~toichiometricl~ with respect to the number of NH4+ and C104- ions. When the surface becomes nonstoichiometric then obviously the NH4+ or C104- has to come from the immediate next layer for compensation and thus the sublimation process is slowed down. (111) The strain in the crystal, caused by doping, may also control the sublimation process. Lester and Somorjai have shown that strain in the crystal desensitizes the sublimation rate.9 Proof of the strain in AP due to doping has been shown from the broadening of infrared peaks.15 Further work is in progress to throw more light on the mechanism of the sublimation process. References and Notes (1) G. A. Somorjai, Science, 162,755 (1968). (2)S. H. Inami, W. A. Rooser, and H. Wise, J. Phys. Chem., 67, 1077 ( 1963). (3)V. R. Pai Verneker, M. McCartey, Jr., and J. N. Maycock, Thermochim. Acta, 3,37 (1971). (4)P. W. M. Jacobs and A. Russel-Jones, J. Phys. Chem., 72,202 (1968). (5) C. Guirao and F. A. Williams, J. Phys. Chem., 73,4302 (1969). (6)J. N. Maycock and V. R. Pai Verneker, Proc. R. SOC.,Ser. A, 307,303-315 (1968). (7) G. A. Somorjai and D. W. Jepsen, J. Chem. Phys., 41,1394 (1964). (8) H. Bethge, Phys. Status Solidi, 2,3,775 (1962);Surf. Sci., 3,33 (1964). (9)J. E. Lester and G. A. Somorjai, J. Chem. Phys., 49,2940 (1968). (IO) S.W. McBain and A. M. Baer, J. Am. Chem. SOC., 48,600 (1926). ( I 1) V. R. Pai Verneker, unpublished work. (12) P. W. M. Jacobs and W. L. Ng, J. Solidstate Chem., 9,315,(1974). (13)G.P. Owen, J. M. Thomas, and J. 0. Williams, J. Chem. Soc., Faraday Trans. 1, 70, 1934(1974). (14)D.L. Howlett, J. E. Lester, and G. A. Somorjai, J. Phys. Chem., 75,4049 119711 '(15) V. R. Pai Verneker and K. Rajeshwar, Thermochim. Acta, 13, 333 I . - .. I .
(1975). High Energy Solids Laboratory Department of Inorganic and Physical Chemistry Indian Institute of Science Bangalore 560 0 12, India
V.
R. Pal Verneker K. Kishore.
M. P. Kannan
Received December 18, 1975
Vibration to Translation Energy Transfer from Excited Cyclobutane Chemically Activated by Nuclear Recoil Reaction Publication costs assisted by the United States Energy Research and Development Administration
Sir: Intermolecular energy transfer studies from thermally and chemically activated molecules provide information of fundamental and practical importance in chemical dynamics. Of particular interest are highly vibrationally excited species which transfer relatively large amounts of energy on collision.1-6 Previous reports for conventional chemical activation systems have shown that the average energy transferred per collision is on the order of a few kilocalories per mole, while the relative energy transfer efficiencies generally range over no more than an order of magnitude in going from simple monatomic to complex polyatomic c o l l i d e r ~ .The ~ , ~ detailed mechanism for vibrational energy transfer, however, as yet is not understood in complex molecular systems. We have measured the vibration to translation energy transfer efficiencies from nuclear recoil chemically activated cyclobutane-t to the homologous series of noble gases He, Ne, Ar, Kr, and Xe in order to further clarify the mechanism for inter-
