PREPARATION AND PROPERTIES OF PALLADIUM AND PLATINUM SUPPORTED ON LAYER SILICATE MINERALS

摘要

The addition of molecular hydrogen at 140(DEGREES)C to Pd(NH(,3))(,4)('2+) and Pt(NH(,3))(,4)('2+) complexes exchanged on Wyoming montmorillonites, at various metal weight to support weight ratios, results in metal reduction according to the mechanism M(NH(,3))(,4)('2+) + H(,2) (--->) M(DEGREES) + 2NH(,4)('+) + 2NH(,3). In a parallel reaction, the concurrent reduction of structural iron (III) occurs, producing an increase in the layer charge of the mineral. Charge balance is maintained by the additional formation of ammonium cations by ammine protonation.;Metal reduction is accompanied by the collective migration of atoms and/or crystallites from the internal surface of the mineral to sites, on the external surface and edges, of low negative charge. At high metal concentrations, large clusters of > 100(ANGSTROM) are easily visible by transmission electron microscopy. Also apparent is a high proportion of the metal crystallites formed in diameters of 50(ANGSTROM) or less. As the metal concentration decreases, mean metal diameters decrease and the particle size distributions become narrower.;Attempts to measure metal dispersions (number of surface metal atoms/total number of metal atoms) by the hydrogen titration of oxygenated surface metal atoms result in a drastic overestimation of this value. The primary cause appears to be related to the spillover of atomic hydrogen from the metal to the support phase. Infrared evidence exists which implicates the role of structural iron, present in the montmorillonite support, as hydrogen atom acceptor sites. Molecular water, formed during the titration reaction, also appears to be involved in the process, acting as a carrier for hydrogen atoms between the metal surface and the acceptor sites.;Values, more indicative of the actual metal dispersion, are obtainable by measuring the gravimetric loss of water after the initial hydrogen titration. This technique offers a simple and convenient method for determining metal dispersion independent of hydrogen spillover.;Finally, the catalytic activity of palladium loaded montmorillonites is demonstrated for the hydrogenation of 1-hexene. The higher specific activity of these catalysts compared to a 5.0% Pd(.)Al(,2)O(,3) would indicate the manifestation of hydrogen spillover once again.