Vibrational spectra of ammonia adsorbed on Fe(110)

EELS spectra of ammonia adsorbed on a Fe(110) surface at 120 K reveal three different adsorption states of molecular ammonia. Thermal processing of the ammonia covered Fe(110) surface to 315 K indicates fragmentation of the NH₃ molecules into atomic hydrogen and nitrogen. Formation of an NH₂ intermediate is not observed whereas the existence of NH_ad species cannot be excluded at present.     

Unusually low stretching frequency for CO adsorbed on Fe(100)

For CO adsorption on Fe(100) different adsorption species are detected with high resolution EELS (electron energy loss spectroscopy) which sequentially fill in with increasing coverage. Up to ～ 350 K and low CO exposure (≦1 L), a predominant molecular species with an unusually low stretching frequency, 1180–1245 cm^?1, is detected. This unusual CO bond weakening is consistent with a “lying down” binding configuration of CO. For higher CO coverages at 110 K, further CO adsorption states with vibrational frequencies of 1900–2055 cm^?1 are populated which are due to CO bound with the molecular axis perpendicular to the surface.     

The adsorption of ammonia on an Fe(110) single-crystal surface studied by high-resolution electron energy-loss spectroscopy (eels)

The EELS spectra of ammonia adsorbed on an Fe(110) surface at 110 K reveal four different adsorption states of molecular ammonia with increasing coverage : chemisorption at “on-top” sites, chemisorption at multi-coordinated sites, adsorption in a second layer, and multilayer condensation.Thermal processing of an ammonia-covered sample to 155 K causes desorption of both the condensed phase and the second layer without any fragmentation of ammonia.Further heating of the sample leads to a much weaker desorption of molecular ammonia up to a temperature of 260 K. EELS spectra recorded after heating to 290 K show only small amounts of atomic nitrogen and hydrogen present on the surface, indicating partial decomposition of ammonia.The formation of species such as NH₂ (ads) or NH (ads) during the thermal-processing experiments could not be observed.     

A vibrational study of ammonia chemisorbed on Ni(110) and Ni(111): whither goest the metal-nitrogen stretching mode on FCC (111) surfaces?

The adsorption of ammonia on the Ni(110) and Ni(111) surfaces has been studied with high resolution (≤ 65 cm_?1) electron energy loss spectroscopy (EELS) combined with thermal desorption spectroscopy. The EELS spectra of the initial chemisorbed layer or α state on each surface are very different. Ammonia chemisorbed on the Ni(110) surface exhibits a strong Ni-N stretching mode at 570 cm^?1 which is absent on the Ni(111) surface. The Ammonia adsorption site appears to be different on the Ni(110) and Ni(111) surfaces. We suggest that the absence of the M-N stretching mode on the Ni(111) surface is a general characteristic of the ammonia adsorption site on the (111) surfaces of fcc Group VIII metals.     

The adsorption of ethylene oxide on Ag(110) and Pt(111): Relevance to selective olefin oxidation

The interactions of ethylene oxide (EtO) with the Ag(110) and Pt(111) surfaces have been studied using XPS, TDS, AES and EELS. On Ag(110), the interaction is very weak, with only molecular desorption observable. The heat of adsorption is ≈ 10.1 kcal mole⁻¹. In contrast, decomposition reactions strongly predominate on Pt(111) at low coverage. Molecular desorption is only seen at high coverages. The heat of adsorption decreases from > 11.9 to 10 kcal mole⁻¹ with increasing coverage. Condensed multilayers desorb at ≈ 140 K. Ultimate decomposition products on Pt(111) include H₂ and CO gas, and carbon residue on the surface. Evidence suggests that adsorbed decomposition intermediates may include atomic hydrogen, CO, acetyl and ethylidyne species, with at least one other, yet unidentified, species. These results imply that, if produced, adsorbed ethylene oxide would be unlikely to escape a reactor containing Pt catalyst without further decomposition reactions. This may help explain the uniqueness of Ag catalysts in ethylene epoxidation.     

