INSITU ELECTRON-MICROSCOPY STUDIES OF SURFACE SEGREGATION IN BIMETALLIC CATALYST PARTICLES

We have used various techniques including in-situ transmission electron microscopy to examine the manner by which the catalytic action of platinum is modified by the introduction of either iron, cobalt, or nickel into the metal. Changes in the behavior of the noble metal have been investigated by the use of the probe reactions, catalytic oxidation, and hydrogenation of single-crystal graphite, which are known to be extremely sensitive to the chemical state of the catalyst surface. In-situ electron diffraction measurements showed that in all cases alloys were formed when the mixed system was treated in hydrogen. When such specimens were subsequently heated in oxygen the composition and chemical state of the particles changed significantly, the ferromagnetic metal tending to segregate to the catalyst-gas interface in the form of an oxide leaving platinum and residual alloy at the graphite interface. Under these conditions the catalyst was observed to undergo a spreading action along the graphite edge regions and it was interesting to find that in this bimetallic state there was a significant decrease in the catalytic activity of the noble metal toward graphite oxidation. © 1992.