Iron Catalyst For Ammonia Synthesis Research Articles

Thermodynamics of Iron Ammonia Synthesis Catalyst Sintering

The sintering of iron ammonia synthesis catalysts (nanocrystalline iron promoted with: Al2O3, CaO and K2O) was studied under a hydrogen atmosphere, in a temperature range of 773 to 973 K to obtain stationary states. The catalysts were characterized by measuring the nitriding reaction rate under an ammonia atmosphere at 748 K to obtain steady states and the measurement of specific surface area. Chemical processes were conducted in a tubular differential reactor enabling thermogravimetric measurements and the chemical composition analysis of a gas phase under conditions allowing experiments to be carried out in the kinetic region of chemical reactions. An extended model of the active surface of the iron ammonia synthesis catalyst was presented, taking into account the influence of the gas phase composition and process temperature. The surface of iron nanocrystallites was wetted using promoters in an exothermic process associated with the formation of the surface Fes-O- bond and the change in the surface energy of iron nanocrystallites. Promoters formed on the surface of iron nanocrystallites with different structures of chemisorbed dipoles, depending on the composition of the gas phase. The occupied sites stabilized the structure, and the free sites were active sites in the process of adsorption of chemical reagents and in sintering. Based on the bonding energy of the promoter oxides and the difference in surface energy between the covered and uncovered surfaces, the wetting abilities of promoters, which can be arranged according to the order K2O > Fe3O4 > Al2O3 > CaO, were estimated. By increasing the temperature in the endothermic sintering process, the degree of surface coverage with dipoles of promoters decreased, and thus the catalyst underwent sintering. The size distribution of nanocrystallites did not change with decreasing temperature. Only the equilibrium between the glass phase and the surface of iron nanocrystallites was then established.

A bioinspired sulfur-surrounded iron catalyst for ammonia synthesis

A nitrogenase-inspired inorganic compound consisting of iron (Fe) and molybdenum disulfide (MoS2) is designed with a promising capability of N2 fixation using first principles.

Development and application of wüstite-based ammonia synthesis catalysts

In the 1980s, Zhejiang University of Technology (ZJUT) started developing a new catalyst for ammonia synthesis which in final form comprises multi-promoters wüstite-based catalysts with Fe1-xO as precursor. This became the first new iron catalyst for ammonia synthesis commercialized since Fritz Haber’s promoted iron catalyst. The catalyst is commercialized in the existing or new low pressure process. In this paper, the basic theory, method, technology and practice of the wüstite-based catalyst in industrial production and application are reviewed. The wüstite-based catalysts are still iron-based, however, they are completely different from the magnetite-based catalyst in the preparation principle, chemical composition, crystal structure, and chemical-physical properties. Wüstite does not exist in nature, and is a thermodynamically meta-stable phase. Therefore, the Fe1-xO must be firstly synthesized by a chemical method and avoid occurrence reactions of oxidation and disproportionation to obtain a commercial wüstite-based catalyst with single phase and stability during the preparation of catalyst. We described the development of the catalyst, starting from the 1980s with studies to elucidate the inhibition mechanism of disproportionation reaction and the effects of temperature and promoters, and to ensure that a single wüstite phase is obtained during preparation. This is a prerequisite for ensuring high activity of the catalyst. Scale up of catalyst production and testing were described briefly, as are some process considerations. Secondly, we described the industrial application of the catalyst with studies to elucidate including the industrial bypass testing, the foundation research on the industrial application technology such as industrial reduction technology due to too fast reduction rate, development of industrial technology taking out the excess reaction heat, optimization of reaction conditions using wüstite-based catalyst, low-pressure ammonia process, and optimization of synthesis loop process and convertor for existing synthesis ammonia plant, etc. The wüstite-based catalysts exhibit about 70 % higher catalytic activity, and 65.9 % higher the TOF. It is the most advanced commercial iron catalyst with the highest activity and low production cost in the world, and it is competitive with ruthenium-based ammonia synthesis catalysts. In industrial application, compared to the classic magnetite-based catalyst for ammonia synthesis, the higher activity allows lower operating temperatures and loop pressures. Existing loops can utilize it by running at minimum possible loop pressure for extended period of time. This allows to increase ammonia plant performance, efficiency and economy without the need of any revamping. New plants could be designed for lower pressure operation and with a smaller synthesis converter. This all together leads to lower capital expend tureen lower energy costs. The activation of this catalyst is fast and easy, which saves time and cost. Wüstite-based catalyst is clearly a perfect low cost alternative to Ru/C catalyst.

