PHOTOCHEMICAL DECOMPOSITION OF NITRONIUM PERCHLORATE
2107
The Photochemical Decomposition of Nitronium Perchlorate1
by J. N. Maycock, V. R. Pai Verneker, and L. Witten Research Institute for Advanced Studies, Baltimore, Margland 21227
(Received November 28, 1966)
The kinetics of gas evolution from nitronium perchlorate irradiated with ultraviolet light from a high-pressure mercury arc has been investigated as a function of intensity, temperature, and time of irradiation. Different gaseous products of decomposition have been identified mass spectrometrically, the predominant species being 0 2 and NO. The data indicate that two primary mechanisms for photolysis are operating simultaneously. The first mechanism starts out rapidly, builds up to a large rate of photolytic decomposition, and then dies off with time. The second slower mechanism then takes over and is re sponsible for the long-time photolytic behavior. An absorption spectrum of a single crystal of nitronium perchlorate has also been measured. The results agree with the threshold energies for photodecomposition.
Introduction Photolysis of metastable materials in the solid state can provide very illuminating information about their stability and electronic structure. Evidence for this statement is found in the wealth of information resulting from photolytic studies of the azides,2 many of which are highly explosive. So far, no photochemical decomposition studies have been made on the perchlorates although the thermal decomposition of these materials, particularly of ammonium perchlorate,* has been thoroughly investigated. One perchlorate which has received relatively little attention, either thermally or photochemically, is nitronium perchlorate. The thermal decomposition of nitronium perchlorate has been studied over a temperature range of 70 to 112" by conventional techniques' and has been found t o be probably dependent on an electron-transfer mechanism. Further work to elucidate this proposed mechanism has been performed in our l a b ~ r a t o r y . ~To complement these thermal decomposition studies, we have investigated the photolytic behavior of pure nitronium perchlorate powder under ultraviolet irradiation. In the next sections, we describe the experimental results of the photolytic study and outline a possible physical mechanism that yields the observed results.
Experimental Section It is very well known that nitronium perchlorate is extremely hygroscopic, nitric acid and perchloric acid
being formed immediately upon contact with water. For this reason, the transference of the Callery Chemical Co. nitronium prechlorate powder from the stock batch to any sample container was always performed in a drybox having a stream of pure, dry nitrogen gas passing through it. However, by differential thermal analysis and mass spectrographic analysis, it was found that all samples invariably absorbed a certain amount of water. Using mass spectroscopy as a guideline, it was found that complete removal of water and hydrolysis products was attained by pumping on each individual sample for a period of 18 hr in a vacuum line maintained at torr. The perchlorate was contained in a Pyrex cell fitted with a flat quartz window and connected to the vacuum line. This consisted in sequence of an outlet to the mass spectrometer, a cold trap, a Pirani gauge, an ionization gauge, a thermocouple gauge, a McLeod gauge, a cold trap, a three-stage silicone oil diffusion pump, and a mechanical backing pump. A roughing line was also installed from the mechanical pump to the ir(1) Supported by the U. S. Army Missile Command, Contract No. DA-Ol-O2l-AMC-l2596(2). (2) B. L. Evans, A. D. Yoffe, and P. Gray, Chem. Rev., 59, 616 (1959). (3) A. K. Galway and P. W. M. Jacobs, J. Chem. Soc., 974, 6031 (1960). (4) H. F. Cordes, J . Phvs. Chem., 67, 1693 (1963). (6)J. N. Maycock, C. 8.Gorzynski, and D. E. Grabenstein, unpublished results.
