Adsorption of CO, CO2 and NO molecules on a BaTiO3 (0 0 1) surface

Since the development of Scanning Tunnelling Microscopy (STM) technique, considerable attention has been devoted to various molecules adsorbed on various surfaces. Also, a new concept emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale are thus particularly invaluable. The present work describes the first Density Functional Theory (DFT) study of adsorption of CO, CO₂ and NO molecules on a BaTiO₃ surface following a first preliminary calculation of O and O₂ adsorption on the same surface. In the previously considered work, we found that a (0 0 1) surface with BaO termination is more stable than the one with TiO₂-termination. Consequently, we extended our study to CO, CO₂ and NO molecules adsorbed on a (0 0 1) surface with BaO termination. The present calculation was performed on a (1 × 1) cell with one monolayer of adsorbed molecules. Especially, a series of cases implying CO molecules adsorbed in various geometrical configurations has been examined. The corresponding adsorption energy varies in the range of −0.17 to −0.10 eV. The adsorption energy of a CO₂ molecule directly located above an O surface atom (called O_s) is of the order of −0.18 eV. The O-C distance length is then 1.24 Å and the O-C-O and O-C-O_s angles are 134.0° and 113.0°, respectively. For NO adsorption, the most important induced structural changes are the followings: (i) the N-O bond is broken when a NO molecule is absorbed on a Ba-O_s bridge site. In that case, N and O atoms are located above an O and a Ba surface atom, respectively, whereas the O-Ba-O_s and N-O_s-Ba angles are 106.5° and 63.0°, respectively. The N-O distance is as large as 2.58 Å and the adsorption energy is as much as −2.28 eV. (ii) In the second stable position, the NO molecule has its N atom adsorbed above an O_s atom, the N-O axis being tilted toward the Ba atom. The N-O_s-Ba angle is then 41.1° while the adsorption energy is only −0.10 eV. At last, the local densities of states around C, O as well as N atoms of the considered adsorbed molecules have also been discussed.     

Reaction mechanism for CO oxidation on Cu(3 1 1): A density functional theory study

The microscopic reaction mechanism for CO oxidation on Cu(3 1 1) surface has been investigated by means of comprehensive density functional theory (DFT) calculations. The elementary steps studied include O₂ adsorption and dissociation, dissociated O atom adsorption and diffusion, as well as CO adsorption and oxidation on the metal. Our results reveal that O₂ is considerably reactive on the Cu(3 1 1) surface and will spontaneously dissociate at several adsorption states, which process are highly dependent on the orientation and site of the adsorbed oxygen molecule. The dissociated O atom may likely diffuse via inner terrace sites or from a terrace site to a step site due to the low barriers. Furthermore, we find that the energetically most favorable site for CO molecule on Cu(3 1 1) is the step edge site. According to our calculations, the reaction barrier of CO + O → CO₂ is about 0.3 eV lower in energy than that of CO + O₂ → CO₂ + O, suggesting the former mechanism play a main role in CO oxidation on the Cu(3 1 1) surface.     

