Gerd Wedler

Photoelectron spectroscopic study of the adsorption of carbon dioxide on Cu(110) and Cu(110)K — as compared with the systems Fe(110)CO2 and Fe(110)K + CO2

Abstract The adsorption of CO 2 on clean Cu(110) and Cu (110) K has been studied in the temperature range from 130 K to room temperature, mainly by application of the photoelectron spectroscopies UPS and XPS. There is no adsorption of CO 2 on the clean Cu(110) face. Pre-adsorption of K (0.75 monolayers), however, leads to strong interaction between CO 2 and the adsorbent. Depending on both exposure to CO 2 and temperature, CO 2 , CO δ− 2 , CO, CO n − 3 and C 2 O m − 4 can be observed as adsorbed species. It is possible to propose a reaction scheme. The systems Cu (110) CO 2 and Cu (110) K + CO 2 on one hand and the systems Fe (110) CO 2 and Fe (110) K + CO 2 on the other hand behave very similarly. Probably the interaction of CO 2 with the two adsorbents is dominated by the interaction of CO 2 with the pre-adsorbed K.

Electron spectroscopic study of the interaction of oxygen with Co(1120) and of coadsorption with water

Abstract Coverage dependence of oxygen adsorption on the hexagonal Co(11 2 0) surface is studied at 100 and 320 K by means of Auger electron spectroscopy (AES), photoelectron spectroscopy (XPS, UPS) and change in work function (Δφ). Adsorption of oxygen at 100 K leads to the formation of Co 3 O 4 -like features together with adsorbed oxygen. An influence of a precursor on the adsorption kinetics is proposed. Oxygen at 320 K builds up CoO, and the uptake behaviour is explained best in terms of chemisorption and oxide nucleation followed by island growth of the oxide. The explanation of the nature of a second oxygen species, observed in the O1s XP spectra, and the characterisation of OH are facilitated by a coadsorption experiment with oxygen and water at 100 K.

Interaction of carbon dioxide with potassium-promoted Fe(110) I. Dependence on potassium coverage and carbon dioxide exposure at 85 K

Abstract Adsorption of carbon dioxide on potassium-covered Fe(110) at 85 K has been studied by means of UV photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and change in work function (Δφ). Only a small amount of carbon dioxide dissociates into carbon monoxide and oxygen. The main reaction occuring in the studied coverage range up to one monolayer of potassium is the formation of a carbonate species and carbon monoxide. After saturation with carbonate, linear carbon dioxide is adsorbed. For potassium coverages exceeding half a monolayer, a further species could be detected with UPS. Its chemical nature is discussed and a model of the reaction paths in the investigated system is developed.

Low-energy electron diffraction and X-ray photoelectron spectroscopy on the oxidation of cobalt (112̄0)

Abstract The structural aspects of the regimes of stepwise oxidation of Co(1120) by admission of oxygen from 0.1 to 5 L were studied by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and change in work function (Δo). Six different superstructures are observed if the oxygen dosage is followed by heating to 450 K. With increasing oxygen coverage, the following order is observed: ( −2 5 −4 0 ) ; ( −2 5 −2 0 ) ; (1 × 3); overlapping (1 × 3) with ( −2 5 −2 0 ) and multiple scattering reflexes; (3 × 2), and a new (1 × 1) pattern, the latter arising from epitaxially grown CoO(100). Based on a geometric analysis of the diffraction patterns and the properties of the XP spectra, the structures are interpreted as oxygen-induced reconstructions of the surface, possibly indicating continuous growth of an oxide layer starting from isolated oxide grains. As an exception, ordered oxygen adsorption is proposed for the (1 × 3) structure. Quantitative analysis of the Co 2p region of the XP spectra was performed to determine the depth of the epitaxial CoO(100) layer. It was found to be 1.2–1.5 nm thick, corresponding to 6–8 monolayers.

Interaction of carbon dioxide with potassium-promoted Fe(110): II. Temperature-dependent reactions

Abstract The temperature-dependent reactions of carbon dioxide and potassium coadsorbed on Fe(110) have been studied between 85 and 700 K by means of UV photoelectron spectroccopy (UPS) and X-ray photoelectron spectroscopy (XPS). Since five different species are formed at 85 K(O ox , CO, CO 3 n − , CO 2 and a further species, probably of the composition C 2 O 4 m − ), the thermal behaviour of the system is rather complex. However, quantification of the O 1s spectra by applying Gaussian fits allows a rather detailed description of the reaction paths. There are two ranges of potassium coverage, which exhibit big differences concerning the thermal stability of the carbonate species: small and medium coverages (up to 60% of a monolayer) and high coverages (exceeding 80% of a monolayer). They are separated by a narrow transition range.

