Nano Design of Alumina Supported Monometallic Catalysts: A Promising Way to Improve the Selective Hydrogenation of Poly-Unsaturated Hydrocarbons

摘要

In the field of catalysis by metals, a new insight for the nanodesign of supported heterogeneous catalysts is the tailoring of metallic nanoparticles. In this work, well-faceted monometallic nanoparticles (Pd, Pt, Ni) exposing mostly the {111} crystallographic facet are obtained in aqueous solution and are deposited on an alumina support. The involved mechanisms of nanoparticles formation are determined and are evidenced to be different as a function of the nature of the metal. In the case of palladium the mechanism consists in an oriented attachment of palladium nanoparticles leading to the energetically most favourable stacking of nanoparticles, at the origin of the early differentiation of the nanoparticles shapes and of the formation of the well-faceted palladium nanoparticles. In the case of platinum, the mechanism seems to be a combination of aggregation of already reduced nuclei and direct reduction depending on the experimental conditions. In the case of the less reductible metal, nickel, well-faceted nanoparticles are not obtained during the synthesis and only a thermal activation under hydrogen can engender their formation. The impact of the {111} crystallographic facet for platinum and nickel is very important and induces a drastic increase of selectivity towards olefins formation with a selectivity close to the one of a palladium catalyst which is the most selective metal for the selective hydrogenation of poly-unsaturated hydrocarbons.