Vibration spectroscopy of benzene adsorbed on Pt(111) and Ni(111)

High resolution electron energy loss spectroscopy has been applied to study the adsorption of benzene (C₆H₆ and C₆D₆) on Pt(111) and Ni(111) single crystal surfaces between 140 and 320 K. The vibrational spectra provide evidence that benzene is chemisorbed with its ring parallel to the surface, predominantly π bonded to the platinum and nickel surface respectively. A significant frequency increase of the CH-out-of-plane bending mode, largest in the case of platinum, is observed compared to the free molecule. On both metals two phases of benzene exist simultaneously, characterized by a different frequency shift. The shifts are explained by electronic interaction between the metal d-orbitals and molecules adsorbed in on top and threefold hollow sites respectively. The vibrational spectra of the multilayer condensed phase of benzene exhibit the infrared active modes of the gasphase molecule as expected.     

Fourier transform infrared studies of cyclohexane and benzene adsorbed on Pt/Al2O3: Evidence for π- and σ-bonded chemisorbed species

Fourier transform infrared spectroscopy has been applied to the study of cyclohexane adsorbed on Al₂O₃ and Pt/Al₂O₃ surfaces. Earlier studies of benzene on these same materials have also been extended to include benzene adsorbed on a Pt/Al₂O₃ surface which contains structured carbon residues. The data provide indirect evidence for the formation of a carbon residue on Pt/Al₂O₃ which retains the six-membered cyclic structure of the parent adsorbates. The carbon residue can be formed upon vacuum heating of the parent C₆ ring molecules chemiorbed on Pt/Al₂O₃. There is spectroscopic evidence that cyclohexane dehydrogenates on Pt/Al₂O₃ at 300 K to form two different chemisorbed species; a π-bonded benzene and a dissociated σ-bonded benzene. These two chemisorbed species have CH stretching vibrations centered at 3030 and 2947 cm^?1, respectively. Benzene added to a clean catalyst surface forms only a π-bonded benzene. However, benzene added to Pt/Al₂O₃ with ordered carbon residues forms both π- and σ-bonded benzenes. The addition of H₂ at 300 K to any of the π- or σ-bonded benzenes or to the carbon residue results in the formation of cyclohexane physisorbed on the catalyst. The absence of CH₃ groups upon hydrogenation suggests the lack of CC bond breaking during adsorption or hydrogenation. Simultaneous infrared and thermal desorption studies on chemisorbed deuterated benzene (from C₆D₁₂) indicate that the a-bonded species exchange H from the surface OH groups of the alumina support more readily than does the π-bonded benzene. In addition to hydrogen exchange with the support, thermal desorption experiments indicate the oxidation of a portion of the chemisorbed hydrocarbons and/or carbon residue by oxygen from the alumina support. Therefore, the support is capable of playing a direct role in reactions occurring on the catalyst surface.     

Surface chemistry of polyimide precursors on Cu(111)

We have investigated the bonding of the monomers pyromellitic dianhydride (PMDA) and oxydianiline (ODA) as well as model compounds, succinic anhydride, phthalic anhydride, and benzoic acid, on Cu(111) in ultrahigh vacuum. Unenhanced surface Raman spectroscopy was used to identify the adsorbed species. ODA was unreactive at 110 K; the surface vibrational features were identical to those in the condensed multilayer. In contrast, PMDA chemisorbed dissociatively to form a bidentate surface carboxylate. Succinic anhydride physisorbed at 110 K, whereas benzoic acid and phthalic anhydride both adsorbed dissociatively forming bridging surface carboxylates as was observed for PMDA. The surface Raman spectrum of PMDA showed resonance enhancement.     

The interaction of D2O with Zr(0001) at 80 K

The adsorption of D₂O on Zr(0001) at 80 K and its subsequent reactions at higher temperatures have been studied by thermal desorption spectroscopy (TDS), work-function measurements (Δф), nuclear reaction analysis (NRA), LEED, infrared reflection spectroscopy (FTIR-RAS), Auger electron spectroscopy (AES), and static secondary ion mass spectroscopy (SSIMS). D₂O adsorption on Zr(0001) at 80 K is accompanied by a Δф of −1.33 eV. The adsorbed D₂O can be characterized into three layers by TDS: a chemisorbed layer (up to 0.23 ML), a second adsorbed layer, and an ice layer. The chemisorbed D₂O dissociates into OD_ad and D_ad at 80 K (possibly also into O_ad) and no desorption products could be detected, implying that the reaction products dissolved into the zirconium at temperatures appropriate for each component. The ice layer and most of the second adsorbed layer desorb as molecular water during heating. The water adsorbed at 80 K did not form any long-range ordered structure, but a (2 × 2) LEED pattern that was formed by heating the sample to temperatures above 430 K is believed due to be an ordered oxygen superstructure.     

