The interaction of oxygen with TiC(001): Photoemission and first-principles studies

Coating of Flexible PDMS Substrates through Matrix-Assisted Pulsed Laser Evaporation (MAPLE) with a New-Concept Biocompatible Graphenic Material

In this study, matrix-assisted pulsed laser evaporation (MAPLE) was used to deposit graphene-like materials (GL), a new class of biocompatible graphene-related materials (GRMs) obtained from a controlled top-down demolition of a carbon black, on silicone slices to test their potential use as functional coating on invasive medical devices as indwelling urinary catheters. Results indicate that the relevant chemical-physical features of the deposit (controlled by FTIR and AFM) were maintained after MAPLE deposition. After deposition, GL films underwent a biological survey toward target cellular lines (murine fibroblast NIH3T3, human keratinocytes HaCAT and the human cervical adenocarcinoma epithelial-like HeLa). Results indicate that the GL films did not lead to any perturbations in the different biological parameters evaluated. The presented results and the possibility to further functionalize the GL or combine them with other functional materials in a hybrid fashion to assure a tighter adhesion onto the substrate for use in harsh conditions open the door to practical applications of these new-concept medical devices (drug delivery, next generation flexible devices, multifunctional coatings) paving the way to the prevention of nosocomial infections driven by catheterization through antibiotics-free approaches.

Structural, electronic, and magnetic properties of Ni nanoparticles supported on the TiC(001) surface

Metals supported on transition metal carbides are known to exhibit good catalytic activity and selectivity, which is interpreted in terms of electron polarization induced by the support. In the present work we go one step further and investigate the effect that a titanium carbide (TiC) support has on the structural, electronic, and magnetic properties of a series of Ni nanoparticles of increasing size exhibiting a two- or three-dimensional morphology. The obtained results show that three-dimensional nanoparticles are more stable and easier to form than their homologous two-dimensional counterparts. Also, comparison to previous results indicates that, when used as the support, transition metal carbides have a marked different chemical activity with respect to oxides. The analysis of the magnetic moments of the supported nanoparticles evidences a considerable quenching of the magnetic moment that affects mainly the Ni atoms in close contact with the TiC substrate indicating that these atoms are likely to be responsible for the catalytic activity reported for these systems. The analysis of the electronic structure reveals the existence of chemical interactions between the Ni nanoparticles and the TiC support, even if the net charge transfer between both systems is negligible.

Boosting the activity of transition metal carbides towards methane activation by nanostructuring

Molybdenum carbide breaks methane by going nano.

An electronic perturbation in TiC supported platinum monolayer catalyst for enhancing water-gas shift performance: DFT study

The water-gas shift (WGS) reaction behaviors over the TiC(0 0 1) supported Pt monolayer catalyst (Pt_ML/TiC(0 0 1)) are investigated by using the spin-unrestricted density functional theory calculations. Importantly, we find that the Pt_ML/TiC(0 0 1) system exhibits a much lower density of Pt-5d states nearby the Fermi level compared with that for Pt(1 1 1), and the monolayer Pt atoms undergo an electronic perturbation when in contact with TiC(0 0 1) support that would strongly improve the WGS activity of supported Pt atoms. Our calculations clearly indicate that the dominant reaction path follows a carboxyl mechanism involving a key COOH intermediate, rather than the common redox mechanism. Furthermore, through the detailed comparisons, the results demonstrate that the strong interactions between the monolayer Pt atoms and TiC(0 0 1) support make Pt_ML/TiC(0 0 1) a highly active catalyst for the low-temperature WGS reaction. Following the route presented by Bruix et al (2012 J. Am. Chem. Soc. 134 8968-74), the positive effect derived from strong metal-support interaction in the metal/carbide system is revealed.

