XPS,UPS and thermal desorption studies of alcohol adsorption on Cu(110): I. Methanol

The adsorption of methanol on clean and oxygen dosed Cu(110) surfaces has been studied using temperature programmed reaction spectroscopy (TPRS), ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Methanol was adsorbed on the clean surface at 140 K in monolayer quantities and subsequently desorbed over a broad range of temperature from 140 to 400 K. The UPS He (II) spectra showed the 5 highest lying emissions seen in the gas phase spectrum of methanol with a chemisorption bonding shift of the two highest lying orbitais due to bonding to the surface via the oxygen atom with which these orbitals are primarily associated. A species of quite a different nature was produced by heating this layer to 270 K. Most noticeably the UPS spectrum showed only 3 emissions and the maximum coverage of this state was approximately $\text{1}\text{2}$ monolayer. The data are indicative of the formation of a methoxy species, thus showing that methanol is dissociated on the clean Cu(110) surface at 270 K. The same dissociated species was observed on the oxygen dosed surface, the main difference in this ease being the production of large amounts of H₂CO observed in TPRS at 370 K.     

Adsorption of N2O on Ru(001)

The adsorption of N₂O on Ru(001) at ~ 100K has been studied using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and thermal desorption spectroscopy (TDS). At low exposures, N₂O partly dissociates leaving atomic oxygen on the surface and desorbing N₂. With increasing N₂O exposures, molecular adsorption becomes dominant. He II UPS of the gas phase, solid and monolayer adsorbed molecular N₂O are compared. To within experimental error, the peak spacings in all three are the same. The distributions of intensities in the gas and solid phase spectra are the same. In the monolayer spectra, the 7~σ (terminal nitrogen lone pair) orbital intensity is decreased significantly indicating that it is more strongly coupled to the surface than the other valence orbitals. No molecular N₂O remains after heating to above 180 K and no detectable amount of dissociated nitrogen appears. Molecularly adsorbed N₂O is easily dissociated by an electron beam to give N₂(g), NO(g) and O(a).     

Photoemission studies of H2S,H2 and S adsorbed on Ru(110): Evidence for an adsorbed SH species

H₂S, H₂ and S adsorbed on Ru(110) have been studied by angle-integrated ultraviolet photoemission (UPS) as part of a study of the effect of adsorbed sulfur, a common catalytic poison, on this Ru surface. For low exposures of H₂S at 80 K, the work function rises to a value 0.16 eV above that of clean Ru(110) while the associated UPS spectra (hν = 21.2 eV) exhibit features similar to those of H(ads) and S(ads) and different from those of molecular H₂S. We conclude that H₂S dissociates completely at low coverages on Ru(110) at 80 K. At intermediate exposures the work function drops and the UPS spectra show new features which are attributed to the presence of an adsorbed SH species. This appears to be the first direct observation of this surface complex. At higher exposures the work function saturates at a value 0.36 eV below the clean value; the UPS spectra change markedly and indicate the adsorption of molecular H₂S. Heating adsorbed H₂S leaves a stable layer of S(ads) on Ru(110). The surface with adsorbed sulfur strongly modifies the adsorption at 80 K of a number of molecules relative to the clean Ru(110) surface.     

Decomposition pathways of C1C4 alcohols adsorbed on platinum (111)

The adsorption and decomposition of methanol, ethanol, propan-1-ol, propan-2-ol and butan-1-ol has been studied on clean, and oxygen pre-covered Pt(111) surfaces. Temperature Programmed Reaction Spectroscopy (TPRS), Surface Potential Measurements (ΔV), UPS and XPS were used to characterise the adsorbed layer as a function of temperature. Each alcohol adsorbed into two states, a monolayer phase and a multilayer phase which were distinguishable by TPRS and Spectroscopy measurements. The monolayer alcohol adsorption heats increased sequentially from methanol to n-butanol (11.5–15 kcal mole^?1). On the clean surface, less than 10% of the adsorbed monolayer dissociated, with 90% of the alcohol desorbing intact. Two competing dissociative pathways were observed: complete dissociation to adsorbed CO, H and C, and with propan-1-ol and butan-1-ol, scission of the CC bond nearest the CO group to form adsorbed CO, H and ethylidyne and propylidyne species respectively. The latter reaction probability was constant at 30% for n-propanol and n-butanol. In all cases the final desorption products were the parent alcohol, CO and H₂ with carbon remaining on the surface for the higher alcohols. Atomic oxygen removed hydrogen from the alcohols as water but did not change the final reaction products.     

