Hcp Sites Research Articles

Activity, Selectivity, and Durability of Ruthenium Nanoparticle Catalysts for Ammonia Synthesis by Reactive Molecular Dynamics Simulation: The Size Effect.

We report a molecular dynamics (MD) simulation employing the reactive force field (ReaxFF), developed from various first-principles calculations in this study, on ammonia (NH3) synthesis from nitrogen (N2) and hydrogen (H2) gases over Ru nanoparticle (NP) catalysts. Using ReaxFF-MD simulations, we predict not only the activities and selectivities but also the durabilities of the nanocatalysts and discuss the size effect and process conditions (temperature and pressure). Among the NPs (diameter = 3, 4, 5, and 10 nm) considered in this study, the 4 nm NPs show the highest activity, in contrast to our intuition that the smallest NP should provide the highest activity, as it has the highest surface area. In addition, the best selectivity is observed with the 10 nm NPs. The activity and selectivity are mainly determined by the hcp, fcc, and top sites on the Ru NP surface, which depend on the NP size. Moreover, the selectivity can be improved more significantly by increasing the H2 pressure than by increasing the N2 pressure. The durability of the NPs can be determined by the mean stress and the stress concentration, and these two factors have a trade-off relationship with the NP size. In other words, as the NP size increases, its mean stress decreases, whereas the stress concentration simultaneously increases. Because of these two effects, the best durability is found with the 5 nm NPs, which is also in contrast to our intuition that larger NPs should show better durability. We expect that ReaxFF-MD simulations, along with first-principles calculations, could be a useful tool in developing novel catalysts and understanding catalytic reactions.

Diffusion and adsorption of Au and Pt adatoms on ideal and missing row reconstructed surfaces of Au(110): DFT and EAM calculations

Density-functional theory (DFT) and embedded atom method (EAM) are used to study the adsorption and diffusion on unreconstructed (1 × 1) and reconstructed (1 × 2), (1 × 3) and (1 × 4) surfaces for homogenous Au/Au(110) and heterogeneous Pt/Au(110) systems. Adsorption energies show a strong binding for Pt than Au on all considered geometries. Additionally, adsorption on the (111) microfacet of the (1 × 2), (1 × 3) and (1 × 4) geometries is investigated. Surprisingly the adsorption of Pt on the top hcp sites of the (111) microfacet of the (1 × 2), (1 × 3) and (1 × 4) geometries leads to the exchange phenomenon. The diffusion via hopping and metastable walk is investigated for both the homo and heterogeneous systems. The hopping diffusion barrier show a slight dependence on the reconstruction order and the diffusion is found to be more costly on the missing row reconstructed surface than the unreconstructed surfaces. The potential energy profile for the diffusion via metastable walk mechanism on the (1 × 2) missing row reconstructed geometry indicates that the metastable walk diffusion is more favorable to homogeneous system than to heterogeneous system.The results of this paper are important to understand the properties of metallic reconstructed surfaces.

The nature of the Pt(111)/α-Fe2O3(0001) interfaces revealed by DFT calculations.

Density functional theory calculations are performed to give a thorough description of structural, energetic, and electronic properties of Pt(111)/α-Fe2O3(0001) systems by spin-polarized calculations, accounting for the on-site Coulomb interaction. Toward the better understanding of Pt(111)/α-Fe2O3(0001) interfaces, two terminations of α-Fe2O3(0001) surface, namely, the single Fe- and the O3-termination, are considered and coupled with the four possible (top, hcp, fcc, and bridge) sites on Pt(111). The effect of the strain on clean hematite surfaces due to the lattice mismatch between the substrate and the overlayer is included in the analysis. Among the possible adsorption configurations, bridge sites are unstable, while the most favorable configurations are the ones at hollow sites. The stability of the interfaces is not only influenced by the termination of the overlayer but also influenced by the degree of its structural relaxation and the relative position of the first layer of O atoms in hematite with respect to Pt. To elucidate the different nature of the two terminations of the overlayer on Pt, projected density of states and 3D charge density difference plots are also discussed.

