Platinum-gold and platinum-tin bimetallic catalysts: Correlation between microstructure, miscibility, and activity.

Abstract

The correlation between the microstructure and the activity and product selectivity of catalysts is central to the underst and ing of a working catalyst. This is especially true in supported bimetallic catalyst systems such as Pt-Au and Pt-Sn, two systems which are known to be miscible in the bulk to varying extents. The possibility of the presence of bimetallic alloys is an added complexity to the task of determining the relative distribution of the metallic components in the small metal particles. This microstructural information is vital in the interpretation of the catalytic reactivity trends. A series of bimetallic platinum-gold catalysts supported on high surface area non-porous Aerosil was prepared by the incipient wetness technique. The reducibility of the various metal components in these catalysts was determined by temperature programmed reduction (TPR). Microstructural characterization of the catalysts was performed by using analytical and high resolution electron microscopic techniques. The combination of elemental analysis by EDX and lattice fringe imaging by HREM of individual metal particles was used to determine the presence and location of the various components of this bimetallic system. The kinetic behavior for the n-hexane conversion reaction and the product selectivities towards dehydrocyclization, isomerization and hydrogenolytic cracking were measured. The product selectivity trends in n-hexane conversion can be attributed mainly to geometric effects related to the relative distribution and interdispersion of the platinum and gold atoms in the catalysts. Bimetallic Pt-Sn/Al$\\sb2$O$\\sb3$ catalysts were found to have beneficial characteristics in the activity maintenance for the production of cyclical compounds in the n-hexane conversion reaction. TPR studies indicated that the average oxidation state of tin in these catalysts was Sn(II). HREM and AEM investigations showed that the metal particles in bimetallics with low tin content were elemental platinum surrounded by a patch of ionic tin. However, at higher tin loadings evidence of PtSn alloy formation was found. From these characterization techniques a comprehensive structural model was obtained for these catalysts. An attempt at correlating this structural information with the catalytic trends was made.