Two-fold Bridge Site Research Articles

First principles investigation on adsorption mechanism of As(OH)3 on the kaolinite (001) surface

The environmental fate of arsenic (As) and arsenic-containing compounds are garnering more attention as a result of the global As contamination. Arsenous acid (As(OH)3) exists as a common arsenic-containing compound in the aqueous phase. Natural clay minerals like kaolinite have the potential to be adsorbents. First-principles calculations based on DFT were used to investigate the adsorption mechanism between As(OH)3 molecules and the kaolinite (001) surface. The two-fold bridge sites of the kaolinite (001) surface were found to be most stable for As(OH)3 adsorption. Three representative stable adsorption configurations, B2, H6, and T3, were selected to analyze. Strong hydrogen bonds formed between As(OH)3 molecules and the kaolinite (001) surfaces made the main contribution to the adsorption. The coverage (Θ) dependence to the adsorption stability was methodically investigated with a wide coverage range of 0 < Θ ≤ 1.0 monolayers (ML). The adsorption energy of As(OH)3 increased in the coverage range of 0 < Θ ≤ 0.5 ML and then decreased in the coverage range of 0.5 < Θ ≤ 1.0 ML. Other properties of the As(OH)3/kaolinite (001) system, including the bonding interaction, atom population/charge, electronic density of states, and lattice relaxation were also discussed and analyzed in detail.

Periodic DFT study on heavy metals Cu(II) and Pb(II) atoms adsorption on Na-montmorillonite (010) edge surface

Heavy metal (HM) pollution is of great concern currently because it has been recognized as a potential threat to air, water,and soil. Adsorption is one of the most effective strategies for removing heavy metals. Montmorillonite as the adsorbent with a low cost and high thermal stability has received much attention. The adsorption of heavy metal Copper [Cu(II)] and Plumbum [Pb(II)] atoms on Na-montmorillonite hydroxylated (010) edge surface was analyzed using the generalized gradient approximation and density functional theory in the supercell technique. The coverage dependence of the adsorption structure and energetics was systematically investigated for a wide range of coverage from 0 to 1.0 monolayer (ML). Among all the possible adsorption sites, the most stable adsorption site for Cu(II) atom was a three-fold hollow site while the most favorable adsorption site for Pb(II) atom was a three-fold hollow site and followed by a two-fold bridge site. The adsorption energy increased with increasing coverage of Pb(II) atoms, indicating higher stability of surface adsorption and a tendency for the formation of adsorbate island clusters with increasing coverage. However, the adsorption energy of Cu(II) atoms decreased with increasing coverage. In the coverage range of 0 < Θ ≤ 0.5 ML the recovery capacity of the Na-montmorillonite for the heavy metal atoms was in the order of Cu(II)>Pb(II), while the recovery capacity of the Na-montmorillonite in the coverage range of 0.5 < Θ ≤ 1 ML was in the order of Pb(II)>Cu(II). The other aspects of the Cu(II) and Pb(II)/Na-montmorillonite (010) systems, including adsorption geometry, differential charge distribution, and electronic density of states were also studied and discussed in detail.

Theoretical study on water adsorption and dissociation on the nickel surfaces.

Using density functional theory methods, H2O dissociation was investigated on the Ni(111), Ni(100), and Ni(110) surfaces. H and O atom as well as OH species adsorb stably at the high coordination sites. While on the Ni(110) surface, the OH species prefers at the twofold short bridge site because the adsorption on the fourfold hollow site is less feasible due to the increased distances between the nickel atoms. The amount of charge transfer is related to the adsorption stability. The more charge transfer, the more stable the adsorption. The charge transfer decreases in the order of O > OH > H. H2O molecule adsorbs at the top site in a configuration parallel to the surface. The final products are different for H2O dissociation due to the different mechanisms. On the Ni(111) surface, the final product is the O atom. On the Ni(100) and Ni(110) surfaces, the most abundant species are OH and H, but the reaction mechanisms were different. It is not necessary to linear BEP relationship for a given reaction on different surfaces. These results could provide fundamental insights into water behaviors and a favorable theoretical basis for further understanding and research on the interaction between water and metal surfaces.

