Palladium-silver sol-gel catalysts for selective hydrodechlorination of 1,2-dichloroethane into ethylene - I. Synthesis and characterization

Low-density xerogel Pd-Ag/SiO2 catalysts were prepared by a cogelation of tetraethoxysilane with organically substituted alkoxides capable of forming chelates with palladium and silver ions, followed by a drying under vacuum, a calcination, and a reduction. The resulting Pd-Ag alloy particles have diameters of about 2-3 nm and are dispersed inside the silica particles which exhibit a monodisperse micropore size distribution centered at 0.8 nm. At low silver loading, the composition of the bimetallic particles is almost identical to the overall composition, but when this loading is increased, the percentage of silver in the alloy particles is appreciably lower than the overall percentage, In all bimetallic samples, a few large pure silver particles are observed. When dried under vacuum, the xerogels exhibit a texture similar to that of aerogels dried under supercritical conditions with pore volumes in the range 2-6 cm(3)/g and pores ranging from micro- to macropores of several hundred nanometers. When the silver loading is increased, xerogels containing palladium show aggregates of increasingly interpenetrated silica particles. This is explained bq an aggregation, during the gelation reaction, of colliding SiO2 particles which is limited by an increasingly slow sticking reaction due to the growing basicity in the precursor solution when the concentrations of Ag and associated compounds are increased. During thermal treatments, the Pd-Ag alloy seems to form by the migration, through the porous network, of silver which finally alloys with Pd crystallites trapped inside SiO2 particles. Whereas ethane is the main product when hydrodechlorination of 1,2-dichloroethane is performed over pure palladium, the dilution of this active metal by silver leads to a strong increase in ethylene selectivity. (C) 1997 Academic Press.