Passivation effect of Cl, F and H atoms on CuIn0.75Ga0.25Se2 (1 1 2) surface

Using the first-principles calculations within the density functional-theory (DFT) framework, we theoretically investigated the surface reconstruction, surface states near the Fermi level and their passivation on CuIn0.75Ga0.25Se2 (112) (CIGS) surface by chlorine, fluorine and hydrogen. Surface reconstruction appears on CIG-terminated CIGS (112) surface and it is a self-passivation. For the locations of Cl, F and H atoms adsorbing on Se-terminated CIGS (112) surface, four high symmetry adsorption sites: top sites, bridge sites, hexagonal close-packed (hcp) sites and faced centered cubic (fcc) sites were studied respectively. With the coverage of 0.5 monolayer (ML), Cl, F and H adatoms energetically occupy the top sites on the CIGS (112) surface. The corresponding adsorption energies were −2.20 eV, −3.29 eV, −2.60 eV, respectively. The bond length and electronic properties were analyzed. We found that the surface state density near the Fermi level was markedly diminished for 0.5 ML Cl, F and H adsorption on Se-terminated CIGS (112) surface at top sites. It was also found that H can more efficiently passivate the surface state density than Cl and F atoms, and the effect of adsorption of Cl atoms is better than that of F.

Investigation of fcc and hcp island nucleated during homoepitaxial growth of copper by molecular dynamics simulation

We studied thin film growth of Cu on Cu (111) surface by molecular dynamics (MD) simulations using the Embedded Atom Method (EAM) potential and an incidence energy of 0.06 eV at 300 K. At the beginning of the deposition, we observe a coexistence of atoms nucleated on hcp and fcc sites in the first layer. The connection of these islands produces a border zone in rectangular form. The coexistence of fcc and hcp geometries has generated energy competition and eventually transitioned to fcc sites implying a large collective movement of several layers. However, the transition to the coexistence of these two structures, can promote coalescence by linking two large mounds.

First-principles study of H, Cl and F passivation for Cu2ZnSnS4(112) surface states

The first-principles calculation method is used to systematically investigate the lattice structure, energy band, density of states of the bulk Cu2ZnSnS4, surface reconstruction, and mechanism of adsorption and passivation of F, Cl and H atoms on Cu2ZnSnS4 (112) surface. We find that the surface reconstruction occurs on the Cu-Zn-Sn-terminated Cu2ZnSnS4 (112) surface and this reconstruction introduces surface self-passivation. By analyzing the partial density of states of the atoms on the S-terminated Cu2ZnSnS4 (112) surface, it can be seen that surface states near the Fermi level are mainly contributed by 3d orbitals of Cu atoms and 3p orbits of S atoms at the top of the valence band. When a single F, Cl or H atom is adsorbed on the S-terminated Cu2ZnSnS4 (112) surface, all three kinds of atoms exhibit an optimal stability at a specific top adsorption site in comparison with at the bridge, hcp and fcc sites. And this top position is also the position of the S atom that has the greatest influence on the surface states. When two atoms of the same kind are adsorbed on the surface, H, Cl or F atoms occupy the top sites of two S atoms that cause surface states on the Cu2ZnSnS4 (112) surface, which have the lowest adsorption energy. And the surface states near the Fermi level are partially reduced. Therefore, two S atoms that cause the surface states are the main targets of S-terminated Cu2ZnSnS4 (112) surface passivation. It has also been found that the passivation effect of H atom for surface states is the most significant and the effect of Cl atom is better than that of F atom. Comparing the partial density of states, the Bader charge and the differential charge of the atoms before and after adsorption, we find that the main reason for the decrease of the surface states is that the adsorption atoms obtain electrons from the S atoms, and the state density peaks of the Cu and S atoms at the Fermi level almost disappear completely. In the surface model, the F atom obtains the same number of electrons from the two S atoms, while the two S atoms have different effects on the surface states. And the H and Cl atoms obtain fewer electrons from the S atoms, that have less influence on the surface states. It may be the reason why the passivation effect of F atom is slightly less than that of H and Cl atoms.

