KINETIC ISOTOPE EFFECTS AND TUNNELING IN C-H BOND ACTIVATION OF ETHYLENE AND ETHYLIDYNE ON PT(111)

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.