X-ray emission spectroscopy and density functional study of CO/Fe(100)

We report x-ray emission and absorption spectroscopy studies of the electronic structure of the predissociative α(3) phase of CO bound at hollow sites of Fe(100) as well as of the on-top bound species in the high-coverage α(1) phase. The analysis is supported by density functional calculations of structures and spectra. The bonding of "lying down" CO in the hollow site is well described in terms of π to π? charge transfer made possible through bonding interaction also at the oxygen in the minority spin-channel. The on-top CO in the mixed, high-coverage α(1) phase is found to be tilted due to adsorbate-adsorbate interaction, but still with bonding mainly characteristic of "vertical" on-top adsorbed CO similar to other transition-metal surfaces.     

Dissociation of CH3I on the Al111 surface--an STM and density functional theory study

The reaction of methyl iodide with the Al(111) surface was studied by room-temperature scanning tunneling microscopy (STM) and by first principles calculations. It was found that at 300 K methyl iodide decomposes on the Al(111) surface, forming methyl (CH(3)), methylidyne (CH), and adsorbed iodine. Methyl groups are observed to occupy atop sites by STM. The occupation of the hollow site by methylidyne was observed in STM measurements. Total energy density functional theory calculations have shown that methyl species occupy atop Al sites (E(A) = 45.3 kcal/mol), methylidyne species adsorb on fcc hollow sites (E(A) = 155.0 kcal/mol), while individual iodine atoms can bind on both on-top or hollow sites with adsorption energies between 54 and 56 kcal/mol.     

Modelling of benzene-1,4-dithiol on a Au(1 1 1) surface

The bonding of benzene-1,4-dithiol to a Au(1 1 1) surface is studied using a Au₂₅–SC₆H₄SH model. We find that the most favorable structure has the S directly above an Au atom, i.e., an on-top site. If the molecule is constrained to be perpendicular to the surface, the on-top site is the least favorable site and the threefold hollow is the most favorable one.     

Density function theory study of CO adsorption on Fe3O4(111) surface

Density functional theory calculations have been carried out for CO adsorption on the Fe(oct2)- and Fe(tet1)-terminated Fe(3)O(4)(111) surfaces, which are considered as active catalysts in water-gas shift reaction. It is found that the on-top configurations are most stable on these two surfaces. Some bridge configurations are also stable in which the new C-O bond formed between the surface O atom and the C atom of CO. The adsorption on the Fe(oct2)-terminated surface is more stable than on the Fe(tet1)-terminated surface. The density of state reveals the binding mechanism of CO adsorption on the two surfaces. Our calculations have also shown that the absorbed CO can migrate from the on-top site to the bridge site or 3-fold site. The oxidation of CO via surface oxygen atoms is feasible, which is in good agreement with experimental results.     

Structure and stability of Al13H clusters

We have performed calculations on the structures and stabilities of Al13H at the density functional and coupled-cluster levels of theory. There are low-symmetry (Cs on-top) isomers energetically comparable to well-known high-symmetry (C2nu bridge and C3nu hollow) isomers. The shape of the Al13 moieties in the Cs isomers is significantly distorted from icosahedral, and similar to Al13 cationic structures. Despite the high stability of the Al13H cluster, Al13H appears to be highly fluxional, as evidenced by multiple close-lying structures.     

Why h atom prefers the on-top site and alkali metals favor the middle hollow site on the Basal plane of graphite

In this work, the different adsorption properties of H and alkali metal atoms on the basal plane of graphite are studied and compared using a density functional method on the same model chemistry level. The results show that H prefers the "on-top site" while alkali metals favor the "middle hollow site" of graphite basal plane due to the unique electronic structures of H, alkali metals, and graphite. H has a higher electronegativity than carbon, preferring to form a covalent bond with C atoms, whereas alkaline metals have lower electronegativity, tending to adsorb on the highest electrostatic potential sites. During adsorption, there are more charges transferred from alkali metal to graphite than from H to graphite.     