Ordering and interactions between Cl adatoms on Cu(111) and their influence on the local electronic properties as measured by STM and STS

We present a scanning tunneling microscopy/spectroscopy study of compressed Cl adlayers on Cu(111) under ultrahigh-vacuum conditions. We describe a rational scheme to assign Cl adatoms to different surface sites. The dominant electronic state visible in scanning tunneling spectroscopy (STS) corresponds to an antibonding interaction between the Cl adlayer and the copper surface. This state was observed to be $200\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ higher in energy at hcp sites compared to fcc sites, and it is attributed to the greater charge transfer to Cl adatoms at hcp sites. Although there was no STS signature associated with bridging sites, the presence of bridging Cl adatoms along the periphery of fcc domains caused a shift in the energy of the interface state in the latter. These results shed important light on the ordering and interaction between Cl adatoms on Cu(111) and their influence of the local electronic structure of the surface.

Thickness-dependent energetics for Pb adatoms on low-index Pb nanofilm surfaces: First-principles calculations

Adsorption, interaction, and diffusion of adatoms on surfaces control growth and relaxation of epitaxial nanostructures and nanofilms. Previous reports of key diffusion barriers for Pb diffusion on low-index Pb surfaces are limited in scope and accuracy. Thus, we apply density functional theory (DFT) to calculate the adsorption and diffusion energetics for a Pb adatom on Pb(111), Pb(100), and Pb(110) nanofilms with different thicknesses. We find that these quantities exhibit damped oscillatory variation with increasing film thickness. For Pb(111) films, energetics along the minimum energy path for Pb adatom diffusion between adjacent fcc and hcp sites varies significantly with film thickness, its form differing from other metal-on-metal(111) systems. For Pb(111) and Pb(100) nanofilms, diffusion barriers obtained for both adatom hopping and exchange mechanism differ significantly from previous DFT results. Hopping is favored over exchange for Pb(111), and the opposite applies for Pb(100). For Pb(110) nanofilms, Pb adatom hopping over an in-channel bridge is most facile, then in-channel exchange, then cross-channel exchange, with cross-channel hopping least favorable. We also assess lateral Pb adatom interactions, and characterize island nucleation during deposition on Pb(111).

Scanning Tunneling Microscopy Study of the Structure and Interaction between Carbon Monoxide and Hydrogen on the Ru(0001) Surface.

We use scanning tunneling microscopy (STM) to investigate the spatial arrangement of carbon monoxide (CO) and hydrogen (H) coadsorbed on a model catalyst surface, Ru(0001). We find that at cryogenic temperatures, CO forms small triangular islands of up to 21 molecules with hydrogen segregated outside of the islands. Furthermore, whereas for small island sizes (3-6 CO molecules) the molecules adsorb at hcp sites, a registry shift toward top sites occurs for larger islands (10-21 CO molecules). To characterize the CO structures better and to help interpret the data, we carried out density functional theory (DFT) calculations of the structure and simulations of the STM images, which reveal a delicate interplay between the repulsions of the different species.

Adsorption and desorption of propane on Pd (111): A van der Waals density functional study. Energy binding sites and geometries

Palladium supported catalysts used for the partial oxidation of propane reaction aiming the H2 production deserves specific characterizations and theoretical modeling for the explanation of the transition phase and energy needed for the adsorption and desorption of propane on top of the palladium atoms. On the other hand, the product distribution will depend on the adsorption and desorption capacity of the different compounds present during the reaction. In this work, the adsorption of propane on a Pd (111) surface was studied by using different approximations. A periodic method based on the Density Functional Theory (DFT) formalism employing vdW-DF functional was investigated for determining preferred binding sites of propane on palladium. The results show that the adsorption on hcp site is more stable than on top site and predictions fit well the experimental results.

