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On the reaction between xenon and fluorine
J. inorg, nucl. Chem., 1974, Vol. 36, pp. 997 1001. Pergamon Press. Printed in Great Britain.
O N THE R E A C T I O N B E T W E E N X E N O N A N D F L U O R I N E * J. LEVEC, J. SLIVNIK and B. ~EMVA~ Faculty of Natural Sciences and Technology and J. Stefan Institute, University of Ljubljana, Lj ubljana, Yugoslavia (Received 30 April 1973) Abstract The influence of nickel (34-8 m2/g and 10-4 m2/g), silver (1.4 mZ/g), and magnesium (57.8 m2/g) difluorides and nickel(Ill) and silver(l) oxides on the reaction between xenon and fluorine was investigated. The experiments were performed using a xenon to fluorine mole ratio of I : 4.5, with a total pressure amounting to 36 atm as measured at 22°C while the reaction temperature was 120°C. In some cases the reaction proceeds in the presence of nickel and silver difluorides explosively. Similarly, the reaction proceeds explosively in the presence of small amounts (1 m-mote) of nickel(Ill) or silver(1) oxide at 0°C. The reaction can also be initiated by local application of heat (e.g. by the addition of small amounts, 1 m-mole, of elemental sulphur or simply by an electrically heated wire). The explosion phenomena indicate that the reaction can be considered as a homogeneous reaction.
INTRODUCTION A RECENTLY published study o f the reaction between x e n o n a n d f l u o r i n e [ l ~ ] , to form x e n o n fluorides, s h o w e d that the reaction is h e t e r o g e n e o u s a n d occurs on the walls of the metallic nickel reaction vessels or on the surface of a d d e d metallic fluorides. It was also s h o w n that nickel difluoride is the one of the m o s t effective catalyst[2,5]. T h e e x p e r i m e n t s were e x t e n d e d to survey the catalytic activity o f nickel, silver a n d m a g n e s i u m difluorides. In addition, the influence of silver(I) a n d nickel(III) oxides was investigated. The total pressure o f reaction mixture was m u c h greater t h a n in the p a p e r s r e p o r t e d previously[I-4]. Since the f o r m a t i o n of x e n o n difluoride p r o c e e d s with a measurable rate already in range b e t w e e n 110 ° and 130°C[5,6], the reaction rate was m e a s u r e d at 120°C. T h e course of the reaction was followed by m e a s u r i n g time dep e n d e n c e of pressure. O n the basis of the results o b t a i n e d the n a t u r e of the x e n o n - f l u o r i n e reaction u n d e r pressure is discussed here. EXPERIMENTAL
Materials Fluorine was produced and purified in this laboratory as described elsewhere[7,8]. Fluorine used in the experiments was 99-0 + 0.5~o pure. When a large amount of fluorine (5 atm in 100 ml container) was distilled from a container * Presented at 4th European Symposium on Fluorine Chemistry, Ljubljana, 28 August-1 September 1972. t J- Stefan Institute, University of Ljubljana, Ljubljana, Yugoslavia.
cooled with liquid nitrogen, the i.r. spectrum of residue showed only traces of CF4. Xenon was supplied by L'Air Liquide (Paris, France) in 99.5 ~ purity (the balance was krypton). Nickel difluoride with specific surface area of around 208 mZ/g was obtained from C.E.N. (Dr. Ehretsman, Saclay, France). Another sample of NiFE (13.1 mE/g) was produced by Koch-Light (Colnbrook, England). Magnesium difluoride with specific surface area of around 170 mZ/g was prepared as described elsewhere[9]. Before the experiments, the fluorides were conditioned by fluorine under pressure of about 30 atm at 120°C. The specific surface areas dropped : their values (B.E.T.) are shown in Table 1. Silver difluoride was prepared by the fluorination of silver(1) oxide (92.4~o Ag) with elemental fluorine[10]. The chemical analysis of the product was in good agreement with the calculated composition of silver difluoride (Table 1), but unfortunately its specific surface area was rather low, 1.4 m2/g. Nickel(IIl)(70-1% Ni) and silver(I) (92-4~ As) oxides were supplied by Riedel de l-l~ien (Hanover, W. Germany) in p.a. purity. Prior to use in the experiments, the oxides and sulphur were dried 2 hr in vacuum (10 -5 mm Hg) at 120°C. Apparatus Reactions were carried out in 100 ml argon-arc welded nickel pressure vessels with a Teflon packed brass valves (for work under pressure and vacuum--developed in this laboratory). The reaction vessels were tested hydrostatically to 210 atm pressure at room temperature. To reduce the dead volume of the reaction vessels, pressure transmitters with nickel bellows[ 11] in conjunction with oil-filled Bourdon gauges (160 mm dia. face, 0-60 atm ((~250 arm), VDO-OTA, Frankfurt/Main, W. Germany, accuracy _+0.3 atm (+ 1.5 atm)) were used for the measurements of pressure. The reaction vessel was immersed completely in the liquid bath. 997
998
J. LEVEC, J. SLIVNIKand B. ZEMVA Table 1. Chemical compositions and specific surface areas of difluorides after 24 hr fluorination (30 atm F2) at 120°C MF2
Calcd. comp. [ %]
Found. comp. E%]
Spec. surface area [m2/g]
NiF* NiF2t AgF2 MgF2
Ni 60.6; F 39.4 Ni 60.6; F 39.4 Ag 73.9; F 26.l Mg 39.0; F 61-0
Ni 60.4; F 39.3 Ni 60.2; F 39.0 Ag 73.9; F 25.4 Mg 36.1; F 58.0
34.8 10.4 1.4 57.8
* C.E.N., Saclay, France. t Koch & Light, Colnbrook, England. evolved were monitored by i.r. spectroscopy. At each of the mentioned temperature the weight of vessel was controlled and from the difference the quantity of volatiles were determined.
