We study the adsorption and reaction of CO* as a function of temperature between 100 and 700 K in the presence of Na on a Pd(ll1) surface using high resolution electron energy loss spectroscopy. While COa does not react with a clean Pd(ll1) surface, we find various reaction channels on the Na precovered Pd(ll1) surface depending on the Na coverage. At iutetmediate coverage a bent CO!species with characteristic vibrational bands can be unambiguously identified. This species is stable up to 200 K, and dissociates into CO and oxygen similar to its behaviour on other surfaces, and as reported in a previous photoemission study [Wambach et al., Surface Sci. 209 (1989) 1591. In case the surface has been oxygen contaminated before Na and CO, exposure surface carbonates can be observed.

Hydroxyl groups at the surfaces of NtO ( 100)) N10 ( 111) , and Cr203 ( 111) have been studled usmg different surface sensttlve spectroscopies The OH groups are readdy formed by the mteractlon of the oxide surfaces Hrlth the residual gas atmosphere or by dosing of water They can be removed by annealmg at temperatures T>, 600 K (NIO) or T> 540 K (Cr,O,) OH does not bond to regular NtO( 100) sites so that for a cleaved NlO( 100) single crystal surface no OH adsorption could be observed For the more defect contammg NIO( lOO)/Nl( 100) film the existence of OH could be venfied by Isotope exchange wtth OD As mdlcated by TDS (thermal desorption spectroscopy) of an NO adsorbate, OH groups fully block the ( 111) onented surface of N10 for NO adsoorptlon which mdlcates that OH groups bond to regular NlO( 111) surface sites For Cr203( 111) thermal decomposition of water at defect sates and photochemIcal dlssoclatlon IS observed The latter path seems to mvolve water molecules m the second layer and leads most ltkely to an occupation of regular surface sites

We have investigated the electronic and geometric structure of sinfaces of transition metal oxides and simple metal oxides applying electron spectroscopic methods. In order to avoid charging problems, we have resorted to the preparation of thin (5 50 A) metal oxide films grown on metallic substrates via several oxidation techniques. We have studied NiO, COO, Cr,O,, and A&O,. The thin films have the advantage that they may be easily cooled to liquid nitrogen and liquid helium temperatures. Another interesting feature of the thin films is the possibility to prepare thermodynamically unstable surfaces, such as (111) surfaces of ionic rock salt structures, and study the adsorption and reaction at such surfaces. Adsorption and reaction of molecules has not only been investigated on the clean oxide substrates but also on the surfaces modified through deposited ultrathin metal films. Such systems may be considered as models for heterogeneous catalysts.

The present paper summarizes the possibilities to use electron energy loss spectroscopy (EELS) to investigate the electronic properties of systems with highly localized electronic states, such as oxide surfaces. Surface excitations may be clearly distinguished from bulk excitations in EELS, and eventually a ligand field spectroscopy may be developed on this basis. This ligand field spectroscopy is used to study surface phase transitions at oxide surfaces. Adsorbates influence the surface properties of oxides dramatically. Examples are discussed.

Metal oxides may be prepared as thin (5-50 Angstrom) films on top of metallic substrates. By such means oxide substrates with properties identical to bulk oxides may be formed which can be studied via electron spectroscopies without being hindered by charging, as well as cooling problems. We report here on results on NiO and on Cr2O3 surfaces. We discuss some structural aspects of oxide surfaces such as surface reconstruction of polar rock salt-type surfaces, and structural phase transitions on corundum type structures. The nature of the phase transition will be discussed with respect to the magnetic properties of the oxide. Furthermore we report on the interaction of those surfaces with molecules from the gas phase. In particular we study the interaction with small molecules such as CO, NO, O-2, CO2, H2O and C2H4. We observe via various surface sensitive techniques such as thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), angle resolved photoemission (ARUPS), electron energy loss spectroscopy (HREELS), infrared-reflection-absorption-spectroscopy (IRAS), and near-edge-X-ray-absorption-fine-structure spectroscopy (NEXAFS), associative as well as dissociative adsorption and in the case of ethylene also polymerization reactions. Via isotopic labelling techniques combined with IRAS we study in detail the interaction of oxygen with the oxide surfaces, a process of general interest in connection with oxidation reactions.

