Communications to the Editor
1118
3.5
t
1
Od
3.0
‘._
E
I
1
I 2.51
i
0.6
-
n
.$ 1.0
.
Yo SOz-
III
Y-*0+3”0‘ I
1
+l+3+0+2*3+ 1
I
I
30 40 TIME, min Figure 3. Reaction of SOz with stable surface oxides at 800 flowing Nz.
20
IO
50
“C in
sulfur was found on the cooler walls downstream of the reactor. Gas samples were analyzed by mass spectrometry during the transient perturbations (which lasted for several minutes) in the experiments the results of which are presented in Figures 2 and 3. The analyses showed that SO2was partially consumed and the gaseous products were mainly Cog and a small amount of COS. Other sulfur compounds and CO were not detected and are assumed to be negligible. The major differences in the analytical results for the two cases, i.e., in Figures 2 and 3, were that quantities of the sulfur deposit and the net SOz comsumption for the latter case were much greater. Presumably most of the sulfur formed in the former case was oxidized in the gas phase by oxygen. A solid-phase analysis was also made for the sulfur content in the “control experiment” and it showed a gain of 0.1%; it increased from 1.7 to 1.8%. From the above, it is clear that SOz interacts with the thermally stable surface oxides, dissociates and removes COz, releases S and COS resulting in a net weight loss of carbon. Although the reaction mechanisms involved are obviously very complex, the following represents the major reactions that are consistent with the observations: C(0) t
so,
-*
+ C*(SO)
CO,(g)
C*(SO) + C(O), C*O&) t S(g) C*(SO) -+ COS(g) t Cf -+
Cf t 0, -t C ( 0 ) t Cf t SO,
slow -+
..
+ Cf
*
CO t CO, t S t C(S species)
(1) (2a)
Acknowledgment. We thank Mr. R. Smol for the entire experimental work and Drs. J. W. Sutherland and C. R. Krishna for helpful discussions. The carbon sample was kindly supplied and characterized by the Physical Chemistry Division of Alcoa Laboratories. This work was performed under the auspices of the Office of Molecular Sciences, Division of Physical Research, U.S. Energy Research and Development Administration, Washington,
D.C. References and Notes (1) R. T. Yang and M. Steinberg, Carbon, 13, 411 (1975). (2) N. R. Laine, F. J. Vastola, and P. L. Walker, Jr., J. Pbys. Cbem., 67, 2030 (1963). (3) G. R. Hennig, Proc. Conf. Carbon, 5tb, 1, 143 (1962). (4) E. J. Evans, R. J. M. Griffiths, and J. M. Thomas, Science, 171, 174 (1971). (5) R. T. Yang and M. Steinberg, J. Pbys. Cbem., EO, 965 (1976). (6) P. L. Walker, Jr., F. J. Vastoia, and P. J. Hart in “Fundamentals of Gas-Surface Interactions”, H. Saitzburg, J. N. Smith, and R. Rogers, Ed., Academic Press, New York, N.Y., 1967, p 307. (7) H. Marsh and A. D. Foord, Carbon, 11, 421 (1973). (8) W. 0. Stacy, F. J. Vastoia, and P. L. Walker, Jr., Carbon, E, 917 (1968). (9) H. Abramowltz, R. Inslnga, and Y. K. Rao, Carbon, 14, 84 (1976). Department of Applied Science Brookhaven Natlonal Laboratory Upton, New York 11973
Ralph T. Yang” Meyer Stelnberg
Received December 20, 1976
(2b) (3)
(4)
The parentheses indicate the adsorbed state, the asterisk indicates the new surface site, and the subscript f denotes the free site. The subscript m indicates a mobile oxygen The Journal of Physical Chemistry, Vol. 81, No. 11, 1977
which remains in the surface force field but is energetic enough to jump from site to site, and which is believed to be important in the C-O2 m e c h a n i ~ m . ~Reaction ,~ 2a is thought to be more important than reaction 2b. Reaction 3 could serve as a chain propagator until a stable species, e.g., an oxide, sulfide, or sulfoxide, is formed. This probably explains why the transient perturbations are longer when O2 is present; especially when the surface is first exposed to SO2 (Figure 2), and the transients are always shorter when SOz is not present in the gas phase (Figure 3). Reaction 4 has been studied and it proceeds quite slowly as compared to rates measured in this study.81’ The fact that a steady-state rate is restored at each SO2 concentration indicates that an equilibrium is reached between the gas phase and surface species involving sulfur. Results shown in Figure 3, however, indicate the lack of such an equilibrium. Therefore, O2 in the gas phase must be involved in the equilibrium. The nature of the equilibrium and the interactions between SO2and the surface oxides, in general, is far from being understood at this point. Results presented in this report are qualitative. Further work is in progress in our laboratory and the results should also shed some light on the carbon combustion mechanism. In passing, it should be noted that a similar phenomenon has also been observed with a nuclear graphite (ash content