The gauges, complete with their pressure transmitters, were calibrated by an absolute method at reaction temperature. The corrected pressure values were used in the estimation of pressure differences. Reaction vessels were heated up to 120°C using a thermostated glycerine bath. Monel, 10 cm path length, gas absorption cells fitted with silver chloride windows, and a Zeiss UR-20 infrared spectrometer were used to obtain i.r. spectra of reaction products.
RESULTS In a series of experiments the same sample of silver difluoride was used. After the reaction was completed, the volatiles were p u m p e d away a n d a new x e n o n fluorine mixture was prepared in the reaction vessel. The pressure measurements in this series are shown in Fig. 1. The reaction in presence of 20 m-mole of silver difluoride proceeded explosively in the first experiment. A typical reaction procedure of that kind is shown in Fig. 2(a). After the reaction vessel has been immersed into thermostated b a t h the pressure of the gaseous mixture was increasing normally a n d reached the value expected for that particular temperature (49 atm). After 3 min, however, the pressure increased instantaneously to approximately 80 atm a n d then dropped to 28 atm. W h e n the excessive fluorine was pumped off, it was found out that practically all xenon was consumed
Procedure
130 m-mole of fluorine and 29 m-mole of xenon were metered with an adequate amount of each of the fluorides or oxides. The amount of fluorine and xenon was checked by weight and determined to + 1 mg. The mole ratio Xe:F2 was 1:4.5 and the total pressure measured at room temperature 36 atm. The reaction vessel was immersed afterwards into the thermostated bath which was kept at 120~C ( + I°C). As the result of the reaction between xenon and fluorine the pressure dropped. After the reaction stopped the reaction vessel was taken out and quenched to the liquid nitrogen temperature. The volatiles were separately pumped off at liquid nitrogen temperature, and at -80°C. Ultimately the reaction vessel was warmed to room temperature and the reaction products
First e x p e r i m e n t - e x p l o s i v e l y ,
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j~o
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• Fourth e x p e r i m e n t
T e m p e r a t u r e : 120°C Mole r a t i o : X e : F2= 1:4.5
' Fifth e x p e r i m e n t
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1.5
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2.5
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5.5
6-0
Time,
hr
Fig. 1. Reaction rate curves for five xenon-fluorine reaction runs in the presence of 20 m-mole of AgF2.
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The xenon-fluorine reaction
"presence of 6, 4 and 2 m-moles of nickel difluoride (34.8 mZ/g) (Fig. 2a). In these cases the explosive reaction courses were observed on the same sample even three times in sequence after the products of the proceeding reaction have been removed. Always a very good agreement in conversion of fluorine and the reaction course was observed. The reaction proceeded with a measurable rate always in the fourth run as is shown in Fig. 3. A very similar xenon-fluorine reaction course, that is approaching an explosion (Fig. 2b), was observed in presence of 40 m-mole of nickel difluoride with the specific surface area 10.4m2/g whereas the reaction proceeded with a measurable rate when only 20 m-mole of nickel difluoride were employed (Fig. 4). Unexplosive courses gave xenon difluoride in most part (about 80%--mass balance--and xenon tetra- and hexafluoride). The course of the reaction between xenon and fluorine in the presence of magnesium difluoride is shown in Fig. 5. Reaction shaped when xenon difluoride was formed. The yield was higher than 80~o, the rest of the product consisting of xenon tetrafluoride (without xenon hexafluoride). The explosive course of the reaction was observed also in presence of small amounts of silver(I) and nickel(III) oxides, 1 m-mole. The explosions occurred already at temperatures below 0°C. A similar reaction course was observed when the oxides have been replaced in same amounts by elemental sulphur. The reaction could be simply initiated by an electrically heated nickel wire in a mixture of fluorine and xenon (Xe:F2 = 1:4.5) at 36 atm as measured at 22°C. Experiment was carried out in 100 ml reaction vessel, equipped with special nickel wire electrical heater; 35 cal was required to initiate explosion. All explosive courses gave as principal reaction product xenon tetrafluoride (about 80~o-mass balance--the rest of the product consisting of xenon di- and hexafluoride).
