A. Reizer

Bio: A. Reizer is an academic researcher from Jagiellonian University. The author has contributed to research in topics: Catalysis & Ammonia production. The author has an hindex of 7, co-authored 11 publications receiving 115 citations.

Kinetics of activation of the industrial and model fused iron catalysts for ammonia synthesis

Abstract: The paper summarizes our results published for many years and also adds new information. Kinetic data concerning the reduction of the oxidized forms of model as well as industrial catalysts used in ammonia synthesis were obtained in dry and in wet atmosphere containing 1% water vapour. The data for the industrial catalyst have been reassessed using three kinetic models. Modifications applied to the classical Seth-Ross model of the shrinking-core type resulted in the best fitting of this equation to the experimental data. The failure of the crackling core model to describe the kinetic data in a quantitative way is tentatively explained. The reduction of model catalysts containing enhanced amounts of wustite and/or potassium proceeds initially linearly with time. The effect of promoters, and especially of potassium, is discussed in more detail. The magnetite-alumina subsystem is responsible for the retardation effect of the water vapour on the reduction rate. A hypothesis is formulated that — in the presence of wustite or potassium — the inhibitive, Al-rich, hydrated surface layer is not effective in hindering the progress of the reduction process.

Kinetics of reduction of an iron catalyst for ammonia synthesis

Abstract: The kinetics of reduction of an iron catalyst have been studied at 450–550 °C. The overall kinetic equation was of the “mixed-control” type. The equation of the surface reaction was of the Langmuir-Hinshelwood type with the adsorption of only water vapor taken into account.

The Haber–Bosch Process Revisited: On the Real Structure and Stability of “Ammonia Iron” under Working Conditions

Abstract: Ammonia synthesis is one of the largest processes in chemical industries. It was first operated at BASF one hundred years ago based on the fundamental work of Fritz Haber and process engineering by Carl Bosch. Haber combined feed gas recycling with application of high pressure (P = 200 bar) and a Ruthenium catalyst to achieve sufficiently high conversions of nitrogen according to N2 + 3 H2 .2 NH3. This success enabled the large scale production of artificial fertilizers, which was a prerequisite to face the world’s increase in population and is known as the “extraction of air from bread” – a term that was coined later by Max von Laue. Today, contrary to the generation of syngas for ammonia, only little has changed in the industrial process for the actual synthesis of ammonia.The process is operated at typical temperatures of 500 °C and pressures around 200 bar, resulting in ammonia concentrations in the exhaust gas of up to 17 vol.%. Approximately 80% of the worldwide ammonia output of 136 Mtons (2011) is used for the production of fertilizers. A key development for the modern Haber-Bosch process, however, has been the catalyst development at BASF that was led by Alwin Mittasch in the early 20 century. After testing 22 000 different formulations in a gigantic effort, the work was concluded in 1922 with the identification of a very unique catalyst synthesis. To achieve a highly active iron catalyst, magnetite, Fe3O4, was promoted by fusing it together with irreducible oxides (K2O, Al2O3, later also CaO) in an oxide melt at temperatures around 1000 °C. The fused magnetite is mechanically granulated and its reduction need to be conducted with great care in the syngas feed to finally give the active α-Fe catalyst. This special synthesis leads to certain crucial properties of the resulting α-Fe phase, which is commonly termed “ammonia iron”. In addition to its outstanding economic relevance, ammonia synthesis acts as a “drosophila reaction” for catalysis research and has always been a test case for the maturity of catalysis science in the context of a technologically mature application. Today, due to the enormous efforts in surface science, physical and theoretical chemistry, and chemical engineering a consistent picture of the reaction mechanism and the role of the Fe catalyst and its promoters has emerged. Key contributions to the modern understanding of the ammonia synthesis reactions came from the teams lead by Gerhard Ertl, Michel Boudart, Gabor Somorjai, Haldor Topsoe and Jens K. Norskov, just to mention a few. However, even after 100 years of application and research there still is scientific interest in the Haber-Bosch process, mainly because of two aspects. Firstly, catalysts with improved lowtemperature activity, higher specific surface area and higher tolerance against poisons and on-off operations are generally desirable. Also the development of a more elegant synthesis route for the Fe-based catalyst without the melting step and the extremely critical activation procedure could foster the potential application of ammonia as an energy storage molecule. Secondly, there still is a gap between the model studies conducted with well-defined simplified materials with clean surfaces at low pressures to elaborate the current knowledge of ammonia synthesis and the industrial process. These so-called pressure and materials gaps often prevent straightforward extrapolation of model studies to real industrial processes. Thus, the question of a dynamical change of the catalyst under true reaction conditions remains to be studied and calls for in situ experimentation. This point requires special attention in case of the ammonia synthesis over iron catalysts, because it is well known and has been studied for decades in the context of steel hardening and catalytic ammonia decomposition that iron can be easily nitrided by ammonia. Ertl and co-workers described the reaction mechanism of ammonia synthesis. 14] He and other authors showed that the reaction is structure sensitive. The dissociative chemisorption of di-nitrogen on the iron surface is the rate limiting step in ammonia synthesis and opens possibilities for sub-surface diffusion of the atomic nitrogen. Ertl et al. proposed the surface dissolution of nitrogen into iron forming a surface nitride of the approximate composition Fe2N and the presence of in-situ formed metastable γFe4N. [6a] Thus, for experimental conditions remote from the HaberBosch process, participation of stoichiometric bulk nitrides like FeN has been excluded. Instead, Herzog et al. proposed formation of [∗] Timur Kandemir, Dr. Manfred.E. Schuster, Dr. Malte Behrens, Prof. Dr. Robert Schlogl Department of Inorganic Chemistry Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4-6, D-14195 Berlin, Germany Fax: (+)49-(0)30-8413-4401 E-mail: behrens@fhi-berlin.mpg.de, acsek@fhi-berlin.mpg.de

