Interactions of halogen atoms with shock-heated hydrogen fluoride

INTERACTIONS OF HALOGEN ATOMS WITH SHOCK-HEATED HYDROGEN FLUORIDE JAY BLAUER, V. S. ENGLEMAN AND WAYNE C. SOLOMON

Air Force Rocket Propulsion Laboratory, Air Force Systems Command, United States Air Force, Edwarc~% California The thermal decomposition of H F in the presence of added halogen atoms was studied in the temperature range of 3800 ~ to 5500~ and at optical densities for H F in the vicinity of 0.1 atm-cm. The course of the reaction was followed by monitoring the infrared emission intensity, at various frequencies, from the 1-0 vibration-rotation band of HF. The spectral properties of H F were found to be significantly altered in the presence of halogen atoms other than F. Also, for a concentration ratio [-XJ/[-Ar] of approximately 0.019, the apparent efficiencies of the halogen atoms X relative to argon, in promoting the dissociation are given by: kF//kAr-"

kcl/kAr : kBr/kAr: kI//kAr = 1 : 150: 500: 850.

Evidence is presented that supports the conclusion that the catalytic species are primarily the hydrogen halides and not the halogen atoms themselves. The net result should manifest itself as an increase in the over-all reaction rate.

Introduction Although the thermal decomposition of H F has been the subject of at least three separate studies, 1-3 the experiments were all conducted with an argon bath and, except for only two species, no attempt was made to determine the effects of gaseous additives. Both Jacobs I and Blauer 2 determined the effect of H atoms, and Blauer 2 determined the effect of added F atoms. The present work has been undertaken to study the dissociation reaction in the presence of halide atoms which are produced within a shock wave by decomposition of the appropriate halogen. Due to the possibility of exchange reactions of the type HF+X=

HX+F

(1)

H F + X = H -{- FX,

(2)

which are analogous to the important exchange reaction

HF~-H----H~+F,

(3)

Experimental Purity of Materials Argon, hydrogen fluoride, chlorine, and bromine in research grades, were obtained from the Matheson Co. Mass-spectrometric analysis of the argon used revealed trace amounts ( ( 0 . 0 1 % ) of N2 and 02 as the only impurities. The hydrogen fluoride and chlorine were further purified by chilling to --75~ and applying a vacuum until the residual pressure was below 10 torr. Massspectrometric analysis, with Ar as an internal standard, indicated the presence of <: 0.01% each of N2, 02, and H2 in either gas which had been treated in the above manner. The bromine used was also treated in this manner; however, the treated liquid showed an additional impurity of 0.2% C12, which was removed by distillation at 0~ Resublimed iodine having a stated purity of 99.95% was purchased from Mallinckrodt and was used without further purification.

the presence of these halogen atoms may provide paths of decomposition having lower energy requirements than simple dissociation

HF+M

= H-~Fna

M.

(4)

Method The present experiments were carried out in a stainless-steel shock tube at temperatures in the 109

110

KINETICS OF ELEMENTARY CHEMICAL REACTIONS 1,0

6.8 u

0.6-

0.2

0

i 3460

3600

3800

4060

4260

w {cm'l I Fro. 1. Spectral slit function based on spin reversal of Br atoms at 3680 cm-1 and 4500~ range of 3800 ~ to 5500~ and at optical densities near 0.1 atm-cm for the shock-heated HF. The course of the reaction was followed by monitoring the infrared emission from the 1-0 vibrationrotation band of HF. Correlation between emission intensity and H F concentration was made by recourse to the calculations of Malkmus 1'2'4 concerning emissivities of the Doppler-broadened rotational lines. Before using a gaseous mixture its HF, C12, and Brs contents were determined by measurement of its optical density at 2.5 ~, 325 m~, and 439 m~, respectively. Mixtures containing Is were prepared over solid Is and allowed to reach thermal equilibrium before use. The spectral slit function for the spectrometer was determined by monitoring the emission intensity from the spin reversal line (2P1/~--~ sPa/s) of Br atoms (at 3680 cm - I ) which were shock heated to 4500 2= 50~ The results indicated a spectral half-width of 300 cm-1 as shown in Fig. 1.

