Palladium, palladium-tin, and palladium-silver catalysts in the selective hydrogenation of hexadienes: TPR, Mossbauer, and infrared studies of adsorbed CO

Alumina-supported Pd-Sn catalysts prepared by successive impregnation of the precursors, calcination, and reduction contain SnO2 and SnAl2O5, Pd-Sn solid solutions, and intermetallic phases Pd2Sn and Pd3Sn. The formation of Pd3Sn is favored not only by lowering the Sn/Pd ratio but also by introducing tin through ethanolic impregnation. The dilution of surface Pd atoms is confirmed by infrared spectroscopy of adsorbed CO: when the Sn/Pd ratio increases, the linear/bridged intensity ratio increases and the dipole-dipole interactions decrease. TPR experiments show the influence of the support acidity and of the preparation procedure on the reducibility of the various Pd-Sn and Pd-Ag species; in the case of Pd-Ag catalysts, the presence of silver postpones the reduction of PdO in calcined samples. (C) 2000 Academic Press.