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1.
Y. Song  J.H. Dai  R. Yang 《Surface science》2012,606(9-10):852-857
We studied the adsorption behavior of oxygen on low index surfaces of γ-TiAl via first principles to investigate the mechanism that drives the adsorption behavior. The (100) surface is the most stable surface energetically followed by the (111), (110) and (001) surfaces. A study of the adsorption of a single oxygen atom on surfaces of TiAl showed that the O atom prefers the Ti-rich environment that has a high potential of generating TiO2. Competition between O-Al bonding and O-Ti bonding was observed in the O adsorbed surface regions. However, the O-Ti interaction dominates the adsorption behavior in all considered systems except when O is adsorbed on an Al-terminated (001) surface as the O–Al bond is stronger than O–Ti bond. A linear relationship between adsorption energy and integration of orbital overlaps between the O atom and the metals is obtained, which indicates that the electronic structure controls the adsorption behavior of an O atom on a γ-TiAl surface — an opportunity to improve the oxidation resistance of γ-TiAl based alloys.  相似文献   

2.
《Surface science》1986,176(3):491-504
The effect of a sodium and sodium oxide overlayer on a Pd(100) surface has been investigated by AES, LEED, XPS, UPS and TPRS methods. Ordered surface structures of (4 × 4), (2 × 2) and c(2 × 2) were observed for a sodium overlayer, which was contracted by the adsorption of oxygen. The increase of bonding energy of CO, H2 as well as CO2 was confirmed on both sodium and sodium oxide covered surfaces, and three adsorption states were distinguished for CO2 on these surfaces. For the adsorption of methanol, formaldehyde and formic acid, the interaction was also strengthened on the sodium covered surface. As a result, the concentration as well as the stability of surface methoxide and formate was significantly increased and the strong interaction between formaldehyde and surface oxygen was confirmed.  相似文献   

3.
G. Pilania  R. Ramprasad 《Surface science》2010,604(21-22):1889-1893
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.  相似文献   

4.
The adsorption of H2O on the surface of a single-crystal sphere of silver with exposed (111), (100) and (112) facets has been examined using ESDIAD (electron stimulated desorption ion angular distribution), LEED (low energy electron diffraction) and TDS (thermal desorption spectroscopy). The purpose of the study was (a) to examine the influence of substrate geometry for adsorption of H2O on a metal surface for which the adsorbate-substrate interaction is weak, and (b) to study the influence of a surface impurity, oxygen, on the surface chemistry and local bonding structure of H2O on Ag. We have found no evidence for either long-range or short-range local bonding order for adsorbed H2O at 80 K on any of the surfaces studied. This appears to be a consequence, in part, of the lattice mismatch between the Ag crystal structure and the two-dimensional H2O ice crystal structure. Adsorbed H2O reacts with preadsorbed oxygen to form OH species which are bonded with the molecular axis perpendicular to Ag(111) and (100) but “inclined” on (112) surfaces, as identified using ESDIAD. The “inclined” OH species are associated with atomic steps on the (112) surface.  相似文献   

5.
The adsorption of CO, O2, and H2O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H2O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O2 was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O2 on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.  相似文献   

6.
Adsorption of the methoxy radical on clean and on low oxygen precovered Ru(0 0 0 1) metallic surfaces has been studied by density-functional theory cluster calculations. Methoxy is predicted to be preferentially chemisorbed on both hollow sites (hcp and fcc) of such surfaces, and adopts an upright orientation (C3ν local symmetry). Such prediction is supported by the good agreement found between the calculated vibrational frequencies and the experimentally observed RAIRS spectra. Regarding the charge transfer process between the adsorbate and the surface, our results suggest that the bonding is dominantly polar covalent which arises from a charge donation from the ruthenium surface to the radical, and the co-adsorbed electronegative oxygens deplete slightly the surface electron density. However, the major difference is not induced through much O-Ru bonding, but indirectly, by lowering the valence d-band center of the system. This results in a lower adsorption energy for methoxy than on the clean Ru(0 0 0 1) surface, in accordance with experimental data. Further, accordingly to the present calculations, the radical is expected to dissociate or desorb more easily on the modified surface but with no participation from the co-adsorbed oxygen atoms.  相似文献   

