Oxygen adsorption on Mo(112) surface studied by ab initio genetic algorithm and experiment

Density functional theory in combination with genetic algorithm is applied to determine the atomic models of p(1x2) and p(1x3) surface structures observed upon oxygen adsorption on a Mo(112) surface. The authors' simulations reveal an unusual flexibility of Mo(112) resulting in oxygen-induced reconstructions and lead to more stable structures than any suggested so far. Comparison of the stabilities of the predicted models shows that different p(1x2) and p(1x3) structures may coexist over a wide range of oxygen pressures. A pure p(1x2) structure can be obtained only in a narrow region of oxygen pressures. In contrast, a pure p(1x3) structure cannot exist as a stable phase. The results of simulations are fully supported by a multitude of experimental data obtained from low energy electron diffraction, x-ray photoelectron spectroscopy, and scanning tunneling microscopy.     

Diffusion of lithium and strontium on Mo(112)

Surfaces with the furrowed atomic corrugation induce a strong anisotropy both in lateral interaction of adatoms and in surface diffusion. This gives rise to some interesting peculiarities in structure and diffusion kinetics of adsorbed layers. In particular, chain-like adatom structures are typical of such surfaces, and surface diffusion of adatoms may have a quasi-one-dimensional character.

In our work we report on the data obtained in a study of the surface diffusion of electropositive adsorbates, Li and Sr, on the (112) surface of Mo which is made of close packed rows of atoms separated by atomically deep furrows. The method used was a contact potential technique with a narrow (˜ 15μm) electron beam providing an accuracy of the work function measurements of ˜ 10⁻³eV. The diffusional evolution of adsorbate coverage profiles, initially shaped as sharp-edged steps, stripes and trenches, results in a complex self-organization of the diffusion zone. The most extended 2D phases emerging in the zone are those having the highest diffusion rate. We have also observed an unusual nonmonotonic variation of adatom concentration in space and time which may be caused by the strong nonlinearity of the system under study. Diffusion mechanisms operating under various conditions are discussed.     

Density functional theory study of pyrrole adsorption on Mo(110)

The objective of the present study is to identify possible adsorption configurations of pyrrole on Mo(110) using density functional theory (DFT) calculations. Several adsorption configurations were studied including pyrrole and pyrrolyl adsorption as parallel, perpendicular, and tilted adsorption modes relative to the Mo(110) surface plane. Based on the DFT calculations, pyrrole is suggested to adsorb in a parallel mode with respect to the Mo(110) surface through its pi-orbital as mu3,eta(5)-Pyr-0 degrees with an adsorption energy of -28.7 to -31.5 kcal mol(-1). The possibility of a coexisting mode where pyrrole adsorbs on the surface with a slightly tilted molecular plane as mu3,eta(4)(N,C2,C3,C4)-Pyr-5 degrees is also likely to occur, particularly at higher pyrrole coverages. The slightly tilted configuration is suggested to arise from the lateral interactions of adsorbed pyrrole on Mo(110), and not the result of a phase transformation on the surface since this requires a relatively high activation energy as indicated by additional linear synchronous transit (LST)/quadratic synchronous transit (QST) calculations. Both adsorption geometries bond to three surface Mo atoms, and Mo(110) did not promote hydrogen abstraction. Pyrrolyl adsorption on Mo(110) is energetically possible, but unlikely to occur because gas-phase hydrogen has not been previously experimentally observed as a pyrrole decomposition product on Mo(110).     

