A periodic density functional theory study of the dehydrogenation of methanol over CuCl(1 1 1) surface

The decomposition of methanol on clean and oxygen-precovered CuCl(1 1 1) surface have been studied with the method of density functional theory-generalized gradient approximation (DFT-GGA) and the periodic slab models. The effects of different methanol coverages up to one monolayer are investigated. The activation of the O-H bond of methanol to form the methoxide intermediate, the activation of the C-H bond to form the hydroxymethyl intermediate and the activation of the C-O bond to form methyl are examined. These intermediates can subsequently react to form methoxide, hydroxymethyl, methyl, formaldehyde, formyl, and finally CO on the surface. The chemisorption energies of CH₃OH, CH₃O, H₂COH, CH₃, H₂CO, HCO, OH and CO at their most favorable adsorption sites are predicted to be −57.9, −235.3, −172.9, −170.5, −67.8, −192.4, −309.5 and −105.7 kJ/mol, respectively. We also confirm that the O-H bond-breaking paths have lower energy barrier, compared to the C-O and C-H bond-breaking paths. However, these reactions need a lower energy barrier when precovered oxygen atoms participate in these reactions.     

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.     

A theoretical study of Ge adsorption on Si(0 0 1) covered with Bi nanolines

The energetic stability and the equilibrium geometry of Ge adsorption on the Si(0 0 1) surface covered with Bi nanolines were examined by ab initio total energy calculations. We find that there is an energetic preference of Ge atoms lying on the Si(0 0 1) terraces, forming Si^down-Ge^up mixed dimers. Further investigations reveal a repulsive interaction between the mixed dimers and the Bi nanolines, suggesting that the formation of Si^down-Ge^up dimers can be tailored by the presence of the Bi nanolines.     

Study of vacancy on diamond (1 0 0) (2 × 1) surface from first-principles

Structure and energy related properties of neutral and charged vacancies on relaxed diamond (1 0 0) (2 × 1) surface were investigated by means of density functional theory. Calculations indicate that the diffusion of a single vacancy from the top surface layer to the second layer is not energetically favored. Analysis of energies in charged system shows that neutral state is most stable on diamond (1 0 0) (2 × 1) surface. The multiplicity of possible states can exist on diamond (1 0 0) surface in dependence on the surface Fermi level, which supports that surface diffusion of a vacancy is mediated by the change of vacancy charge states. Analysis of density of states shows surface vacancy can be effectively measured by photoelectricity technology.     

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.     

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).     

A theoretical study of c-C5H8 adsorption on Ge (0 0 1)-2 × 1 and on dimer vacancies on the surface: Electronic structure and bonding

In this work we analyzed the geometry and the chemical interactions for c-C₅H₈ adsorption on Ge (0 0 1), using density functional theory calculations (DFT). We examined the changes in the atomic interactions using a slab model. We considered two cases, the cyclopentene adsorption on Ge(0 0 1) and on dimer vacancies on the surface. We found an average distance H-Ge, -C-Ge and C-Ge of 1.50, 1.70 and 1.65 Å, respectively, on dimer vacancies; and an average C-Ge distance of 2.05 Å on Ge-Ge dimer. We also computed the density of states (DOS) and the DOS weighted overlap populations (OPDOS) corresponding to C-C, C-Ge, C-H, and Ge-Ge bonds. During adsorption the main contribution are the CC double bond in both cases, and the next C and the H's belonging to this bonds in the case of adsorption on dimer vacancies. The orbital contribution includes participation of the 2p_y and 2p_z orbitals corresponding to unsaturated C atoms, 2p_z corresponding to side saturated C, and the 4p orbitals of Ge for the adsorption on dimer vacancies; 2s and 2p_z orbitals corresponding to double bond C atoms, 4s and 4p_z orbitals of Ge for the adsorption on Ge(0 0 1).     

Characterization of diamond (1 0 0) surface with oxygen termination

Ab initio density functional theory (DFT) was employed to study reconstructions of diamond (1 0 0) surfaces in the presence of hydrogen, oxygen and hydroxyl. Clean and (2 × 1):1H surfaces are taken as reference. The properties of oxidization diamond surfaces with several adsorption structures, namely, O-on-top (OT) site, O-bridge (BR) site, hydroxyl (-OH), hydroxyl/hydroxyl, OT/hydroxyl, BR/hydroxyl have been considered. The calculated results indicate that the BR model is much more stable than the OT model, and the most energetically favorable structures of oxygenated surfaces are those with chemisorbed hydroxyl (-OH) group. Furthermore, the stability of the structures is also discussed from the point of HOMO-LUMO gap. Analysis of electronic structures shows that the presence of hydrogen induces surface conductivity whereas oxygen weakens it.     