Highly Efficient Ni-Doped Iron Catalyst for Ammonia Synthesis from Quantum-Mechanics-Based Hierarchical High-Throughput Catalyst Screening

To discover more efficient industrial catalysts for ammonia synthesis via the Haber–Bosch (HB) process, we employed quantum mechanics (QM)-based hierarchical high-throughput catalyst screening (HHTCS) to test a wide group of elements (34) as candidates to dope the Fe(111) catalyst subsurface. The QM free-energy reaction network of HB over Fe(111) yields ten barriers as potentially rate-determining, of which we select four as prototypical, arrange them hierarchically, and define a corresponding set of screening criteria, which we then use to screen candidate catalysts. This leads to two promising candidates (Co and Ni), from which we selected the most promising (Ni) for a complete QM and kinetic study. The kinetic Monte Carlo (kMC) simulations predict a 16-fold increase in HB turn-over frequency (TOF) for the Ni-doped catalyst compared to the pure Fe(111) surface under realistic conditions. The 16-fold increase in HB TOF is a significant improvement and may trigger future experimental studies to validate our prediction. This TOF improvement could lead to similar reaction rates as with pure Fe but at a reaction temperature decreased by 100° from 773 to 673 K and a total reactant pressure decreased by 6 times from 201 to 34 atm. We interpret the reasons underlying this improvement using valence bond and kinetic analyses. We suggest this Ni-doped Fe(111) catalyst as a candidate to reduce the world energy consumption for the HB process while satisfying future needs for energy and environment.

TOF-SIMS study of morphology and chemical composition of wustite-based precursor and iron catalyst for ammonia synthesis

This study focuses on the application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) for analysis of the morphology and chemical composition of a wustite-based precursor and iron catalyst for ammonia synthesis. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used as supplementary methods. TOF-SIMS imaging was capable of visualizing the micrometer-scale spatial distribution of elements on the surface of the catalyst precursor, which was composed of wustite crystallites separated by spaces, with high concentrations of K, Al and Ca used as promoters. Moreover, aluminum and calcium were detected in the wustite crystallites. Potassium, despite having a vanishingly low concentration in wustite phase, was able effectively to wet the wustite surfaces upon annealing at 350 °C.The potassium layer that formed on the surface of the wustite precursor had a different composition than that on the surface of the iron nanocrystallites of the iron catalyst. The most important finding concerns the composition of the surface of the iron catalyst.TOF-SIMS analysis showed a layer containing oxygen in the subsurface region of the iron catalyst obtained from the wustite-based precursor. This structure on the surface of the iron catalyst enabled the formation of active sites for the adsorption of nitrogen on iron atoms during ammonia synthesis.

Study of the Iron Catalyst for Ammonia Synthesis by Chemical Potential Programmed Reaction Method

A new method, entitled chemical potential programmed reaction, for determining the physicochemical properties of iron ammonia synthesis catalyst has been proposed. Two model reactions were applied: nitriding of the iron catalyst and reduction of the obtained nitrides. Measurements of the rates of those reactions were carried out at 350 °C in a differential tubular reactor. The reactor is equipped with a system that allows us to perform simultaneous thermogravimetric measurements and a catharometric system to determine hydrogen concentration in the gas phase. The reactor was fed with a mixture of ammonia and hydrogen of varying composition, which was changing in a controlled way. Different accelerations of the nitriding potential change were applied. During the processes of nitriding of nanocrystalline iron and reduction of the obtained nanocrystalline iron nitrides rates of these processes were measured. The minimum nitriding potential, at which the phase transformation of nanocrystallites of a certain si...

A method of determining nanoparticle size distribution in iron ammonia synthesis catalyst by measuring mass changes during the nitriding process

A new method for determination of nanoparticle size distribution, on the basis of measurements of nanomaterial conversion degree as a function of the chemical potential of gas reagent, was presented. The method was applied for determination of nanocrystallite size distribution in a pre-reduced iron catalyst for ammonia synthesis being an example of a nanomaterial. Samples were reduced with hydrogen at 500°C and then nitrided at 300°C in gaseous ammonia-hydrogen mixtures of different nitriding potentials. Conversion degree measurements, viz., measurements of mass changes of the catalyst were performed in a differential reactor equipped with systems that enable thermogravimetric measurements and analysis of gas phase chemical composition. Based on the performed measurements, a relationship binding the conversion degree with size distribution was proposed. The resulting size distribution of nanocrystallites was compared to distributions determined by known techniques based on XRD or on measurements of the nitriding reaction rate. The size distribution determined using the new method has a higher resolution, revealing more details of the sample morphology.