A photoelectron spectroscopic investigation of the interaction between H2O and oxygen on Ag(110)

The adsorption and reaction of H₂O with adsorbed oxygen atoms on Ag(110) was examined by UPS. In agreement with previous EELS results, H₂O formed multilayers of ice upon adsorption at 140 K. The ice layers could be easily distinguished from monolayer coverages of chemisorbed H₂O (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 H₂O valence levels to higher binding energy and (3) strong attenuation of emission from the H₂O 3a₁ orbital. H₂O 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 H₂O are discussed.     

Co adsorption on potassium promoted Fe(110)

CO adsorption on potassium covered Fe(110) has been studied using UPS, XPS, AES and flash desorption. It was found that CO adsorbs molecularly at room temperature with a larger binding energy than on clean Fe(110). The CO saturation coverage increases and the sticking coefficient decreases with increasing potassium coverage. On heating, the probability of adsorbed CO dissociating increases with the amount of potassium present. The UPS spectra show that the CO 4σ peak is shifted by 0.8 eV to higher binding energies on Fe(110) + K and that at 21.2 eV the peak due to the 1π + 5σ orbitals is split into a double peak. The catalytic relevance of the measurements is discussed with reference to the Fischer-Tropsch synthesis.     

Ag(110)表面吸附酞菁铜分子的紫外光电子谱研究

用紫外光电子能谱(UPS)研究了酞菁铜分子在Ag(110)单晶表面上的吸附,随着酞菁铜分子覆盖度增加,衬底Ag的3d电子信号逐渐减弱,在此能带区域出现两个新的谱峰,这两个与吸附有机分子轨道有关的谱峰的束缚能分别为4.45 和6.36 eV.随着覆盖度的增加,在结合能为1.51和9.20 eV处又出现了两个谱峰,它们同样来自吸附有机分子的轨道.随着覆盖度的继续增加,上述四个谱峰的强度逐渐增加,其能量位置均发生了明显的偏移.根据角分辨光电子能谱的实验结果,酞菁铜分子的分子平面基本与衬底表面平行.密度泛函理论计 关键词： 酞菁铜 紫外光电子谱 吸附电子态 密度泛函理论     

UPS spectra of H2O,CH3OH and C5H9OH adsorbed onto Cu(111)/Na and Na(cp)

The present communication presents ultraviolet photoemission spectra (UPS) of three different “alcohols”; water (H₂O), methanol (CH₃OH), and cyclopentanol (C₅H₉OH), chemisorbed onto a Cu(111) surface partially covered by sodium atoms as well as onto closely packed sodium films, a free electron adsorbent. Whereas all three alcohols ROH bind reversibly and associatively to Cu(111) they react with adsorbed sodium atoms to metal bound alcoxides RO. The chemisorption bond, characterized by the interaction between O 2pπ orbitals and metal atoms as an electron donor, the alcoxide being the acceptor, is similar for all groups R. The O 2pπ orbitals shift to higher UPS binding energies with increasing electron density, i.e. decreasing r_s/a_o of the sodium overlayer. Only for HONa, the sterically smallest group R, does the alcoxide growth continue in three dimensions. Although, possibly failing to reproduce the electron density profile of a free electron surface, Hartree-Fock-Slater cluster calculations of small models ROH and RONa₃ enable correlations to be made between UPS intensity peaks and one electron orbitals.     

The adsorption of water on clean and oxygen covered Cu(110)

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.     

An X-ray photoelectron spectroscopic study of the adsorption of N2, NH3, NO,and N2O on dysprosium

The adsorption of N₂, NH₃, NO, and N₂O onto clean polycrystalline dysprosium at 300 and 115 K and the changes undergone by the adsorbed species upon heating from 115 K have been investigated using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). At 115 K, N₂ adsorbs dissociatively, vielding two peaks in the N 1s region at 396.2 and 398.2 eV corresponding respectively to a nitride and to chemisorbed nitrogen N(a). No peaks corresponding to molecularly adsorbed N₂ (BE 400.2 eV [10]) were observed. Upon heating the sample the N(a) is converted into the nitride species, as evidenced by a decrease in the 398.2 eV peak and a corresponding increase in the 396.2 eV peak. At a warm-up temperature of 300 K, the N(a) species accounts for only ～10% of the total nitrogen on the surface. Ammonia adsorbed at 115 K shows three distinct peaks, at 401.7, 399.3 and 396.2 eV, corresponding to molecular, partly dissociated, and completely dissociated (nitride) ammonia. Upon heating multilayer ammonia to 175 K, it desorbs to leave predominantly the peak corresponding to the partly dissociated species. Upon further heating the molecular and partly dissociated ammonia is converted into the nitride species. At 400 K only nitride-type nitrogen remains on the surface. The adsorption of NO and N₂O at 115 K is predominantly dissociative. NO has N 1s peaks at 403.1 and 396.3 eV corresponding possibly to molecularly adsorbed NO, and to nitride species. After N₂O adsorption there is very little nitrogen on the surface. Adsorption of N₂ and NO at 300 K yields only the peak at 396.2 eV, whereas NH₃ yields, in addition to this peak, a small intensity (～20%) of the peak at 398.2 eV (partly dissociated ammonia).     

