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Reaction Mechanisms, Kinetics, and Improved Catalysts for Ammonia Synthesis from Hierarchical High Throughput Catalyst Design

Fuller, Jon and An, Qi and Fortunelli, Alessandro and Goddard, William A., III (2022) Reaction Mechanisms, Kinetics, and Improved Catalysts for Ammonia Synthesis from Hierarchical High Throughput Catalyst Design. Accounts of Chemical Research, 55 (8). pp. 1124-1134. ISSN 0001-4842. doi:10.1021/acs.accounts.1c00789. https://resolver.caltech.edu/CaltechAUTHORS:20220407-359886241

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Abstract

The Haber–Bosch (HB) process is the primary chemical synthesis technique for industrial production of ammonia (NH₃) for manufacturing nitrate-based fertilizer and as a potential hydrogen carrier. The HB process alone is responsible for over 2% of all global energy usage to produce more than 160 million tons of NH₃ annually. Iron catalysts are utilized to accelerate the reaction, but high temperatures and pressures of atmospheric nitrogen gas (N₂) and hydrogen gas (H₂) are required. A great deal of research has aimed at increased performance over the last century, but the rate of progress has been slow. This Account focuses on determining the atomic-level reaction mechanism for HB synthesis of NH₃ on the Fe catalysts used in industry and how to use this knowledge to suggest greatly improved catalysts via a novel paradigm of catalyst rational design. We determined the full reaction mechanism on the two most active surfaces for the HB process, Fe(111) and Fe(211)R. We used density functional theory (DFT) to predict the free-energy barriers for all 12 important reactions and the 34 most important 2 × 2 surface configurations. Then we incorporated the mechanism into kinetic Monte Carlo (kMC) simulations run for several hours of real time to predict turnover frequencies (TOFs). The predicted TOFs are within experimental error, indicating that the predicted barriers are within 0.04 eV of experiment. With this level of accuracy, we are poised to use DFT to improve the catalyst. Rather than forming bulk alloys with uniform concentration, we aimed at finding additives that strongly prefer near-surface sites so that minor amounts of the additive might lead to dramatic improvements. However, even for a single additive, the combinations of surface species and reactions multiplies significantly, with ∼48 reaction steps to examine and nearly 100 surface configurations per 2 × 2 site. To make it practical to examine tens of dopant candidates, we developed the hierarchical high-throughput catalysis screening (HHTCS) approach, which we applied to both the Fe(111) and Fe(211) surfaces. For HHTCS, we identified the most important 4 reaction steps out of 12 for the two surfaces to examine >50 dopant cases, where we required performance at each step no worse than for pure Fe. With HHTCS, the computational cost is about 1% of that for doing the full reaction mechanism, allowing us to do ≈50 cases in about 1/2 the time it took to do pure Fe(111). The new leads identified with HHTCS are then validated with full mechanistic studies. For Fe(111), we predict three high-performance dopants that strongly prefer the second layer: Co with a rate 8 times higher, Ni with a rate 16 times higher, and Si with a rate 43 times higher, at 400 °C and 20 atm. We also found four dopants that strongly prefer the top layer and improve performance: Pt or Rh 3 times faster and Pd or Cu 2 times faster. For Fe(211), the best dopant was found to be second-layer Co with a rate 3 times faster than that for the undoped surface. The DFT/kMC data were used to predict reshaping of the catalyst particles under reaction conditions and how to tune dopant content so as to maximize catalytic area and thus activity. Finally, we show how to validate our mechanistic modeling via a comparison between theoretical and experimental operando spectroscopic signatures.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1021/acs.accounts.1c00789DOIArticle
ORCID:
AuthorORCID
Fuller, Jon0000-0003-1233-7842
An, Qi0000-0003-4838-6232
Fortunelli, Alessandro0000-0001-5337-4450
Goddard, William A., III0000-0003-0097-5716
Additional Information:© 2022 American Chemical Society. Received 20 December 2021. Published online 6 April 2022. A.F. gratefully acknowledges the contribution of the International Research Network IRN on Nanoalloys (CNRS). A.F. and W.A.G. received support from NSF (CBET-1805022 and CBET-2005250). The authors declare no competing financial interest.
Funders:
Funding AgencyGrant Number
Centre National de la Recherche Scientifique (CNRS)UNSPECIFIED
NSFCBET-1805022
NSFCBET-2005250
Other Numbering System:
Other Numbering System NameOther Numbering System ID
WAG1513
Issue or Number:8
DOI:10.1021/acs.accounts.1c00789
Record Number:CaltechAUTHORS:20220407-359886241
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20220407-359886241
Official Citation:Reaction Mechanisms, Kinetics, and Improved Catalysts for Ammonia Synthesis from Hierarchical High Throughput Catalyst Design Jon Fuller, Qi An, Alessandro Fortunelli, and William A. Goddard Accounts of Chemical Research Article ASAP DOI: 10.1021/acs.accounts.1c00789
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:114194
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:07 Apr 2022 09:58
Last Modified:23 Apr 2022 05:14

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  • Fuller, Jon and An, Qi and Fortunelli, Alessandro and Goddard, William A., III Reaction Mechanisms, Kinetics, and Improved Catalysts for Ammonia Synthesis from Hierarchical High Throughput Catalyst Design. (deposited 07 Apr 2022 09:58) [Currently Displayed]

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