The adsorption of ammonia on a Fe(110) single crystal surface studied by high resolution electron energy loss spectroscopy (EELS)

EELS spectra of ammonia adsorbed on a Fe(110) single crystal surface at 120 K reveal four different molecular adsorption states:1. At very low exposures (0.05 L) three vibrational losses at 345 cm^?1, 1170 and 3310 cm^?1 are observed which are attributed to the symmetric Fe-N stretching-, N-H₃ deformation and N-H₃ stretching modes of chemisorbed molecular ammonia, respectively. The observation of only three vibrational losses indicates an adsorption complex of high symmetry (C_3v).2. Further exposures up to 0.5 L cause the appearance of additional losses at 1450 cm^?1, 1640 cm^?1 and 3370 cm^?1. The latter two are interpreted as the degenerate NH₃ deformation and - stretching modes of molecularly adsorbed NH₃. The 1450 cm^?1 loss is a combination of the losses at 345 cm^?1 and 1105 cm^?1. The observation of 5 vibrational losses is consistent with an adsorption complex of C_s symmetry.3. In the exposure range from 0.5 to 2 L adsorption of molecular ammonia in a second layer is observed. This phase is characterized by a symmetric deformation mode at 1190 cm^?1 and by two additional very intense modes at 160 cm^?1 and 350 cm^?1 which are due to rotational and translational modes.4. Exposures above 2 L cause multilayer condensation of ammonia characterized by translational and rotational bands at 190 cm^?1, 415 cm^?1 and 520 cm^?1, and a symmetric deformation mode at 1090 cm^?1. A broad loss feature around 3300 cm^?1 is attributed to hydrogen bonding in the condensed layer.Thermal processing of a Fe(110) surface ammonia covered at 120 K leads to decomposition of the ammonia into hydrogen and nitrogen above 260 K. No vibrational modes due to adsorbed NH or HN₂ species were detected.     

On the structure of chemisorbed acetylene and ethylene on Ni,Pd and Pt surfaces

Filtered He II (hv = 40.8 eV) photoemission spectra for acetylene and ethylene molecularly chemisorbed at T ～ 100 K on Ni(111), Ni(110), Pd(111) and Pt(111) have been obtained. The resulting vertical ionization potentials are presented and used within the framework of an approximate model to obtain information of the geometric structure of these molecules. Two initial state effects are discussed which are found to be important in deducing the molecular structures. These include an initial state shift of the lowest lying carbon-2s derived orbital and a metal atom induced shift of the σ_CC valence orbital for strongly distorted species. The magnitudes of both effects are estimated — the latter using Hartree — Fock LCAO calculations of Be interacting with acetylene or ethylene. The deduced geometries of chemisorbed ethylene are found to differ only slightly from those determined without considering these effects, but for acetylene two classes of structures are found. One class of structures is weakly distorted while the other is strongly distorted (～sp^2.5 hybridization). The latter structure is consistent with recent vibrational loss studies of chemisorbed acetylene on Ni(111) and Pt(111). In contrast to chemisorbed acetylene, chemisorbed ethylene on Ni shows relatively weak distortions. More subtle crystallographic and structural effects for acetylene and ethylene on (111), (100) and (110) Ni surfaces are also discussed.     

Interaction of O2, CO2, CO,C2H4 and C2H4O with Ag(110)

The interaction of O₂, CO₂, CO, C₂H₄ AND C₂H₄O with Ag(110) has been studied by low energy electron diffraction (LEED), temperature programmed desorption (TPD) and electron energy loss spectroscopy (EELS). For adsorbed oxygen the EELS and TPD signals are measured as a function of coverage (θ). Up to θ = 0.25 the EELS signal is proportional to coverage; above 0.25 evidence is found for dipole-dipole interaction as the EELS signal is no longer proportional to coverage. The TPD signal is not directly proportional to the oxygen coverage, which is explained by diffusion of part of the adsorbed oxygen into the bulk. Oxygen has been adsorbed both at pressures of less than 10^-4 Pa in an ultrahigh vacuum chamber and at pressures up to 10³ Pa in a preparation chamber. After desorption at 10³ Pa a new type of weakly bound subsurface oxygen is identified, which can be transferred to the surface by heating the crystal to 470 K. CO₂ is not adsorbed as such on clean silver at 300 K. However, it is adsorbed in the form of a carbonate ion if the surface is first exposed to oxygen. If the crystal is heated this complex decomposes into O_ad and CO₂ with an activation energy of 27 kcal/mol(1 kcal = 4.187 kJ). Up to an oxygen coverage of 0.25 one CO₂ molecule is adsorbed per two oxygen atoms on the surface. At higher oxygen coverages the amount of CO₂ adsorbed becomes smaller. CO readily reacts with O_ad at room temperature to form CO₂. This reaction has been used to measure the number of O atoms present on the surface at 300 K relative to the amount of CO₂ that is adsorbed at 300 K by the formation of a carbonate ion. Weakly bound subsurface oxygen does not react with CO at 300 K. Adsorption of C₂H₄O at 110 K is promoted by the presence of atomic oxygen. The activation energy for desorption of C₂H₄O from clean silver is ～ 9 kcal/mol, whereas on the oxygen-precovered surface two states are found with activation energies of 8.5 and 12.5 kcal/mol. The results are discussed in terms of the mechanism of ethylene epoxidation over unpromoted and unmoderated silver.     

