Surface reactivity of strongly interacting alloys and its influence on their catalytic properties

Abstract

The intermetallic compounds of the Brewer-Engel type represent a unique case in novel heterogeneous catalyst systems. The components of these alloys are known to have altered electronic structures described as sharing of d orbitals from the electron rich to the electron poor member. The strong bonding in the bulk of such compounds affects surface segregation phenomena and causes variations in surface composition relative to the bulk. Such modification, coupled with the changes possible in surface atomic geometry through compound stoichiometric variation and the high reactivity of the compounds with hydrogen or oxygen at elevated temperatures, that occurs in the catalyst life cycle can affect catalytic activity, specificity and stability. This investigation examines the catalytic properties and structure relationships induced and controlled by surface reactivity in Zr/Ni and other intermetallic compounds. The Brewer-Engel intermetallic compound system Zr/Ni has been investigated to compare the surface chemistry among known ordered phases. The variation in surface composition has been correlated with bulk composition, for samples before and after reactive gas treatments at a variety of temperatures. Comparison is made to samples with similar thermal treatments under ultra high vacuum conditions. This variation in surface composition with sample treatment history has led to an explanation for the catalytic behavior of the Zr/Ni in the hydrogenation of ethylene. Samples of known ordered phases of Zr/Ni intermetallic compounds were prepared by arc melting the elements in appropriate stoichiometries. Uniformly sized powders of the intermetallic phases were prepared for catalyst testing along with planar samples for surface characterization. These powder and planar samples were examined by X-ray photoelectron spectroscopy, Auger electron spectroscopy and scanning electron microscopy, under varied conditions of vacuum, hydrogen and oxygen treatments at elevated temperatures...