80
(a)
75
(b)
7O
E 0
£5 6O
55 o,. 50
"g 45 I-4O 35 30
o'25
0
12
3
0
Time,
hr
Fig. 2. Pressure-time dependence at explosion of xenonfluorine mixture. Xenon to fluorine mole ratio 1:4.5 and total pressure at 22°C 36 atm. (a) in the presence of AgF2 and NiF 2 (34.8 m2/g). (b) in the presence of NiFz (10.4 ml/g). in the reaction. I.R. spectrum of the volatile products revealed the presence of xenon tetrafluoride, xenon difluoride, and xenon hexafluoride. According to the mass balance the product contained about 80 % xenon tetrafluoride. Time period of about 3 rain can be considered as the induction period. An exact reproduction of this experiment was observed when a new batch of silver difluoride synthesized and reaction carried out for the first time. Similarly, the reaction proceeds explosively in 50.0~-
o 2 m-mole NiF 2 " 4 m-mole NiF 2 ~ 6 m-mole NiF2
~
o{? 45'0
Tempera~rure : 120*C Mole ratio: Xe : F2= 1:4.5 T o t a l pressure a t 2 2 * C : 3 6 a t m
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7
8
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l
I0
I
II
hr
Fig. 3. Xenon-fluorine reaction rate curves in the presence of several amounts of NiF z (34.8 m2/g).
I
12
I
13
J. LEVEE,J. SLIVNIKand B. ZEMVA
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Time,
hr
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I 9
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I 12
Fig. 4. Xenon-fluorine reaction rate curves in the presence of several amounts of NiF2 (10.4 m2/g). that the reaction can be considered homogeneous. Initiation with oxides, sulphur, supposedly since the fluorination of these materials is a strongly exothermic processE12], and by electrical heat showed that one needs only locally enough energy to start an explosion. This is confirmed by observation with vanadium(V) oxide--fluorination of V 2 0 5 is namely an endothermic process [ 12J--xenon-fluorine reaction does not start explosively in presence of vanadium(V) oxide even at 120°C. It is also possible that the initial, very rapid, heterogeneous reaction on the surface of fresh silver and nickel
DISCUSSION As shown in Figs. 3-5 the rate of the reaction between xenon and fluorine is increased by the increased contact surface. These results indicate that the reaction is heterogeneous and occur for the most part on the surface of added metal fluorides. Reaction is first order in regard to xenon and zero order in regard to fluorine, but only in the narrow time interval. This interval corresponds to the formation of xenon difluoride. In these cases the reaction shaped when xenon difluoride was formed. On the other hand the explosion phenomena indicate
50-0 ~lt*4 ~, E
%
Temperature ,120"C Mole r a t i o : X e : F 2 f f i l : 4 . 5
a 30
m-mole MgF 2
T o t a l pressure a t 2 2 ° C : 3 6 o t m
• 15
m-mole MgF 2
, wit,,out M,f2
,,
45'0
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24
hr
Fig. 5. Xenon-fluorine reaction rate curves in the presence of several different amounts of MgF z.