7.
The adsorption of hydrogen, ethylene, acetylene, cyclohexane and benzene was studied on both the (111) and stepped [6(111) × (100)] crystal surfaces of iridium. The techniques used were low energy electron diffraction, Auger electron spectroscopy, and thermal desorption mass spectrometry. At 30°C, acetylene, ethylene and benzene are adsorbed with a sticking probability near unity. The sticking probability of cyclohexane is less than 0.1 on both surfaces. Heating the (111) surface above 800°C, in the presence of the hydrocarbons, results in the formation of an ordered carbonaceous overlayer with a diffraction pattern corresponding to a (9 × 9) surface structure. No indication for ordering of the carbonaceous residue was found on the stepped iridium surface in these experimental conditions. The hydrocarbon molecules form only poorly ordered surface structures on both iridium surfaces when the adsorption is carried out at 30°C. Benzene is the only gas that can be desorbed from the surfaces in large amounts by heating. Ethylene remains largely on the surface, only a few percent is removed by heating while acetylene and cyclohexane cannot be desorbed at all. When adsorption is carried out at 30°C and the crystal is subsequently flashed to high temperature, hydrogen is liberated from the surface. The hydrogen desorption spectra from the iridium surfaces exposed to C2H4, C2H2, or C6H6 exhibit two hydrogen desorption peaks, one around 200°C and the second around 350°C. The temperatures where these peaks appear vary slightly with the type of hydrocarbon. The relative intensities of these two peaks depend strongly on the surface used. Arguments are presented that decomposition of the hydrocarbon molecules (C-H bond breaking nd possibly also C-C bond breaking) occurs easier on the stepped iridium surface than on the (111) surface. Hydrogen is desorbed at a higher temperature from an iridium surface possessing a high density of surface imperfections than from a perfect iridium (111) surface. The results are compared with those obtained previously on similar crystal surfaces of platinum. It appears that C-H bond breaking occurs more easily on iridium than on platinum.  相似文献   

8.
Density functional theory is used to study surface atomic distributions on slabs of PtCo and Pt3Co overall compositions, as well as water molecule adsorption on PtCo(1 1 1) and Pt-skin structures. Pt-rich surfaces are energetically favored under vacuum in the PtCo and Pt3Co alloys. The adsorption trend on the studied structures agrees with the d-band model, with stronger adsorption at higher surface Co composition. The most stable adsorption site for a water molecule on PtCo surfaces is on top of Co atoms, with the dipole vector parallel to the surface. This water/surface interaction is as strong as that of water molecule on Pt(1 1 1), whereas bonding to Pt-skin monolayers is found much weaker than that on Pt(1 1 1). It is found that water interacts mainly through its 1b1 and 3a1 orbitals with d orbitals of the Pt(1 1 1), PtCo(1 1 1) and Pt-skin surface atoms. Compared to the sum of the electron densities of the separated systems, the electron density of the water/surface gets depleted along O-Pt on Pt-skin surfaces while it becomes richer in the O-Co bonding region of PtCo.  相似文献   

9.
Masoud Nahali 《Molecular physics》2013,111(13):1437-1445
Density functional theory is used in a spin-polarized plane wave pseudopotential implementation to investigate molecular oxygen adsorption and dissociation on graphite and nickel-doped graphite surfaces. Molecular oxygen physisorbs on graphite surface retaining its magnetic property. The calculated adsorption energy is consistent with the experimental value of ?0.1?eV. It is found that substituting a carbon atom of the graphite surface by a single doping nickel atom (2.8% content) makes the surface active for oxygen chemisorption. It is found that the molecular oxygen never adsorbs on doping nickel atom while it adsorbs and dissociates spontaneously into atomic oxygens on the carbon atoms which are bound to the nickel. The adsorption energy of ?1.4?eV and zero activation energy barrier indicate that O2 dissociative adsorption is both thermodynamically and kinetically favoured over the surface. The large electric field near the doping nickel atom along with the excess electrons on the neighbouring carbon atoms, which are bound to the nickel induce molecular oxygen to adsorb and dissociate favourably.  相似文献   