Chemisorption of (CHx and C2Hy) hydrocarbons on Pt(111) clusters and surfaces from DFT studies

We used the B3LYP flavor of density functional theory (DFT) to study the chemisorption of all CH(x) and C(2)H(y) intermediates on the Pt(111) surface. The surface was modeled with the 35 atom Pt(14.13.8) cluster, which was found to be reliable for describing all adsorption sites. We find that these hydrocarbons all bind covalently (sigma-bonds) to the surface, in agreement with the studies by Kua and Goddard on small Pt clusters. In nearly every case the structure of the adsorbed hydrocarbon achieves a saturated configuration in which each C is almost tetrahedral with the missing H atoms replaced by covalent bonds to the surface Pt atoms. Thus, (Pt(3))CH prefers a mu(3) hollow site (fcc), (Pt(2))CH(2) prefers a mu(2) bridge site, and PtCH(3) prefers mu(1) on-top sites. Vinyl leads to (Pt(2))CH-CH(2)(Pt), which prefers a mu(3) hollow site (fcc). The only exceptions to this model are ethynyl (CCH), which binds as (Pt(2))C=CH(Pt), retaining a CC pi-bond while binding at a mu(3) hollow site (fcc), and HCCH, which binds as (Pt)HC=CH(Pt), retaining a pi bond that coordinates to a third atom of a mu(3) hollow site (fcc) to form an off center structure. These structures are in good agreement with available experimental data. For all species we calculated heats of formation (DeltaH(f)) to be used for considering various reaction pathways on Pt(111). For conditions of low coverage, the most strongly bound CH(x) species is methylidyne (CH, BE = 146.61 kcal/mol), and ethylidyne (CCH(3), BE = 134.83 kcal/mol) among the C(2)H(y) molecules. We find that the net bond energy is nearly proportional to the number of C-Pt bonds (48.80 kcal/mol per bond) with the average bond energy decreasing slightly with the number of C ligands.     

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.     

Decomposition of methylamine on Mo(100) surface: A DFT study

The initial decomposition of methylamine on Mo(100) surface has been investigated by self-consistent (GGA-PW91) density functional theory combined with periodic slab model. The adsorption energies of possible species and the activation energies for possible elementary reactions involved are obtained in the present work. Our results indicate that the barriers decreased with the order of C-N>N-H>C-H. In addition, metastable adsorption of the abstracted hydrogen atom on the hollow site in the final state is also considered for the N-H and C-H bond breaking. For the C-H bond cleavage, the reaction barrier that the abstracted hydrogen located on the hollow site in the final state is lower than that on the bridge site. However, for the N H bond breaking, the barriers are alike for the abstracted hydrogen on both hollow and bridge sites in the final state.     

Adsorption characteristics of arylisocyanide on Au and Pt electrode surfaces: surface-enhanced Raman scattering study

It is very important to understand metal-molecule interface characteristics for the development of efficient molecular wires in molecular electronics. Because isocyanide is potentially a good alligator clip, we have investigated in this work the adsorption characteristics of 2,6-dimethylphenylisocyanide (DMPI) on Au and Pt electrodes by recording the potential-dependent surface-enhanced Raman scattering (SERS) spectra. First of all, we confirmed that Pt nanoaggregate films were efficient SERS-active substrates, not only in ambient conditions, but also in electrochemical environments. Second, we confirmed that aryl isocyanide should adsorb on Au and Pt by forming exclusively metal-CN bonds, via a pure sigma type interaction in the case of gold compared with a sigma/pi synergistic interaction on Pt. This implies that DMPI should adsorb on Au only via the on-top site, whereas not only the on-top site, but also the 2-fold bridge and 3-fold hollow sites, could be used in the surface adsorption of DMPI on Pt. Despite these differences, DMPI was assumed to possess a vertical orientation with respect to the Au and Pt substrates, irrespective of the potential variation between +0.2 and -0.6 V relative to the Ag/AgCl reference electrode. The latter characteristics of the Au-CN and Pt-CN combinations are presumed to be useful in designing molecular-scale wires.