Molecular-level studies of transition metal-support interactions during the first steps of catalysts preparation: platinum speciation in the hexachloroplatinate/alumina system

This paper summarizes recently published results on the mechanisms of chlor oplatinate adsorption onto alumina, as revealed by the joint use of Pt-195 NMR and other spectroscopic techniques. Molecular models of solution specie s and conceivable adsorbed species are presented. It is shown that the adso rption of hexachloroplatinate-derived species is initially electrostatic, b ut accompanied by weak specific bonding to the surface (probably H-bonding; these are the "specifically adsorbed" ions of triple-layer theory). This i nitial electrostatic adsorption can be followed by a slow, thermally activa ted grafting, i.e., by the formation of inner-sphere complexes with two sur face (AlOH) groups. Upon drying at elevated temperatures, grafting becomes irreversible. However, modification of the alumina surface by chlorination prior to Pt deposition efficiently prevents grafting, opening the possibili ty of a chemical control of adsorbed Pt speciation. (C) 2000 Elsevier Scien ce B.V. All rights reserved.