Adsorption of chlorophenol on the Cu(1 1 1) surface: A first-principles density functional theory study

The interaction between a 2-chlorophenol (C₆H₄OHCl) molecule and the Cu(1 1 1) surface has been investigated using density functional theory as an initial step in gaining a better understanding of the catalyzed formation of dioxin compounds on a clean copper surface. The 2-chlorophenol molecule is found to form several weakly bonded, horizontally and vertically oriented configurations. Dissociative modes have also been investigated. For the latter, the formation of phenyl and benzyne fragments is found to be more energetically favourable than the formation of 2-chlorophenoxy radicals.     

Adsorption and dissociation of ammonia on Au(1 1 1) surface: A density functional theory study

Periodic density functional theory (DFT) calculations using plane waves had been performed to systematically investigate the stable adsorption amine and its dehydrogenated reaction on Au(1 1 1) surface. The equilibrium configuration including on top, bridge, and hollow (fcc and hcp) sites had been determined by relaxation of the system. The adsorption both NH₃ on top site and NH₂ on bridge site is favorable on Au(1 1 1) surface, while the adsorption of NH on hollow (fcc) site is preferred. The adsorbates are adsorbed on the gold surface with the interaction between p orbital of adsorbate and the d orbital of gold atoms. The interaction between adsorbate and gold slab is more evident on the first layer than on any others. Furthermore, the dissociation reaction of NH₃ on clean gold surface, as well as on the pre-covered oxygen atom and pre-covered hydroxyl group surface had been investigated. The results show that the dehydrogenated reaction energy barrier on the pre-covered oxygen gold surface is lower. The adsorbed O can promote the dehydrogenation of amine. Additionally, OH as the product of the NH₃ dissociation reaction participates in continuous dehydrogenation reaction, and the reaction energy barrier is the lowest (22.77 kJ/mol). The results indicated that OH_ads play a key role in the dehydrogenated reaction on Au(1 1 1) surface.     

Adsorption of Ag on Si(1 0 0) surface

The chemisorption of one monolayer Ag atoms on an ideal Si(1 0 0) surface is studied by using the self-consistent tight-binding linear muffin-tin orbital method. The adsorption energies (E_ad) of different sites are calculated. It is found that the adsorbed Ag atoms are more favorable on C site (fourfold site) than on any other sites on Si(1 0 0) surface, the polar covalent bond is formed between Ag atom and surface Si atom, a Ag and Si mixed layer does not exist and does form an abrupt interface at the Ag–Si(1 0 0) interface. This is in agreement with the experiment results. The layer-projected density of states is calculated and compared with that of the clean surface. The charge transfer is also investigated. Comparing with the Au/Si(1 0 0) system, the interaction is weaker between Ag and Si than between Au and Si.     

Adsorption and dissociation of O2 on CuCl(1 1 1) surface: A density functional theory study

The adsorption and dissociation of O₂ on CuCl(1 1 1) surface have been systematically studied by the density functional theory (DFT) slab calculations. Different kinds of possible modes of atomic O and molecular O₂ adsorbed on CuCl(1 1 1) surface and possible dissociation pathways are identified, and the optimized geometry, adsorption energy, vibrational frequency and Mulliken charge are obtained. The calculated results show that the favorable adsorption occurs at hollow site for O atom, and molecular O₂ lying flatly on the surface with one O atom binding with top Cu atom is the most stable adsorption configuration. The O-O stretching vibrational frequencies are significantly red-shifted, and the charges transferred from CuCl to oxygen. Upon O₂ adsorption, the oxygen species adsorbed on CuCl(1 1 1) surface mainly shows the characteristic of the superoxo (O₂⁻), which primarily contributes to improving the catalytic activity of CuCl, meanwhile, a small quantity of O₂ dissociation into atomic O also occur, which need to overcome very large activation barrier. Our results can provide some microscopic information for the catalytic mechanism of DMC synthesis over CuCl catalyst from oxidative carbonylation of methanol.     

Adsorption of molecular SiO2 on a clean Si(1 0 0) surface

We have investigated the adsorption of molecular (gaseous) SiO₂ on a clean Si(1 0 0) p(2 × 2) reconstructed surface using density functional theory based methods. The SiO₂ molecule is found to be chemisorbed on various sites on the Si surface and the most energetically favourable structure is on top of the dimers. The minimum energy pathways for the various adsorption channels indicate that the reaction is barrierless in all cases. The corresponding vibrational spectrum is also calculated and the adsorbed molecules are, as expected, found to have red-shifted vibrational frequencies. The energetically favourable adsorption sites and adsorption energies are comparable to the results found for SiO.     

