MOLECULAR MODELING OF ETHYLENE DECOMPOSITION ON PLATINUM SURFACES

We examine the process of dehydrogenating ethylene over the (111) and (100) platinum single crystal surfaces from a modeling point of view. In order to establish the reaction pathways and the important reaction coordinates, the stability and concentration of intermediate surface species must be known. We use a simple semi-empirical tight-binding scheme, based on the extended Huckel theory. In order to model the potential energy hyper-surface (PES) on which the energy minima are found, we use a pair-potential model to describe the repulsion, similar to the ASED-MO and other methods. We fit the parameters of the pair-potential to the vibrational properties of simple molecules and adsorbates. The energy is minimized with respect to all the coordinates of the hydrocarbons on the surface using the conjugate gradient method. On the Pt(111) surface ethylidyne (CCH3) is the most stable species. On the unreconstructed Pt(100) surface we find that the situation is different: ethylidyne is very unstable, in agreement with a recent HREELS study. This is because there are no threefold coordinated sites on the (100) surface and the repulsive part of the potential dominates in the fourfold adsorption site. The reaction barriers have been estimated by smoothly transforming the coordinates of the reactants into those of the products, while the energy of the transition state was simultaneously minimized with respect to the relevant coordinates. In this way we believe that the barriers have been determined with sufficient accuracy within the model. The general trend is that the barriers for hydrogenation-dehydrogenation from the adsorbate to the surface are considerably lower than the barriers for hydrogen transfer within the adsorbate (isomerization), which in turn is lower than that of a concerted reaction, like CHCH + 2H --> CH2CH2. From this we can determine the decomposition pathway for the ethylene to ethylidyne transition, in agreement with the findings of the ASED-MO method by Anderson.