THERMOANALYTICAL AND SPECTROSCOPIC STUDIES OF THE SYNTHESIS OF SUPPORTED PALLADIUM CATALYST IN A H2/PDCL2/SIO2 PARENT SYSTEM

A 5 wt% Pd/SiO2 catalyst was synthesized by heating PdCl2-impregnated SiO2 in H2 at 300-degrees-C for 2 h. It was found that the metal particle dispersion is improved when the reduction step is preceded by calcination at 300-degrees-C for 2 h. Thermogravimetry of the impregnated support in air, N2 and H2 atmospheres was used to monitor the interactions occurring during the various preparative steps (i.e. drying, calcination and reduction) of the catalyst. The solid product of each preparative step was characterized by X-ray diffractometry and UV/Vis diffuse reflectance spectroscopy. The results indicate that following the drying step (at 110-degrees-C in air) the palladium occurs in two detectable forms: PdCl2 particles and Si-O-Pd(n+) surface species. The calcination appears to transform the PdCl2 particles into the latter surface species. The H2-reduction eventually converts the surface species into finely-dispersed Pd-degrees metal particles (average size = 8-14 nm). No other reduction products, such as PdySix, were detected.