Promoter segregation in Pt and Ru promoted cobalt model catalysts during oxidation–reduction treatments

Abstract

A flat model approach was used to study segregation phenomena in Pt and Ru-promoted cobalt catalysts prepared by co-impregnation. In the calcined state the cobalt (Co3O4) and promoter phases are well mixed and in the oxidic form (PtO2 and RuOx). Upon reduction the promoter alloys with the metallic cobalt and it is evenly distributed over the cobalt particles. During reoxidation of the metallic system Pt and Ru segregate: Kirkendall voids of Co3O4 are formed, and the promoter is left inside the hollow oxide shell. Reduction of the reoxidized state results in re-alloying of the promoter with the cobalt phase, but the promoter concentration in the surface region of the sample is lower than after reduction of the calcined catalyst. This is explained by incomplete remixing, a result of the segregation in the re-oxidized state. The results suggest that break-up of the Pt-containing hollow oxide shells leads to uneven distribution of the promoter over the cobalt phase, forming both particles with high promoter concentration and promoter-free particles. Our approach provides a simple method to generate encapsulated noble metal particles, which are potentially interesting as sinter-resistant catalysts.