Mass transfer in low-density xerogel catalysts

Texture and morphology of a Pd-Ag/SiO2 hydrodechlorination low-density xero gel catalyst prepared by a cogelation sot-gel process were characterized in detail to examine mass transfer in such catalysts. The catalyst consists o f active Pd-Ag nanocrystallites trapped inside elementary 20 nm microporous silica particles arranged in larger aggregates, which constitute the macro scopic pellet. To reach active sites, reactants must first diffuse through large pores located between aggregates of SiO2 Particles and then through s maller pores located between those elementary particles inside the aggregat es. Finally, they, diffuse through micropores located inside silica particl es. Diffusion in such a "funnel" structure cannot be described assimilating the pellet to a pseudo-continuum. Diffusion should be examined carefully a t three levels of decreasing size: the macroscopic pellet, the aggregate of silica particles, and the elementary silica particle. This approach shows that cogelled xerogel catalysts have remarkable mass-transfer properties.