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1.
The adsorption and reaction of acetonitrile (CH 3CN) on clean and oxygen covered Ag(110) surfaces has been studied using temperature programmed reaction spectroscopy (TPRS), isotope exchange, chemical displacement reactions and high resolution electron energy loss spectroscopy (EELS). On the clean Ag(110) surface, CH 3CN was reversibly adsorbed, desorbing with an activation energy of 10 kcal mol -1 at 166 K from a monolayer state and at 158 K from a multilayer state. Vibrational spectra of multilayer, monolayer and sub-monolayer CH 3CN were in excellent agreement with that of gas phase CH 3CN indicating that CH 3CN is only weakly bonded to the clean Ag(110) surface. On the partially oxidized surface CH 3CN reacts with atomic oxygen to form adsorbed CH 2CN, OH and H 2O in addition to forming another molecular adsorption state with a desorption peak at 240 K. This molecular state shows a CN stretching frequency of 1840 cm -1, which is indicative of substantial rehybridization of the CN bond and is associated with side-on coordination via the π system. The CH 2CN species is stable up to 430 K, where C-H bond breaking and reformation begins, leading to the formation of CH 3CN at 480 K and HCN at 510 K and leaving only carbon on the surface. In the presence of excess oxygen atoms C-H bond breaking and reformation is more facile leading to additional desorption peaks for CH 3CN and H 2O at 420 K. This destabilizing effect of O (a) on Ch 2CN (a) is explained in terms of an anionic (CH 2CN -1) species. Comparison of the vibrational spectra from CH 2CN (a) and CD 2CN (a) supports the following assignment for the modes of adsorbed CH 2CN: ν(Ag-C) 215: δ(CCN) 545; ϱ t(CH 2) 695; ϱ w(CH 2) 850; ν(C-C) 960; ϱ r(CH 2) 1060; δ(CH 2) 1375; ν(CN) 2075; and ν(CH 2) 2940 cm -1. These results serve to further indicate the wide applicability of the acid-base reaction concept for reactions between gas phase Brönsted acids and adsorbed oxygen atoms on solver surfaces. 相似文献
2.
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
3.
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
4.
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 H 2O 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 H 2O 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
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.
The adsorption and reaction of H 2O with adsorbed oxygen atoms on Ag(110) was examined by UPS. In agreement with previous EELS results, H 2O formed multilayers of ice upon adsorption at 140 K. The ice layers could be easily distinguished from monolayer coverages of chemisorbed H 2O (present above 160 K) by UPS. The ice layers produced (1) strong attenuation of the emission from the Ag d-bands, (2) a nearly 2 eV shift of H 2O valence levels to higher binding energy and (3) strong attenuation of emission from the H 2O 3a 1 orbital. H 2O was observed to react stoichiometrically with O(a) above 250 K to produce a pure layer of adsorbed hydroxyl species. The UPS spectra for these species exhibited features at ?5.8 and ?8.7 eV, as well as strong features above the d-bands. These spectra were compared with those for OH(a) on other surfaces, and the difficulties of identifying OH by UPS due to contamination by excess H 2O are discussed. 相似文献
7.
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
8.
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
9.
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
10.
The adsorption of H 2O 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 H 2O 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 H 2O on Ag. We have found no evidence for either long-range or short-range local bonding order for adsorbed H 2O 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 H 2O ice crystal structure. Adsorbed H 2O 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
11.
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
12.
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.
The kinetics of H 2 desorption from H/W(110) and H/Fe 1/W(110) were studied by measuring work function changes Δø vs time at a number of temperatures. Combination with previously determined Δø vs coverage data and differentiation at various fixed coverages gave rate vs T data from which activation energies of desorption could be obtained. E vs coverage results agree well with previously determine Δ Hdes results. In the case of H/Fe 1/W(110) this includes a rise from 20 to 30 kcal mol −1 of H 2 at H/Fe = H/W > 0.3. Plots of rate −dθ/d t vs θ (θ being coverage in units of H/W) vary much more steeply than θ 2 at most coverages for both systems. The θ dependence can be explained almost quantitatively in terms of the variations of Δ Hdes and surface entropy Ss with coverage, by assuming that rates of desorption are equal to the equilibrium rates of adsorption. The latter can be formulated thermodynamically, except for a sticking coefficient, s. Values for s(θ, T) can also be obtained and show relatively little temperature dependence. 相似文献
14.
