1,720,978 research outputs found
Structural and electronic properties of bulk and low-index surfaces of zincblende PtC
No full textTransition metal carbides have been extensively used in diverse applications over the past decade. Their versatility is in part thanks to their unique bonding, which displays a mixture of ionic, metallic and covalent character. While the bulk structure of zincblende (ZB) PtC has been investigated several times, a detailed understanding of the electronic and structural properties of its low-index surfaces is lacking. In this work, we present an ab initio investigation of the properties of five crystallographic ZB PtC surfaces (Pt/C-terminated PtC(1 0 0), PtC(1 1 0) and Pt/C-terminated PtC(1 1 1)). Upon geometry optimization, both polar and nonpolar surfaces undergo a mild interlayer relaxation, without extensive reconstructions. Calculated vacancy formation energies indicate facile C removal on the (1 1 1) surface while Pt-vacancy formation is endothermic. Finally, atomic O adsorption energies on all surfaces reveal a high affinity of the C-terminated surfaces towards this species
First principles investigation of NO2and SO2adsorption on γ-Al2O3supported mono- and diatomic metal clusters
No full textNOx storage/reduction catalysts have been developed in the 1990's to minimize the emission of harmful NO and NO2 gases as a result of lean burning in diesel engines. Sulfur poisoning of the catalyst occurs when SOx (x = 2-3) species present in the fuel react aggressively with both the storage and the precious metal redox components. In the present work, DFT calculations within the plane wave, pseudopotential GGA framework were performed to study NO2 and SO2 adsorption on mono and diatomic clusters of Pt and Rh supported on the gamma-Al2O3 (100) surface. The most stable adsorption geometries for the clusters on the surface were identified and used as anchoring points for the adsorption of NO2 and SO2 molecules. Binding energies of a large number of NO2 and SO2 adsorption geometries were reported. In all cases where direct comparison between NO2 and SO2 binding geometries was possible, NO2 binding energies were observed to be larger than SO2 binding energies, in some cases by more than 1 eV
Covalent and noncovalent functionalization of pristine and defective graphene by cyclohexane and dehydrogenated derivatives
No full textThe interaction of cyclohexane (C6H12), cyclohexyl (C6H11*) and cyclohexene (C6H10) with both pristine and defective graphene (single vacancy and a carbon adatom), is systematically investigated within the density functional theory framework. C6H12 physisorbs on both pristine and defective graphene while C6H10 chemisorbs on graphene in the presence of an adatom. The C6H11* radical binds covalently with the graphene substrate in all adsorption geometries considered
Active role of the support in NOx storage and reductioncatalytic systems
No full textWe present first-principles density functional theory calculations of the adsorption properties of NO2 and SO2 on isolated (BaO)(n) (n = 1, 2, 4, 6, 8, 9) clusters as well as on small BaO clusters ((BaO)(n) with n = 1, 2, 4) supported on the anatase TiO2(001) surface. The TiO2 support influences binding indirectly by enhancing the electron donation from the BaO clusters to both chemisorbed NO2 and the support. This support-mediated increase in stability is not observed for SO2. We describe in detail and highlight the role played by TiO2 on the charge transfer mechanism, which can be used to control the catalytic properties of the active components of nitrogen storage and reduction catalytic systems. The relatively larger activity of the supported BaO clusters towards NO2 adsorption in comparison to SO2 could in principle offer protection against sulfur poisoning
A Classical Molecular Dynamics Study of the Effect of the Atomic Force Microscope Tip Shape, Size and Deformation on the Tribological Properties of the Graphene/Au(111) Interface
Full text linkAtomic force microscopes are used, besides their principal function as surface imaging tools, in the surface manipulation and measurement of interfacial properties. In particular, they can be modified to measure lateral friction forces that occur during the sliding of the tip against the underlying substrate. However, the shape, size, and deformation of the tips profoundly affect the measurements in a manner that is difficult to predict. In this work, we investigate the contribution of these effect to the magnitude of the lateral forces during sliding. The surface substrate is chosen to be a few-layer AB-stacked graphene surface, whereas the tip is initially constructed from face-centered cubic gold. In order to separate the effect of deformation from the shape, the rigid tips of three different shapes were considered first, namely, a cone, a pyramid and a hemisphere. The shape was seen to dictate all aspects of the interface during sliding, from temperature dependence to stick–slip behavior. Deformation was investigated next by comparing a rigid hemispherical tip to one of an identical shape and size but with all but the top three layers of atoms being free to move. The deformation, as also verified by an indentation analysis, occurs by means of the lower layers collapsing on the upper ones, thereby increasing the contact area. This collapse mitigates the friction force and decreases it with respect to the rigid tip for the same vertical distance. Finally, the size effect is studied by means of calculating the friction forces for a much larger hemispherical tip whose atoms are free to move. In this case, the deformation is found to be much smaller, but the stick–slip behavior is much more clearly seen
Effect of Surface Pt Doping on the Reactivity of Au(111) Surfaces towards Methanol Dehydrogenation: A First-Principles Density Functional Theory Investigation
