EFFECT OF THE COMPOSITION OF THE GAS ATMOSPHERE ON THE THERMAL DECOMPOSITION OF IONIC SALTS

V. F. KOMAROV, V. V. BOLDYREV, V. T. SHIPATOV, V. V. NAKHALOV,
R. I. AKIMOVA

(Presented by the scientific seminar of the sector of radiation-chemical analysis
of the Scientific Research Institute of Nuclear Physics)

The decomposition of ionic salts in the presence of a gas atmosphere may be accompanied by participation of this atmosphere in individual elementary acts of decomposition. Depending on the mechanism of the decomposition process, the gas may take part in the following stages:

1. The gas may alter the catalytic activity of the solid reaction product by changing its crystal structure or by entering with it into oxidation-reduction reactions.

2. The gas may participate in the chemical reaction, changing the equilibrium concentration of products in reversible processes, or changing the conditions of stationarity of the process, or, finally, changing the heat balance of the system.

3. The gas may participate in electronic processes occurring in the solid during heating by changing the concentration of electrons.

The last case is of special interest, as the least studied.

The possibility that a gas participates in electronic stages occurring in a solid substance during heating is based on the following. If, upon heating a solid, reversible transfer of electrons occurs from an anion to a cation or to an intermediate trap of the type of an impurity ion or vacancy, followed by irreversible decomposition of one of the constituents, then the lifetime of such an excited state may determine the rate of the process as a whole. In this case the lifetime of the excited state can be affected by changing the concentration of free electrons in the substance. Chemisorption of gases on the surface of the solid substance or of the product of its decomposition leads to such changes [1]. In the first case the effect will be exerted directly through a change in the equilibrium concentration of free electrons in the solid substance, and in the second—through a change in the catalytic activity of the solid reaction product according to the mechanism [2—3].

How well these ideas agree with experiment will be considered in the present communication using as examples the thermal decomposition of silver and potassium permanganates, and of ammonium perchlorate and chlorate.

Experimental Part

All the experimental data presented below on the kinetics of decomposition were obtained by the thermogravimetric method at atmospheric pressure.

in a stream of the corresponding gases at temperatures of 300–350°C for ammonium perchlorate, 65–80°C for ammonium chlorate, 200–220°C for potassium permanganate, and 100–120°C for silver permanganate. The processing of the experimental results for calculation of the kinetic parameters was carried out according to equations [4–5].

\[ \alpha = 1 - \exp[-\kappa t^n], \]

\[ K = n\kappa^{\frac{1}{n}}. \]

By isothermal decomposition of silver permanganate at 115°C it was established that the reaction rate decreases in the series of gases \(O_2 > N_2 > CO_2\). If the reaction

\[ \mathrm{MnO_4^-} \mathop{\rightleftarrows}^{\kappa_1}_{\kappa_2} \mathrm{MnO_4^0} + \bar e \quad \begin{matrix} \\[-0.6em] \downarrow \kappa_3 \end{matrix} \]

\[ \mathrm{MnO_2} + \mathrm{O_2}, \]

is now taken as the leading stage of the decomposition process of silver permanganate, then, taking into account the stationarity of the concentrations, we arrive at an expression for the reaction rate in terms of the yield of one of the products

\[ \frac{dx}{dt\,(O_2)} = \frac{\kappa_1 \cdot \kappa_3 \gamma}{\kappa_2[\bar e] + \kappa_3}, \tag{1} \]

where

\[ [\bar e] \text{ is the concentration of free electrons;} \]

\[ \gamma \text{ is the concentration of ions on the surface of the reaction zone.} \]

Then the increase in the reaction rate in the series of gases \(O_2 > N_2 > CO_2\) is consistent with their donor–acceptor properties. The basis for adopting the above mechanism of decomposition of silver permanganate was provided by data obtained by us through direct measurement on the presence of electronic transitions in the system ZnO—decomposition product of silver permanganate—\(\mathrm{AgMnO_4}\), from the decomposition product to ZnO and \(\mathrm{AgMnO_4}\), and by the relationship of the electron work functions \(\varphi_{\mathrm{ZnO}} > \varphi_{\mathrm{pr}} \leq \varphi_{\mathrm{AgMnO_4}}\), as well as by the fact that the decomposition process is accelerated by zinc oxide, which decreases the electron concentration in the decomposition product, thereby inhibiting the reaction [6]. Although \(\mathrm{AgMnO_4}\) also has \(n\)-type conductivity [6–7], the effect of the gases on the decomposition process is not accompanied by an analogous effect of their influence on electrical conductivity. Thus, we did not detect a significant difference in the resistance of the samples in vacuum, nitrogen, and oxygen, while only an increase in the activation energy from 1.60 eV in vacuum to 2.2 eV in gases was noted (temperature interval 55–90°C). The former is possibly connected with compensation of the number of charge carriers by impurity ions of manganese of lower valence and by generation of the latter during heating.

In addition to influencing the rate of the process, the gas also affects the temperature dependence of the reaction rate. Thus, if in a nitrogen atmosphere the process is described by the equation \(K = 10^{14.5}\cdot e^{-29100/RT}\), then in an oxygen atmosphere it is described by the equation \(K = 10^4 \cdot e^{-9200/RT}\). In the presence of gases, the catalytic activity of heterogeneous additives also changes (Table 1). The latter is possibly connected with the difference in the temperature course of the work functions of the electron of the additive and of the substance in the presence of gases.

