N2O decomposition on TiO2 (110) from dynamic first-principles calculations

We have carried out a systematic study of N(2)O dissociation on a TiO(2) (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have made use of both static and dynamic calculations in order to elucidate N(2)O decomposition mechanisms. We find that dissociation is not favorable on the stoichiometric surface. On the other hand, the presence of oxygen bridging vacancies make the N(2)O decomposition possible. The role of the defective surface is to provide electrons to the adsorbed molecule. We find two channels for decomposition, depending on whether the molecule is adsorbed with the O or the N end of the molecule on a vacancy. The first case is energetically downhill and proceeds spontaneously, leading to N(2) ejection from the surface and vacancy oxidation. The second case relies on the formation of an intermediate bridging configuration of the adsorbed molecule and is hindered by a small energy barrier. In this case, molecule breaking produces N(2) in the gas phase and leaves oxygen adatoms on the surface. We relate our results to recent experimental findings.     

The rutile TiO2 (110) surface: obtaining converged structural properties from first-principles calculations

We investigate the effects of constraining the motion of atoms in finite slabs used to simulate the rutile TiO2 (110) surface in first-principles calculations. We show that an appropriate choice of fixing atoms in a slab eliminates spurious effects due to the finite size of the slabs, leading to a considerable improvement in the simulation of the (110) surface. The method thus allows for a systematic improvement in convergence in calculating both geometrical and electronic properties. The advantages of this approach are illustrated by presenting the first theoretical results on the displacement of the surface atoms in agreement with experiment.     

NO adsorption on the stoichiometric and reduced SnO2(110) surface

The adsorption of NO molecules on the perfect and defective (110) surfaces of SnO₂ was studied with first-principles methods at the density-functional theory level. It was found that NO mainly interacts via the nitrogen atom with the bridging oxygens of the stoichiometric surface while the coordinatively unsaturated surface Sn atoms are less reactive. On the oxygen-deficient surface, NO is preferentially adsorbed at the vacancy positions, with the nitrogen atom close to the former surface oxygen site. Regardless of the adsorption site, the unpaired electron is located mainly on the NO molecule and only partly on surface Sn atoms. The results for the SnO₂ surface are compared to literature results on the isostructural TiO₂ rutile (110) surface. Dedicated to Professor Karl Jug on the occasion of his 65th birthday     

Enhanced adsorption energy of Au1 and O2 on the stoichiometric TiO2(110) surface by coadsorption with other molecules

During heterogeneous catalysis the surface is simultaneously covered by several adsorbed molecules. The manner in which the presence of one kind of molecule affects the adsorption of a molecule of another kind has been of interest for a long time. In most cases the presence of one adsorbate does not change substantially the binding energy of another adsorbate. The calculations presented here show that the stoichiometric rutile TiO(2)(110) surface, on which one of the compounds -OH, Au(3), Au(5), Au(7), Na, K, or Cs or two different gold strips was preadsorbed, behaves differently: the binding energy of Au(1) or O(2) to such a surface is much stronger than the binding to the clean stoichiometric TiO(2)(110) surface. Moreover, the binding energy of Au(1) or O(2) and the amount of charge they take from the surface when they adsorb are the same, regardless of which of the above species is preadsorbed. The preadsorbed species donate electrons to the conduction band of the oxide, and these electrons are used by Au(1) or O(2) to make stronger bonds with the surface. This suggests that adding an electron to the conduction band of the clean stoichiometric TiO(2)(110) slab used in the calculation will affect similarly the adsorption energy of Au(1) or O(2). Our calculations show that it does. We have also studied how the preadsorption of Au(4) or Au(6) affects the binding of Au(1) or O(2) to the surface. These two gold clusters do not donate electrons to the surface when they bind to it and therefore should not influence substantially the binding energy of Au(1) or O(2) to the surface. However, adsorbing O(2) or Au(1) on the surface forces the clusters to change their structure into that of isomers that donate charge to the oxide. This charge is used by Au(1) or O(2) to bind to the surface and the energy of this bond exceeds the isomerization energy. As a result the surface with the isomerized cluster is the lowest energy state of the system. We believe that these results can be generalized as follows. The molecules that we coadsorbed with Au(1) or O(2) donate electrons to the oxide and are Lewis bases. By giving the surface high energy electrons, they turn it into a Lewis base and this increases its ability to bind strong Lewis acids such as Au(1) and O(2). We speculate that this kind of interaction is general and may be observed for other oxides and for other coadsorbed Lewis base-Lewis acid pairs.     

