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Nanoparticles
要旨:
Nanoparticles are widely used in heterogeneous catalysis and generally, they are supported on oxides such as silica or alumina. Unfortunately, strong interactions between support and particles exist and lead to the modification of the particles’ properties which means that no unique active sites are obtained. Other side phenomena like diffusion or migration of adsorbed atoms towards the support surface are observed and undesired reactions (isomerization, cyclization,…) with the acid sites take place. Unsupported metal nanoparticles do not interact with a support. Their properties can be modified depending on their size especially in the order of magnitude between 1 and 10 nm. This makes unsupported nanoparticles interesting candidates for fine chemical catalytic applications since no side phenomena occur and active sites are well defined.
Surface Organometallic Chemistry on Metals allows controlling the coordination sphere of the active metal by an organometallic fragment. By changing “at will” the steric and electronic properties of the organometallic fragment, it is possible to influence the chemio- , stereo- and regio-selectivity of reactions catalyzed by metallic surfaces.
Pt is considered to be a very good catalyst for selective hydrogenation; in particular for enantioselective hydrogenation of alpha ketoesters and it is believed to be less hydrolyzing than Ru. Unsupported Pt nanoparticles were prepared following the procedure already described for Ru, using Pt(dba)2 as precursor and octylsilane (C8SiH3) or vinylphenylmethylsilane (VPMSiH) as stabilizer. The size and the structure of the nanoparticles were determined by TEM and the grafted organometallic fragments were characterized by solid state 1H and 13C CP-MAS NMR, IR and elementary analysis. Crystalline Pt nanoparticles (see Figure) of about 2 nm corresponding to a cubo-octahedron with 3 edge atoms, covered by organosilane fragments were obtained. These nanoparticles are active and chemio-selective in the hydrogenation of cetopentoylactone into the corresponding alcohol and their properties were compared to silica supported Pt.
