4397
J. Phys. Chem. 1986, 90, 4397-4402 energy origin may be expected to have longer lifetimes than at 3000 8, and be more prone to quenching by 02.If indeed this is the case and TI quenching does not result in C O formation, as has been suggested by Weaver et al.? then it should be reflected in a differenceS between $, estimates based on CH3 scavenging
by HCl and those derived from HCO scavenging by 02.Studies aimed a t the testing of this hypothesis are now in progress. Registry NO. CH,CHO, 75-07-0; HCI, 7647-01-0; C02, 124-38-9; 02, 7782-44-7; CH.,, 74-82-8; CO, 630-08-0; H2, 1333-74-0.
Reactions of Chlorine wlth Liquid Metals. 4. Tin D. R. Olander,*t M. Balooch: and W. J. Siekhaust Materials and Molecular Research Division of the Lawrence Berkeley Laboratory and Department of Nuclear Engineering, University of California, Berkeley, California 94720, and Chemistry Division of the Lawrence Livermore National Laboratory, Livermore, California 94550 (Received: January 13, 1986)
The reaction of molecular chlorine with solid and liquid tin was studied by modulated-molecular-beam mass spectrometry and Auger electron spectrometry. The temperature range was 350-700 K and equivalent chlorine pressures between 2 X and 5 X lo4 Torr were used. At constant pressure, the production rate of the sole reaction product (SnC12)increased with temperature with no sign of a discontinuity at the melting point. The reaction was nonlinear with respect to chlorine equivalent pressure. The model developed for the lead-chlorine system was applicable to the tin-chlorine system. However, solution diffusion of chlorine in the liquid metal did not appear to be significant for the tin reaction. A comparison of all four metal-chlorine reactions studied in this series is presented.
Introduction The present work is a continuation of the series on the reactions between low-melting metals and molecular chlorine over the temperature range bracketing the solid-liquid phase transition. Like the previous investigations of In,' Pb,2 and Bi,3 the present study of tin utilizes the modulated-molecular-beam technique with phase-sensitive detection of reaction products by an in situ mass spectrometer. In a separate chamber, the surface of the metal is monitored by Auger electron spectroscopy (AES) under conditions simulating those in the molecular-beam tests. Details of the experimental method and the data analysis procedures are given in ref 1. Results The tin-containing ions observed in the mass spectrometer are SnC12+,SnCl+ and Sn+. All have the same temperature dependence and the same phase angles, indicating that they all originate from the neutral species SnCl2. This molecule is thus the sole product of the reaction. All measurements utilize the SnCl' peak, which has the highest intensity at a nominal electron energy of 75 eV, and are then corrected for the observed fragmentation pattern. The reaction probabilities reported below are based on the SnC12 product signals divided by the sum of the C12+signal and half of the C1+ signal for a surface at room temperature. Signal ratios are converted to flux ratios (i.e., apparent reaction probabilities) by using ionization cross section ratios obtained from ref 4 and other instrumental efficiency factors and transit time effects discussed in ref 5. Figure 1 shows the apparent reaction probability (c) and phase lag ($) of the SnClz product as functions of ternperdure at the maximum incident chlorine beam intensity and for a modulation frequency of 20 Hz. Unlike the previous reactions studied, no discontinuity is observed in either e or $ as the melting point is *Address correspondence to this author at the Department of Nuclear En ineering, University of California, Berkeley, CA 94720. ?Materials and Molecular Research Division of the Lawrence Berkeley Laboratory and the Department of Nuclear Engineering, University of California, Berkeley, CA 94720. *Chemistry Division of the Lawrence Livermore National Laboratory, Livermore, CA 94550.
0022-3654/86/2090-4397$01.50/0
TABLE I: Parameters of the Sn-CI?Reaction Model preexponential activation parameter factor enerav. kcal/mol ll
KkLH KqER/Ns
0.06 1.2 x io-' cm2/s 3.5 X lo-" cm2
11 10
crossed. The reactivity increases with temperature up to 600 K and then levels off a t higher temperatures. The phase monotonically decreases with temperature. The effect of beam intensity on c and $ is shown in Figures 2 and 3 for constant modulation frequency and three surface temperatures, two above and one below the melting point. These results clearly indicate that the overall reaction is nonlinear and that there is no distinction that can be attributed to the phase change of the tin. The frequency dependences of the reaction product vector are shown in Figures 4 and 5 for two temperatures above the melting point and two below. The phase lag decreases with increasing frequency for all temperatures but 600 K, where it remains approximately constant. This is a clear indication of a mechanism containing parallel reaction paths for the production of SnCl2, one of which provides a rapid channel for converting incident Clz to product. The slow step is demodulated a t high frequencies, leaving only the rapid step that exhibits the low phase lag. The surface coverage of chlorine as a function of temperature for an incident flux of 5.5 X 10l6molecules/(cmz s), as measured in the AES apparatus, is shown in Figure 6. The surface appears to be saturated up to about 350 K, after which a rapid decrease with increasing temperature ensues. (1) Balooch, M.;Siekhaus, W. J.; Olander, D. R. J . Phys. Chem. 1984, 88, 3522. (2) Balooch, M.; Siekhaus, W. J.; Olander, D. R. J . Phys. Chem. 1984, 88, 3530. (3) Balooch, M.; Siekhaus, W. J.: Olander, D. R. J . Phys. Chem. 1986, 90, 1671. (4) Mann, J. B. In Recent Developments in Mass Spectroscopy; Ogata, K., Hayakawa, T., a s . ; University Park Press: Baltimore, MD, 1970; p 814. (5) Jones, R. H.; Olander, D. R.;Siekhaus, W. J.; Schwarz, J. A. J. Vac. Sei. Technol. 1972, 9, 1429.
