H. Kuhlenbeck

Formation of a well-ordered aluminium oxide overlayer by oxidation of NiAl(110)

We have investigated the electronic and geometric structure of a thin oxide film grown by oxidation on NiAl(110) using electron spectroscopic techniques, i.e., LEED, EELS, XPS and ARUPS. This film is inert to adsorption of, respectively reaction with many molecules up to temperatures of about 800 K. It is well ordered as deduced from the LEED pattern and covers the whole surface. We find that the oxide film is about 5 A thick, consisting of aluminium oxide as shown by EELS, XPS and ARUPS. It is most likely formed of two aluminium layers and two quasihexagonal oxygen layers with oxygen surface termination. Since the oxide film is rather thin it only shows a two-dimensional band structure which has been investigated using ARUPS. For the electronic levels of the oxide strong periodic dispersions are observed with bandwidths compatible to dispersion bandwidths calculated for the ΓX direction of α-Al2O3.

Hydroxyl groups on oxide surfaces: NiO(100), NiO(111) and Cr2O3(111)

Abstract 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 gas atmosphere or by dosing of water. They can be removed by annealing at temperatures T ⩾ 600 K (NiO) or T ⩾ 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.

Adsorption and reaction of CO2 and CO2/O CO-adsorption on Ni(110): Angle resolved photoemission (ARUPS) and electron energy loss (HREELS) studies

Abstract Molecular adsorption is observed on a Ni(110) surface at 80 K. The relative binding energies of the valence ion states as determined by ARUPS are consistent with those in the gas as well as in the condensed phase, and indicate that the electronic structure of the adsorbed molecule is only slightly distorted upon adsorption at this temperature. The adsorbate spectra show E versus k ∥ dispersions indicating some long-range order in the adsorbate. The variations in relative ionisation probabilities of the ion states as a function of electron emission angle suggest that the molecular axis is oriented parallel to the surface within ± 20°. Upon heating the adsorbate to above 100 K. (i.e. 140 K) the spectrum changes. A new species causing an increase in work function by 1 eV can be identified. Comparison with calculations suggests that it is an anionic bent CO; molecule. Electron energy loss studies on this intermediate species support the proposed bent CO 2 geometry and favour a coordination site with C 2v symmetry. The bent CO 2 moiety is stable up to 230 K. Further heating to room temperature leads to dissociation of the bent CO 2 molecule into adsorbed CO and O. The CO molecule is oriented with its axis perpendicular to the surface. The bent CO 2 species appears to be a precursor to dissociation. Results on CO 2 adsorption on an oxygen precovered surface show that CO 2 interacts with oxygen at 85 K. Upon heating the co-adsorbate to near room temperature a reaction product is formed the nature of which cannot yet be clearly identified.

Infrared spectroscopic investigation of CO adsorbed on Pd aggregates deposited on an alumina model support

Abstract We show that CO adsorption on Pd aggregates of varying size and order gives rise to several absorption bands in the range of CO stretching frequencies which we assign to different absorption sites. At low temperature (90 K) and saturation coverage we find the population of terminal as well as bridging sites. The CO molecules are preferentially terminally bound, but these exhibit the lower binding energies in agreement with earlier TDS studies. The CO-molecules on two-fold bridging sites are more tightly bound and the relative intensity of the corresponding absorption band increases with increasing size and order of the Pd aggregates. The observed bands may be assigned according to IR results on Pd(111) single crystals which is the orientation of the aggregate surfaces observed for the present deposits. The bands previously assigned to Pd(100), which gain intensity for the well-ordered aggregates with preferentially (111) oriented surfaces, we reassign to CO molecules bound to edges of the (111) facets.

SMART: a planned ultrahigh-resolution spectromicroscope for BESSY II

Abstract A new UHV spectromicroscope called SMART (spectromicroscope for all relevant techniques) is currently under construction for a soft X-ray undulator beamline at BESSY II. The instrument consists of a plane-grating monochromator with an aspherical focusing mirror and an ultrahigh-resolution, low-energy electron microscope containing an energy filter. It can be used as a photoemission microscope for a variety of electron spectroscopies (XAS, XPS, UPS, XAES) and has a calculated spatial resolution of better than 1 nm. A maximum energy resolution of about 0.1 eV will be provided by a corrected omega filter. The high lateral resolution of the electron microscope will be achieved through the correction of the chromatic and spherical aberrations of the objective lens by means of an electrostatic mirror in combination with a corrected magnetic beam separator. An additional electron source placed on the other side of the beam separator opposite the electrostatic mirror will also allow LEEM, MEM and small-spot LEED investigations to be carried out. The basic ideas, the various modes of operation and the electron optical design of the instrument are outlined.

