Studying the Mechanisms of Hydrogen-Based Iron Oxide Reduction for Green Steel Making at the Atomic-And Nanoscale Using Atomprobe Tomography

1.9 billion tons of steel have been produced last year, making it the most important material in terms of volume and environmental impact. Steel is a sustainability enabler, in lightweight vehicles, wind mills and magnets. However, its primary production is the oppposite. Iron is today reduced from oxide ores using fossil reductants, translating to about 2t CO₂ per ton of steel produced [1,2]. This stands for more than 30% of the global CO₂ emissions in manufacturing. These emissions can be reduced when replacing carbon by hydrogen or ammonia as reductants [1-4].<br/>The lecture discusses some key mechanisms of hydrogen- and ammonia-based solid-state direct reduction and hydrogen-based plasma-based liquid state reduction from a near-atomistic perspective using atom probe tomography [3-5].<br/>Our perception of such reactions has not only been limited so far by the availability of state-of-the-art techniques which can delve into the structure and near-atomic scale chemistry of the reacted solids, but we continue to miss an important reaction partner that defines the thermodynamics and kinetics of gas phase reactions: the gas molecules. We present here results from cryogenic-atom probe tomography to study the quasi in-situ evolution of iron oxide in the solid and gas phases of the direct reduction of iron oxide by deuterium gas at 700°C [6]. We observed several so far unknown atomic-scale characteristics, including, D2 accumulation at the reaction interface; formation of a core (wüstite)-shell (iron) structure; inbound diffusion of D through the iron layer and partitioning of D among phases and defects; outbound diffusion of oxygen through the wüstite and/or through the iron to the next free available inner/outer surface; and the internal formation of heavy nano-water droplets at nano-pores.<br/><br/>References<br/>[1] Raabe D, Tasan CC, Olivetti EA. Strategies for improving the sustainability of structural metals. Nature. 2019 Nov; 575 (7781): 64-74.<br/>[2] Raabe, D. The Materials Science behind Sustainable Metals and Alloys. Chem. Rev. 2023, 123, 2436–2608.<br/>[3] Kim, S. H.; Zhang, X.; Ma, Y.; Souza Filho, I. R.; Schweinar, K.; Angenendt, K.; Vogel, D.; Stephenson, L. T.; El-Zoka, A. A.; Mianroodi, J. R.; Rohwerder, M.; Gault, B.; Raabe, D. Influence of Microstructure and Atomic-Scale Chemistry on the Direct Reduction of Iron Ore with Hydrogen at 700°C. Acta Mater. 2021, 212, 116933.<br/>[4] Ma, Y.; Bae, J. W.; Kim, S.; Jovi, M.; Li, K.; Vogel, D.; Ponge, D.; Rohwerder, M.; Gault, B.; Raabe, D. Reducing Iron Oxide with Ammonia: A Sustainable Path to Green Steel. Adv. Sci. 2023, 2300111, 1–7.<br/>[5] Souza Filho, I. R.; Ma, Y.; Kulse, M.; Ponge, D.; Gault, B.; Springer, H.; Raabe, D. Sustainable Steel through Hydrogen Plasma Reduction of Iron Ore: Process, Kinetics, Microstructure, Chemistry. Acta Mater. 2021, 213, 116971.<br/>[6] El-Zokaa, A.A., L.T. Stephenson, S.–H. Kim, B. Gault, D. Raabe, The fate of water in hydrogen-based iron oxide reduction. Adv. Sci. 2023, in press.