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Abstract

Cobalt and bimetallic Co-Pd systems are well-known Fischer-Tropsch catalysts. Compared to Co, the bimetallic systems exhibit an increased activity towards CO-hydrogenation and methane conversion, attributed to resistance against oxidation. In order to study the oxidation behavior, model catalysts have been generated by depositing either Co or first Co and subsequently Pd onto a thin epitaxial alumina film grown on NiAl(110). Pure Co particles and bimetallic particles with a Co core and a Pd shell have been studied before and after exposure to oxygen and after thermal treatments, using x-ray photoelectron spectroscopy (XPS), temperature-programmed CO desorption (TPD), ferromagnetic resonance (FMR), and infrared reflection absorption spectroscopy (IRAS) in ultra-high vacuum.
Large doses of O2 (1000 L) at 300 K lead to complete oxidation of Co particles. Upon annealing to temperatures above 530 K, XPS indicates that the Co oxide is mostly reduced by transfer of oxygen to the alumina support, resulting in its thickening. TPD, however, indicates the existence of persistent surface oxygen species, reducing the CO adsorption energy on the particles. Also exposures to small doses of O2 (30-50 L) were studied by a careful comparison of XPS, TPD, and FMR data. In this case, XPS indicates Co in a metallic state whereas TPD and FMR indicate oxidic behavior. We conclude that small amounts of non-stoichiometric subsurface oxygen or subsurface and surface oxygen are present which are not detectable in the Co 2p XPS signal but have a pronounced effect on the surface chemistry and the magnetism, i.e., on certain bulk properties. In case of bimetallic Co/Pd particles, an incomplete Pd shell on the Co particles even promotes oxygen uptake while only a complete Pd shell inhibits oxidation.