XPS CHARACTERIZATION OF THE REDUCTION AND SYNTHESIS BEHAVIOR OF CO MNOXIDE CATALYSTS FOR FISCHER-TROPSCH SYNTHESIS/

The surface composition of Co/Mn oxide catalysts after calcination, re duction and during CO hydrogenation experiments were investigated by X PS. The bulk changes during reduction were studied by Temperature Prog rammed Reduction (TPR). The calcined catalysts showed Co/Mn oxides of different compositions in their surfaces. The Co surface concentration , of the catalysts with high Co content, decreased after calcination c ompared to the bulk composition, but after reduction the bulk concentr ation was almost reached again. The catalysts with low Co content show ed no decrease in the surface concentrations after calcination. Signif icant differences in surface concentrations for the catalysts Co20 and Co5 were observed by analysing the Co 2p and 3p levels, respectively; these can be explained by an internal reduction model. After reductio n the sample Co100 was completely reduced to metallic cobalt. In the m anganese-containing catalysts, after in-situ reduction, Co3+ and Co2+, were found; all manganese was reduced to Mn2+. A comparison of the re sults of the in-situ reduction and the TPR profiles led to the develop ment of a so called ''internal reduction'' model. This model assumes m igration of the Co2+ ions to the reduction front in inner layers of th e catalysts, where they will be reduced to metallic cobalt; this is en riched in the bulk and cannot be found at the surface. The manganese m atrix stabilizes the surface oxide layer so that all catalysts exhibit ed Co/Mn spinels in the surface. Synthesis experiments in the reaction chamber of the XPS apparatus did not lead to changes of the catalyst surfaces as a function of the reaction pressure, synthesis time or syn thesis gas composition. The differences in the synthesis behaviour obs erved for the catalysts must be due to other effects, (i.e. a change i n adsorption of hydrogen connected with a change in hydrogenation acti vity or the different cobalt concentrations).