Jndian Journal of ChemistryVol. 16A, February 1978, pp. 109-111

Role of Transition Metal Fluorides in Ammonium PerchlorateDecomposition

K. C. PAUL & V. R. PAl VERNEKERHigh Energy Solids Laboratory, Department of Inorganic & Physical Chemistry

Indian Institute of Science, Bangalore 560012and

S. R. JAINPropellant Chemistry Laboratory, Department of Aeronautical Engineering

Indian Institute of Science, Bangalore 560012

Received 8 June 1977; accepted 3 August 1977

The effect of various transition metal fluorides on the thermal decomposition of ammoniumperchlorate (AP) has been studied using isothermal thermogravimetry (TG) and differentialthermal analysis (DTA). The overall decomposition of AP is sensitized. A similar behaviour isobserved when transition metal perchlorate amrnines are used as additives. The observedcatalytic activity of the various metal ions appears to be in the order, Co2+>Mn2+>Zn2+>Cu2+> Ni2+as shown by isothermal studies at 230°. The transition metal oxides appear to have bettercatalytic activity than the corresponding fluorides.

THE thermal decomuosition of ammoniumpercblorate (AP) is known to be catalysedby the addition of certain metal salts- .. This

catalytic behaviour of metal IOnShas been attributedto the formation of metal-ammonia complexess.",which facilitate proton transfer step in the decom-position of AP. Dauerrnan'' bas 'predicted theeffectiveness of metal Ions III catalysing AP decom-position to be in the order, transition metals z-alkalineearths> alkali metals. Earlier, we reported4,5 tbeeffect of various metal perchlorate amm ines onAP decomJosition. From tbese studies it appearedthat the catalytic activity of metal salts could beexulained in terms of tbe format ion of metalperchlorate amrnine intermediates. Recently,Glaskova? has reported that metal fluorides inhibitAP combustion at high pressures. However, ourstudy on the role of lithium and ammonium fluor!deson AP decomiosition? showed that these fluoridescatalyse AP decomposition. This was rather un-exoected considering the fact that the condensedphase reactions do play an important role in theAP combustion processs-". The nresent studyreports the role of transition metal fluorides on APdecom iosition. The results have been comparedwith those on metal perchlorate ammine-Af' (1 :10)mixtures.

Materials and MethodsThe metal fluorides were prepared by deammoniat-

ing the freshly prepared metal fluoride arnmines asdescribed earher-''. The preparation of metal per-chlorate hexarnines has already been reported-t.The DTA and TG experiments were carried outusinz equipments described earlierll. All experi-ments were carried out in air using platinum sampleholders. The heating rate employed was 12°jmin:in the case of DTA. The kinetic studies were carried

out using metal fluoride-AP (1 :10, by weight)mixtures and metal perchlorate ammine-Al> (1 :10,by weig ht) mixtures. Combustion studies of metalfluoride-AP mixtures were carried out by heatingthe mixtures in a test-tube and analysing theresidues qualitatively.

Results and DiscussionThe ignition temperature of metal fluoride-AP

mixtures were determined by DTA and are presentedIII Table 1. The ignition temperature of metalperchlorate ammine-Af', and metal oxide-AP (1 ;10by weight) mixtures are also included for comparison.The DTA of pure AP shows an endotherm at 240°corresponding to orthorhombic to cubic phasechanve, an exotherrn at 3000 attributed to partialdecomoosition and another exotherm at 400° dueto the complete decomposition. In the presence?f metal fluorides the decomposition takes placeIII one step only, and this too in the temperaturerange 280-310°. The metal fluorides thereforeact as catalysts. However, the ignition tempera-ture of metal fiuoride-AP mixtures are highercompared to the corresponding metal oxide-APmixtures. The thermal stability of the metal per-chlorate amrnines and metal salt-AP mixtures in-creases in the order, C02+~Cu2+<Mn2+<Ni2+<Zn2+.

The isotherms of metal fluoride-AP mixtureswere obtained at 2300 (in the orthorhombic regionof AP), and the fractional decomposition (IX) versus~ime V) curves are shown in Fig. 1. In all tbe cases,Includllll? pure AP, an induction period of about50 mill IS observed before any appreciable decom-position of AP begins. The extent of AP decom-position in the presence of metal fluorides over thesame period (250 min) being of the order: CoF2(95%).. MnF2 (90%), ZnF2 (82%), CuF2 (43%)and N1F2 (30%) (pure AP, 25%), the catalyses of

109

INDIAN J. CHEM., VOL. 16A, FEBRUARY 1978

CoTABLE 1 - IGNITION TEMPERATURES (DC)

Metal Metal Metal Metal Metal(M) perchlorate perchlorate oxide-AP fluoride-Al>

arnminc ammine-AP (1: 10) (1: 10)(1: 10)

Co 222 240 240 282Cu 222 258 258 282Mn 234 265 265 288Ni 240 280 280 307Zn 330 307 307

(307 m.p.)

AP decomposition by the metal ions is shown to bein the order: C02+ > Mn2+ > Zn2+ > Cu2+ > Ni2+.Zinc fluoride however appears to be an anomalouscase; although the overall effect is that of sensit i-zation, the decomposition curve initially showshigher catalytic activity than those of COF2 andand MnF2, which subsequently gets lowered.

