Microstraining in titania-, alumina- and silica-supported V₂O₅-catalysts

Commonly used catalysts in industry are compositions of highly dispersed particles. Typical systems consist of precious metals or transition metal oxides like V₂O₅ on oxide supports, especially TiO₂, Al₂O₃ and SiO₂. Support and active compound show a different chemical and material behaviour. A very important influence of the support on the active compound is the formation of microstrains due to the different thermal expansion behaviour. On the surface of a stiff linear elastic support the active compound is certainly strain hindered. In order to monitor the development of strain hindrance and further effects, in situ experiments were carried out at temperature both in an X-ray and in a neutron powder diffractometer. The width of the reflexes indicated a strong influence of the thermal expansion mismatch on peak width. The strain hindrance creates mismatch stresses high enough to overcome the yield stress of V₂O₅. The Williamson-Hall plots showed both a particle size effect as well as a stress widening but the measurements were difficult because of the materials anisotropy. The TEM work showed again very fine particles which agrees with the X-ray measurements. The support effect may be seen as thermal stress induced formation of a mosaic structure in the active compound. Moreover, the peak width correlated with the catalytic activity. The low ordered regimes in the mosaic structure are acting as further active centres for the catalytic reaction.