Periodic Density Functional Study Research Articles

The acidic properties of H-MeAlPO-5 (Me = Si, Ti, or Zr): A periodic density functional study

The relative strength for Brønsted acid sites of isomorphously substituted H-MeAlPO-5 molecular sieves (Me = Si, Ti, or Zr) is investigated using density functional theory employing periodic models. The results are consistent with the experimental observation and show that H-ZrAlPO-5 has the weakest Brønsted acid sites. Furthermore, the Brønsted acidity of H-MeAlPO-5 (Me = Si or Ti) is compared with that of H-MeAlPO-34 (Me = Si or Ti) [M. Elanany, K. Koyama, M. Kubo, P. Selvam, A. Miyamoto, Micropor. Mesopor. Mater. 71 (2004) 51].

Different support effect of M/ZrO 2 and M/CeO 2 (M = Pd and Pt) catalysts on CO adsorption: A periodic density functional study

CO adsorption over Pd 4 and Pt 4 cluster supported by c-ZrO 2(1 1 1) and CeO 2(1 1 1) catalyst systems was investigated using periodic density functional method in order to clarify the support effect on CO activation. We found that the support increases the CO activation for bridge and three-fold sites but decreases for the atop site. Moreover, it was found that the support changes the site preference for the CO adsorption. Bridge site on both the Pt 4/c-ZrO 2 and Pt 4/CeO 2 show larger CO adsorption energies than those on the other sites while the atop site is energetically preferable on isolated Pt 4 cluster. c-ZrO 2 supported Pd shows the largest CO activation with large charge transfer from the catalyst to the CO molecule. This reveals that ZrO 2 supported Pd can be a good catalyst for CO activation because of its higher probability to the three-fold site CO adsorption. We also found that positively charged M 4 clusters on the support keep their strong electron-donating properties and have enough charge density to contribute to the activation of an adsorbed CO molecule by a charge transfer.

CO adsorption on monometallic Pd, Rh, Cu and bimetallic PdCu and RhCu monolayers supported on Ru(0 0 0 1)

This work presents a systematic periodic density functional study of the changes in CO adsorption on metallic Rh, Pd, Cu monolayers supported on Ru(0 0 0 1) relative to CO adsorption on their corresponding Rh(1 1 1), Pd(1 1 1), Cu(1 1 1) single metal surfaces. Moreover, CO adsorption on various compositions of bimetallic PdCu and RhCu overlayers, also supported on Ru(0 0 0 1) have been considered. The metallic support affects each metallic crystal surface in a different way. However, the changes in the ability of back-donation from each metal surface to the CO(2π*) antibonding molecular orbital explain the variation of the adsorption energies. On the contrary, the presence of Ru does not affect the CO adsorption geometry. For the supported bimetallic overlayers, the CO–Pd and CO–Rh bond strength is enhanced by the presence of Cu. It is suggested that this effect is due to charge transfer from Cu the other metals thus enhancing their ability to back-donate to CO. The study of adsorption on different sites of the different bimetallic monolayers indicates that the key factor controlling CO adsorption energy is the nature of the metallic atoms constituting the active sites. These findings have been confirmed for two different CO coverage situations.

Periodic density functional and tight-binding quantum chemical molecular dynamics study of surface hydroxyl groups on ZrO 2(111)-supported Pt catalyst

Periodic density functional and tight-binding quantum chemical molecular dynamics calculations were employed to clarify the characteristics and dynamics of surface hydroxyl groups on Pt/ZrO 2 catalysts. We applied the periodic density functional theory calculations to the investigations on the effect of hydroxyl groups on the adsorption characteristics of Pt atom on ZrO 2(111) support and hydroxyl groups were found to decrease the adsorption energy of Pt atom on the ZrO 2(111) support. Furthermore, we applied our original tight-binding quantum chemical molecular dynamics method to the clarification of the dynamic behaviors of the hydroxyl group on the Pt/ZrO 2(111) and ZrO 2(111) surface. We found that the Pt atom triggers the migration and hopping of the H atom of the hydroxyl group. The migration of H atoms on the Pt/ZrO 2 catalyst was discussed in the sight of their catalytic activity.

