SYNTHESIS OF PT CLUSTERS IN ZEOLITE BEA - EFFECT OF REDUCTION RATE ONCLUSTER-SIZE AND LOCATION

The relation between the overall reduction rate of the intermediate Pt O species in zeolite BEA, created upon calcination of the tetraamminep latinum(II) exchanged zeolite, and the size and location of the Pt clu sters formed has been studied. When the reduction is carried out in ex cess hydrogen, small dusters (less than or equal to 15 Angstrom) are f ormed which are homogeneously distributed over the zeolite matrix. Thi s 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 distribu tion and a larger fraction of clusters on the external surface is obta ined. In this case the reduction starts at the external surface and gr adually proceeds towards the core of the zeolite crystals. This non-ho mogeneous formation of nucleation sites results in the enhanced probab ility of collisions between neutral Pt atoms and aggregates going from the core of the zeolite crystal to the external surface. This, togeth er 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 stick ing probability because growth beyond the spatial limits requires part ial destruction of the zeolite lattice. Migration to the external surf ace, however, provides the possibility of unrestricted growth.