COMBUSTION AND FLAME 33, 311-314 (1978)
Mechanism of Catalytic Activity of Transition Metal Oxides on Solid Propellant Burning Rate K. KISHORE and M. R. SUNITHA High-Energy Solids Laboratory, Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-12, India
Transition metal oxides are known to promote the thermal decomposition of ammonium perchlorate (AP) [1, 2], AP deflagration [3-5], AP-binder sandwich burning [6, 7], and the combustion of AP based on composite solid propellants [8-10]. Many attempts have been made in the past to understand the mechanism of the action of these catalysts on decomposition and combustion, but no clearcut picture has emerged so far particularly on the latter process. The effect of the oxide catalyst on the thermal decomposition has been explained as follows [12, 13]. (1) Semiconducting properties (degree o f p or n typeness) of the catalysts are related to the catalytic activity. (2) Charge transfer process (electronic/ionic/ vacancy diffusion) is promoted. (3) Redox cycle promotes the high temperature electron-transfer process. With respect to (1) most oxides that are catalytically active for perchlorate decomposition are of p-type, but FezO a is of n-type , and MnO2 is doubtful . On this ground the relationship of the semiconducting properties to the catalytic activity does not appear to be fully established. Rudloff and Freeman  have measured the electrical conductivity of various transition metal oxides and potassium perchlorate mixtures and have found that the activation energies (E) for the conductance are between 10-20 kcal mole - 1 , showing that the conductance cannot be purely Copyright ~91978 by The Combustion Institute Published by Elsevier North-Holland, Inc.
electronic and is associated with the ionic transport process or the vacancy diffusion process. The conductance data on AP + transition metal oxide mixture are not available in the literature, but Owen et al.  have shown that there is a similarity of electrical conductance between AP and other isomorphous alkali metal perchlorates like KC1Oa, RbC104, and CsC104. It may therefore be inferred, as observed in KC104 + transition metal oxide mixture , that the charge transfer mechanism cannot be visualised clearly in the AP + transition metal oxide mixture either. Recent studies by Kishore et al.  on MnOz-catalysed AP decomposition have revealed that the catalyst promotes the electron-transfer process of the AP decomposition, showing that the Redox cycle acts as a bridge for the transfer of electrons. Unpublished results of Kishore et al.  have shown that this also happens in Fe203, Ni208, CozO3 catalysed decomposition of AP. Due to the multivalent nature of these oxides the mechanism of catalysis as the promotion of the electron-transfer process through the redox cycle seems to be most plausible. It is believed that the catalysts which accelerate the AP decomposition enhance the burning rate of AP-based composite propellants, but no concrete proof of the same is available. Recently Pai Verneker et al.  have shown that the thermal decomposition of the AP is related to the thermal decomposition and the burning rate of the APbased composite propellants. If this relationship is valid then the mechanism of the AP-based propel-
K. KISHORE and M. R. SUNITHA 1.6
Redox potentiol (Volts)
3. Fe20-3 4.
A N r (Kcal. mole"1 ot 208°K)
Fig. 1. Dependence of'rbe/rbu on Redox potential and AH r. lants could be explained in a similar way to that of the catalysed AP decomposition. If this is so then the Redox potential of the oxides, which is a measure of the extent of the facilitation of the electron transfer process, should be correlated with the burning rate, i.e., lower the Redox potential the higher should be the burning rate. Figure l(a) very clearly shows the linear dependence of the redox potential and ~ e / ~ / b u , the burning-rate ratio (i.e., the ratio of the burning rate of the catalysed propellant to that of the uncatalysed one). The making o f the polystyrene/ AP(75%) propellant with and without the catalysts employed in this investigation and the determination of the burning rate at ambient pressure were accomplished as described earlier [19, 20]. Strands of 1 cm diameter and 6 cm length were used for burning-rate measurements. Figure l(a) also suggests that the ease with which the valence states of the transition metal oxides change should be correlated with the catalytic activity. The relevant heats of reaction (z2ff/r) have been
calculated as follows. Thermochemical data [21, 22] used in the present work are given in Table 1. (1/2)CrzOa + (3/4)02 -~ CrOa ;
A H r = 2.4
(1/3)Fea04 + (1/12)02 ~ (1/2)Fe20a ; A H r = 8.9 (1/2)Mn2Os + (1/4)O2 -+ MnO2;
CoO + (1/6)O2 -+ (1/3)CoaO4 ;
AHr = 12.5
(1/2)Cu20 + (1/4)O2 -+ CuO;
AH,. = 15.1
Figure l(b)represents the plot of the burning-rate ratio to that of zSJ/r (heat of reaction per gram ion in lower valent oxide), showing that the two quantities are related to each other. This gives credence to the belief that the burning rate enhancement is due to the promotion o f electron-transfer process of the oxidiser through the redox cycle. Figure l(b) could have been better appreciated if more points had been there, but due to the lack of thermodynamic data this was not possible.
BRIEF COMMUNICATIONS TABLE 1 Heat of Formation (AHf) and Redox Potential Data 1. AHf (kcal mole- 1 ) at 298°K in solid phase Cr203 02 CrO 2 Fe20 3 Fe304 Mn203 MnO 2 CoO Co30 4
= = = --= = -=
271.0 2.8 140.0 196.3 267.0 229.0 124.4 56.0 207.0
Cu20 CuO Ni(C 104) 2-2H20 Mn(C 104) 2 • 2H20 Co(CI04)2"2H20 Fe(C104)2"2H20
40.4 36.0 252.8 294.8 269.2 263.8
2. Standard or Redox potentials (volts) at 298°K Half reactions Cr2072-- + 14 H + + 6 e - --- 2Cr 3+ + 7H20 Fe(III) + e--= Fe(II) NiO2 + 2H20 + 2e-- = Ni(OH) 2 + 2OH-MnO 2 + 4H + + 2e-- = Mn 2+ + 2H20 Cu 2+ + e-- = Cu + Co(llI) + e-- = Co(II)
The catalyst action m e c h a n i s m could also be explained on the basis o f the f o r m a t i o n o f metal perchlorate intermediates. The metal perchlorates, being more thermally unstable, m a y bring d o w n the d e c o m p o s i t i o n threshold energy. The heat-off o r m a t i o n data o f these metal perchlorates (ease w i t h which these metal perchlorates are f o r m e d ) are given in Table 1. The d i h y d r a t e d f o r m o f the metal perchlorate has been q u o t e d because the propellant during d e c o m p o s i t i o n and c o m b u s t i o n gives o f f water vapour and also because the dihydrate is thermally m o r e stable. The heat-of-formation data o f metal perchlorates presented in Table 1 show that values for Ni, Fe, and Co are in the same range and do n o t indicate any perceptible t r e n d as observed for R e d o x potentials. REFERENCES
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Received 20 November 1977; revised 23 March 1978