I
26
I
28
r
30
The xenon-fluorine reaction difluoride generates enough heat to turn it into explosion. Since the specific surface areas of silver difluoride and nickel difluoride in the first six experiments - - t h e first three proceeded explosively--were practically unchanged, this possibility was not supported. Another fact which contradicts this hypothesis is a measurable rate of the reaction in the presence of 20 m-mole of nickel difluoride (10-4 mZ/g) with correspondingly greater contact surface whilst in the presence of only 2 m-mole of nickel difluoride (34.8 mZ/g) the reaction proceeded explosively. Because of the relatively low exothermicity of the reaction between xenon and fluorine the occurrence of thermal explosion is not very probable, and the explosion phenomena may be explained by chain explosion. Initiation of chain by silver difluoride and nickel difluoride could be most plausibly discussed in terms of the idea that the chain started at the centres located on the surface of added fluoride, and then traversed the gas. For the explosive course of the reaction a minimum of such active centres is required. This condition was fulfilled with only 2m-mole of nickel difluoride (34.8mZ/g) while 40m-mole of nickel difluoride (20.4 m2/g) were needed. After three successive explosions on the same sample the centers have been destroyed and the reaction proceeded with a measurable rate in the following exl~eriments. Such an explanation speaks in favour of the induction period which was observed. Initiation by heat could be discussed in terms of formation of fluorine atoms. Regardless of the origin of radicals, the reaction between xenon and fluorine under pressure can be considered as a non-stationary chain reaction. The preliminary experiment, however showed that there exists a lower explosion limit : at 8 atm of total pressure and equal mole ratio of the reactants the reaction was proceeded with a measurable rate. For the energy branching chain reaction the following scheme could be supposed : Xe XeF XeF$ XeF* XeF3
+ + + + +
F2 F Fz Fz F 2
F2
~ 2F --* XeF' ~ XeF* ~ XeF2 ~ XeF~ ~ XeF~,
J.1.N.C., Vol. 36, No. 5 D
+ + + +
F F + F' F F
(1) (2) (3) (4) (4a) (5)
XeF* XeF* XeF~ XeF* XeF* F XeF;, XeF;,
+ + + + + + + +
F2 F2 F2 F2 M F' F XeF;,
1001 --* XeF4 + F + F (6) ~ XeF~ + F" (6a) --* XeF* + F (7) --, XeF6 + F" + F" (8) ~ XeF, + M, n = 2,4,6, M = wall (9) ~ F2 (10) -~ X e F , + l , n - - 1,3,5 (11) ~ XeF,+,, + Xe, n,m = 1,3 (12)
Equation (1) represents the homogeneous or heterogeneous initiation. Fluorine atoms and XeF, radicals are chain carriers in this reaction and an obvious way by .which xenon difluoride might be formed is represented by Eqn (3). This reaction is exothermic and xenon difluoride molecule formed will be excited. If the XeF'~ encounters a fluorine molecule the latter may be dissociated, see Eqns (4) and (4a). Equations (9)-(12) represent the termination of chain reaction. Since all the xenon was consumed in the reaction, the reaction (12) is not probable. Acknowledgements~The financial support of the Research Community of Slovenia is gratefully acknowledged. The authors are grateful to Miss B. Sedej for the analytical work.
REFERENCES
1. B.G. Baker and P. G. Fox, Nature, Lond. 204, 466 (1964). 2. B. H. Davis, J. L. Wishlade and P. H. Emmett, J. Catalysis 10, 266 (1968). 3. B. G. Baker and P. G. Fox, J. Catalysis 16, 102 (1970) 4. C. F. Weaver, Thesis, University of California, Berkeley, 1966. 5. J. Slivnik, B. Zemva, B. Frlec and T. Ogrin, Fifth Int. Symp. on Fluorine Chem.. Moscow (July 1969). 6. J. Slivnik, A. gmalc, B. Zemva and A. N. Mosevi~, Croat. Chem. Aeta 40, 49 (1968). 7. J. Slivnik, A. ~malc and A. Zemlji~, Vest. SIov. kem. dru~tva 9, 61 (1962). 8. J. Slivnik, A. ~malc and A. Zemlji~, Vest. Slov. kern. dru.~tva 12, 17 (1965). 9. J. Slivnik, M. Zvanut and B. Sedej, Mh. Chem. 99, 1713 (1968). 10. J. Levec and J. Slivnik, Unpublished work. 11. B. Frlec and A. ~malc, Patent requested dated 26 May 1971, Patent Office, Ljubljana, Yugoslavia. 12. A. Glassner, ANL Report 5750, Argonne, October 1965.
O N THE R E A C T I O N B E T W E E N X E N O N A N D F L U O R I N E * J. LEVEC, J. SLIVNIK and B. ~EMVA~ Faculty of Natural Sciences and Technology and J. Stefan Institute, University of Ljubljana, Lj ubljana, Yugoslavia (Received 30 April 1973) Abstract The influence of nickel (34-8 m2/g and 10-4 m2/g), silver (1.4 mZ/g), and magnesium (57.8 m2/g) difluorides and nickel(Ill) and silver(l) oxides on the reaction between xenon and fluorine was investigated. The experiments were performed using a xenon to fluorine mole ratio of I : 4.5, with a total pressure amounting to 36 atm as measured at 22°C while the reaction temperature was 120°C. In some cases the reaction proceeds in the presence of nickel and silver difluorides explosively. Similarly, the reaction proceeds explosively in the presence of small amounts (1 m-mote) of nickel(Ill) or silver(1) oxide at 0°C. The reaction can also be initiated by local application of heat (e.g. by the addition of small amounts, 1 m-mole, of elemental sulphur or simply by an electrically heated wire). The explosion phenomena indicate that the reaction can be considered as a homogeneous reaction.