CO adsorption on the Cu(1 1 1) surface: A density functional study

The adsorption of CO on the Cu(1 1 1) surface has been studied with ab initio density functional theory. The adsorbate-metal system was analyzed with the local density approximation, the gradient corrected functional of Perdew and Wang and the B3LYP hybrid functional, for comparison. A slab model was used for the pattern at a coverage of 1/3. The local density approximation and the gradient corrected functional give the fcc site as the favorable adsorption site. In contrast, the B3LYP functional results in the preference of the top site, in agreement with the experiment. These results confirm the suggested explanation for the failure of standard functionals, based on the position of the highest occupied and lowest unoccupied molecular orbital. The results of total energy calculations are presented, together with projected densities of states and Mulliken populations. In addition, the basis set superposition error is discussed for CO/Cu(1 1 1) and for CO/Pt(1 1 1).     

Acetaldehyde photochemistry on TiO2(1 1 0)

The ultraviolet (UV) photon induced decomposition of acetaldehyde adsorbed on the oxidized rutile TiO₂(1 1 0) surface was studied with photon stimulated desorption (PSD) and thermal programmed desorption (TPD). Acetaldehyde desorbs molecularly from TiO₂(1 1 0) with minor decomposition channels yielding butene on the reduced TiO₂ surface and acetate on the oxidized TiO₂ surface. Acetaldehyde adsorbed on oxidized TiO₂(1 1 0) undergoes a facile thermal reaction to form a photoactive acetaldehyde–oxygen complex. UV irradiation of the acetaldehyde–oxygen complex initiated photofragmentation of the complex resulting in the ejection of methyl radical into gas phase and conversion of the surface bound fragment to formate.     

Adsorption and dissociation of H2 on the Cu2O(1 1 1) surface: A density functional theory study

Interactions of atomic and molecular hydrogen with perfect and deficient Cu₂O(1 1 1) surfaces have been investigated by density functional theory. Different kinds of possible modes of H and H₂ adsorbed on the Cu₂O(1 1 1) surface and possible dissociation pathways were examined. The calculated results indicate that O_SUF, Cu_CUS and O_vacancy sites are the adsorption active centers for H adsorbed on the Cu₂O(1 1 1) surface, and for H₂ adsorption over perfect surface, Cu_CUS site is the most advantageous position with the side-on type of H₂. For H₂ adsorption over deficient surface, two adsorption models of H₂, H₂ adsorbing perpendicularly over O_vacancy site and H₂ lying flatly over singly-coordinate Cu-Cu short bridge, are typical of non-energy-barrier dissociative adsorption leading to one atomic H completely inserted into the crystal lattice and the other bounded to Cu_CUS atom, suggesting that the dissociative adsorption of H₂ is the main dissociation pathway of H₂ on the Cu₂O(1 1 1) surface. Our calculation result is consistent with that of the experimental observation. Therefore, Cu₂O(1 1 1) surface with oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of H₂.     

Density functional study of the adsorption of aspirin on the hydroxylated (0 0 1) -quartz surface

In this study the adsorption geometry of aspirin molecule on a hydroxylated (0 0 1) α-quartz surface has been investigated using DFT calculations. The optimized adsorption geometry indicates that both, adsorbed molecule and substrate are strongly deformed. Strong hydrogen bonding between aspirin and surface hydroxyls, leads to the breaking of the original hydroxyl–hydroxyl hydrogen bonds (Hydrogenbridges) on the surface. In this case new hydrogen bonds on the hydroxylated (0 0 1) α-quartz surface appear which significantly differ from those at the clean surface. The 1.11 eV adsorption energy reveals that the interaction of aspirin with α-quartz is an exothermic chemical interaction.     

Adsorption of NO on Au atoms and dimers supported on MgO(1 0 0): DFT studies

The adsorption of NO on single gold atoms and Au₂ dimers deposited on regular O²⁻ sites and neutral oxygen vacancies (F_s sites) of the MgO(1 0 0) surface have been studied by means of DFT calculations. For Au₁/MgO the adsorption of NO is stronger when the Au atom is supported on an anionic site than when it is on a F_s site, with adsorption binding energies of 1.1 and 0.5 eV, respectively. In the first case the spin density is mainly concentrated on the metal atom and protruding from the surface. In such a way, an active site against radicals such as NO is generated. On the F_s site, the presence of the vacancy delocalizes the spin into the substrate, weakening its coupling with NO. For Au₂/MgO, as this system has a closed-shell configuration, the NO molecules bonds weakly with Au₂. Regarding the N–O stretching frequencies, a very strong shift of 340–400 cm⁻¹ to lower frequencies is observed for Au₁/MgO in comparison with free NO.     