A polar Cr2O3(0001) surface is prepared as an epitaxial film on a Cr(110) substrate. The film is thick enough to represent the bulk surface. Applying a variety of surface sensitive techniques [thermal desorption spectroscopy (TDS), reflection absorption infrared spectroscopy (RAIRS), electron energy loss spectroscopy (EELS) and photoelectron spectroscopy (PES)] we have studied adsorption of molecular oxygen, ethene and sodium.

Structural rearrangements of the (111) surface in the system Cr2O3(111)/Cr(110) as a function of temperature were investigated by means of low-energy electron diffraction (LEED) and electron energy loss spectroscopy (EELS). At room temperature, one observes a simple (1*1) LEED pattern of the clean (111) surface. If the temperature was lowered to 150 K a ( square root (3)* square root (3))R30 degrees superstructure was observed. The structure reached its maximum intensity at about 150 K substrate temperature. Below 150 K the superstructure vanished again and the simple (1*1) LEED pattern of the (111) surface was recovered at 90-100 K. Parallel to this, a considerable change in the electron energy loss spectra with varying temperature was observed. With the help of quantum-chemical cluster calculations the low-energy excitations in the range between 0.8 and 2.5 eV were assigned to local d-d excitations of Cr3+ ions at the Cr2O3(111) surface. Some of these peaks were quenched upon adsorption of gases such as CO, NO or CO2. We propose a model of two successive phase transitions the first of which is a disorder-to-order transition above 150 K whereas the second is an order-to-order transition below 150 K. The transitions may be driven by antiferromagnetic coupling of the surface chromium ions to those in the second layer.

We have investigated the electronic and geometric structure of surfaces of transition metal oxides and simple metal oxides applying electron spectroscopic methods. In order to avoid charging problems, we have resorted to the preparation of thin (5 – 50 Å) metal oxide films grown on metallic substrates via several oxidation techniques. We have studied NiO, CoO, Cr2O3, and Al2O3. The thin films have the advantage that they may be easily cooled to liquid nitrogen and liquid helium temperatures. Another interesting feature of the thin films is the possibility to prepare thermodynamically unstable surfaces, such as (111) surfaces of ionic rock salt structures, and study the adsorption and reaction at such surfaces. Adsorption and reaction of molecules has not only been investigated on the clean oxide substrates but also on the surfaces modified through deposited ultrathin metal films. Such systems may be considered as models for heterogeneous catalysts.

Hydroxyl groups at the surfaces of NiO(100), NiO(111), and Cr2O3(111) have been studied using different surface sensitive spectroscopies. The OH groups are readily formed by the interaction of the oxide surfaces with the residual ps atmosphere or by dosing of water. They can be removed by annealing at temperatures T greater-than-or-equal-to 600 K (NiO) or T greater-than-or-equal-to 540 K (Cr2O3). OH does not bond to regular NiO(100) sites so that for a cleaved NiO(100) single crystal surface no OH adsorption could be observed. For the more defect containing NiO(100)/Ni(100) film the existence of OH could be verified by isotope exchange with OD. As indicated by TDS (thermal desorption spectroscopy) of an NO adsorbate, OH groups fully block the (111) oriented surface of NiO for NO adsorption which indicates that OH groups bond to regular NiO(111) surface sites. For Cr2O3(111) thermal decomposition of water at defect sites and photochemical dissociation is observed. The latter path seems to involve water molecules in the second layer and leads most likely to an occupation of regular surface sites.

Molecular adsorption on oxide surfaces is gaining increasing interest both experimentally and theoretically. Adsorption studies on model systems, where well ordered thin oxide films grown on a metal substrate to avoid sample charging in connection with electron spectroscopic measurements, were used, are reported. Two oxide systems are compared: (i) a reactive transition metal oxide surface of Cr2O3(111) where it is shown that the surface contains Cr2+ ions which trigger its reactivity; (ii) a non-reactive simple metal oxide surface of γ-Al2O3(111) which is used as a support model surface. The adsorption of various molecules on both surfaces has been examined, and how the properties of the surface are modified when metals are deposited on the oxide surface have been studied. The results of alkali metal deposits on Cr2O3(111) and Pt deposits on γ-Al2O3(111) are presented. The applied methods include LEED, STM, TPD, ARUPS, ELS, XPS, HREELS and ISS.

We have investigated the adsorption of CO and CO 2 on epitaxially grown Cr 2 O 3 (111) by means of EELS. LEED, ARUPS, NEXAFS and XPS. CO is found to adsorb on the oxide surface in an ordered (√3 × √3)R30° structure with the molecular axis oriented approximately parallel to the surface. CO 2 on the other hand reacts with the chromium oxide to form a surface carbonate. Adsorption of CO, respectively reaction of CO 2 only takes place on a clean, freshly flashed oxide surface. Preadsorption of oxygen leads to a surface which is rather inert to adsorption, likely due to electronic or steric reasons.