Calculations The shock-performance parameters were obtained by means of a solution of the RankineHugoniot equations. Initial radiancies of H F were estimated by means of linear extrapolation of the emission trace to the shock front. The results of Malkmus* indicate t h a t if the temperature is higher than 3500~ and the

optical density of H F is below 0.5 atm-cm, the radiancy will be linearly dependent upon the H F concentration. All results reported herein refer to conditions within these limits. Calculations indicate that under the conditions of these experiments the halogens will be completely dissociated5,6 in times shorter than the response time of the detection system. Consequently, the initial halogen atom concentrations are easily obtained and the appropriate corrections to the shock performance parameters can be made. Calculations based on the results of Duffv for argon indicate that the boundary-layer thickness, (0.95), will not exceed 1% of the tube diameter at 15 ~sec for a temperature of 4500~ provided the incident shock pressure exceeds 0.4 atm. Since no d a t a were taken at pressures below this limit, initial radiancy and slope d a t a will not be significantly affected by boundary-layer buildup; however, equilibrium radiances will be subject to an error from this source which will depend upon the time required to reach equilibrium (ca. 50-200 ~sec). Accordingly, equilibrium radiancies can be expected to show greater uncertainty than the corresponding values taken from the initial conditions.

Results Initially, all d a t a were obtained a t a spectral frequency (ca. 3982 cm- I ) in the vicinity of the band center. A typical oscillogram is shown in Fig. 2. The apparent radiancies for mixtures containing various amounts of chlorine are shown plotted as a function of temperature in Fig. 3. These results show the independence of temperature predicted by the results of Malkmus4; however, they show a very marked dependence upon the Cl-atom concentration. Similar results

iiD|ia| Lii ImUllllllUllRglimllHIIlflllHll FIG. 2. Emission trace for Run No. 115: 2.1% HF, 1.0% CI~ in argon; 4527~ 3.2 atm. Ordinate: 0.2 volt/cm; abscissa: 20 ~s/cm.

INTERACTIONS OF HALIDE ATOMS WITH HF

111

10 8 0%

6 lID

0

cI2-o

o

o

o

0.5% CI 2 - - - Q "

[~----~

1.0% Ct 2 _A_. . . . . . . . .

Z~__Z~

~

[]

4

z~) 2.0% CI 2 " E ~ - O - - - ~ ) - - - - O - o - - < > O - - - - - - O "

5.65- [] -~" "4.3 7" 0

2.94

2 o u.I

I 4000

3500

I 4500

I 5000

I 5500

TEMPERATURE [OK]

FIG. 3. Initial radiancy of hydrogen fluoride vs temperature. Effect of added chlorine. All measurements were taken at 3982 cm-I. ~ represents data for a mix having 5% Cl~. All mixes contained approximately 2% HF. were obtained for mixtures containing Br atoms or I atoms. To eliminate the possibility that these results are complicated by a very rapid initial reaction, the entire vibration-rotation band was mapped. The observations are illustrated in comparison to the results of Malkmus4 in Figs. 4, 5, and 6. Direct comparison was achieved by vertical adjustment of the theoretical curve until it

matched the results for binary mixtures of H F and Ar. From Fig. 5 it is apparent that a significant difference between observed and predicted radiancies for mixtures containing added chlorine occurs in the vicinity of the band center and possibly in the P branch at low frequencies. The fact that the radiancy is not decreased by the same amount at all frequencies appears to rule out fast reaction when coupled to the fact that initial and final radiancies agree with the initial

p APPARENTRADIANCYAFTER MALKMUS { Ref. 4} o 2% HF IN ARGON[INITIAL RADIANCY/

APPARENTRADIANCYOF HF AFTER MALKMUS[Ref. 4) Z~ 2.0%HF & 1.0% CI 2 IN ARGON

6.0

---

m

BAND CENTER DEPRESSIONDUE TO ATOMIC CHLORINE 8.0

4.0 ~-

4.0

'-

2.0

z

2.0

"=

-~

=

1.0 N

0.8

E ,,3 ~

0.6

B

1.0 3

03

0.4

0.8 08

Co:

1,.

2800

I

I

3600

4400

cm -1

Fzo. 4. Apparent radiancy as a function of frequency at 4300~ for an optical density of 0.1 atm-cm. Emissivities taken from Malkmus (Ref. 4). Values herein refer to conditions at the shock front.

0.4

I 2800

I 3600

4400

cm1

FIG. 5. Effect of chlorine atoms upon the apparent initial radiancy of HF at 4300~ and an optical density of 0.1 atm-cm. Emissivity of HF taken from Malkmus (Ref. 4).