Acetonitrile adsorption and decomposition on the SnO2 (110) surface: a first-principles computation

The adsorption and decomposition of acetonitrile on the SnO₂ (110) surface were investigated by means of first-principles computations. It is found that acetonitrile could be relatively easier decomposed into CH₃ and CN fragments on the SnO₂ (110) surface than on TiO₂ (110), which agrees with the experimental results. The higher activity of the SnO₂ (110) surface than the TiO₂ (110) surface can be attributed to its higher work function and closer molecular orbital energies.     

First-row transition metal atoms adsorption on rutile TiO2(110) surface

We performed periodic DFT calculations for adsorption of metal atoms on a perfect rutile TiO₂(110) surface (at low coverage, ???=?1/3) to investigate the interaction of an individual metal atom with TiO₂ and to compare it with a study previously done on MgO(100). We considered partial period of Mendeleev??s table from K to Zn. The overall evolution of the adsorption energies shows two maxima as for MgO(100). Two main differences, however, exist: the adsorption energy is much stronger and the first maximum is enhanced relative to the second one. This is attributed to the reducibility of the surface titanium cation. When the adsorbed metal is electropositive, it is oxidized under adsorption transferring electrons to titanium cations. We present the effect of introducing a Hubbard term to the gradient-corrected approximation band-structure Hamiltonian (GGA?+?U). The introduction of a reasonable Hubbard correction preserves the trends and allows localizing the electron of the reduction on Ti atoms in the near surface region. Finally, our results conclude that for heavier M atoms of the period, insertion is energetically favored relative to adsorption.     

Oxygen vacancies on TiO2 (110) from first principles calculations

We have carried out a systematic study of oxygen vacancy formation on the TiO2 (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have used models with the mean number of vacancies per surface unit cell being theta=0.25 and theta=0.5. The study comprises several kind of vacancies within the outermost layers of the surface. The use of a suitable set of technical parameter is often essential in order to get accurate results. We find that the presence of bridging vacancies is energetically favored in accordance to experimental data, although the formation of sub-bridging vacancies might be possible at moderate temperatures. Surprisingly, the spin state of the vacancy has little influence on the results. Atomic displacements are also analyzed and found to be strongly dependent on the particular arrangement of vacancies.     

Structure, stability, and mobility of small Pd clusters on the stoichiometric and defective TiO2 (110) surfaces

We report on the structure and adsorption properties of Pd(n) (n = 1-4) clusters supported on the rutile TiO(2) (110) surfaces with the possible presence of a surface oxygen vacancy or a subsurface Ti-interstitial atom. As predicted by the density functional theory, small Pd clusters prefer to bind to the stoichiometric titania surface or at sites near subsurface Ti-interstitial atoms. The adsorption of Pd clusters changes the electronic structure of the underlying surface. For the surface with an oxygen vacancy, the charge localization and ferromagnetic spin states are found to be largely attenuated owing to the adsorption of Pd clusters. The potential energy surfaces of the Pd monomer on different types of surfaces are also reported. The process of sintering is then simulated via the Metropolis Monte Carlo method. The presence of oxygen vacancy likely leads to the dissociation of Pd clusters. On the stoichiometric surface or surface with Ti-interstitial atom, the Pd monomers tend to sinter into larger clusters, whereas the Pd dimer, trimer, and tetramer appear to be relatively stable below 600 K. This result agrees with the standard sintering model of transition metal clusters and experimental observations.     

A DFT study of acetonitrile adsorption and decomposition on the TiO2 (110) surface

The adsorption and decomposition of acetonitrile on the TiO₂ (110) surface have been investigated with first principles calculations. Our results reveal that both C?N and C? C bonds of acetonitrile become weakened after adsorption. Acetonitrile behaves as an electron donor, and electrons transfer from acetonitrile to substrate is obvious. The reaction mechanism of further decomposition of acetonitrile on TiO₂ (110) surface is also investigated, and the result shows that acetonitrile can decompose into CH₃ and CN fragments and form OCH₃ and NCO groups on the TiO₂ (110) surface, which consists with the experimental results. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Adsorption of platinum on the stoichiometric RuO2(110) surface