INTRODUCTION A RECENTLY published study o f the reaction between x e n o n a n d f l u o r i n e [ l ~ ] , to form x e n o n fluorides, s h o w e d that the reaction is h e t e r o g e n e o u s a n d occurs on the walls of the metallic nickel reaction vessels or on the surface of a d d e d metallic fluorides. It was also s h o w n that nickel difluoride is the one of the m o s t effective catalyst[2,5]. T h e e x p e r i m e n t s were e x t e n d e d to survey the catalytic activity o f nickel, silver a n d m a g n e s i u m difluorides. In addition, the influence of silver(I) a n d nickel(III) oxides was investigated. The total pressure o f reaction mixture was m u c h greater t h a n in the p a p e r s r e p o r t e d previously[I-4]. Since the f o r m a t i o n of x e n o n difluoride p r o c e e d s with a measurable rate already in range b e t w e e n 110 ° and 130°C[5,6], the reaction rate was m e a s u r e d at 120°C. T h e course of the reaction was followed by m e a s u r i n g time dep e n d e n c e of pressure. O n the basis of the results o b t a i n e d the n a t u r e of the x e n o n - f l u o r i n e reaction u n d e r pressure is discussed here. EXPERIMENTAL
Materials Fluorine was produced and purified in this laboratory as described elsewhere[7,8]. Fluorine used in the experiments was 99-0 + 0.5~o pure. When a large amount of fluorine (5 atm in 100 ml container) was distilled from a container * Presented at 4th European Symposium on Fluorine Chemistry, Ljubljana, 28 August-1 September 1972. t J- Stefan Institute, University of Ljubljana, Ljubljana, Yugoslavia.
cooled with liquid nitrogen, the i.r. spectrum of residue showed only traces of CF4. Xenon was supplied by L'Air Liquide (Paris, France) in 99.5 ~ purity (the balance was krypton). Nickel difluoride with specific surface area of around 208 mZ/g was obtained from C.E.N. (Dr. Ehretsman, Saclay, France). Another sample of NiFE (13.1 mE/g) was produced by Koch-Light (Colnbrook, England). Magnesium difluoride with specific surface area of around 170 mZ/g was prepared as described elsewhere[9]. Before the experiments, the fluorides were conditioned by fluorine under pressure of about 30 atm at 120°C. The specific surface areas dropped : their values (B.E.T.) are shown in Table 1. Silver difluoride was prepared by the fluorination of silver(1) oxide (92.4~o Ag) with elemental fluorine[10]. The chemical analysis of the product was in good agreement with the calculated composition of silver difluoride (Table 1), but unfortunately its specific surface area was rather low, 1.4 m2/g. Nickel(IIl)(70-1% Ni) and silver(I) (92-4~ As) oxides were supplied by Riedel de l-l~ien (Hanover, W. Germany) in p.a. purity. Prior to use in the experiments, the oxides and sulphur were dried 2 hr in vacuum (10 -5 mm Hg) at 120°C. Apparatus Reactions were carried out in 100 ml argon-arc welded nickel pressure vessels with a Teflon packed brass valves (for work under pressure and vacuum--developed in this laboratory). The reaction vessels were tested hydrostatically to 210 atm pressure at room temperature. To reduce the dead volume of the reaction vessels, pressure transmitters with nickel bellows[ 11] in conjunction with oil-filled Bourdon gauges (160 mm dia. face, 0-60 atm ((~250 arm), VDO-OTA, Frankfurt/Main, W. Germany, accuracy _+0.3 atm (+ 1.5 atm)) were used for the measurements of pressure. The reaction vessel was immersed completely in the liquid bath. 997
998
J. LEVEC, J. SLIVNIKand B. ZEMVA Table 1. Chemical compositions and specific surface areas of difluorides after 24 hr fluorination (30 atm F2) at 120°C MF2
Calcd. comp. [ %]
Found. comp. E%]
Spec. surface area [m2/g]
NiF* NiF2t AgF2 MgF2
Ni 60.6; F 39.4 Ni 60.6; F 39.4 Ag 73.9; F 26.l Mg 39.0; F 61-0
Ni 60.4; F 39.3 Ni 60.2; F 39.0 Ag 73.9; F 25.4 Mg 36.1; F 58.0
34.8 10.4 1.4 57.8
* C.E.N., Saclay, France. t Koch & Light, Colnbrook, England. evolved were monitored by i.r. spectroscopy. At each of the mentioned temperature the weight of vessel was controlled and from the difference the quantity of volatiles were determined.
