Preparation Of Non-Pyrophoric Metallic Catalysts

Крылова, А. В.; Ustimenko, G.A.; Torocheshnikov, N.S.

doi:10.1016/s0167-2991(09)60040-9

Cited by 6 publications

(5 citation statements)

References 2 publications

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“…The possibility of stabilizing iron catalyst with carbon oxides, water vapor, oxygen, and their mixtures was also checked. ,,− It was ascertained that the interaction of iron catalyst with air or pure oxygen in temperature ranges −85 to −195 °C and 450 to 550 °C led to a low oxidation of the catalyst, which is a common feature in passivation. Although oxidation in temperatures below 0 °C does not directly lead to obtaining a continuous passivation film, it creates oxide islands that do not have protective properties.…”

Section: Introductionmentioning

confidence: 99%

Passivation versus Oxidation of Iron Catalyst with Carbon Dioxide

Ekiert

Arabczyk

2015

J. Phys. Chem. C

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The rate of iron catalyst passivation is strongly limited because the reaction of iron with oxygen is exothermic. It was proposed to perform passivation process using endothermic reaction of Fe oxidation with CO 2 . A thorough insight into the mechanism of the reaction of nanocrystalline iron with carbon dioxide was required. Therefore, oxidation of commercial iron catalyst for ammonia synthesis with CO 2 was investigated (T=300-500 o C, p CO2 =3.75-101.325 kPa). Thermogravimetry, XRD, SEM/EDX and GC were used in measurements.The only solid product of the Fe catalyst oxidation with CO 2 was magnetite Fe 3 O 4 , whereas the only gaseous one -CO. At the initial stage of oxidation of Fe catalyst with CO 2 the process occurred in accordance with the adsorption range model. In this scope, kinetics was described using Langmuir-Hinshelwood model for p CO2 <50 kPa. Above this value the oxidation rate is independent of p CO2 . With the progress of the oxidation process, the surface reaction ceased to be the slowest step, which limited the process rate, and the process started to be driven by gas diffusion in pores. The apparent activation energy was determined as ca. 120kJ/mol.It was assumed, that low temperature oxidation with CO 2 led to creation of magnetite layer, which had no passive properties. The complementary stage of passivation with oxygen at low pressure was needed to obtain a stable, protective oxide layer. However, the time spent on the whole process of passivation was significantly shorter than that in the classic route.

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Section: Introductionmentioning

confidence: 99%

Passivation versus Oxidation of Iron Catalyst with Carbon Dioxide

Ekiert

Arabczyk

2015

J. Phys. Chem. C

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“…[4][5][6] Further investigations showed, that in the reaction between reduced iron catalyst and oxygen of different partial pressures in the range of temperature -195 to -85 °C as well as 450-550 °C the oxidation degree characteristic for passivation process had been obtained, and the oxidized material was no longer pyrophoric. [7][8][9][10][11] The dependence of mass increments vs temperature had the form of a so-called "bell curve". The behavior of the catalyst under oxygen atmosphere was surprising, viz.…”

Section: Introductionmentioning

confidence: 99%

“…The influence of promoters on passivation process was not explained, either. 6 It was suggested, that the catalyst pyrophority depended on the promoters content 7 and the catalyst composition influenced the passive layer structure. 3 However, formation of ferrite clusters with aluminum, calcium or potassium was excluded.…”

Section: Introductionmentioning

confidence: 99%

“…However, this procedure is energy- and time-consuming and uses great amounts of nitrogen. Searching for the new methods of iron catalyst passivation some trials to explain the mechanism of this process were undertaken. − Experiments were carried out to study the interaction of oxygen, nitrogen, and hydrogen with the ammonia synthesis catalyst . Initially, it was proposed, that passive layer had been composed of adsorbed water and oxygen monolayer formed as a result of the reaction between oxygen added and hydrogen left in catalyst after its reduction during activation process.…”

Section: Introductionmentioning

confidence: 99%

“…Initially, it was proposed, that passive layer had been composed of adsorbed water and oxygen monolayer formed as a result of the reaction between oxygen added and hydrogen left in catalyst after its reduction during activation process. However, it has been found later that the passive layer with the multilayer structure was formed. − Further investigations showed, that in the reaction between reduced iron catalyst and oxygen of different partial pressures in the range of temperature −195 to −85 °C as well as 450−550 °C the oxidation degree characteristic for passivation process had been obtained, and the oxidized material was no longer pyrophoric. − The dependence of mass increments vs temperature had the form of a so-called “bell curve”. The behavior of the catalyst under oxygen atmosphere was surprising, viz.…”

Section: Introductionmentioning

confidence: 99%

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Studies of the Oxidation of Nanocrystalline Iron with Oxygen by means of TG, MS, and XRD Methods

2008

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The oxidation of the nanocrystalline iron containing small amounts of structural promotersssuch as aluminum, calcium, and potassium oxidesshas been studied. The oxidation processes were carried out under atmospheric pressure at pure oxygen atmosphere at various temperatures in the range of 573-773 K, in the differential tubular reactor with thermogravimetric (TG) measurement. Nanocrystalline promoted iron did not reveal the pyrophority under pure oxygen atmosphere at temperatures higher than 573 K. The oxide layer stable at a given temperature was formed and protected the iron against further oxidation. The higher oxidation temperature, the thinner was the oxide layer. X-ray diffraction analysis (XRD) and Mo ¨ssbauer spectroscopy (MS) were used to characterize the oxidized samples in order to determine the structure of formed oxides. The XRD analysis proved the magnetite structure of obtained iron oxides. The Mo ¨ssbauer studies revealed that magnetite layer formed at temperatures higher than 573 K had got the disturbed structure, derived from aluminum ions incorporation into the tetrahedral sites of magnetite's lattice. The ratio of the number of iron atoms to the number of aluminum atoms located in tetrahedral sites was 2:1 whereas the ratio of the number of iron atoms located in tetrahedral sites to the number of iron atom located in octahedral sites was 1:3.

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