V. Nehasil

Influence of substrate structure on activity of alumina supported Pd particles : CO adsorption and oxidation

Abstract Recently the unexpected effects of partial CO dissociation were reported for small Pd particles deposited on γ-alumina, prepared by thermal oxidation of aluminium whilst this behaviour was not observed on Pd α- alumina model catalysts. In this study we compared the CO adsorption and oxidation properties of Pd particles deposited on α-alumina substrates of different surface stoichiometry with those of Pd on γ-alumina. It was shown that the oxygen adsorption capacity as well as the reactivity of α-alumina supported catalysts was lower than those of γ-alumina supported catalysts. The CO and oxygen sticking probability measurements indicated the CO and O diffusion over the support that in the case of CO decreased with T. The temperature dependent CO oxidation rate was explained in terms of its limitation by the CO diffusion process which is less important in the case of the aluminium rich alumina surface.

Study of CO desorption and dissociation on Rh surfaces

Abstract The adsorption of CO on Rh foil and small Rh particles supported by Al 2 O 3 polycrystalline surfaces was studied using temperature programmed desorption (TPD). The supported Rh particles were prepared by evaporation, using the principles of electron bombardment. The activation energy of desorption was calculated. The results show the dependence on the CO exposure of the sample as well as the size effect. Desorption and recombination peaks were measured. During CO desorption the CO 2 production was followed. The dependence of CO oxidation rate on the kind of sample is discussed.

Size effect study of carbon monoxide oxidation by Rh surfaces

Abstract The catalytic oxidation of CO on Rh foil and small Rh particles supported by Al 2 O 3 polycrystalline surfaces was studied by the transient experiment of molecular beam surface scattering technique. The supported Rh particles were prepared by evaporation, using the principle of electron bombardment. The results show the dependence of the CO 2 production rate on surface morphology and particle size. The defect rich Rh foil surface and smaller particles were found to be more active. The high oxygen adsorption capacity of 2.5 nm particles as well as their time dependent form of the CO 2 production rate is explained by the oxygen diffusion into the subsurface region.

Miniature electron bombardment evaporation source: evaporation rate measurement

Miniature electron beam evaporation sources which operate on the principle of vaporization of source material, in the form of a tip, by electron bombardment are produced by several companies specialised in UHV equipment. These sources are used primarily for materials that are normally difficult to deposit due to their high evaporation temperature. They are appropriate for special applications, like heteroepitaxial thin films growth that require very low and well controlled deposition rate. We propose a simple and easily applicable method of evaporation rate control. The method is based on the measurement of ion current produced by electron bombardment of evaporated atoms. In order to be able to determine the ion current – evaporation flux calibration curves we measured the absolute values of evaporation flux by means of Bayard-Alpert ion gauge.

Investigation of behaviour of Rh deposited onto polycrystalline SnO2 by means of TPD, AES and EELS

Rh films deposited onto a SnO2 polycrystalline substrate were used to study CO adsorption in dependence on the amount of deposited rhodium. The sample purity and composition were investigated by means of Auger electron spectroscopy, the development of a surface electronic structure by electron energy loss spectroscopy. Temperature programmed desorption (TPD) was used to follow the adsorption of CO. The behaviour of Rh deposit stimulated by the annealing and CO adsorption was monitored by a variation of CO desorption energy and an area of the TPD peak. The CO adsorption capacity of sample and stimulation energy of CO desorption decrease if the TPD experiments are repeated. During these experiments the Auger spectra exhibited the presence of a metallic Sn on the surface.

Molecular beam study of CO and O2 sticking coefficients on Rh model catalysts

Abstract The adsorption of CO and O 2 on Rh foil and Rh small particles supported by an Al 2 O 3 polycrystalline substrate were investigated by means of a molecular beam technique. The supported Rh particles of different sizes were prepared by evaporation using the principle of electron bombardment. The Rh foil used was plane annealed and Ar + ion bombarded. The effect of Rh surface morphology on the results is discussed. All the sticking coefficients obtained on small particles show that the part of adsorbed molecules reach the Rh particles by diffusion across the Al 2 O 3 substrate. The diameters of the diffusion zones are calculated.

Abilities of elastic peak electron spectroscopy in the field of thin films growth investigation: Au on Al and Al2O3

Since the electron elastic reflection coefficient depends on the target atomic number, elastic peak electron spectroscopy (EPES) may be employed to monitor inhomogeneous thin films growth. In frame of this work, growth of gold on aluminium and alumina was studied using an originally designed EPES spectrometer at various temperatures. To optimise the experimental parameters, the elastically reflected electron current from a gold sample normalised to that one from an aluminium sample was measured as a function of the incident energy. A maximum at the energy of 550 eV was found. It corresponded to the result of an independent EPES measurement on gold carried out using another spectrometer. Then gold was grown on aluminium and the elastically reflected electron current was followed. The signal intensity vs. deposition time curve exhibited equidistant break points indicating the layer-by-layer growth mode. The saturation value was reached as soon as three atomic layers of gold had been completed. This result is in a good agreement with a Monte Carlo simulation confirming an excellent surface sensitivity of EPES. Final experiments were performed on gold grown on alumina. The EPES signal intensities raised fractionally again. It proved the layer-by-layer growth mode of gold on alumina and applicability of EPES to its investigation.

XPS study of Pd particle growth on different alumina surfaces

Abstract X-ray Photoelectron spectroscopy (XPS) was used to investigate the Pd particle formation on Al2O3 substrates. Palladium was deposited step by step in situ using an electron beam evaporator on single crystalline alumina specimens which had been cleaned by heating in air. The (0001), (1–102) and (11–20) orientations of alumina were used for Pd deposition with no further treatment. In addition, the (0001) and (1–102) substrates were also Ar∗ ion sputtered before deposition, to change the surface stoichiometry and order. The Pd growth process was monitored by means of the modified Auger parameter and the Full Width at Half Maximum of the Pd3d 5 2 peak. The results obtained on the different substrates are compared and the influence of the substrate on the Pd particle growth is discussed.

Influence of the alumina surface orientation to the Rh particle growth and reconstruction

Growth and reconstruction of Rh particles deposited onto different Al 2 O 3 substrates were studied. Three single crystalline α-alumina substrates, having (0001), (1102) and (1120) surface orientations, and polycrystalline γ-alumina were positioned simultaneously on the same sample holder. Consequently, all performed procedures (Rh deposition, sample annealing, CO and O 2 adsorption) were completely identical. The sample conditions were checked after every particular procedure by means of electron spectroscopy. especially AES and EPES. Morphology parameters (Rh particle size and density) of Rh deposit were measured ex situ using transmission electron microscopy of the sample carbon replica. The difference among the electron spectra obtained on the identically treated samples is discussed to study the influence of the substrate orientation on the properties of Rh/ Al 2 O 3 model catalysts.

Study of CO adsorption on Rh/alumina model catalysts in dependence on substrate orientation

Abstract The interaction of a CO molecular beam with rhodium vapour-deposited onto (0001), (1–102) and (11–20) α-alumina was investigated. The dependence of the CO sticking coefficient on the sample temperature and the relative CO coverage for different Rh deposits were calculated from the intensity of the reflected CO molecular beam. The diffusion of CO over the uncovered substrate to the metal is discussed in relation to the Rh particle size and density.