Rovibrational state distribution of deuterium molecules resulting from D-atom interaction with Ni(110)

Using the REMPI technique we have studied the internal state distribution of deuterium molecules produced by the interaction of atomic deuterium with chemisorbed deuterium on Ni(110) at 180 K. We observed molecules in vibrational states up to v = 3 with a mean vibrational energy of 220 meV. The mean rotational energies of the molecules in the vibrational states v = 0 to 3 are 185 meV, 133 meV, 75 meV and 37 meV, respectively. The overall mean rotational energy amounts to 150 meV, again far in excess of the mean rotational energy of molecules accommodated to the surface. The data are consistent with direct interactions of the impinging particles with the adsorbed particles (Eley-Rideal reaction and collision induced desorption), for which it is assumed that a considerable amount of the potential and kinetic energy of the impinging atoms is channeled into translational and internal energy of the reaction products.     

Experimental study of the vibrations of acetylene chemisorbed on Ni(111)

High resolution electron energy-loss measurements of normal and deuterated acetylene chemisorbed on Ni(111) have been obtained. Observed vibrational modes are identified using the frequency shifts for the deuterated species and comparisons to the free molecule and a di-cobalt compound of acetylene. These vibrational frequencies indicate that chemisorbed acetylene is strongly rehybridized having a state of hybridization between ~sp^2.5 and sp³. Consideration of the types of modes observed, their assignments and the surface selection rule suggests a molecular orientation with the C-C bond axis slightly skewed relative to the surface and with the plane of the distorted molecule normal to the surface. A bonding geometry is proposed which has the carbon atoms residing above two adjacent 3 fold hollow sites of the Ni surface. This molecular geometry differs from that deduced previously by electron energy-loss spectroscopy for molecularly adsorbed acetylene on Pt(111).     

Study of reactive film heterostructures with several interfaces using the thermal desorption spectroscopy

The thermal desorption spectroscopy is used to study the interaction of the chemisorbed oxygen and carbon monoxide molecules with the nanometer-thick ytterbium films that are formed on the surfaces of silicon substrates at room temperature. In accordance with the results at a temperature of 300 K, the O₂ and CO molecules are chemisorbed on the surface of a metal film and do not exhibit dissociation to atoms under such conditions. The molecular dissociation is observed at higher temperatures. The liberated oxygen is involved in reactions with ytterbium and silicon that lead to the formation of complicated silicates, which dissociate at even higher temperatures.     

Photoelectron spectroscopy study of oxygen adsorption on Mo2C(0 0 0 1)

Oxygen adsorption on the α-Mo₂C(0 0 0 1) surface has been investigated with X-ray photoelectron spectroscopy and valence photoelectron spectroscopy utilizing synchrotron radiation. It is found that oxygen adsorbs dissociatively at room temperature, and the adsorbed oxygen atoms interact with both Mo and C atoms to form an oxycarbide layer. As the O-adsorbed surface is heated at ≧800 K, the C-O bonds are broken and the adsorbed oxygen atoms are bound only to Mo atoms. Valence PES study shows that the oxygen adsorption induces a peculiar state around the Fermi level, which enhances the emission intensity at the Fermi edge in PES spectra.     

The adsorption of nitrogen dioxide on the Ag(110) surface and formation of a surface nitrate

The adsorption of NO₂ on the Ag(110) surface has been characterized by temperature programmed reaction spectroscopy and high resolution electron energy loss spectroscopy. At 95 K the NO₂ is dimerized to N₂O₄ in a multilayer and a distinct molecular layer, both of which desorb below 200 K. The first adsorption layer contains chemisorbed NO₂ and NO₃, the latter formed by a reaction between NO₂ and its decomposition products. Part of the NO₂ is molecularly chemisorbed via the oxygen atoms with a proposed symmetric bidentate geometry, desorbing at 270 K. Nitrogen dioxide also undergoes partial dissociation to nitrogen and oxygen adatoms. An NO₃ species is formed by the reaction of NO₂ with the oxygen adatoms produced from the partial dissociation of NO₂. The NO₃ is attached to the surface via one oxygen atom and has C_2v symmetry; it decomposes below 500 K. The geometry of both the chemisorbed NO₂ and NO₃ have analogues among inorganic metal complexes.     

