Some Aspects of Catalysis of the Thermal Decomposition of Ammonium Perchlorate

R. N. Isaev, Yu. A. Zakharov

(Presented by the scientific seminar of the Department of Radiation Chemistry)

A considerable number of studies has been devoted to questions of the catalysis of the thermal decomposition of ammonium perchlorate (AP) by heterogeneous additives (see, for example, [1, 2]).

In our previous work [3], on the basis of mass-spectrometric analysis of the gaseous products of AP decomposition, derivatography, and DTA of mixtures of additives with AP, it was proposed to divide the additives studied into three groups, depending on the nature of their influence on thermal decomposition.

In accordance with this classification, the first group of catalysts consists of oxides of certain metals of variable valence ($Co_2O_3$, $Ni_2O_3$, apparently other oxides of iron, cobalt, and nickel) and $Cr_2O_3$ [2], which do not interact chemically (in the solid phase) with AP.

The second group includes additives that react with perchlorate (among those tested, $CaO$ and other oxides and salts of alkaline-earth metals). The third group of additives includes oxides whose effect on the rate of AP thermal decomposition is not due to a solid-phase reaction with it, but to other causes. Among those studied, these include $ZnO$, $CdO$ [4–6]. As a result of the interaction of AP with additives of the second and third groups, metal perchlorates are formed [3, 6]. The reason for the acceleration in this case of AP decomposition is considered to have been clarified [5, 6] and is associated with the formation of a readily melting eutectic in the AP–metal perchlorate system, i.e., with the transition of AP thermolysis into the melt. Along with the stated explanation, there are apparently also other reasons for the observed changes in the rate of thermal decomposition (for example, the polarizing power of the cation [6]) by additives of these two groups.

Obviously, the mechanism and causes of the effect of additives of group I on the rate of thermal decomposition of AP must be different, not connected with chemical interactions in AP–additive systems.

In a number of works on the thermal decomposition of AP it is considered that the rate-limiting stage of AP decomposition is the decomposition of \(HClO_4\) [7], formed as a result of proton transfer [8]:

\[ NH_4^+ + ClO_4^- \longrightarrow NH_3 + HClO_4 . \]

In this connection it is reasonable to suppose that additives which very significantly (by tens or even hundreds of times) increase the rate of thermal decomposition of AP affect precisely this (rate-limiting) stage of the process.

The acceleration of the thermal decomposition of \(HClO_4\) by inorganic semiconductors was indeed discovered recently in the works of Koroban [9] and Polyak [10]. In work [9] the decomposition of 70% \(HClO_4\) was carried out in a closed volume, whereas in [10] the acid decomposed in the gas phase; the range of catalysts tested in these studies is therefore very limited.

In view of the above, in the present work we carried out an initial study of the catalytic decomposition of an \(HClO_4\) solution on various catalysts (containing oxides of all three groups) and under conditions analogous to those used in work [3].

First of all, experiments on the decomposition of mixtures of AP with additives in the ion source of an MI-1305 mass spectrometer showed that the amount of perchloric acid in the decomposition products of AP mixtures with additives is considerably smaller than in experiments on the thermal decomposition of pure AP. Thus, the intensity of the \(HClO_4\) peak in the mass spectra of the decomposition products of a mixture of 90% AP + 10% \(Co_2O_3\) is approximately 50 times, in a mixture of 90% AP + 10% \(ZnO\) approximately 6 times, in a mixture of 90% AP + 10% \(CdO\) approximately 5 times, and in a mixture of 90% AP + 10% \(CaO\) approximately 4.5 times lower than the intensity of the acid peak during decomposition of pure AP under analogous conditions.

Thus, these data already indicate catalysis by additives of the thermal decomposition of \(HClO_4\).

The decomposition of a 57% solution of \(HClO_4\) was studied by us in the apparatus described in [11]; the gaseous products of acid decomposition were analyzed on an MKh-1303 mass spectrometer. The decomposition was carried out at a residual air pressure of 205 mm Hg and a temperature of \(220^\circ C\).

