Theoretical studies on the adsorption and decomposition of H2O on Pd(1 1 1) surface

To provide information about the chemistry of water on Pd surfaces, we performed density functional slab model studies on water adsorption and decomposition at Pd(1 1 1) surface. We located transition states of a series of elementary steps and calculated activation energies and rate constants with and without quantum tunneling effect included. Water was found to weakly bind to the Pd surface. Co-adsorbed species OH and O that are derivable from H₂O stabilize the adsorbed water molecules via formation of hydrogen bonds. On the clean surface, the favorable sites are top and bridge for H₂O and OH, respectively. Calculated kinetic parameters indicate that dehydrogenation of water is unlikely on the clean regular Pd(1 1 1) surface. The barrier for the hydrogen abstraction of H₂O at the OH covered surface is approximately 0.2-0.3 eV higher than the value at the clean surface. Similar trend is computed for the hydroxyl group dissociation at H₂O or O covered surfaces. In contrast, the O-H bond breaking of water on oxygen covered Pd surfaces, H₂O_ad + O_ad → 2OH_ad, is predicted to be likely with a barrier of ∼0.3 eV. The reverse reaction, 2OH_ad → H₂O_ad + O_ad, is also found to be very feasible with a barrier of ∼0.1 eV. These results show that on oxygen-covered surfaces production of hydroxyl species is highly likely, supporting previous experimental findings.     

Initial stages of H2O adsorption and hydroxylation of Fe-terminated α-Fe2O3(0 0 0 1) surface

The adsorption modes of H₂O on a Fe-terminated hematite(0 0 0 1) surface have been investigated by first principles Density Functional theory within a periodic slab model and the generalized gradient approximation. Molecular adsorption and dissociative adsorption in monolayer coverage, one H₂O per surface Fe, were both considered. Five plausible orientations were studied to determine the most favorable adsorption position. Molecular adsorption is shown to have a small effect on the underlying surface structure, while hydroxylation has a strong effect on the surface geometry. Electronic densities of state calculations reveal details of these different interactions. The heterolytic dissociation, which produces two types of surface hydroxyls, is the preferable adsorption mode, being slightly favored energetically over the molecular adsorption. Homolytic dissociative adsorption, forming a single hydroxyl on surface Fe, is energetically unfavored, even though strong binding interaction (∼3 eV) is found between the OH radical and surface. Dissociative adsorption on an oxidized ferryl site was also studied to investigate suggested local reactivity enhancement.     

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.     

Photoemission study of the (1 0 0) surface of Ni2MnGa and Mn2NiGa ferromagnetic shape memory alloys

The (1 0 0) surface of Ni₂MnGa and Mn₂NiGa ferromagnetic shape memory alloys have been studied by photoelectron spectroscopy and low energy electron diffraction (LEED). It is shown that by sputtering and annealing, it is possible to obtain a clean, ordered and stoichiometric surface that shows a four-fold 1 × 1 LEED pattern at room temperature. For both Ni₂MnGa and Mn₂NiGa, the surface becomes Ni-rich and Mn deficient after sputtering. However, as the annealing temperature is increased Mn segregates to the surface and at sufficiently high annealing temperature the Mn deficiency caused by sputtering is compensated. The (1 0 0) surface of Ni₂MnGa is found to have Mn-Ga termination. The valence band spectra of both Ni₂MnGa and Mn₂NiGa exhibits modifications with surface composition. For the stoichiometric surface, the origin of the spectral shape of the valence band is explained by calculations based on first principles density functional theory.     