Adsorption properties of CO on low-index Pt3Sn surfaces

The adsorption properties of CO on Pt₃Sn were investigated by utilizing quantum mechanical calculations. The (1 1 1), (1 1 0) and (0 0 1) surfaces of Pt₃Sn were generated with all possible bulk terminations, and on these terminations all types of active sites were determined. The adsorption energies and the geometries of the CO molecule at those sites were found. Those results were compared with the results obtained from the adsorption of CO on similar sites of Pt(1 1 1), Pt(1 1 0) and Pt(0 0 1) surfaces. The comparison reveals that adsorption of CO is stronger on Pt surfaces; this may be the reason why catalysts with Pt₃Sn phase do not suffer from CO posioning in experimental works. Aiming to understand the interactions between CO and the metal adsorption sites in detail, the local density of states (LDOS) profiles were produced for atop-Pt adsorption, both for the carbon end of CO for its adsorbed and free states, and for the Pt atom of the binding site. LDOS profiles of C of free and adsorbed CO and Pt for corresponding pure Pt surfaces, Pt(1 1 1), Pt(1 1 0) and Pt(0 0 1) were also obtained. The comparison of the LDOS profiles of Pt atoms of atop adsorption sites on the same faces of bare Pt₃Sn and Pt surfaces showed the effect of alloying with Sn on the electronic properties of Pt atoms. Comparison of LDOS profiles of the C end of CO in its free and atop adsorbed states on Pt₃Sn and LDOS of Pt on bare and CO adsorbed Pt₃Sn surface were used to clear out the electronic changes occurred on CO and Pt upon adsorption. The study showed that (i) inclusion of a Sn atom at the adsorption site structure causes dramatic decrease in stability which limits the number of possible CO adsorption sites on Pt₃Sn surface, (ii) the presence of Sn causes angles different from 180° for M-C-O orientation, (iii) the presence of Sn in the neighborhood of Pt on which CO is adsorbed causes superposition of the 5σ/1π derived-state peaks at the carbon end of CO and changes in adsorption energy of CO, (iv) Sn present beneath the adsorption site strengthens the CO adsorption, whereas neighboring Sn on the surface weakens it for all Pt₃Sn surfaces tested and (v) the most stable site for CO adsorption is the atop-Pt site of the mixed atom termination of Pt₃Sn(1 1 0).     

The configuration and electronic state of SO3 adsorbed on Au surface

We evaluated the adsorption of SO₃ molecule on Au (1 1 1) surface using first principles calculation by a slab model with a periodic boundary condition. We find that there are six stable adsorption configurations on an Au surface, where the SO₃ molecule is adsorbed above the three-fold fcc and hcp hollow sites and on the atop site. In two of these configurations, S and two O atoms are bound to the Au atoms, the next two configurations have all the three O atoms bound to the Au surface atoms, and the last two configurations have the S atom bound to an Au surface atom on the atop site and O atoms situated above the hollow sites. In these configurations, the electronic structures of SO₃ on the Au surface show that molecular orbitals of SO₃ and those of the Au surface are hybridized in the active metal d-band region, that the localized molecular orbitals in SO₃ are stabilized, and that charge is transferred from Au to S 3p by SO₃ adsorption on the Au surface though there is little other interaction of the S and O (bound to Au) component with Au. Moreover, the bond between the S and O atoms bound to Au is weakened due to SO₃ adsorption on the Au surface due to the charge polarization of the O-Au bond. This interaction is likely to encourage the S-O bond to break.     

Ab-initio pseudopotential study of electronic structure and chemisorption of oxygen on aluminium surface

Chemisorption of oxygen atom on aluminium (1 1 1), (1 1 0) and (1 0 0) surfaces is studied using ab-initio plane wave pseudopotential method based on density functional theory (DFT). Oxygen atom chemisorbed on three different high symmetry sites; top, short-bridge and hollow sites on the aluminium surfaces are examined. It has been found that the O-adatom adsorbed at the hollow site on aluminium (1 1 1), (1 1 0) and (1 0 0) plane yield energetically most stable structure. Calculation of chemisorption energies of O-adatom on aluminium surfaces shows that oxygen is most strongly bound to aluminium atoms on Al(1 1 1) plane and the calculated value of the chemisorption energy of O-adatom at the hollow site on Al(1 1 1) surface is 4.8 eV. In this work, the chemisorption energies calculated for O-adatom on Al(1 1 0) and Al(1 0 0) surfaces are reported for the first time. The electronic structures and the electronic charge density distributions of the oxygen chemisorbed aluminium surfaces are also investigated. Calculations show that for aluminium, p orbitals also contribute significantly along with the s orbitals during the bond formation with oxygen atom. Therefore, the possibilities of hybridizations lead to the strong bonding configurations.     

