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1.
The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H 2O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below EF (6.8, 9.4 and 12.7 eV below EF). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H 2O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level ( ΔEmax = 1.5 eVat 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H 2O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below EF (He(II)), being interpreted as due to hydroxyl species (OH) δ? on the surface. A kinetic model for the H 2O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H 2O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH) δ? recombines and desorbs as H 2O at ? 320 K, leaving behind an oxygen covered Ni surface. 相似文献
2.
The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H 2O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm ?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H 2O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H 2O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H 216O (a) and 18O (a). 相似文献
3.
The UHV cleaved (110) face has been exposed to water in the range from 10 L to 2 × 10 4 L. The main TDS peak in H 2O desorption appears at 350 K, independent of coverage. The low desorption energy of 0.7 eV (16 kcal/mol) is reasonable for oxygen atoms bound via the lone pair orbital to As as was earlier derived from UPS measurements. A broad spur between 450 and 600 K may be related to O-Ga bonds. The sticking probability shows values below 10 -4; only near 4.8 × 10 3 L (6 × 10 15 cm -2 s -1 H 2O molecules for 300 s) corresponding to a coverage of about 0.4 monolayes a steep maximum appears. At about one monolayer saturation is observed. Exposures to more than 10 4 L of water quench the intensity of the (10) LEED spot considerably stronger than the intensity of the (11) spot. A comparison of the I( E) curves with existing model calculations suggests that the observed behaviour of the LEED spots is caused by a change in surface structure towards the unrelaxed configuration. The higher sticking coefficient observed near 0.4 monolayers may be connected with this rearrangement of surface atoms. 相似文献
4.
The reaction of CO 2 and H 2O to form bicarbonate (HCO −3) was examined on the nearly perfect and vacuum annealed surfaces of TiO 2(110) with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS) and high resolution electron energy loss spectrometry (HREELS). The vacuum annealed TiO 2(110) surface possesses oxygen vacancy sites that are manifested in electronic EELS by a loss feature at 0.75 V. These oxygen vacancy sites bind CO 2 only slightly more strongly (TPD peak at 166 K) than do the five-coordinated Ti 4+ sites (TPD peak at 137 K) typical of the nearly perfect TiO 2(110) surface. Vibrational HREELS indicates that CO 2 is linearly bound at the latter sites with a νa(OCO) frequency similar to the gas phase value. In contrast, oxygen vacancies dissociate H 2O to bridging OH groups which recombine to liberate H 2O in TPD at 490 K. No evidence for a reaction between CO 2 and H 2O is detected on the nearly perfect surface. In sequentially dosed experiments on the vacuum annealed surface at 110 K, CO 2 adsorption is blocked by the presence of preadsorbed H 2O, adsorbed CO 2 is displaced by postdosed H 2O, and there is little or no evidence for bicarbonate formation in either case. However, when CO 2 and H 2O are simultaneously dosed, a new CO 2 TPD state is observed at 213 K, and the 166 K state associated with CO 2 at the vacancies is absent. SSIMS was used to tentatively assign the 213 K CO 2 TPD state to a bicarbonate species. The 213 K CO 2 TPD state is not formed if the vacancy sites are filled with OH groups prior to simultaneous CO 2+H 2O exposure. Sticking coefficient measurements suggest that CO 2 adsorption at 110 K is precursor-mediated, as is known to be the case for H 2O adsorption on TiO 2(110). A model explaining the circumstances under which the proposed bicarbonate species is formed involves the surface catalyzed conversion of a precursor-bound H 2O–CO 2 van der Waals complex to carbonic acid, which then reacts at unoccupied oxygen vacancies to generate bicarbonate, but falls apart to CO 2 and H 2O in the absence of these sites. This model is consistent with the conditions under which bicarbonate is formed on powdered TiO 2, and is similar to the mechanism by which water catalyzes carbonic acid formation in aqueous solution. 相似文献
5.