Density functional theory was used to calculate the geometries and electronic structures of Pt adsorption on the stoichiometric RuO(2)(110) surface at different coverages. The calculated results revealed that the Pt atoms strongly adsorb on RuO(2), and two-dimensional growth up to 1.25 ML deposition is energetically favorable. At low coverage, the binding between Pt and RuO(2) is very strong, accompanied by a significant transfer of electron density from Pt to the support and a large downshift of the d-band compared to that of the unsupported Pt. At high coverage, a weak interaction of RuO(2) with the Pt cluster is observed, and the electronic structure of Pt is only slightly modified with respect to that of the unsupported material. Our results suggest that among the systems investigated, the RuO(2)-supported Pt at a coverage of 1 ML may become one of the best alternatives to pure Pt as a catalyst because it combines a high stability and a moderate activity similar to Pt.     

CO2 adsorption on TiO2(110) rutile: insight from dispersion-corrected density functional theory calculations and scanning tunneling microscopy experiments

Adsorption of CO(2) on the rutile(110) surface was investigated using dispersion-corrected density functional theory and scanning tunneling microscopy (STM). On the oxidized surface the CO(2) molecules are found to bind most strongly at the five-fold coordinated Ti sites adopting tilted or flat configurations. The presence of bridging oxygen defects introduces two new adsorption structures, the most stable of which involves CO(2) molecules bound in tilted configurations at the defect sites. Inclusion of dispersion corrections in the density functional theory calculations leads to large increases in the calculated adsorption energies bringing these quantities into good agreement with experimental data. The STM measurements confirm two of the calculated adsorption configurations.     

Probing organic layers on the TiO2(110) surface

In this work, we use first principles simulations to provide features of the dynamic scanning force microscopy imaging of adsorbed organic layers on insulating surfaces. We consider monolayers of formic (HCOOH) and acetic (CH(3)COOH) acid and a mixed layer of acetic and trifluoroacetic acids (CF(3)COOH) on the TiO(2)(110) surface and study their interaction with a silicon dangling bond tip. The results demonstrate that the silicon tip interacts more strongly with the substrate and the COO(-) group than the adsorbed acid headgroups, and, therefore, molecules would appear dark in images. The pattern of contrast and apparent height of molecules is determined by the repulsion between the tip and the molecular headgroups and by significant deformation of the monolayer and individual molecules. The height of the molecule on the surface and the size of the headgroup play a large role in determining access of the tip to the substrate and, hence, the contrast in images. Direct imaging of the molecules themselves could be obtained by providing a functionalized tip with attraction to the molecular headgroups, for example, a positive potential tip.     

First-principles calculations of hydrogen diffusion on rutile TiO2(110) surfaces

Density functional calculations are performed to study the H-atom diffusion on titanium dioxide (110) surface in the cases of water-molecule dissociation and splitting of the adjacent hydroxyl OH pair. It is shown that, when a water molecule is adsorbed at a surface oxygen-vacancy site, a fragment H atom of the water molecule tends to diffuse toward the nearest-neighboring bridging-oxygen sites by using a straight-line or relay-point path. As the result, a pair of surface hydroxyl OH is formed on the same oxygen row. In a thermal process, on the other hand, such OH pair favorably splits only by using a relay-point path, i.e., by transferring one H atom from a bridging-oxygen site to a next-neighboring one along the same oxygen row by way of another in-plane oxygen site. We found that the latter splitting reaction is activated around room temperature.     

First-principles characterization of formate and carboxyl adsorption on stoichiometric CeO2(111) and CeO2(110) surfaces

Molecular adsorption of formate and carboxyl on stoichiometric CeO₂(111) and CeO₂(110) surfaces was studied using periodic density functional theory (DFT+U) calculations. Two distinguishable adsorption modes (strong and weak) of formate are identified. The bidentate configuration is more stable than the monodentate adsorption configuration. Both formate and carboxyl bind at the more open CeO₂(110) surface are stronger. The calculated vibrational frequencies of two adsorbed species are consistent with the experimental measurements. Finally, the effects of U parameters on the adsorption of formate and carboxyl over both CeO₂ surfaces were investigated. We found that the geometrical configurations of two adsorbed species are not affected by different U parameters (U = 0, 5, and 7). However, the calculated adsorption energy of carboxyl pronouncedly increases with the U value while the adsorption energy of formate only slightly changes (<0.2 eV). The Bader charge analysis shows the opposite charge transfer occurs for formate and carboxyl adsorption where the adsorbed formate is negatively charge while the adsorbed carboxyl is positively charged. Interestingly, with the increasing U parameter, the amount of charge is also increased.     