Theoretical study of adsorption mechanism of heavy metals As and Pb on the calcite (104) surface

Heavy metal pollution has attracted much attention because it has long-term serious impact on the environment. Arsenic (As) and lead (Pb) are two kinds of heavy metals that are widely existing and highly toxic. Adsorption method has been proved to be an economical and efficient treatment to remove or reduce their toxicity. The adsorption of As and Pb atoms on the calcite (104) surface has been studied by density functional theory. The coverage dependence of the adsorption energetics and structures were comprehensively discussed at different adsorption sites for a wide range of coverage Θ from 0 to 1.0 monolayers (ML). Among all the adsorption sites, the one-fold top site cannot be adsorbed, and the three-fold hollow site was the most stable, followed by the two-fold bridge site. For the adsorption of As atom, the adsorption energy increased first and then decreased in the coverage range of 0–1.0 ML, indicating that the interaction between As atoms leaded to the decrease of adsorption energy. However, the adsorption energy of adsorbed Pb atom increased with the increase of coverage, implying that the Pb atoms tended to form clusters on calcite surface, thus improving adsorption stability. The calculation results showed that calcite minerals could be used to remove heavy metal atoms, which had high removal ability to As and Pb atoms. Besides, the lattice relaxation, different charge distribution and electron density of states for As or Pb / calcite (104) system were also investigated in depth.

First-principles calculations of methane adsorption at different coverage on the kaolinite (001) surface

Understanding gas adsorption capacity on shale sediments is great significance for the evaluation of shale gas reservoir potential and the exploration of potential gas shale. Clay minerals have an important impact on the adsorption of methane according to previous research. Density-functional theory was employed to explore the process of methane molecules adsorption on the kaolinite (001) surface. The coverage dependence of the adsorption position and energetics was systematically studied for a wide range of coverage Θ [from 1/16 to 1 monolayers (ML)]. It is found that the two-fold bridge site is the most stable, followed by the hollow and top sites. The adsorption energy of CH4 decreased with increasing coverage, thus indicating the lower stability of surface adsorption due to the repulsion of neighboring CH4 molecules. The coverage has obvious effects on the methane adsorption process. Other properties of the CH4/kaolinite (001) system, including the lattice relaxation and changes of electronic density of states, were also studied and discussed in detail.

Benzene Adsorption on Rh(111): A New Perspective on Intermolecular Interactions and Molecular Ordering

The adsorption of benzene on the Rh(111) substrate was investigated through scanning tunneling microscopy imaging and density functional theory calculations. Experiments were carried out at various surface coverages, with the amount of benzene adsorbed determined to influence the molecular adsorption site, the intermolecular interactions, and the interaction between the molecule and the substrate. At a sub-monolayer coverage of the surface, the molecules are disordered and kept apart by a strong interadsorbate repulsion, with a preference for the molecule to adsorb on a threefold hexagonally close-packed hollow site. At high coverage, the preferred adsorption site becomes the twofold symmetric bridge site, whether as part of the two dense ordered structures that form at high coverage ((2√3 × 3)rect or (√19 × √19)R23.4°) or as part of the disordered array of benzene molecules, which are arranged in formations which resemble the “building blocks” of the ordered overlayers. Despite the adsorption energy for ...

Density functional theory investigation of carbon monoxide adsorption on the kaolinite (001) surface**Project supported by the Young Elite Scientist Sponsorship Program by CAST and the National Natural Science Foundation of China (Grant No. 51574296).

Carbon monoxide (CO) is a gaseous pollutant with adverse effects on human health and the environment. Kaolinite is a natural mineral resource that can be used for different applications, including that it can also be used for retention of pollutant gases. The adsorption behavior of carbon monoxide molecules on the (001) surface of kaolinite was studied systematically by using density-functional theory and supercell models for a range coverage from 0.11 to 1.0 monolayers (ML). The CO adsorbed on the three-fold hollow, two-fold bridge, and one-fold top sites of the kaolinite(001) was tilted with respect to the surface. The strongest adsorbed site of carbon monoxide on the kaolinite (001) surface is the hollow site followed by the bridge and top site. The adsorption energy of CO decreased when increasing the coverage, thus indicating the lower stability of surface adsorption due to the repulsion of neighboring CO molecules. In addition to the adsorption structures and energetics, the lattice relaxation, the electronic density of states, and the different charge distribution have been investigated for different surface coverages.

