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1.
The interaction of oxygen with polycrystalline niobium has been studied using both Auger electron spectroscopy and low-energy secondary ion mass spectrometry in the temperature range from 300–1250 K. At higher temperatures there is oxygen dissolution into the bulk but a preferential surface segregation on recooling. Between 300 and 1250 K, there is a rapid initial adsorption into a very stable state which is associated with increases in the Nb+ and NbO+ yields that are linear with coverage. At 1250 K, further changes are very slow. At 900 K, the initial stage is followed by the adsorption with a lower sticking coefficient (<0.1) as coverage increases from θ = 0.5 to 0.7. This produces an additional larger increase in the yield of Nb+ but a much smaller change in NbO+. At 300 K, the sticking probability falls more slowly with coverage above θ = 0.5 and the amount of oxygen continues to increase slowly with exposure. The SIMS spectrum shows dramatic increases in Nb+, NbO+ and NbO+2 yields and the successive appearance of small yields of ions such as Nb2O+2 and Nb2O+3 as oxide formation begins. The Nb+ yield slowly decreases as further oxidation occurs. Each stage of oxidation has a characteristic secondary ion mass spectrum.  相似文献   

2.
Adsorption isotherms have been measured at 77.5 K for nitric oxide and nitrogen on Al2O3, MgO, ZnO and NiO, and at 90.2 K. for nitric oxide on A12O3 and NiO. Three isotherm measurements at 77.5 K were made on the Al2O3 sample for each adsorbate to examine the effect of different degrees of surface dehydroxylation. The latter was assessed by means of infrared absorption studies on an Al2O3 disc. Isosteric heats for NO adsorption on Al2O3 and NiO increase from ca. 8 kJ mol?1 and 6 kJ mol?1 (respectively) at half monolayer coverage to near the value of the enthalpy of sublimation (16.6 kJ mol?1) at monolayer completion. These results are discussed in terms of adsorbate dimerisation. Anomalous adsorption-desorption behaviour for the NONiO system is discussed. Effective adsorbate molecular cross-sectional areas and results for N2 adsorption on preadsorbed NO do not support the existence of either localisation or micro-porosity.  相似文献   

3.
O2 exposure of polycrystalline nickel at 300 K results in characteristics changes of secondary ion emission. These can be described by a model which is in good agreement with corresponding LEED, AES, XPS, and ΔΦ results of other authors. According to this model, oxygen can be bonded on Ni in at least five different phases:
  1. chemisorption, indicated by a rapid increase of Ni+, Ni 2 + , and Ni2O+ (≦5 L);
  2. a rearranged chemisorption layer, characterized by a drastic decrease of Ni+, Ni 2 + , and Ni2O+ (5–15 L);
  3. nickel oxide (NiO) responsible for a strong NiO?- and NiO 2 ? -emission (≦40 L);
  4. oxygen on top of this NiO layer, producing a final increase of Ni+ and NiO+ and a O2-flash signal at 400 K (>40 L);
  5. bulk dissolved oxygen in thermal equilibrium with a chemisorption layer (after several exposure/heating cycles).
During ion bombardment of a 100 L O2 exposed Ni surface these different binding states occur in a reversed order of succession. O2-flash signals at 400 and 1100 K, related to drastic changes in secondary ion emission at 400, 700, and 1100 K, reflect the disappearance of various oxygen binding states. The exchange between different oxygen phases was studied by16O2/18O2 isotope experiments.  相似文献   

4.
《Solid State Ionics》2006,177(17-18):1421-1428
Spinel lithium manganese oxides can be used as Li+ adsorbent with topotactical extraction of lithium. In this paper, the solid state methods were introduced to prepare spinel lithium manganese precursors with Li2CO3 and LiOH·H2O as different Li sources. The Li+ uptake was studied to clarify the correction between Li+ adsorption capacity and the preparation conditions of precursors, including different Li sources, Li/Mn mole ratios and heating time. The results indicated that the Li+-extracted materials prepared with LiOH·H2O and MnCO3 usually have higher Li+ adsorption capacity than Li2CO3 and MnCO3, and an ascending trend was found in Li+ uptake with increasing Li/Mn mole ratio in the preparation of the precursor, but it is not proportional. The Mn2O3 impurities could be the primary reason for decreasing Li+ adsorption capacity. Furthermore, it is concluded that the Li+-extracted materials obtained from spinel manganese oxides synthesized with Li/Mn = 1.0 can serve as selective Li+ absorbents due to its high selectivity and large adsorption capacity.  相似文献   

