Role of translational and vibrational energy in the dissociative chemisorption of methane on Pd{1 1 0} M. Hirsim aki a , S. Paavilainen b , J.A. Nieminen b , M. Valden a, * a Surface Science Laboratory, Institute of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland b Computational Physics Group, Institute of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland Abstract Dissociative adsorption of methane has been investigated on Pdf110g by using molecular beam surface scattering. The initial sticking probability has been determined in the translational energy range of 7±95 kJ/mol and at selected vibrational energies from 300 to 700 K. The measured initial sticking probability is found to increase strongly with both translational and vibrational energy of CH 4 molecules. The activation of the dissociative chemisorption of CH 4 induced by the vibrational energy is shown to depend on the translational energy and is attributed to the excitation of the bending modes of the incident molecule. We have also performed molecular dynamics simulations to investigate the dissociation mechanism theoretically. The simulations clearly demonstrate that an ecient energy transfer occurs upon adsorption between the translational and vibrational energies of the incident CH 4 molecule, which thereby facilitates the deformation of the molecular structure of CH 4 resulting in dissociation. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Alkane; Chemisorption; Palladium; Molecular dynamics 1. Introduction Conversion of methane to higher hydrocar- bons and oxygenates is a challenging problem both from the scienti®c and economic viewpoints. However, due to its closed-shell structure CH 4 molecule is extremely stable and its chemisorption on solid surfaces is a highly activated process. As a result, the initial and often rate-limiting step in catalytic conversion of methane to higher hydro- carbons is the C±H bond activation. Dissociative adsorption of CH 4 on metal sur- faces has been extensively studied with molecular beam methods. It has been found experimentally for Ruf0001g [1], Ptf111g [2], Nif100g [3] and Wf110g [4] that the CH 4 dissociation is a direct and activated process and strongly enhanced by the translational and vibrational energies of the incident molecule. Only a weak surface tem- perature dependence has been observed on most surfaces, which is a consequence of the direct adsorption mechanism in which the molecule does not have to accommodate to the surface before dissociating. A variety of models have been developed to describe the CH 4 adsorption dynamics. In the quantum mechanical model by Luntz and Harris Surface Science 482±485 2001) 171±176 www.elsevier.nl/locate/susc * Corresponding author. Tel.: +358-3-365-2555/358-31- 3162600; fax: +358-3-365-2600/358-31-3162555. E-mail address: valden@ee.tut.® M. Valden). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602801)00818-4