Departures from the Adsorption Energy Scaling Relations for Metal Carbide Catalysts

The activity, of heterogeneous catalysts is often limited by a strong correlation between the chemisorption energies of reaction intermediates described by the "scaling relations" among the transition metals. We present electronic structure calculations that suggest that metal carbides do not in general follow the transition-metal scaling relations and tend to exhibit a carbophobic departure relative to the transition metals, meaning they tend to bind carbon-bound species weakly compared to oxygen-bound species. This contrasts with the oxophobic departure exhibited by Pt and Pd. Relative to the parent metals, carbides tend to bind carbon and oxygen more weakly and hydrogen more strongly. The departures are rationalized with the adsorbate-surface valence configuration and the energy of the metal sp-states. We employ these general trends to aid in the understanding of various catalytic properties such as the high activity of iron carbides for Fischer-Tropsch synthesis and Pt-group catalysts for partial oxidation of methane. These conclusions 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.