Chemisorption properties of thin films of Pd evaporated onto Nb recrystallized-foil surface have been studied by temperature-programmed desorption (TPD) and by measurement of the work function changes (WFC) at room temperature. TPD results indicate two states of CO adsorbed in dependence on the thickness of the palladium layer. The low temperature state (330 K) occurs at low Pd coverages (about one monolayer). Second state, occurring at slightly higher temperature (370 K), is attributed to bulk Pd. WFC data show negligible change of the work function after deposition of small amounts of Pd onto a clean Nb surface. With increasing amount of Pd, ∆ϕ rises and reaches a saturation value. Adsorption of CO onto Pd/Nb surface caused the work function increase at all coverages of Pd deposited.
Thin and thick palladium overlayers were deposited onto clean and oxidized niobium surfaces. Thermally programmed desorption of carbon monoxide, chemisorbed at room temperature on these layers has been investigated. Qualitatively similar results, i.e., weak interaction of CO with thin Pd overlayers and a stronger one with thick overlayers both for Pd/Nb and for Pd/NbOJNb systems have been obtained.
Thermally programmed desorption technique has been applied to the investigation of "virgin" and repeated adsorption of carbon monoxide on Pd/Nb and Pd/Nb2O5/Nb systems. The desorbed products of the CO interaction with these surfaces, CO and CO2, were monitored with a quadrupole mass spectrometer. On the basis of presented results some of the observed differences between the metal- and oxide-supported active metal (Pd) have been ascribed to the morphological rather than to chemical (electronic) effects in these systems. The different excess kinetic energy dissipation of Pd atoms colliding with metallic or oxidic surface is considered to influence the initial growth of the Pd layer, resulting afterwards in a different morphology of the final Pd layers.
New arrangement of temperature measurement by W-Ta thermocouple yields a slightly shifted temperature scale in comparison with that reported in the previous paper (Jirsak T., Nikolajenko V., Knor Z.: Collect. Czech. Chem. Commun. 59, 1709 (1994)). Temperatures resulting from the corrected thermoelectric voltages of the W-Ta thermocouple are given and figures with corrected temperature scales are presented.
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