The structure of thin NiO(100) films grown on Ni(100) as determined by low-energy-electron diffraction and scanning tunneling microscopy

Abstract A Ni(100) surface exposing terraces of approximately 100 A width which are separated from each other by monatomic steps descending along the [010] direction has been oxidized above room temperature. Via intermediate formation of the well-known p(2 × 2) and c(2 × 2) chemisorbed phases, which are identified by LEED (low energy electron diffraction) and STM (scanning tunneling microscopy) in the present study, a thin film of 4–5 layers of NiO(100) builds up on the surface. The NiO layer consists of crystallites with a typical lateral extension of 50 A as revealed by the STM data. SPA-LEED (LEED spot profile analysis) measurements allowed us to determine that the crystallite surfaces are tilted preferentially along the [011] and [011] directions of the Ni(100) plane by an average angle of 8° with a half width of the angular distribution of 6°. We show that the development of the oxide islands most probably starts at the terrace edges of the metal surface. While the islands grow in size the strain between oxide and metal increases due to the large differences in the lattice constants of Ni and NiO. Part of the strain is compensated by a tilt of the islands induced via migration of Ni atoms from the step edges underneath the oxide islands. The generated NiO surface is characterized by two types of regions, namely the regions on the islands which are basically flat and contain regular NiO sites, covering 75–80% of the crystal surface, and the regions between the islands with many defect sites (20–25% of the surface area). The consequences of the structural properties of the NiO film on the adsorption of molecules, i.e., NO, are discussed in line with results of a previous study.

Adsorption and reaction on oxide surfaces: NO, NO2 on Cr2O3(111)/Cr(110)

Abstract We report results of electron spectroscopic measurements, i.e., LEED, EELS, ARUPS, XPS, and NEXAFS on NO, and NO2 adsorbed on a thin Cr2O3 film with (111) orientation grown on top of a Cr(110) single-crystal surface via an oxidation procedure. It is shown that the Cr2O3(111) surface is likely to consist of Cr-terminated and O-terminated terraces and that the Cr-atoms located within the oxide surface are in oxidation states different from the bulk. Our results indicate that these sites are involved in the dissociation of NO2 at rather low temperature to yield adsorbed NO and adsorbed oxygen.

CO2 adsorption and reaction on Fe(111): An angle resolved photoemission (ARUPS) study

Abstract Molecular CO 2 adsorption is observed on an Fe(111) surface at 85 K. For the main fraction of molecules the relative binding energies of the valence ion states as determined by ARUPS are consistent with those in the gas as well as in the condensed phase, and indicate that the electronic structure of that fraction of adsorbed molecules is only slightly distorted upon adsorption. There is a fraction of adsorbed molecules at 85 K that can be identified as bent, anionic CO 2 − species. While the weakly adsorbed, linear CO 2 molecules desorb at low temperature, the CO 2 − species is stable up to 160–180 K. The latter is proposed to be a precursor to dissociation. Above this temperature adsorbed carbon monoxide and oxygen are observed on the surface, and at room temperature the CO 2 − signals have disappeared. Heating above room temperature dissociates the CO molecules into carbon and oxygen.

Interaction of oxygen with palladium deposited on a thin alumina film

The interaction of oxygen with Pd particles, vapor deposited onto a thin alumina film grown on a NiAl(1 1 0) substrate, was studied by STM, AES, LEED, XPS, TPD and molecular beam techniques. The results show that O2 exposure at 400–500 K strongly influences the oxide support. We suggest that the oxygen atoms formed by dissociation on the Pd surface can diffuse through the alumina film and react with the NiAl substrate underneath the Pd particles, thus increasing the thickness of the oxide film. The surface oxygen inhibits hydrogen adsorption, and readily reacts with CO at 300–500 K. For large and crystalline Pd particles, the system exhibits adsorption–desorption properties which are very similar to those of the Pd(1 1 1) single crystal surface. The molecular beam and TPD experiments reveal that, at low coverage, CO adsorbs slightly stronger on the smaller Pd particles, with an adsorption energy difference of � 5–7 kJ mol � 1 for 1 and 3–5 nm Pd particles studied. 2002 Elsevier Science B.V. All rights reserved.

Bridging the pressure and materials gaps between catalysis and surface science: clean and modified oxide surfaces.

The preparation of model systems based on thin epitaxial oxide films and oxide single crystals is discussed. A variety of surface sensitive techniques has been applied to study the geometric and electronic properties of these systems. The findings are correlated with adsorption and reaction of probe molecules on the surfaces. Metal vapor deposition under controlled conditions leads to the formation of metal aggregates with narrow size distributions. Their properties have been characterized, establishing that we can begin to bridge the materials gap between catalysis and surface science. While mainly performed under UHV conditions, adsorption measurements can be pushed to ambient conditions using non-linear optical techniques such as sum frequency generation. Results for systems with deposited metal aggregates will be discussed.