Multiply-twinned particles in silver catalysts

Abstract

It has been known for several years that anomalous structures can occur in the early stages of particulate growth. Ino¹ and Ino and Ogawa² for gold on rocksalt and Allpress and Sanders³ for nickel on mica found small polycrystalline nuclei which they interpreted as multiply-twinned particles (MTPs). These particles can be considered as 5 or 20 f.c.c. tetrahedra joined by twin boundaries to give decahedra or icosahedra (Fig. 1). As the tetrahedra do not assemble to a completely space filling structure, either elastic strains or grain boundaries are required to complete the particles. More recently, MTPs have been observed in the early stages of growth for all f.c.c. metals in certain conditions. Gillet⁴ has recently reviewed the field whilst Hayashi et al.⁵ give extensive results for f.c.c. metals in argon smokes. The possible significance of MTPs in heterogeneous catalysts was first suggested by Allpress and Sanders⁶ and investigated by Avery and Sanders⁷. They found, however, that at most 2% of particles 100–350 Å in size for Ni, Au, Pt and Pd on γ-alumina were multiply twinned. We report here the presence of considerable numbers of multiply-twinned particles in various heterogeneous metal catalysts, in particular silver on α-alumina. The particles can have quite large dimensions up to ∼2,000 Å. Attention is drawn to the possible significance of some special sites on these particles which have no counterpart on single crystal surfaces, and to the unusual distribution of surface faces.