Volume 71, Number 7 June 1067
2108
radiation cell so that the cell could be quickly dismantled while keeping the main line a t torr and then pumped out when reassembled without having to break the high vacuum. The mass spectrometer, a Consolidated Electrodynamics 21-613 residual gas analyzer, was connected to the main high-vacuum line through a Varian 951-5100 adjustable leak valve having a minimum leak rate of torr l./sec. The working volume was determined by calibration with nitrogen using the McLeod gauge for pressure measurement. The thermocouple and Pirani gauges were also calibrated against the hfcLeod gauge. All ground-glass joints and stopcocks were lubricated with Kel-F grease which does riot interact with nitronium perchlorate. Unfortunately, the vapor pressure of the Kel-F grease prevented experimentation at pressures much below torr. A typical sample size weighed about 500 mg; the area of irradiation was about 3 cm2. The sample was photolytically decomposed into a working volume of 1.3 1. For typical decomposition runs (approximately 1 hr long), there was no visible change in the sample size. Weighing before and after a photolytic decomposition run was impracticable due to the unstable properties of nitronium perchlorate. The irradiation source was a water-cooled General Electric AH-6 high-pressure mercury arc whose radiant intensity was varied by placing neutral density filters between the sample cell and the Hg lamp. The transmittance of these filters was calibrated by replacing the cell with a thermopile; intensities are recorded in terms of the thermopile emf since the spectral output of the lamp is more nearly a continuum than a line spectrum due to the high-pressure broadening of the spectral lines. Thus the number of photons received by the perchlorate is unknown; all intensities are relative to the unfiltered lamp. It was found necessary to monitor the light output due to occasional voltage fluctuations. The low-temperature baths were ice and water for 0". For -20 and -40", CaCl2.6H20 plus ice were used in different proportions. In each case efficient stirring of the baths was achieved by bubbling helium through the solutions; in this way, the baths maintained constant temperature. All temperature measurements were made with a copper-constantan thermocouple.
Results Absorption spectra studies of nitronium perchlorate crystals grown from a nitric acid solution were made prior to the photochemical decomposition studies. After being grown, the crystals were cleaved, polished The Journal of Phyaical Chemiatry
J. N. MAYCOCK, V. R. P. VERNEKER,AND L. WITTEN
I
I
I
I
I
I
I
t
320 330 340 3!X 360 370 380 390 40C WAVELENGTH, A(rnF)
Figure 1. Absorption spectrum of a nitronium perchlorate crystal a t 25'. This crystal had been grown from a nitric acid solution of nitronium perchlorate.
in nitric acid, dried in vacuo, and then used for the absorption spectra studies. The room temperature absorption spectrum, Figure 1, was obtained by using a Cary Model 14 spectrophotometer. The main features of the absorption spectrum are that nitronium perchlorate is transparent in the visible region and has its absorption edge at about 340 mp. Prior to the irradiation experiments, possible decomposition products were considered to be oxygen, nitrogen oxides, and chlorine oxides. For this reason, methanol and solid C02 were used for the cold traps, since this cold-trap mixture would trap out all possible species except O2and NO. The rate of evolution of O2 and NO from the irradiation of nitronium perchlorate with the high-pressure lamp as a function of time is shown in Figure 2. It is apparent that this rate, after the initial periods of acceleration and deceleration, is constant with time. Long irradiations of the order of 8 hr have been performed with no change in rate, which appears to imply that the rate will remain essentially constant until all of the salt is completely transformed. It is also apparent upon careful investigation of the pressure vs. time curve, Figure 3, that there is, in every case, a small but definite induction period of the order of 2 min. When a shutter is placed between the light source and the sample, photolysis immediately ceases and there is absolutely no evidence of any "dark rate" even a t 25". Upon reirradiation of the same sample, the same constant first rate is achieved after an initial induction period consisting of only an acceleratory period to this constant rate; no deceleratory period exists. We have also allowed an irradiated sample to stand under the gaseous product atmosphere for 18 hr, after which we pumped the atmosphere out and reirradiated the sample. This again produced no effect
PHOTOCHEMICAL DECOMPOSITION OF NITRONIUM PERCHLORATE
I
"'"I
3
I 0.346 :
8
4
12
16
20 24 28 TIME (MINS)
I=0.159
32
Figure 2. Rate of gas (On with a relatively small admixture of NO) evolution as a function of time from nitronium perchlorate being irradiated with a high-pressure Hg lamp at 25". The different intensities of irradiation were obtained by using neutral density filters; Z indicates relative intensity.
