Iron Catalyst For Ammonia Synthesis Research Articles

Model of active surface of iron catalyst for ammonia synthesis

The AES method has been used to investigate the iron-potassium-oxygen system. A new model of the iron catalyst for the ammonia synthesis reaction has been proposed, on the grounds of studies on the interactions between potassium, oxygen, and nitrogen on the iron surface. In this simple model the active sites are located under the layer of potassium atoms and on the layer of iron atoms. A part of those sites are occupied by oxygen atoms which stabilize the potassium atoms on the catalyst surface at the temperature of the ammonia synthesis reaction. The promoting action of potassium in this reaction is effected by the electron transfer from potassium to the iron surface covered with oxygen.

Emission of excited potassium species from an industrial iron catalyst for ammonia synthesis

The angular dependence of potassium emission-desorption is studied from a fused iron catalyst of the type used for ammonia synthesis. The excited species (K*, Kn*, etc.) and positive ions K+ have strongly different angular distributions. The bilobular distribution measured for ion desorption is concluded to be either due to excited atoms, so-called Rydberg atoms, or excited clusters. Both types of species have to desorb from the edges of the sample and become field ionized and deexcited just outside the sample, as reported in previous studies on an iron oxide catalyst. The peak in the normal direction measured for excited species is due to excited cluster formation outside the catalyst surface. Similarities with previous results for other catalysts are observed. The possibility that the promoter function of potassium in the ammonia synthesis is due to excited species is pointed out.

Study of the activation process and catalytic behaviour of a supported iron ammonia synthesis catalyst

A 3 wt% iron-supported catalyst has been prepared by free adsorption of K 4[Fe(CN) 6] precursor on acid-modified γ-alumina. The catalyst activation was studied by in situ X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and mass-spectrometry techniques. Thermal treatments under vacuum of H 2 : He flow produce the removal of cyanide ligands mainly as HCN and the formation of well dispersed Fe 2+ species which easily interact with the support. However, an activation under H 2:N 23:1 flow up to 673 K produces the elimination of cyanide ligands and formation of ammonia; small, well dispersed iron particles are obtained simultaneously. This activation process raises the initial K/Fe atomic surface ratio from 4.37 to 6.1. Chemisorption of dinotrogen species on potassium-rich small iron particles is related with an infrared absorption found at 2043 cm -1, which appears under H 2:N 23:1 activation conditions above 573 K. Catalytic behaviour in the ammonia-synthesis reaction has been studied in a flow reactor under pressure. A catalytic activity at 30 bar and 723 K as 405 mmol NH 3 per iron gram per hour has been determined.

Effect of the cooling rate on the structure of the wustite phase in the model iron catalyst for ammonia synthesis

Model catalysts for ammonia synthesis containing large amount of wustite (∼50 wt%) were prepared and cooled in various ways. Then, the structure of the wustite phase was studied by Mössbauer spectroscopy and X-ray diffraction technique. The Mössbauer spectra of the wustite phase revealed the presence of two different Fe 2+ surroundings which are attributed to two kinds of wustite with different defect structures. The high resolution scans over the wustite reflections in the X-ray diffraction patterns confirmed the Mössbauer results. The kind of defect structure is almost independent of the cooling rate. The cooling procedure affects mainly the relative contributions of the wustites. Higher contents of wustite having the structure close to the stoichiometric compound were found in samples exposed to prolonged cooling.

Influence of aluminium and potassium on activity and texture of fused iron Catalysts for ammonia synthesis

BET, scanning electron microscopy and X-ray diffraction were used to investigate the texture of fused iron Catalysts which had been previously reduced and passivated, and into which potassium hydroxide was introduced by impregnation or aluminium removed by extraction. Activity tests were carried out under 10.0 MPa in the 400–520 °C temperature range. Investigations were performed on the doubly promoted (Al, Ca) and industrial, triply promoted (K, Al, Ca) Catalysts. Introduction of potassium hydroxide into the doubly promoted Catalyst increases the process rate by about 15–20 times per unit surface area. Sintering and recrystallization are observed in the Catalyst, proving that interaction takes place during the process between the potassium hydroxide introduced and the alumina present on the iron surface. Partial removal of aluminium from the surface of the doubly and triply promoted Catalysts also leads to sintering and recrystallization under ammonia synthesis conditions. This shows that the presence of alumina on the surface, and not within the iron, is responsible for textural promotion. Loss of Catalysts activity is proportional to reduction in total surface area.

