Coordination chemistry of transition metal carbide surfaces: Detailed spectroscopic and theoretical investigations of CO adsorption on TiC and VC (100) surfaces

Detailed spectroscopic studies of the interaction of carbon monoxide (CO) w ith the (100) surfaces of titanium carbide (TiC) and vanadium carbide (VC) have been performed for the first time and analyzed to provide insight into the nature of the surface chemical interactions. The carbide materials are technologically important in extreme applications due to their remarkably high hardness and melting points. This work was pursued to develop a fundam ental understanding of the surface bonding and reaction properties to enhan ce the use of TiC and VC as tribological materials and to gain insight into their potential use as catalysts. VC and TiC are both rocksalt materials b ut differ fundamentally in their electronic structure as the additional ele ctron present in a formula unit of VC presents a significantly different su rface bonding environment. CO has been used as a probe molecule to determin e the relative electron accepting and donating tendencies of the substrates . Temperature-programmed desorption (TPD) has demonstrated that CO has a si gnificantly higher heat of desorption on VC compared to TiC. High-resolutio n energy loss spectroscopy (HREELS) was used to measure surface vibrational frequencies, and the C-O stretch of reversibly adsorbed C-O is 2060 cm(-1) on VC, and 2120 cm(-1) on TiC, indicative of greater pi -back-bonding on t he VC surface. This enhanced back-bonding interaction is also observed in c ore level X-ray photoelectron spectroscopy satellite structure, and in vale nce band perturbations observed with ultraviolet photoelectron spectroscopy , Detailed analyses of these data show that CO has a slightly stronger a-do nor interaction with VC. but the stronger VC-CO bond is due primarily to th e pi -interaction that is essentially absent on the TiC surface. Density fu nctional theory (DFT) has also been applied to small MC clusters that quali tatively reproduce the observed experimental trends. DFT also provides comp elling evidence of the impact of the electronic structure difference on the CO interaction, as occupied d-orbitals in VC participate in the back-bondi ng interaction, but these levels are unoccupied in TiC. The results are ent irely consistent with a simplified molecular orbital description of the mat erials that results in the surface metal atoms of TIC behaving like d(0) sp ecies and those of VC as d(1) species. These formal occupations are greatly tempered by covalent mixing with carbon atoms in the lattice, but the elec tronic structure clearly plays a dominant role in the surface bonding of th e carbides, controlling their reactivity with lubricants and reactants with which they come into contact.