Surface Structure of Hydroxylated and Sulfated Zirconia. A Periodic Density-Functional Study

The surface structure of sulfated zirconia (SZ) is examined by density-functional theory (DFT) with periodic boundary conditions. Adsorption of H2O and SO3 (or H2SO4) on the (101) surface of tetragonal zirconia is studied for different loadings up to H2SO4·3H2O and 2H2SO4·2H2O per two surface unit cells (four Zr surface sites). The considered surface species include H2O, [H+,OH-], SO3, [H+,HSO4-], [2H+,SO42-], [H+,HS2O7-], and [2H+,S2O72-]. Statistical thermodynamics is used to evaluate the relative stability of different surface structures for different temperatures and pressures of H2O and SO3 (or H2SO4). The simulated surface phase diagrams show a strong dependency on the considered sulfur species (H2SO4 or SO3) as well as on pressure and temperature. Monosulfates and pyrosulfates may occur, but higher condensated sulfates are not observed. In agreement with infrared experiments, we predict transformation of water-rich structures, [SO42-,2H+,3H2O], into pyrosulfate structures, [S2O72-,2H+,H2O], during calcination. Further increase of the temperature yields adsorbed SO3 before the clean surface is reached. Water adsorbed on the t-ZrO2(101) surface leaves in three steps upon heating from 250 to 730 K at 0.01 bar pressure: physisorbed water below room temperature, the first chemisorbed water at about 440 K and the last water at about 730 K.

Periodic density functional study on structural and vibrational properties of vanadium oxide aggregates

We present periodic density-functional calculations within the generalized gradient approximation (Perdew-Wang 91) on structures and vibrational properties of different vanadium oxide aggregates, namely, bulk ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ and its (001) surface, as well as thin vanadium oxide films supported by $\ensuremath{\alpha}$-alumina. Vanadium is differently coordinated by oxygen in the different systems. The calculated vibrational frequencies of bulk ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ are in good agreement with observed IR and Raman frequencies, for stretching modes the rms deviation is $40{\mathrm{cm}}^{\ensuremath{-}1}.$ The calculations for the ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$(001) surface suggest modifications of previous assignments of high-resolution electron-energy-loss spectroscopy (HREELS) data. In agreement with HREELS, vanadyl frequencies shift to higher wave numbers on surface formation. The calculated frequencies for bulk ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ are systematically lower than the observed IR data (by about $30{\mathrm{cm}}^{\ensuremath{-}1}).$ Models for ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ supported on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ are obtained when in the outermost layers of ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$(0001) slabs Al is replaced by V. These films do not show vibrations above $930{\mathrm{cm}}^{\ensuremath{-}1}.$ Oxygen adsorption on top of the vanadium sites on these supported films creates very stable vanadyl groups with binding energies of about 450 kJ/mol $(\frac{1}{2}{\mathrm{O}}_{2}).$ Bond distances, vibrational frequencies, and oxygen binding energies are compared with those of vanadyl groups at the ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$(001) surface and in $({\mathrm{V}}_{2}{\mathrm{O}}_{5}{)}_{n}$ clusters $(n=2,$4). The relevance of the findings for experiments on vanadia particles supported on alumina is discussed.

Simulation of phenol formation from benzene with a Pd membrane reactor: ab initio periodic density functional study

Phenol formation from benzene with a Pd membrane has been simulated by ab initio periodic density functional theory (DFT) method (DMol 3) to investigate the nature of the active oxygen species and the energy profile of phenol formation. An oxygen atom that is produced by the reaction of hydrogen permeating through the Pd membrane is the most likely active species. The reaction of benzene and oxygen atom to form phenol on the Pd surface is energetically probable (the total energy gain of the reaction is about 0.6 eV). Adsorption of phenol is not stronger than that of benzene, but rather a little less stable. In the presence of a larger amount of benzene in the gas phase, benzene can expel phenol from the surface, which thus represses the successive oxidation of phenol and continues the catalytic cycle.

Electronic properties of metal–molecule–metal systems at zero bias: A periodic density functional study

We have studied the electronic properties of conjugated and saturated dithiol molecules sandwiched between two Au(111) electrodes using first principles density functional calculations with a slab geometry. Relaxation of the molecule/surface adsorption geometry as well as the extended character of the metal electrode states are fully taken into account by our approach. Investigated quantities include the alignment of molecular energy levels with the Fermi energy (EF) of the metal, the charge transfer and electrostatic potential profile, and the local density of electronic states (LDOS) at EF. The behavior of the LDOS for benzene–, dibenzene–, and xylyl–dithiol molecules is analyzed and compared with that of alkane–dithiols of various lengths.

