In-Situ XAFS Investigation of K-Promoted Co Catalysts

Abstract

In situ XAFS spectroscopy studies have been conducted at reaction conditions on potassium-promoted cobalt catalysts supported on SiO2 and Al2O3, and on similar samples without potassium. Measurements were conducted at temperature up to 200°C under both reducing (flowing H2) and simulated Fischer-Tropsch (F-T) reaction (flowing CO/H2) conditions. K and Co K-edge XANES spectra indicate that K interacts with both the Co phase and the SiO2 support, and that K promotion inhibits the reduction of the Co/SiO2 catalyst in H2 at 200°C. However, under CO hydrogenation conditions in the presence of water vapor, the differences in degree of reducibility are less since the unpromoted catalyst becomes more oxidized. For the K-promoted Co/Al2O3 catalyst, Co K-edge XANES and EXAFS spectra show that Co is initially present as a mixture of Co3O4 and CoO. Under H2 at 200°C, these oxides are reduced to a mixture of CoO and Co metal. During reaction at 200°C in H2/CO, the Co phases are reconverted to a mixture of Co3O4 and CoO. Without K, the Co in Co/Al2O3 is reduced almost completely to Co metal under H2 at 200°C and remains in that form even during reaction at 200°C in H2/CO. Only a minor trace of CoO is observed.