Synthesis of Pt clusters in zeolite BEA - Effect of reduction rate on cluster size and location

The relation between the overall reduction rate of the intermediate PtO species in zeolite BEA, created upon calcination of the tetraammineplatinum(II) exchanged zeolite, and the size and location of the Pt clusters formed has been studied. When the reduction is carried out in excess hydrogen, small dusters (less than or equal to 15 Angstrom) are formed which are homogeneously distributed over the zeolite matrix. This is explained by the rapid and homogeneous formation of a high number of nucleation sites (PtO atoms) which results in a rapid formation of small Pt aggregates. When the supply of hydrogen is rate determining, a larger average Pt cluster size, a non-uniform cluster size distribution and a larger fraction of clusters on the external surface is obtained. In this case the reduction starts at the external surface and gradually proceeds towards the core of the zeolite crystals. This non-homogeneous formation of nucleation sites results in the enhanced probability of collisions between neutral Pt atoms and aggregates going from the core of the zeolite crystal to the external surface. This, together with steric considerations, explains the increase in the amount of clusters located on the external surface. Pt atoms which collide with full grown clusters, located in the zeolite matrix, have a lower sticking probability because growth beyond the spatial limits requires partial destruction of the zeolite lattice. Migration to the external surface, however, provides the possibility of unrestricted growth.