Study on the adsorption of fluorescein on Ag(1 1 0) substrate

The adsorption of fluorescein on the Ag(1 1 0) surface has been investigated by the first-principles pseudopotential method. Various adsorption geometries have been calculated and the energetically most favorable structure of fluorescein/Ag(1 1 0) was identified. The fluorescein molecule, in most favorable structure, is on hollow site, and the adsorption energy is 2.34 eV. Here the adsorption sites refer to the positions at the first layer of the substrate where the middle carbon atom of the fluorescein molecule is located. The bonding strength of the fluorescein molecule to the Ag substrate is site selective, being determined by electron transfer to the oxygen atoms of the molecule and local electrostatic attraction between the oxygen atoms and the silver atoms.     

Dissociation of water molecule at three-fold oxygen coordinated V site on the InVO4 (0 0 1) surface

Water molecule adsorption properties at the surface of InVO₄ have been investigated using an ab initio molecular dynamics approach. It was found that the water molecules were adsorbed dissociatively to the three-fold oxygen coordinated V sites on the (0 0 1) surface. The dissociative adsorption energy was estimated to be 0.8-0.9 eV per molecule. The equilibrium distance between V and O of the hydroxyl -OH was almost the same as the V-O distance of tetrahedra VO₄ in the InVO₄ bulk crystal (1.7-1.8 Å).     

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.     

CO interaction with Au atoms adsorbed on terrace, edge and corner sites of the MgO(1 0 0) surface. Electronic structure and vibrational analysis from DFT

The interaction of CO with Au atoms adsorbed on terrace and low-coordinates sites (edge and corner) of the MgO(1 0 0) surface was studied using the density functional theory (DFT) in combination with embedded cluster models. Surface anionic (O²⁻) and neutral oxygen vacancy (F_s) sites were considered. In all the cases, the CO stretching frequencies are shifted with respect to free CO with values between −232 and −358 cm⁻¹. In particular, the values for Au on F_s at edge and corner are shifted to higher stretching frequencies by 100 and 59 cm⁻¹, respectively, with respect to the value on a perfect MgO(1 0 0) surface. This result is in agreement with recent scanning tunneling microscopy and infrared spectroscopy experiments where a corresponding shift of 70 cm⁻¹ was observed by comparing the measurements on perfect and O-deficient MgO(1 0 0) surfaces. However, these results are different than expected because Au atoms on F_s centers are negatively charged and, therefore, according to the generally accepted scheme the CO frequency should be red-shifted with respect to the adsorption on anionic five-coordinated site where the Au atom is essentially neutral. The following picture emerges from the present results: the single occupied HOMO(α) of Au atom on F_s at low-coordinated sites consists in two lobes extended sideward the Au atom. For symmetry reasons, this MO overlaps efficiently with the 2π^∗ MO of CO. This bonding contribution to the Au-CO link is counteracted by a Pauli repulsion between the 5σ MO of CO and more internal orbitals (the HOMO-1(α) and the HOMO(β)) centered on Au. In consequence, CO is forced to vibrate against a region with a high electron density. This is the so-called “wall effect” which by itself contributes to higher CO frequency values.     

Photoelectron spectroscopy study of oxygen adsorption on Mo2C(0 0 0 1)

Oxygen adsorption on the α-Mo₂C(0 0 0 1) surface has been investigated with X-ray photoelectron spectroscopy and valence photoelectron spectroscopy utilizing synchrotron radiation. It is found that oxygen adsorbs dissociatively at room temperature, and the adsorbed oxygen atoms interact with both Mo and C atoms to form an oxycarbide layer. As the O-adsorbed surface is heated at ≧800 K, the C-O bonds are broken and the adsorbed oxygen atoms are bound only to Mo atoms. Valence PES study shows that the oxygen adsorption induces a peculiar state around the Fermi level, which enhances the emission intensity at the Fermi edge in PES spectra.     