Adsorption of water at 100 K. on clean and oxygen-covered Cu(110) has been studied using UPS, TDS, Δφ and LEED measurements. The results indicate that two-dimensional hydrogenbonded islands are formed on the clean surface. The long-range order in these islands is in registry with the substrate lattice and gives rise to a c(2×2) LEED pattern. Upon the formation of multilayer ice, the ordering disappears. The presence of oxygen on the surface disrupts the hydrogen bonding, and composite oxygen-water layers are formed. A model of the arrangement of oxygen atoms and water molecules is presented, based upon the LEED observations for these layers and an estimate of the relative oxygen and water coverages. The intensity variation of a thermal desorption peak at 290 K, attributed to adsorbed OH species, with oxygen coverage is in accordance with this model. For low oxygen coverages, the TDS and Δφ results indicate that small oxygen-water clusters with an enhanced ratio of water molecules per adsorbed oxygen atom are present. 相似文献
6.
New Inelastic Neutron Scattering findings on homologues disaccharides (C 12H 22O 11)/H 2O mixtures are presented. The comparison among the spectra of trehalose, maltose and sucrose/H 2O mixtures, besides evidencing a different destructuring effectiveness on the H 2O hydrogen bond network, and hence different cryoprotectant properties, shows a higher ‘crystallinity’ degree for the trehalose/H 2O system which accounts for its higher ‘rigidity’. This result justifies the better cryptobiotic action of trehalose in respect to maltose and sucrose. 相似文献
7.
The adsorption and reaction of H 2O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature ( T<150 K), a multilayer adsorption of H 2O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H 2O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H 2O into O ads and H 2. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state. 相似文献
8.
A study of the adsorption/desorption behavior of CO, H 2O, CO 2 and H 2 on Ni(110)(4 × 5)-C and Ni(110)-graphite was made in order to assess the importance of desorption as a rate-limiting step for the decomposition of formic acid and to identify available reaction channels for the decomposition. The carbide surface adsorbed CO and H 2O in amounts comparable to the clean surface, whereas this surface, unlike clean Ni(110), did not appreciably adsorb H 2. The binding energy of CO on the carbide was coverage sensitive, decreasing from 21 to 12 as the CO coverage approached 1.1 × 10 15 molecules cm ?2 at 200K. The initial sticking probability and maximum coverage of CO on the carbide surface were close to that observed for clean Ni(110). The amount of H 2, CO, CO 2 and H 2O adsorbed on the graphitized surface was insignificant relative to the clean surface. The kinetics of adsorption/desorption of the states observed are discussed. 相似文献
9.
Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H 2 and D 2 on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s0, irrespective of surface temperature Ts and mean kinetic energy normal to the surface 〈 E⊥ 〉. The desorbing molecules exhibit a cos θ e angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s0 is only about 0.05 if both the gas and the surface are at room temperature. s0 is again independent of Ts, but increases continuously with 〈 E⊥ 〉 up to a value of ~0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos 5θ e angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages. 相似文献
10.
The reactions of H 2S with predosed surface oxygen on Ni(110) surfaces were studied for a variety of coverage conditions. The primary reaction product is H 2O, but the details of the water formation and desorption depends on the coverage of both O and H 2S. For high coverages of oxygen (p(2 × 1)−O; 0.5 ML), the reaction to form water is quantitative. The loss of oxygen from the surface (as measured by AES) is equal to the increase in sulfur coverage. XPS and HREELS measurements indicate the presence of chemisorbed H2O immediately following large exposures of H2S on the oxygen predosed surface at 110 K. Deuterium incorporation results suggest that the primary mechanism for these coverage conditions involves direct transfer of hydrogen from SH or H2S moieties to the oxygen. A second mechanism involving reaction of surface hydroxyl groups with surface hydrogen was also identified. This mechanism is particularly important for high coverages of oxygen (0.5 ML) and low coverages of H2S (0.15 ML), where water desorption was observed at 235 K, but was not observed spectroscopically at 110 K. The sequential addition of two surface hydrogen atoms to surface oxygen is not an important mechanism in this system. These reactions were modeled using a bond-order conservation method, and the model successfully reproduced the important mechanistic conclusions. 相似文献
11.
