Adjacent Hollow Sites Research Articles

Insights into the mechanism in electrochemical CO2 reduction over single-atom copper alloy catalysts: A DFT study

Copper single-atom alloy catalysts (M@Cu SAAs) have shown great promise for electrochemical CO2 reduction reaction (CO2RR). However, a clear understanding of the CO2RR process on M@Cu SAAs is still lacking. This study uses density functional theoretical (DFT) calculations to obtain a comprehensive mechanism and the origin of activity of M@Cu SAAs. The importance of the adsorption mode of M@Cu is revealed: key intermediates either adsorbed in the adjacent hollow site around Cu atoms (AD mode) or adsorbed directly on the top site of M (SE mode). AD mode generally exhibits finely tuned binding strengths of key intermediates, which significantly enhances the activity of the catalysts. Increasing the coverage of ∗CO on the M@Cu with SE mode leads to relocation of the active site, resulting in improved activity of C2 products. The insights gained in this work have significant implications for rational design strategy toward efficient CO2RR electrocatalysts.

Strain-Assisted Single Pt Sites on High-Curvature MoS 2 Surface for Ultrasensitive H 2 S Sensing

Strain-Assisted Single Pt Sites on High-Curvature MoS ₂ Surface for Ultrasensitive H ₂ S Sensing

The role of Cr atom in the early steam oxidation of Fe‐based alloys: An atomistic simulation

AbstractThis study employed the density functional theory to capture the atomic‐level dissociation processes of steam and investigate the ions migration on Fe(001) and FeCr(001) surfaces, revealing the role of Cr atom in the early oxidation. Various coadsorption structures with different steam‐derived species have been systematically examined to find the most energetically favored surface site. The results showed that the steam dissociation on the alloy surface underwent two steps. First, H2O molecule on the top site was dissociated into OH group and H atom, which further combined with metal atoms on the bridge site and hollow site. Second, the OH group was decomposed into O and H atoms, which moved to two adjacent hollow sites and generated an oxide. On further oxidation, the Fe atom migrated outward and formed an outer Fe oxide, whereas the Cr oxide could only grow inward as O atom passed through oxide. It was found that the presence of Cr atom on the surface was thermodynamically beneficial, which could promote steam oxidation. The Cr atom could effectively block ion diffusion across the oxide scale and protect the underlying substrate from further oxidation. These results were in good agreement with experimental observation.

Ab initio molecular dynamics study of wet H2S adsorption and dissociation on Fe(100) surface

Oil and gas pipeline transportation media often contain acidic media, such as H2S and CO2. The corrosion behavior of oil and gas equipment in wet H2S environment affects its service life. In this paper, ab initio molecular dynamics (AIMD) is used to study the dynamic behavior of wet H2S on Fe(100) surface in 300 K, including adsorption, dissociation and diffusion. Besides, the influences of surface defects and H2S coverage are also considered. The calculation results show the primary dissociation and secondary dissociation of H2S occur successively in perfect and defective Fe(100) surface. The presence of surface defects will reduce the dissociation energy of H2S, making it more prone to dissociation to generate H and S atoms, while the increase of H2S coverage will promote the primary dissociation of H2S but inhibit the secondary dissociation. The entire simulation process suggests that the SH, H and S generated from the dissociation of H2S adsorbed preferentially in the hollow site. The adsorption of S atoms is more stable than H atoms since H could diffuse between adjacent hollow site. In addition, no H atoms diffusion into subsurface region was observed during the whole simulation process.

The interaction of oxygen with the α-U(001) surface: an ab initio study

First-principles calculations based on density functional theory have been performed to investigate the adsorption, dissociation and diffusion of oxygen on the α-Uranium(001) surface. The results showed that O2 tends to dissociate followed by the occupation of the two adjacent three-fold hollow sites. The weak molecular adsorption was found for O2 adsorbed perpendicularly at the top of a uranium (U) atom. The investigation of the dissociation for O2 from the top site with the O atoms falling into the two adjacent hollow sites indicated the spontaneous dissociation of O2. The surface diffusion of atomic oxygen between two adjacent hollow sites encountered an activation barrier of 0.55 eV, indicating the slow motion for O on the α-U(001) surface.

