Direct dissociative chemisorption of methane and ethane on Ir(110): isotope effects and vibrationally assisted chemisorption

Abstract

We have investigated the direct dissociative chemisorption of methane and ethane on the reconstructed Ir(110) surface using supersonic molecular beams. We have considered the direct chemisorption of both CH 4 and C 2H 6 and their fully deuterated isotopes at beam energies between 1 and 35 kcal/mol, and have observed a substantial kinetic isotope effect, both in the translational energy required for the onset of measurable chemisorption, and for the initial probability of dissociative chemisorption at a given translational energy. The translational energy needed for a measurable (> 0.01) probability of chemisorption varies between 6 and 11 kcal/mol for the four molecules. The initial probability of dissociative chemisorption increases rather linearly with beam energy above this onset up to the highest beam energies employed. We obtain reasonable agreement between the experimental data and a model of tunneling through an Eckart barrier. The tunneling model gives a barrier height of 36 kcal/mol for both methane and ethane, with ethane having a slightly more narrow barrier width (0.13 Å) than methane (0.15 Å). We have also investigated the effect of vibrational energy on dissociative chemisorption by using vibrationally hot molecular beams. The results show that there is substantial vibrational assistance for CD 4 and C 2D 6 (~ 0.05), but that there is no measurable effect for CH 4 and C 2H 6 (< 0.02). We show that these results are most consistent with attributing the enhanced chemisorption probability to the asymmetric C-D and C-H stretching modes, although it is possible that other vibrational modes are contributing to the observed enhancement. We also observe a lowering of the translational energy required for measurable chemisorption of the vibrationally hot beams by approximately 4 kcal/mol. Such an effect is again shown to be consistent (although not uniquely) with the asymmetric C-D stretching mode. We have also measured the variation of the initial probability of chemisorption with the polar angle of incidence between the beam and the surface. In all cases, alkane chemisorption is shown to obey normal-energy scaling, within experimental error. Finally, we report that there is no observable dependence of the initial probability of direct chemisorption on surface temperature between 550 and 1150 K under the conditions investigated here, which is consistent with the traditional picture of direct dissociative chemisorption.