Adsorption kinetics and absolute coverages for C2H2 and C2H4 on a tantalum (110) surface held at 465 and 680 K

Auger spectroscopy has been used to measure the adsorption kinetics of acetylene and ethylene gases at 300 K on a clean tantalum (110) surface held at either 465 or 680 K, at gas pressures near 10^?6 Pa. Adsorption occurs irreversibly with identical initial sticking probabilities for both gases leading to the conclusion that its value is unity. This assumption permits a calculation of absolute coverage of approximately 2 carbon atoms for each tantalum atom in both cases. The fact that this value is the same for both gases suggests that ethylene may dehydrogenate to an acetylenic species as claimed for W(110). Heating a sample previously saturated at 465 K results in desorption of hydrogen accompanied by an increase in the carbon Auger signal. This is interpreted as evidence that at least some of the hydrogen atoms lie further from the surface than do the carbon.     

Vibrational spectra of ethylene and acetylene on metal surfaces - an electron energy loss study of ethylene adsorbed on Ni(110) and its carbided surface,and the use of metal-cluster analogies

An analysis has been made of on- and off-specular electron energy loss spectra (EELS) from C₂H₄ and C₂D₄ adsorbed on a clean Ni(110) and also a carbided Ni(110) surface. The carbided surface was prepared by heating the clean Ni surface in ethylene to 573 K or above. EELS spectra were obtained using a Leybold-Heraeus spectrometer at a beam energy of 3.0 eV and with a resolution of ca. 6.5 meV (ca. 50 cm^?1).The loss spectrum from ethylene at low temperatures (110 K) showed principal features at 3000 (w), 1468 (w), 1162 (s), 879 (w) and 403 cm^?1 (s) (C₂D₄ adsorption) and 2186 (w), 1258 (ms), 944 (ms), 645 (w) and 400 cm^?1 (s) (C₂D₄ adsorption). The overall pattern of wavenumbers and intensifies of the C₂H₄/C₂D₄ loss peaks is very similar in form (although systematically different in positions) to those previously observed on Ni(111) (ref.1) and Pt(111) (ref.2) surfaces at low temperatures. Like these earlier spectra,the EELS results for C₂H₄/C₂D₄ adsorbed on clean Ni(110) can be well interpreted in terms of a MCH₂CH₂M/MCD₂CD₂M species (M = metal) with the CC bond parallel to the surface.After adsorption on the carbided Ni(110) surfaces at 125 K,the main loss features occur at 3065 (m), 2992 (m), 1524 (ms), 1250 (s), 895 (s), and 314 cm^?1 (vs) (C₂H₄ adsorption) and 2339 (m), 2242 (m), 1395 (s), 968 (s), 661 (m) and 314 cm^?1 (vs). With the exceptions of reduced intensities of the bands at 895 cm^?1 (C₂H₄) and 661 cm^?1 (C₂D₄) this pattern of losses - particularly the 1550-1200 cm^?1 features which can be assigned to coupled νCC and δCH₂/δCD₂ modes - is well related to similar results on Cu(100) (ref.3) and Pd(111) (ref.4) which have been interpreted convincingly in terms of the presence of π-bonded species, (C₂H₄)M or (C₂D₄)M on the surface. This structural assignment is supported by comparison with the vibrational spectra of Zeise's salt, K[PtCl₃(C₂H₄)].H₂O (refs.5&6).Spectral changes occur on warming C₂H₄ on the clean Ni(110) surface with a growth of a feature near 895 cm^?1 at 200 K. At 300 K a rather poorly-defined spectrum occurs, which differs substantially from those found on (111) surfaces of Pt (ref.2), Rh (ref.7) or Pd (ref.8) at room temperature. These latter have been attributed to the ethylidyne, CH₃.CM₃, surface species (ref.9). For adsorption on Ni(110) there is clearly a mixture of species at room temperature.The analysis of the vibrational spectra of selected metal-cluster compounds of known structure with selected hydrocarbon ligands has helped substantially to assign the spectra of surface species in terms of bonding structures of the adsorbed species, as in the cases of the identification of (C₂H₄)M π-adsorbed (refs.5&6) and the ethylidyne CH₃.CM₃ species (ref.9). We have recently analysed the infrared and Raman spectra of the cluster compound (C₂H₂)Os₃(CO)₁₀ and its deuterium-containing analogue. The infrared frequency and intensity pattern for the A′ modes (C_S symmetry) of the two isotopomers bears a remarkable resemblance to EELS spectra previously obtained at low temperature for C₂H₂/C₂D₂ adsorbed on Pt(111) (ref.2) and (after taking into account systematic frequency shifts) for Pd(111) (ref.4). There is good evidence for believing that the structure of the hydrocarbon ligand interacting with the osmium complex takes the form

