Influence of pH on the preparation of monometallic rhodium and platinum, and bimetallic RhodiumPlatinum Catalysts supported on γ-Alumina

Abstract 195 PtNMR and Raman experiments indicated that H 2 PtCl 6 adsorbed monomolecularly on the surface of alumina up to a loading of 1 μmol m −2 during pore volume impregnation. Above this limit crystalline H 2 PtCl 6 was formed during the subsequent drying procedure. Adsorption experiments showed that RhCl 3 exhibited the same behaviour, with an even larger monomolecular coverage. The coverage of both metal complexes was dependent on the pH during adsorption and decreased with decreasing pH, due to a shift in the bonding equilibrium between the metal complexes in solution and the complexes adsorbed on the support surface. Because of the acidic properties of RhCl 3 and H 2 PtCl 6 , the amounts of rhodium and platinum adsorbed during co-adsorption were smaller than during adsorption of the separate metal complexes. The reduction of RhCl 3 +H 2 PtCl 6 supported on Al 2 O 3 was governed by mobile rhodium atoms and small bimetallic clusters. Large metal salt crystals smothered the reduction process.