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 d rying. In the first, PdCl42- was adsorbed onto a positively charged Al 2O3 surface and (NH3Pd2+ Onto negatively charged SiO2. It is shown tha t there is a maximum adsorption density and that the anion adsorbs wit h at least one hydration sheath intact while the cation appears to ret ain two hydration sheaths. These results are supported by a comparison of adsorption density (mu mole/m(2)) with literature values. In the s econd, the stability of the precursor was studied as a function of dry ing temperature. Both the electrostatically adsorbed chloride and ammi ne precursors could be induced to desorb after drying at room temperat ure. At elevated temperatures, desorption of the chloride precursor co uld not be induced at 60 degrees C or higher (presumably the adsorbed precursor had decomposed and become anchored to the surface), while th e ammine precursor could be induce to desorb to a significant extent f rom silica up to calcination temperatures of 185 degrees C. In the fin al experiment, migration through pellets during drying of an initially homogeneously dispersed precursor (AHM) was studied. If electrostatic s are favorable (oppositely charged precursor and support) then there is strong adsorption and no migration. If the precursor and support ha ve like charges, migration occurs during drying. In sum, the maximum a dsorption density of Pd and Pt precursors can be calculated by a steri c monolayer of hydrated complexes, and these adsorbed precursors appea r to maintain their aqueous-like environment in which electrostatic ef fects persist, even through the drying step.