The gauges, complete with their pressure transmitters, were calibrated by an absolute method at reaction temperature. The corrected pressure values were used in the estimation of pressure differences. Reaction vessels were heated up to 120°C using a thermostated glycerine bath. Monel, 10 cm path length, gas absorption cells fitted with silver chloride windows, and a Zeiss UR-20 infrared spectrometer were used to obtain i.r. spectra of reaction products.
RESULTS In a series of experiments the same sample of silver difluoride was used. After the reaction was completed, the volatiles were p u m p e d away a n d a new x e n o n fluorine mixture was prepared in the reaction vessel. The pressure measurements in this series are shown in Fig. 1. The reaction in presence of 20 m-mole of silver difluoride proceeded explosively in the first experiment. A typical reaction procedure of that kind is shown in Fig. 2(a). After the reaction vessel has been immersed into thermostated b a t h the pressure of the gaseous mixture was increasing normally a n d reached the value expected for that particular temperature (49 atm). After 3 min, however, the pressure increased instantaneously to approximately 80 atm a n d then dropped to 28 atm. W h e n the excessive fluorine was pumped off, it was found out that practically all xenon was consumed
Procedure
130 m-mole of fluorine and 29 m-mole of xenon were metered with an adequate amount of each of the fluorides or oxides. The amount of fluorine and xenon was checked by weight and determined to + 1 mg. The mole ratio Xe:F2 was 1:4.5 and the total pressure measured at room temperature 36 atm. The reaction vessel was immersed afterwards into the thermostated bath which was kept at 120~C ( + I°C). As the result of the reaction between xenon and fluorine the pressure dropped. After the reaction stopped the reaction vessel was taken out and quenched to the liquid nitrogen temperature. The volatiles were separately pumped off at liquid nitrogen temperature, and at -80°C. Ultimately the reaction vessel was warmed to room temperature and the reaction products
First e x p e r i m e n t - e x p l o s i v e l y ,
,50"0 88 8
8 1~8
45-0
not plotted
o Second e x p e r i m e n t = Third e x p e r i m e n t
j~o
°
• Fourth e x p e r i m e n t
T e m p e r a t u r e : 120°C Mole r a t i o : X e : F2= 1:4.5
' Fifth e x p e r i m e n t
Total
p r e s s u r e of 220C: 3 6 a r m
==
40.0
-
i
°a~WiIi ooo %,~mmiWQQgo ° I-
°Oo o
3,5.0
-
==~lg
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°
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o o e
A •
I
I
I
P
I
I
i
I
I
I
I
I
0.5
1.0
1.5
2.0
2.5
3-0
3.5
4-0
4.5
.5.0
5.5
6-0
Time,
hr
Fig. 1. Reaction rate curves for five xenon-fluorine reaction runs in the presence of 20 m-mole of AgF2.
A
o
j
= •
a
o ,e
a A
A
I
I
I
6"5
7-0
7- 5
999
The xenon-fluorine reaction
"presence of 6, 4 and 2 m-moles of nickel difluoride (34.8 mZ/g) (Fig. 2a). In these cases the explosive reaction courses were observed on the same sample even three times in sequence after the products of the proceeding reaction have been removed. Always a very good agreement in conversion of fluorine and the reaction course was observed. The reaction proceeded with a measurable rate always in the fourth run as is shown in Fig. 3. A very similar xenon-fluorine reaction course, that is approaching an explosion (Fig. 2b), was observed in presence of 40 m-mole of nickel difluoride with the specific surface area 10.4m2/g whereas the reaction proceeded with a measurable rate when only 20 m-mole of nickel difluoride were employed (Fig. 4). Unexplosive courses gave xenon difluoride in most part (about 80%--mass balance--and xenon tetra- and hexafluoride). The course of the reaction between xenon and fluorine in the presence of magnesium difluoride is shown in Fig. 5. Reaction shaped when xenon difluoride was formed. The yield was higher than 80~o, the rest of the product consisting of xenon tetrafluoride (without xenon hexafluoride). The explosive course of the reaction was observed also in presence of small amounts of silver(I) and nickel(III) oxides, 1 m-mole. The explosions occurred already at temperatures below 0°C. A similar reaction course was observed when the oxides have been replaced in same amounts by elemental sulphur. The reaction could be simply initiated by an electrically heated nickel wire in a mixture of fluorine and xenon (Xe:F2 = 1:4.5) at 36 atm as measured at 22°C. Experiment was carried out in 100 ml reaction vessel, equipped with special nickel wire electrical heater; 35 cal was required to initiate explosion. All explosive courses gave as principal reaction product xenon tetrafluoride (about 80~o-mass balance--the rest of the product consisting of xenon di- and hexafluoride).
