ON THE RELATIONSHIPS BETWEEN CATALYTIC ACTIVITY AND MORPHOLOGY OF THE METALLIC PHASE IN CATALYSTS PREPARED BY CHEMISORPTION-HYDROLYSIS
Creators: name: SCOTTI, NICOLA
Publisher: name: Universit� degli Studi di Milano
Year: 2012

description: Chemisorption?hydrolysis (CH) represents a fruitful preparation method, as it combines the simplicity of impregnation techniques, both in terms of handiness and cheapness, with the high metal dispersion obtained by anchoring techniques. In this work different Cu, Co and Co-Cu catalysts were prepared by CH. The characterization of CH copper silica materials agree on the formation of finely dispersed phase of CuO, easily reducible to metal copper at any copper loading. In particular CH method allows to keep a great dispersion obtaining very small copper particles also to an higher metal loading (up to 15%). Moreover CH offers the possibility to tune the nature of the copper phase using different supports during catalyst preparation. In fact, if on SiO2 small and well-dispersed and easily reducible CuO particles are formed either at low and high metal loading, whereas on SiO2-Al2O3 the nature of Cu species depends on the copper amount: up to about 5 wt% only hardly reducible Cu?+ (+1 ? ? ? +2) is formed on the catalyst, while after this loading a CuO phase easy reducible to well formed Cu (0) begins to form on the surface. Copper CH catalysts were tested in the direct epoxidation of propene with O2. Catalysts exposing only metallic copper (thus silica based ones) were those that revealed the best activity: Cu (0) appears to be the active site for the reaction. Copper catalyst are well known to catalyze hydrogenation reaction, moreover Zaccheria et al. reported a bifunctional process of etherification starting from 4-methoxyacetophenone in presence of 2-propanol. Two steps are involved in the process: the hydrogenation of ketone to the alcohol and the subsequent acid reaction with 2-propanol and 1-(4-methoxyphenyl)ethanol to give the corresponding ether. The structure sensitivity of these two separate reactions was studied on 8.5 and 15 wt% reduced Cu/SiO2 Chrom CH catalysts. The results indicates that both the reactions depend on terrace atoms. Also well dispersed Co/SiO2 and CO-Cu/SiO2 catalysts were successfully prepared with a modified CH method. A Co (II) hardy reducible phase was generated on Co/SiO2, while on the bimetallic materials more easily reducible Co species were formed. Catalytic experiments on methanol decomposition indicate that Co-Cu/SiO2 catalysts show higher CH3OH conversion and higher CO selectivity compared to single Co and Cu catalysts with the same total amount of metal. The control of morphology (size, shape and surface structures) is a very important method in order to tune catalytic, electronic and optical proprieties of metal nanoparticles. An ideal heterogeneous catalyst should be composed by exactly the same nanometric size metal nanoparticles, uniformly dispersed in the support. Bovine serum albumin (BSA) was used as surfactant for the shape controlled synthesis of Au nanoparticles by a modified polyol processs. We successfully prepared Au nanocrystals of different shape: octahedra, cubes and tetrahxaedra. In particular the innovative use of BSA allows one to synthesize tetrahexahedra, an unusual shape for metal nanocrystals. The shape controlled synthesis of metal nanocrystals bounded by high-index facets (such as tetrahexahedra) is a potential route to obtain materials with enhanced catalytic performances, since high-index crystal planes generally show much higher activity than that of the most common stable low-index planes, due to high surface energy and roughness.