Role of Promoters on Tungstated Zirconia Catalysts

The isomerization of straight-chain alkanes is effectively catalyzed by tungstated zirconia promoted by Pt or Pt and Fe. In the present paper, it is attempted to better understand the role of these promoters. The materials were characterized by CO chemisorption and by transmission electron microscopy (TEM) in combination with EDX, and their catalytic performance was tested by the isomerization of n-pentane to isopentane in the presence of H2. At low calcination temperature, very high Pt dispersion is observed with Pt atoms/clusters being “embedded” in or decorated by oxide material. The highly dispersed Pt moieties appear to catalyze the hydrogenolysis reaction, thus leading to low isoalkane selectivities. Higher calcination temperatures yield larger Pt particles (lower dispersion), which results in high catalytic activity and selectivity toward the isoalkane target product. It is inferred that the dissociative H2 chemisorption occurs on the Pt particles and facilitates the reduction of the surface tungstate, which is responsible for the dehydrogenation and isomerization of n-alkane. The role of the additional promoter iron oxide still remains unresolved.