Sulfided CO-mo/AL2O3 Catalysts: Carbon Monoxide Chemisorption and Surface Structures

The interaction between Co and Mo ions has been examined on a series of Co-Mo/Al2O3 catalysts with constant Mo (6.3 % MoO3) and variable Co concentrations (0.5 - 4.3 % CoO) by comparison with Co/Al2O3 and Mo/Al2O3 catalysts. The type of surface structures present in the systems in infered from the fraction of cobalt or molybdenum which can be dissolved in acidic or basic medium, and from carbon monoxide adsorption. The CO chemisorption is measured by pulses at 0°C and adsorbed species are studied by infrared spectroscopy. The interaction between Co and Mo ions in the sulfided state is shown by the appearance of a band at 2065 cm−1, partially irreversible in our conditions. It could correspond to highly reduced sites of the catalysts. This new type of site correlates with HDS activity. Results are consistent with the build up of a Co-Mo-S type phase. This phase is well dispersed and consists of small entities associating Mo and Co in the approximate ratio 3/2. Mo/Al2O3 catalysts are found to be promoted essentially the same with Co or Ni atoms.