Interaction of 2,2,6,6-tetramethyl-3,5-heptanedione with the Si(1 0 0)-2 × 1 surface: Scanning tunneling microscopy and density functional theory study

Room temperature adsorption and reaction of 2,2,6,6-tetramethyl-3,5-heptanedione (dpmH) on the Si(1 0 0)-2 × 1 surface has been studied with ultra-high vacuum scanning tunneling microscopy (UHV-STM) and temperature programmed desorption (TPD). The molecule is found to chemisorb as a mixture of at least five distinct species. Density functional theory (DFT) was used to calculate the structures and adsorption energies of 12 possible addition products. Unique bonding assignments for each experimental feature are proposed by consideration of a common intermediate reaction network, and a comparison of possible reaction pathways leading to the final products. These assignments are: OH inter-dimer dissociation, OH intra-dimer dissociation, 1,5 intra-dimer addition, 1,5 inter-dimer addition, and intra-dimer [2 + 2]CO addition with OH dissociation on an adjacent dimer. TPD and STM results show that the molecule dissociates completely upon annealing to 700 °C with formation of the c(4 × 4) phase at low exposures, and SiC islands for exposures exceeding 0.15 L.     

Density functional theory calculations of surface properties and H2 adsorption on the Cu2O (1 1 1) surface

First-principles calculation on the basis of the density functional theory (DFT) and generalized gradient approximation have been applied to study the adsorption of H₂ on the stoichiometric O-terminated Cu₂O (1 1 1), Cu₂O (1 1 1)-Cu_CUS and Cu-terminated Cu₂O (1 1 1) surfaces. The optimal adsorption position and orientation of H₂ on the stoichiometric O-terminated Cu₂O (1 1 1) surface and Cu-terminated Cu₂O (1 1 1) surface were determined and electronic structural changes upon adsorption were investigated by calculating the Local Density of States (LDOS) of the Cu_CUS 3d and Cu_CUS 4s of stoichiometric O-terminated Cu₂O (1 1 1) surface. These results showed that H₂ molecule adsorption on Cu_CUS site parallel to stoichiometric O-terminated Cu₂O (1 1 1) surface and H₂ molecule adsorption on Cu₂ site parallel to Cu-terminated Cu₂O (1 1 1) surface were the most favored, respectively. The presence of surface copper vacancy has a little influence on the structures when H₂ molecule adsorbs on Cu_CSA, O_CUS and O_CSA atoms and the H₂ molecule is only very weakly bound to the Cu₂O (1 1 1)-Cu_CUS surface. From the analysis of stoichiometric O-terminated Cu₂O (1 1 1) Local Density of States, it is observed that Cu_CUS 3d orbital has moved to a lower energy and the sharp band of Cu_CUS 4s is delocalized when compared to that before H₂ molecule adsorption, and overlapped substantially with bands due to adsorbed H₂ molecule. The Mulliken charges of H₂ adsorption on Cu_CUS site showed that H₂ molecule obtained electron from Cu_CUS which was consistent with the calculated electronic structural changes upon H₂ adsorption.     

A density functional theory study on the adsorption and dissociation of N2O on Cu2O(1 1 1) surface

Density functional theory has been employed to investigate the adsorption and the dissociation of an N₂O at different sites on perfect and defective Cu₂O(1 1 1) surfaces. The calculations are performed on periodic systems using slab model. The Lewis acid site, Cu_CUS, and Lewis base site, O_SUF are considered for adsorption. Adsorption energies and the energies of the dissociation reaction N₂O → N₂ + O(s) at different sites are calculated. The calculations show that adsorption of N₂O is more favorable on Cu_CUS adsorption site energetically. Cu_CUS site exhibits a very high activity. The Cu_CUS-N₂O reaction is exothermic with a reaction energy of 77.45 kJ mol⁻¹ and an activation energy of 88.82 kJ mol⁻¹, whereas the O_SUF-N₂O reaction is endothermic with a reaction energy of 205.21 kJ mol⁻¹ and an activation energy of 256.19 kJ mol⁻¹. The calculations for defective surface indicate that O vacancy cannot obviously improve the catalytic activity of Cu₂O.     

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.     

First-principle study of NO adsorption on the LaFeO3 (0 1 0) surface

The adsorption of NO molecule on the LaFeO₃ (0 1 0) surface was studied using first-principle calculations based on density functional theory. The calculated results indicate that the Fe-top site is the most favorable for NO adsorption. The N-O bond length, Mulliken charge, and the N-O vibration frequency of the NO molecule are discussed after adsorption. The analysis results of the density of the states show that when NO is adsorbed with the Fe-NO configuration, the bonding mechanism is mainly from the interaction between the NO and the Fe d orbit.     

