Coefficient silica formers

If reaction (1) is fast, a concentration gradient of O2 inwards and SiO outwards must exist. Then the interdiffusion coefficients and the effective thiekness of the boundary layer will control whether the eritical i (SiO) of the condensation reaction (3) is reached or not. From tables and estimations of those physieal constants we can relate back to the oxygen pressure of the bulk gas necessary to induce the critical P(SiO) on the surface. The calculation 3nelded reasonable results for the active-passive boundary of Si in streaming atmospheres with low oxygen contents and accordingly the theory was later applied to other silica-formers [14,15]. 

Although the immobilization of Rh2POM in microporous silica yielded a stable electrode, the disparity in molecular size relative to pore width is insufficient to extend this approach to use of mesoporous sol-gel materials as supports for these catalysts. Mesoporous structures have an intrinsic advantage over microporous materials in that effective diffusion coefficients are greater in the former materials [36]. Macromolecular dendrimers have several attractive features for use in sol-gel materials other than projected stable encapsulation. They may provide a template around which mesoporous silica can form [37]. Metal centers that are potential catalysts can be incorporated readily into dendrimeric compounds. Further, the structure of dendrimers assures spatial distribution of metal centers, which is optimum geometry for most types of catalysis. 

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