Dynamics and Energetics of Methane on the Surfaces of Transition Metal Oxides

In the very early part of this chapter, we review the Langmuir model of adsorption to understand the dynamics of the competition between adsorption and desorption of methane on a surface. Then, the kinetics model introduced is further developed so that one can apply it to the dissociation dynamics of the C–H bond of methane on surface whether it happens in a high-pressure limit or low-pressure one. In the latter limit, an interesting concept of the apparent activation energy is introduced and its correlation with the desorption barrier or the adsorption energy is clarified. In order to reduce the apparent activation energy associated with the dissociation of the C–H bond of methane on a surface, one needs to find a surface which enjoys a substantial affinity to methane; such a candidate can be found in a class of metal oxides. In such a situation, what is so-called trapping-mediated mechanism can apply. The (101) surface of PdO and the (110) and (100) surfaces of IrO2 and RuO2 have been experimentally and theoretically characterized as a good candidate for such a reaction mechanism.We carry out a comprehensive survey of their surface features, stressing an importance of possessing a coordinatively unsaturated (cus) metal site as well as a cus O site nearby on the surface so that a H atom of methane can be abstracted by the cusOsite and the resultantCH3 fragment can be significantly stabilized by an interaction with the cus metal site. Taking the IrO2 (110) surface as an example, we conduct a thorough scrutiny on the electronic structure of methane on the surface at the level of the extended Hückel method, which can provide us with a qualitatively correct insight into orbital interaction. We make good use of a band decomposition technique called crystal orbital overlap population (COOP) to unveil some important orbital interactions between the C–H bond dissociated and the surface cus Ir and O atoms.