The adsorption of oxygen on the Ag(110) surface was examined at temperatures down to 123 K. In addition to the dissociatively adsorbed state which desorbed at 590 K, a second oxygen state desorbed at 190 K following adsorption at 150 K and below. This high temperature state appeared to form prior to the development of the low temperature state. The ratio of coverages of the two states was a strong function of both exposure and adsorption temperature. Isotopic exchange experiments indicated that the low temperature state was molecularly adsorbed. The desorption of the molecularly adsorbed oxygen exhibited complex kinetics due to interaction with adsorbed oxygen atoms. 相似文献
16.
The adsorption of SO 2 on Ag(110) and the reaction of SO 2 with oxygen adatoms have been studied under ultrahigh vacuum conditions using low energy electron diffraction, temperature programmed reaction spectroscopy and photoelectron spectroscopy. Below 300 K, SO 2 adsorbs molecularly giving p(1×2) and c(4×2) LEED patterns at coverages of one half and three quarter monolayers. respectively. At intermediate coverages, streaked diffraction patterns, similar to those reported for noble gas and alkali metal adsorption on the (110) face of face-centered cubic metals were observed, indicating adsorption out of registry with the surface. A feature at low binding energy in the ultraviolet photoemission spectrum appeared which was assigned to a weak chemisorption bond to the surface via the sulfur, analogous to bonding observed in SO 2-amine charge transfer complexes and in transition metal complexes. SO 2 exhibited three binding states on Ag(110) with binding energies of 41, 53 and 64 kJ mol ?1; no decomposition on clean Ag(110) was observed. On oxygen pretreated Ag(110), SO 2 reacted with oxygen adatoms to form SO 3(a), as determined by X-ray photoelectron spectroscopy. Reacting preadsorbed atomic oxygen in a p(2 × 1) structure with SO 2 resulted in a c(6 × 2) pattern for SO 3(a). The adsorbed SO 3(a) decomposed and disproportionated upon heating to 500 K to yield SO 2(g), SO 4(a) and subsurface oxygen. 相似文献
17.
Electron energy loss spectra of clean and oxygen covered GaAs(110) surfaces have been measured with a four grid retarding field analyser. Loss spectra of clean cleaved p- and n-type surfaces are slightly different and different states of adsorption for the oxygen on the two surfaces are found. The loss peaks which are common in the spectra obtained from clean surfaces of both types of material have been interpreted in terms of bulk and surface excitations. The data associated with the bulk excitations are in good agreement with previous optical and electron transmission data while loss peaks at 11.5 and 18.5 eV are interpreted as the surface plasma loss and a surface state transition respectively. For n-type material extra loss peaks were observed. In the case of oxygen adsorption on these surfaces new loss peaks were found at 13.5, 17.2 and 28.1 eV in both spectra and are assumed to be characteristic of the oxygen. Further, for n-type material an extra peak occurs at 8.2 eV. 相似文献
18.
The behavior of oxygen on Pd 1/W(110) has been investigated from 25 to 200 K by thermal desorption, UPS, XPS, and work function measurements. At 25 K only dioxygen species are present. A weakly bound O 2layer, containing O2/ Pd = 0.31 or 4.4 × 10 14 O2 molecules/cm 2 is desorbed at 35 K, leaving a coverage of O2/ Pd = 0.35 or 5 × 10 14 O2 molecules/cm 2. Heating to 200 K results in desorption of molecular O 2 as well as conversion to O, with O/Pd = 0.3. The molecular states, except the very weakly bound one, have high dipole moments with electron transfer to O 2, and thus correspond to Superoxide and peroxide species. These have UPS spectra quite different from physisorbed O 2. At 90 K adsorption is still mostly molecular with a sticking coefficient s near unity. At 200 K, adsorption is atomic with an initial s0 = 0.8. This must be contrasted with Cu 1/W(110) where s0 is unity even at 300 K. The difference can be explained by the much better size match of Pd and W, than Cu and W which makes it easier for Cu to take up momentum of impinging O 2 molecules. The behavior of oxygen on Pd1/W(110) is very similar to that on bulk Pd(111), suggesting that for oxygen the former surface resembles bulk Pd. This is not so for CO adsorption which is much weaker on Pd1/W(110) than on bulk Pd. The reasons for this difference are not presently understood. 相似文献
19.
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
20.
H 2O adsorption on clean Ni(110) surfaces at T ≦ 150 K leads at coverages below to the formation of chemisorbed water dimers, bound to the Ni substrate via both oxygen atoms. The linear hydrogen bond axis is oriented parallel to the [001] surface directions. With increasing H 2O coverage , the accumulation of further hydrogen bonded water molecules induces some modification of the dimer configuration, producing at a two-dimensional hydrogen bonded network with a slightly distorted ice lattice structure and long range order. 相似文献
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