X-ray diffraction study of nickel oxide reduction by hydrogen

Abstract: Hydrogen reduction of porous bulk NiO particles has been studied with in situ hot-stage X-ray diffraction (XRD) in the temperature range 175–300 °C. This technique has the ability to measure NiO disappearance and Ni appearance simultaneously, together with the crystallite size of each. Since the sample was a very thin, 50-μm slab of dispersed 20-μm diameter grains, textural and morphological features normally encountered during studies with fixed beds of NiO particles were absent and measurements reflected only the chemical mechanism and kinetics. The results indicated that reduction in the absence of water added to the reducing gas followed a series of steps: (1) an induction period associated with the initial reduction of NiO and the appearance of Ni metal clusters; (2) acceleration of the reduction rate as the size of the clusters increase; and (3) a pseudo-first-order (excess H 2 ) process in which NiO disappeared and Ni appeared in concert until reduction slowed at a fractional conversion of about 0.8. Crystallite size measurements showed NiO crystallites of about 3 nm in size were transformed into Ni crystallites of more than 20 nm, implying that Ni 0 ion transport following reduction was very fast due to the close proximity of the NiO crystallites being reduced. When 2.2×10 −2 atm of H 2 O was added to the reducing gas, induction times increased by approximately a factor of two and reduction rates decreased (increasingly at lower temperatures) with an apparent activation energy of 126±27 kJ mol −1 compared to 85±6 kJ mol −1 without added water. The lag between NiO reduction and Ni growth observed in previous studies was not seen, indicating that textural and morphological factors are very important in establishing the role of water vapor in the reduction process.

Bulk and surface phases of iron oxides in an oxygen and water atmosphere at low pressure

Abstract: Thermodynamic stability ranges of different iron oxides were calculated as a function of the ambient oxygen or water gas phase pressure (p⩽1 bar) and temperature by use of the computer program EquiTherm. The phase diagram for Fe–H2O is almost completely determined by the O2 pressure due to the H2O dissociation equilibrium. The formation of epitaxially grown iron oxide films on platinum and ruthenium substrates agrees very well with the calculated phase diagrams. Thin films exhibit the advantage over single crystals that bulk diffusion has only limited influence on the establishment of equilibrium phases. Near the phase boundary Fe3O4–Fe2O3, surface structures are observed consisting of biphase ordered domains of FeO(111) on both oxides. They are formed due to kinetic effects in the course of the oxidation to hematite or reduction to magnetite, respectively. Annealing a Fe3O4(111) film in 5 × 10−5 mbar oxygen at 920–1000 K results in a new γ-Fe2O3(111)-like intermediate surface phase during the oxidation to α-Fe2O3(0001). A model is suggested for the growth of iron oxides and for redox processes involving iron oxides. The formation of several equilibrium surface phases is discussed.