Theoretical study of CO adsorption on the illuminated TiO2 (0 0 1) surface

A quantum modeling of the CO adsorption on illuminated anatase TiO₂ (0 0 1) is presented. The calculated adsorption energy and geometries of illuminated case are compared with the ground state case. The calculations were achieved by using DFT formalism and the BH and HLYP. Upon photoexcitation, an electron-hole pair is generated. Comparing of natural population in the ground state and the exited state, shows that an electron is trapped in a Ti⁴⁺ ion and a hole is localized in an oxygen ion. The photoelectron helps generation of a CO₂ molecule on the TiO₂ surface. As shown by optimization of these systems, the CO molecule adsorbed vertically on the TiO₂ (0 0 1) surface in the ground state case while the CO molecule made an angle of 134.3° to this surface at the excited state case. Based on the here used model the obtained adsorption energy was 0.36 eV which is in excellent agreement with the reported experimental value. In the present work the C-O stretch IR frequencies are calculated which are 1366.53 and 1423.16 cm⁻¹. These results are in good agreement with the earlier reported works for the surface carbonaceous compounds, and oxygenated carbon species.     

A density functional study of atomic oxygen and water molecule adsorption on Ni(1 1 1) and chromium-substituted Ni(1 1 1) surfaces

Density functional theory (DFT) for generalized gradient approximation calculations has been used to study the adsorption of atomic oxygen and water molecules on Ni(1 1 1) and different kind of Ni-Cr(1 1 1) surfaces. The fcc hollow site is energetically the most favorable for atomic oxygen adsorption and on top site is favorable for water adsorption. The Ni-Cr surface has the highest absorption energy for oxygen at 6.86 eV, followed by the hcp site, whereas the absorption energy is 5.56 eV for the Ni surface. The Ni-O bond distance is 1.85 Å for the Ni surface. On the other hand, the result concerning the Ni-Cr surface implies that the bond distances are 1.93-1.95 Å and 1.75 Å for Ni-O and Cr-O, respectively. The surface adsorption energy for water on top site for two Cr atom substituted Ni-Cr surface is 0.85 eV. Oxygen atoms prefer to bond with Cr rather than Ni atoms. Atomic charge analysis demonstrates that charge transfer increases due to the addition of Cr. Moreover, a local density of states (LDOS) study examines the hybridization occurring between the metal d orbital and the oxygen p orbital; the bonding is mainly ionic, and water bonds weakly in both cases.     

Structure and reactivity of Pd doped Ag surfaces

We have performed first principles calculations for clean and Pd doped Ag(1 1 1) and Ag(1 0 0) surfaces, with and without adsorbed O and CO. Our results for the structure of the Pd doped Ag surfaces indicate that Pd atoms are located lower than the surrounding Ag surface atoms. We find that O atoms adsorbed on Pd doped Ag(1 1 1) reside at the fcc hollow sites, the site next to Pd being slightly favored. Moreover, we provide results for O and CO co-adsorption on the clean and Pd doped Ag(1 1 1) surfaces, indicating that Pd can act as an electronic promoter for the CO oxidation reaction.     

DFT study of BaTiO<Subscript>3</Subscript> (001) surface with O and O<Subscript>2</Subscript> adsorption

Progress of scanning tunneling microscopy (STM) allowed to handle various molecules adsorbed on a given surface. New concepts emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale is thus particularly invaluable. In this work, within the framework of density functional theory (DFT), we present an electronic and structural ab initio study of a BaTiO₃ (001) surface (perovskite structure) in its paraelectric phase. As far as we know the atomic and molecular adsorption of oxygen at surface is then analyzed for the first time in the literature. Relaxation is taken into account for several layers. Its analysis for a depth of at least four layers enables us to conclude that a reasonable approximation for a BaTiO₃ (001) surface is provided with a slab made up of nine plans. The relative stability of two possible terminations is considered. By using a kinetic energy cut off of 400 eV, we found that a surface with BaO termination is more stable than with TiO₂ termination. Consequently, a surface with BaO termination was chosen to adsorb either O atom or O₂ molecule and the corresponding calculations were performed with a coverage 1 on a (1×1) cell. A series of cases with O₂ molecule adsorbed in various geometrical configurations are also analyzed. For O₂, the most favorable adsorption is obtained when the molecule is placed horizontally, with its axis, directed along the Ba-Ba axis and with its centre of gravity located above a Ba atom. The corresponding value of the adsorption energy is -9.70 eV per molecule (-4.85 eV per O atom). The molecule is then rather extended since the O–O distance measures 1.829 ?. By comparison, the adsorption energy of an O atom directly located above a Ba atom is only -3.50 eV. Therefore we are allowed to conclude that the O–O interaction stabilizes atomic adsorption. Also the local densities of states (LDOS) corresponding to various situations are discussed in the present paper. Up to now, we are not aware of experimental data to be compared to our calculated results.     

