76 Annals New York Academy of Sciences

70. T ~ T ~ N Y I , P., L. BABERNICH, K. SCHACHTER & Z. PAAL. 1969. Research in Iso-

71. GUCZI, L. & P. TBT~NYI. 1972. Acta Chim. Hung. 71: 341. 72. GUCZI, L. & P. TBTTBNYI. 1968. Z. Phys. Chem. 237: 356. 73. DEBRENTZEV, Yu. I., 2. PALL & P. TBTBNYI. 1973. Z. Phys. Chem. In press. 74. GUCZI, L., B. S. GUDKOV & P. TBTBNYI. 1972. J. Catal. 24: 187. 75. Guczr, L., A. S ~ ~ N Y & P. T B ~ N Y I . 1973. Fifth Congress Catalysis, Miami

76. HORIUTT, J. 1957. Advances in Catalysis. Vol. 9: 1. Academic Press. New York,

77. KANEKO, Y. & H. ODANAKA. 1967. J. Res. Inst. Catal. Hokkaido Univ. 14: 213. 78. CSUHA, R. S. & J. HAPPEL. 1971. Amer. Inst. Chem. Eng. J. 17: 927. 79. HAPPEL, J. , H. ODANAKA & P. ROSCHE. 1971. Amer. Inst. Chem. Eng. J. Symp.

80. HAPPEL, J. & R. S. ATKINS. 1970. Ind. Eng. Chem. Fund. 9: 11. 81. HAPPEL, J. , M. A. HNATOW & R. MEZAKI. 1970. Advan. Chem. 97: 92. 82. OKI, S. & Y. KANEKO. 1970. J. Res. Inst. Catal. Hokkaido Univ. 18: 93. 83. TAKEUCHI, T., M. SAKACUCHI & Y. TOGASHI. 1966. Bull. Chem. Soc. Jap. 39

tope Chemistry. P. Tktknyi, Ed. Publ. Inst. Isotopes. Budapest, Hungary.

Beach. North Holland Publ. Amsterdam, The Netherlands. In press.

N.Y.

Ser. 67: 60.

1437.

DISCUSSION

Kenneth Kamholz, Moderator

Merck, Sharp Ce Dohme Laboratories Rahway, New Jersey 07070

DR. KAMHOLZ: Dr. Gal, I would like to know whether you’ve considered the use of a combination of tracers, for example, 14C and an oxygen tracer simultaneously, to see if you could determine additional information?

DR. GAL: I mentioned the use of 180, but this was not used as a tracer, and not as a kinetic application in the sense of the KIM, because if you use only initial compound labeling, during the reaction, all compounds will have the same specific activity.

DR. KAMHOLZ: Because you have the technology for the use of both l 8 0

and 14C, you might be able to conduct some of your experiments with alcohol, for example, instead of hydrocarbon, and doubly-tag it.

DR. GAL: Yes, however, KIM equations are based on the assumption that the concentration of the isotope is negligible. This is true if you use radioactive isotopes, where the concentration of the labeled species is extremely low. If you use oxygen, you have to modify all the equations.

DR. HAPPEL: There are points of similarity between your work and that of Horiuti. Essentially, you both used the continuity equation and the steady- state approach. But, Horiuti’s treatment has an additional feature in that he used the transition-state theory to develop a relationship that applies to reactions that are reasonably close to equilibrium and in which you can measure the free energy change of the reaction. This provides an extra relationship in which this can be used. Have you developed any of your theories to consider the effect of reactions that were not-or steps that were not-completely irre- versible? Also, do you think that some of these transition-state relationships may be used to give some additional insight into the mechanistic equation?