低覆盖度CO分子在 Ni（110）面的吸附研究

利用密度泛函理论研究了低覆盖度下CO分子在Ni（110）表面的吸附结构和电子态。研究结果表明：在低覆盖度情况下, CO分子优先垂直吸附在短桥位,其次是顶位和长桥位。垂直短桥位吸附、顶位吸附相应的振动频率分别是1850.52 cm-1、1998.08cm-1。态密度的研究结果表明：CO分子和Ni原子在-10 eV -8 eV,-8 eV—-6 eV及1 eV -5 eV能量范围内发生了杂化作用。-10ev -8ev能量范围内的杂化主要来源于C、O原子的s轨道、pz轨道与Ni原子s、p、d轨道的杂化作用。-8ev—-6ev能量范围内的杂化作用主要来源于C、O原子的py、 px轨道与Ni原子d、s轨道的杂化作用。轨道间的杂化作用是吸附作用的主要来源。 我们计算的吸附位置与相应的振动频率与相关实验结果基本一致。     

The adsorption and condensation of oxygen on aluminium at low temperaturf

The interaction of oxygen with an Al(111) surface at 30 K leads initially to dissociative adsorption, followed by the physisorption of molecular oxygen. The latter is characterised by HeI and HeII UPS spectra similar to the gas phase. The physisorbed molecules partly convert into the dissociated phase on standing in vacuum. There appears to be no difference between the UPS spectrum of a condensed layer of oxygen and that of the physisorbed species.     

Methanol and formaldehyde molecular states on copper surfaces at 300 K?

In a recent paper, Kojima, Sugihara, Miyazaki and Yasumori concluded that methanol and formaldehyde adsorb molecularly (non-dissociatively) on polycrystalline copper at 300 K. Methanol and methyl formate were also found to produce adsorbed formaldehyde. We demonstrate that the “ formaldehyde” UPS spectrum in their study was incorrectly assigned, and is identical to that of adsorbed formate generated during dissociative exposure of formaldehyde to a Cu(110) surface. We have measured the He II spectra of formaldehyde (120 K) and formate (300 K) on clean Cu(110) and show that they are distinctly different. No evidence is found in the present work for stabilization of molecular formaldehyde, methanol or methyl formate on the Cu(110) surface at 300 K.     

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).     

Metal (Cu; Pd) adsorption on MgO: investigations with MIES and UPS

MgO films (2-nm thick) were grown on W(110) while metastable impact electron (MIES) and photoelectron (UPS(HeI)) spectra were collected in situ; apart from the valence-band emission no additional spectral features could be detected. The oxide surface was exposed to metal atoms (Cu; Pd) (substrate at 300 K). For Cu, but not for Pd/MgO, a characteristic initial decrease of the surface work function by about 0.4 eV is observed for small exposures. Metal-induced intensity develops above the top of the O2p valence band in UPS caused by 3dCu (4dPd) emission. The emission seen for Cu/MgO in the MIES spectra above the 2pO valence band is attributed to the ionization of Cu4s states of neutrally adsorbed Cu species; the shape of the MIES spectra suggests island growth even at the lowest studied exposures. For Cu/MgO the critical coverage for the transition from 2D to 3D island growth, as determined with MIES, is estimated as 0.15 monolayers. PACS 79.20; 79.60.Dp; 73.22.-f; 82.80.Pv     

Orientation dependence of oxygen adsorption on a cylindrical GaAs sample: II. Photoemission measurements

