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19, Iron oxide model catalysis: adsorption and catalysis
Abstract:
Dehydrogenation of ethylbenzene to styrene is usually run over potassium promoted iron oxide based catalysts at 870 K in presence of steam. Here we present conversion yield measurements on unpromoted single crystalline α-Fe2O3 (0001) model catalysts by combining surface science techniques with an in-situ micro flow reactor. The influence of H2O and O2 on the reaction was investigated by varying the composition of the feed. The initial conversion over Fe2O3 is always high (5-8%), independent of the type of the feed composition. Only the length of this period depends on the feed composition. In presence of O2 (EB:H2O:O2 = 2:20:1), the high yield period can be maintained, in absence of O2 (EB: H2O = 1:10) it decreases in two steps of about a factor of 2-3 each. The reaction was interrupted in the different yield regimes, and the sample structure and composition was analyzed. The high yield is related to Fe2O3 with almost no carbon deposits. O2 in the feed maintains this phase. Without O2, Fe2O3 is reduced to Fe3O4 and the yield drops to the intermediate region. The same yield is observed on clean Fe3O4. Carbon deposits increase but do not yet limit conversion. This happens at the transition to the low yield regime where a thick layer of carbon deposits is observed. With H2O in the feed, the oxide below the carbon deposits remains Fe3O4, without H2O, it is reduced to metallic Fe. We ascribe the low yield to catalysis by carbonaceous species. The study shows that the high yield is typical for Fe2O3 and can be maintained by proper admixture of O2 to the feed.