Ab initio study of molecular oxygen adsorption on Pu (111) surface

Using the generalized gradient approximation to density functional theory (DFT), molecular and dissociative oxygen adsorptions on a Pu (111) surface has been studied in detail. Dissociative adsorption with a layer‐by‐layer alternate spin arrangement of the plutonium layer is found to be energetically more favorable, and adsorption of oxygen does not change this feature. Hor1 (O₂ is parallel to the surface and lattice vectors) approach on the center2 (center of the unit cell, where there is a Pu atom directly below on the third layer) site, both without and with spin polarization, was found to be the preferred chemisorbed site among all cases studied with chemisorption energies of 8.365 and 7.897 eV, respectively. The second‐highest chemisorption energy occurs at the Ver (O₂ is vertical to the surface) approach of the bridge site with chemisorption energies of 8.294 eV (non‐spin‐polarized) and 7.859 eV (spin‐polarized), respectively. We find that 5f electrons are more localized in the spin‐polarized case than the non‐spin‐polarized counterparts. Localization of the 5f electrons is higher in the oxygen‐adsorbed plutonium layers compared with the bare layers. The ionic part of O? Pu bonding plays a significant role in the chemisorption process, along with Pu 5f? O 2p hybridization. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

First-principles study of hydrogen adsorption on Mo(1 1 0)

First-principles calculations based on density functional theory-generalized gradient approximation method have been performed for hydrogen (H) adsorption on Mo(1 1 0) surface. For various coverages, the hollow (hol) site was found to be the most stable binding site. The adsorption energy of this site was slightly increased as the increasing of hydrogen coverage. Subsurface (sub) occupation at low and medium coverages was ruled out while it became to be stable at the coverage of 1 ML. This is also supported by the potential energy surface (PES) study for hydrogen diffusing from hol to sub site. It’s interesting to find a surface reconstruction at the coverage of 1 ML, which is characterized by the lateral shift of the topmost layer for the sub adsorption. At higher coverage, the local density of states (LDOS) analysis showed that a new peak was clearly visible which was ascribed to a surface state induced by hydrogen adsorption. This surface state was mostly localized on the hydrogen atom and the first Mo layer, implying the hybridization of the hydrogen 1s states and the Mo metal states.     

Low temperature adsorption of oxygen on Ni(100)

LEED, AES and data indicate the nonreconstructive character of oxygen adsorption on Ni(100) at 170 K and reconstructive chemisorption at T>270 K with the formation of a c(2×2)0 structure.

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, , Ni (100) 170 >270 (2×2)0.

     

Density-functional study of the cycloaddition of acrylonitrile on the Si(100) surface

Using a density functional approach, we have explored the cycloaddition of acrylonitrile on the Si(100) surface. The buckling of the surface dimers characteristic for the (2x1) reconstructed surface is shown to favor structures with a dipolar moment such as the resonant form of acrylonitrile with cumulative double bonds. The bond of acrylonitrile via a single C atom is a possible intermediate leading to the nitrile structure of the adsorbed molecule.     

改性凹凸棒土对Mo(Ⅵ)的吸附性能研究

研究了改性凹凸棒土吸附Mo(Ⅵ)的性能和机理,探讨了Mo(Ⅵ)溶液的起始浓度、pH值、吸附时间以及温度对此吸附剂吸附Mo(Ⅵ)性能的影响,得出了适宜的吸附条件.用红外对适宜条件下的吸附产物进行了表征.结果表明:改性凹凸棒土吸附Mo(Ⅵ)的适宜条件为:Mo(Ⅵ)溶液初始浓度1.86 mg·L-1,pH值2.20,时间20 min,温度35 ℃.改性凹凸棒土与MoO42-离子间通过静电作用(化学吸附)以及物理吸附相结合.     

Density-functional theory study of Iron(III) hydrolysis in aqueous solution

Fe(III) hydrolysis in aqueous solution has been investigated using density-functional methods (DFT). All possible structures arising from different tautomers and multiplicities have been calculated. The solvation energy has been estimated using the UAHF-PCM method. The hydrolysis free energies have been estimated and compared with the available experimental data. The different hydrolysis species have distinct geometries and electronic structures. We have shown that improvement of theory level in calculating the electronic energy does not necessarily improve the estimated free energies in aqueous solution since the UAHF-PCM is a simple method that neglects specific interactions with the solvent. Therefore, it is important to have the correct balance between theory level used in the electronic calculation and the UAHF-PCM. The PBE/TZVP/UAHF-PCM method has been found to describe correctly the hydrolysis energies of Fe(III), deviating about 3.0 kcal mol(-1) from experimental values.     