The effect of cobalt on the activity of iron catalyst supported on magnesium hydroxide carbonate (MHC) in the ammonia synthesis

AbstractThe effect of cobalt on the activity of the magnesium hydroxide carbonate supported iron catalysts for ammonia synthesis was studied. The addition of cobalt has been found to increase considerably the activity of the catalyst samples. A sample containing ca 20 wt% Co as metallic phase showed maximum activity in the ammonia synthesis (X‐ray fluorescence (XRFS) was used to analyze the sample compositions). The precursor reduction temperature proved an important factor affecting properties of the samples studied, and catalyst reduced at the relatively high temperature of 873 K was the most active. On the basis of the activity measurements, phase structure studies with the X‐ray diffraction (XRD) method, temperature‐programmed reduction studies, and a Mössbauer spectroscopic investigation, a conclusion has been drawn that the formation of an intermetallic iron‐cobalt compound is crucial to the catalyst activity. It has been postulated that this might be due to the surface restructuring by exposing the most active iron surface, Fe(111). Considering the support and promoter natures the catalysts studied can be called ‘super‐basic’. Negligible potassium addition (0.2 wt% K) in the form of KOH can cause very high increase in the catalytic activity in ammonia synthesis.

Kinetics of wet atmosphere reduction of a fused iron catalyst for ammonia synthesis

An iron catalyst, used for ammonia synthesis, is activated by means of the reduction of its oxide precursor. The catalyst is reduced in situ by a mixture of gases containing hydrogen in the industrial reactors or in especially built installations. Water vapour retards the reduction rate at conditions far beyond the thermodynamic expectations, and, hence, its partial pressure is an essential factor in influencing the technology of the catalyst activation [ 11. Surprisingly, an explanation of the retardation effect at the molecular level is not known, though the catalyst has been used for almost 80 years [ 21. This is also in contrast with the fact that the mechanism of ammonia synthesis on the already reduced catalyst is known in detail [ 31. The effect of water vapour has been studied in our laboratory for many years. The dramatic decrease of the reduction rate of the iron catalyst [4] was linked with the presence of promoters [ 51. The role of alumina [ 61 and, later on, the role of a magnetite-alumina catalyst sub-system [7] was emphasized. This agrees well with the earlier data of Dry [ 81 and Rozowskii [ 91. It has also been found that the effect was reversible [lo] in the sense that the removal of water from the gas-phase instantaneously accelerated the retarded reduction rate.

ChemInform Abstract: Fused Iron Catalyzed Conversion of Benzene to Toluene.

AbstractBenzene hydrogenolysis is studied over a conventional fused iron ammonia synthesis catalyst (BASF‐R).

Spectrophotometric determination of Fe(II) in milligram solid samples

The analytical procedure of determination of Fe(II) in the presence of Fe(III) in milligram samples is proposed. A solid sample is dissolved in hydrochloric acid in argon atmosphere. Subsequently, the pH-value of the solution is fixed and Fe(III) complexed by phosphoric buffer. At the next step Fe(II) is oxidized by α-molybdosilicic acid. Finally, the intensity of colouring caused by the reduced form of the acid is measured at the wavelength 770 nm. The proper treatment of the sample during oxidation, and immediately before, is decisive for the accuracy of the analysis. The method was verified by means of the analysis of synthetic magnetite. The theoretical content of Fe(II) was 24.12%, the determined one was 24.02 ± 0.10%. The method was applied to the determination of the homogeneity of an iron catalyst for ammonia synthesis.

