Density functional theory study of NO2-sensing mechanisms of pure and Ti-dopedWO3 （002） surfaces

Density functional theory (DFT) calculations are employed to explore the NO₂-sensing mechanisms of pure and Ti-doped WO₃ (002) surfaces. When Ti is doped into the WO₃ surface, two substitution models are considered: substitution of Ti for W_6c and substitution of Ti for W_5c. The results reveal that substitution of Ti for 5-fold W forms a stable doping structure, and doping induces some new electronic states in the band gap, which may lead to changes in the surface properties. Four top adsorption models of NO₂ on pure and Ti-doped WO₃ (002) surfaces are investigated: adsorptions on 5-fold W (Ti), on 6-fold W, on bridging oxygen, and on plane oxygen. The most stable and likely NO₂ adsorption structures are both N-end oriented to the surface bridge oxygen O_1c site. By comparing the adsorption energy and the electronic population, it is found that Ti doping can enhance the adsorption of NO₂, which theoretically proves the experimental observation that Ti doping can greatly increase the WO₃ gas sensor sensitivity to NO₂ gas.     

Adsorption of water on (0 0 1) surface of SrTiO3 and SrZrO3 cubic perovskites: Hybrid HF-DFT LCAO calculations

Hybrid HF-DFT LCAO simulations of (0 0 1) surface properties and water adsorption on cubic SrZrO₃ and SrTiO₃ perovskites are performed using a single slab model framework. Three slab models with the different surface termination including 6-11 atomic planes were used for calculations. The effect of the symmetry reduction and the role of an extra atomic layer basis set have been considered for the bare surface slabs. The optimized structures and water adsorption energies have been calculated for the various types of surface coverage. It is shown that the formation of H-bonds between the water hydrogens and surface oxygens, as well as between the water molecules themselves, controls the structure of the water adsorption layers on the perovskite surfaces. Obtained results indicate that the dissociative type of water adsorption is the energetically more favourable for SrO-terminated zirconate surface than for similar titanate surface giving evidence to the more basic nature of oxygen atoms on SrO-terminated SrZrO₃ surface.     

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.     

First-principles study on the catalytic role of Ag in the oxygen adsorption of LaMnO3(0 0 1) surface

In the present paper, the catalytic role of Ag in the oxygen adsorption of LaMnO₃(0 0 1) surface has been theoretically investigated using first-principles calculations based on the density functional theory (DFT) and pseudopotential method. The O₂ adsorption energy is larger for the vertical adsorption and the covalent bond was formed between O₂ molecule and surface Mn. The calculation of electronic properties of interaction between Ag atom and LaMnO₃(0 0 1) surface demonstrates that the most stable position for Ag adsorption is hollow site. The O₂ adsorption energy dramatically increased from 0.298 eV to 1.108 eV due to Ag pre-adsorbed. It is Ag pre-adsorbed that facilitates O₂ adsorption on surface. The bond length and bond population of O₂ molecule indicate that Ag atom facilitates O₂ molecule dissociative adsorption. The Ag atom strengthens LaMnO₃(0 0 1) substrate activity and activity center was formed on surface, which enhances the electrocatalytic activity of LaMnO₃ as solid oxide fuel cells cathode material at low temperature.     

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.     

Density functional study of oxygen adsorption on the Mg3Nd (0 0 1) surface

The adsorption of oxygen atoms on Mg₃Nd (0 0 1) surface was studied based on density function theory (DFT), in which the exchange-correlation potential was chosen as the generalized gradient approximation (GGA) in the Perdew and Wang (PW91). The most preferred adsorption position was at the top-hollow site. Upon the optimization on top-hollow site with different coverage, it was found that the adsorption energy decreased with oxygen coverage. The density of states analysis showed that obvious charge transfer took place between O atom and the nearest Nd atom and chemical bond formed between O atom and the nearest Nd atom after O adsorption. The result of surface energy as a function of chemical potential change of oxygen indicated the clean Mg₃Nd (0 0 1) surface was easy to adsorb oxygen and form 1.00 ML surface.     

A photoelectron spectroscopic observation of iron surfaces exposed to N2, N2O,NO, NO2 and air at 200 torr or 1 atm

Photoelectron spectra from core levels are presented for adsorption of nitrogen-containing gases at 200 torr or 1 atm on iron surfaces. Assignments of each band and the adsorption process are discussed. On the substrate at normal temperature, each gas forms nitride and nitrogen oxide groups (NO, NO₂ and NO₃) which appear at 398.6, 400.0, 404.5 and 407.1 eV, respectively. The relative intensifies of each species depend on the gases adsorbed. NO and NO₂ are considered to dissociate on the surface and the oxygen atom adsorb preferentially. The oxygen on the surface can be considered to contribute to the formation of each surface ligand.     