Density Functional Theory Study Of The Interaction Of Hydroxyl Groups With Iron Surface

AbstractThe electronic interaction of hydroxyl groups with Fe(100) surface is modelled using a density functional theory (DFT) approach. The adsorption energies and structures of possible adsorption sites are calculated. According to our calculations of the adsorption energies, the interaction between oxygen atom of OH species and surface iron atom is shown to be strong. It is likely to be due to the interaction of the lone-pair electrons of oxygen and the 3dorbital electrons of iron atom. At low coverage (0.25ML), the most favorable adsorption sites are found to be two-fold bridge sites, and the orientation of the O-H bond is tilted to the surface normal. Further, the adsorption energy is found to be decreasing with the increasing OH group coverage.

Analysis of hydrogen adsorption and surface binding configuration on tungsten using direct recoil spectrometry

In this work, we apply low energy ion beam analysis to examine directly how the adsorbed hydrogen concentration and binding configuration on W(100) depend on temperature. We exposed the tungsten surface to fluxes of both atomic and molecular H and D. We then probed the H isotopes adsorbed along different crystal directions using 1–2keV Ne+ ions. At saturation coverage, H occupies two-fold bridge sites on W(100) at 25°C. The H coverage dramatically changes the behavior of channeled ions, as does reconstruction of the surface W atoms. For the exposure conditions examined here, we find that surface sites remain populated with H until the surface temperature reaches 200°C. After this point, we observe H rapidly desorbing until only a residual concentration remains at 450°C. Development of an efficient atomistic model that accurately reproduces the experimental ion energy spectra and azimuthal variation of recoiled H is underway.

Theoretical study of heavy metal Cd, Cu, Hg, and Ni(II) adsorption on the kaolinite(0 0 1) surface

Heavy metal pollution is currently of great concern because it has been recognized as a potential threat to air, water, and soil. Adsorption was one of the most popular methods for the removal of heavy metal. The adsorption of heavy metal Cd, Cu, Hg, and Ni(II) atoms on the hydroxylated (001) surface of kaolinite was investigated using density-functional theory within the generalized gradient approximation and a supercell approach. The coverage dependence of the adsorption structures and energetics were systematically studied for a wide range of coverage Θ [from 0.11 to 1.0 monolayers (ML)] and adsorption sites. The most stable among all possible adsorption sites for Cd(II) atom was the two-fold bridge site followed by the one-fold top site, and the top site was the most favorite adsorption site for Cu and Ni(II) atoms, while the three-fold hollow site was the most stable adsorption site for Hg(II) atom followed by the two-fold bridge site. The adsorption energy increases with the coverage for Cd, Cu, and Hg(II) atoms, thus indicating the higher stability of surface adsorption and a tendency to the formation of adsorbate islands (clusters) with increasing the coverage. However, the adsorption energy of Ni(II) atoms decreases when increasing the coverage. The adsorption capabilities of the kaolinite clay for the heavy metal atoms were in the order of Ni>Cu>Cd>Hg(II). The other properties of the Cd, Cu, Hg, and Ni(II)/kaolinite(001) system including the different charge distribution, the lattice relaxation, and the electronic density of states were also studied and discussed in detail.

Adsorption and diffusion of Pb(II) on the kaolinite(001) surface: A density-functional theory study

The adsorption and diffusion of Pb(II) atom on the hydroxylated (001) surface of kaolinite were investigated using density-functional theory within the generalized gradient approximation and a supercell approach. The coverage dependence of the adsorption structures and energetics was systematically studied for a wide range of coverage Θ [from 0.11 to 1.0 monolayers (ML)] and adsorption sites. The most stable among all possible adsorption sites was the two-fold bridge site followed by the one-fold top site, and the adsorption energy increased with the coverage, thus indicating the higher stability of surface adsorption and a tendency to the formation of Pb(II) islands (clusters) with increasing coverage. Moreover, the energy barrier for diffusion of Pb(II) atom between the one-fold top and the two-fold bridge adsorption sites on kaolinite(001) surface was 0.23 (0.31) eV, implying that the Pb(II) atom is prone to diffusing on kaolinite(001) surface. The other properties of the Pb(II)/kaolinite(001) system including the different charge distribution, the lattice relaxation, and the electronic density of states were also studied and discussed in detail.

