Study on the formation of Pt/C catalysts by non-oxidized active carbon support and a sulfur-based reducing agent

The formation mechanism of Pt/C catalysts using non-oxidized active carbon support and the weak reducing agent Na₂S₂O₄ was investigated. Platinum on carbon catalysts were fabricated by an impregnation/reduction process of the Pt-precursor H₂PtCl₆ on carbon support. The effect of thermal treatment in argon up to 700°C on the structural characteristics of these catalysts was studied by XRD and TEM analyses. The importance of carbon support properties on Pt/C formation was recognized. Before thermal treatment a very weak internal organization (a very small particle size and amorphous structure) in the metal was obtained. Thermal treatment at relatively low temperatures leads to the growth and then to the crystallization of platinum particles in the well-known face centered cubic structure. The sintering of Pt particles occurs through the migration of Pt atoms on the carbon support, likely by a bridge-bonding mechanism on sulfur atoms. A fast growth of Pt particles occurred in the temperature range 300—400°C. Thermal crystallization, instead, occurred mostly going from 400 to 550°C. Following annealing at 550°C, the formation of platinum sulfide was revealed. The sample thermally treated at 700°C showed an anomalous XRD pattern with Pt reflexions shifted towards high angles and an increase of Pt111]/Pt220] peak intensity ratio.