Hydrogen defects in Forsterite: A test case for the embedded cluster method

We present a theoretical study of several hydrogen defects in the upper mantle mineral forsterite using a quantum mechanical embedded cluster approach. We make extensive comparisons with results obtained using Mott–Littleton calculations and data from periodic density functional studies by ourselves and other authors. We show that the embedded cluster method gives reliable energies for a range of defect reactions, including cation vacancy formation, binding between defects, and formation of hydrogen defects by reaction with water. We propose that the embedded cluster techniques should be the method of choice when studying point defects in minerals.

Periodic Density Functional Study of CO and OH Adsorption on Pt−Ru Alloy Surfaces: Implications for CO Tolerant Fuel Cell Catalysts

In this work, we present results of a periodic density-functional theory study of the adsorption of carbon monoxide (CO) and hydroxyl (OH) on platinum, ruthenium, and a series on platinum−ruthenium alloys. The surfaces are modeled as four-layer slabs. The binding energies and geometries of CO and OH are computed, as well as the vibrational properties of chemisorbed CO. We find that the mixing of Pt by Ru leads to a weaker bond of both CO and OH to the Pt sites, whereas mixing of Ru by Pt causes a stronger bond of CO and OH to the Ru sites. The binding energy trends for CO do not show a clear-cut relationship with its vibrational characteristics. The binding energy changes are electronic alloying effects that can be explained by the d band shift model of Hammer and Nørskov. From our calculations, we can conclude that for a good CO oxidation fuel cell catalyst, it is important to have both Pt sites (which bind CO weakly) and Ru sites (which bind OH strongly) on the surface. However, if a low surface coverage of CO is required, which may be case for the oxidation of H2 in the presence of a small amount of CO, Ru with a monolayer of Pt might be more advantageous, as this is the Pt−Ru surface that shows the weakest CO binding.

Periodic density functional study on adsorption properties of organic molecules on clean Al (111) surface

The knowledge of the details of an interaction between the metal surface and organic compounds is important in various fields such as catalysis, lubrication, electron chemistry, and so on. As for the lubrication process, the metal–molecule interaction plays an important role in the formation of a protective film on the metal surface. In the present study, the first principle density functional theory (DFT) calculations have been carried out in order to investigate the adsorption properties of various organic molecules on the clean Al surfaces. We have calculated the adsorption energies for methanol, ethylene, ketone, and dimethyl ether molecules, which are widely used in the lubrication field. It has been found that the calculated adsorption energies are in a good agreement with the experimental data. The adsorption energy order of the organic compounds can be presented as follows: methanol>ketone>dimethyl ether>ethylene.

Periodic Density Functional LDA and GGA Study of CO Adsorption at the (001) Surface of MgO

The deficiencies of the local density approximation (LDA) to describe weakly adsorbed species on ionic surfaces are exposed in the current study of CO on the (001) surface of MgO. The authors applied a periodic formulation of density functional theory with crystal orbitals and charge density expanded as linear combinations of atomic orbitals of Gaussian form. The binding was investigated for four orientations of the CO molecule; either the carbon or oxygen atom of CO down over either the surface magnesium cation or oxygen anion. The authors find that the carbon down over the magnesium cation is the only noticeably attractive configuration, when basis set superposition error is corrected for, and estimate the binding energy of CO to be 1.9 kcal/mol for the Perdew, Burke, and Enzerhof (PBE96) functional. The binding energy obtained in the LDA approximation is as large as 7.4 kcal/mol. For the C-down-over-Mg configuration, the CO stretching frequency shift is only 4 cm{sup {minus}1} for the PBE96 functional. This binding energy, vibrational shift, and analysis of density of states indicate that little chemical bonding accompanies the adsorption. The values of binding energy and vibrational shift support a suggestion by Nygren and Pettersson that the CO adsorption discussedmore » by He et al. actually takes place at defect sites on the (001) surface of MgO.« less