(RELATIVE INTENSITY 1'
+
NO) evolution as a function Figure 4. Rate of gas ( 0 2 of 1 2 : 0, after 2 min, Le. in the initial acceleratory period; o, after 6 min, i.e., a t the maximum of the rate us. time curves; A, after 20 min where the rate is constant until complete disappearance of the salt. ~~
Time of irradiation, min Activation energy, kcal mole-'
60-
I
2
4
6
I
8 10 TIME (MINSI
~
Table I: The Values of the Activation Energy, E, for the Photochemical Decomposition of Nitronium Perchlorate
LIGHT LIGHT OFF ON
B go$ ; 3 0
2109
1
1
I
I
0
2
4
6
Figure 3. "Stop-start" experiment showing the reproducibility of the pressure-time curve for photolytic decomposition at 25". The two slopes between 2 and 6 min are equal within the limits of reproduciblity of the data.
on the pressure curve, which reproduced the original data quite closely, implying that the gaseous products do not catalyze the decomposition processes (Figure 3). The rate of product formation both at long times and in the early stages of photolysis is proportional to the square of the intensity of the light. This dependence is valid over the temperature range 25 to -40" which has been investigated. A typical intensity dependence plot for 25" is shown in Figure 4, where the intensity is measured in thermopile emf output. The activation energies of the photochemical decomposition reactions were determined at various times and are displayed in Table I. As is evident from Figure 5, the temperature dependence of the rate of gas evolution obeys an Arrhenius type of plot quite accurately over the complete temperature range covered.
2
6
30
10.5
10.5
8.25
By a mass spectral analysis of the decomposition products not trapped out, it is clear that during a normal 30-min run the only observed species present are O2 and NO, whereas in a long run, 8 hr duration, the data showed that chlorine is also formed. However, a mass spectral analysis of an untrapped decomposition shows 02,NO, NO2, C102, OC1, and Cl2. Of the species observed, oxygen is by far the most abundant species, followed by nitric oxide and chlorine. The oxides of chlorine were only observed as very minor constituents. In all probability, the observed OC1 is due to the cracking of C102 in the mass spectrometer. We have concluded that NO is a primary product and not formed only by cracking of NO2 since the experimentally observed ratios of these two peaks do not agree at all with the known cracking pattern of NO2. Due to the low scanning speed of the mass spectrometer, the mass spectra in different portions of the decomposition rate-time curve was not determined. In an attempt to determine the threshold wavelength necessary for photolysis, we found that the intensity of the output from a Hilger-Watts Model D246/7 spectrometer was not sufficient to produce measurable photolysis. By filtering the output of the high-presVolume 71,Number 7 Juna lQbY
J. N. MAYCOCK, V. R. P. VERNEKER,AND L. WITTEN
2110
sure lamp using optical absorption filters, we were able to establish qualitatively the threshold wavelength necessary for photolysis. The detection of photolysis was achieved by mass spectrometrically gating on the oxygen, 02,peak. Table I1 shows that photolysis is considerable at wavelengths less than approximately 370 mp. This is in very good agreement with the absorption spectrum found for the crystalline nitronium perchlorate. Table 11: Analysis of the Threshold Wavelength (mp) Necessary for the Photolysis of Nitronium Perchlorate Corning filter
3-67 3-71 3-73 3-75 7-37
Wavelength cutoff, mP
Photolysis
-544 -460
None None None Very slight
~416 -370 (transmission a t 365 mp is < 0 . 5 % ) -315-390 (trans& sion band)
+
Figure 5. Log of the rate of gas (02 NO) evolution us. 10*/T(O K ) for the photochemical decomposition of pure nitronium perchlorate 6 min after onset of irradiation.