5.
We have studied the surface chemistry of the nickel-oxygen system using both temperature changes and ion bombardment as techniques for elucidating the surface structure. The spectra of metallic Ni, NiO and Ni2O3 were characterized from samples prepared directly in the spectrometer. The Ni2O3 species could be distinguished from an authentic Ni(OH)2 sample from both the X-ray photoelectron lines and the Auger transitions. The oxides of NiO and Ni2O3 could be prepared by bombardment with low energy (400eV) O2+ ions as well as by exposure of Ni to oxygen at reduced pressure (~ 100 torr). The Ni2O3 was found to be present on most nickel-oxygen surfaces except those prepared by exposing Ni to air for many hours at high temperature (> 600°C), indicating that the stability of Ni2O3 decreased as the temperature increased. Exposure of both NiO and Ni2O3 to 400 eV Ar+ ion bombardment caused reduction to metallic Ni. This observation has also been noted for several other oxides and a prediction of whether or not reduction should be observed is presented by examining the free energy of formation of the molecule.  相似文献   

6.
《Applied Surface Science》1986,25(3):333-340
Electron stimulated desorption of ionic species from CO adsorbed on Pt(111) has been studied and comparison made with EELS results. The “on-top” site which, according to EELS data, fills first is observed to yield O+ ion. The bridge adsorption site appears to release CO+ during electron bombardment. Coadsorption of H2 and CO was also examined and compared with the polycrystalline platinum case. Only very weak coadsorption effects are seen on the Pt(111) surface, as evidenced by presence of a weak low energy component associated with the O+ ESD energy distribution.  相似文献   

7.
P.H Dawson 《Surface science》1978,71(2):247-266
The adsorption of CO on polycrystalline molybdenum was studied using a combination of secondary ion mass spectrometry at low primary energies and electron induced desorption. Auger electron spectroscopy was used as an auxiliary technique. By examining ion yields during adsorption and desorption at various temperatures, it was possible to identify several different states of adsorption in agreement with earlier flash desorption studies. There are two strongly bound states which fill simultaneously, followed by the occupation of a third state. Continued exposure to CO results in the formation of weakly held states. Electron induced desorption gives O+ from one of these states but CO+ from the most weakly bound state. A characteristic of the latter is the production of MoCO+ on ion bombardment. The different states of adsorption have quite different influences on the secondary ion yields. The nature of the adsorption states and the significance of SIMS in characterizing them is discussed.  相似文献   

8.
We present a study based on first-principles calculations of the adsorption of CO on selected equilibrium configurations of MAunO2 + (M = Ti, Fe; n = 1, 6, 7) complexes resulting from the adsorption of O2 on doped cationic gold clusters MAun +. Empirical rules for the formation of CO2-MAunO+ complexes are outlined. The desorption energy of CO2 is calculated. The adsorption of a second CO molecule on the residual MAunO+ complex leads in some cases to the formation of CO2-MAun +. The desorption of a second CO2 molecule brings back to the initial doped gold cluster MAun +.  相似文献   

9.
Mott-Hubbard anti-ferromagnetic insulator, NiO shows p-type semiconducting behaviour due to vacancy at Ni2+ site in its bunsenite structure. We report the modification of structural and magnetic order in NiO on Fe doping. NiO samples at different Fe concentrations in the range 0 to 5 at.% have been prepared by chemical co-precipitation and post thermal decomposition method. Both structural and magnetic characterization reveal that with increasing Fe doping concentration, NiO evolves as a magnetically inhomogeneous state out of the parent homogeneous antiferromagnetic state. In addition, structural inhomogeneity was also observed with Fe precipitating to γ-Fe2O3 phase, the signature of which could be clearly seen for Fe content beyond 2 at.%. At lower Fe content however, some amount of Fe occupies lattice and interstitial sites in the NiO matrix and drive the latter to acquire ferromagnetic ordering, which was evident from a clear hysteresis loop at 300 K.   相似文献   

10.
A structural study of the different stages during NiO(100) sulphurization by H2S was carried out by RHEED, LEED and AES. On exposure to H2S (PH2S < 10?5 Torr) The “clean” surface, obtained by UHV cleavage, was found to react with H2S to produce islands of Ni(100) covered with an ordered c(2 × 2) S structure up to 300°C. Growth of Ni3S2 islands occurs on increasing the temperature and the exposure to H2S.  相似文献   