Theoretical study on adsorption of thiophenethiolate molecule on Au(1 1 1) surface

We have theoretically studied the adsorption of a thiophenethiolate (C₄H₃S-S) molecule on the Au(1 1 1) surface by first-principles calculations. It is found that the bridge site is the most stable adsorption site with the adsorption energy of 1.02 eV. In the optimized adsorption geometry, the bond between the head S atom and the connected C atom in the tail thiophene molecule is tilted by 57.2° from the surface normal. In addition, the adsorption of thiophenethiolate induces large relaxations of the surface Au atoms around it. Furthermore, weak interactions between the S atom in the tail thiophene ring and the Au atoms also contribute to the adsorption on the Au surface.     

Density functional and dynamics study of the dissociative adsorption of hydrogen on Mg (0 0 0 1) surface

A first principles study is performed to investigate the adsorption characteristics of hydrogen on magnesium surface. Substitutional and on-surface adsorption energies are calculated for Mg (0 0 0 1) surface alloyed with the selected elements. To further analyze the hydrogen-magnesium interaction, first principles molecular dynamics method is used which simulates the behavior of H₂ at the surface. Also, charge density differences of substitutionally doped surface configurations were illustrated. Accordingly, Mo and Ni are among the elements yielding lower adsorption energies, which are found to be −9.2626 and −5.2995 eV for substitutionally alloyed surfaces, respectively. In light of the dynamic calculations, Co as an alloying element is found to have a splitting effect on H₂ in 50 fs, where the first hydrogen atom is taken inside the Mg substrate right after the decomposition and the other after 1300 fs. An interesting remark is that, elements which acquire higher chances of adsorption are also seen to be competent at dissociating the hydrogen molecule. Furthermore, charge density distributions support the results of molecular dynamics simulations, by verifying the distinguished effects of most of the 3d and 4d transition metals.     

Effects of metal-support interactions on the electronic structures of metal atoms adsorbed on the perfect and defective MgO(1 0 0) surfaces

The electronic structures of Ni, Pd, Pt, Cu, and Zn atoms adsorbed on the perfect MgO(1 0 0) surface and on a surface oxygen vacancy have been studied at the DFT/B3LYP level of theory using both the bare cluster and embedded cluster models. Ni, Pd, Pt, and Cu atoms can form stable adsorption complexes on the regular O site of the perfect MgO(1 0 0) surface with the binding energies of 19.0, 25.2, 46.7, and 17.3 kcal/mol, respectively, despite very little electron transfer between the surface and the metal atoms. On the other hand, adsorptions of Ni, Pd, Pt, and Cu atoms show strong interaction with an oxygen vacancy on the MgO(1 0 0) surface by transferring a significant number of electron charges from the vacancy to the adsorbed metal atoms and thus forming ionic bonds with the vacancy site. These interactions on the vacancy site for Ni, Pd, Pt, and Cu atoms increase the binding energies by 25.8, 59.7, 85.2, and 19.1 kcal/mol, respectively, compared to those on the perfect surface. Zn atom interacts very weakly with the perfect surface as well as the surface oxygen vacancy. We observed that the interaction increases from Ni to Pt in the same group and decreases from Ni to Zn in the same transition metal period in both perfect and vacancy systems. These relationships correlate well with the degrees of electron transfer from the surface to the adsorbed metal atom. The changes in the ionization potentials of the surface also correlate with the adsorption energies or degrees of electron transfers. Madelung potential is found to have significant effects on the electronic properties of metal atom adsorptions on the MgO(1 0 0) surface as well as on an oxygen vacancy, though it is more so for the latter. Furthermore, the Madelung potential facilitates electron transfer from the surface to the adsorbed metal atoms but not in the other direction.     

