The effect of calcination on metal oxide oxidation catalysts

The potential of a variety of physical techniques in catalyst characterization and especially in the understanding of the types of changes which may occur on calcination is described using as examples the Sn-P-O and Ln-doped CeO2 catalyst sytems. Sn-P-O catalysts produced by coprecipitation have been examined by transmission electron microscopy, diffuse reflectance infrared, XRD, solid-state magic angle spinning nmr, and Mössbauer spectroscopies. The freshly prepared catalysts comprise uniform nanosized particles of tin (IV) oxide and isolated orthophosphate [PO4] groups. Progressive calcination results in loss of molecular water and condensation of surface hydroxyl groups on the tin oxide microcrystallites allowing the binding of [PO4] groups at the oxide particle surface, and finally the formation of a distinct tin phosphate phase. Some reduction of tin to the bivalent state is also observed. Running under catalytic conditions results in similar gross changes in catalyst constitution accounting for the observed fall-off in activity. X-ray photoelectron spectroscopy (XPS) has been used to show that trivalent cations in lanthanide-doped ceria catalysts segregate to the surface on calcination giving a surface coverage of up to two monolayers after aging at 1450° for 24 hours. The segregation is dependent upon the method of preparation: materials obtained by coprecipitation undergo segregation at relatively low temperatures, whilst materials produced via sol-gel techniques are much more thermally robust. It is concluded that in catalysts produce by coprecipitation the mechanism of segregation is via macrodefects in the lattice whereas in the sol-gel derived catalyst the mechanism is one of bulk diffusion.