Study of the reaction mechanisms between Pt(acac)2 and alumina surface sites

A new preparation technique is presented allowing the controlled variation of the average size of platinum particles in small steps of 0.2-0.5 nm in the size range between 1 and 2 nm. The particle growth is achieved by refilling, i.e. by increasing the metal loading of a parent catalyst in a separate preparation step. The technique makes use of the selective decomposition of a platinum complex, platinum bis-acetylacetonate, Pt(acac)2, on reduced platinum particles. Anchoring of the complex on the surface of the support and thus appearance of new small particles is completely avoided by appropriate surface treatments developed on the basis of a study of the reaction mechanisms between Pt(acac)2 and the support. Three types of surface sites could be identified in this study by their specific reactivity towards Pt(acac)2. A model is presented describing the nature of these sites, the occurrence of which is correlated with the degree of dehydroxylation of the alumina surface. The efficiency of the new technique in increasing the particle size while reducing to a minimum any broadening of the initial narrow size distribution is demonstrated. The results are discussed in terms of particle shapes and two- and three-dimensional growth mechanisms.