Laser Raman studies of the preparation of platinum and iridium catalyst systems

Steps in the preparation of Pt, Ir, and PtIr on alumina catalysts have been followed by laser Raman spectroscopy. The samples investigated were prepared by impregnation of alumina with the appropriate chloroplatinic acid and chloroiridic acid solutions. The resulting materials were then heated in air at varying temperatures. Laser Raman spectra indicated significant differences in the nature of the surface species remaining when PtAl2O3 and IrAl2O3 preparations were subjected to such thermal treatments. After a drying step at 110 °C, the PtAl2O3 preparation exhibits Raman bands at frequencies very close to those at which bands have been reported for the PtCl2−6 ion, whereas the IrAl2O3 preparation yields a spectrum significantly different from that reported for the IrCl2−6 ion. With increasing temperature, both the platinum and the iridium species decompose with loss of chlorine ligands. After being heated in air at 500 °C, the IrAl2O3 sample exhibits a Raman spectrum characteristic of crystalline IrO2. The presence of platinum in the bimetallic PtIrAl2O3 sample inhibits the formation of crystalline IrO2 to some degree.