Theoretical study of hydrogen adsorption on Co clusters

We present results of a systematic study of the adsorption of one to five hydrogen atoms on Conn=1–5 neutral clusters. The density functional (DFT) formalism has been adopted to perform this study with the use of the hybrid functional B3LYP. We present results for their binding, vertical and adiabatic detachment energies, magnetic moments, HOMO–LUMO gaps and their infra red spectra. The results show that by the use of an hybrid functional we obtain different ground state geometries as well as magnetic moments than some of the previous theoretical studies. We test the methodology comparing the calculated Infra Red (IR) spectra for the cationic clusters: Co4Ar+, Co5Ar+, Co4H2+ and Co5H6+ with the available Far Infra Red Multiple Adsorbed Photon (FIR-MPD) experimental spectra, and we obtained a good agreement. We found the adsorption of H2 exclusively dissociative for all neutral ConHmn=1–5, m=1–5 clusters. The twofold bridge site is identified to be the most favorable for adsorption on neutral Con (n>2). Therefore hydrogen adsorption in clusters is different than it is on surfaces, where it is known that hydrogen adsorbs in high coordination sites. We found that as we add H atoms: first, a structural transition from three to two dimensions occur in the cluster geometry, second an enhanced but non-monotonic change in their magnetic moments is observed, and finally the stability of the system is greatly increased.

Preparation and characterization of planar Ni–Au bimetallic model catalysts

Ni–Au bimetallic model catalysts were prepared as thin films on Re(0001) or Ru(0001) single crystal substrates. Surface compositions and electronic structures of the Ni–Au thin films were characterized by low energy ion scattering spectroscopy and X-ray photoelectron spectroscopy, respectively. Surface alloys were prepared by annealing Ni–Au thin films from 500 to 800K, resulting in substantial surface enrichment of Au. Annealing a Ni–Au thin film with a 1:1 bulk composition ratio at 700K for 10min resulted in a surface alloy with 84% (atomic concentration) of Au in the outermost surface layer. The surface atomic structure was investigated using CO as probe molecules, which exclusively adsorbs on the Ni atoms rather than on the Au atoms at room temperature. Polarization modulation infrared reflection absorption spectroscopy of CO adsorption on Ni–Au surface alloys showed that CO adsorption on two-fold bridge sites decreased and finally disappeared with an increase of Au surface concentration. The absence of Ni bridge site CO adsorption indicated that Ni atoms were isolated by Au atoms on Ni–Au alloyed surface.

Simulation of scanning tunneling microscope image of benzene chemisorbed on a Pd(111) electrode surface by density functional theory

A computational method based on density functional theory was used to simulate the scanning tunneling microscopy (STM) images of benzene chemisorbed on a Pd(111) electrode in order to confirm the adsorption site of the aromatic molecule on the metal surface held at a certain applied potential. The simulated STM images on various adsorption sites were obtained and compared with the experimental electrochemical STM images. The simulation results indicate that when the potential of the Pd electrode is held at 0.3 V, benzene is chemisorbed on a threefold hollow site; at 0.55 V, the molecule is adsorbed on a position between a threefold and a twofold bridge site. These findings corroborate previously published experimental elec - trochemical STM results.