A valence band photoemission study of Pb adsorption on Rh(1 0 0) and Rh(1 1 0)

We have studied the adsorption of Pb on the Rh(1 0 0) and (1 1 0) surfaces by photoemission and low energy electron diffraction (LEED), and tested the chemical properties by adsorption of CO. Pb forms two distinct c(2 × 2) phases on Rh(1 0 0), according to the temperature of the substrate. The phase formed below about 570-620 K, denoted α-c(2 × 2), reduces the coverage of adsorbed CO but does not affect the valence band spectrum of the molecule. The phase formed above this temperature, denoted β-c(2 × 2), also reduces the coverage of adsorbed CO but the valence band spectrum of the adsorbed CO is strongly affected. The two phases are also characterised by a slightly different binding energy of the Pb 5d_5/2 level, 17.54 eV for the α phase and 17.70 for the β phase. The Pb/Rh(1 1 0) surface shows two ordered Pb induced phases, c(2 × 2) and p(3 × 1). CO adsorbs on the first with reduced heat of adsorption and with a valence band spectrum that is strongly altered with respect to CO adsorbed on clean Rh(1 1 0), but does not adsorb on the p(3 × 1) structure at 300 K. We compare the present results with previous results from related systems.     

Photoemission study of glycine adsorption on Cu/Au(1 1 1) interfaces

The adsorption of glycine on Au(1 1 1) pre-deposited with different amounts of Cu was investigated with both conventional X-ray photoelectron spectroscopy (XPS) and synchrotron-based photoemission. In the Cu submonolayer range, glycine physically adsorbs on the Cu/Au(1 1 1) surfaces in its zwitterionic form and completely desorbs at 350 K. The C 1s, O 1s and N 1s core level binding energies monotonically increase with Cu coverage. This indicates that, in the Cu submonolayer range, the admetal is alloyed with Au rather than forming overlayers on the Au(1 1 1) substrate, consistent with our recent experimental and theoretical results [X. Zhao, P. Liu, J. Hrbek, J.A. Rodriguez, M. Pérez, Surf. Sci. 592 (2005) 25]. Upon increasing the amount of deposited Cu over 1 ML, part of the glycine overlayer transforms from the zwitterionic form to the anionic form (NH₂CH₂COO⁻) and adsorbs chemically on the Cu/Au(1 1 1) surface with the N 1s binding energy shifted by −2.3 eV. When the amount of deposited Cu is at 3.0 or 6.0 ML, the intensity of the N 1s chemisorption peak increases with aging time at 300 K. It indicates that glycine adsorption induces Cu segregation from the subsurface region onto the top layer of the substrate. Judging from the initial N 1s peak intensities, it is concluded that 64% and 36% of the top layer are still occupied by Au atoms before glycine adsorption even when the amounts of deposited Cu are 3.0 and 6.0 ML, respectively. On the Au(1 1 1) surface pre-dosed with 6.0 ML of Cu, part of the chemisorbed glycine will desorb and part will decompose upon heating to 450-500 K. In addition, about 20% of the glycine exists in the neutral form when the glycine overlayer was dosed on Cu/Au(1 1 1) held at 100 K.     

Electronic structure of the Sb-induced Si(1 1 3)2 × 5 surface

The surface electronic structure of Sb/Si(1 1 3)2 × 5 was investigated by angle-resolved photoemission spectroscopy experiments. This reveals Sb/Si(1 1 3)2 × 5 to have three surface bands with anisotropic two-dimensional characteristics. The band widths of the surface bands along is larger than along . The number of surface bands of Sb/Si(1 1 3)2 × 5 and their band dispersions along and are quite analogous with those of Sb/Si(1 1 3)2 × 2 composed of Sb adatom and Si tetramer chains. The electronic structure analogy suggests that Sb/Si(1 1 3)2 × 5 and Sb/Si(1 1 3)2 × 2 have common building blocks such as Sb adatom and Si tetramer chains.     

DFT study on dissociative adsorption of SiH4 and GeH4 on SiGe(1 0 0)-2 × 1 surface

