Coordination chemistry of transition metal carbide surfaces:detailed spectroscoppic and theoretical investigation of CO adsorption on TiC and VC(100) surfaces

摘要

Detailed spectroscopic studies of the interaction of carbon monoxide (CO) with 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 fundamental understanding of the surface bonding and reaction properties to enhance 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 but differ fundamentally in their electronic structure as the additional electron present in a formula unit of VC presents a significantly different surface bonding environment. CO has been used as a probe molecule to determine the relative electron accepting and donating tendencies of the substrates. Temperature-programmed desorption (TPD) has demonstrated that CO has a significantly higher heat of desorption on VC compared to TiC. High-resolution 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 sr-back-bonding on the VC surface. This enhanced back-bonding interaction is also observed in core level X-ray photoelectron spectroscopy satellite structure, and in valence band perturbations observed with ultraviolet photoelectron spectroscopy. Detailed analyses of these data show that CO has a slightly stroner (7-donor interaction with VC, but the stronger VC-CO bond is due primarily to the r-interaction that is essentially absent on the TiC surface. Density functional theory (DFT) has also been applied to small MC clusters that qualitatively reproduce the observed experimental trends. DFT also provides compelling evidence of the impact of the electronic sti-ucture difference on the CO interaction, as occupied d-orbitals in VC participate in the back-bonding interaction, but these levels are unoccupied in TiC. The results are entirely consistent with a simplified molecular orbital description of the materials that results in the surface metal atoms of TiC behaving like d” species 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 electronic structure clearly plays a dominant role in the surface bonding of the carbides, controlling their reactivity with lubricants and reactants with which they come into contact.