Metal-support interaction in the Pd/SiO2 system: Influence of the support pretreatment

Transmission electron microscopy, electron diffraction, and convergent beam diffraction have been used to study strong metal-support interaction in the Pd/SiO2 system. It has been shown that heating of the palladium/silica system in a hydrogen atmosphere may lead to chemical (strong) interaction between metal and support and growth of palladium silicide (Pd2Si). Microdiffraction analysis shows an oriented growth of the Pd2Si precipitate with respect to the Pd matrix. The magnitude of metal-support interaction, the agglomeration of metal particles, and the formation of an intermetallic compound are strongly influenced by the thermal pretreatment of the SiO2 substrate. The presence of silanol groups (SiOH) on the silica surface seems to facilitate the metal particle migration and growth by coalescence, in contrast to OH-depleted silica where the change in the particle size proceeds by atomic diffusion. The presence of silanol groups may facilitate chemical metal-support interaction and formation of an intermetallic compound.