10.
2 and CO on Ni(100), benzene on Ni(100) and Cu(110), and glycine adsorbed on Cu(110). New types of molecular states are observed which are directly related to the surface chemical bond. The long-accepted Blyholder model which is based on a frontier orbital concept cannot explain our results for N2 and CO chemisorption. We find it necessary to offer a new picture where changes in the whole molecular orbital framework have to be considered. We show that both π and σ type interactions are important in describing the bonding in benzene to metal surfaces. The future prospect is illustrated by the adsorption of the simplest amino acid, glycine, on Cu(110). The adsorbate has four different atomic centers where X-ray emission spectra are obtained, providing a unique view of the local electronic structure. Accepted: 6 March 1997  相似文献   

11.
Electron energy loss spectroscopy (ELS) with primary energies e0 ? 80 eV has been performed on ultrahigh vacuum (UHV) cleaved nonpolar (11?00) and polar zinc (0001) and oxygen (0001?) surfaces of ZnO to study the adsorption of oxygen and carbon monoxide. Except for CO on the nonpolar surface where no spectral changes in ELS are observed a surface transition near 11.5 eV is strongly affected at 300 K on all surfaces by CO and O2. At 300 K clear evidence for new adsorbate characteristic transitions is found for oxygen adsorbed on the Zn polar surface near 7 and 11 eV. At 100 K on all three surfaces both CO and O2 adsorb in thick layers and produce loss spectra very similar to the gas phase, thus indicating a physisorbed state.  相似文献   

12.
《Current Applied Physics》2015,15(11):1303-1311
Spin-polarized density functional theory calculations were performed to investigate the magnetism of bulk and Cu2O surfaces. It is found that bulk Cu2O, Cu/O-terminated Cu2O(111) and (110) surfaces have no magnetic moment, while, the O-terminated Cu2O(100) and polar O-terminated Cu2O(111) surfaces have magnetism. For low index surfaces with cation and anion vacancy, we only found that the Cu vacancy on the Cu2O(110) Cu/O-terminated surface can induce magnetism. For atomic and molecular oxygen adsorption on the low index surfaces, we found that atomic and molecular oxygen adsorption on the Cu-terminated Cu2O(110) surface is much stronger than on the Cu/O-terminated Cu2O(111) and Cu-terminated Cu2O(100) surfaces. More interesting, O and O2 adsorption on the surface of Cu/O terminated Cu2O(111) and O2 adsorption on the Cu-terminated Cu2O(110) surface can induce weak ferromagnetism. In addition, we analysis origin of Cu2O surfaces with magnetism from density of state, the surface ferromagnetism possibly due to the increased 2p–3d hybridization of surface Cu and O atoms. This is radically different from other systems previously known to exhibit point defect ferromagnetism, warranting a closer look at the phenomenon.  相似文献   

13.
J.H. Dai  Y. Song  R. Yang 《Surface science》2011,605(13-14):1224-1229
First principle calculations have been performed to explore the adsorption characteristics of water molecule on (001) and (110) surfaces of magnesium hydride. The stable adsorption configurations of water molecule on the surfaces of MgH2 were identified by comparing the total energies of different adsorption states. The (110) surface shows a higher reactivity with H2O molecule owing to the larger adsorption energy than the (001) surface, and the adsorption mechanisms of water molecule on the two surfaces were clarified from electronic structures. For both (001) and (110) surface adsorptions, the O p orbitals overlapped with the Mg s and p orbitals leading to interactions between O and Mg atoms and weakening the O–H bonds in water molecule. Due to the difference of the bonding strength between O and Mg atoms in the (001) and (110) surfaces, the adsorption energies and configurations of water molecule on the two surfaces are significantly different.  相似文献   

14.
We report on the interaction of benzene with zirconium (0001) surfaces. Following adsorption at 150 or 170 K benzene desorbs near 715 K at exposures above one Langmuir. The high desorption temperature of benzene is indicative of the complicated kinetics that zirconium surfaces exhibit. For lower exposures benzene dissociates during heating and an increase of the oxygen content at the surface is detected. We propose that hydrogen from the dissociated layer(s) attracts subsurface oxygen and that an exchange of adsorbed carbon with this oxygen takes place.  相似文献   