Probing the properties of the (1 1 1) and (1 0 0) surfaces of LaB6 through infrared spectroscopy of adsorbed CO

The adsorption of carbon monoxide on the LaB₆(1 0 0) and LaB₆(1 1 1) surfaces was studied experimentally with the techniques of reflection absorption infrared spectroscopy and X-ray photoelectron spectroscopy. The interaction of CO with the two surfaces was also studied with density functional theory. Both surfaces adsorb CO molecularly at low temperatures but in markedly different forms. On the LaB₆(1 1 1) surface CO initially adsorbs at 90 K in a form that yields a CO stretching mode at 1502-1512 cm⁻¹. With gentle annealing to 120 K, the CO switches to a bonding environment characterized by multiple CO stretch values from 1980 to 2080 cm⁻¹, assigned to one, two, or three CO molecules terminally bonded to the B atoms of a triangular B₃ unit at the (1 1 1) surface. In contrast, on the LaB₆(1 0 0) surface only a single CO stretch is observed at 2094 cm⁻¹, which is assigned to an atop CO molecule bonded to a La atom. The maximum intensity of the CO stretch vibration on the (1 0 0) surface is higher than on the (1 1 1) surface by a factor of 5. This difference is related to the different orientations of the CO molecules on the two surfaces and to reduced screening of the CO dynamic dipole moment on the (1 0 0) surface, where the bonding occurs further from the surface plane. On LaB₆(1 0 0), XPS measurements indicate that CO dissociates on the surface at temperatures above 400 K.     

A density functional theory study on the adsorption of CO and O₂ on Cu-terminated Cu₂O（111） surface

The adsorptions of CO and O2 molecules individually on the stoichiometric Cu-terminated Cu2O(111) surface are investigated by first-principles calculations on the basis of the density functional theory.The calculated results indicate that the CO molecule preferably coordinates to the Cu2 site through its C atom with an adsorption energy of -1.69 eV,whereas the O2 molecule is most stably adsorbed in a tilt type with one O atom coordinating to the Cu2 site and the other O atom coordinating to the Cu1 site,and has an adsorption energy of -1.97 eV.From the analysis of density of states,it is observed that Cu 3d transfers electrons to 2π orbital of the CO molecule and the highest occupied 5σ orbital of the CO molecule transfers electrons to the substrate.The sharp band of Cu 4s is delocalized when compared to that before the CO molecule adsorption,and overlaps substantially with bands of the adsorbed CO molecule.There is a broadening of the 2π orbital of the O2 molecule because of its overlapping with the Cu 3d orbital,indicating that strong 3d-2π interactions are involved in the chemisorption of the O2 molecule on the surface.     

Li adsorption on a Fullerene–Single wall carbon nanotube hybrid system: Density functional theory approach

In this study, we investigate Li adsorption mechanisms on the C₆₀-SWCNT hybrid system using density functional theory. It is found that the Li adsorption energy of the C₆₀-SWCNT hybrid system is increased in comparison to that of the pure SWCNT. The Li adsorption energy ranges from −1.917 eV to −2.642 eV for the single-Li adsorbed system and from −2.351 eV to −2.636 eV for the double-Li adsorbed system. It is also found that the adsorption energy becomes similar at most positions throughout the structure. In addition, the Li adsorption energy of 31-Li system is calculated to be −1.863 eV, which is significantly lower than the Li–Li binding energy (−1.030 eV). These results infer that Li atoms will be adsorbed on the space 1) between C₆₀ and C₆₀; 2) between SWCNT and C₆₀; 3) the rest of the space (e.g. between SWCNTs), rather than form Li clusters. As more Li atoms are adsorbed onto the C₆₀-SWCNT hybrid system due to such improved Li adsorption capability, the metallic character of the system is enhanced, which is confirmed via the band structure and electronic density of states.