Rh inclusion in sol-gel SiO2. Effects of Rh precursors on metal dispersion and SiO2-Rh thermal behavior

RhCl3·nH2O and [RhCl(C2H4)2]2 are used as precursors for the preparation of 1%Rh in sol-gel derived SiO2. The gelling process of Si(OEt)4 is carried out in the absence of solvent and under strong acid catalysis. The thermal behavior of Rh precursors, of SiO2 gel and Rh-SiO2 composites is independently studied by analysing organic species released at definite temperature intervals and concomitantly collecting infrared, XPS, TEM, XRD and porosity data. Results indicate that nanometric Rh particles may be obtained from [RhCl(C2H4)2]2, their dispersion being homogeneous, dense and stable up to 250°C, whereas RhCl3·nH2O affords less metallic dispersion with other crystalline Rh-species; in both cases, well-shaped Rh metal crystallites are obtained at 650°C. The different synthetic approaches used for the preparation of RhCl3- and [RhCl(C2H4)2]2-derived samples, are invoked to account for the features of Rh dispersion obtained by mild temperature treatment. Moreover, the particular procedures for sol-gel SiO2 synthesis are related to the high-temperature maintenance of great porosity and elevated specific surface area.