The fact that metal fluorides react Chemicallywith AP was shown bv combustion studies on metalfluoride-AP mixtures." On heating all the mixturesignited and gaye solid residues in addition to a whitesublimate and a yellow condensate on the coolerparts of the test tube. The white sublimate wasfound to be NH4F and the yellow condensate to beHCI. The solid residues were identified by theircolour, solubility awl other qualitative tests, e.g.CuFz-AP mixture gave a yellow solid, which washighly hy.:sroscopic, soluble in water and gavepositive tests for Cuz+ and CI-. This shows thatCuF2 reacts with AP when heated to give CuClz asthe combustion product. The combustion productof copper perchlorate ammine is also CuCI2. [Earlier

'-0

o 8~co+-.~ 0'6Q.

EouOJ

'"

_-*------0 N iF2_--------AP

02

O~~~ __ ~ __ ~ ~ __ ~ __ ~~~o 100 150 200 250 300

TIME (mini

Fig. 1 - Isothermal TG curves of metal fluoride-AP (1: 10)mixtures at 2300

110

I Zn

IAP-' ----

O~~~~~~~ __ ~ ~ __ L-__L-~o 25 50 75 100 125 150 175

TiME Im in I

Fig. 2 - Isothermal TG curves of metal perchlorate amrnines-AP (1: 10) mixtures at 2300

we reportec.t- that copper perchlorate amrnir.e c'e-corm oses to CuO. However. it is known that CuClzvets converter; into CuO (ref. 12) when heated in air].Sirnilartv the combustion rrocucts in the «'·'e· ofMnF2-AP, CoFz-AP. NiF2-AP and ZnFz'AP mixtures(1:10) were identified as Mn02; C0203. C0304; NiO;and ZnO respect! vely,

The isotherms of metal perchlorate ammine-Af"~lxtures at 2300 are shown in Fig. 2. Except forzinc ,salt, the decomposition of AP begins wi t hou!any induction period. The extent of AP decom-position oyer the same jeriod (150 min) is Co(100%), Mn (98%), Zn (70%), Cu (40%) andNl (30%) (pure AP, 20%), The zinc perchlorate

. arnmine-Al> mixture has an induction period ofabout 60 min: This is probably due to its highthermal stabil itv (rn.p. 307°, decomp., 330°). How-ever, here again the obscr x eel catalyses of AP de-composition by various meta! ions is in the order:~02+ > Mn2+ > Zn2+ > Cu2+ > Ni2+, The similarityIII the decomposition behaviour and evidence ofchemical reaction between the fluorides and AP incombustion experiments, indicate that the catalyticact ivi ty may be due to the formation of metal:'l-mmine perchlorates 25 intermediates, as postulatedIn the case of metal oxidess-". It is interesting tonote tha t the overall order of reactivity remainsunaltered when these meta'! oxides are used as ca ta-Iysts in AP decomposi ti on13-15.

A careful. examination of the catalytic activityof the transit ion metal fluorides and oxides on APdecomposition reveal certain subtle differences.(a) The ignition temperatures of metal fluoride-A]"mixtures are higher than the metal oxide-Af"mixtures (Table 1). (b) In the orthorhombic revionof, AP, metal.oxides13,14 catalyse AP decompositionWithout any Induction period, whereas with metalfluorides an induction period of about 50 min is.observed (Fig. 1).

PATIL et al.: DECOMPOSITION OF METAL FLUORIDE-AMMONIUM PERCHLORATE MIXTURES

Thus, it appears that transition metal oxides arebetter catalysts than the corresponding metalfluorides. This lower reactivity of the fluoridescould be explained by understanding therole played by the F- ions in AP decomposition.Assuming that AP decomposition is controlled by itsdissociation,

~H4Cl04~NH:+CI04~NH3+HCl04the role of F- ions could be visualized as follows.Due to the strong affinity of F- ions for the protons,the proton transfer step of AP decomposition isfacilitated with the preferential formation of HFand rnetal-ammines complex,

2NH4CI04 + MF 2-+M(NH3W +2CI04 +2HFthe formation of HF inhibits or suppresses theformation of HCI04 leading to the delay in theignition temperature. This may also explain theobserved induction periods in isotherms of metalfluoride-AP mixtures.

Although the order of catalytic activity of varioustransition metals in the AP decomposition is thesame with the fluorides, oxides or the perchlorateamrnines, it is difficult to correlate the DTA ignitiontemperatures and isothermal TG results. Theignition temperatures show the order of reactivityto be, C02+~CU2+ > Mn2+ > Ni2+ > Zn2+. Thistrend is not observed in the isothermal runs, whereCu2+ shows lower activity than ME2+, and Zn2+considerably higher activity than Cu2+ or Ni2+ ions.Also, the decomposition of AP is incomplete whenCu2+ and Ni2+ are used as catalysts. The observedtrend in the isothermal runs can not be explainedeither in terms of thermal stability or in terms ofinstability constants of the first row transition metalammines-P as predicted by Dauermans. It is tobe noted, however, that whereas the isothermal

decomposition has been carried out in the ortho-rhombic region of AP, the observed ignition tempe-ratures are all in the cubic region. Copper andnickel salts are good catalysts in the cubic regionbut not so in the orthorhombic region. It, there-fore, appears as shown by Shidlovskii ei at.15 thatcertain catalysts which may catalyse AP decom-position in the high temperature (cubic) region maynot act as good catalysts in the low temperature(orthorhombic) region.

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