0 1986 American Chemical Society
4398
Olander et al.
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
I
I
f
=20 HZ
a
0 IO” Intensity, ~ , ( m o ~ e c u ~ e s / c.s) m~
10‘6
CI, Beom
Figure 3. Beam-intensity dependence of phase lag of SKI2with respect to room-temperature-scattered C12 for three different target temperatures. I
I
I
I I
f=20Hz
1
1
1
I
I ~ = I .x I~ ~ “ r n o ~ e c u ~ e s / c r n ~ ~ s
445
2 i
10
10”
10‘6
2 -
C I eeorn ~ I n t e n s i t y , I,[molecdes/cm‘s)
Figure 2. Beam-intensity dependence of apparent reaction probability of the SnCI2 product for three different target temperatures.
Reaction Model Based on the clear signs from the raw data of nonlinearity and branched steps, the following reaction model is proposed: Clz(g)
Cl(ads) Cl(ads) Cl,(g)
2Cl(ads)
(1)
K
+ Sn =, SnCl(ads)
+ SnCl(ads)
~ L H
SnCl,(g)
+ SnCl(ads) -% SnClz(g) + Cltads)
(2) (3) (4)
Reaction 1 represents the dissociative chemisorption of molecular chlorine with sticking probability 7. Reaction 2 denotes equilibrium between adsorbed chlorine and the monochloride surface intermediate. The production and desorption of SnClz via Langmuir-Hinshelwood branch with a rate constant kLHis shown in step 3. Reaction 4 is the Eley-Rideal branch, which provides the fast pathway for production of SnCl,. It is char-
I
102 Modulation Frequency (HI)
I
I
Figure 4. Frequency dependence of apparent reaction probability of
SnCI, for four different target temperatures.
acterized by a reactive sticking probability vER. This model is identical with that used to interpret the leadchlorine reaction2 except that no solution-diffusion step is needed for the case of tin. The theoretical apparent reaction probability and phase lag can be obtained from eq 10-12 of ref 2 by ignoring the solution-diffusion terms. The three parameters of the model, ~ , determined by a Monte Carlo namely 7,KkLH,and K ~ E R I Nare least-squares method. The results of this procedure are the reaction parameters shown in Table I. The sticking probability is close to that obtained for the lead reaction. The rate constant for the Langmuir-Hinshelwood reaction is slightly larger than that for lead but has the same activation energy. The Eley-Rideal parameter in the last row of Table I differs from that observed for the lead reaction by its nonzero activation energy; its magnitude is also larger than that of lead. The fit of the model using the parameters in Table I to the data is shown as the curves in Figures 1-5. In general the agreement is very good.
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4399
Reactions of Chlorine with Liquid Metals
,
TABLE II: Volatile Products from the Reactions of Chlorine and
I
I
Four Low-Melting Metals
I,= I . I ~ I Ornolecules/crn2.s '~
~
T (K)
-a-
T: 400K
metal indium lead bismuth tin
400
-0- 450 --A-- 523
.....0.... 600
o\D
products observed solid liquid InCI, (InCI2)" InCl PbC12 PbC12 BiCI3 BiCl SnCI, SnC1,
source of equilibrium data b C
b C
"Trace quantity detected. Barin, I.; Knacke, O., Kubaschewski, O., Eds. Thermochemical Properties of Inorganic Substances; SpringerVerlag: New York, 1977, supplement. 'Barin, I., Knacke, 0.;Thermochemical Properties of Inorganic Substances; Springer-Verlag; New York, 1973. 6 00..."
6
-
I
I
I
I
-
8
1
102
Modulation Frequency (Hz)
Figure 5. Frequency dependence of the SnCI2 phase lag for four different target temperatures. 1
I
I
10=5.5x10'6molecules/cm2.s
0
.-
0
t
o300
I
400
m
-
600
700
Surface Temperature IK)
Figure 6. AES measurement of the temperature dependence of the surface chlorine coverage for fixed beam intensity of -5.5 X 10l6 molecules/(cm2 s). The solid line is the prediction of the model with the contribution of SnCl(ads) neglected.
As a final verification of the model, the chlorine coverages from the AES measurements are compared to the model prediction, which is (C1 + SnCl),d, = (1 K ) ( V I ~ / K I C ~ ~ ) ~ / ~( 5 )
+
With the assumption of a saturation concentration of 3 X 1414 cm-2 at low temperature and K
