Coordination Chemistry of Transition Metal Carbide Surfaces:  Detailed Spectroscopic and Theoretical Investigations of CO Adsorption on TiC and VC (100) Surfaces

Abstract

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 π-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 stronger σ-donor interaction with VC, but the stronger VCCO bond is due primarily to the π-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 structure 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 d0 species and those of VC as d 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.

Interaction of oxygen with ZrC(001) and VC(001): Photoemission and first-principles studies

“…11͑c͔͒, the CO molecule desorbs ͓⌬E = 0.4 eV for ZrC͑001͒ and 0.6 eV for VC͑001͒, DF-PSP͔ leaving behind an oxidized system with a larger metal/carbon ratio than in stoichiometric ZrC͑001͒ or VC͑001͒. The adsorption energies of CO on the oxidized carbides are close to those calculated and measured experimentally for CO on pure VC͑001͒, −0.6 and −0.5 eV, 23,48 respectively. The CO molecule is not stable on the VC͑001͒ surface at temperatures above 250 K 48 and, thus, it could not be detected in the photoemission experiments of Figs.…”

“…Our main interest here is in bonding-energy variations when oxygen moves from one adsorption site to another on the ZrC͑001͒ and VC͑001͒ surfaces. DF calculations using the RPBE functional predicted an adsorption energy for CO on VC͑001͒, 0.63 eV, 23 that is within 0.15 eV of the experimental value, 0.49 eV, 48 and gave a very good description of the bonding interactions of oxygen with TiC͑001͒. 36 Nevertheless, even if one expects the RPBE functional to provide adsorption energies closer to experiment than those obtained by the PW91 method, it has also been pointed out that it tends to lead to worse results for bulk properties like lattice parameters and bulk moduli.…”

Interaction of sulfur dioxide with titanium–carbide nanoparticles and surfaces: A density functional study

“…8,17,20,32 In the present paper, a set of DFT calculations is carried out to study SO 2 adsorption on Ti carbides, which are representative of transition metal carbides with good catalytic performances. 25,33,34 Several kinds of geometrical structures with different C/Ti a͒ Author to whom correspondence should be addressed. Fax: 631-344-5815; Electronic mail: rodrigez@bnl.gov ratios are taken into account.…”

Influence of Carbon Vacancies on CO Chemisorption on TiC(001): A Theoretical Study

“…For the surface-CO interactions, the Ti-CO bond distance is 2.20 Å in both clusters. 3,4 Usually the metal-CO bond is described in terms of the Blyholder model, 8 which involves electron donation from the CO 5σ orbital to the metal and the back-donation from the metal to the CO 2π* orbitals. In the CO bonded clusters, carbon monoxide can interact with the substrates as the σ-donor and π-acceptor molecule, exhibiting a similar interaction as is expected for metal surfaces.…”

“…Didziulis et al used density functional theory (DFT) to examine interactions of CO with TiC(001). 3,4 They report that CO adsorption on TiC(001) prefers on top Ti sites. Depending on the cluster model selected, the adsorption energies of CO are quite different.…”