112

KINETICS OF ELEMENTARY CHEMICAL REACTIONS

~x

r~"

0

o~

I [ [ I I I[11[I

11

I1~

0

I I I I I I I I I I [ ~

Cq

O ..Q

I I I I ~ ~

I f=~llt

I I I I

ttt

I I I I r

~D

~

~ ~ cD ~

r-~'~ X

,io-~ II . ~ v

INTERACTIONS OF HALIDE ATOMS WITH H F frozen and final equilibrium shock-performance calculations over the entire temperature range (see Table I). Although the radiances are diminished at all frequencies for mixes containing Br atoms (see Fig. 6), the initial and equilibrium radiancies are still in agreement (see Table I). Accordingly, it is tentatively concluded t h a t fast reaction can be ruled out in this instance also. The spectrum was not mapped for mixes containing I atoms; however, the radiancy for these mixes is also depressed at the band center. Here again there is agreement between equilibrium and initial radiancies. To check the possibility that the halogen atom forms a complex with the H F molecule, the emission intensity at 5000 ~. from the radiative recombination 5 of C1 atoms present in the ternary mixture was monitored. These results were compared to the intensities obtained from binary mixtures of Cle and Ar. The presence of H F was found to have no significant effect upon either this radiant intensity or the dissociation rate of C12. Accordingly, any. complex formed must he of a very transient nature. The initial reaction-rate constants were computed from the expression

113

IO0

..~ 10

i

%HF ~

-I1

2 %Br 2 ~ 2

I o- 1.6 I A- 1.6

0.0 1.0

0.0 0.0

l a - 1.6 J<>- 1.6

0.0

1.0

0.0

0.0

0.0 / 0.0 0.0 1.0

I 1.8

I

I

2.0 2.2 :2.4 2.6 [TEMPERATURE~ • 4

FIG. 7. Experimentally determined rate constants for the initial dissociation of H F in the presence of halogen atoms. The optical density of HF was always approximately 0.1 atm-cm.

where M is the sum of all possible collision partners. Appropriate corrections were made for density and temperature changes occurring during the course of the reaction, s The results are shown plotted in Fig. 7 for mixes t h a t have a --APPARENT RAOIANCYOF HF AFTER MALKMUS[Ref. 4} concentration ratio, EX~/[Ar-], of 0.019. a 2.0% HF & 1.0% Br 2 IN ARGON Since fluorine atoms have previously been ---APPARENT RADIANCYIN THE PRESENCEOF ATOMICBROMINE shown to have no effect upon the rate of the reaction, 2 the results illustrated in Fig. 7 indicate 6.0that the halogen atoms accelerate the reaction in the order k = ( 1 / E M J ) (d In

3

EHF]/dt),

(5)

4.0-

kAr = kF ~ kcl <: kBr < kz.

i

In Ref. 2 it was shown t h a t the dissociation rate constant for H F in an Ar bath can be represented by the expression

,~ !2.0-

,.oi-I_ i "1 2800

A k = -~exp

i

o.I:/ r 0.4|

(6)

I 3600

I 4400

cm'l

Fio. 6. Effect of bromine atoms upon the apparent initial radiancy of H F at 4300~ and an optical density of 0.1 atm-cm. Emissivity of I-IF taken from Malkmus (Ref. 4).

(--Do/RT)

cc/mole-sec,

(7)

where Do is the bond dissociation energy (ca. 134.1 kcal/mole) and A is an adjustable parameter. For comparative purposes, this expression was fit to all of the data for which E X J / [ A r ] -0.019. The values of A X 10- ~ for X = Ar, F, C1, Br, and I were 0.25, 0.25, 1.02, 2.78, and 4.5, respectively. These results can be transformed into apparent efficiencies for the different collision partners M in Reaction (4) of

k:JkAr =

1, 1, 150, 500, 850,

(8)