Density functional theory study of selenium adsorption on Fe(1 1 0)

The adsorption of atomic Se on a Fe(1 1 0) surface is examined using the density functional theory (DFT). Selenium is adsorbed in high-symmetry adsorption sites: the -short and long-bridge, and atop sites at 1/2, 1/4, and 1 monolayer (ML) coverages. The long bridge (LB) site is found to be the most stable, followed by the short bridge (SB) and top sites (T). The following overlayer structures were examined, p(2 × 2), c(2 × 2), and p(1 × 1), which correspond to 1/4 ML, 1/2 ML, and 1 ML respectively. Adsorption energy is −5.23 eV at 1/4 ML. Se adsorption results in surface reconstruction, being more extensive for adsorption in the long bridge site at 1/2 ML, with vertical displacements between +8.63 and −6.69% -with regard to the original Fe position-, affecting the 1st and 2nd neighbours. The largest displacement in x or y-directions was determined to be 0.011, 0.030, and 0.021 Å for atop and bridge sites. Comparisons between Se-adsorbed and pure Fe surfaces revealed reductions in the magnetic moments of surface-layer Fe atoms in the vicinity of the Se. At the long bridge site, the presence of Se causes a decrease in the surface Fe d-orbital density of states between 4 and 5 eV below Fermi level. The density of states present a contribution of Se states at −3.1 eV and −12.9 eV. stabilized after adsorption. The Fe-Fe overlap population decrease and a Fe-Se bond are formed at the expense of the metallic bond.     

Ge adsorption on SiC(0 0 0 1): An ab initio study

In this work we have performed an ab initio total energy investigation of the Ge adsorption process on the Si-terminated SiC(0 0 0 1)- and (3 × 3) surfaces. We find that Ge adatoms lying on the topmost sites of the and (3 × 3) surfaces represent the energetically more stable configurations at the initial stage of the Ge adsorption on the SiC(0 0 0 1) surface. The Si → Ge substitutional adsorption processes have been examined as a function of the Si and Ge chemical potentials. Increasing the Ge coverage, we verify that the formation of Ge wetting layer on the surface, and Ge nanocluster on the (3 × 3) surface are the energetically more stable configurations, in accordance with recent experimental findings.     

Optical anisotropy of the In/Si(1 1 1)(4 × 1)/(8 × 2) nanowire array

Ab initio calculations of the reflectance anisotropy of Si(1 1 1)-In surfaces are presented. A very pronounced optical anisotropy around 2 eV is found that is related to In-chain states. The distortion of the indium chains characteristic for the (4 × 1) → (8 × 2) phase transition results in a splitting of the 2 eV peak, as observed experimentally. The splitting occurs irrespective wether the phase transition occurs according to the trimer or hexamer model.     

Chemisorption of 1,1-dichloroethene on the Si(1 1 1)-7 × 7 surface

Chemisorption of 1,1-dichloroethene (Cl₂CCH₂) to a Si(1 1 1)-7 × 7 surface was studied by means of X-ray photoelectron spectroscopy using synchrotron radiation, recording chlorine 2p and carbon 1s spectra. For carbon 1s, spectral assignment of the chemisorbed species is based on quantum chemical calculations of chemical shifts in model compounds.The results confirm the identity of covalently bonded 1-chlorovinyl (-CClCH₂) and vinylidene (CCH₂) adspecies. Upon chemisorption at room temperature it was found that about one-third of the molecules break one C-Cl bond while about two-thirds of the adsorbates break two C-Cl bonds. We do not, however, find evidence for isomerization of CCH₂ to di-bonded vinylene (-CHCH-).     

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.     

Adsorption and reaction of methanethiol on the Ru(0 0 0 1)-p(2 × 2)O surface: A TPD and XPS study

Methanethiol adsorbed on Ru(0 0 0 1)-p(2 × 2)O has been studied by TPD and XPS. The dissociation of methanethiol to methylthiolate and hydrogen at 90 K is evidenced by the observation of hydroxyl and water. The saturation coverage of methylthiolate is ∼0.15 ML, measured by both XPS and TPD. A detailed analysis suggests that only the hcp-hollow sites have been occupied. Upon annealing the surface, water and hydroxyl desorb from the surface at ∼210 K. Methylthiolate decomposes to methyl radical and atomic sulphur via C-S cleavage between 350 and 450 K. Some methyl radicals (0.05 ML) have been transferred to Ru atoms before they decompose to carbon and hydrogen. The rest of methyl radicals desorb as gaseous phase. No evidence for the transfer of methyl radical to surface oxygen has been found.     

Vibrationally-assisted dissociative adsorption of oxygen on Ru(0 0 0 1)-p(2 × 1)-O

Supersonic molecular beam technique combined with high resolution X-ray photoelectron spectroscopy using synchrotron radiation was applied to the study of the dynamics of dissociative adsorption of oxygen on Ru(0 0 0 1) surface in high coverage region. The Ru(0 0 0 1) surface pre-covered with oxygen atoms of 0.5 monolayer, which corresponds to the p(2 × 1)-O structure, was dosed to oxygen molecules with translational energy of 0.5 eV. Oxygen uptake was compared between the cases with and without the beam source heated in order to verify the effects of internal energy of oxygen. We found drastic enhancement in initial sticking probability of oxygen when the beam source was heated to 1400 K. We concluded that the enhancement of sticking probability is mainly caused by molecular vibrational excitation, indicating that dissociation barrier is located in the exit channel on potential energy surface.