First-principles calculations based on density functional theory and the pseudopotential method have been used to investigate the energetics of H 2O adsorption on the (110) surface of TiO 2 and SnO 2. Full relaxation of all atomic positions is performed on slab systems with periodic boundary conditions, and cases of full and half coverage are studied. Both molecular and dissociative (H 2O→OH −+H −) adsorption are treated, and allowance is made for relaxation of the adsorbed species to unsymmetrica configurations. It is found that for both TiO 2 and SnO 2 an unsymmetrical dissociated configuration is the most stable. The symmetrical molecularly adsorbed configuration is unstable with respect to lowering of symmetry, and is separated from the fully dissociated configuration by at most a very small energy barrier. The calculated dissociative adsorption energies for TiO 2 and SnO 2 are in reasonable agreement with the results of thermal desorption experiments. Calculated total and local electronic densities of states for dissociatively and molecularly adsorbed configurations are presented, and their relation with experimental UPS spectra is discussed. 相似文献
13.
Adiabatic potential energy surfaces (PESs) for three major isotopologues of water, H 216O, H 217O, and H 218O, are constructed by fitting to observed vibration-rotation energy levels of the system using the nuclear motion program DVR3D employing an exact kinetic energy operator. Extensive tests show that the mass-dependent ab initio surfaces due to Polyansky et al. [O.L. Polyansky, A.G. Császár, S.V. Shirin, N.F. Zobov, P. Barletta, J. Tennyson, D.W. Schwenke, P.J. Knowles, Science 299 (2003) 539-542.] provide an excellent starting point for the fits. The refinements are performed using a mass-independent morphing function, which smoothly distorts the original adiabatic ab initio PESs. The best overall fit is based on 1788 experimental energy levels with the rotational quantum number J = 0, 2, and 5. It reproduces these levels with a standard deviation of 0.079 cm −1 and gives, when explicit allowance is made for nonadiabatic rotational effects, excellent predictions for levels up to J = 40. Theoretical linelists for all three isotopologues of water involved in the PES construction were calculated up to 26 000 cm −1 with energy levels up to J = 10. These linelists should make an excellent starting point for spectroscopic modelling and analysis. 相似文献
14.
Electron energy-loss spectroscopy has been applied to the study of Si(111) surfaces covered with H 2S, H 2O and O 2 at room temperature and the surfaces annealed at ~ 600°C. The experimental results strongly suggest that H 2S and H 2O adsorb in the molecular states at room temperature. It is proposed that O 2 is first adsorbed in a molecular state, then adsorbs as atoms, and finally oxidizes forming SiO 2. 相似文献
15.
The electron-stimulated desorption (ESD) of D − and H − ions from condensed D 2O and H 2O films is investigated. Three low-energy peaks are observed in the ESD anion yield, which are identified as arising from excitation of 2B 1, 2A 1 and 2B 2 dissociative electron attachment (DEA) resonances. Additional structure is observed between 18 and 32 eV, which may be due to ion pair formation or to DEA resonances involving the 2a 1 orbital. The ion yield resulting from excitation of the 2B 1 resonance increases as the film is heated. We attribute the increase in the ion yield to thermally induced hydrogen bond breaking near the surface, which enhances the lifetimes of the excited states that lead to desorption. 相似文献
16.
