Structures and performance of Rh–Mo–K/Al 2O 3 catalysts used for mixed alcohol synthesis from synthesis gas

Abstract

A series of rhodium-modified Mo–K/Al 2O 3 catalyst samples was prepared by varying the rhodium loading between 0 and 1.0 wt.% and maintaining molybdenum and potassium contents as constants. The structures of the samples were characterized by techniques of XRD, LRS, TPR, XPS and EXAFS and correlated to the catalytic properties of the samples for alcohol synthesis from synthesis gas. It was found that, in the oxidic rhodium-modified samples, a strong interaction of the rhodium modifier with the supported K–Mo–O species occurs. This interaction facilitates the sulfidation and reduction of the supported oxo-molybdenum and leads to a decrease in the size of the molybdenum species and stabilization of the cationic rhodium species on the samples during sulfidation. Upon sulfidation, the sulfided molybdenum species in the rhodium-free sample is mainly present as large patches of MoS 2-like slabs with their basal sulfur planes interacting with the support surface. With the modification of rhodium to the samples, the supported MoS 2-like species becomes highly dispersed, as revealed by the decrease in the average size of the sulfided molybdenum species. The interaction of the rhodium species with the molybdenum component may cause the basal planes of the MoS 2-like species to become oriented perpendicular to the support surface due to favorable bonding of the MoS 2edge planes to the support through Mo–O–Al bonds. In comparison with the sulfided sample free of rhodium, the properties of the rhodium-modified samples for alcohol synthesis from synthesis gas are much improved. It most probably results from the synergic interaction of the rhodium with the molybdenum species that gives rise to the appearance of the catalytically active surfaces or sites. The co-existence of cationic and metallic rhodium stabilized by this interaction may be responsible for the increased selectivity for the formation of C 2+ alcohols.