PREPARATION OF PD/SIO2 CATALYSTS FOR METHANOL SYNTHESIS .3. EXPOSED METAL FRACTION AND HYDROGEN SOLUBILITY

Tetramminepalladium complex was ion exchanged (IE) on purified, well characterized macro- and microporous Davison G-59 and G-03 silica gels (Pd loadings: 1.1-7.7% Pd w/w). These catalytic materials were reduced in hydrogen (473 to 723 K) after calcining them under both oxidating and inert atmospheres, in a wide range of experimental conditions. Two different regions were identified, each one showing its own dependence for the exposed metal fraction (FE) vs the maximum calcining temperature (Tc): (i) at Tc < 423 K the FE depends upon the Pd loading and also upon the type of for Tc > 423 K the FE is a function of the metal loading and the support structure, but not of the complex species mentioned. Hydrogen solubility on these Pd/SiO2 systems diminishes with the mean diameter of the metal crystallites, reaching a constant value for d < 20 A ̊. The fraction of soluble hydrogen can be quantitatively decomposed into that of β-HPd and another one corresponding to hydrogen sorbed onto subsurface sites. It is shown that a suppression of H2 solubility exists for the metal crystallites occluded by the microporous support. © 1992.