ABSTRACT
Transmission electron micrographs [12] have revealed that during heating in oxygen, Pd crystallites extended on an alumina substrate, changed their shape and exhibited tearing and fragmentation. They also showed that for various metals supported on alumina, films with thicknesses in the nanometer range coexisted with crystallites during heating in oxygen [13-15]. These films connected several crystallites and/or formed a contiguous phase on the substrate. They have sometimes been detected by electron microscopy, but frequently they were too thin to detect. During subsequent heating in hydrogen, the interconnecting films either fractured and merged with the particles or even triggered the coalescence of neighboring particles. The undetectable films that covered the substrate in regions containing only few particles could also fracture, thus generating patches, the contraction of which led to particles in regions initially lacking them.