ELS study on chemisorbed state of CO on Cr(110)

The chemisorbed state of CO on a Cr(110) surface has been investigated at 300 K by electron energy loss spectroscopy (ELS) with the in-situ combined supplementary techniques. The ELS spectrum of the Cr(110) surface after CO adsorption is characterized by the peaks at 2, 4.4, 6–7, 9, 11, 14.5, 19 and 23 eV, and is found to be practically the same as that of the oxygen covered surface. The C-KLL Auger spectra obtained in the range 0.1–900 L CO agree with those of metal carbides. These results are considered to indicate that CO is dissociatively chemisorbed on the Cr(110) surface throughout the whole exposure region examined. The average sticking probability of CO on Cr(110) is 0.7 at below 0.5 L, and the maximum work function increase at 1 L is ~0.1 eV. The adsorbed state of O atoms produced from dissociative adsorption of CO is also discussed.     

Partial oxidation of higher olefins on Ag(1 1 1): Conversion of styrene to styrene oxide, benzene, and benzoic acid

Temperature programmed reaction spectroscopy (TPRS), X-ray photoelectron spectroscopy (XPS), and reflection absorption infrared spectroscopy (RAIRS) were used to study the partial oxidation of styrene on Ag(1 1 1). Styrene oxide, benzene, and benzoic acid were identified as partially oxidized reaction products. XPS and RAIRS provide evidence for the formation of a styrene-derived surface oxametallacycle that either forms styrene oxide or is further oxidized in a branching reaction to form benzoate, which is the intermediate responsible for the formation of both benzene and benzoic acid. The strong dependence of the product distribution on oxygen coverage suggests that O monomers adsorbed on Ag(1 1 1) provide a higher selectivity for partial oxidation than oxygen from the Ag(1 1 1)-p(4 × 4)-O reconstruction.     

Low temperature adsorption of oxygen on reduced V2O3(0 0 0 1) surfaces

Well ordered V₂O₃(0 0 0 1) films were prepared on Au(1 1 1) and W(1 1 0) substrates. These films are terminated by a layer of vanadyl groups under typical UHV conditions. Reduction by electron bombardment may remove the oxygen atoms of the vanadyl layer, leading to a surface terminated by vanadium atoms. The interaction of oxygen with the reduced V₂O₃(0 0 0 1) surface has been studied in the temperature range from 80 to 610 K. Thermal desorption spectroscopy (TDS), infrared reflection absorption spectroscopy (IRAS), high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) were used to study the adsorbed oxygen species. Low temperature adsorption of oxygen on reduced V₂O₃(0 0 0 1) occurs both dissociatively and molecularly. At 90 K a negatively charged molecular oxygen species is observed. Upon annealing the adsorbed oxygen species dissociates, re-oxidizing the reduced surface by the formation of vanadyl species. Density functional theory was employed to calculate the structure and the vibrational frequencies of the O₂ species on the surface. Using both cluster and periodic models, the surface species could be identified as η²-peroxo () lying flat on surface, bonded to the surface vanadium atoms. Although the O-O vibrational normal mode involves motions almost parallel to the surface, it can be detected by infrared spectroscopy because it is connected with a change of the dipole moment perpendicular to the surface.     

Cleavage of N-H bonds by active oxygen on Ag(110): I. Ammonia

Ammonia adsorbs without dissociation on clean Ag(110) with a binding energy of 11 kcal/mol. Coadsorption of ammonia and atomic oxygen at 105 K produces adsorbed hydroxyl groups and NH_x species. Coadsorption of ammonia and molecular oxygen leads to the stabilization of molecular oxygen, as is shown by the increase in the desorption peak temperature of dioxygen from 180 to 210 K. The reaction of ammonia with both forms of adsorbed oxygen produces the same products at the same temperatures. Water desorbs in a series of peaks at 310, 340, and 400 K resulting from hydroxyl recombination and hydrogen transfer from NH_x species to adsorbed oxygen atoms. NO and N₂ desorb together at 530 K. Oxygen recombination at 590 K only occurs following small ammonia doses such that excess oxygen persists on the surface. No hydrogen was seen to desorb under any reaction conditions. Vibrational spectroscopy shows that NH groups persist on the surface at temperatures well into the water desorption peak at 310 K and possibly to significantly higher temperatures, indicative of the difficulty of N-H bond cleavage by metallic silver.     

