Palladium, Palladium–Tin, and Palladium–Silver Catalysts in the Selective Hydrogenation of Hexadienes: TPR, Mössbauer, 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.