The orientation dependence of oxygen adsorption has been investigated by XPS and UPS on the surface of a cylindrically shaped GaAs single crystal with [11̄0] as its axis. This sample exposes the main low-index orientations (001), (111)Ga, (110), (111̄)As, etc. and all intermediate orientations. It was prepared by ion bombardment and annealing [see part I, Surface Sci. 120 (1982) 67). XPS was used to separate the known two forms of adsorbed oxygen quantitatively. The less tightly chemisorbed a-oxygen has an O 1s binding energy 1.50 ± 0.05 eV larger than the β-oxygen for all surface orientations. Comparison with UPS shows that for hv =40.8 eV the excitation probability of the O 2p orbitals for α-oxygen is four times larger than for β-oxygen. The Ga 3d peak shows the known chemical shift of 1.1 ± 0.1 eV induced by β-oxygen whereas α-oxygen causes no Ga 3d shift. Most of the α-oxygen desorbs thermally starting near room temperature, only a portion smaller than 20% is converted to β-oxygen; β-oxygen desorbs starting at 300–350°C slightly depending on orientation. These results confirm the interpretation of β-oxygen as dissociated oxygen. However, the nature of α-oxygen still is not clear. The strong orientation dependence of the total amount of oxygen is consistent with the Auger results published recently (see part I). The nature of α- and β-oxygen is found to be the same for all orientations. Also the ratio of adsorbed α- to β-oxygen depends surprisingly weakly on the orientation and on the exposure varying only between 0.35 and 0.7 as the extreme values. This suggests that the adsorption of α- and β-oxygen is connected. A possible model is that adsorption, probably of β-oxygen, starts from edges or other energetically less favourable sites and that this disturbance creates stresses or defects which serve as adsorption site for the α-oxygen.     

An XPS study of the water adsorption on Cu(110)

The adsorption of H₂O on clean and oxygen precovered Cu(110) is studied at temperatures between 90 and 300 K by XPS. On the oxygen precovered surface three O(1s) emission lines are detected at lower temperature. They originate from adsorbed atomic oxygen, from OH groups, and from H₂O molecules. For an oxygen coverage of half a monolayer, XPS indicates that during H₂O decomposition the preadsorbed oxygen does not directly participate in the OH formation. After water adsorption on the clean surface three O(1s) emission lines are found, which are due to H₂O molecules, “disturbed” H₂O molecules, and OH groups. The XPS results are directly correlated with information about the adsorbates obtained by UPS. Coverages are determined from the XPS spectra for the detected species.     

Photoemission study of oxygen adsorbed on coldly deposited silver films

UPS spectra of coldly deposited silver films differ from those of films deposited at room temperature by electronic states localized at surface defects with an energy about 4.2 eV below E_F. Changes after exposure at 140 K to oxygen only occur in the presence of these defects, demonstrating that oxygen is only adsorbed at defects. Raman vibrational spectroscopy shows that oxygen is adsorbed nominally as O₂^? and O₂^2?. Possible assignments of the oxygen related UPS structures are discussed.     

The adsorption of methanol on Pt(111), an IR reflection and UV photoemission study

IR reflection (absorption) spectroscopy (IRS) under grazing incidence and UV photoemission (UPS) with He I and He II radiation were used to study the adsorption of CD₃OD and CH₃OH, respectively, on Pt(111) at temperatures between 90 and 350 K. At 90 K a first chemisorbed layer of methanol consists of molecules being strongly chemisorbed at their oxygen end. On top of this first layer two other phases of methanol are detected. For both the second and third phase the IR spectra indicate hydrogen bonding between the molecules. From UPS a stronger contribution to chemical bonding at the oxygen end of the molecules is derived for the second phase. On top of the second phase amorphous multilayers condense which exhibit a phase transition into the crystalline α-ice modification of methanol upon annealing to 125 K. Thermal desorption proceeds via loss of the α-ice multilayers (135 K), desorption of the second (“amorphous”) methanol phase to a final dissociation of molecules in the first layer into CO and H. About 3% of a monolayer of CO remains on the surface at temperatures above 230 K. No stable intermediate in the decomposition of methanol into CO is detected.     

H2O分子在五边形BCN上的吸附与解离特性研究

基于密度泛函理论的第一性原理计算方法,研究了H₂O分子在五边形BCN上的吸附与解离过程.研究结果表明,五边形BCN结构的B原子是H₂O分子的最稳定的活性吸附位点. H₂O分子在该活性位点极易解离,其初步解离过程为放热反应且分解势垒仅为0.191 eV,并形成稳定的OH/H产物.深入研究发现,H₂O分子初步解离后的五边形BCN表面,可直接分解后续吸附的H₂O分子.该研究结果为五边形BCN对H₂O分子的吸附解离机制提供理论借鉴.