Dissociative adsorption of carbon monoxide on Mo(110): first-principles theory

The adsorption and dissociation of carbon monoxide on Mo (110) surface is studied with density functional theory. The results at different sites (atop, short bridge, long bridge, and hollow) are presented. The hollow site is found to be the most stable adsorption site for CO. The CO molecule is found to adsorb in end-on configurations (alpha states) at high coverage and inclined configurations (beta states) at low coverage. The dissociation activation energy from beta states is found to be approximately 1 eV lower than from alpha state. The adsorption of dissociation products, C and O, on Mo(110) has also been studied. The most stable adsorption site for C and O is long bridge and hollow site, respectively. The adsorption of C and O at low coverage is, in general, stronger than at high coverage, which is partly responsible for the high reactivity of CO dissociation at low coverage, since the binding energy of CO is not very sensitive to the coverage.     

Density-functional calculations of HCN adsorption on the pristine and Si-doped graphynes

Graphyne, a lattice of benzene rings connected by acetylene bonds, is one-atom-thick planar sheet of sp- and sp²-bonded carbons differing from the hybridization of graphene (considered as pure sp²). Here, HCN adsorption on the pristine and Si-doped graphynes was studied using density-functional calculations in terms of geometric, energetic, and electronic properties. It was found that HCN molecule is weakly adsorbed on the pristine graphyne and slightly affects its electronic properties. While, Si-doped graphyne shows high reactivity toward HCN, and, in the most favorable state, the calculated adsorption energy is about ?10.1 kcal/mol. The graphyne, in which sp-carbon was substituted by Si atom, is more favorable for HCN adsorption in comparison with sp²-carbon. It was shown that the electronic properties of Si-doped graphyne are strongly sensitive to the presence of HCN molecule and therefore it may be used in sensor devices.     

Au and Pd atoms adsorbed on pure and Ti-doped SiO2/Mo(112) films

The interaction of Pd and Au atoms with a silica surface and SiO2Mo(112) ultrathin films has been studied with periodic density-functional theory-generalized gradient approximation calculations. On both unsupported and supported silica, Pd and Au are weakly bound. No charge transfer occurs to the empty Pd and Au orbitals. Differently from Au, Pd can easily penetrate with virtually no barrier into the hexagonal rings of the supported silica film and binds strongly at the SiO2-Mo interface. The same process for Au implies overcoming a barrier of 0.9 eV. Completely different is the behavior of Ti-doped silica films. Au forms a direct covalent bond with substitutional Ti at the expense of the Ti...O-Mo interface bond which breaks. The global process is exothermic by 1 eV and nonactivated, showing that Ti doping results in solid anchoring points for the adsorbed Au atoms and for nucleation and growth of small gold particles. The effect of Ti doping is less pronounced for Pd but still visible with substantial enhancement of the Pd adsorption strength.     

Theoretical modeling of oxygen adsorption on the PbTe(001) surface

For the semiconducting compound PbTe, the initial stages of oxidation, which are important for technology of IR-and thermoelectric devices, have been theoretically studied. The structure, stability, and changes in the electrostatic potentials at the oxidized sites in lead telluride have been calculated in the framework of the cluster approach by the hybrid density functional theory B3LYP method. Different variants of attachment of one to six oxygen atoms to the atoms of the surface and subsurface layers have been considered. The most stable oxidation products have been found. The calculation results are quantitatively consistent with experimental XPS data on chemical shifts.     

Study of oxygen adsorption on Pt(110) surface with EELS

The forms of oxygen adsorbed on Pt(110) are characterized using Electron Energy loss Spectroscopy (EELS).

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Pt(110).

