Departures from the Adsorption Energy Scaling Relations for Metal Carbide Catalysts

ABSTRACT: The activity of heterogeneous catalysts is often limited by a strong correlationbetween the chemisorption energies of reaction intermediates described by the “scaling relations”among the transition metals. We present electronic structure calculations that suggest that metalcarbides do not in general follow the transition-metal scaling relations and tend to exhibit acarbophobic departure relative to the transition metals, meaning they tend to bind carbon-boundspecies weakly compared to oxygen-bound species. This contrasts with the oxophobic departureexhibited by Pt and Pd. Relative to the parent metals, carbides tend to bind carbon and oxygen moreweakly and hydrogen more strongly. The departures are rationalized with the adsorbate−surfacevalence configuration and the energy of the metal sp-states. We employ these general trends to aidin the understanding of various catalytic properties such as the high activity of iron carbides forFischer−Tropsch synthesis and Pt-group catalysts for partial oxidation of methane. Theseconclusions are shown to extend beyond atomic probe adsorbates to molecular fragments of relevance to catalysis, making these concepts generally useful for the theory-based design of catalytic materials.