The electron affinity of Al13H cluster: high level ab initio study

Al13H clusters have been considered candidates for cluster assembled materials. Here we have carried out benchmark calculations for the Al13H cluster, both neutral and anionic, with the aim of verifying the nature of stationary points on the potential energy surface, studying dynamics of H atom and determining an adiabatic electron affinity. A range of correlated methods applied include second-order perturbation theory (MP2), spin-component-scaled MP2, coupled electron pair (CEPA) and coupled cluster singles and doubles with perturbative triple corrections (CCSD(T)). These methods are used in combination with the correlation consistent basis sets through aug-cc-pVTZ including extrapolation to the complete basis set (CBS) limit. Performance of several different flavours of density functional theory (DFT) such as generalised gradient approximation (GGA), hybrid GGA, meta-GGA and hybrid-meta-GGA is assessed with respect to the ab initio correlated reference data. The harmonic force constant analysis is systematically performed with the MP2 and DFT methods. The MP2 results show that for neutral Al13H only the hollow structure is a potential energy minimum, with the bridged structure being a transition state for the H shift from the hollow site to the adjacent hollow site. The CCSD(T)/aug-cc-pVTZ (CCSD(T)/CBS) estimate of the energy barrier to this H shift is 2.6 (2.9) kcal/mol, implying that the H atom movement over the Al13H cluster surface is facile. By contrast, the DFT force constant analysis results suggest additional terminal and bridged minima structures. For the anion Al13H−, exhibiting ‘stiffer’ potential energy surface compared to the neutral, the existence of the hollow and terminal isomers is consistent with the earlier photoelectron spectroscopy assignment. The adiabatic electron affinity of Al13H is determined to be 2.00 and 1.95 eV (the latter including the ΔZPE correction) based on the CCSD(T) energies extrapolated to the CBS limit, whereas the respective CCSD(T)/CBS thermodynamic EA values are 2.79 and 2.80 eV.

Electronic Structure and Bonding of an Ionic Molecular Adsorbate: c-C5H5 on Cu{111}

Self-assembled monolayers containing conjugated π systems find application in organic electronics to functionalize and modify the electronic properties of metals and metal oxides. Isolated cyclopentadienyl is an aromatic molecular anion similar in size to benzene that, unlike benzene, adsorbs quite strongly even on coinage metal surfaces. In this study, the electronic structure, bonding, and minimum energy configuration of cyclopentadienyl (c-C5H5 or Cp) adsorbed on Cu{111} are calculated via first-principles density functional theory (DFT). The Cu{111} surface has been modeled within a (2√3 × 2√3)R30° cell, and the adsorbed Cp has been found to reside preferentially on the hollow sites, with a binding energy of 1.73 eV. Electronic population analysis reveals a net charge transfer of ∼1.1 electrons from the metal to the Cp, indicating that the adsorption is dominated by ionic bonding. The surface diffusion barrier between two adjacent hollow sites was calculated to be 55 meV, in good agreement with previously reported measurements by helium spin echo (HeSE) spectroscopy. It was found that lateral interactions do not significantly influence the binding energy and mobility of the adsorbate. The physical–chemical properties of this strongly bound but weakly mutually interacting molecular adsorbate suggest that Cp could become a model system for ionically adsorbed molecular adsorbates.

Ab initio investigation of interfacial layer formation in the Mo∕Si boundary for extreme ultraviolet lithography

The energetics and the electronic structure of the Si∕Mo(110) surface were investigated using the density functional theory calculations in an attempt to clarify the initial stage of silicide layer formation. The energy barriers for the migration of Si adatom to an adjacent hollow site passing the bridge site were calculated to be 0.64eV [Mo (110)] and 0.74eV [Si-substituted Mo (110)]. In order to analyze the bonding nature between Si adatom and Mo (110) surface, layer density of states and total valence electron density were obtained. It can be reasonably inferred that Mo 4d states mainly contributed to the bonding between Mo and Si and, consequently, four Mo–Si covalent bonds were formed around the Si atom at the hollow site.