114

K I N E T I C S OF E L E M E N T A R Y

CHEMICAL REACTIONS

TABLE II Initial reaction-rate parameters for various experimental conditions A. Constant value of the Ratio [-C1]/[-Ar] [CI]/EM]

n

log A

E4~ (kcal)

m

log B

E~ (kcal)

f:~

0.0 0.0092 0.019 0.042

1.0 =E 0.1 ----

14.3 • 0.2 ----

116 =t= 5 ----

-0 . 4 • 0.3 0.3 • 0.3 0.1 =t= 0.1

-11.1 • 0 . 3 10.1 • 0.3 9.1 • 0.1

-84 • 7 89 • 7 91 • 4

8 6 15 15

B. Constant pressure FAr] )< l0 s

m

log B

E:~

0.42 0.82

1.0 ~ 0.1 1.0 • 0.2

14.8 -4- 0.3 --

84 ~ 5 --

f 8 3:~

C. Constant concentration of atomic chlorine I-C1] X 107

**

log B

0.40 0.82

- - 1 . 0 -4- 0 . 2 - - 1 . 0 =E 0 . 2

2 . 0 -4- 0.3 2 . 6 4- 0.2

E$ (kcal)

f

84 =i= 7 88 • 5

5 5

* N u m b e r of d a t a points represented. t All at 4740~ ** Exponent of r A r ] in second term.

respectively. T h e s e results were f o u n d t o b e completely i n d e p e n d e n t of t h e f r e q u e n c y of o b s e r v a t i o n in a g r e e m e n t w i t h t h e results of M a l k m u s . 4 T h i s is s h o w n in T a b l e I where t h e A of Eq. (7) is t a b u l a t e d as a f u n c t i o n of frequency. F o r each v a l u e of t h e ratio [-Cl~/[-Ar~ considered, a n analysis of v a r i a n c e was used to fit t h e corresponding initial r e a c t i o n - r a t e d a t a to t h e following expression:

20.0

1o.o E ""

8.0 D

e.O

O

%

4.0

- - d In _t~ ~

2.0

(Ar]~1051moles cc

o

-

\

-- 0.10-0.11. - . 1.0

0.8 1.9

I 2.1

I 2.3

[-ArJ~A exp ( - - E 4 ~ / R T )

-~ I-ClimB e x p ( - - E ~ / R T ) .

_

oko-o.5o

[HF]/dt =

I 2.5

I 2.7

[Temperature ' K}-I ~10 4

Fro. 8. Initial decomposition rates for H F in the presence of large quantities of atomic chlorine [ C 1 ] / [ A r ] = 0.042.

(9)

D a t a corresponding to c o n s t a n t FAr-] b u t v a r i a b l e [C1] a n d to v a r i a b l e [-Ar-I b u t c o n s t a n t EC1] were t r e a t e d i n a like m a n n e r . T h e values of n, A, a n d E4 ~ were o b t a i n e d from a c o n s i d e r a t i o n of d a t a t a k e n in t h e absence of a d d e d halogen a t o m s . T h e results are s h o w n in T a b l e I I a n d are illust r a t e d for one case in Fig. 8. Clearly, t h e second t e r m of E q . (9) is a n e a r linear f u n c t i o n of t h e ratio FC1]/FAr~. Accordingly, all of t h e d a t a (ca. 48 cases) c a n b e des-

INTERACTIONS OF HALIDE ATOMS WITH HF

out the formation of large quantities of an intermediate complex and suggests that the presence of halogen atoms offers an efficient path for deactivation of vibrationally excited H F which competes with the process

cribed by the equation - -

115

d In [HF]/dt = [Ar]. 1014"3~~'e 9exp [--- (116,000 4- 5000)/RT] -5 (ECI]/EArJ)" 10~'e~x~ 9exp [--- (87,100 4- 4000)/RTJ;

(10)

see Fig. 9. Empirically, it was discovered that the radiancy R at the band center is related to the concentration ratio [-Cl]/rAr] by means of the equation

F = R/Ro = {1 -5 b([-ClJ/[-Ar])} -z,

(11)

where R0 is the radiancy of a binary mixture of H F and Ar. The constant b was found to have a value of 36 4- 2 for EClJ/[M] ~ 0.05.

ka HF* -5 M = HF -5 M.

One possibility is the formation of a collision complex of long life, i.e., kb

HF* -5 X = H F ' X * .