Potassium ferrites of KFeO 2 and K 1+ xFe 11O 17 systems ( x = 0–1): Studies on leaching behavior in fused iron catalysts

Two series of potassium ferrites, KFeO 2 and K 1+ x Fe 11O 17 ( x = 0–1), were prepared by several methods and their leaching process was studied. Because of its structural characteristics, KFeO 2 can be easily hydrolyzed and its structure can be completely destroyed during leaching in water, whereas for K 1+ x Fe 11O 17 only a portion of the potassium ions shifts out of the system and its structure still remains after soaking in water. The kinetic parameters for potassium leaching from the K 1+ x Fe 11O 17 system have also been determined. It is suggested that both types of potassium ferrites coexist in fused iron catalysts for ammonia synthesis: KFeO 2 occurs at boundaries, while K 1+ x Fe 11O 17 mainly remains in the crystal grains of the catalyst. The former belongs to the so-called soluble and the latter to the insoluble potash of the catalyst. The catalytic activity largely depends on the latter.

Fused iron catalyzed conversion of benzene to toluene

Whereas benzene hydrogenation catalyzed over group VIII metals has been investigated, intensively, very few reports have considered ring hydrogenolysis of this simple aromatic hydrocarbon. The authors have recently discovered that surface carbon reincorporation to produce toluene and xylenes is an important reaction operating during benzene hydrogenolysis over a conventional fused iron ammonia synthesis catalyst.

Disproportionation phenomena of wustite phase in the model iron catalyst for ammonia synthesis studied by Mössbauer spectroscopy

A model iron catalyst for ammonia synthesis containing a large amount of wustite (≈35 wt%) has been studied during the annealing in an inert atmosphere in the temperature range of 573–773 K. Changes in magnetite and wustite phases wer followed by Mössbauer spectroscopy (MS). Before starting the thermal treatment, two kinds of different structures have been found by MS and X-ray diffraction. The behaviour of both kinds of wustite during the annealing was different. One of them, closer to the stoichiometric compound, disproportionated at once into magnetite and iron; its content decreased systematically without changes in the Mössbauer parameters. In the second wustite, only qualitative changes were observed at first (its nonstoichiometry decreased without changes in content and, next, this phase started to disproportionate too.

Activity and thermoresistance of fused iron catalysts for ammonia synthesis

The activity and thermoresistance of five industrial catalysts were examined. Kinetic measurements were carried out in an isothermal, flow differential reactor under 10.0 MPa in the 400–520°C temperature range. In thermoresistance tests, overheating of the catalysts was carried out at 650°C for 24 h, or at 620°C for 24 to 192 h. The relationships obtained during both experimental tests were similar, with 24-h operating at 650°C giving a similar activity decrease as ageing for 192 h at 620°C. The chemical composition of the catalysts (XRF) was determined.

Effect of pressure on the reduction rate of a fused iron catalyst for ammonia synthesis

The reduction kinetics of an iron catalyst for ammonia synthesis of the KM I type with a grain size of 1.00–1.20 mm was studied at 773 K within the pressure range 0.1–3.8 MPa in a thermo-balance flow system using a hydrogen-nitrogen (3:1) mixture. With a 40-fold increase in the pressure, the initial reduction rate is approximately doubled. The reduction rate is also doubled with a 4-fold increase in the flow-rate. The flow-rate effect is due to a build-up of the water content (ca. 700–1700 ppm, v/v) in the thermobalance chamber during the reduction. It is well know that water vapour results in a dramatic decrease in the reduction rate. At higher flow-rates, a decrease in water content accelerates the reduction. On the above basis, the rate of reduction of an iron catalyst can be determined at an a priori fixed temperature and pressure and at an a posteriori experimentally determined humidity of the gas phase.