Static investigation of adsorption and hetero‐diffusion of copper, silver, and gold adatoms on the (111) surface

In this work, we have used the static molecular simulations combined with an interatomic potential derived from the embedded‐atom method to study the adsorption and hetero‐diffusion on the (111) surface of Cu, Ag, and Au adatoms by using LAMMPS code. The investigation is performed for six heterogeneous systems such as Ag/Au(111), Ag/Cu(111), Au/Ag(111), Au/Cu(111), Cu/Ag(111), and Cu/Au(111). First, we have investigated the relaxation trends and the bond lengths of the atoms in the systems. The calculation results show that, the top layer spacing between the first and second layers of the Au(111), Ag(111), and Cu(111) substrates is contracted. This contraction is found to be more important in the Au(111) substrate. On the other hand, the strong reduction of the binding length is found in Au/Cu(111) for the different adsorption sites. In addition, the binding, adsorption, and static activation energies for all studied systems were examined. The results indicated that the binding and adsorption energies reached their maximum values in the Au/Cu(111) and Au/Ag(111) systems, respectively. Moreover, the static activation barriers for hopping diffusion on the (111) surfaces are found to be low compared with those found in the (100) and (110) surfaces. Therefore, our calculations showed that the difference in energy between the hcp and fcc sites on the (111) surfaces is very small. Copyright © 2017 John Wiley & Sons, Ltd.

Study of Adsorption of Hydrogen on Al, Cu, Mg, Ti Surfaces in Al Alloy Melt via First Principles Calculation

Adsorption of hydrogen on Al(111), Cu(111), Mg(0001), and Ti(0001) surfaces have been investigated by means of first principles calculation. The calculation of surface energy indicates that Mg(0001) is the most stable surface, while Ti(0001) is the most unstable surface among all the four calculated surfaces. The obtained adsorption energy shows that the interaction between Al and H atoms should be energetically unfavorable, and the adsorption of hydrogen on Mg(0001) surface was found to be energetically preferred. Besides, the stability of hydrogen adsorption on studied surfaces increased in the order of Al(111), Ti(0001), Cu(111), Mg(0001). Calculation results also reveal that hydrogen adsorption on fcc and hcp sites are energetically stable compared with top and bridge sites for Ti(0001), Cu(111), and Mg(0001), while hydrogen adsorbing at the top site of Al(111) is the most unstable state compared with other sites. The calculated results agreed well with results from experiments and values in other calculations.

Mechanistic Insight into H<sub>2</sub>S Adsorption and Dissociation on MoP(010): A Density Functional Investigation

H2S adsorption and dissociation on MoP(010) were investigated using density functional theory (DFT) together with periodic slab models. Several different possibilities for H2S, SH, S and H adsorption were considered. Our results show that the H2S, SH and H prefer to adsorb at bridge site, while S adsorbs preferentially at hcp and bridge sites. Additionally, the optimum co-adsorption configurations for SH/H and S/H were determined. The results indicate that the co-adsorbed species repel each other slightly on MoP(010) surface. Finally, the potential energy profile of H2S dissociation on MoP(010) surface was given out. The dissociation energy barriers of the S–H bond scission exhibit that H2S prefers to dissociate on MoP(010) surface. When compared with MoP(001) surface, the obvious differences in H2S decomposition arise demonstrate that the MoP-based catalysts are structure-sensitive.

Structure of NO dimer multilayer on Rh(111)

Molecular self-assembly is the spontaneous organization of molecules under thermodynamic equilibrium conditions into well-defined arrangements via cooperative effects between chemical bonds and weak noncovalent interactions. Molecules undergo self-association without external instruction to form hierarchical structures. Molecular self-assembly is ubiquitous in nature and has recently emerged as a new strategy in chemical biosynthesis, polymer science and engineering. NO monomer is apt to be absorbed on the surfaces of some metals such as Ir(111), Ni(111), Pd(111), Pt(111), Rh(111) and Au(111), and the interactions of NO monomer with the metal surfaces have been extensively studied. When NO monomer is weakly adsorbed on the noble-metal surface, it cannot be reduced completely but forms a stable structure, which is named NO dimer. The first-principle technique is employed to determine the structures of NO dimer ((NO)2) molecular chains and monolayers on virtual Rh(111), as well as (NO)2 monolayer and multilayer on Rh(111). First, (NO)2 monomers are assembled into two stable molecular chains on the virtual Rh(111) surface, whose bind energies are 0.309 and 0.266 eV, respectively. The molecular chains are self-assembly systems, in which (NO)2 monomers are parallel and ordered, and the O atoms and N atoms are shown to be of (100) and (111) structures, respectively. Then, the two molecular chains are assembled into two stable monolayers (denoted as M1 and M2) on the virtual Rh(111)-(13), and the coverage is 1.00 ML. In the M1 monolayer, the angle between the NN bond of (NO)2 monomer and the substrate is in a range of 70-90, and in the M2 monolayer, the NN bond is parallel to the substrate.In the adsorption system of M2/Rh(111), (NO)2 molecules can be adsorbed on the top as well as the hcp and fcc hollow sites. When (NO)2 molecules are adsorbed on the top site, the adsorption system is best described by the electron structure Rh+0.14N0=O-0.14, and when (NO)2 molecules are absorbed on the two hollow sites, the adsorption system is described by the electron structure Rh+0.34N-0.18=O-0.16. Therefore, (NO)2 molecules are more apt to be adsorbed on the two hollow sites than on the top site. In the adsorption systems of M1+M2/Rh(111) and M1+(M1+M2)/Rh(111), (NO)2 molecules are adsorbed vertically on the two hollow sites, the NN bond is parallel to the substrate in the first monolayer, and the angle between the NN bond and the substrate is in a range of 70-90 in the second and third monolayers. The interaction between the neighbor monolayers is about 0.01 eV, and the thickness of the vacuum layer is 0.31 nm0.02 nm.