Promotion of Nb2O5 on the wustite-based iron catalyst for ammonia synthesis

Niobium was selected and investigated as a potential promoter for wustite-based catalyst (WBC) for ammonia synthesis. Experiments on reduction performance, activity test and H2-TGA, in situ XRD as well as XPS were carried out to obtain the promotion effect and mechanism involved. Niobium as a promoter was confirmed to enhance the reduction of WBC significantly. This behavior is highly desired for industry in terms of catalyst regeneration and lesser pretreatment time for fabrication regardless the unimproved catalytic performance for Nb2O5-doped wustite-based catalyst (Nb-WBC). Possible reasons for these phenomena are discussed. It is suggested that Nb2O5 is not favorable for the segregation or formation of solid solutions on the catalyst surface, which are difficult to be reduced. However, it seems that niobium does not promote the growth of [211] plane, which is active for ammonia synthesis.

A New Method for Determining the Nanocrystallite Size Distribution in Systems Where Chemical Reaction between Solid and a Gas Phase Occurs

The proposed method, based on measuring the chemical reaction rate in solid phase, is, therefore, limited to such systems where reaction between nanocrystalline materials and a gas phase occurs. Additionally, assumptions of the model of reaction between nanocrystalline materials and a gas phase, where the surface chemical reaction rate is the rate limiting step, are used. As an example of such a reaction, nitriding (with ammonia) of the prereduced industrial iron catalysts for ammonia synthesis of different average crystallite sizes was used. To measure the reaction rate, the differential reactor equipped with systems for thermogravimetric measurements and analysis of the chemical composition of the gas phase was used. The crystallites mass and size distributions for the analyzed samples of catalyst were determined.

The Study of Iron Catalyst for Ammonia Synthesis in Chemical Engineering

The temperature programmed desorption technique is used to study the hydrogen adsorption on the catalytic surface of fused iron catalysts with different oxide precursors in chemical engineering. The different catalysts desorption active energy, desorped temperature and their amouts desorped have been attained. The desorption energies, desorped temperatures and desorption amounts have been related to the iron ratio (Fe2+/Fe3+). It is compared with the curve of ammonia synthesis activity against iron ratio. The inhibition of hydrogen in the ammonia synthesis reaction is verified, the adsorption intensity of hydrogen controls the inhibition to nitrogen adsorption

The effect of manganese on the structural and surface properties of nanocrystalline cobalt catalyst for ammonia synthesis

Abstract Based on the active surface model of the iron catalyst for ammonia synthesis, the assumptions which led to obtaining the cobalt catalyst for ammonia synthesis were described. The incorporation into the system small amounts of manganese, an element which binds oxygen stronger than cobalt, has influence the development of the catalyst’s specific surface area. The activity of manganese modified catalysts is higher than that of cobalt catalyst without manganese addition. The obtained catalysts were characterized with the following methods: ICP-OES, XRD, BET

Influence of chemical composition of nanocrystalline iron’s surface on the rates of two parallel reactions: nitriding and catalytic decomposition of ammonia

AbstractInfluence of chemical composition of nanocrystalline iron’s surface on its activity in the nitriding process of iron and catalytic decomposition of ammonia was studied. The rate of the nitriding reaction was measured by the thermogravimetric method using a tubular differential reactor. Hydrogen concentration in the reactor was also measured. The rate of the catalytic decomposition of ammonia was determined using the reactor’s mass balance. Experiments were conducted at different temperatures within the range from 300°C to 525°C. Iron catalyst for ammonia synthesis was studied. Two sorts of samples with a different content of potassium oxide (0.16 mass % and 0.64 mass % of K2O) were used. The composition of samples from the second group was modified by an addition of different amounts of sulfur. At temperatures above 400°C, when the effect of ammonia decomposition on the gas phase composition was experimentally measured, the presence of potassium and sulfur on the iron surface influenced the rate of the iron nitriding process. The ammonia decomposition rate was higher for samples with a greater amount of potassium. The rate of reactions depended also on the sulfur concentration and dropped when the sulfur content increased. The value of apparent activation energy of ammonia decomposition was in the range of 150 kJ mol−1 to 180 kJ mol−1 while the content of sulfur in the samples increased.

ChemInform Abstract: A Study of Iron Catalyst for Ammonia Synthesis by Electron Spectroscopy Methods

AbstractReview: 63 refs.