The adsorption and condensation of H 2O(D 2O) on ZnO(101̄0), (0001)Zn and (0001̄)O surfaces was investigated by means of thermal desorption (TDS) and UV photoelectron spectroscopy (UPS). The clean ZnO single-crystal surfaces were prepared by Ar-ion sputtering and annealing and characterised by Auger electron spectroscopy, LEED, UPS and work-function measurements. On all three surfaces six different adsorption states were found. In the monolayer regime there is a stronger bonding to Zn sites (desorption temperature 340 K) than to O sites (190 K), The bonding to the Zn sites seems to be accompanied by some clustering. Before the chemisorption layer is completed a first ice state is found whose desorption temperature shifts from 162 to 168 K with increasing exposures. At higher exposures the multilayer ice state is found at 152 K. On the (0001̄)O face defect-induced features were identified. The water lone-pair orbital 1b 1, whose energy falls between the O p and the Zn 3d emission of the substrate and which is known to show bonding shifts, was analysed using angle-resolved UPS. In the monolayer, the main chemisorption states are found at EBV(1b 1) = ?9.6 eV for the (0001)Zn face and at ? 10.6 eV for the (0001̄)O face and are compared with the multilayer ice emission at 1̄1.1 eV. The difference in binding energies shows the same trend as the TDS data. For the (101̄0) face the 1b 1 emission is very broad, indicating some overlap between different states. 相似文献
17.
We have investigated the bonding of the monomers pyromellitic dianhydride (PMDA) and oxydianiline (ODA) and their subsequent polymerization to form polyimide on Ag(110) in ultrahigh vacuum. Unenhanced surface Raman spectroscopy was used to identify adsorbed species and to follow the polymerization reaction. ODA was unreactive on the cold (140 K) surface; spectral features were identical to those in the condensed multilayer indicating that the monolayer is physisorbed. PMDA, on the other hand, adsorbed dissociatively, the evidence being consistent with a bidentate surface carboxylate mode of bonding. Finally, heating a 30 Å film of codosed monomers to 473 K resulted in the formation of a polyimide film as shown by characteristic features in the carbonyl stretching region of the Raman spectrum. 相似文献
18.
The very low pressure adsorption kinetics of H 2S on the clean and oxygen covered Cu(110) face have been examined by Auger Electron Spectroscopy (AES) and Mirror Electron Microscopy (MEM, used for continuous surface potential variations of the copper surface). The AES experimental curves on the clean copper face have been interpreted using a model of island growth by surface diffusion. The presence of an adsorbed oxygen layer on the copper surface changes notably the induction times observed on both AES and MEM measurements. 相似文献
19.
We have measured Ru 3d, 4s, 4p and O 1s high-resolution core-level X-ray photoelectron spectra, along with Ru 3d and O 1s scanned-angle X-ray photoelectron diffraction angular distributions, for RuO 2(110). The surfaces were prepared by oxygen-plasma-assisted molecular beam epitaxial growth of RuO 2 on TiO 2(110). XPS spectral interpretation and the nature of the XPD scans strongly suggest that the complex line shapes are due to final-state screening effects, rather than the presence of Ru in oxidation states other than +4. 相似文献
20.
Adsorbed H 2S decomposes on Ni(110) to form primarily surface S and H for coverages of less than 0.5 ML. The hydrogen evolves in two separate TPD peaks, characteristic of hydrogen recombination and desorption from the clean surface and from regions perturbed by chemisorbed sulfur. XPS and HREELS indicate the presence of SH and possibly H 2S groups on the surface at 110 K. The XPS data indicates that for coverages less than about 0.5 ML, the concentration of molecular H 2S is small, but it is difficult to asess the coverage of SH groups. However, all of the molecular species decompose prior to hydrogen desorption (for high coverage, 180 K). Physisorbed H 2S is observed on the surface for coverages greater than about 0.5 ML. The sulfur Auger lineshape was observed to be a function of both coverage and temperature. The changes in the lineshape were attributed to perturbations in local bonding interactions between the S and the Ni surface, perhaps involving some change in either bonding sites or distances but not involving SH bond scission. The decomposition reaction was modeled using a bond order conservation method which successfully reproduced the experimental results. 相似文献
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