The Haber-Bosch process revisited: on the real structure and stability of "ammonia iron" under working conditions.

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