(552a) Feasibility Study - the Benefits and Drawbacks of Group 11 Promoted Co/Alumina Catalysts for Fischer-Tropsch Synthesis
Group 11 (i.e., Cu, Ag, and Au) promoted 15%Co/alumina catalysts were prepared by an aqueous impregnation method. Catalysts were characterized by temperature programmed reduction, in-situ EXAFS/XANES spectroscopies, and by adsorption methods (e.g., BET surface area and hydrogen chemisorption/pulse reoxidation). All of the Group 11 promoters clearly facilitated the reduction of cobalt oxide species interacting with the support. However, the promoters were most effective only after an initial reduction step, and should be transferred directly to the reactor and not passivated prior to use. EXAFS/XANES results indicate that the promoting effect is significantly damaged due to structural changes in the catalyst promoter-cobalt interaction at the atomic level if the catalysts are first passivated prior to re-activation and use. The catalysts were tested in a CSTR slurry reactor, and significant improvements in CO conversion rates on a per g catalyst basis were observed with the addition of the Group 11 promoters Ag and Au, due to increases in surface cobalt metal active site densities (i.e., arising from the improved extent of reduction). In the loading range tested, the Cu promoted catalysts hindered the CO conversion rates, probably due to Co site blocking by Cu. Interestingly, relative to the unpromoted catalyst, the Ag and Au promoted catalysts displayed similar or slightly lower light product selectivities at the same level of conversion (i.e., ~50%). Ag is of special interest, since it is significantly less expensive than some reduction promoters currently in commercial use.