SiH₄ and GeH₄ dissociative adsorptions on a buckled SiGe(1 0 0)-2 × 1 surface have been analyzed using density functional theory (DFT) at the B3LYP level. The Ge alloying in the Si(1 0 0)-2 × 1 surface affects the dimer buckling and its surface reactivity. Systematic Ge influences on the reaction energetics are found in SiH₄ and GeH₄ reactions with four dimers of Si^∗-Si, Ge^∗-Si, Ge^∗-Ge, and Si^∗-Ge (∗ denotes the protruded atom). On a half H-covered surface, the energy barriers for silane and germane adsorption are higher than those on the pristine surface. The energy barrier for silane adsorption is higher than the corresponding barrier for germane adsorption. Rate constants are also calculated using the transition-state theory. We conclude that the SiGe surface reactivity in adsorption reaction depends on the Ge presence in dimer form. If the surface Ge is present in form of Ge^∗-Ge, the surface reactivity decreases as the Ge^∗-Ge content increases. If the surface Ge prefers to be in form of Ge^∗-Si at low Ge contents, the surface reactivity increases first, then decreases at high Ge surface contents when Ge^∗-Ge prevails. The calculated rate constant ratio of GeH₄ adsorption on Si^∗-Si over Ge^∗-Ge at 650 °C is 2.1, which agrees with the experimental ratio of GeH₄ adsorption probability on Si(1 0 0) over Si(1 0 0) covered by one monolayer Ge. The experimental ratio is 1.7 measured through supersonic molecular beam techniques. This consistency between calculation and experimental results supports that one monolayer of Ge on Si(1 0 0) exists in form of Ge^∗-Ge dimer.     

Adsorption of phenylalanine on single crystal rutile TiO2(1 1 0) surface

The adsorption of the aromatic amino acid, phenylalanine on a TiO₂ rutile (1 1 0) single crystal surface has been investigated with photoemission and NEXAFS (near edge X-ray absorption fine structure) spectroscopy. The results indicate initial adsorption via the carboxylate group in a bidentate configuration with the phenyl ring oriented at approximately 25° to the surface normal. The amino group remains as NH₂. Subsequent layers of phenylalanine appear to adsorb as neutral molecules with H-bonding between NH₂ and CO groups.     

First principles study of H2S adsorption and dissociation on Fe(1 1 0)

We report first principles density functional theory (DFT) results of H₂S and HS adsorption and dissociation on the Fe(1 1 0) surface. We investigate the site preference of H₂S, HS, and S on Fe(1 1 0). H₂S is found to weakly adsorb on either the short bridge (SB) or long bridge (LB) site of Fe(1 1 0), with a binding energy of no more than 0.50 eV. The diffusion barrier from the LB site to the SB site is found to be small (∼0.10 eV). By contrast to H₂S, HS is predicted to be strongly chemisorbed on Fe(1 1 0), with the S atom in the LB site and the HS bond oriented perpendicular to the surface. Isolated S atoms also are predicted to bind strongly to the LB sites of Fe(1 1 0), where the SB is found to be a transition state for S surface hopping between neighboring LB sites. The minimum energy paths for H₂S and HS dehydrogenation involve rotating an H atom towards a nearby surface Fe atom, with the S-H bonds breaking on the top of one Fe atom. The barrier to break the first S-H bond in H₂S is low at 0.10 eV, and breaking the second S-H bond is barrierless, suggesting deposition of S on Fe(1 1 0) via H₂S is kinetically and thermodynamically facile.     

SiO adsorption on a p(2 × 2) reconstructed Si(1 0 0) surface

We have investigated the adsorption mechanism of SiO molecule incident on a clean Si(1 0 0) p(2 × 2) reconstructed surface using density functional theory based methods. Stable adsorption geometries of SiO on Si surface, as well as their corresponding activation and adsorption energies are identified. We found that the SiO molecule is adsorbed on the Si(1 0 0) surface with almost no activation energy. An adsorption configuration where the SiO binds on the channel separating the dimer rows, forming a Si-O-Si bridge on the surface, is the energetically most favourable geometry found. A substantial red-shift in the calculated vibrational frequencies of the adsorbed SiO molecule in the bridging configurations is observed. Comparison of adsorption energies shows that SiO adsorption on a Si(1 0 0) surface is energetically less favourable than the comparable O₂ adsorption. However, the role of SiO in the growth of silicon sub-oxides during reactive magnetron plasma deposition is expected to be significant due to the relatively large amount of SiO molecules incident on the deposition surface and its considerable sticking probability. The stable adsorption geometries found here exhibit structural properties similar to the Si/SiO₂ interface and may be used for studying SiO_x growth.     

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.     