     

Pulse-chromatographic determination of molybdenum surface by oxygen adsorption in Co?Mo catalysts

A pulse-chromatographic method has been developed for the estimation of molybdenum surface by selective adsorption of oxygen at –78°C in cobalt-molybdenum-alumina catalysts. The surface concentrations of molybdenum in a series of catalysts of different compositions were compared with the activities for cyclohexene hydrogenation and for thiophene hydrogenolysis.

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- –78°C -- . - .

     

Mechanism of Charging of Au Atoms and Nanoclusters on Li Doped SiO2/Mo(112) Films

We present the results of supercell DFT calculations on the adsorption properties of Au atoms and small clusters (Au_n, n≤5) on a SiO₂/Mo(112) thin film and on the same system modified by doping with Li atoms. The adsorbed Li atoms penetrate into the pores of the silica film and become stabilized at the interface where they donate one electron to the Mo metal. As a consequence, the work function of the Li‐doped SiO₂/Mo(112) film is reduced and results in modified adsorption properties. In fact, while on the undoped SiO₂/Mo(112) film Au interacts only very weakly, on the Li‐doped surface Au atoms and clusters bind with significant bond strengths. The calculations show that this is due to the occurrence of an electron transfer from the SiO₂/Mo(112) interface to the adsorbed gold. The occurrence of the charge transfer is related to the work function of the support but also to the possibility for the silica film to undergo a strong polaronic distortion.     

A density functional theory study of atomic oxygen and nitrogen adsorption over alpha-alumina (0001)

The interaction of atomic oxygen and nitrogen on the (0001) surface of corundum (alpha-alumina) is investigated from first-principles by means of periodic density functional calculations within the generalized gradient approximation. A large Al(2)O(3) slab model (18 layers relaxing 10) ended with the most stable aluminium layer is used throughout the study. Geometries, adsorption energies and vibrational frequencies are calculated for several stationary points for two spin states at different sites over an 1 x 1 unit cell. Two stable adsorption minima over Al or in a bridge between Al and O surface atoms are found for oxygen and nitrogen, without activation energies. The oxygen adsorption (e.g., E(ad) = 2.30 eV) seems to be much more important than for nitrogen (e.g., E(ad) = 1.23 eV). Transition states for oxygen surface diffusion are characterized and present not very high-energy barriers. The computed geometries and adsorption energies are consistent with similar adsorption theoretical studies and related experimental data for O, N or alpha-alumina. The present results along with our previous results for beta-cristobalite do not support the assumption of an equal E(ad) for O and N over similar oxides, which is commonly used in some kinetic models to derive catalytic atomic recombination coefficients for atomic oxygen and nitrogen. The magnitude of O and N adsorption energies imply that Eley-Rideal and Langmuir-Hinshelwood reactions with these species will be exothermic, contrary to what happens for beta-cristobalite.     

DFT study of oxygen adsorption on modified nanostructured gold pyramids

Periodic DFT calculations are used to predict and investigate the adsorption behavior of molecular oxygen on Au, Au/Pt, and Pt surfaces. To obtain an array of pyramids containing surface atoms with the lowest possible coordination number, a nano-modified surface consisting of a symmetrically "modified" (100) surface was used. The effect of atom substitution (organized alloying) is investigated. The adsorption of molecular oxygen on a pure gold pyramid is exothermic by 0.77 eV for the end-on adsorption mode. In the case of a pure platinum pyramid, the end-on adsorption mode was found to dissociate; however, a side-on geometry was encountered with an energy of adsorption of 2.3 eV. This value is in line with the fact that the adsorption energy of small molecules does not vary much on Pt surfaces with different indices. Additionally, some geometrically related trends of the surface deformation in relation to its composition and after adsorption of molecular oxygen are highlighted.     

XPS study of ethylene adsorption on Ir (110)

The state of adsorption layers and adsorption kinetics of C₂H₄/Ir (110) at 300–1000 K has been studied using XPS method.

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C₂H₄/Ir (110) 300–1000 K.