The simultaneous effect of promoters and water on the reduction of an iron catalyst for ammonia synthesis
Abstract
The rate of reduction of magnetite by gaseous hydrogen is slightly affected by water vapour (1%). However, this effect of water vapour is significant in iron catalysts for ammonia synthesis of the KM I type. Promoted magnetite is the main component of the iron catalyst and it is concluded therefore that the influence of water is applicable only when promoters are present.
The validity of the core-and-shell reduction model, assuming a Langmuir-Hinshelwood kinetic equation which describes the reaction at the oxide/iron interface, is discussed on the basis of the kinetic data for unpromoted and promoted iron catalysts. It is found that the model is generally valid, except for the case of advanced reduction of promoted catalyst in a moist atmosphere.
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Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres
2007, Applied Catalysis A: GeneralThe reduction of various iron oxides in hydrogen and carbon monoxide atmospheres has been investigated by temperature programmed reduction (TPRH2 and TPRCO), thermo-gravimetric and differential temperature analysis (TG-DTA-MS), and conventional and “in situ” XRD methods. Five different compounds of iron oxides were characterized: hematite α-Fe2O3, goethite α-FeOOH, ferrihydrite Fe5HO8·4H2O, magnetite Fe3O4 and wüstite FeO. In the case of iron oxide-hydroxides, goethite and ferrihydrite, the reduction process takes place after accompanying dehydration below 300 °C. Instead of the commonly accepted two-stage reduction of hematite, 3 α-Fe2O3 → 2 Fe3O4 → 6 Fe, three-stage mechanism 3Fe2O3 → 2Fe3O4 → 6FeO → 6Fe is postulated especially when temperature of reduction overlaps 570 °C. Up to this temperature the postulated mechanism may also involve disproportionation reaction, 3Fe2+ ⇌ 2Fe3+ + Fe, occurring at both the atomic scale on two-dimensional interface border Fe3O4/Fe or stoichiometrically equivalent and thermally induced, above 250 °C, phase transformation—wüstite disproportionation to magnetite and metallic iron, 4FeO ⇌ Fe3O4 + Fe. Above 570 °C, the appearance of wüstite phase, as an intermediate of hematite reduction in hydrogen, was experimentally confirmed by “in situ” XRD method. In the case of FeO–H2 system, instead of one-step simple reduction FeO → Fe, a much more complex two-step pathway FeO → Fe3O4 → Fe up to 570 °C or even the entire sequence of three-step process FeO → Fe3O4 → FeO → Fe up to 880 °C should be reconsidered as a result of the accompanying FeO disproportionation wüstite ⇌ magnetite + iron manifesting its role above 150 °C and occurring independently on the kind of atmosphere—inert argon or reductive hydrogen or carbon monoxide. The disproportionation reaction of FeO does not consume hydrogen and occurs above 200 °C much easier than FeO reduction in hydrogen above 350 °C. The main reason seems to result from different mechanistic pathways of disproportionation and reduction reactions. The disproportionation reaction wustite ⇌ magnetite + iron makes simple wüstite reduction FeO → Fe a much more complicated process. In the case of thermodynamically forced FeO disproportionation, the oxygen sub-lattice, a closely packed cubic network, does not change during wüstite → magnetite transformation, but the formation of metallic iron phase requires temperature activated diffusion of iron atoms into the region of inter-phase FeO/Fe3O4. Depending on TPRH2 conditions (heating rate, velocity and hydrogen concentration), the complete reduction of hematite into metallic iron phase can be accomplished at a relatively low temperature, below 380 °C. Although the reduction behavior is analogical for all examined iron oxides, it is strongly influenced by their size, crystallinity and the conditions of reduction.
The mechanism of reduction of cobalt by hydrogen
2004, Materials Chemistry and PhysicsCobalt catalysts supported on silica were prepared by an impregnating method and characterized using temperature-programmed reduction (TPR). H2 was used as the reduction agents. The two-stage reduction was observed. Co3O4 was reduced to CoO and then reduced to metallic Co. The activation energies for the two reduction steps of cobalt oxide are 94.43 and 82.97 kJ mol−1, respectively. The simulation by reduction models of the TPR patterns presents well-fitting of two-dimensional nucleation according to Avarmi-Erofeev model for the reduction of Co3O4 to CoO and unimolecular model for the reduction of CoO to Co.
The mechanism of reduction of iron oxide by hydrogen
2003, Thermochimica ActaPrecipitated iron oxide samples were characterized using temperature-programmed reduction. H2 was used as the reduction agents. The two-stage reduction was observed: Fe2O3 was reduced to Fe3O4 and then reduced to metallic Fe. The activation energy for the two reduction steps of iron oxide are 89.13 and 70.412 (kJ mol−1), respectively. The simulation by reduction models of the TPR patterns presents well fitting of unimolecular model for Fe2O3→Fe3O4 reduction and two-dimensional nucleation according to Avarmi–Erofeev model for Fe3O4→Fe.
The micromorphology of the activated iron catalyst used for ammonia synthesis
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Catalytic dehydrogenation of 10,11-dihydro-5H-dibenz[b.f]azepine (iminodibenzyl) to 5H-dibenz[b.f]azepine (iminostilbene) over potassium-promoted iron oxides: effect of steam, potassium promotion and carbon dioxide treatment
1995, Applied Catalysis A, GeneralThe dehydrogenation of 10,11-dihydro-5H-dibenz[b.f]azepine (iminodibenzyl) to 5H-dibenz[b.f]azepine (iminostilbene) was investigated over potassium-promoted iron oxide catalysts. The catalytic behaviour of the catalysts was found to be strongly influenced by the oxidation state of iron and by the amount of carbon deposits on the catalyst surface. Dilution of the iminodibenzyl feed by steam prevents reduction of the active iron oxide phase and reduces drastically the deposition of carbon, leading to high steady-state activities of potassium-promoted catalysts. Promotion of the iron oxides with 0.5 wt.-% potassium increases significantly the activity of the catalysts. Potassium improves the reducibility of iron oxides probably by enhancing the electron exchange between Fe3+ and Fe2+ ions, and results in an apparent increase of the number of sites active for the dehydrogenation of iminodibenzyl. Potassium carbonate and magnetite represent the main phases under reaction conditions in the presence of steam and are proposed as the active phases in the dehydrogenation of iminodibenzyl. Gasification of carbonaceous deposits by steam leads to the formation of carbon dioxide and potassium carbonate under normal reaction conditions. Therefore, the active centres are not sensitive to further carbon dioxide treatment.
Kinetics of activation of the industrial and model fused iron catalysts for ammonia synthesis
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