Small Cluster Models of the Surface Electronic Structure and Bonding Properties of Titanium Carbide, Vanadium Carbide, and Titanium Nitride

Density functional theory (DFT) calculations on stoichiometric, high-symmetry clusters have been performed to model the (100) and (111) surface electronic structure and bonding properties of titanium carbide (TiC), vanadium carbide (VC), and titanium nitride (TiN). The interactions of ideal surface sites on these clusters with three adsorbates, carbon monoxide, ammonia, and the oxygen atom, have been pursued theoretically to compare with experimental studies. New experimental results using valence band photoemission of the interaction of O2 with TiC and VC are presented, and comparisons to previously published experimental studies of CO and NH3 chemistry are provided. In general, we find that the electronic structure of the bare clusters is entirely consistent with published valence band photoemission work and with straightforward molecular orbital theory. Specifically, V9C9 and Ti9N9 clusters used to model the nonpolar (100) surface possess nine electrons in virtually pure metal 3d orbitals, while Ti9C9 has no occupation of similar orbitals. The covalent mixing of the valence bonding levels for both VC and TiC is very high, containing virtually 50% carbon and 50% metal character. As expected, the predicted mixing for the Ti9N9 cluster is somewhat less. The Ti8C8 and Ti13C13 clusters used to model the TiC(111) surface accurately predict the presence of Ti 3d-based surface states in the region of the highest occupied levels. The bonding of the adsorbate species depends critically on the unique electronic structure features present in the three different materials. CO bonds more strongly with the V9C9 and Ti9N9 clusters than with Ti9C9 as the added metal electron density enables an important π-back-bonding interaction, as has been observed experimentally. NH3 bonding with Ti9N9 is predicted to be somewhat enhanced relative to VC and TiC due to greater Coulombic interactions on the nitride. Finally, the interaction with oxygen is predicted to be stronger with the carbon atom of Ti9C9 and with the metal atom for both V9C9 and Ti9N9. In sum, these results are consistent with labeling TiC(100) as effectively having a d0 electron configuration, while VC- and TiN(100) can be considered to be d1 species to explain surface chemical properties.