11.
The adsorption of CO on the (110) plane of tungsten has been studied using electron impact desorption, thermal desorption, and work function measurements in a single apparatus combining these various techniques. It is concluded that a single molecular adsorption state exists at 20–250 K (virgin-CO). At 300–400 K, 60% of the low temperature layer desorbs, the remainder converting principally to a beta-1 state, which has very small electron impact cross section; in addition to beta-1 an O+ yielding state, which we call beta-precursor is formed. The beta-1 state is stable to 900 K, where some desorption and conversion of the remaineder to a beta-2 state occurs. The O+ yielding state decays with increasing T and is gone at 800 K. Readsorption on beta-1 leads to two types of adsorption states called alpha and gamma, which seem to be site specific. Electron impact desorption yields mostly CO+ and CO for virgin, O+ for beta-precursor, and CO+ and CO for the readsorption states. There is no isotopic mixing in virgin or in readsorbed CO, nor does readsorbed CO exchange with beta-1 or beta precursor. There is complete isotopic mixing in beta desorption. In addition, massive EID creates another state, characterized by a large dipole moment, also yielding O+ in EID. This state can be converted to beta-1 by heating to 400 K. The total disappearance cross sections for the various states are virgin-CO5 × 10?17cm2; γ-CO 1.6 × 10?16cm2; α-CO 5 × 10?17cm2; β-precursor 6 × 10?18cm2and 1.2 × 10?19cm2; EID induced state 8 × 10?18cm2. In addition, cross sections for ion production are determined and found to be several orders of magnitude less than total disappearance cross sections. These results, and Leed and coverage data obtained in parallel investigations are used to formulate models of the various adsorption states. It is concluded that virgin and readsorbed CO are molecular and beta-precursor and beta dissociated, although strong interactions between C and O remain. The electron impact desorption of physisorbed CO was investigated and found to yield C+, O+, and neutral CO, but very little CO+. These results suggest primary dissociation of CO by electron impact, and desorption of neutral physisorbed CO by the energetic fragments. Physisorbed CO+, although undoubtedly created, lies on the attractive part of its potential curve relative to the surface, and thus does not desorb as CO+.  相似文献   

12.
We have studied electronic excitations at the surfaces of NiO (100), Cr2O3(111), and Al2O3(111) thin films with Electron Energy Loss Spectroscopy (EELS). On NiO (100) we observe surface electronic excitations in the energy range of the band gap which shift upon adsorption of NO. Ab initio cluster calculations show that these excitations occur within the Ni ions at the oxide surface. The (111) surface of Cr2O3 is characterized by distinct excitations which are also strongly influenced by the interaction with adsorbates. Temperature-dependent measurements show that two different states of the surface exist which are separated by an activation energy of about 10 meV. For Al2O3(111) we present data for a CO adsorbate. The oxide is quite inert with respect to CO adsorption as indicated by desorption temperatures between 38 K and 67 K. Due to the weak interaction with the substrate the a3II valence excitation of CO shows a clearly detectable vibrational splitting which has not been observed previously for a CO adsorbate in the (sub)monolayer coverage range. For several different adsorption state the lifetimes of the a3II state could be estimated from the halfwidths of the loss peaks, yielding values between 10–15 s for the most strongly bound species and 10–14 s for the CO multilayer.  相似文献   

13.
Secondary ion emission from a beryllium surface is studied in the presence of hydrogen and oxygen. For submonolayer coverages of oxygen, the adsorption follows site-exclusion statistics. On this basis, the Be+ secondary ion yield and energy distribution can be separated into oxygen-dependent and oxygen-independent processes governed by the local properties of the surface. Ionic molecular species with hydrogen or oxygen present include Be2O+, Be2O?, BeO?, and BeH+ but not BeO+ or BeOH+. This result is inconsistent with emission in which the oxide molecule is formed in the near-surface vacuum region by agglomeration of individually sputtered surface atoms. For BeH+emission the opposite conclusion is reached.  相似文献   

14.
ESCA has been used to monitor alterations of catalytically and electrochemically important metal-oxygen surfaces following exposure to Ar+ and O2+ ion bombardment. This treatment resulted not only in sputtering, but also, in many cases, in reduction to the corresponding metal or lower oxide. A model based on bulk thermodynamic free energy considerations Is proposed to explain this phenomenon. We have also exploited this approach to obtain an in-depth concentration profile of various oxidation states of an element, to selectively prepare desired surface oxide compositio and to aid in interpreting complex O ls spectra. Results obtained from metal-oxygen surfaces for Ni, Ru and Mo are presented. Ni2O3 and RuO3, which are gross defect structures of the bulk species, are present on NiO and RuO2 respectively, with the former being confined to the surface layers. The MoO2, on the other hand, is covered with a surface layer of MoO3 present as a regular crystal structure.  相似文献   

