Electronic structure of clean and carbon-covered closed-packed rhodium and ruthenium surfaces

Abstract

In order to determine the nature of the active species which several groups have found to be important in catalytic carbon hydrogenation reactions, self-consistent linear combination of atomic orbitals (LCAO) calculations have been performed of the electronic structure of clean and C-covered Rh(111) and Ru(0001) surfaces. Apart from the shift of the Fermi level to higher energy to accomodate an extra electron per atom in Rh, the results are quite similar for the two metals. A (1×1) layer of C's, occupying threefold-coordination sites binds by forming tetrahedral bonds with its neighbors, leaving a nonbonding $p_z$ band at the Fermi energy. Thus the model C species examined here should be very reactive. Additionally, the $\mathrm{C} 2s-\mathrm{C} 2p_z$ binding-energy difference is about 10 eV, in accord with the Augerpeak separation seen by Goodman et al. for a reactive carbidic layer on Ni. Partial local density of states (LDOS) and energy-band dispersion curves are presented to permit the verification of the model carbidic overlayer. Results presented for the clean surfaces include work functions that are in excellent accord with the measured values, and band dispersions, notably a surface resonance band that has been observed on Ru, by Himpsel et al. Finally, surface-bulk core-level binding-energy shifts have been calculated. For the clean surfaces, they are in reasonable agreement with empirical heats of segregation. For the C-covered surfaces, they indicate that C binds most strongly to Ru, followed by Rh and Pd.