CALCINATION-INDUCED SURFACE ENRICHMENT OF IMPURITIES AND ITS EFFECT ON CATALYTIC PROPERTIES OF IRON-OXIDE

The possible role of surface heterogeneities as activity sources on α-Fe2O3 was studied by high-temperature calcination, which resulted in surface smoothing and selective enrichment of impurities at the surface. Among the six impurities, Ca, Mn, Mg, Zn, Ni, and Si, found in the bulk of a commercial α-Fe2O3 (99.9%), only Si and Ca were enriched at the surface after high-temperature calcination. Silicon contamination at the surface (up to about 30% surface cation composition) had little effect on the catalytic performance of iron oxide in butene oxidative dehydrogenation. Calcium, on the other hand, showed a strong poisoning effect an both the dehydrogenation and tbe combustion site, thus served as a probe in locating the active sites on α-Fe2O3. Washing the high-temperature-calcined sample with water or nitric acid resulted in the removal of surface calcium and the restoration of the activity. Based on a high-magnification SEM comparison between a just-calcined and a water-washed sample, it was suggested that the active sites on α-Fe2O3 are located at the surtace steps. © 1993 Academic Press, Inc.