Oxidation and faceting of polycrystalline tungsten ribbons

We have measured the oxidation rate of tungsten and the evaporation rate of tungsten oxide in the temperature range from 900 to 1200 K at an oxygen pressure from 5 × 10^?4 to 5 × 10^?3 Torr. The oxidation rate increases steadily with coverage in the whole range studied. The evaporation rate decreases at high pressure and is strongly dependent on the initial conditions of the experiments. These kinetic measurements support a qualitative model of oxidation. The surface is composed of oxide islands surrounded by oxide-free regions covered only by chemisorbed oxygen atoms. On the bare regions beside the chemisorbed oxygen atoms we suppose the existence of a dilute chemisorbed oxide layer which can either enter the condensed oxide phase or evaporate. The number of the growing islands is set up at the beginning of the reaction and does not increase further. This model, consistent with kinetic results during oxidation, has been proposed first to explain results obtained by Auger electron spectroscopy and thermal desorption spectroscopy under vacuum. Faceting is particularly important in the early stages of the experiment because it can hinder the nucleation of the oxide which is a necessary step for growth. In a narrow range of temperature and oxygen pressure this inhibited nucleation leads to an enhanced evaporation rate so that the growth rate is lower. Recording this growth rate allows us to follow faceting. The parameters studied are the oxygen coverage and the temperature, experimental results are in agreement with LEED and RHEED results. Reconstruction and faceting are discussed and are believed to be caused by a smoothing of the surface during the chemisorption step.     

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).     

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.     

Investigation of oxygen adsorption on Cu(110) by low-energy ion bombardment

The interaction of molecular oxygen with a Cu(110) surface is investigated by means of low energy ion scattering (LEIS) and secondary ion emission. The position of chemisorbed oxygen relative to the matrix atoms of the Cu(110) surface could be determined using a shadow cone model, from measurements of Ne⁺ ions scattered by adsorbed oxygen atoms. The adsorbed oxygen atoms are situated 0.6 ± 0.1 Å below the midpoint between two adjacent atoms in a 〈100〉 surface row. The results of the measurements of the ion impact desorption of adsorbed oxygen suggest a dominating contribution of sputtering processes. Ion focussing effects also contributes to the oxygen desorption. The ion induced and the spontaneous oxygen adsorption processes are studied using different experimental methods. Sticking probability values obtained during ion bombardment show a strong increase due to the ion bombardment.     

The interaction of oxygen with aluminium (111)

The initial uptake of oxygen by an aluminium (111) surface at 300 K has been studied by several experimental techniques including XPS, AES, Δφ, ellipsometry and surface plasmon spectroscopy. The interaction falls into two stages. Oxygen exposure up to 30–50 L in the first stage results in a chemisorbed layer with oxygen atoms located in the immediate surface region. In the second stage increased exposure produces a thin, relatively homogeneous oxide film.     

Étude de la chimisorption de O2 À 77 K à l'aide d'isothermes d'adsorption physique à marches: Systèmes CdOKr et CdOCF4

The evolution of stepwise isotherms of CF₄ and Kr on cadmium at 77.4 K, when increasing oxygen quantities are prechemisorbed at the same temperature, brings out some information about that chemisorption. It can be deduced from the splitting of steps that, at 77.4K, oxygen chemisorbs as rather homogeneous patches. It seems to be islands spreading in proportion to the chemisorbed quantities. As a mean, 4 oxygen atoms seem to be adsorbed for every surface cadmium atom. Such a number involves a reaction with slightly deeper Cd atoms. Over these oxidized patches, oxygen seems to physisorb (precursor state of oxidation) more readily than upon the bare part of the metal. This suggests one modification in the set of hypotheses usually acknowledged for the analysis of oxidation kinetics in that temperature range. A brief comparison is made with the somewhat different islands that are known to grow on metals when oxidization, or sulfurization, occurs at higher temperatures. Some remarks are aimed at the effects of “thermal regeneration” under vacuum.     

Molecular beam studies of ethanol oxidation on Pd(110)

The adsorption and decomposition of ethanol on Pd(110) has been studied by use of a molecular beam reactor and temperature programmed desorption. It is found that the major pathway for ethanol decomposition occurs via a surface ethoxy to a methyl group, carbon monoxide and hydrogen adatoms. The methyl groups can either produce methane (which they do with a high selectivity for adsorption below 250 K) or can further decompose (which they do with a high selectivity for adsorption above 350 K) resulting in surface carbon. If adsorption occurs above 250 K a high temperature (450 K) hydrogen peak is observed in TPD, resulting from the decomposition of stable hydrocarbon fragments. A competing pathway also exists which involves C---O bond scission of the ethoxy, probably caused by a critical ensemble of palladium atoms at steps, defects or due to a local surface reconstruction. The presence of oxygen does not significantly alter the decomposition pathway above 250 K except that water and, above 380 K, carbon dioxide are produced by reaction of the oxygen adatoms with hydrogen adatoms and adsorbed carbon monoxide respectively. Below 250 K, some ethanol can form acetate which decomposes around 400 K to produce carbon dioxide and hydrogen.