Effect of carbon concentration on changing the morphology of titanium carbide nanoparticles from cubic to cuboctahedron

Titanium carbide nanoparticles were synthesized by flowing methane through a plasma generated from an arc discharge between two titanium electrodes. Different methane concentrations were employed in studies made to investigate the effects of carbon concentration on particle morphology. Transmission electron microscopy and X-ray diffraction were used to investigate the synthesized TiC nanopowders, whereupon it was found that nanocrystalline TiC nanoparticles prefer a cubic morphology at low concentrations of methane and a cuboctahedron morphology at high concentration of methane. The change in particle morphology is attributed to carbon affecting the relative growth rates of the {111} and {100} facets on a TiC seed crystal.

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

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.

Structure and Energy of Mo27SxCy Clusters: A Density Functional Theory Study

For understanding the carburization processes of MoSx catalysts, the structures and energies of Mo27SxCy cluster models have been computed at the level of density functional theory. The surface sulfur atoms on the Mo edge and S edge as well as bulky sulfur atoms have been replaced by atomic carbon, and the corresponding structures have S/C ratios in the range of 0.8-55. The formation of all Mo27SxCy structures is favored thermodynamically. It is also found that the formation of CS and C2 bridging units is more favored than the individual or separated replacements and that the formation of C2 bridging units is more favored than that of CS units. In contrast, the replacement of sulfur on the Mo edge is least favored. Furthermore, the replacement of the bulky sulfur on the Mo edge is equally favored as those of sulfur on the S edge. For aiding further experimental studies, the C=S and C=C stretching frequencies have been computed.

Role of Kinetics in the Selective Surface Oxidations of Transition Metal Carbides

The different oxidation behavior of TiC and VC(100) surfaces by molecular oxygen has been investigated by density functional theory with a slab model. From the thermodynamic stability of the final states that involve dissociated O(2), one cannot well explain the experimental observations. Two different oxidation pathways of TiC and VC(100) surfaces have been explored in this work, and the results indicate that two channels share the same precursor state. However, from the precursor, only the pathway leading to the formation of a C-O bond is energetically feasible for the TiC(100) surface, while on VC(100) the O atoms tend to occupy the metal surface sites due to a smaller energy barrier for this channel. Further band structure calculations reveal that the additional d electron of V atom favors the stability of the molecularly adsorbed species. The oxidation mechanism unveiled from the present calculations clearly evidences that the kinetic effects introduced by one additional d electron of the V atom play a crucial role in explaining the different surface chemistry between TiC and VC (100) surfaces.

Desulfurization Reactions on Ni2P(001) and α-Mo2C(001) Surfaces: Complex Role of P and C Sites

X-ray photoelectron spectroscopy and first-principles density-functional calculations were used to study the interaction of thiophene, H(2)S, and S(2) with Ni(2)P(001), alpha-Mo(2)C(001), and polycrystalline MoC. In general, the reactivity of the surfaces increases following the sequence MoC < Ni(2)P(001) < alpha-Mo(2)C(001). At 300 K, thiophene does not adsorb on MoC. In contrast, Ni(2)P(001) and alpha-Mo(2)C(001) can dissociate the molecule easily. The key to establish a catalytic cycle for desulfurization is in the removal of the decomposition products of thiophene (C(x)H(y) fragments and S) from these surfaces. Our experimental and theoretical studies indicate that the rate-determining step in a hydrodesulfurization (HDS) process is the transformation of adsorbed sulfur into gaseous H(2)S. Ni(2)P is a better catalyst for HDS than Mo(2)C or MoC. The P sites in the phosphide play a complex and important role. First, the formation of Ni-P bonds produces a weak "ligand effect" (minor stabilization of the Ni 3d levels and a small Ni --> P charge transfer) that allows a high activity for the dissociation of thiophene and molecular hydrogen. Second, the number of active Ni sites present in the surface decreases due to an "ensemble effect" of P, which prevents the system from deactivation induced by high coverages of strongly bound S. Third, the P sites are not simple spectators and provide moderate bonding to the products of the decomposition of thiophene and the H adatoms necessary for hydrogenation.