A.J.M. Mens

XPS analysis of palladium oxide layers and particles

The thermal reduction of both oxidised palladium foil and SiO2Si(100) supported palladium oxide particles, ranging in size fro 3.5 to 13 nm, was investigated with XPS. Equations were derived for the XPS intensities, measured at normal emission angles, of the particles which consisted of a metallic core and an oxidic skin. By applying these equations on the spectra measured after each reduction step, the particle size and the size of the metallic core were calculated. Measurements on palladium foil showed that the oxide layer thickness decreases linearly with the reduction time up to the last monolayer oxide. The reduction rate of the surface oxide is about eight times lower than the reduction rate of the bulk oxide. The growth of the metallic core in palladium oxide particles appeared to be linearly proportional to the surface area. The reduction rate of the smallest particles was comparable to the reduction rate of the surface oxide of the palladium foil. The larger particles behave identical to the palladium foil.

Adsorption of oxygen and surface oxide formation on Pd(111) and Pd foil studied with ellipsometry, LEED, AES and XPS

Abstract The interaction of oxygen with Pd(111) and polycrystalline palladium foil has been studied with ellipsometry, LEED, AES and XPS in the temperature range of 300 to 770 K and pressures up to 1 Pa. Ellipsometry was used to monitor the adsorption of oxygen and gave indication for the formation of a surface oxide at higher temperatures ( T ≥ 470 K) and pressures ( p ≥ 10 −4 Pa). The presence of a surface oxide is supported by a complex LEED pattern, ascribed to a square lattice with a = 7.5 ± 0.5A and domains in six orientations. It was not possible to match this structure with a simple overlayer structure on the (111) plane or with an unreconstructed crystal plane of PdO. XPS measurements on palladium foil, after the same treatment, showed the presence of ≈0.5 ML PdO on the surface. Bulk oxide was not formed. The amount of oxygen on the surface could not be determined with AES because during AES the electron beam easily removed adsorbed oxygen, especially on Pd(111). On palladium foil the oxygen is removed less effectively by the electron beam, which indicates that oxygen is bound more tightly to defects. Bulk palladium oxide, produced by heating the palladium foil in air, was not affected by the electron beam, even at high current densities. The ellipsometric parameters δΔ and δΨ never exceeded 0.40° and 0.08° respectively, both on Pd(111) and palladium foil. This indicates that the diffusion of oxygen is limited to surface layer(s) under the conditions studied. Diffusion to the bulk did not occur.

XPS/NRA investigations of particle size effects during the oxidation of Cu particles supported on oxidised Si (100)

Abstract In this study we investigate the effects of the particle size on the oxidation of supported Cu particles under equivalent experimental conditions with respect to temperature and pressure. We compare the results of oxidation of 4 and 15 nm Cu particles supported on oxidised Si(100) at room temperature and 573 K. At room temperature and p = 10 Pa, the oxidation of Cu0 to Cu2+ at the surface of 4 nm particles proceeds significantly slower than in the case of 15 nm particles. At 573 K and p = 0.1 Pa, a reverse trend is observed: oxidation of 4 nm particles from Cu0 to a Cu2+-oxide proceeds significantly faster than in the case of 15 nm particles. At both temperatures Cu0 was oxidised to Cu2+-oxide through an intermediate Cu+-oxide.

Real-Time Quantitative Operando Raman Spectroscopy of a CrOx/Al2O3 Propane Dehydrogenation Catalyst in a Pilot-Scale Reactor

Combined operando UV/vis–Raman spectroscopy has been used to study the deactivation of CrOx/Al2O3 catalyst extrudates in a pilot scale propane dehydrogenation reactor. For this purpose, UV/vis and Raman optical fiber probes have been designed, constructed and tested. The light absorption measured by operando UV/vis spectroscopy was used to correct for the effect of catalyst darkening. This makes operando Raman spectroscopy a quantitative technique to follow the formation of coke deposits on‐line during the first hour of catalytic propane dehydrogenation process. The probe was used to study the coke deposition at the top and bottom of the reactor. Differences in the rate of coke deposition were observed, which were related to the local temperature of the catalyst bed. The chemical nature of the coke deposits formed on a catalyst extrudate, as expressed by the D1/G ratio, was independent of reaction time as well as of the position of the catalyst extrudate within the reactor bed.

AES study of electron beam induced damage on TiO2 surfaces

Abstract The damage induced by an electron beam on several TiO 2 surfaces has been studied with Auger Electron Spectroscopy. We studied single crystal specimens of TiO 2 (rutile) of the (100) and (110) orientation as well as pressed powder specimens consisting of rutile and a 65%–35% mixture of anatase and rutile. All surfaces acquired a (sometimes extremely high) negative charge upon electron irradiation. All surfaces did loose a significant amount of oxygen due to electron stimulated desorption. In the presence of background oxygen this process did not occur. Carbon (graphite) was deposited on the samples from residual gases. This is due to the decomposition of adsorbed carbonaceous species on the damaged surface and to the decomposition of positive gas phase ions on the negatively charged (irradiated) region of the surface. Significantly less carbon was deposited on the powder samples as compared to the single crystals. Carbon deposition is inhibited by the presence of oxygen in gas phase.

The effect of chemical composition and structure on XPS binding energies in zeolites

The effect of the composition and structure of zeolites on the XPS core level binding energies has been studied for a large class of zeolites, viz. FAU, MFI, MOR and LTA with Si:Al ratio chabging from 1 to 160. Also the effect of the difference in the counter ions (Na, K, Rb, Ca, Mg, Ba, La) was investigated. As absolute binding energies cannot be determined to any reasonable degree of accuracy we focuss in this work on relative peak positions, which can be found with high precision. The main effect on binding energy differences between silicon, aluminium and oxygen in the zeolites is caused by their aluminium content. Effects of the zeolite structure and the co-cation are far less important. The Si(2p) to Al(2p) XPS peak separation decreases regularly with increasing aluminium content. We performed quantum chemical calculations that enable us to rationalise these findings in terms of a non-homogeneous charge distribution in the zeolite framework.

Unraveling the Structure of Mn‐Promoted Co/TiO2 Fischer‐Tropsch Catalysts by In Situ X‐Ray Absorption Spectroscopy

Combination of in situ X‐ray absorption spectroscopy (XAFS) at the Co and Mn K‐edges with electron microscopy (STEM‐EELS) has allowed to unravel the complex structure of a series of unpromoted and Mn promoted TiO2‐supported cobalt Fischer‐Tropsch catalysts prepared by homogeneous deposition precipitation (HDP), both in their calcined and reduced states. After calcination the catalysts are generally composed of large Co3O4 aggregates (13–20 nm) and a MnO2‐type phase that is either dispersed on the TiO2 surface or, for the major part, covering the Co3O4 particles. Additionally Mn is also forming a spinel‐type Co3−xMnxO4 solid solution at the surface of the Co3O4 particles. In pure Co or when small amount of this spinel‐type phase are formed during calcination, reduction in H2 at 350 °C produces Co0 particles of variable sizes (3.5–15 nm) otherwise Co reduction is limited to the Co2+ state. Manganese that exists entirely in a Mn2+ state in the reduced catalysts is forming (1) a highly dispersed Ti2MnO4‐type ...

Quantitative Analysis of Palladium Oxide by Means of the Palladium M4,5N1N2,3Transition

The palladium M 4,5 N 1 N 2,3 transition, which is almost absent in palladium metal, becomes quite intense in palladium oxide. The intensity of the transition is linearly proportional to the amount of palladium oxide. This means that AES can be used for quantitative analysis of palladium oxide layers or supported palladium oxide particles.