NATURE OF METAL CATALYST PRECURSORS ADSORBED ONTO OXIDE SUPPORTS

In this work, three sets of experiments were conducted to investigate the nature of adsorbed catalyst precursor complexes during and after drying. In the first, PdCl42- was adsorbed onto a positively charged Al2O3 surface and (NH3Pd2+ Onto negatively charged SiO2. It is shown that there is a maximum adsorption density and that the anion adsorbs with at least one hydration sheath intact while the cation appears to retain two hydration sheaths. These results are supported by a comparison of adsorption density (mu mole/m(2)) with literature values. In the second, the stability of the precursor was studied as a function of drying temperature. Both the electrostatically adsorbed chloride and ammine precursors could be induced to desorb after drying at room temperature. At elevated temperatures, desorption of the chloride precursor could not be induced at 60 degrees C or higher (presumably the adsorbed precursor had decomposed and become anchored to the surface), while the ammine precursor could be induce to desorb to a significant extent from silica up to calcination temperatures of 185 degrees C. In the final experiment, migration through pellets during drying of an initially homogeneously dispersed precursor (AHM) was studied. If electrostatics are favorable (oppositely charged precursor and support) then there is strong adsorption and no migration. If the precursor and support have like charges, migration occurs during drying. In sum, the maximum adsorption density of Pd and Pt precursors can be calculated by a steric monolayer of hydrated complexes, and these adsorbed precursors appear to maintain their aqueous-like environment in which electrostatic effects persist, even through the drying step.