RETRACTED ARTICLE: Preparation of highly active ruthenium based catalysts and performance comparison of Ru catalyst and iron catalyst for ammonia synthesis

RETRACTED ARTICLE: Preparation of highly active ruthenium based catalysts and performance comparison of Ru catalyst and iron catalyst for ammonia synthesis

Heterogeneity of ingot of the fused iron catalyst for ammonia synthesis

The research focused on changes in the structure, phase and chemical composition, activity and overheating resistance of the fused iron catalyst for ammonia synthesis in the ingot of the melted catalyst. The ingot is characterized with high heterogeneity. The distribution of promoters in the ingot is determined by the progress of crystallization. Aluminium is evenly distributed over the whole volume of the ingot. Other promoters shift along with the crystallization face. Their highest concentration is to be found in the middle part of the ingot. The content of promoters in the magnetite phase depends on the cooling rate of various parts of the ingot. When the temperature of cooling decreases, the concentration of aluminium in the magnetite also decreases while the concentration of calcium increases. Catalysts which are more slowly cooled, with a smaller amount of aluminium and a larger amount of calcium in the magnetite are less resistant to overheating. Catalysts from the upper and lower parts of the ingot, with the highest content of the magnetite phase, are more active than catalysts from the middle part with a significantly smaller portion of the magnetite phase.

Effects of Reaction Conditions on Performance of Ru Catalyst and Iron Catalyst for Ammonia Synthesis

Activated carbon-supported Ru-based catalyst and A301 iron catalyst were prepared, and the influences of reaction temperature, space velocity, pressure, and H 2/N 2 ratio on performance of iron catalyst coupled with Ru catalyst in series for ammonia synthesis were investigated. The activity tests were also performed on the single Ru and Fe catalysts as comparison. Results showed that the activity of the Ru catalyst for ammonia synthesis was higher than that of the iron catalyst by 33.5%–37.6% under the reaction conditions: 375–400 °C, 10 MPa, 10000 h −1, H 2 : N 2 = 3, and the Ru catalyst also had better thermal stability when treated at 475 °C for 20 h. The outlet ammonia concentration using Fe-Ru catalyst was increased by 45.6%–63.5% than that of the single-iron catalyst at low temperature (375–400 °C), and the outlet ammonia concentration increased with increasing Ru catalyst loading.

Measurements of the relative number of active sites on iron catalyst for ammonia synthesis by hydrogen desorption

Analysis of relative number of active sites of ammonia synthesis iron catalysts was performed by TPD-H 2 method. Samples of the industrial pre-reduced catalyst, which were reduced and heated at different temperatures from the range 500 °C to 780 °C in order to modify their specific surface area, were used. Chemical composition of samples to be tested was modified as well by adding sulfur compound or reducing the amount of potassium oxide. The process of hydrogen adsorption was conducted at 25 °C. Then hydrogen adsorption occurs on the surface together with absorption in the volume of iron. On the spectrum of H 2-TPD the presence of peaks α 1, α 2 and α 3 was observed at temperatures 130 °C, 300 °C and 450 °C, respectively. Peak α 1, associated with adsorption on the surface, is proportional to the total amount of absorbed as well as adsorbed hydrogen. Height of α 1 peak varies with the shift of catalysts’ activity. This peak gives the information about the number of active sites on the catalyst's surface. Heights of peaks α 2 and α 3 depend mainly on the size of sorption of hydrogen in the volume of iron. Changes in activity of the samples did not significantly affect the heights of peaks α 2 and α 3.

ChemInform Abstract: Influence of Aluminum and Potassium on Activity and Texture of Fused Iron Catalysts for Ammonia Synthesis.

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A study of iron catalyst for ammonia synthesis by electron spectroscopy methods

A concise review of former studies into iron catalyst using electron spectroscopy methods is presented. A model of iron catalyst’s surface, the so-called double layer model is presented. On the basis of this model both a method of determining surface content of a real iron catalyst and a method of determining the rate of potassium surface diffusion were developed. A double layer model was used to interpret thermal stability loss of potassium atoms on iron surface subjected to sulphur compounds’ exposure.

Iron catalyst for ammonia synthesis doped with lithium oxide

Abstract Iron catalysts doped with Al2O3, CaO were obtained by melting iron oxide with 2.2 wt.% of Al2O3, 2.1 wt.% of CaO. The reduced catalyst was impregnated with lithium hydroxide water solution. Activity measurements were carried out in the laboratory installation in the temperature range 623–773 K under the pressure of 10 MPa. The activity of catalyst containing 0.79 wt.% of Li2O and reduced at 773 K was similar to the activity of an industrial iron catalyst doped with potassium oxide. After reduction at 923 K the catalyst containing 0.48 wt.% of Li2O was about 15% more active than the industrial catalyst. Increasing Li2O concentration results in the decrease of the surface area of a catalyst reduced at 923 K. The most active catalysts doped with lithium oxide were more active than the industrial catalyst when their activity was calculated and scaled down to surface area units.