Effect of Chlorine Atoms for Development of Aluminum Corrosion

Computational density functional theory (DFT) model of the adsorption of chlorine atoms onto the perfect Al (111) surface has been performed. The structural and electronic properties of chlorine atoms adsorbed on the surface are investigated within a supercell approach for chlorine coverages of 0.25, 0.33, 0.5 and 1 ML respectively. It is found that the adsorbates prefer on-top sites over bridge, hcp and fcc sites in low coverage while fcc sites in high coverage, and the binding energy decrease with increase of coverage due to the interactions of chlorine atoms. The discussion of geometrical and electronic analysis by plotting differential charge density distribution and projected density of states (PDOS) are presented.

A first-principles study of oxygen adsorption on Ir(111) surface

In order to understand deposition mechanism of iridium thin film by atomic layer deposition, the adsorption of oxygen on Ir(111) surface was studied by use of density functional theory and a periodical slab model. By calculating the adsorption energy and structure of oxygen at four adsorption sites (top, bridge, fcc-hollow and hcp-hollow) on Ir(111) surface, the most stable adsorption site was determined. On this basis, the banding mechanism of O and Ir atoms was studied by density of states of oxygen and iridium atoms. Oxygen adsorbed at hcp(parallel) site on Ir(111) surface was the most stable one according to the adsorption energy calculation results. Orbital charge analysis indicate that charge transferred from 5p and 5d orbit to 2p orbit of adsorbed O atoms, and 6s orbit of iridium atoms. Meanwhile, density of state study indicated that adsorption of oxygen on Ir(111) surface is mainly due to the interaction between 2p orbit of O atoms and 5d orbit of iridium atoms.

First-principles study of Mg(0001)/MgO(1-11) interfaces

By means of first-principles density-functional calculations, we studied the surface energy of a nonstoichiometric MgO(1-11) slab, the interfacial energy and interfacial bonding characteristics of Mg-terminated and O-terminated Mg/MgO(1-11) interfaces with three stacking-site (TOP, HCP and FCC sites) models, and the effect of the thickness of Mg films on the O-terminated MgO(1-11) surface. The results indicate that the surface energies of the nonstoichiometric MgO(1-11) slab and interfacial energies of Mg/Mg(1-11) interface depend on Mg chemical potential. We found that the Mg-terminated MgO(1-11) surface is more stable than the O-terminated MgO(1-11) surface at high Mg chemical potential, and Mg/MgO(1-11) with FCC stacking-site model is the most stable configuration in the Mg/MgO(1-11) interfaces. The results of the electronic structure reveals that the interfacial bonding of Mg-terminated interface with FCC site model mainly consists of metallic bond and of the O-terminated interface with FCC site model is mainly ionic with a small degree of [Formula: see text]-type covalent bond. Although the interfacial energy of Mg-terminated Mg/MgO interface with FCC stacking-site model is slightly higher than that of O-terminated Mg/MgO interface, the molten Mg would epitaxially grow on the FCC sites of the Mg-terminated MgO(1-11) surface because of the high evaporation pressure of Mg at high temperature.