Discussion 77

DR. GAL: I was studying your paper in Catalysis Review 6(2) :221-260 (1972). YOU mentioned in previous work that the Neiman approach is valid only for irreversible reactions. However, we have seen that there is no need to say whether a reaction step is reversible or irreversible. It is important to distinguish between total activity and specific activity. If compound B is labeled and is formed from an unlabeled compound (A) , its specific activity will be changed, but its total activity remains unchanged. Therefore, if we write the reaction from A to B as a reversible step, consumption changes only total activity and formation changes specific activity. Thus, the same expressions can be used for reversible and irreversible reactions. This I would like to demonstate in TABLE 1 in which I have been comparing the two different treat- ments (Horiuti’s and the KIM) for a reversible step. It can be seen that both yield entirely the same expressions. DR. HAPPEL: Both methods give the same results as far as the material

balance is concerned. But, Horiuti’s treatment goes one step further; he uses the transition state to derive an entirely independent useful relationship. Has this ever been applied in the KIM? DR. GAL: I don’t know-not in homogeneous reactions as far as I know.

There is no doubt that Horiuti’s new treatment is a far-reaching development of the whole procedure. However, I was considering the tracer; I still have a problem with the heterogeneous surface.

DR. HAPPEL: Your method and Horiuti’s method can be applied efficiently if you only measure the homogeneous (terminal) species. The only problem in the application of either Horiuti’s or the kinetic isotope method in hetero- geneous systems is with an intermediate. You don’t know what the concentra- tion is if the intermediate is on the surface, unless you have additional informa- tion. Horiuti’s method supplies the necessary information by the transition-state relationship. You can avoid this problem by measuring the movement of tracer from one terminal species to the other terminal species, without considering what happens to the intermediate. DR. GAL: I agree completely, but is it possible to have a system where only

these two components are present-no intermediate? DR. HAPPEL : With or without intermediates, the tracer material balance

equations hold. DR. GAL: What happens if, for instance, when A is transformed into B,

you have a parallel reaction? DR. HAPPEL: The simple treatment says that you have one path, one route.

If you have two routes, two paths, two mechanisms, whether two mechanisms for a single reaction or more than one reaction, this becomes more complicated.

DR. GAL: How do you know it is only one? You can’t measure activity on the surface. DR. HAPPEL: Correct. DR. GAL: If there are two ways, two complexes on different surfaces, with

different energetics, how do you know their specific activities? With specific activity, you measure the gas state and assume that it is the same on the surface.

DR. HAPPEL: Well, you don’t have to assume that it is the same as on the surface if you use material balance equations that involve only the homogeneous reactants and products. You can have either a nonuniform or a uniform surface; as long as one surface species does not migrate to another location during steady-state reaction conditions.

If the steady-state is reached, you can draw some conclusions. Also, if you

78 Annals New York Academy of Sciences

use this method, you must assume some mechanism. Whenever you use a method, you must presuppose that you know what the mechanism is or what the possible mechanisms are.

DR. GUCZI: I think the problem of labeling the starting material has been raised, and I would like to comment about the usefulness of this method. Consider the hydrogenolysis of propane. If you want ?o simultaneously measure the hydrogenolysis and the isomerization, it is useful to label the starting mate- rial with [14C]propane. If the hydrogenolysis occurs, there are two ways to split the two bonds (FIGURE 1 ) . However, only one sequence produces a labeled effect in the gas phase. The specific activity of ethane is nearly one-half that of propane.

I4 c-c-c No Isomerization

14c-c- + -c-c

I4 c-c-c -+ C-I4C-C

Isomerization

I4 c- c- + - c-c FIGURE 1.

Now, if during hydrogenolysis isomerization is occurring, that is, the labeled molecule from the terminal carbon enters the molecule, then both halves of the molecule will be labeled after the carbon-carbon bond splits. Thus, the specific radioactivity of ethane, if there is any isomerization during hydrogenolysis, will be larger. This is a crude method to decide whether isomerization occurs during hydrogenolysis.

DR. HAPPEL: Dr. Guczi has demonstrated something different from what both Horiuti and Gal discussed, because they were only tracing velocities of reactions by using a small amount of tracer, whereas we now have an example of the effect of tracing one of the carbon atoms and comparing it with a carbon atom in a different position.