D-alaninol adsorption on Cu(100): photoelectron spectroscopy and first-principles calculations

The adsorption of a single molecule of the D-enantiomer of alaninol (2-amino-1-propanol) on the surface of Cu(100) is investigated through density functional theory calculations. Different possible adsorption sites for D-alaninol are tested, and it is found that the most stable configuration presents both amino and hydroxyl group covalently interacting with "on top" copper atoms. The electronic structure is analyzed in detail and compared with experimental photoelectron spectra. Another adsorption structure in which a dehydrogenation process is assumed to occur on the amino group is analyzed and provides a possible explanation of the valence band electronic structure and of the experimentally observed N 1s core-level shift at full coverage, where a self-assembled ordered chiral monolayer is formed on the copper surface.     

Dissociative adsorption of NO on TiO2 (110)-(1 x 2) surface: Ti2O3 rows as actives sites for the adsorption

The interaction of NO with TiO2 (110)-(1 x 2) surface has been studied by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, and low-energy electron diffraction, with the aim to clarify the role of ordered defects in NO reactivity toward TiO2. The interaction was studied for exposures up to 2000 L. However, the main effects occur already in the first 2 L. The exposure of the surfaces to NO resulted in the healing of defect sites without adsorption of N and low-energy electron diffraction shows that the surface (1 x 2) symmetry is not lost after the NO dose.     

Initial stage of Si(001) surface oxidation from first-principles calculations

A comprehensive density-functional theory (DFT) study of the atomic structure, electronic properties, and optical response of the Si(001) surface at the initial stages of oxidation is presented. The most favored adsorption position of a single O atom on top of the (4 x 2)-reconstructed Si(001) surface is found at the back-bond of the "down" Si dimer atom. There is no energy barrier for oxygen insertion into this bond. The ionization energy of the surface reaches a maximum when the oxidation of the second Si monolayer starts. Oxidation leads to an increase of the energy gap between occupied and empty surface states. The calculated reflectance anisotropy spectroscopy (RAS) data in comparison with experiment suggest a considerable amount of surface disorder already after oxidation of the first monolayer.     

Adsorption of acetic and trifluoroacetic acid on the TiO2(110) surface

We use the first-principles static and dynamic simulations to study the adsorption of acetic (CH(3)COOH) and trifluoroacetic (CF(3)COOH) acid on the TiO(2)(110) surface. The most favorable adsorption for both molecules is a dissociative process, which results in the two oxygens of the carboxylate ion bonding to in-plane titanium atoms in the surface. The remaining proton then bonds to a bridging oxygen site, forming a hydroxyl group. We further show that, by comparing the calculated dipoles of the molecules on the surface, it is possible to understand the difference in contrast over the acetate and trifluoroacetate molecules in the atomically resolved noncontact atomic force microscopy images.     

Theoretical study of adsorption of O((3)P) and H(2)O on the rutile TiO(2)(110) surface

The adsorption of oxygen atoms O(3P) on both ideal and hydrated rutile TiO(2)(110) surfaces is investigated by periodic density functional theory (DFT) calculations within the revised Perdew-Burke-Ernzerhof (RPBE) generalized gradient approximation and a four Ti-layer slab, with (2 x 1) and (3 x 1) surface unit cells. It is shown that upon adsorption on the TiO(2) surface the spin of the O atom is completely lost, leading to stable surface peroxide species on both in-plane and bridging oxygen sites with O-binding energies of about 1.0-1.5 eV, rather than to the kinetically unstable terminal Ti-O and terminal O-O species with smaller binding energies of 0.1-0.7 eV. Changes in O-atom coverage ratios between 1/3 and 1 molecular layer (ML) and coadsorption of H(2)O have only minor effects on the O-binding energies of the stable peroxide configurations. High O-atom diffusion barriers of about 1 eV are found, suggesting a slow recombination rate of adsorbed O atoms on TiO(2)(110). Our results suggest that the TiOOTi peroxide intermediate experimentally observed in photoelectrolysis of water should be interpreted as a single spinless O adatom on TiO(2) surface rather than as two Ti-O* radicals coupled together.     

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.