where the arrow denotes a π-bond to the third metal atom. This strongly confirms the structure for the low-temperature acetylene species on Pt(111) as proposed by Ibach and Lehwald (ref.2).Finally the room-temperature spectra for ethylene adsorbed on finely-divided silica-supported Pt and Pd catalysts have previously been interpreted in terms of the presence of MCH₂CH₂M (ref.10) and π-bonded (C₂H₄)M species (ref.11). However comparisons with the more recent EELS spectra from ethylene on Pt(111) at room temperature (ref.2) now leads to a reassignment of the 2880 cm^?1 band, on Pt, previously assigned to MCH₂CH₂M, together with a new, related,band at 1340 cm^?1 (ref.12), to the ethylidyne species CH₃CPt₃ found on the single crystal surface.More detailed analyses of the spectra reported here will be published later. Acknowledgement is given to substantial assistance for this programme of research from the Science and Engineering Research Council.     

Adsorption of potassium on iron

Adsorption of K on Fe(110), (100) and (111) surfaces was studied by means of LEED, AES, thermal desorption and work function measurements. The monolayer capacity is about 5.5 × 10¹⁴ K-atoms/cm² in all three cases. With Fe(111) an ordered 3 × 3 overlayer was found at fairly low coverages. The work function decreases to a minimum and the initial dipole moments were determined to μ₀ = 7.0 Debye for Fe(110), μ₀ = 4.4 Debye for K/Fe(100) and μ₀ = 3.9 Debye for K/Fe(111). The heat of adsorption decreases from its initial value (Fe(110): 57; Fe(100): 54; Fe(111): 52 kcal/mole) continuously with increasing coverage which parallels the continuous decrease of the dipole moment of the adsorbate complex.     

Thermal evolution of acetylene and ethylene on Pd(111)

The thermal evolution of acetylene and ethylene and their deuterated counterparts on a palladium (111) surface has been studied by high-resolution electron energy loss spectroscopy in the temperature range 150–500 K. Analysis of the vibrational spectra indicates that chemisorbed acetylene evolves at 300 K in the presence of surface hydrogen to mainly ethylidyne, CCH₃, and a small amount of residual acetylene. Spectra obtained with and without preadsorbed hydrogen provide evidence for a 〉C CH₂ intermediate in the reaction. Chemisorbed ethylene also evolves to ethylidyne after heating from 150 to 300 K but much of the ethylene desorbs. The high temperature (400–500 K) behavior of C₂H₂ and C₂H₄ involves formation of a CH species. Although a small amount of the CH species may be formed from the dehydrogenation of ethylidyne, it is found that carbon-carbon bond scission of acetylene near 400 K is the dominant mechanism in CH formation.     

Vibrational excitations and structure of oxygen on Fe(110)

Vibrational spectra of oxygen adsorbed on a clean Fe(110) surface at 300 K have been measured by high resolution electron energy loss spectroscopy (EELS). In the exposure range up to 6 L a single loss around 500 cm^-1 is observed which is interpreted to be due to the stretching vibration of atomic oxygen adsorbed at a 2-fold long-bridge site. Above exposures of 6 L a second loss around 400 cm^-1 appears which is attributed to the formation of a disordered oxide layer. Subsequent heating of the sample leads to the observation of a (5×12) LEED pattern which is explained by a mixed oxygen-iron surface structure which is nearly identical to the (111) face of bulk FeO. A weak loss around 910 cm^-1 appears after oxygen exposures at elevated sample temperatures. This loss is attributed to the formation of bulk oxide.     

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

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 H₂O 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 H₂O 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 H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).     