Coadsorption of CN and O on Cu (1 0 0) surface: A density functional study

The adsorption of cyanide (CN) or oxygen atom, as well as the coadsorption of CN + O on Cu (1 0 0) surface is studied by using density functional theory (DFT) and the cluster model method. Cu₁₄ cluster is used to simulate the surface. Perpendicular and parallel bonding geometries of CN adsorbed on Cu (1 0 0) surface are considered, respectively. The present calculations show that the CN may be absorbed on top and bridge sites by carbon atom of cyanide (C-down), and C-down on top site is the most favorable. The adsorbed C-N stretch frequencies compared with that of the gaseous CN species are all red-shifted, except the C-down on top site. The charge transfer from the surface to the CN species leads to an increase in work function for the Cu surface. The oxygen atom adsorbed on the four-fold hollow site of Cu (1 0 0) is the most favorable, and is consistent with the experimental study. The coadsorption of O at a four-fold hollow site tends to block adsorption of CN at the nearby sites. If O coverage increases, the CN may be adsorbed on the top and bridges sites with the C-down model. The reaction CN + O → OCN on the Cu (1 0 0) is predicted to be exothermic, and formed OCN species may be stably absorbed on the Cu (1 0 0).     

Adsorption of 2-chlorophenol on Cu2O(1 1 1)-CuCUS: A first-principles density functional study

First-principles density functional theory and a periodic-slab model have been utilized to investigate the adsorption of a 2-chlorophenol molecule on a CuO(1 1 1) surface with a vacant Cu surface site, namely Cu₂O(1 1 1)-Cu_CUS. Several vertical and flat orientations have been studied. All of these molecular configurations interact very weakly with the Cu₂O(1 1 1)-Cu_CUS surface, an observation which also holds for clean copper surfaces and the Cu₂O(1 1 0):CuO surface. Hydroxyl-bond dissociation assisted by the surface was found to be endoergic by 0.42-1.72 eV, depending predominantly on the position of the isolated H on the surface. In addition, the corresponding adsorbed 2-chlorophenoxy moiety was found to be more stable than a vacuum 2-chlorophenoxy radical by about 0.76 eV. Despite these predicted endoergicities, however, we would predict the formation of 2-chlorophenoxy radicals from gaseous 2-chlorophenol over the copper (I) oxide Cu₂O(1 1 1)-Cu_CUS surface to be a feasible and important process in the formation of PCDD/Fs in the post-flame region where gas-phase routes are negligible.     

Computational study of the adsorption of dimethyl methylphosphonate (DMMP) on the (0 1 0) surface of anatase TiO2 with and without faceting

The adsorption of dimethyl methylphosphonate (DMMP) on the (0 1 0) surface of anatase TiO₂, which is isostructural with the (1 0 0), has been studied using density functional theory and two-dimensionally-periodic slab models. The experimentally-observed faceting of this surface has, for the first time, been included in the modeling. The relaxations of bare surfaces both with and without faceting are similar, leading to an atomic-scale roughening due to inward (outward) displacement of fivefold-coordinated Ti_5c (sixfold-coordinated T_6c) atoms together with outward displacement of threefold-coordinated O_3c atoms. Molecular adsorption occurs by formation of a Ti_5c?OP dative bond with one or more CH?O_2c bonds between CH₃ groups and unsaturated, twofold-coordinated (O_2c) sites. The energies for molecular adsorption, obtained using the B3LYP functional, are virtually identical (about ?21.0 kcal/mol) for the two surfaces and are also close to those found elsewhere for the rutile (1 1 0) and anatase (1 0 1) surfaces. A possible first step in the dissociative adsorption of DMMP has also been modeled and is found to be thermodynamically favored over molecular adsorption to a degree which depends on faceting.     

Ab initio density functional study of O on the Ag(0 0 1) surface

The adsorption of oxygen on the Ag(1 0 0) is investigated by means of density functional techniques. Starting from a characterization of the clean silver surfaces oxygen adsorption in several modifications (molecularly, on-surface, sub-surface, Ag₂O) for varying coverage was studied. Besides structural parameters and adsorption energies also work-function changes, vibrational frequencies and core level energies were calculated for a better characterization of the adsorption structures and an easier comparison to the rich experimental data.     