The experimental evidence indicates that halogen atoms alter the radiation process of H F without themselves being consumed. This rules

(13)

If the complex HF. X* is deactivated or destroyed by subsequent reactions before it spontaneously decomposes, a steady-state treatment for HF* gives the following result for the radiancy in comparison to that of a binary mixture of H F and Ar:

R/Ro = {1 -5 (kb/ka)([-XJ/[-M])} -~,

Discussion

(12)

(14)

which is identical in form to Eq. (11). The derivation of Eq. (14) presupposes that the process ko

HF*--+ HF -5 h, -

D

~ ,

5.5

~

IDEN(CL)/(M) 0

A rn~

,

0.0092

~

o.o19o

i-1

0:0420

x

does not compete effectively with Eq. (12). An examination of the absolute intensity measurements of Kuipers, 9 substantiates this assumption, i.e., k~ ( 108 sec-z. Although the complex may contribute to the total radiant intensity, an examination of Fig. 5 shows that the effect appears minimized in the vicinity of the band center. Consequently, we estimate ]~b/~ at 36 4- 2. The bimolecular rate constant for the process HF -5 C1 -- HC1-5 F

~

5.o

kl~ = 1013"sexp (--33,200/RT).

-a I 1.8

(16)

has been estimated by Mayer and Schieler 1~ as

"u

4.s

(15)

2.0

I

I

I

2,2

2.4

2.6

104/T

Fro. 9. Illustration of temperature and composition dependencies of initial reaction rat~s in the presence of atomic chlorine.

(17)

The present data give an activation energy of 87 kcal for the decomposition of HF in the presence of atomic chlorine. These facts suggest that a second process having a high activation energy (ca. E ~ > 87 kcal) competes successfully with Reaction (16). Since Eq. (4) has an apparent activation energy of only 116 kcal/mole, it is likely that this second process is simply an example of the holpogeneous catalysis of atomic

116

KINETICS OF ELEMENTARY CHEMICAL REACTIONS

recombination as described by Benson. ~1 The proposed reaction sequence for recombination of H and F atoms in the presence of a catalyst C, is as follows: C+F=

C-F*

C-F* ~- M = C'F-F

M

]ca C'F Jr H = C Jr HF.

~IEC1EF1 1 + (L~/k2EM]){ 1 + ( L , EMJ/k3EH]} " (19)

We consider the intermediate case described by Benson, 13i.e., ]C_l>> k2FM] and k-2[-M] < k3FH]. Under these conditions, Eq. (19) reduces to Rc =

k2(/,:,/Li)EClEM]EF1.

(20)

Since the termolecular recombination rate is given by R, =

k,EM]EH]EF],

(21)

the catalytic efficiency is E = R c / R t = (]r

H -~- C1 + M -- HC1 -{- M

(23)

CI~ -{- M = 2 C1 -{- M

(24)

(18)

Application of the quasi-stationary-state condition to C. F* and C. F leads to the following expression for the recombination rate: Re

exist in appreciable quantities under the conditions of the experiment, the only alternative is HC1. In order to test the assumption that HC1 has the proper steady-state concentration to act as the proposed catalyst, the rate expressions for Steps (3), (4), (16), and the following:

(22)

For an efficient catalyst, kl will exceed 1013 cc/mole/sec. We estimate k-1 at 1011 sec-1 and k2 at approximately 1013 cc/mole-sec. The value of let (ca. 1018/T cc2/mole2-sec) was taken from our previous work. 2 For one test at 4861~ the initial concentrations of H F and C1 were 1.3X 10-7 and 1.2X 10-7 moles/cc, respectively. Since the initial reaction rates quoted herein generally correspond to conditions maintaining at 5--15% reaction, the H-atom concentration would be approximately 1.3 X 10-8 moles/cc. These estimates given an over-all catalytic efficiency of 0.4 X 109 FC]. Since the observed efficiency for this case was 2.4, the concentration of the catalyst C should be 6 X 10-9 moles/cc. The requirement that the excited complex C.F* have a lifetime in the neighborhood of 10-100 vibrations rules out any monatomic species as the catalyst, C. Since C12 does not

were numerically integrated by means of a nonequilibrium computer program TM which assumes the process to be isentropic. The rate constant for Step (16) was taken from Mayer and Schieler, 1~ whereas those for Reactions (3) and (4) were taken from Blauer 2 and those for Steps (23) and (24) were taken from Cohen. 13 The results indicated that Reaction (16) has 1012'~ cc/mole-sec as its maximum pre-exponential factor, and furthermore, that it achieves equilibrium at about 15% reaction, after which it contributes little to subsequent decomposition. The steady-state HC1 concentration at this point was 1.0 X 10-8 moles/cc. Accordingly, catalysis by HC1 is reasonable. The enhanced catalytic efficiency in the presence of Br and I atoms can be accounted for by assuming that the corresponding complexes HBr. F* and H I . F* have longer lifetimes than HC1. F*. This is in keeping with the fact that the exothermicity of Reaction (--1) increases in the appropriate order. The negative dependence upon pressure depicted by the second term of Eq. (10) can be accounted for, at least in part, by a consideration of the factors that determine the steady-state concentration of HC1. The primary path for the production of HC1 is via Eq. (16) and its destruction proceeds primarily via Steps (--16) and (--23). Consequently, at steady state,

