Adsorption properties of cobalt and cobalt-manganese catalysts studied by in situ diffuse reflectance FTIR using CO and CO+H-2 as probes

The adsorption properties of the precipitated Co and CoMn catalysts with di fferent ratios of cobalt to manganese reduced by Hz were studied by in situ diffuse reflectance FTIR using CO and CO + H-2 (syngas) as probe molecules . The adsorption measurements were performed by flowing the probe gases ove r the samples at a high pressure of 1.2 MPa. It has been found that metalli c cobalt particles form on both Co and CoMn samples after reduction. This i s manifested by the occurrence of the bands corresponding to the probes int eracting with the metallic cobalt species. On the reduced Co sample itself, however, the molecularly adsorbed CO species are rather difficult to detec t upon CO and CO + H-2 adsorption because of the dissociation of the adsorb ed probes on the fine active metallic cobalt particles formed. On the manga nese-incorporated samples, distinct bands appear in the region of 2100 and 1750 cm(-1) and their intensities increase with increasing the loading of m anganese in the samples. They correspond to the species of CO Linearly adso rbed, bridge-adsorbed, and multiply bridge-adsorbed on Co-0 sites. The appe arance of these adsorption species, especially the species of multiply brid ge-bonded CO, indicates that the size of the formed metallic cobalt particl es is larger than that on the reduced Co sample. Because of the moderate ad justment in size of the formed metallic particles by the incorporated manga nese, the stability of the adsorption species are remarkably enhanced, as r evealed by the spectra recorded from the thermal desorption of the adsorbed species. The size effect of the metallic cobalt particles formed may also be reflected by the improved properties of the catalysts to which manganese was added, such as reactivity, deactivation on stream and sulfur-tolerance in Fischer-Tropsch reaction. (C) 2001 Elsevier Science B.V. All rights res erved.