MOSSBAUER STUDY OF IRON FISCHER-TROPSCH CATALYSTS DURING ACTIVATION AND SYNTHESIS

Mossbauer data indicate that activating an iron Fischer-Tropsch cataly st containing Cu, K, and kaolin with syngas for 24 h at 270 degrees C and 1.14 MPa produces a material that has about 60% of-the iron in the form of Fe3O4 and the remainder as a mixture of chi-Fe3C4 and epsilon '-Fe2.2C. A similar activation with CO produces a material that contai ns more than 80% of the iron in the form of chi-Fe5C2 and the remainde r as Fe3O4. The CO-activated catalyst exhibits a high activity, wherea s the syngas-activated sample has a low activity. Moreover, the CO-act ivated sample converts during 50 h of synthesis to a mixture that cont ains more Fe3O4 (40%) and less chi-Fe5C2 (60), whereas the syngas acti vated sample contains about 80% chi-Fe5C2 after 50 h of exposure to sy nthesis gas. Thus, both catalysts underwent significant changes in the bulk composition without a parallel change in CO conversion. Another catalyst formulation, containing more Cu, much less K, and no binder, showed significantly higher syngas conversion after activation by CO t han by syngas even though the bulk phase compositions were similar. A catalyst, initially activated with syngas and exhibiting low activity, showed evidence for only Fe3O4. This low-activity material after a 24 h treatment in CO exhibited a dramatic increase of the carbide phase and catalytic activity. The analysis of phase composition is informati ve about changes in the bulk composition of iron catalysts, but these changes do not necessarily track directly catalytic activity.