Ruthenium adsorption and diffusion on the GaN(0 0 0 1) surface

We report first principles calculations to analyze the ruthenium adsorption and diffusion on GaN(0 0 0 1) surface in a 2×2geometry. The calculations were performed using the generalized gradient approximation (GGA) with ultrasoft pseudopotential within the density functional theory (DFT). The surface is modeled using the repeated slabs approach. To study the most favorable ruthenium adsorption model we considered T₁, T₄ and H₃ special sites. We find that the most energetically favorable structure corresponds to the Ru- T₄ model or the ruthenium adatom located at the T₄ site, while the ruthenium adsorption on top of a gallium atom (T₁ position) is totally unfavorable. The ruthenium diffusion on surface shows an energy barrier of 0.612 eV. The resultant reconstruction of the ruthenium adsorption on GaN(0 0 0 1)- 2×2 surface presents a lateral relaxation of some hundredth of Å in the most stable site. The comparison of the density of states and band structure of the GaN(0 0 0 1) surface without ruthenium adatom and with ruthenium adatom is analyzed in detail.     

First-principles studies of the oxygen adsorption on unreconstructed and reconstructed Ni(1 1 0) surfaces

First-principles calculations have been performed to investigate the adsorption of oxygen on unreconstructed and reconstructed Ni(1 1 0) surfaces. The energetics, structural, electronic and magnetic properties are given in detail. For oxygen adsorption on unreconstructed surface, (n×1)(n=2,3) substrate with oxygen atom on short-bridge site is found to be the most stable adsorption configuration. Whereas energetically most favorable adsorption phase of reconstructed surface is p(n×1) substrate with oxygen atom located at long-bridge site. Our calculations suggest that the surface reconstruction is induced by the oxygen adsorption. We also find there are redistributions of electronic structure and electron transfer from the substrate to adsorbate. Our calculations also indicate surface magnetic moment is enhanced on clean surfaces and oxygen atoms are magnetized weakly after oxygen adsorption. Interestingly, adsorption on unreconstructed surface does not change surface magnetic moment. However, adsorbate leads to reduction of surface magnetic moment in reconstructed system remarkably.     

Effects of N adsorption on the structural and electronic properties of SrTiO3(0 0 1) surface

The atomic configurations, bonding characteristics, and electronic structures of the N-adsorbed (directly and substitutionally) SrTiO₃(0 0 1) surface are studied by using first-principles method based on the density functional theory. From the analysis of the energetics and density of states, it is found that the stability of the directly adsorbed N depends on the relative position of N atom to the surface. To better understand the effects of the substitutionally adsorbed N on the surface, as an example, the behavior of Au atoms adsorbed on the N-substituted surface is discussed in detail. There is clearly a synergy effect between the substitution of N for O_s(I) and the adsorption of Au atoms on the SrTiO₃(0 0 1) surface.     

Methanol adsorption on silicon (0 0 1)

Using a first-principles pseudopotential technique, we have investigated the adsorption of CH₃OH on the Si(0 0 1) surface. We have found that, in agreement with the overall experimental picture, the most probable chemisorption path for methanol adsorption on silicon (0 0 1) is as follows: the gas phase CH₃OH adsorbs molecularly to the electrophilic surface Si atom via the oxygen atom and then dissociates into Si-OCH₃ and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. Other possible adsorption models and dissociation paths are also discussed. Our calculations also suggest that the most probable methanol coverage is 0.5 ML, i.e., one molecule per Si-Si dimer, in agreement with experimental evidences. The surface atomic and electronic structures are discussed and compared to available theoretical and experimental data. In addition, we propose that a comparison of our theoretical STM images and calculated vibrational modes for the adsorbed systems with detailed experimental investigations could possibly confirm the presented adsorption picture.     

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.     

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).     

Adsorption and dissociation of O2 on the Cu2O(1 1 1) surface: Thermochemistry, reaction barrier

The adsorption and dissociation of O₂ on the perfect and oxygen-deficient Cu₂O(1 1 1) surface have been systematically studied using periodic density functional calculations. Different kinds of possible modes of atomic O and molecular O₂ adsorbed on the Cu₂O(1 1 1) surface are identified: atomic O is found to prefer threefold 3Cu site on the perfect surface and O_vacancy site on the deficient surface, respectively. Cu_CUS is the most advantageous site with molecularly adsorbed O₂ lying flatly over singly coordinate Cu_CUS-Cu_CSA bridge on the perfect surface. O₂ adsorbed dissociatively on the deficient surface, which is the main dissociation pathway of O₂, and a small quantity of molecularly adsorbed O₂ has been obtained. Further, possible dissociation pathways of molecularly adsorbed O₂ on the Cu₂O(1 1 1) surface are explored, the reaction energies and relevant barriers show that a small quantity of molecularly adsorbed O₂ dissociation into two O atoms on the deficient surface is favorable both thermodynamically and kinetically in comparison with the dissociation of O₂ on the perfect surface. The calculated results suggest that the presence of oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of O₂ and can obviously improve the catalytic activity of Cu₂O, which is in agreement with the experimental observation.     