ChemInform Abstract: Iron Catalyst for Ammonia Synthesis: Moessbauer Spectroscopy Studies

ChemInformVolume 20, Issue 32 Reviews ChemInform Abstract: Iron Catalyst for Ammonia Synthesis: Moessbauer Spectroscopy Studies A. PATTEK-JANCZYK, A. PATTEK-JANCZYK Dep. Chem., Jagiellonian Univ., PL-30-060 Krakow, Pol.Search for more papers by this author A. PATTEK-JANCZYK, A. PATTEK-JANCZYK Dep. Chem., Jagiellonian Univ., PL-30-060 Krakow, Pol.Search for more papers by this author First published: August 8, 1989 https://doi.org/10.1002/chin.198932334Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume20, Issue32August 8, 1989 RelatedInformation

LR spectroscopic study of chemisorbed dinitrogen species on ammonia synthesis iron catalysts

The results of the LRS study of N-containing chemisorbed species on the promoted iron catalyst for ammonia synthesis have further substantiated the existence of two species of N2(a) as the dominant N-containing chemisorbed species under the functioning catalyst conditions. A model of active site, as 3-Fe cluster on (111) or (211) surface of α-Fe, and two modes of multinuclear coordination activation for the observed two species of N2(a) were proposed. It was further illustrated from reaction energetics that the mechanism of the dominant reaction pathway for ammonia synthesis/decomposition may be associative, rather than dissociative.

Oxygen sorption on reduced iron catalysts with different contents of promoters

Oxygen sorption has been studied in the temperature range from 78 to 773 K on molten iron catalysts for ammonia synthesis with different contents of promoters. Factors have been established that are responsible for the secondary pyrophoricity of passivated catalyst samples.

Simultaneous effect of phase composition and water vapour on the reduction of iron catalyst for ammonia synthesis

Water vapour causes a dramatic decrease in the reduction rate of an iron catalyst for ammonia synthesis and also affects the topochemistry of the reduction. Core-and-shell behaviour and reduction of an internal type have been observed in dry and wet atmospheres, respectively. Magnetite is the main component of the catalyst. Some industrial catalysts, e.g. Topsøe KM I, also contain a small amount (ca. 10%) of wustite. A sample of singly aluminium-promoted catalyst was divided into two parts; one part was oxidized to magnetite and the other was reduced to wustite. It has been shown that the reduction of the magnetite sample was completely retarded by 10 000 ppm of water vapour, whereas that of the wustite was only slightly affected. The results indicate that the retardation effect of water vapour is due to the magnetite phase promoted with aluminium. This result, supported by already published porosimetric and Mössbauer data and microscopic observations, explain the topochemistry of reduction of the iron catalyst in dry and wet atmospheres.

Characterization of unreduced fused iron catalyst for ammonia synthesis

A procedure for the characterization of iron catalysts is proposed. For this purpose, some model iron catalysts (obtained by fusion) for ammonia synthesis, in unreduced form, were studied. In order to characterize the catalysts, the following properties have to be established: (i) the content of major and minor cations, including Fe2+, and the total amount of iron; (ii) phase composition, insofar as the major phases are considered; (iii) homogeneity on the macro- and micro-scale, insofar, at least, as the major cations and major phases are considered; (iv) non-stoichiometry of the major iron oxides. In order to check the proposed procedure, five samples of the model catalysts (unpromoted, potassium oxide-promoted, alumina-promoted and potassium oxide/alumina-promoted) were prepared under industrial conditions and analysed according to the programme outlined above. It was found that the Fe2+ content varies significantly on either the macro- or micro-scale, this property being reflected in the wustite abundance. Wustite is present in two separate phases, one being rich in iron. This feature is due to slow cooling of the fused material. On the other hand, the magnetite phase is almost stoichiometric and well crystallized. The magnitude of the effective fields acting on iron nuclei at room temperature is a measure of the amount of alumina built into the magnetite phase. Mössbauer spectroscopy was found to be a very useful tool for the determination of the composition of the major phases and of the stoichiometry of iron-containing phases. The contents of impurities can be well approximated by the content of unreducible oxides at their highest degree of oxidation.

Fused iron surface composition as measured by low energy ion scattering

Low energy He+-scattering was used to simply demonstrate that the surface of a typical fused iron ammonia synthesis catalyst is largely covered by promoter oxides of calcium and/or potassium. Complementary photoemission results indicate that atomic nitrogen is deposited in the near surface region during catalyst activation in NH3/H2-mixtures.