Adsorption and reaction of organic molecules on solid surfaces – ab-initio density functional investigations

Abstract

The state-of-the-art of density-functional studies of the adsorption and reaction of organic molecules on solid surfaces is illustrated at three examples chosen to cover different aspects of the field. The first concerns the adsorption and subsequent hydrogenation of benzene on nickel surfaces. Here the essential points are the description of the adsorption geometry and the determination of the barriers and heats of reaction for the hydrogenation to cyclohexene. It is shown that although in the gas-phase, the energy differences between the different cyclic hydrocarbon species are correctly described only at the level of a meta-generalized-gradient-approximation (meta-GGA) to the exchange-correlation functional (providing an improved description of electron localization in double bonds), for the adsorbed species a conventional GGA leads to accurate geometries and energetics. The second example serves to study the interplay between surface reconstruction and the molecular geometry of the adsorbate. It is demonstrated that the buckling of the Si–Si dimers in the Si(100) surface (and the associated charging of the surface Si atoms) has a decisive influence on the energetically most favorable adsorption configuration of acrylonitrile, based on the resonant form of the molecule with cumulative C=C and C=N double bonds. As the formation of the energetically most favorable configuration involves a complex rearrangement of the adsorbate, it is argued that kinetic effects are also essential for interpreting the experimental information. The third case study describes the Beckmann rearrangement of cyclohexanone oxime to ɛ-caprolactam catalyzed by an acid zeolite. It is shown that, in combination with harmonic transition-state theory, the DFT calculations allow to uniquely identify Bronsted acid sites as the catalytically active center. Finally we discuss briefly the current status of the implementation of the hierarchy of DFT functionals and their application to the interaction between molecules and solids (with particular reference to the performance of hybrid functionals) and attempts to include entropic effects in the ab-initio modeling of catalyzed molecular reactions.