Metal Particle Size in Silica-Supported Copper Catalysts. Influence of the Conditions of Preparation and of Thermal Pretreatments

The size of metal copper particles supported on silica was studied as a function of the following parameters: the preparation route, the nature of the metallic precursor, the copper content, and the thermal pretreatments before reduction. Two modes of preparation were investigated: (i) incipient wetness impregnation with copper nitrate; (ii) cationic exchange with copper tetraammine and copper bis-ethanediamine complexes. The key step for the obtention of small metal particles after reduction to 700 °C is the drying step at room temperature for the Cu/SiO2 samples prepared by impregnation and the presence of chemical bonds between Cu2+ and the support for the samples prepared by cationic exchange. The chemical bonding is established during drying or calcination, depending on the metal complex. The characterization studies clearly showed that different Cu(II) species (copper nitrate hydroxide, CuO, copper phyllosilicate, or isolated−grafted Cu2+ ions) can exist depending on the preparation method and the thermal pretreatment. For the first time, the formation of copper phyllosilicate is pointed out in Cu/SiO2 samples prepared by cationic exchange.