Activation and deactivation of cobalt catalysts in the hydrogenation of carbon dioxide

Abstract Activation and deactivation of Co foils during hydrogenation of carbon dioxide have been studied in dependence on pretreatment of the catalyst and time on stream, temperature and composition of the educt gas. Modifications of the surface due to activation and deactivation have been investigated by means of a combination of various methods, including temperature programmed processes as well as Auger electron spectroscopy and electron microscopy. Both oxidation/reduction of the surface and incorporation of oxygen and carbon in the bulk lead to marked changes in the surface structure and a considerable increase in surface area. The catalytic activity of the oxidized surface concerning the hydrogenation of CO2 is small, but increases strongly with reduction. Deactivation is accompanied with structural changes and proved to be reversible. Chemical poisoning was only observed after addition of hydrogen sulfide to the educt gas.

Interaction of oxygen with the system Si(100)Fe(film)

The interaction of oxygen with the systems Si(100) and Si(100)Fe(film) has been studied by means of the photoelectron spectroscopies UPS and XPS. Exposure of the clean Si(100) 2 × 1 surface to oxygen under ultrahigh vacuum conditions leads only to the formation of a very thin layer of suboxide. SiO2 cannot be observed even at higher temperatures. When Si(100) surfaces are precovered with an iron film (23 ML thick) and then exposed to oxygen at room temperature a large amount of iron oxide is formed. At temperatures exceeding 550 K this iron oxide reacts with the silicon under formation of SiO2 and metallic iron. At temperatures above 630 K formation of FeSi2 is superimposed. It is not the aim of the paper to add one more example of enhancement of oxidation of silicon by deposited metals but to study the mechanisms of the above mentioned processes in the special case of Si(100)Fe.

Interaction of H2O with Co(112¯0): a photoelectron spectroscopic study

Abstract Adsorption of water on the hexagonal Co(112¯0) surface is studied at temperatures between 100 and 500 K by means of photoelectron spectroscopy (XPS, UPS) and change in work function (Δφ). Molecular adsorption at 100 K is accompanied by the formation of small amounts of OH in the submonolayer range. In XPS, mono and bilayer show O 1s binding energies different from those of the multilayer. A calibration of the coverage can be obtained from work function data. When the multilayer builds up, charging of the sample is observed in UPS. When the temperature is increased, desorption of the multilayer occurs at 150 K. After desorption of the monolayer, OH remains on the surface. This species seems to be able to stabilize small amounts of molecular water. Disproportionation of OH takes place at 270 K, leaving oxygen on the surface.

LEED study of stepped Co(112̄0) prepared by ion bombardment

The effect of ion bombardment with Ar+ and Kr+ on Co(1120) has been studied by means of low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and UV photoelectron spectroscopy (UPS). At 300 K, the ion bombardment leads to the formation of a regularly stepped surface, as determined by the diffraction pattern. A geometrical evaluation of this pattern allowed development of a model of the stepped surface. The step width is found to vary between 1.4 ± 0.5 nm and 2.8 ± 0.5 nm corresponding to 3–5 or 6–8 unit cells, respectively. The step depth of 0.12 ± 0.02 nm is identical to the atomic distance between two consecutive layers of the Co(1120) surface. The orientation of the edges is preferentially parallel to the [1100] direction. As formation of these steps due to stabilisation by implanted noble gas atoms can be excluded, an alternative explanation is discussed, which combines anisotropic properties of the surface and kinetic effects, similar to the layer restricted diffusion (LRD) model.

Structural aspects of activation and deactivation of cobalt catalysts in hydrogenation of carbon dioxide

Publisher Summary The hydrogenation of carbon dioxide to methane and other hydrocarbons is an interesting catalytic alternative to the Ftscher–Tropsch reaction. One of the metals known to be active for this process is cobalt. To get a better understanding of the catalytic process, it is of great interest to elucidate the microscopic processes taking place in the various catalytic steps. Included are structural and chemical changes of the catalyst surface, which most likely influence the overall catalytic performance or the activation and deactivation of the catalyst. Concerning the hydrogenation of carbon dioxide on cobalt, pioneering work by Amariglio et al . revealed interesting correlations between activation/deactivation behavior and oxidizing pretreatments of the catalysts. Intensive studies were also performed on CO hydrogenation on supported Co catalysts providing a comprehensive background for the understanding of CO 2 hydrogenation. However, resent results also showed that the catalytic behavior of supported or compact material reveals different aspects. Thus, based on the earlier work, this chapter focuses on a thorough investigation of the respective properties of the catalytic system of compact materials. To achieve extensive information, the experimental work includes a broad variety of methods performed by both groups in Erlangen and Krakow.