Theoretical study on AlnO2 (n = 1–10) clusters and O2 adsorption on the Al(111) surface

AbstractThe structural properties of neutral and ionic AlnO2 (n = 1–10) clusters have been systematically investigated using the density functional method B3LYP with a standard 6‐311+G(d) basis set. The calculated results show that in the AlnO, AlnO2, and AlnO (n ≥ 3) clusters, O atoms tend to penetrate into the aluminum clusters with some Al atoms moving outward. The binding energies and natural charges populations indicate that the oxygen‐etching is generally stronger in the order Al < Aln < Al for n < 3, and Al > Aln > Al for n ≥ 3. To further understand the mechanism of interaction between Al and O2, the adsorption of O2 on the Al(111) surface was studied using the density functional theory with plane wave pseudopotential method. The calculated results are consistent with the experimental observation that the O2 molecule would dissociate on the Al(111) surface and be adsorbed in adjacent hollow sites, forming a local structure of Al3O–Al3O. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Shell model calculations of the adsorption and diffusion of K +, Cl −, and KCl on KCl(0 0 1)

Equilibrium adsorption positions and diffusion pathways of the ions K + and Cl − as well as of the molecule KCl on the terrace of the (0 0 1) surface of KCl were determined by shell model calculations allowing relaxations of the crystal lattice in the vicinity of the adsorbed species. For the ions each one adsorption position was found, in which the ions are located above the hollow site at the center of a slightly distorted square formed by two cations and two anions of the uppermost surface layer of the KCl crystal. Adsorption energies of −1.52 eV for K + and −1.61 eV for Cl − were calculated. Jumps of the ions occur from these positions to adjacent hollow positions in the ±[1 0 0] and ±[0 1 0] directions with a jump distance of a 0/2. The activation energies for the jumps result as 0.142 for K + and 0.152 eV for Cl − and the mean diffusion lengths as x s K ( nm ) = 0.315 exp [ 1.379 ( eV ) / 2 kT ] and x s Cl ( nm ) = 0.315 exp [ 1.462 ( eV ) / 2 kT ] . For the KCl molecule four distinct adsorption minima with energies between −0.932 and −0.825 eV were found. Because of the smaller lattice relaxation caused by the molecule the adsorption energies are considerably lower than for the single ions. In the position with the largest adsorption energy the ions of the admolecule are again placed above adjacent hollow sites. In two more adsorption positions only one ion is at the hollow site and the other one in a top position above an oppositely charged ion of the surface. In the fourth position with the smallest adsorption energy both ions are in top positions. Jumps between the different adsorption positions proceed by rotations of the molecule, in which one of its ions remains essentially attached to a local minimum position. The diffusion and desorption of a KCl molecule was studied by a Monte Carlo method, resulting in a mean diffusion length x s (nm) = 0.39 exp[0.84 (eV)/2 kT], which agrees rather well with an experimental value of x s exp ( nm ) = 0.20 exp [ 0.82 ( eV ) / 2 kT ] . Values for the mean stay time as well as for the surface diffusion coefficient are derived.

Hydrogen Adsorption on Nickel (100) Single-Crystal Face. A Monte Carlo Study of the Equilibrium and Kinetics

