Dynamics of the activated dissociative chemisorption of CH4 and implication for the pressure gap in catalysis: A molecular beam–high resolution electron energy loss study

Abstract

The dynamics of the activated dissociative chemisorption of CH4 on Ni(111) are studied by molecular beam techniques coupled with high‐resolution electron energy loss spectroscopy. The probability of the dissociative chemisorption of CH4 increases exponentially with the normal component of the incident molecule’s translational energy and with vibrational excitation. The dissociative chemisorption probability of CD4 exhibits the same trends with a large kinetic isotope effect. High‐resolution electron energy loss spectroscopy identifies the nascent products of the dissociative chemisorption event as an adsorbed methyl radical and a hydrogen atom. These results, which have shown that there is a barrier to the dissociative chemisorption, are interpreted in terms of a deformation model for the role of translational and vibrational energy in promoting dissociative chemisorption. The barrier likely arises largely from the energy required to deform the molecule sufficiently to allow a strong attractive interaction between the carbon and the Ni surface atoms. Tunneling is suggested as the final process in the C–H bond cleavage. The presence of this barrier to dissociative chemisorption presents a plausible explanation for the pressure gap in heterogeneous catalysis.