E H C l l . --

k16EClJErIF] (k-16EF] + k-~3EAr])"

(25)

Since we find that k-16 is not greater than 1012 cc/mole-sec, we have k_16[-F] < k_2~[-Ar] at 4860~ and the desired result is obtained. Although there may be alternative explanations to those presented here to account for the experimental facts, several other possibilities have been considered and discarded. Among those considered is the possibility that halogen atoms are simply efficient collision partners for excitation of HF. Unfortunately, the pre-exponential term in the Arrhenius expression must be unreasonably large (ca. 1017 cc/mole-sec) for the latter treatment.

INTERACTIONS OF HALIDE ATOMS WITH H F

117

these discussions proved very useful in the interpretation of our results.

Conclusions The decomposition of H F in the presence of halogen atoms appears to proceed via the formation of a collision complex, H F . X , having properties which differ significantly from those of the parent H F molecule. The change in spectral properties suggests that the dipole moment for H F is significantly altered during collision with a halogen atom. A dramatic acceleration of the over-all reaction rate in the presence of various halogen atoms is also observed. For mixtures having a concentration ratio of [-Xl/[-M ~ = 0.019, the apparent relative efficiencies are ]~F//kAr: kCl/]~Ar:kBr/kAr: kl/~Ar = 1:150: 500: 850.

The results are interpreted in terms of the homogeneous catalysis of the recombination of H and F atoms with HC1 acting as the catalytic species.

Acknowledgments The authors gratefully acknowledge their many discussions with Dr. T. A. Jacobs of Aerospace Corporation concerning this work. The results of

REFERENCES 1. JACOBS, T. A., GEIDT, R. R., AND COHEN, N.: J. Chem. Phys. 43, 3688 (1965). 2. BLAUER, J. A.: J. Phys. Chem. 72, 79 (1968). 3. SPINNLER,J. F. : Rohm and Haas Co. Technical Report S-204 (1969). 4. MALXMUS,W. : General Dynamics--Convair Report No. ZPH-119 (1961). 5. CARABETTA,R. A. AND PALMER, H. B. : J. Chem. Phys. ~46, 1325 (1967). 6. BOYD, R. K., BURNS, G., LAWRENCE, T. R., AND LIPPIATT, J. H. : J. Chem. Phys..~9, 3804 (1968). 7. DuFf, R. E.: Phys. Fluids 1, 546 (1958). 8. BRITTON, D. AND JOHNSON, J.: J. Phys. Chem. 64, 742 (1960). 9. KUIPERS, G. A.: J. Mol. Spectry. Z, 75 (1958). 10. MAYER, S. W. and SCHIELER, L.: J. Phys. Chem. 72, 236 (1968). 11. BENSON, S. W.: J. Chem. Phys. 38, 2285 (1963). 12. Furnished by Dr. T. A. Jacobs of Aerospace Corp., E1 Segundo, Calif. 13. COHEN, N., JACOBS, T. A., EMANUEL, G., AND WlLKINS, R. L.: Intern. J. Chem. Kin. 1, 551 (1969).

COMMENTS A. L. Myerson, Esso Research. Were any efforts made to identify HC1 as the catalyst in the chlorine experiments? J. A. Blauer. The rapid dissociation of HC1 under the conditions of our experiments makes it difficult to study its catalytic behavior unless very dilute mixes are used. Accordingly, we investigated the behavior of a mixture containing

2.0% H F and 0.2% HC1 in Ar. Tests conducted at 4460~ and 5050~ gave catalytic efficiencies E of 8.4 and 6.2, respectively. These results compare with a catalytic efficiency of 9.0 at 4500~ for a mixture containing 2.0% H F and 2.0% C12 in Ar. These tests were all conducted at a frequency of 4000 em-1, allowing complete spectral isolation of emission of H F from that of HC1 by our apparatus.