We propose a model of the dissociative adsorption of hydrogen on nickel single-crystal face. In this model, we treat the Ni(100) surface as a strongly correlated energetically heterogeneous surface, because the density functional theory (DFT) studies indicate that hydrogen atoms may adsorb either on hollow sites (energetically more favorable, binding energy 48 kJ/mol H) or bridge sites with the binding energy less by 11 kJ/mol H. The essential assumption of the proposed model is that the dissociation of the hydrogen molecule is possible only over the topmost Ni atom, and the resulting H atoms may adsorb either on two free hollow sites (but the adjacent bridge sites must be free) or two bridge sites (the adjacent hollow sites must be free). If the above condition is not fulfilled, then the dissociation and adsorption are impossible. The second assumption is that the rate (probability) of the associative desorption is limited by the rate of diffusion of H atoms on the surface. This is because the two H atoms desorb, giving an H2 molecule, only when they meet on two adjacent hollow-bridge sites. Our model recovers very well the behavior of the experimental equilibrium adsorption isotherms as well as kinetic isotherms. As a result, we stated that hydrogen atoms are not completely free on the surface, but they cannot also be considered localized at room and elevated temperatures. Additionally, while analyzing the kinetic adsorption isotherms, we stated that the rate-limiting step during the dissociative adsorption of H2 is the disintegration of the activated complex and the subsequent adsorption of hydrogen atoms.

Surface diffusion and incorporation of adatom in Co/Al (0 0 1) system

The surface diffusion and the incorporation of an adatom on early stage of Co/Al (001) interface formation was investigated using the density functional theory (DFT). The energy barrier for the surface diffusion (migration of Co adatom to an adjacent hollow site passing the bridge site) was calculated as 1.01 eV. Large displacement of neighboring Al atoms was accompanied by the surface diffusion of Co adatom. For incorporation process, the energy barrier was 0.39 eV, only 38.6% of the barrier for surface diffusion and the energy gain of the system was 0.43 eV. After the incorporation process was completed, the Co adatom and Al atoms formed seven Co–Al bonds of highly coordinated B2-like configuration.

A density functional study of Br on Cu( [formula omitted]) at low coverages

Density functional theory has been used to study the adsorption of Br on Cu(1 0 0) at low coverages. As expected, the most stable binding site for Br atoms is the fourfold hollow site. The barrier for Br hopping between adjacent hollow sites along 〈0 1 1〉 directions via twofold saddle points is estimated to be 0.23 eV, whereas the barrier for hopping along 〈0 1 0〉 directions via an on-top site is 0.37 eV. The low barriers for motion between hollow sites indicate that it should not be possible to image low coverages of Br on Cu(1 0 0) at room temperature in scanning tunnelling microscopy. It is suggested that features previously attributed to Br atoms in hollow sites should in fact be assigned to Br bound in surface copper vacancy sites.

Surface structures of sulfur and carbon overlayers on Mo(100): A detailed analysis by automated tensor LEED

Detailed surface structures have been determined for three ordered sulfur overlayers and one ordered carbon overlayer on Mo(100), by automated tensor low-energy electron diffraction (LEED). The three ordered sulfur structures occur at 0.5, 0.67 and 0.75 monolayers (ML) and have c(2 × 2), c(3√2 × √2)R45° and c(4 × 2) periodicities with one, two and three atoms per unit cell, respectively, while the carbon structure has c(2 × 2) periodicity at 0.5 ML. In addition, a previously reported p(2 × 1) structure at high sulfur coverage is shown to actually be a poorly ordered version of the c(4 × 2) structure. In all four ordered structures the substrate is unreconstructed (unlike the clean surface) but relaxed down to its second layer, while the adatoms reside at or near fourfold hollow sites. In the c(2 × 2) structures, the smaller carbon penetrates deeper into the hollow sites than does sulfur: C approaches the second-layer Mo atoms closer than the first-layer Mo atoms, while S does the opposite. Except for the c(4 × 2) structure, the second-layer Mo atoms are pushed deeper into the substrate by adatoms directly above them. In the c(4 × 2) structure, large lateral displacements due to crowded S packing complicate this picture. In the two higher-coverage S structures, adatoms in adjacent hollow sites repel each other, forcing each other away from the hollow centers by 0.13 to 0.2 Å. Comparison with results of parallel STM experiments is made. A brief discussion is also given of a very high-coverage sulfur structure interpreted as MoS 2-like.