80
(a)
75
(b)
7O
E 0
£5 6O
55 o,. 50
"g 45 I-4O 35 30
o'25
0
12
3
0
Time,
hr
Fig. 2. Pressure-time dependence at explosion of xenonfluorine mixture. Xenon to fluorine mole ratio 1:4.5 and total pressure at 22°C 36 atm. (a) in the presence of AgF2 and NiF 2 (34.8 m2/g). (b) in the presence of NiFz (10.4 ml/g). in the reaction. I.R. spectrum of the volatile products revealed the presence of xenon tetrafluoride, xenon difluoride, and xenon hexafluoride. According to the mass balance the product contained about 80 % xenon tetrafluoride. Time period of about 3 rain can be considered as the induction period. An exact reproduction of this experiment was observed when a new batch of silver difluoride synthesized and reaction carried out for the first time. Similarly, the reaction proceeds explosively in 50.0~-
o 2 m-mole NiF 2 " 4 m-mole NiF 2 ~ 6 m-mole NiF2
~
o{? 45'0
Tempera~rure : 120*C Mole ratio: Xe : F2= 1:4.5 T o t a l pressure a t 2 2 * C : 3 6 a t m
D~
%\ 40.0
°%°
% %
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00
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0
I I
I 2
L 3
I
4
I
5
I
6 Time,
I
I
7
8
I
9
l
I0
I
II
hr
Fig. 3. Xenon-fluorine reaction rate curves in the presence of several amounts of NiF z (34.8 m2/g).
I
12
I
13
J. LEVEE,J. SLIVNIKand B. ZEMVA
I000
5 ° ° r ~oAji l~ •~
o 6 m-mole • 12 m-mole 2 0 m-mole 4 ~U m-mole not p l o t t e d
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o
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T e m p e r a t u r e : 1200C NiF z NiF 2 Mole ratio :Xe ; F2: I: 4.5 NiF 2 Total pressure at 2 2 " C : 3 6 arm NiF2-explosively
oo
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I 2
q
0
I
I 3
r 4
I 5
, I 6
I 7
Time,
hr
I 8
I 9
I I0
F II
I 12
Fig. 4. Xenon-fluorine reaction rate curves in the presence of several amounts of NiF2 (10.4 m2/g). that the reaction can be considered homogeneous. Initiation with oxides, sulphur, supposedly since the fluorination of these materials is a strongly exothermic processE12], and by electrical heat showed that one needs only locally enough energy to start an explosion. This is confirmed by observation with vanadium(V) oxide--fluorination of V 2 0 5 is namely an endothermic process [ 12J--xenon-fluorine reaction does not start explosively in presence of vanadium(V) oxide even at 120°C. It is also possible that the initial, very rapid, heterogeneous reaction on the surface of fresh silver and nickel
DISCUSSION As shown in Figs. 3-5 the rate of the reaction between xenon and fluorine is increased by the increased contact surface. These results indicate that the reaction is heterogeneous and occur for the most part on the surface of added metal fluorides. Reaction is first order in regard to xenon and zero order in regard to fluorine, but only in the narrow time interval. This interval corresponds to the formation of xenon difluoride. In these cases the reaction shaped when xenon difluoride was formed. On the other hand the explosion phenomena indicate
50-0 ~lt*4 ~, E
%
Temperature ,120"C Mole r a t i o : X e : F 2 f f i l : 4 . 5
a 30
m-mole MgF 2
T o t a l pressure a t 2 2 ° C : 3 6 o t m
• 15
m-mole MgF 2
, wit,,out M,f2
,,
45'0
o 120 m - m o l e MgF 2 • 6 0 m-mole MgF 2
'
o
~
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eo %...
a
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°
,
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,
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30-0
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4
t
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8
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ill
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u2
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[
04 Time,
I
I
t6
18
I
20
J
2z
I
24
hr
Fig. 5. Xenon-fluorine reaction rate curves in the presence of several different amounts of MgF z.