Evolution of Co n Al clusters and chemisorption of hydrogen on Co n Al clusters

The growth behavior of ConAl (n = 1–15) and the chemisorptions of hydrogen on the ground state geometries have been studied using the density functional theory (DFT) within the generalized gradient approximation (GGA). The growth pattern for ConAl is Al-substituted Con+1 clusters, and it keeps the similar frameworks of the most stable Con+1 clusters except for n = 2, 3, and 6. The Al atom substitutes the surface atom of the Con+1 clusters for n ≤ 13. Starting from n = 14, the Al atom completely falls into the center of the Co-frame. The dissociation energy, the second-order energy differences, and the HOMO–LUMO gaps indicate that the magic numbers of the calculated ConAl clusters are 7, 9, and 13, corresponding to the high symmetrical structures. To my knowledge, this is the first time that a systematic study of chemisorption of hydrogen on cobalt aluminum clusters. The twofold bridge site is identified to be the most favorable chemisorptions site for one hydrogen adsorption on ConAl (n = 1–6, 8, 10), and two hydrogen adsorption on ConAl (n = 1–7), while threefold hollow site is preferred for one hydrogen adsorption on ConAl (n = 7, 9, 11–15) and two hydrogen adsorption on ConAl (n = 8–10, 12–15) clusters. The ground state structure of two hydrogen adsorption on Co11Al is exceptional. In general, the binding energy of both H and 2H of ConAl (n = 1–12) is found to increase with the cluster size. And the result shows that large binding energies of the hydrogen atoms and large fragmentation energies for Co11AlH and Co12AlH make these species behaving like magic clusters.

Adsorption of carbon monoxide on Pd (210) and (510) surfaces

Adsorption of carbon monoxide on Pd (210) and (510) stepped surfaces has been investigated by the extended London‐Eyring‐Polyani‐Sato method constructed using a five‐parameter Morse potential. Pd (210) and (510) stepped surfaces consist of terrace with (100) structure and step with (110) character. These results show that there exist common characteristics of CO adsorption on these two surfaces. At low coverage, CO adsorbs in twofold bridge site of the (100) terrace. The critical characteristics inherit that of CO molecule adsorbed in twofold bridge site of (100) original surface. When the coverage is increased, the top site of (110) step is occupied. The critical characteristics resemble that of CO molecule adsorbed in top site of (110) original surface. A number of new sites are exposed on the boundary regions, for example, the fivefold hollow site (H) of these two surfaces. There are stable adsorption sites at high coverage. Because of the different length of the (100) terrace, the (210) and (510) stepped surfaces have some different characteristics. First, CO is tilted adsorption on bridge site of terrace of (210), but perpendicular on terrace of (510) surface. Second, the bridge site (B1) where one Pd atom at the top of the step and the other at the bottom of the step is a stable adsorption site on (210), but the same type of site on Pd (510) surface is not. Copyright © 2012 John Wiley &amp; Sons, Ltd.

A Density Functional Theory Study on the Effect of Zero‐Point Energy Corrections on the Methanation Profile on Fe(100)

The thermodynamics and kinetics of the surface hydrogenation of adsorbed atomic carbon to methane, following the reaction sequence C+4H(-->/<--)CH+3H(-->/<--)CH(2)+2H(-->/<--)CH(3)+H(-->/<--)CH(4), are studied on Fe(100) by means of density functional theory. An assessment is made on whether the adsorption energies and overall energy profile are affected when zero-point energy (ZPE) corrections are included. The C, CH and CH(2) species are most stable at the fourfold hollow site, while CH(3) prefers the twofold bridge site. Atomic hydrogen is adsorbed at both the twofold bridge and fourfold hollow sites. Methane is physisorbed on the surface and shows neither orientation nor site preference. It is easily desorbed to the gas phase once formed. The incorporation of ZPE corrections has a very slight, if any, effect on the adsorption energies and does not alter the trends with regards to the most stable adsorption sites. The successive addition of hydrogen to atomic carbon is endothermic up to the addition of the third hydrogen atom resulting in the methyl species, but exothermic in the final hydrogenation step, which leads to methane. The overall methanation reaction is endothermic when starting from atomic carbon and hydrogen on the surface. Zero-point energy corrections are rarely provided in the literature. Since they are derived from C-H bonds with characteristic vibrations on the order of 2500-3000 cm(-1), the equivalent ZPE of 1/2 hν is on the order of 0.2-0.3 eV and its effect on adsorption energy can in principle be significant. Particularly in reactions between CH(x) and H, the ZPE correction is expected to be significant, as additional C-H bonds are formed. In this instance, the methanation reaction energy of +0.77 eV increased to +1.45 eV with the inclusion of ZPE corrections, that is, less favourable. Therefore, it is crucial to include ZPE corrections when reporting reactions involving hydrogen-containing species.