The decomposition of ammonium perchlorate can also be described by equation (1) [8], which gives grounds, as in the case of silver permanganate, to expect an increase in the reaction rate in the presence of electron-acceptor gases and a decrease in the presence of electron-donor gases. Experiment confirmed that the decomposition rate of ammonium perchlorate increases in the presence of \(O_2\), NO, and \(NO_2\) and decreases in the presence-

in the form of CO, CO₂, and N₂O. There are grounds to believe that such a division of gases according to the effect of their influence on the reaction rate corresponds to their donor–acceptor properties. Thus, by infrared spectroscopy it has been established that, upon adsorption on ammonium perchlorate, nitrogen oxide becomes negatively charged, whereas carbon monoxide becomes positively charged [9]. In addition, it should be noted that gases that exert an accelerating effect on the decomposition of ammonium perchlorate possess paramagnetic properties, while those that retard it are diamagnetic.

Table 1

Change in the catalytic effect of a 5% additive on the decomposition of AgMnO₄ in different gases

Potassium permanganate decomposes upon heating by a mechanism similar to that of the two preceding substances, with the sole distinction that electronic transitions occur in the anion sublattice [10]. It is possible that this leads to a certain peculiarity in the influence of gases on the decomposition of potassium permanganate, expressed in the fact that, at temperatures below 205°C, a decrease in the reaction rate is observed in the series of gases O₂ > N₂ > CO₂, corresponding to the change in the donor–acceptor properties of these gases. The temperature dependence is described in the interval 200–220°C by the equation $K = 10^{14}\cdot e^{-36600/RT}$ in the case of decomposition in a nitrogen atmosphere, and by the equation $K = 10^{10}\cdot e^{-27600/RT}$ for decomposition in an oxygen atmosphere. However, as in the case of silver permanganate, measurements of the electrical conductivity of potassium permanganate in atmospheres of different gases and in vacuum did not establish differences in the resistances of the samples up to the temperatures at which decomposition begins. The activation energy of electrical conductivity in vacuum proved to be 2.81 eV and 2.48 eV in gas atmospheres (temperature interval 125–180°C).

Raising the decomposition temperature of KMnO₄ to 210°C led to qualitatively different results, presented in Table 2,

Table 2

Influence of gases on the decomposition of potassium permanganate at $t = 210^\circ\mathrm{C}$

in which the first row gives data on the change in the decomposition rate in gases relative to the rate in nitrogen, the second row gives the relative change in the diameter of the gas molecule, calculated from the equation for the viscosity of gases at the temperatures of the experiments [11],

$$ \eta=\frac{1}{\pi\sigma^{2}}\cdot\sqrt{\frac{kTm}{\pi}} $$

and in the third line—the catalytic effect of the addition of 5% zinc oxide in an atmosphere of various gases.

It follows from the data in Table 2 that the rate of the decomposition reaction of KMnO$_4$ at 210°C decreases in the series of gases He $>$ Ar $>$ N$_2$ $>$ CO$_2$ $>$ O$_2$, which, with the exception of oxygen, agrees with the increase in the size of the molecules of the sorbed gases. Decomposition under vacuum leads to an increase in the reaction rate, compared with the rate in nitrogen, by a factor of 9.7. This character of the change in rate may be due either to a change in the diffusion conditions of the decomposition products or to some other, as yet unclear, causes.

The rate of thermal decomposition of NH$_4$ClO$_3$ at 70°C decreases in the series of gases CO$_2$ $>$ CO $>$ O$_2$ $>$ H$_2$ $>$ N$_2$ $>$ NO $>$ NO$_2$ $>$ C$_2$H$_4$, which is in no way consistent with the change in the donor–acceptor properties of these gases, although such properties are indeed manifested upon adsorption [9]. The activation energy of decomposition in accelerating gases increases from 18.5 kcal/mole in nitrogen to 25 kcal/mole in accelerating gases, whereas for decomposition in an ethylene atmosphere it remains unchanged.

The absence of a correlation between the influence of gases on the decomposition of NH$_4$ClO$_3$ and their donor–acceptor properties may be due to a peculiarity of the decomposition mechanism of this salt. According to [12], the decomposition of ammonium chlorate is ionic-radical in character and includes a stage of formation and consumption of H° radicals, whose deactivation leads to inhibition of the process. Agreeing with the proposed decomposition mechanism, the reaction rate can be expressed through a function of the concentration of hydrogen radicals.

\[ \frac{dx}{dt_{(\mathrm{O}_2)}} = f\{K, \gamma, [\mathrm{H}^{\circ}]\}. \]

The fact, known from radiation chemistry, of acceptance of H° radicals by molecules of the gases NO and C$_2$H$_4$ [13] coincides with the inhibiting effect of these gases.

Conclusions

1. Using the thermal decomposition of silver and potassium permanganates, and of ammonium perchlorate and chlorate, as examples, a substantial influence of the composition of the gaseous atmosphere on the reaction rate has been established. The effect of the gases depends both on the nature of the gas and on the mechanism of decomposition of the solid substance.

LITERATURE

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