The kinetic isotope effect in the dissociative chemisorption of methane

Investigations involving the dissociative chemisorption of CH4 and CD4 on clean tungsten have now been extended to include the other partially deuterated methanes. The ratio of the sticking probabilities S (CD3H)/ S (CD4), S (CD2H2)/S (CD4), S (CDH3)/(SCD4), and S (CH4)/ S (CD4) are approximately 1.7, 2.7, 3.5, and 4.5, respectively, for 2250 °K≳T≳1250 °K. These large ratios have an unexpectedly small temperature dependence. The apparent activation energies (i.e., the slope of R lnS vs 1/T) are all 9.3±1 kcal/mole for 1350 °KT2359 °K. It is shown that a model involving (1) the tunneling of hydrogen atoms through a potential barrier, (2) vibrational excitation, and (3) the lifetime of the undissociated molecule on the surface can qualitatively explain the experimental results. Model calculations suggest that the dissociative chemisorption reaction, and in particular the kinetic isotope effect, is dominated by tunneling. The apparent activation energies are primarily determined by vibrational excitation while the change in activation energy observed at lower temperatures can be accounted for by considering the lifetime of the undissociated molecule on the surface.