A density functional theory study on the Ag n H (n = 1–10) clusters

Density functional GGA-PW91 method with DNP basis set is applied to optimize the geometries of Ag n H (n = 1–10) clusters. For the lowest energy geometries of Ag n H (n = 1–10) clusters, the hydrogen atom prefers to occupy the two-fold coordination bridge site except the occupation of single-fold coordination site in AgH cluster. After adsorption of hydrogen atom, most Ag n structures are slightly perturbed and only the Ag6 structure in Ag6H cluster is distorted obviously. The Ag–Ag bond is strengthened and the strength of Ag–H bond exhibits a clear odd–even oscillation like the strength of Au–H bond in Au n H clusters, indicating that the hydrogen atom is more favorable to be adsorbed by odd-numbered pure silver clusters. The adsorption strength of small silver cluster toward H atom is obviously weaker than that of small gold cluster toward H atom due to the strong scalar relativistic effect in small gold cluster. The pronounced odd–even alternation of the magnetic moments is observed in Ag n H systems, indicating that the Ag n H clusters possess tunable magnetic properties by adsorbing hydrogen atom onto odd-numbered or even-numbered small silver cluster.

Deduction of a three-phase model for the (√3 × √3)R30°-Cu 2Si/Cu(1 1 1) surface alloy

The (√3 × √3)R30°-Cu 2Si/Cu(1 1 1) surface alloy that forms during high temperature dosing of silane (SiH 4) on Cu(1 1 1) has been investigated using LCAO–DFT. Simulated STM images have shown that experimental images may be interpreted as a mixed phase system consisting of Si ion cores bound in HCP, FCC and twofold bridge sites with a ratio of 25:25:50 rather than previously proposed models where the Si ion cores were bound in only FCC and HCP sites. The new model is shown to be consistent with previously published NIXSW studies.

Molecular adsorption and metal-support interaction for transition-metal clusters in zeolites: NO adsorption on Pdn (n=1–6) clusters in mordenite

The adsorption of NO molecules on Pd(n) clusters of varying size (n=1-6) located in the main channel of mordenite and the interaction of the metallic clusters with the zeolitic framework were investigated using ab initio density-functional calculations under periodic boundary conditions. The supported clusters are created by binding Pd(n) (2+) cations to the inner cavity of a deprotonated Al-exchanged zeolite with an Al/Si ratio of 1/11, such that a charge-neutral system is created. Compared to the highly symmetric structures of the gas-phase clusters, the clusters bound to the zeolitic framework undergo appreciable geometric distortions lowering their symmetry. The distortions are induced by strong interactions with "activated" framework oxygens located close to the charge-compensating Al/Si substitution sites, but the cluster forms also weaker bonds to "nonactivated" oxygen atoms. The interaction with the framework also affects the electronic and magnetic properties of the clusters. While in the gas phase all clusters (except the isolated Pd atom with a closed d(10) ground state) have a paramagnetic moment of 2mu(B), in the zeolite clusters with two to four atoms have zero magnetic moment, while the Pd(5) cluster has a magnetic moment of 2mu(B) and for the Pd(6) cluster, it is even enhanced to 4 mu(B) (but the magnetic energy differences relative to low-spin configurations are modest). Analysis of the magnetization densities shows that in all clusters with zero total moment (singlet ground state), there are sites with excess spin densities of opposite sign. The influence of the cluster-support interaction on the chemical properties of the clusters has been tested by the adsorption of NO molecules. The results demonstrate the interplay between the molecule-cluster and cluster-framework interactions, which can lead to an increase or decrease in the adsorption energy compared to NO on a gas-phase cluster. While on the gas-phase cluster adsorption in low-coordination sites (vertex or bridge) is preferred, for the cluster in the zeolite adsorption in threefold coordinated hollow or twofold bridge sites is preferred. The magnetic properties of the clusters and of the paramagnetic NO molecule play an important role. For the supported clusters with zero magnetic moment, upon adsorption the spin of the molecule is transferred to the cluster (and induces also a modest polarization of the framework). For magnetic clusters, spin pairing induces a reduced magnetic moment of the NO-Pd(n) complex. The redshift of the NO stretching frequencies is reduced compared to the free clusters by the cluster-support interaction for the smaller clusters, while it remains essentially unchanged for the larger clusters. A detailed electronic analysis of the cluster-support interactions and of the adsorption properties is presented.