Adsorption of CO on the O2 pre-adsorbed LaFeO3 (0 1 0) surface: A density functional theory study

The adsorption of CO molecule on the O₂ pre-adsorbed LaFeO₃ (0 1 0) surface has been investigated using a density functional theory calculation. The calculated results show that the most appropriate reaction occurs between the CO and the pre-adsorbed O₂. After CO adsorption, the bonding mechanism between Fe site and the pre-adsorbed O₂ is not modified, and the HOMO–LUMO energy gap of the M1 mode is narrowed, which is caused by the redistribution of electron density in the surface.     

The dissociative adsorption of borane on the Ge(1 0 0)-2 × 1 surface: A density functional theory study

By means of cluster models coupled with density functional theory, we have studied the hydroboration of the Ge(1 0 0)-2 × 1 surface with BH₃. It was found that the Ge(1 0 0) surface exhibits rather different surface reactivity toward the dissociative adsorption of BH₃ compared to the C(1 0 0) and Si(1 0 0) surfaces. The strong interaction still exists between the as-formed BH₂ and H adspeices although the dissociative adsorption of BH₃ on the Ge(1 0 0) surface occurs readily, which is in distinct contrast to that on the C(1 0 0) and Si(1 0 0) surfaces. This can be understood by the electrophilic nature of the down Ge atom, which makes it unfavourable to form a GeH bond with the dissociating proton-like hydrogen. Alternatively, it can be attributed to the weak proton affinity of the Ge(1 0 0) surface. Nevertheless, the overall dissociative adsorption of BH₃ on group IV semiconductor surfaces is favourable both thermodynamically and kinetically, suggesting the interesting analogy and similar diversity chemistry of solid surface in the same group.     

Solvent effects on adsorption of CO over CuCl(1 1 1) surface: A density functional theory study

DFT calculations have been performed to investigate the effect of dielectric responses of the solvent environment on the CO adsorption over CuCl(1 1 1) surface by using COSMO (conductor-like solvent model) model in Dmol³. Different dielectric constants, including vacuum, liquid paraffin, methylene chloride, methanol and water solution, are considered. The effects of solvent model on the structural parameters, adsorption energies and vibrational frequency of CO adsorption over CuCl(1 1 1) surface have been investigated. The calculation results suggest that solvent effects can improve the stability of CO adsorption and reduce the intensity of C-O bond, which might mean that solvent is in favor of C-O bond activation and improve the reaction activity of oxidative carbonylation in a slurry reactor.     

Eley–Rideal reaction dynamics between O atoms on β-cristobalite (1 0 0) surface: A new interpolated potential energy surface and classical trajectory study

We present a theoretical study of the collisions of atomic oxygen with O-precovered β-cristobalite (1 0 0) surface. We have constructed a multidimensional potential energy surface for the O₂/β-cristobalite (1 0 0) system based mainly on a dense grid of density functional theory points by using the interpolation corrugation-reducing procedure. Classical trajectories have been computed for quasithermal (100–1500 K) and state-specific (e.g., collision energies between 0.01 and 4 eV) conditions of reactants for different O incident angles (θ_v). Atomic sticking and O_2(adsorbed) formation are the main processes, although atomic reflection and Eley–Rideal (ER) reaction (i.e., O₂ gas) are also significant, depending their reaction probabilities on the O incident angle. ER reaction is enhanced by temperature increase, with an activation energy derived from the atomic recombination coefficient (γ_O(θ_v = 0°, T)) equal to 0.24 ± 0.02 eV within the 500–1500 K range, in close agreement with experimental data. Calculated γ_O(θ_v = 0°, T) values compare quite well with available experimental γ_O(T) although a more accurate calculation is proposed. Chemical energy accommodation coefficient β_O(T) is also discussed as a function of ER and other competitive contributions.     

Adsorption and NiO(1 0 0) formation by atomic and molecular oxygen on Ni(1 1 0)

The structure of the ordered p(2 × 1) and p(3 × 1) phases of adsorbed oxygen as well as the formation of ultrathin NiO(1 0 0) layers on a Ni(1 1 0) single crystal are investigated by grazing scattering of fast hydrogen atoms. Via ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of oxygen adsorbates and ultrathin nickel oxide layers. For oxidation using atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude. We compare the experimental results with computer simulations based on classical projectile trajectories for grazing scattering of fast hydrogen atoms and test structure models for oxygen adsorbed on Ni(1 1 0) and NiO(1 0 0).