Preparation of a rhodium catalyst from rhodium trichloride on a flat,conducting alumina support studied with static secondary ion mass spectrometry and monochromatic X-ray photoelectron spectroscopy

A rhodium catalyst has been prepared by electrostatic adsorption of RhCl₃-derived species in aqueous solution on a model support, consisting of a 4–5 nm thick layer of aluminum oxide on an aluminum foil. The conversion of the rhodium precursor species into metallic rhodium has been studied by monochromatic XPS and static SIMS. Freshly prepared catalysts contain adsorbed Rh-complexes with only one chloro ligand; this is explained by a mechanism in which chloro ligands of the initially adsorbed complex, of the form [RhCl_n(OH)_4-n(H₂O)₂]^–, are displaced by surface OH groups. Analysis of molecular secondary cluster ions of the type RhCl^– shows that the Rh-Cl species decompose at reduction temperatures below 200 °C, whereas reduction temperatures well in excess of 200 °C are needed to remove chlorine from the alumina support.