Rare-earth metal overlayers

Catalysis of rare-earth metal overlayers or bimetallic compounds... 

In the rare-earth metal overlayers, the introduction of Eu or Yb onto the Co and Ni surface leads to a continuous decrease in hydrogenolysis activity, whereas the activity of hydrogenation, H2-D2 exchange, C6Hi2-D2 exchange and dehydrogenation shows a tendency to increase. A marked decrease in suitably arranged sites with the fractions of... 

This paper contains a simple introduction to the topic hydrogen on metals, a sketch of the recent progress in surface analytical experimental techniques and theoretical models and a review of recent results on hydrogen at the surface of a large variety of metallic substrates. Included are clean and precovered transition metals, rare earth metals. Mg, alloys and inter-metallies, glassy metals and overlayers. We show that the aspects structure of adsorbate and substrate, electronic and magnetic properties and hydrogen-metal bond, surface dynamics as well as transitions from surface to bulk have to be studied. ... 

H ON TRANSITION AND RARE EARTH METALS, NORMAL METALS, CRYSTALLINE AND AMORPHOUS ALLOYS AND IN METALLIC OVERLAYERS... 

In this context, rare earths on transition metal substrates attracted considerable research attention from two directions i) to understand the overlayer growth mechanisms involved [3] and ii) to prepare oxide-supported metal catalysts from bimetallic alloy precursor compounds grown in situ on the surface of a specific substrate [4,5]. The later studies are especially significant in terms of understanding the chemistry and catalytic properties of rare earth systems which are increasingly used in methanol synthesis, ammonia synthesis etc. In this paper, we shall examine the mechanism of Sm overlayer and alloy formation with Ru and their chemisorption properties using CO as a probe molecule. 

Thin films of a rare earth on another metal (or the other way round) were investigated by various authors. Flowever, real alloys were rarely formed and most of the time such studies were performed from a purely surface science point of view (electronic structure, spectroscopic properties. ..) and with no direct relevance to catalysis. One may quote, for example, the oxidation studies of tantalum and aluminum with thin cerium overlayers, carried out at low temperature, which showed that cerium enhances the oxide growth on both substrates (Braaten et al. 1989). However, the mechanism was not identical. No alloy was formed with tantalum and a catalytic oxidation took place. On aluminum, the formation of an intermetallic Ce—Al-O oxide layer was evidenced. 

The interaction of deuterium water vapor with cerium has been studied in XPS recently by Koel et al. (1980). A very similar behavior as compared to the heavy rare earths was found, in that a nonlinear uptake was obtained after exposures of about SOL. Calculations of overlayer thickness using the Ce 3d metal peaks indicated, however, that the ratio corresponds to more than one monolayer of OD. Another argument for the formation of a bulk hydroxide or oxy-hydroxide was derived from the different splitting of the peaks associated with each component of the 3d spin-orbit doublet in the metal. This splitting was 3.7 eV upon oxidation by O2 and 4eV upon exposure to water, indicating a different chemical species. 

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