The controlled synthesis of metal-acid bifunctional catalysts: Selective Pt deposition and nanoparticle synthesis on amorphous aluminosilicates

Abstract

The rational synthesis of metal-acid bifunctional catalysts based on aluminosilicates will involve control over which domain – alumina or silica – the metal can be deposited on, as well as the control of particle size. These factors determine the ratio and proximity of the metal and acid sites. The control of adsorption selectivity and particle size has been studied by measuring the uptake of anionic and cationic Pt precursors as a function of pH over various types of silica-alumina composites and pure oxide supports, followed by characterization of the nanoparticles resulting from reduction of the precursors.Results indicate that electrostatic adsorption can be exploited to achieve selective deposition. Ion exchange of cationic precursors also appears to occur over the acid sites. Cationic tetraammine Pt precursors, either electrostatically adsorbed onto silica domains at high pH, or ion exchanged at the acidic alumina-silica adlineation (interface between two oxides) at neutral pH, lead to small (1.7–3.0nm) nanoparticles. The size of Pt nanoparticles resulting from anionic Pt hexachloride electrostatically deposited onto alumina domains at low pH depends on the size of the alumina domain; a critical domain size is needed to anchor the Pt precursors against sintering.In the companion paper, the factors controlling metal-acid site intimacy and ratio are demonstrated for the isomerization of n-heptane.