D. Borgmann

XPS studies of oxidic model catalysts : internal standards and oxidation numbers

Abstract X-ray photoelectron spectroscopy was used to investigate the behaviour of model systems representing DeNOx catalysts and their individual components (V6Mo4O25, MoO3, V2O5, TiO2). Particular attention was directed to the behaviour of arsenic. The spectrometer energy scale was calibrated by gold vapour deposition samples. The Au4f 7 2 signal can also be used to determine the effects of slight signal shifts which influence the full widths at half maximum (FWHMs) of the investigated signal. It can be established that As2O3 exhibits typical oxidation behaviour on TiO2 (anatase) surfaces. The As5+ formed is reduced at different rates as a function of the radiation used (Mg Kα1,2, Al Kα1, 2). Neither oxidation nor reduction are observed in the absence of TiO2 (anatase). Although these redox processes are coupled on TiO2 (anatase), no chemical change from pure TiO2 (anatase) can be established for titanium. Molybdenum in V6Mo4O25 is present in oxidation number +6 and vanadium in oxidation numbers +4 and +5. Formation of a composite oxide which contains arsenic can largely be ruled out on the basis of the FWHMs of the As3d signals.

New setup for in situ x-ray photoelectron spectroscopy from ultrahigh vacuum to 1 mbar

In an effort to extend the pressure range for electron-based spectroscopies from ultrahigh vacuum into the so-called pressure gap region, we have built a new apparatus for in situ x-ray photoelectron spectroscopy up to 1mbar. The principle of the experimental setup is based on a modified hemispherical electron energy analyzer, a modified twin anode x-ray source, and several differential-pumping stages between sample region and electron detection. The reaction gas is provided in situ either by background dosing or, as a new feature, by beam dosing, using a directed gas beam from a small tube. The latter allows for higher local pressures. The performance of the new setup is discussed, deriving normalization procedures from the analysis of the attenuation of the substrate photoemission intensity by the increasing gas phase pressure. In addition, the change of the work function due to changes in surface composition can be evaluated in situ by analyzing the binding energy shift of the gas phase core-level peak...

Microscopic models of PdZn alloy catalysts: structure and reactivity in methanol decomposition

We review systematic experimental and theoretical efforts that explored formation, structure and reactivity of PdZn catalysts for methanol steam reforming, a material recently proposed to be superior to the industrially used Cu based catalysts. Experimentally, ordered surface alloys with a Pd : Zn ratio of approximately 1 : 1 were prepared by deposition of thin Zn layers on a Pd(111) surface and characterized by photoelectron spectroscopy and low-energy electron diffraction. The valence band spectrum of the PdZn alloy resembles closely the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1 : 1 stoichiometry. Among the issues studied with the help of density functional calculations are surface structure and stability of PdZn alloys and effects of Zn segregation in them, and the nature of the most likely water-related surface species present under the conditions of methanol steam reforming. Furthermore, a series of elementary reactions starting with the decomposition of methoxide, CH(3)O, along both C-H and C-O bond scission channels, on various surfaces of the 1 : 1 PdZn alloy [planar (111), (100) and stepped (221)] were quantified in detail thermodynamically and kinetically in comparison with the corresponding reactions on the surfaces Pd(111) and Cu(111). The overall surface reactivity of PdZn alloy was found to be similar to that of metallic Cu. Reactive methanol adsorption was also investigated by in situ X-ray photoelectron spectroscopy for pressures between 3 x 10(-8) and 0.3 mbar.

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 CO2 with Cs-promoted Fe(110) as compared to Fe(110)/K + CO2

Abstract The interaction of CO2 with Cs-promoted Fe(110) at 85xa0K as well as temperature-dependent reactions between 100 and 700xa0K have been studied by means of ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Several surface species could be detected at 85xa0K, i.e. carbon monoxide (CO), carbonate (COn−3), physisorbed linear carbon dioxide (COlin2) and very small amounts of oxidic oxygen (Oox). An oxalate species (C2Om−4) could not be identified definitively, but from comparison with the literature there is evidence that C2Om−4 is present. Increasing the temperature after saturation with CO2 leads to a complicated reaction behaviour. CO2 either desorbs or dissociates into CO and COn−3 or forms C2Om−4 at temperatures between 85 and 160xa0K. Above 160xa0K C2Om−4, decomposes in parallel reactions into CO2, COn−3 and CO. Above 320xa0K, adsorbed CO either desorbs into the gas phase or dissociates into C and O. In the temperature region between 500 and 700xa0K a recombination of C and O to CO and the desorption of Cs take place. As in the case of Fe(110)/K+CO2, at high alkali coverages two carbonate species could be detected which dissociate upon heating at different temperatures. The system Fe(110)/Cs+CO2 is proved to be very similar to the system Fe(110)/K+CO2.

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.

A fast x-ray photoelectron spectroscopy study of the adsorption and temperature-dependent decomposition of propene on Ni(100)

Using synchrotron radiation, the adsorption and decomposition of propene (C3H6) on the Ni(100) surface has been investigated in situ by time-resolved and temperature-programmed x-ray photoelectron spectroscopy. At 105 K, high-resolution C 1s spectra indicate precursor mediated occupation of a single adsorption state from submonolayer to monolayer coverage with evidence of adsorbate–adsorbate interactions and dispersed phase growth. High exposures lead to the formation of multilayers which desorb above 105 K leaving a chemisorbed monolayer. Between 105 and 150 K, a shift of the binding energies in the C 1s spectra is attributed to the transition from π- to di-σ-bonded propene. An abrupt change in the photoemission spectra occurs at 200 K due to the conversion of di-σ-bonded propene to a C3 intermediate containing a methyl group. Formation of this C2HxCH3 surface species is complete at 300 K and is immediately proceeded by dehydrogenation to carbidic carbon which is the final decomposition product above 370 K.