Ti原子在Al(110)表面吸氢过程中催化作用的第一性原理研究

运用第一性原理方法对H₂分子在Ti掺杂和纯的Al(110)表面的吸附情况进行了研究,发现有Ti原子掺杂时,存在一个H₂分子的吸附路径,即位于Al(110)面顶位Ti原子上方的H₂分子会发生解离,并与Ti原子形成TiH₂分子,然后TiH₂分子向能量更低的空位移动并接近Al(110)表面.态密度与电荷布居分析显示,吸附完成后H原子与表面Al原子存在较强的共价键作用,这为Al-H类物质及Na_{3 关键词： 钛 吸附 解离能 第一性原理}     

C2H4在清洁和有Cs覆盖的Ru(0001)表面吸附的TDS研究

用热脱附谱(TDS)方法研究了乙烯(C₂H₄)在Ru(0001)表面上的吸附.在低温下(200K以下)乙烯可以在清洁及有Cs的Ru(0001)表面上以分子状态稳定吸附,在衬底温度升高至200K以上时,乙烯发生了脱氢分解反应,乙烯分解后的主要产物为乙炔(C₂H₂).在清洁的Ru(0001)表面,乙烯有两种吸附状态,脱附温度分别为275K和360K.而乙炔的脱附温度为350K.在Ru(0001)表面有Cs的存在时,乙烯分解 关键词： 乙烯 钌(0001)表面 铯钌(0001)表面乙烯 钌(0001)表面 铯钌(0001)表面     

Low temperature electron energy loss spectra of acetylene chemisorbed on metal single-crystal surfaces; Cu(111), Ni(110) and Pd(110)

Vibrational spectra of acetylene chemisorbed on Cu(111), Ni(110) and Pd(110) at 110–120 K were measured using electron energy loss spectroscopy. Loss peaks were assigned to vibrational modes of the non-dissociatively adsorbed molecules with the aid of the corresponding C₂D₂ spectra. The spectra show that the molecules undergo significant rehybridisation on adsorption. Comparisons are made with the spectra of acetylene adsorbed on a range of other transition metal surfaces at low temperature. Taking into account these and earlier literature results, two distinct patterns of spectra are observed (Type A and Type B) for specular spectra. The Cu(111) spectrum is classified as Type A while the Ni(110) and Pd(110) spectra are classified as Type B. Suggestions are made for the structures of the surface species corresponding to the two spectral types.     

Effects of adsorbates on the formation of doubly excited He atoms during impact of He2+ ions on Fe(110) and Ni(110) surfaces

The formation of doubly excited states of He atoms during collisions of He²⁺ ions with projectile energies between 74 eV and 124 eV with an Fe(110) and a Ni(110) surface is studied via Auger electron spectroscopy. We observe that the electron spectra from autoionization of doubly excited states of 2s², 2s2p, 2p² configurations show a pronounced dependence on the coverage with oxygen for both surfaces. For a controlled O₂ adsorption on the Fe(110) and Ni(110) surfaces we can explain the resulting changes in the electron spectra by the modification of the measured work functions of the target surfaces. In terms of thermal desorption and dissolution into the bulk of surface contaminations at elevated temperatures, we present an alternative interpretation of similar previous studies by another group, where the local electron spin polarization of Fe(110) and Ni(110) surfaces was deduced from changes in the electron spectra as function of target temperature.     

The adsorption of cyclic hydrocarbons on Ru(001): II. Cyclohexane

The adsorption of cyclohexane on Ru(001) at 90 K has been investigated by thermal desorption mass spectrometry, EELS, UV photoemission and LEED. Thermal desorption indicates the adsorption of the undissociated molecule first in a chemisorbed monolayer (T_d = 200 K) with subsequent formation of multilayers (T_d = 165 K) at higher exposures. The vibrational spectrum obtained by EELS is characterized by a frequency shift of the C-H stretching mode from 2920 cm^?1 (multilayer) to 2560 cm^?1 for the chemisorbed monolayer. Off-specular EELS data indicate two different electron scattering mechanisms for the C-H stretching mode. Whereas for the C-H stretching mode of the multilayer, large angle electron impact scattering is observed, the C-H soft-mode of the monolayer is largely due to small angle dipolar scattering. The He I photoelectron spectra of cyclohexane multilayers are characteristic of the undissociated molecule. A new assignment of C(2s) and the lowest C(2p) level, based on a comparison with benzene, shows that the chemisorbed monolayer is characterized by the absence of emission or broadening of the 2a_1u level. This is attributed to C_3v symmetry of the chemisorbed layer and to a possible interaction of the 2a_Iu orbital with the metal surface.     

Magnetic order in Y6(57Fe:Mn)23H26

The ferrimagnetic compound Y₆Mn₂₃ and its hydride Y₆Mn₂₃H₂₆, both doped with 0.5%⁵⁷Fe, have been investigated using the ⁵⁷Fe Mössbauer resonance and dc field magnetization measurements. For the hydride a small ⁵⁷Fe magnetic hyperfine field is observed to increase abruptly below 110 K whereas the bulk magnetization results suggest antiferromagnetic ordering at T_N≈ 180 K.