I
26
I
28
r
30
The xenon-fluorine reaction difluoride generates enough heat to turn it into explosion. Since the specific surface areas of silver difluoride and nickel difluoride in the first six experiments - - t h e first three proceeded explosively--were practically unchanged, this possibility was not supported. Another fact which contradicts this hypothesis is a measurable rate of the reaction in the presence of 20 m-mole of nickel difluoride (10-4 mZ/g) with correspondingly greater contact surface whilst in the presence of only 2 m-mole of nickel difluoride (34.8 mZ/g) the reaction proceeded explosively. Because of the relatively low exothermicity of the reaction between xenon and fluorine the occurrence of thermal explosion is not very probable, and the explosion phenomena may be explained by chain explosion. Initiation of chain by silver difluoride and nickel difluoride could be most plausibly discussed in terms of the idea that the chain started at the centres located on the surface of added fluoride, and then traversed the gas. For the explosive course of the reaction a minimum of such active centres is required. This condition was fulfilled with only 2m-mole of nickel difluoride (34.8mZ/g) while 40m-mole of nickel difluoride (20.4 m2/g) were needed. After three successive explosions on the same sample the centers have been destroyed and the reaction proceeded with a measurable rate in the following exl~eriments. Such an explanation speaks in favour of the induction period which was observed. Initiation by heat could be discussed in terms of formation of fluorine atoms. Regardless of the origin of radicals, the reaction between xenon and fluorine under pressure can be considered as a non-stationary chain reaction. The preliminary experiment, however showed that there exists a lower explosion limit : at 8 atm of total pressure and equal mole ratio of the reactants the reaction was proceeded with a measurable rate. For the energy branching chain reaction the following scheme could be supposed : Xe XeF XeF$ XeF* XeF3
+ + + + +
F2 F Fz Fz F 2
F2
~ 2F --* XeF' ~ XeF* ~ XeF2 ~ XeF~ ~ XeF~,
J.1.N.C., Vol. 36, No. 5 D
+ + + +
F F + F' F F
(1) (2) (3) (4) (4a) (5)
XeF* XeF* XeF~ XeF* XeF* F XeF;, XeF;,
+ + + + + + + +
F2 F2 F2 F2 M F' F XeF;,
1001 --* XeF4 + F + F (6) ~ XeF~ + F" (6a) --* XeF* + F (7) --, XeF6 + F" + F" (8) ~ XeF, + M, n = 2,4,6, M = wall (9) ~ F2 (10) -~ X e F , + l , n - - 1,3,5 (11) ~ XeF,+,, + Xe, n,m = 1,3 (12)
Equation (1) represents the homogeneous or heterogeneous initiation. Fluorine atoms and XeF, radicals are chain carriers in this reaction and an obvious way by .which xenon difluoride might be formed is represented by Eqn (3). This reaction is exothermic and xenon difluoride molecule formed will be excited. If the XeF'~ encounters a fluorine molecule the latter may be dissociated, see Eqns (4) and (4a). Equations (9)-(12) represent the termination of chain reaction. Since all the xenon was consumed in the reaction, the reaction (12) is not probable. Acknowledgements~The financial support of the Research Community of Slovenia is gratefully acknowledged. The authors are grateful to Miss B. Sedej for the analytical work.
REFERENCES
1. B.G. Baker and P. G. Fox, Nature, Lond. 204, 466 (1964). 2. B. H. Davis, J. L. Wishlade and P. H. Emmett, J. Catalysis 10, 266 (1968). 3. B. G. Baker and P. G. Fox, J. Catalysis 16, 102 (1970) 4. C. F. Weaver, Thesis, University of California, Berkeley, 1966. 5. J. Slivnik, B. Zemva, B. Frlec and T. Ogrin, Fifth Int. Symp. on Fluorine Chem.. Moscow (July 1969). 6. J. Slivnik, A. gmalc, B. Zemva and A. N. Mosevi~, Croat. Chem. Aeta 40, 49 (1968). 7. J. Slivnik, A. ~malc and A. Zemlji~, Vest. SIov. kem. dru~tva 9, 61 (1962). 8. J. Slivnik, A. ~malc and A. Zemlji~, Vest. Slov. kern. dru.~tva 12, 17 (1965). 9. J. Slivnik, M. Zvanut and B. Sedej, Mh. Chem. 99, 1713 (1968). 10. J. Levec and J. Slivnik, Unpublished work. 11. B. Frlec and A. ~malc, Patent requested dated 26 May 1971, Patent Office, Ljubljana, Yugoslavia. 12. A. Glassner, ANL Report 5750, Argonne, October 1965.