Full text linkThe surprisingly high catalytic activity of gold has been known to the heterogeneous catalysis community since the mid-1980s. Significant efforts have been directed towards improving the reactivity of these surfaces towards important industrial reactions. One such strategy is the introduction of small amounts of other metals to create Au-based surface alloys. In this work, we investigated the synergistic effect of the Pt doping of a Au(111) surface on decreasing the activation barrier of the methanol dehydrogenation elementary step within first-principles density functional theory. To this end, we constructed several models of Pt-doped Au(111) surfaces, including a full Pt overlayer and monolayer. The effect of Pt surface doping was then investigated via the computation of the adsorption energies of the various chemical species involved in the catalytic step and the estimation of the activation barriers of methanol dehydrogenation. Both the electronic and strain effects induced by Pt surface doping substantially lowered the activation energy barrier of this important elementary reaction step. Moreover, in the presence of preadsorbed atomic oxygen, Pt surface doping could be used to reduce the activation energy for methanol dehydrogenation to as low as 0.1 eV
Effect of Platinum, Gold, and Potassium Additives on the Surface Chemistry of CdI2‑Antitype Mo2C
Full text linkTransition metal carbides are versatile materials for diverse industrial applications including catalysis, where their relatively low cost is very attractive. In this work, we present a rather extensive density functional theory study on the energetics of adsorption of a selection of atomic and molecular species on two Mo terminations of the CdI2 antitype phase of Mo2C. Moreover, the coadsorption of CO in the presence of preadsorbed metal atoms and its dissociative adsorption in the absence and presence of preadsorbed Pt and K were investigated. By using CO as a probe to understand the structural/electronic effects of the preadsorption of the metal atoms on the Mo2C(001) surface, we showed that K further enhances CO adsorption/activation on the surface, in contrast to the precious metals considered. Moreover, it was observed that the presence of both Pt and K stabilizes the transition state for the C–O bond dissociation, lowering the activation barrier for the dissociation of the C–O bond by about 0.3 and 0.4 eV, respectively
Electronic Structure of Rh and Ir Single Atom Catalysts Supported on Defective and Doped ZnO: Assessment of Their Activity Towards CO Oxidation
Full text linkThis study investigated the electronic structure of single-atom Rhodium (Rh) and Iridium (Ir) adsorbed on defective and impurity-doped ZnO(0001) surfaces, and assessed their activity towards the CO oxidation reaction. Our findings reveal that surface impurities significantly influence the binding energies and electronic properties of the metal atoms, with Al and Cr serving as particularly effective promoters. While Rh and Ir acquire a positive charge upon incorporation on the unpromoted Zn(0001) surface, adsorption directly on the promoter results in a net negative charge, thus facilitating the activation of both CO and O2 species. These results highlight the potential of impurity-promoted ZnO surfaces in modulating and tailoring the electronic properties of SACs, which can be used for a rational design of active single-atom catalysts
First-principles investigation of CO and CO2 adsorption on γ-Al2O3 supported monoatomic and diatomic Pt clusters
No full textSupported clusters of transition metal atoms are key components of heterogeneous catalysts. Understanding their interaction with small molecular species is therefore an important step in catalyst design. In this work, we provide a detailed first-principles investigation of the adsorption properties of CO and CO2 on
clusters supported by a
-Al2O3 (1 0 0) substrate. In particular, important parameters such as the stability of the Pt clusters, molecular adsorption energies and vibration frequencies were investigated. All Pt2 clusters exhibit lower adsorption energies than their monoatomic counterparts, especially single Pt atoms embedded in the surface of the support. Atomically preadsorbed Pt acts as an anchor for the CO molecule, increasing its adsorption energy compared to the bare surface. The support actively participates in stabilizing the adsorbates and a markedly different behaviour can be expected depending on the adsorption site being on the surface of small Pt clusters or on the Pt/support interface. Vibrational frequencies of CO and CO2 adsorbed on the supported clusters can be used to gain insight into the degree of dispersion of the metallic component of the catalyst, and can be profitably used in the design of novel single-atom catalysts (SACs)
Nanotribological Properties of the h-BN/Au(111) Interface: A DFT Study
Full text linkUnderstanding the quantum-mechanical origins of friction forces has become increasingly important in the past decades with the advent of nanotechnology. At the nanometer scale, the universal Amontons–Coulomb laws cease to be valid, and each interface requires individual scrutiny. Because of the well-known lubricating properties of two-dimensional materials, a significant amount of research has been performed in an effort to understand interfaces they form with one another. However, the interfaces between these two-dimensional materials and metals red from a tribological point of view, important for such applications as friction force microscopy, have yet to be thoroughly investigated. In the current work, we present a detailed density functional theory investigation of the hexagonal BN/Au(111) interface. Because of a good agreement between their characteristic lengths, a high level of commensurability is achieved in a suitably constructed model between the bulk surfaces of the two materials. As a result of our calculations, we find that the corrugation in the potential energy surface and the lateral forces in this interface are low compared to other similar interfaces. The friction coefficient falls rapidly with increasing load down to 0.005 for the largest loads considered. In contrast, Aun clusters (n = 1, 4, 13, and 19) sliding on the h-BN surface exhibit much larger lateral forces, indicating strong size and edge effects. The reduction of energy corrugation in going from the Au4 to the Au19 cluster may already indicate a decreasing trend with increasing size even at this very small scale
- …
