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

The adsorption of atomic Au on the (001) surface of TiC, ZrC, HfC, VC, NbC, TaC, and delta-MoC and the mechanism of diffusion of this adatom through the surface have been studied in terms of a periodic density functional theory based approach. In all the cases, the Au adsorption energies are in the range of 1.90-2.35 eV. The moderately large adsorption energies allow the Au diffusion before desorption could take place. For TiC(001), ZrC(001), and HfC(001), atomic Au is adsorbed directly on top of C atoms and diffusion takes place along the diagonal of the squares formed by M-C-M-C atoms with the transition state located above the hollow sites. For the rest of transition metal carbides the situation is less simple with the appearance of more than one stable adsorption site, as for NbC and TaC, of a small energy barrier for diffusion around the most stable adsorption site and of a more complex diffusion pathway. The small energy barrier for diffusion around the most stable site will result in a highly mobile Au species which could be observed in scanning tunnel microscope experiments. After depositing Au on metal-carbide surfaces, there is a noticeable charge transfer from the substrate to the adsorbed Au atom. The electronic perturbations on Au increase when going from TiC to ZrC or TaC. Our results indicate that metal carbides should be better supports for the chemical activation of Au than metal oxides.

Section: Computational Details and Surface Modelsmentioning

confidence: 99%

Adsorption and diffusion of Au atoms on the (001) surface of Ti, Zr, Hf, V, Nb, Ta, and Mo carbides

Flórez

Viñes

Rodríguez

et al. 2009

“…The formation of oxycarbides constitutes a common problem when atomic oxygen or some O-containing species interact with the surface of TMCs. [19][20][21][22] In recent times, Au/TiC systems have been shown to perform extraordinarily well in the low temperature water gas shift reaction 23 and to provide active sites for CO 2 hydrogenation to methanol. 24,25 Regarding catalyzed CO 2 conversion to methanol, joint experimental and theoretical work indicated that CO 2 adsorption energy appears to be a good reactivity descriptor since it nicely follows the trend β-Mo 2 C > δ-MoC > TiC >> Cu.…”

Section: Introductionmentioning

confidence: 99%

Structure and electronic properties of Cu nanoclusters supported on Mo2C(001) and MoC(001) surfaces

Posada‐Pérez

Viñes

Rodríguez

2015

The atomic structure and electronic properties of Cun nanoclusters (n = 4, 6, 7, and 10) supported on cubic nonpolar δ-MoC(001) and orthorhombic C- or Mo-terminated polar β-Mo2 C(001) surfaces have been investigated by means of periodic density functional theory based calculations. The electronic properties have been analyzed by means of the density of states, Bader charges, and electron localization function plots. The Cu nanoparticles supported on β-Mo2 C(001), either Mo- or C-terminated, tend to present a two-dimensional structure whereas a three-dimensional geometry is preferred when supported on δ-MoC(001), indicating that the Mo:C ratio and the surface polarity play a key role determining the structure of supported clusters. Nevertheless, calculations also reveal important differences between the C- and Mo-terminated β-Mo2 C(001) supports to the point that supported Cu particles exhibit different charge states, which opens a way to control the reactivity of these potential catalysts.

“…Two recent works examining the interaction of oxygen with the ͑001͒ surface of a closely related family of compounds, early transition monocarbides, have shown complex trends in the reactivity of the metal and carbon sites depending on the electronic properties of the carbide and the filling of its valence band. 36,37 In this work, we present a detailed experimental and theoretical study of the oxidation of TiN͑001͒ using a combination of synchrotron-based photoemission and density functional theory ͑DFT͒. To the best of our knowledge, this is the first experimental work examining the adsorption of O 2 on a well-defined metal nitride surface.…”

Section: Introductionmentioning

confidence: 99%

Interaction of oxygen with TiN(001):N↔O exchange and oxidation process

Graciani

Sanz

Asaki

et al. 2007

Self Cite

This work presents a detailed experimental and theoretical study of the oxidation of TiN(001) using a combination of synchrotron-based photoemission and density functional theory (DFT). Experimentally, the adsorption of O2 on TiN(001) was investigated at temperatures between 250 and 450 K. At the lowest temperature, there was chemisorption of oxygen (O(2,gas)-->2O(ads)) without significant surface oxidation. In contrast, at 450 K the amount of O2 adsorbed increased continuously, there was no evidence for an oxygen saturation coverage, a clear signal in the Ti 2p core level spectra denoted the presence of TiOx species, and desorption of both N2 and NO was detected. The DFT calculations show that the adsorption/dissociation of O2 is highly exothermic on a TiN(001) substrate and is carried out mainly by the Ti centers. A high oxygen coverage (larger than 0.5 ML) may induce some structural reconstructions of the surface. The exchange of a surface N atom by an O adatom is a highly endothermic process (DeltaE=2.84 eV). However, the overall oxidation of the surface layer is thermodynamically favored due to the energy released by the dissociative adsorption of O2 and the formation of N2 or NO. Both experimental and theoretical results lead to conclude that a TiN+mO2 -->TiOx + NO reaction is an important exit channel for nitrogen in the oxidation process.