M. Yata et Rj. Madix, KINETIC ISOTOPE EFFECTS AND TUNNELING IN C-H BOND ACTIVATION OF ETHYLENE AND ETHYLIDYNE ON PT(111), Surface science, 328(3), 1995, pp. 171-180
The importance of tunneling in the dehydrogenation of ethylene on Pt(1
11) was assessed with temperature programmed reaction spectroscopy. Ra
te constants for hydrogen (deuterium) evolution during the formation a
nd decomposition of ethylidyne were quantified, and the kinetic isotop
e effects measured. At low ethylene coverage hydrogen is evolved in a
desorption-limited step near 340 K. At higher coverage a first-order p
rocess gives rise to ethylidyne formation near 300 K with rate constan
ts of 3 x 10(16) exp(-22.4(kcal/mol)/RT) s(-1) and 2 x 10(16) exp(-23.
0(kcal/mol)/RT) s(-1) for C2H4 and C2D4, respectively, yielding a norm
al kinetic isotope effect of 4.0 at 300 K. The rate constants for rate
-limiting C-H(D) bond activation in CCH3 and CCD3 are 2.9 X 10(16) exp
(-37.3(kcal/mol)/RT) s(-1) and 1 X 10(16) exp(-37.8(kcal/mol)/RT) s(-1
), respectively, giving a kinetic isotope effect of 3.1 at 300 K. All
of the criteria normally applied to judge the importance of tunneling
indicate that tunneling is unimportant for these reactions.