15.
A mechanism for the dissociation of the ozone dication formed by double ionisation of neutral O3 is determined by ab initio calculations. The dication ground singlet state is found to have a linear equilibrium geometry that is stable with respect to dissociation to O+ and O2+. However, at the Franck-Condon zone for formation of O32+ from the neutral molecule the singlet potential energy surface intersects with a dissociative triplet state. We propose that crossing to this dissociative triplet state can account for the absence of any long-lived O32+ ions in the electron-impact mass spectrum of ozone. Further calculations of the kinetic energy release for the fragmentation of O32+ to O++O2+ indicate that considerable vibrational excitation may be present in the O2+ ion.  相似文献   

16.
研究了通过有机金属化学气相沉积技术及单源分子前躯体方法制备的Ni/Al2O3纳米复合材料的氢吸附(存储). 在冷壁的有机金属化学气相沉积反应器中,通过降解Ni(acac)2粉末基底上的[H2Al(OtBu)]2制备的Ni/Al2O3纳米复合材料. 通过X射线粉末衍射、扫描电镜、透射电镜以及能量色散型X射线荧光光谱等技术表征该复合材料. 采用自制Sievert's设备研究该复合材料的氢吸附(存储),可以储存约2.9%(重量比)的氢.  相似文献   

17.
当一束具有一定能量和强度的电子束轰击超高真空系统中残余的水汽、一氧化碳和二氧化碳时,将导致这些气体分子通过如下反应:H2O→Oad+H2,CO2→Oad+CO,CO→Oad+Cad分解并共吸于镍表面。碳和氧的原子各自占据镍(001)面部份四重吸附位置,形成结构为p(2×2)或c(2×2)的许多独立的吸附畴,电子束轰击促进畴的成核、长大、连结和有序化。当氧和碳的原子占据了镍(001)面约一半的四重吸附位后,上述吸附反应将与导致氧和碳的脱附反应:C*+Oad→CO,O*+Cad→CO平衡,氧化镍与碳化镍开始成核。由于残余含氧气体中氧的含量超过碳,氧化镍成核占优势,使碳的吸附被排斥,已吸附的碳被排挤,形成电子束斑内氧高碳低、束斑外碳高氧低的“互补”分布。电子束轰击过程中碳的俄歇峰形的变化反映着碳原子与基底原子的不同结合状态。电子束的解离效应在吸附的初始阶段起重要作用,而其热效应对氧化镍的长大起重要作用。 关键词:  相似文献   

18.
The adsorption and coadsorption of CO and H2 have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as β2, β1, and β0 were detected after adsorption at 250 K. The population of β2 and β1 states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD O+ and CO+ ions observed was discussed. Hydrogen is dissociatively adsorbed on Pd(111) and no ESD H+ signal was recorded following H2 adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H+ ESD signal, a decrease of hydrogen surface population and an arisement of a broad H2 TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO β2 state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO coverages, the “cooperative” species being responsible for the H+ ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from the surface.  相似文献   

19.
J.H. Craig 《Surface science》1983,134(3):745-754
Electron and ion induced desorption has been used to study release of ionic species from CO on nickel. O+, CO+, and O? are found to be released by electron impact. Thermal desorption spectra indicate the presence of two binding states. The O+ species is observed to be released from state 1, the state of higher binding energy, while state 2, a more loosely bound state is the source of CO+. O? species appear to be released from both states. The cluster ions NiCO+ and Ni2CO+ released by argon ion impact were observed to originate from state 2.  相似文献   

20.
We report on Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS) investigations on ErSi2 : the silicide behaviour under Ar+ bombardment (1–5 KeV) and oxygen exposure at room temperature was studied. The ion beam processed surface shows a Si enrichment, resulting in an ErSi-like stoichiometry. This result suggests a predominant role of the mass difference between Er and Si in the sputtering mechanism of ErSi2 At exposure up to 103 langmuirs, both components of the silicide react with oxygen. Firstly, Er oxide is formed, in an Er2O3-like state. After the consumption of the available Er atoms in the surface layer, SiO2 starts to grow. These results are interpreted in terms of a greater heat of formation for Er oxide than for Si oxide.  相似文献   

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