15.
A. Spitzer  H. Lüth 《Surface science》1982,120(2):376-388
The water adsorption on clean and oxygen precovered Cu(110) surfaces is studied by means of UPS, LEED, work function measurements and ELS. At 90 K on the clean surface molecular water adsorption is indicated by UPS. The H2O molecules are bonded at the oxygen end and the H-O-H angle is increased as compared with the free molecule. In the temperature range between 90 and 300 K distorted H2O molecules and adsorbed hydroxyl species (OH) are detected, which are desorbed at room temperature. On an oxygen covered surface hydroxyl groups are formed by dissociation of adsorbed water molecules at a lower temperature than on the clean surface. Multilayers of condensed water are found below 140 K in both cases.  相似文献   

16.
The adsorption of oxygen at exposures of up to 105 L on differently oriented, ion-bombarded and annealed GaAs surfaces was investigated by UPS. Coverages θAs for the clean surfaces and oxygen coverages for the oxygen exposed surfaces were estimated by additional SXPS measurements. It was concluded that at small exposures molecular and atomic adsorption are comparable in quantity and that atomic adsorption (oxidation) becomes maximum at θAs, ≈ 0.2 for (111)Ga and (001) surfaces. Bonding of oxygen molecules should involve Ga sites. Specific bonding of oxygen atoms (O-Ga or O-As) was not indicated by the two stable UPS peaks as they occurred for arsenic coverages from 0 to 0.5 and did not shift their energy positions. They simply indicate the two states of the adsorbate atoms, single and double bonds, to substrate atoms. For the surfaces prepared here, monolayer coverage by oxygen was obtained at about 1010L. Likewise adsorption of H2O was investigated.  相似文献   

17.
胡明  王巍丹  曾晶  秦玉香 《中国物理 B》2011,20(10):102101-102101
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 W6c and substitution of Ti for W5c. 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 O1c 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.  相似文献   

18.
王芒芒  宁华  陶向明  谭明秋 《物理学报》2011,60(4):47301-047301
用密度泛函理论(DFT)研究了金属Au(110)表面结构以及氧原子的吸附状态.计算得到Au(110)-(1×2)缺列再构表面原子的弛豫分别是-15.0%(Δd12/d0)和-1.1%(Δd23/d0),表面能为52.7 meV/2,功函数Φ=5.00 eV;Au(110)-(1×3)缺列再构表面的Δd1 关键词: 缺列再构Au(110)表面 STM图像 氧原子吸附  相似文献   

19.
Sergio R. Calvo 《Surface science》2007,601(21):4786-4792
The reactivity of Pt-Pd alloy surfaces towards the oxygen reduction reaction is studied as a function of the alloy overall composition and surface atomic distribution and compared to that on pure Pt surfaces. The systems include Pd and Pt monolayers on various substrates and Pt3Pd, PtPd and PtPd3 surfaces of ordered alloys. Reactivity is evaluated on the basis of the adsorption strength of oxygenated compounds which are intermediate species of the four-electron oxygen reduction reaction, separating the effect of the first electron-proton transfer from that of the three last electron-proton transfer steps. None of the alloys are found to provide better sites than those of pure Pt both for O2 dissociation and for the reduction of O and OH to water; with the skin surfaces being the closest to pure Pt. The results are discussed in relation to those found in 10-atom clusters of similar compositions and to experiments.  相似文献   

20.
Near-edge x-ray absorption fine structure (NEXAFS) and surface extended x-ray absorption fine structure (SEXAFS) spectroscopies and their application to the determination of the adsorption geometry and bonding of low-Z molecules on surfaces are discussed. NEXAFS is characterized by intramolecular resonances and probes the internal structure of the molecule (intramolecular bond lengths and possibly bond angles) as well as its orientation relative to the surface. SEXAFS provides information about the adsorption site and the molecule-substrate distances. After demonstrating the full power of SEXAFS in the analysis of oxygen adsorption on Cu(110) and on Ag(110) an example is given of a complete structure determination for the formate species (HCO2) on Cu(110) using NEXAFS and SEXAFS.  相似文献   

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