The specific surface area of the additives was measured by the method of thermal desorption of argon in the gas-adsorption laboratory of the Institute of Catalysis, Siberian Branch of the USSR Academy of Sciences.

The results presented in Fig. 1 show that all additives accelerate the thermal decomposition of \(HClO_4\) to varying degrees. At the same time, the catalytic activity of the additives in the reactions of thermal decomposition of \(HClO_4\) and thermal decomposition of AP is found to be the same for the series of catalysts studied and decreases with decreasing values of the specific surface areas of the oxides:

\[ Co_2O_3 > ZnO > Cd > CaO \]

\[ S_{\mathrm{ud}},\ \mathrm{m}^2/\mathrm{g}\quad 23.6\qquad 17.2\qquad 7.2\qquad 3.2 \]

The results presented show that the magnitude of the specific surface area of an additive plays an important role. Possibly, it is precisely with the fact of catalysis by additives of the decomposition of \(HClO_4\) that the facts observed in [11] of “remote” catalysis by additives of the thermal decomposition of AP and the dependence of catalytic activity on the magnitude of the specific surface area of the additives are connected.

It follows from Fig. 1 that the thermal decomposition of \(HClO_4\) is also catalyzed by the surface of AP. This latter point indicates the heterogeneity of the course of the limiting stage of AP thermolysis. It may be considered, in a certain approximation, that one (possibly the principal) cause of the acceleration by additives of AP decomposition consists in the introduction into the system of the surface of a catalyst whose activity in the reaction of thermal decomposition of AP is higher than the activity of the surface of the perchlorate itself.

It is obvious that during the thermal decomposition of mixtures of AP with additives of groups II and III, the catalytic decomposition of \(HClO_4\) on the additive takes place only until it is completely consumed in the solid-phase reaction with AP. The data of Fig. 1 show that catalytic decomposition proceeds in this case as well. However, it cannot be the cause of the observed acceleration of the thermal decomposition of AP,

Fig. 1. Effect of additives on the thermal decomposition of a perchloric acid solution.

1. \(\mathrm{HClO_4} + 10\%\,\mathrm{CaO}\)
2. \(\mathrm{HClO_4} + 10\%\,\mathrm{CdO}\)
3. \(\mathrm{HClO_4}\)
4. \(\mathrm{HClO_4} + 10\%\,\mathrm{NH_4ClO_4}\)
5. \(\mathrm{HClO_4} + 10\%\,\mathrm{ZnO}\)
6. \(\mathrm{HClO_4} + 10\%\,\mathrm{Co_2O_3}\)

because the catalytic activity of these additives in the reaction of thermal decomposition of \(NH_4ClO_4\) is lower than the activity of the surface of the ammonium perchlorate itself.

Analyzing, in the general case, the question of possible causes of catalysis of defect formation by semiconductors in the thermal decomposition of ammonium perchlorate, it should be pointed out that, along with the ionization effect, acceleration of the addition reaction of ammonia with the products of thermal decomposition of perchloric acid may also be caused by the oxidation process [3].

A third possible cause of the catalytic influence of additives that has not been analyzed sufficiently up to the present time may consist in the formation, at the boundary of the ammonium perchlorate section—additive—in the process of heating or even during storage of solid solutions. The latter, as is known, can change the rate of thermal decomposition of ammonium perchlorate, for example for the reasons set forth in [13].

But, finally, the possibility cannot be ruled out that the additive catalyzes the process of direct interaction of the thermal-decomposition products of \(HClO_4\) with ammonium perchlorate, for example according to the scheme [9]:

\[ NH_4ClO_4 + Cl_2O_6 \rightarrow NH_4ClO_3 + Cl_2O_7 . \]

Higher chlorine oxides were found by us by mass spectrometry in small amounts in the decomposition products of pure ammonium perchlorate and its mixtures with additives.

References

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