Reaction mechanism and kinetics for ammonia synthesis on the Fe(211) reconstructed surface
Abstract
To provide guidelines to accelerate the Haber–Bosch (HB) process for synthesis of ammonia from hydrogen and nitrogen, we used Quantum Mechanics (QM) to determine the reaction mechanism and free energy reaction barriers under experimental reaction conditions (400 °C and 20 atm) for all 10 important surface reactions on the Fe(211) reconstructed (Fe(211)R) surface. These conditions were then used in full kMC modeling for 30 minutes to attain steady state. We find that the stable surface under Haber–Bosch conditions is the missing row 2 × 1 reconstructed surface (211)R and that the Turn Over Frequency (TOF) is 18.7 s^(−1) per 2 × 2 surface site for 1.5 Torr NH_3 pressure, but changes to 3.5 s^(−1) for 1 atm, values close (within 6%) to the ones on Fe(111). The experimental ratio between (211) and (111) rates at low (undisclosed) NH_3 pressure was reported to be 0.75. The excellent agreement with experiment on two very different surfaces and reaction mechanisms is a testament of the accuracy of QM modeling. In addition, our kinetic analysis indicates that Fe(211)R is more active than Fe(111) at high pressure, close to HB industrial conditions, and that (211)R is more abundant than (111) via a steady-state Wulff construction under HB conditions. Thus, at variance with common thinking, we advocate the Fe(211)R surface as the catalytically active phase of pure iron ammonia synthesis catalyst under HB industrial conditions.
Additional Information
© 2019 the Owner Societies. Received 22nd March 2019, Accepted 14th May 2019, First published on 14th May 2019. This work was supported by the U.S. Department of Energy (USDOE), Office of Energy Efficiency and Renewable Energy (EERE), Advanced Manufacturing Office Next Generation R&D Projects under contract no. DE-AC07-05ID14517 (program manager Dickson Ozokwelu, in collaboration with Idaho National Laboratories, Rebecca Fushimi). A. F. gratefully acknowledges financial support from a Short-Term Mission (STM) funded by Italian Consiglio Nazionale delle Ricerche (CNR). We would like to thank the Information Technology department at the University of Nevada, Reno for computing time on the High Performance Computing Cluster (Pronghorn). Some calculations were also carried out on a GPU-cluster provided by DURIP (Cliff Bedford, program manager). Authors contributions: AF, WAG and QA designed the strategy of this work. JF and QA performed QM calculations. AF performed kMC simulations. All authors wrote the paper. The authors declare no competing financial interest.Attached Files
Accepted Version - 1336-Fe_211_HaberBosch.pdf
Supplemental Material - c9cp01611b1_si.pdf
Supplemental Material - c9cp01611b2_si.xlsx
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Additional details
- Eprint ID
- 95642
- DOI
- 10.1039/c9cp01611b
- Resolver ID
- CaltechAUTHORS:20190521-110908971
- DE-AC07-05ID14517
- Department of Energy (DOE)
- Consiglio Nazionale delle Ricerche (CNR)
- Created
-
2019-05-21Created from EPrint's datestamp field
- Updated
-
2021-11-16Created from EPrint's last_modified field
- Caltech groups
- Astronomy Department
- Other Numbering System Name
- WAG
- Other Numbering System Identifier
- 1336