On the Dispersion and Characteristics of Platinum-Tin Bimetallic Clusters

Abstract

Metallic clusters of platinum in combination with tin dispersed over alumina exhibit interesting catalytic properties for reforming reactions by providing selectivity advantages and lower rates of deactivation. To understand these aspects, model samples of platinum-tin supported on alumina have been studied in detail using transmission electron microscopy and diffraction techniques. Clusters are dispersed on alumina by vacuum evaporation method while maintaining the substrate at 25° and 350°C during deposition of tin and platinum respectively. Pt-Sn system is found to be quite complex in as deposited state but after reduction in hydrogen shows the formation of bimetallic clusters of PtSn intermetallic alongwith a small quantity of SnO. These phases remain intact during sulfurization (i. e. heating in thiophene-hydrogen mixture). However, reduction in hydrogen for extended period leads to disappearance of SnO without affecting PtSn. The accelerated deactivation intentionally caused by passing methylcyclohexane (MCH) decomposes PtSn into platinum clustres with simultaneous formation of SnO₂. Microstructural observations reveal that sulfur addition helps in maintaining high dispersion of bimetallic clusters for prolonged period by suppressing the coarsening process. The electronic interaction, dilution effects and modifications of support, resulting due to addition of tin, taken together seem to be responsible for improved selectivity towards aromatic hydrocarbons and better stability of PtSn/Al₂O₃ system in reforming processes.