Periodic density-functional study on oxidation of diamond (100) surfaces

The chemical reactions of the diamond surfaces with oxygen play important roles in the chemical-vapor deposition process, etching, and wear of the surface. In the present study, periodic density-functional calculations have been performed to clarify the oxidation mechanisms of the hydrogenated diamond (100) surfaces. The oxidation processes have been simulated in terms of the reaction heats. The ether, hydroxyl, and ketone structures are found to be stable on the diamond (100) surfaces. At the initial stage of the oxidation, the ether structures are priory formed at monohydride dimer bonds on the diamond (100) surfaces. The insertions of oxygen atoms into the lower layers are difficult to occur. As the coverage of oxygen atoms on the diamond surface is increased, the formation of ketone structures becomes easier. The stable structure of the oxygen monolayer sensitively depends on the lattice parameters. As the cell parameters are decreased, the bridge ether becomes more stable and the on-top ketone becomes more unstable.

A periodic density functional study of BEDT–TTF salts†

We report the first ab initio computational study of the structural and electronic properties of BEDT–TTF charge transfer salts, which have wide ranging physical properties which are derived primarily from the packing of the donor-radical cations. The calculations accurately reproduce the observed crystal structure of the material and provide new information on the electronic structure of the (BEDT-TTF)[FeBr4] salt.

Oxidation and Stabilization of Unreconstructed Hydrogen- and Fluorine-Terminated Si(100) Surface: A Periodic Density Functional Study

The understanding and control of silicon surfaces is of great importance in the production of silicon-based electronic devices made from semiconductor materials and constructed on silicon single-crystal substrates. This is the first study to see the effect of surface etching by fluorine on the stability of the unreconstructed Si(100) surface using periodic density functional calculations. All the possible fluorine substitution sites are considered, and the results are compared with the existing experimental observation in terms of suitability of fluorine substitution on the surface. The results are compared with H-terminated surfaces to prove the efficiency of fluorine in stabilizing the unreconstructed Si(100) surface. Oxidation of the Si(100) surface for both H- and F-terminated surfaces were also studied to propose the plausible mechanism of oxidation. Two kinds of experimental situations were mimicked for oxidation; (1) oxidation on the surface, i.e., generation of the Si−O−H bond, and (2) oxidation on the bridging bond between two silicons, i.e., generation of Si−O−Si, were compared to show the feasibility of Si−O−Si bond formation. Oxidation through atomic oxygen was followed throughout the calculation. The calculations were performed with smaller clusters using density functional methodology to validate and rationalize the current understanding. The results were further supported by molecular electrostatic potential maps generated from periodic density functional calculation results.

Periodic density functional study on V 2O 5 bulk and (001) surface

Density functional calculations on periodic models are performed to investigate the structural and electronic properties of both V 2O 5 bulk and (001) surface. Full geometry optimizations of both V 2O 5 bulk and (001) surface are presented. For the bulk, the optimized structure is very close to the experimental one, the calculated band gap and binding energy are in very good agreement with experimental values, from population analysis it is observed that vanadyl oxygens are least ionic (O −0.37), doubly coordinated oxygens are ionic (O −0.56), while triply coordinated oxygens become the most ionic (O −0.68). The structural and electronic properties of the surface are very close to those of the bulk. The interlayer interaction is mainly electrostatic and is found to be 4 kcal/mol. Surface acidic and basic properties are described in terms of projected density of states analysis.

First-Principles Study of π-Bonded (100) Planar Defects in Diamond

AbstractA periodic density functional study of the high-energy π-bonded (100) stacking fault in diamond that can serve as a prototype of a twist grain boundary has been carried out. Information on formation energies, geometries and the electronic structure has been obtained. A single point electronic structure calculation of a ∑5 twist grain boundary based on the geometry taken from a molecular dynamics simulation has also been performed.

Periodic density functional studies on Mg(H)x-doped GaN semiconductor

Periodic density functional calculations have been performed to study a mechanism of passivation of Mg-doped GaN by hydrogen. Optimizing different initial positions of hydrogen we found some favorable positions for the hydrogen atoms within the unit cells. We have found that the hydrogen atoms bind to the Mg in agreement with experimental findings and theoretical considerations. In addition the calculations revealed that the hydrogen atoms strongly interact with close nitrogen atoms. Our novel model of the simultaneous bond formation was supported by the charge distributions in the doped crystals.