338
J. Phys. Chem. 1980, 84, 338-339
edges fellowship support from Merck, Inc. References and Notes
(6) Golden, W. 0.; Dunn, D. S.; Overend, J. J . phys. Chern. 1978, 82, ...A
u4.5.
(1) Shigeishi, R. A; King. D. A. Surf. Sci. 1978, 58, 379. (2) Hoffman, P. M.; Bradshaw, A. M. J . Catal. 1978, 44, 328. (3) Chesters, M. A.; Pritchard, J.; Sims, M. L. In “Adsorption-Desorption Phenomena”; Rlcca, F., Ed.; Academic Press: New York; 1972. (4) Ito, M.; Suetaka, W. Surf. Sci. 1977, 62, 308. (5) Campuzano, J. C.; Greenier, R. G. Surf. Sci. 1979, 83, 301.
(7) Golden, W. G.; Dunn, D. S.; Pavilk, C. E.; Overend, J. J. C h m . phys. 1979, 70, 4426. (8) Sokcevic, D.; Lenac, 2.; Brako, R.; Sunjic, M. 2.Phys. B 1977, 28, 273. (9) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. J. Chrn. phys. 1979, 70, 2180. (10) Ibach, H.; Lehwald, S. Surf. Sci. 1978, 76, 1.
COMMUNICATIONS TO THE EDITOR Mechanism of the Thermal Decomposltlon of Monosllane Publlcation costs assisted by the Unlversify of Alberta
Sir: Among the silicon hydrides, the kinetics of thermal decomposition of the simplest representative, monosilane, were the first to be investigated in detail but in spite of the numerous studies since the pioneering work of Purnell and Walshl (PW) 13 years ago, the mechanism of this reaction has remained controversial. PW postulated two possible alternative mechanisms, a molecular (A) and a radical chain (B) scheme, but the question of which one of them is operative could not be clearly established. mechanism A SiH4 SiH2 H2 (1)
-+ -+
SiH2 SiH4 mechanism B
---
SiH4 SiH3 + H H + SiH4 H2 + SiH3 SiH3 + SiH4 Si2& + H
(2) (3) (4)
Several experiments were performed at 380 “C by using SiH4-C2H4mixtures at various ratios and total pressures and the reaction orders for H2and Si&&formation for each mixture of a given ratio were found to be 1.5, in agreement with PW. Although such results have often been cited6P6 in support of radical mechanism B, PW tentatively concluded, on the basis of thermochemical arguments, that molecular mechanism A was operative and that the observed reaction orders were 1.5 because the initial step (1) was in the unimolecular falloff region. However, if we consider the following elementary steps, where M is any diluent gas: SiH4 + SiH4 SiH4* + SiH4 (a)
+ + + - +
SiH4
SiH4* + M
M
SiH4* SiH4 SiH4*
SiH4*
M
(a’)
SiH4 + SiH4
(b)
SiH4 + M
(b’)
:SiH2
(4
H2
steady-state treatment yields the following expression:
(5)
2SiH3 Si2H6 (6) Ethylene is known to be an efficient scavenger of silyl radicals2 but to be relatively inert toward ~ilylenes.~ We have recently shown that the use of this scavenger made it possible to differentiate between free-radical and molecular reaction channels in the thermolysis of monomethyl~ilane.~We report here an extension of these studies to the thermolysis of monosilane; the results unambiguously point to the occurrence of the molecular mechanism (A) in the gas phase. Pyrolysis of SiH4 in a “seasoned” vessel at conversion not exceeding 1% produces two major products, H2 and Si2&. Addition of 20 torr of ethylene to 200 torr of monosilane at 380 “C had no discernible effect onthe product yields, in,marked contrast to the CH3SiH3system, and clearly points to the absence of radical chain reactions leading to formation of H2 and Si2H6.At 380 “C, addition of an eightfold excess of ethane to 59 torr of SiH4enhanced the rates of H2 and Si2H6formation twofold; under the same conditions ethylene effects an identical increase in the H2 yield but the SizH6 yields are decreased by about 40%. It would therefore appear that at sufficiently high concentrations C2H4 is capable, though inefficiently, of scavenging SiH2radicals. Diluent gas accelerating effects were also reported by PW but the results could not be properly interpreted at that time. 0022-3654/80/2084-0338$01 .OO/O
-
-
Thus, when k, kat and k b kb’ as should be the case for C2H4 or C2H6 diluents, this relation can be rearranged to
which predicts first-order dependence on [SiH4] as long as [SiH4] + [MI is constant. We therefore carried out experiments on various SiH4-C2H6mixtures at constant pressures of -590 torr. The resulting order plots for H2 formation (those for Si2H6 were identical), shown in Figure 1, have slopes of 1.0 f 0.1 at all temperatures examined. We have also recalculated P W s data for Si2H6from SiH4-SF6 mixtures (430 OC, 230 f 10 torr’) and found essentially the same results, some of which are also shown in Figure 1. When plotted in the Arrhenius form the data (14 points) yield a good linear relationship. The rate parameters, together with those obtained from other studies, are listed in Table I. Bearing in mind the widely different experimental conditions employed, we find that these results are all in reasonably good agreement. The measured Arrhenius parameters refer to the unimolecular rate constant k1 in the falloff region but the limiting high-pressure value of E , is probably within a few kilocalories of the falloff value of ca. 55 kcal mol-l. On the 0 1980 American Chemlcai Society
The Journal of Physical Chemistry, Vol. 84, No. 3, 7980 339
Communications to the Editor
TABLE I: Arrhenius Parameters for H, and Si,H, --
Formation in the Initial States of SiH, Pyrolysisa disilane hydrogen -
~
-
.
_
_
conditions
_
log A , s-’
-
-
E,, kcal/mol
log A , s-l
E,, kcal/mol
ref
static thermolysis, T = 305-400 ‘C, Ptot 590 torr 13.76 i 0.15 55.05 i 0.44 13.51 f 0.29 54.77 k 0.84 this work static thermolysis,b T = 380-430 “C, Pbt= 230 torr 13.21 * 0.75 53.58 * 2.23 13.21 i 0.22 53.95 i 0.69 1 shock tube, T z - 927-1027 “C 13.7 i 0.3 56.1 f 1.7 8 First-order rate constants for Arrhenius plots were calculated from the initial a Error limits: are standard deviations. rates of H, and Si,H, formation at 230 torr of SiH,, reported by Purnell and Walsh in Figure 7 of ref 1; the initial rates at different temperah& were estimated from the graph.
0.4
400’
355‘ -04
