Organofunctionalized mesoporous silicas as platforms to the controlled achievement of metallic nanostructures: relations between the nanostructuring and the catalytic activity

The obtainment of supported me tallic nanoparticles without stabilizers agents and with expressive reactivity continues a challenge. In order to expand some existing synthetic routes as well as overcome some difficulties, this work was focused in the in situ achievement of mono or bimetallic nanoparticles of palladium, gold and platinum supported on functionalized mesoporous silicas, and establish some correlations existing between structural properties from nanoparticles and their catalytic activity. Metallic nanostructures were synthetized using the SBA-15 silica as support. Two synthetic routes were employed: the first was based on the modification of the silica surface with a cationic alkoxysilane, derived from 1-methylimidazol or from 4,4'-bypiridine, and then the anion exchange capacity of these materials was employed in the adsorption of one or two metallic complex that were subsequently reduced and originated supported monometallic nanoparticles or nanoalloys. The second route was based on the modification of the silica surface with trimethoxysilane and the reducing capacity of this material was employed in the achievement of monometallic nanoparticles by immersing this modified silica on complex metallic solutions. The three functionalized silicas were investigated by N2 physisorption, small angle X-ray scattering, 29Si and 13C nuclear magnetic resonance and thermogravimetric analysis to confirm the SBA-15 synthesis and its modifications. Materials formed by supported metallic nanoparticles were analyzed by atomic emission to the metal quantification, by ultraviolet-visible spectroscopy to confirm the formation of the Au nanoalloys and by transmission electron microscopy to determination of the nanoparticles sized and the structural modifications occurred in the supports. Then, the supported metallic nanoparticles were tested as heterogeneous catalysts in the probe reaction of 4-nitrophenol reduction with sodium borohydride aqueous solution. The analysis of the functionalized silicas and of the supported metallic nanomaterials revealed that the surface modification as well as the nanoparticles achievement did not changed the support textural properties important for catalytic activity. Metallic nanoparticles were formed mostly inside the pores, they had diameter significantly small, and were highly dispersed throughout the porous structure without the presence of other additional surface stabilizing agent. The catalytic tests at 298 K showed that the reactivity of the nanocatalysts are dependent on the nature of the nanoparticles forming metal or on the nanoalloy composition. The reactivity are also dependent on the surface modifier used at the silica functionalization step. The monometallic materials formed by palladium and the nanoalloys rich on this metal are the most active catalysts for the 4-nitrophenol reduction. The nanoparticles synthetized in the cationic silicas are more active too. The experiments with different amounts o f catalyst indicated that the low reactivity of some materials is associated to high catalytic activity in the borohydride hydrolysis, which is an undesired reaction, because for the synthetized materials the reactivity is not associated directly to the total metallic surface added at the tests and the reaction velocity is not limited by diffusion. The catalytic tests at different temperatures conducted with some materials allowed the determination of activation parameters (AU)