Metal particle size in silica-supported copper catalysts. Influence of theconditions of preparation and of thermal pretreatments

The size of metal copper particles supported on silica was studied as a fun ction of the following parameters: the preparation route, the nature of the metallic precursor, the copper content, and the thermal pretreatments befo re reduction. Two modes of preparation were investigated: (i) incipient wet ness impregnation with copper nitrate; (ii) cationic exchange with copper t etraammine and copper bis-ethanediamine complexes. The key step for the obt ention of small metal particles after reduction to 700 degrees C is the dry ing step at room temperature for the Cu/SiO2 samples prepared by impregnati on 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 character ization studies clearly showed that different Cu(II) species (copper nitrat e hydroxide, CuO, copper phyllosilicate, or isolated-grafted CU2+ ions) can exist depending on the preparation method and the thermal pretreatment. Fo r the first time, the formation of copper phyllosilicate is pointed out in Cu/SiO2 samples prepared by cationic exchange.