Coadsorption of Na with CO2 and H2O on a thin Cr2O3(111) single crystal epitaxially grown on a single-crystal Cr(110) substrate has been performed to study the effects of the coadsorbates on the electronic properties of the Na species. High-resolution electron-energy-loss spectroscopy spectra as a function of Na overlayer coverage reveal a continuous attenuation of the Fuchs-Kliewer surface-phonon loss modes with near total attenuation by approximately 2 monolayers (ML). Coadsorption with either H2O or CO2 returns the substrate Fuchs-Kliewer modes to a relative intensity approximately equal to that observed on the clean Cr2O3(111) surface. Angle-resolved photoemission spectroscopy (ARPES) measurements from the Cr2O3(111) surface demonstrate almost total attenuation of the Cr2O3(111) valence-band emission by a Na coverage of approximately 2 ML with no return of Cr2O3(111) Valence-band emission features upon coadsorption. These observations provide strong evidence that the development of a metallic Na overlayer results in a damping of the coupling interaction between the electron and the Fuchs-Kliewer phonons and that upon reaction with the coadsorbed species, a uniform dielectric overlayer is formed which permits the return of the Fuchs-Kliewer losses. The metallic-to-nonmetallic transition of the overlayer is also supported by the ARPES data which reveal suppression of emission from the Fermi level, the disappearance of the LVV Auger transition, and a shifting to higher binding energy of the Na 2p emission upon compound formation during coadsorption.

Coadsorption of Na with ${\mathrm{CO}}_{2}$ and ${\mathrm{H}}_{2}$O on a thin ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$(111) single crystal epitaxially grown on a single-crystal Cr(110) substrate has been performed to study the effects of the coadsorbates on the electronic properties of the Na species. High-resolution electron-energy-loss spectroscopy spectra as a function of Na overlayer coverage reveal a continuous attenuation of the Fuchs-Kliewer surface-phonon loss modes with near total attenuation by \ensuremath{\sim}2 monolayers (ML). Coadsorption with either ${\mathrm{H}}_{2}$O or ${\mathrm{CO}}_{2}$ returns the substrate Fuchs-Kliewer modes to a relative intensity approximately equal to that observed on the clean ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$(111) surface. Angle-resolved photoemission spectroscopy (ARPES) measurements from the ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$(111) surface demonstrate almost total attenuation of the ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$(111) valence-band emission by a Na coverage of \ensuremath{\sim}2 ML with no return of ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$(111) valence-band emission features upon coadsorption. These observations provide strong evidence that the development of a metallic Na overlayer results in a damping of the coupling interaction between the electron and the Fuchs-Kliewer phonons and that upon reaction with the coadsorbed species, a uniform dielectric overlayer is formed which permits the return of the Fuchs-Kliewer losses. The metallic-to-nonmetallic transition of the overlayer is also supported by the ARPES data which reveal suppression of emission from the Fermi level, the disappearance of the LVV Auger transition, and a shifting to higher binding energy of the Na 2p emission upon compound formation during coadsorption.

We present CO2 adsorption studies on a Pd(111) surface as a function of Na precoverage using high resolution electron energy loss spectroscopy. For low Na coverage we find dissociation of CO2 into CO and O even at 90 K. At high Na coverage as well as on oxygen contaminated Pd surface, formation of surface carbonates can be observed. At intermediate Na coverage the reaction proceeds via a bent anionic COδ−2 species which dissociates at higher temperatures into CO and O.

We study the adsorption and reaction of CO2 as a function of temperature between 100 and 700 K in the presence of Na on a Pd(111) surface using high resolution electron energy loss spectroscopy. While CO2 does not react with a clean Pd(111) surface, we find various reaction channels on the Na precovered Pd(111) surface depending on the Na coverage. At intermediate coverage a bent CO28− species with characteristic vibrational bands can be unambiguously identified. This species is stable up to 200 K, and dissociates into CO and oxygen similar to its behaviour on other surfaces, and as reported in a previous photoemission study [Wambach et al., Surface Sci. 209 (1989) 159]. In case the surface has been oxygen contaminated before Na and CO2 exposure surface carbonates can be observed.