Interaction of Gold with a Pinwheel TiO∼1.2 Film Formed on Rh(111) Facet: STM and DFT Studies

The atomic structure of “pinwheel” TiO∼1.2 ultrathin oxide (w-TiO-UTO) layer and its reaction with gold are studied by scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations. The UTO film was formed as an encapsulation layer on the top facet (111) of stripe-like Rh nanoparticles supported on a TiO2(110) substrate. For proposing a structural model, the previous STM, photoelectron (XPS), and ion scattering spectroscopy (LEIS) results were also taken into account. DFT calculations were carried out within the generalized gradient approximation (GGA-PBE) in the frame of the Quantum Espresso code. A Rh(111) slab of four layers with a TiO1.14 overlayer and a Rh–Ti–O stacking sequence were used. In the starting model, the ratio between hcp and fcc sites filled with Ti atoms was 1.54 (the same value for O atoms was 2.2) on the top of Rh layers. The simulation of the STM images of the relaxed structure was done following the Tersoff–Hamann approximation. The main structural cha...

Structure of BP3S monolayer on Au(111)

The first-principle technique is employed to determine the structure of the BP3S monomer, the structures of the molecular chains and monolayers on virtual Au（111), and the atomic structure of BP3S/Au（111) adsorption system. The results show that the BP3S monomer presents a symmetric structure, and the angle between two benzene rings is 3510. At first, many BP3S monomers are assembled into one stable molecular chain in the virtual Au（111), the distance between the neighbor monmers is 0.516 nm, and the bind energy between the monmer and the molecular chain is 0.071 eV. It is a self-assembly system. Then many molecular chains are assembled into two stable monolayers in the virtual Au（111)-(37) and Au（111)-(313), and their coverages are 0.20 ML and 0.14 ML, respectively. In the virtual Au（111)-(37) and Au（111)-(313), the angles between the molecular chains and the virtual surface are 60 and 30, respectively, and the binding energies between the monmer and two monolayers are 0.101 eV and 0.125 eV, respectively. They are both the self-assembly systems. Finally, two monolayers are adsorbed on the Au（111)-(37) and Au（111)-(313) at four adsorption sites. The S atom is easy to obtain two electrons and turn into S2- ion, and the Au atom is easy to lose one electron and become Au+ ion, so the bridge site（two Au+ ions) is more stable than the top site（one Au+ ion), while the hcp and fcc hollow sites（three Au+ ions) are both unstable. In the Au（111)-(37), the chemisorption energy of the bridge site（-1.879 eV) is lower than that of the top site（-1.511 eV). And in the Au（111)-(313), the chemisorption energy of the bridge site（-1.691 eV) is lower than that of the top site（-1.492 eV). The results are confirmed in the other S-Au adsorption systems, such as the C6H13S/Au（111). A comparison between the structures of the BP3S monolayer before and after being adsorbed on Au（111) clearly shows that the structural parameters of the adsorption system depend mainly on the interaction in the monolayer, and that the contribution of Au（111) to the structure of the monolayer is weak. These results are confirmed in the other self-assembly adsorption systems.

Dibenzyl disulfide adsorption on Cu(111) surface: a DFT study

The adsorption of dibenzyl disulfide (DBDS) on a Cu(111) surface model was investigated by using density functional calculations, considering energetic and electronic aspects. Several complexes were generated, where the bridge, hollow hcp, hollow fcc and top adsorption sites were considered. The results show that the Cu–S interaction guides the final complexes, and a secondary π–Cu weak interaction confers an extra stability. The complexes were grouped as physi- or chemisorption according to their adsorption energy applying a distortion decomposition model, with a preference by a double interaction of S with Cu (i.e., hollow hcp and bridge sites). A degree of disulfide bond dissociation was observed in the complexes, being correlated with adsorption energies. From an electronic aspect, it was found that the electronic flow from copper to DBDS occurs in the most stable complexes, checked with charge analysis. These results are in agreement with experimental revelations of copper corrosion on power transformers.