Structure of Pd-Mo-K/Al2O3 Catalysts and Their Activity for Alcohol Synthesis

A series of palladium-modified Mo-K/Al2O3catalyst samples was prepared by varying the palladiumloading between 0 and 1.0% and maintaining molybdenum and potassium contents as con-stants. The structuresof the sampleswere characterized by techniquesof XRD, EXAFS and correlated to the catalytic properties of the samples for alcohol synthesis fromsynthesisgas. It is found that in the oxi-dic palladium-modified samplesastrong interactionof the palladiummodifierwith thesupported K-Mo-O species occurs. This interaction leads to a decrease in the size of the molybdenum species and stabi-lization of the cationic palladium species on the samples during sulfidation. Upon sulfidation, the sulfided molybdenum species in the palladium-free sample mainly present as large patches of MoS2-like slabs with their basal sulfur planes interacting with the support surface. With the modification of palladium to the samples, the supported MoS2-like species become highly dispersed as revealed by the decrease in the average size of thesulfided molybdenumspecies. The interactionof the palladiumspecieswith the molyb-denum component may cause the basal planes of the MoS2-like species to become oriented perpendicular to the support surface due to favorable bonding of the MoS2edge planes to the support through Mo-O-Al bonds. In comparison with the sulfided sample free of palladium, the properties of the palladium-modifiedsamples for alcohol synthesis from synthesis gas are much improved. It most probably results from the synergic interaction of the palladium with the molybdenum species which gives rise to the ap-pearance of the catalytically active sites.