Solvent effects for CO and H2 adsorption on Cu2O (1 1 1) surface: A density functional theory study

To investigate solvent effects, CO and H₂ adsorption on Cu₂O (1 1 1) surface in vacuum, liquid paraffin, methanol and water are studied by using density functional theory (DFT) combined with the conductor-like solvent model (COSMO). When H₂ and CO adsorb on Cu_cus of Cu₂O (1 1 1) surface, solvent effects can improve CO and H₂ activation. The H-H bond increases with dielectric constant increasing as H₂ adsorption on O_suf of Cu₂O (1 1 1) surface, and the H-H bond breaks in methanol and water. It is also found that both the structural parameters and Mulliken charges are very sensitive to the COSMO solvent model. In summary, the solvent effects have obvious influence on the clean surface of Cu₂O (1 1 1) and the adsorptive behavior.     

Valence band and surface core-level shift studies of the oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

Using synchrotron radiation, W 4f_7/2 and valence-band (VB) photoelectron spectroscopy was used to study the oxidation states of the p(3 × 1) reconstructed surface of W(1 0 0) produced by oxygen adsorption at 1500 K. The W 4f_7/2 study showed two features at 0.40 and 1.50 eV higher binding energies relative to the bulk feature which represent shifts to lower binding energies compared to the W 4f_7/2 features of the as-grown oxygen films on W(1 0 0). Co-existence of these features is a characteristic of the formation of the p(3 × 1) structure. The VB studies of this system and as-grown oxygen films at room temperature (RT) showed an oxygen-induced feature for the p(3 × 1) structure for which the peak maximum had shifted by 0.9 eV to higher binding energies compared to the as-grown films. The direction of shifts in the W 4f_7/2 and VB spectra for the p(3 × 1) structure confirmed the reduction of the charge transfer from W to oxygen compared to the as-grown films. A study of the VB changes with photon energy shows that the main component of the oxygen-induced feature in the p(3 × 1) VB at 6.6 ± 0.2 eV is related to a hybridized state between W 5d and O 2p electrons.     

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.     

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.     

CO adsorption on the reduced RuO2(1 1 0) surface

The O-bridge atoms on a stoichiometric RuO₂(1 1 0) surface were removed by reaction with CO. The resulting reduced surface was then further exposed to CO. By means of thermal desorption spectroscopy and high-resolution electron energy-loss spectroscopy three adsorbed CO states were identified on bridge sites and assigned to double-bonded, single-bonded, and single-bonded species in the vicinity of O-bridge residues, respectively.     

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).     

The electronic structure and reactivity of the oxygen-modified Mo2C(0 0 0 1) surface

Oxygen adsorption on Mo₂C(0 0 0 1) has been investigated with angle-resolved photoemission spectroscopy (ARPES). When the surface is reacted with O₂, the O 2p-induced states are formed at 4.1 and 5.3 eV at the point. The emissions around the Fermi level are also intensified by oxygen adsorption, which is due to the formation of a partially filled state. It is found that the reactivity of the surface toward H₂O adsorption is much enhanced by pre-adsorption of oxygen. The reactivity is found to be maximized at θ_O ∼ 0.2.     

Effect of hydrogenation on the electronic structure of the P/Si(0 0 1)-(1 × 2) surface

Ab initio calculations, based on pseudopotentials and density functional theory (DFT), have been performed to investigate the effect of hydrogenation on the electronic properties of P/Si(0 0 1)-(1 × 2) surface. In parallel with this, the electronic band structure of the hydrogenated and non-hydrogenated P/Si(0 0 1)-(1 × 2) surface have been calculated for half- and full-monolayer P. For the mixed Si-P dimer structure, we have identified two occupied and one unoccupied surface state, which correspond to 0.5 ML coverage of P. When this surface is terminated with H, we see that two occupied states completely disappeared and that one unoccupied state is shifted towards the conduction band. A similar calculations for the 1 ML coverage of P have been also carried out. It is seen that the unoccupied state C₁ appeared in the P/Si(0 0 1)-(1 × 2) surface is passivated when this surface is terminated with the H atoms. To explain the nature of the surface states, we have also plotted the total and partial charge densities at the point of the Surface Brillouin Zone (SBZ).