Dynamics of activated chemisorption: Methane on rhodium

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Abstract
Chemisorption of methane has been examined on highly perfect rhodiumsurfaces by a combination of field emission and molecular beam techniques in order to explore the mechanism of activation. It has been found that dissociative adsorption can be initiated on rhodium at 245 K just by excitation of the gas. Quantitative rate studies as a function of gas temperature have established that the CH4 molecule must overcome an activation barrier of 7 kcal mole−1 for reaction to occur. For gas temperatures 600 < T G < 710 K, the efficiency of chemisorption per impact is at least an order of magnitude smaller for CD4 than for CH4 and a factor of ? 3 smaller for CH2D2. This suggests that translational and rotational motion are not directly involved in passing over the barrier. Vibrational excitation of methane molecules appears to be the significant step in the reaction at the surface. Transition state theory fails to account for the significantly smaller rates observed for CD4 than for CH4. However, Slater’s dynamical model of unimolecular reactions leads to a sizable isotope effect for the rate of dissociation, and the semiquantitative data presently available are best explained by his model.

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