Phase characterization of iron/manganese fischer-tropsch catalysts: effect of compositon and reduction conditions

Directly Converting Syngas to Linear α-Olefins over Core-Shell Fe3O4@MnO2 Catalysts

Converting syngas to value-added chemicals via Fischer-Tropsch synthesis has attracted much attention, whereas the direct hydrogenation of CO to heavy olefins, especially linear α-olefins (LAOs), remains a challenge. In this study, we designed a core-shell Fe₃O₄@MnO₂ catalyst to realize the direct conversion of syngas to LAOs with high efficiency. This catalyst exhibited a high selectivity of 79.60% for total alkenes and 64.95% for C₄⁺ alkenes, 91% of which are LAOs, at a CO conversion of approximately 75%. Promotion of the electron transfer from MnO₂ to Fe₃O₄ inside the core-shell Fe₃O₄@MnO₂ catalyst facilitated the dissociative adsorption of CO molecules on Fe₃O₄ and the spillover of H atoms onto the MnO₂, which enhanced C-C coupling, weakened the hydrogenation activity of the catalyst, and improved the production of LAOs. A superior stability over 100 h was observed, demonstrating the promising potential of this catalyst for industrial applications.

Heats of Adsorption and Reaction of CO on Iron/Manganese Oxide Catalysts

The heats of adsorption and reaction of CO have been measured on reduced Fe/Mn oxide catalysts of different compositions at temperatures between 263 and 543 K. The heats of adsorption depend only to a small extent on the manganese content of the catalyst and are independent of the temperature of reduction. The former fact points to a more structural than electronic effect of the manganese oxide addition. The heats of adsorption are, however, strongly dependent on measurement temperature, increasing from about 60 kJ mol-1 at 263 K to about 200 kJ mol-1 at 493 K and then decreasing again to about 180 kJ mol-1 at 543 K. This temperature dependence can be explained by consecutive reactions following CO adsorption, i.e. the Boudouard reaction, dissociation of CO and diffusion of carbon into the bulk of iron. The amount of CO adsorbed is correlated to the metallic iron content in the surfaces of the catalysts.