Considerable
Discussion The results of the present investigation indicate there are two photolytic processes taking place when nitronium perchlorate is irradiated with ultraviolet light. By an analysis of the results for the rate of gas production with respect to time (Figure 2), temperature (Figure 5 ) , and wavelength (Table 11) of the ultraviolet source, we may conclude that fresh, pure nitronium perchlorate is decomposed by light of wavelength less than 340 mp, at a rate which (i) initially increases to a maximum and then falls to a constant value, (ii) depends in each region on the square of the intensity of the light ( I 2 ) ,and (iii) is associated with an activation energy of about 9 kcal mole-'. One possible explanation of the pressure increase during the initial stages of photolysis above the final steady-state rate is that it is associated with outgassing or desorption phenomena. Outgassing is very unlikely due to the long (18 hr) pumping treatment the sample has undergone. A desorption process is unlikely due to the fact that the initial acceleration in the rate-time curve is reproduced if a photolyzed salt is pumped down and the photolytic process is started again; moreover the rate is proportional to 7 2 in this region. A further argument against a desorption mechanism is that, in our thermal decomposition study6using a time-of-flightiaw. spectrometer, we did not observe any foreign species's% +ny time during the decomposition. This is contrary to the The Journal of Physical Chemistry
4.1
37 lC?/ T O K
33
work of cor de^.^ Our experimental technique differed from that of Cordes in that he condensed the gases and then analyzed, whereas we analyzed by performing the decomposition directly in the analyzing mass spectrometer. The observed decreasing rate of photolysis at about 10 min indicates that the photochemical reaction proceeds at centers or traps which are being consumed. A similar phenomenon has been observed in the aaides16Jin which case the traps have been postulated to be impurity ions or point defects. The data can be qualitatively understood by postulating that two processes or mechanisms, releasing 0 2 and NO, are taking place simultaneously, each mechanism being dependent on 12. The first mechanism occurs quite rapidly and is responsible for the initial fast reaction rate. However, this mechanism depends on the existence of impurity or trapping centers which are consumed in carrying out their part of the reaction mechanism. Hence, this mechanism causes a rapid initial increase of the reaction rate and then dies out, at which time a second reaction process takes over. This second reaction process does not depend on impurity trapping centers and continues presumably until the crystal itself is consumed. The first mechanism proposed is
clod-
hu
*
(c104-)
(6) P. W. M. Jacobs, F. C. Tompkins, and D. A. Young, Discussions Faraday SOC.,28, 234 (1959). (7) P. W. M. Jacobs, F. C. Tompkins, and V. R. P. Verneker, J . Phye. Chem., 66, 1113 (1962).
PHOTOCHEMICAL DECOMPOSITION OF NITRONIUM PERCHLORATE
(c1@4-)*
+T
-
(TC104-)*
hv
c104(TC104-)*
+ (C104->*
2NO2+
(C1O4-)
T
*
+ Cl2 + 402 + 2e-
+ 2e- +2N02 + 2 vacancies
A perchlorate ion is excited by an incoming photon. Ordinarily, it will decay after some time into its ground state; however, in the vicinity of a trap (electron acceptor), it may form a complex with the trap in which the electron is partially or completely transferred t o the trap. The perchlorate-trap complex then reacts with another excited perchlorate ion to give the reaction products indicated above; the free electrons are now absorbed in some fashion. The amount of oxygen evolved is proportional to 12. The trap which participated in the above mechanism is now associated with two anion vacancies and the crystal assumes different properties in the neighborhood of the trap.’ In this new neighborhood, the trap can no longer participate in the mechanism outlined, the number of active traps dtweases as time goes by, and the photolytic rate due to this mechanism decreases. The rise and decline of this mechanism may account for the initial acceleratory and deceleratory parts of the ratetime curve. The observed activation energy of about 9 kcal mole-’ is in reasonable agreement with the values reported for the breakup of similar trap complexes
2111
in silver azide.s The two electrons placed into the conduction band by this process are eventually captured by other traps or more probably by nitronium ions, NO2+. The resultant NO2may be photolyzed directly by the incident radiation, may evolve as a gas, or remain as an impurity in the crystal lattice. During this whole period of time, a second process has been acting slower than the mechanism described but with no self-consuming features. This process can be represented in the following way hv
NOz+ E (NOz+)* hv
c104(NOz+)*
(clod-)*
+ (C104-) * +NO + C10 + 202
Again, the amount of oxygen plus 90 evolved is proportional to 12; the process proceeds until the nitronium perchlorate crystal itself is consumed. Evidence for the existence of two such complementary mechanisms was observed experimentally since a second photolysis of an already photolyzed sample exhibited no deceleratory period. The absence of any dark rate or gas evolution on warmup leads us to deduce that we are not invoking any free-radical photolytic mechanisms in the temperature range studied. ~
~~
(8) F. P. Boden and A. D. Yoffe, “Fast Reactions in Solids,” Butterworth and Co. Ltd., London, 1958, p 96.
Volume 71, Number 7 June 1967
