COLL 82

Cogelation is an effective sol-gel method to produce in one single step finely dispersed metal catalysts supported on highly porous oxides with remarkable structure and properties. It consists in the concomitant hydrolysis and condensation of alkoxides (RO)₃Si-X-A containing an organic group A able to form a chelate with a cation Mⁿ⁺ of a catalytic metal with a silica network-forming reagent such as Si(OC₂H₅)₄. It has been shown that the complex [(RO)₃Si-X-A]_m(Mⁿ⁺) acts as a nucleation agent in the formation of silica particles. After drying, calcination and reduction, the xerogel is constituted of metal crystallites (3 nm) located inside microporous silica particles (10-50 nm) arranged in larger aggregates, themselves constituting the macroscopic pellet. Such a structure allows to avoid diffusional limitations. Moreover, while being accessible through micropores, metal crystallites are trapped in silica particles which makes them sinter-proof. In the case of bimetallic catalysts, cogelation allows to produce alloy nanocrystallites.