Adsorption and dissociation of ammonia on Au(1 1 1) surface: A density functional theory study

Periodic density functional theory (DFT) calculations using plane waves had been performed to systematically investigate the stable adsorption amine and its dehydrogenated reaction on Au(1 1 1) surface. The equilibrium configuration including on top, bridge, and hollow (fcc and hcp) sites had been determined by relaxation of the system. The adsorption both NH₃ on top site and NH₂ on bridge site is favorable on Au(1 1 1) surface, while the adsorption of NH on hollow (fcc) site is preferred. The adsorbates are adsorbed on the gold surface with the interaction between p orbital of adsorbate and the d orbital of gold atoms. The interaction between adsorbate and gold slab is more evident on the first layer than on any others. Furthermore, the dissociation reaction of NH₃ on clean gold surface, as well as on the pre-covered oxygen atom and pre-covered hydroxyl group surface had been investigated. The results show that the dehydrogenated reaction energy barrier on the pre-covered oxygen gold surface is lower. The adsorbed O can promote the dehydrogenation of amine. Additionally, OH as the product of the NH₃ dissociation reaction participates in continuous dehydrogenation reaction, and the reaction energy barrier is the lowest (22.77 kJ/mol). The results indicated that OH_ads play a key role in the dehydrogenated reaction on Au(1 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).     

Adsorption of atomic oxygen on cubic PbTiO3 and LaMnO3 (001) surfaces: A density functional theory study

We present and discuss density functional theory calculations addressing the electronic structure and energetics of isolated oxygen ad-atoms at the (001) surfaces of PbTiO₃ (PTO) and LaMnO₃ (LMO) cubic perovskites. Both AO- and BO₂-type of surface terminations are considered for each perovskite. Difference electron density analysis has been carried out for each surface to probe local electronic charge redistribution upon oxygen adsorption. Our results show that the (001) surfaces of the two perovskites behave quite differently towards oxygen adsorption. In the case of the PTO (001) surfaces, the adsorbate oxygen atom was found to form a peroxide-type molecular species along with a surface lattice oxygen atom on both PbO- and TiO₂-terminated surface facets. On the other hand, the most stable oxygen adsorption site for the LMO (001) surfaces corresponds to the one expected from a natural continuation of the perovskite lattice. Moreover, the dissociative adsorption of molecular oxygen varies from being only slightly exothermic on the PTO (001) facets to being highly exothermic on the LMO (001) facets. The AO-terminated facets, in general, showed a stronger binding to the adsorbed oxygen.     

A density-functional study on the atomic geometry and adsorption of the Cu(1 0 0) c(2 × 2)/N Surface

In this work we have performed total-energy calculations on the geometric structure and adsorption properties of Cu(1 0 0) c(2 × 2)/N surface by using the density-functional theory and the projector-augmented wave method. It is concluded that nitrogen atom was adsorbed on a FFH site with a vertical distance of 0.2 Å towards from surface Cu layer. The bond length of the shortest Cu-N bonding is calculated to be 1.83 Å. Geometry optimization calculations exclude out the possibilities of adsorbate induced reconstruction mode suggested by Driver and Woodruff and the atop structural model. The calculated workfunction for this absorbate-adsorbent system is 4.63 eV which is quite close to that of a clean Cu(1 0 0) surface. The total-energy calculations showed that the average adsorption energy per nitrogen in the case of Cu(1 0 0) c(2 × 2)-N is about 4.88 eV with respect to an isolated N atom. The absorption of nitrogen on Cu(1 0 0) surface yields the hybridization between surface Cu atoms and N, and generates the localized surface states at −1.0 eV relative to Fermi energy E_F. The stretch mode of the adsorbed nitrogen at FFH site is about 30.8 meV. The present study provides a strong criterion to account for the local surface geometry in Cu(1 0 0) c(2 × 2)/N surface.     

The adsorption of and reaction of NO2 on Ag(1 1 1)-p(4 × 4)-O and formation of surface nitrate

The adsorption and reaction of nitrogen dioxide on the Ag(1 1 1)-p(4 × 4)-O surface has been investigated with RAIRS, TPRS and STM. At 300 K NO₂ initially reacts with the oxygen overlayer to form nitrate in p(3 × 3) and p(4 × 4) structures, which convert to a new p(3 × 3) at saturation coverage. Surface pitting during nitrate adsorption is suggestive of the incorporation of silver atoms into the NO₃ structure. With heating NO₃ decomposes into NO₂ and O at 396 K and 497 K, and oxygen desorbs at 578 K.     

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.