Discovery of Dramatically Improved Ammonia Synthesis Catalysts through Hierarchical High-Throughput Catalyst Screening of the Fe(211) Surface

Abstract

In order to improve efficiency of ammonia synthesis using the Haber−Bosch (HB) process with Fe-based catalysts, we employed quantum mechanics (QM)-based hierarchical high-throughput catalyst screening (HHTCS) of 49 possible metal dopants. Here, we consider the Fe( 211) surface (one of the two most active iron catalyst facets) to identify dopants that dramatically increase the turnover frequency (TOF) for HB synthesis. We found that under HB conditions, this surface reconstructs to form the Fe(211)R missing-row surface. Focusing on dopants with a strong preference for the subsurface site, we found that Co is the most promising candidate among the 49. We then examined the full reaction pathway on this Co-doped Fe(211)R surface, considering all 19 important 2 × 2 configurations and calculated the free-energy barriers (ΔG ∫ ) for all 12 important reaction steps. At 673 K and 20 atm, we find a decrease, δ(ΔG ∫ ) = −0.19 eV, in the overall reaction free-energy barrier for the Co-doped case. We then carried out kinetic Monte Carlo simulations for 60−120 min using 100 replicas with the full reaction path using rates from QM free-energy reaction barriers to predict that the TOF for the Co-doped surface increases by a factor of 2.8 with respect to the undoped Fe(211)R surface. Thus, the Co-doped Fe(211)R system could lower the extreme HB pressure of 200 atm to ∼40 atm at 773 K while maintaining the same TOF as that of undoped Fe(211)R. We conclude that Co dopants in the Fe catalyst could significantly improve the catalytic efficiency of ammonia synthesis under industrial conditions. This excellent performance of the Co-doped system is explained in terms of a surface spin analysis on the N 2 -bonded configurations that show how Co dopants shift the N 2 surface-binding mode. This demonstrates that metal surface spins can be used as quantitative descriptors to understand reaction energetics. This study demonstrates that the HHTCS kinetic analysis of the free-energy reaction path in terms of essential configurations can enable discovery of the salient barriers to overcome and best dopant candidates for further improvements.

Vibrational Spectroscopy Signatures of Catalytically Relevant Configurations for N2 Reduction to NH3 on Fe Surfaces via Density Functional Theory

“…This could help distinguish the two surfaces. For Co-doping of the bcc-(211)R surface, we found that the population of H on the surface in the resting states decreases appreciably, 14 for example, at 673 K and P(H 2 ) = 5 atm, P(N 2 ) = 5 atm, and P(NH 3 ) = 1 atm, 30% of sites are occupied with H for undoped and 19% for Co-doped: this change may be experimentally observable.…”

“…Incorporating the reaction mechanisms into a kinetic Monte Carlo (kMC) model allowed us to predict Turnover-Frequency (TOF), in good agreement with single crystal experiments . In order to discover alloying elements to improve rates sufficiently to allow less severe reaction conditions (temperature and pressure) we have developed a hierarchical high-throughput catalyst screening methods that identified multiple doping strategies for improving HB. − Thus, we found that doping Rh prefers the surface, whereas Si and Ni prefer the second layer, so that moderate levels of doping can achieve dramatic enhancements in catalytic rates on Fe-bcc(111). In addition, Co was found to prefer the subsurface for Fe-bcc(211)­R, leading also to improved rates .…”

“…In addition, Co was found to prefer the subsurface for Fe-bcc(211)R, leading also to improved rates. 14 In each case, the presence of the dopant was found to significantly decrease the important reaction barriers for the relevant surface reaction mechanism.…”

“…In order to discover alloying elements to improve rates sufficiently to allow less severe reaction conditions (temperature and pressure) we have developed a hierarchical high-throughput catalyst screening methods that identified multiple doping strategies for improving HB. − Thus, we found that doping Rh prefers the surface, whereas Si and Ni prefer the second layer, so that moderate levels of doping can achieve dramatic enhancements in catalytic rates on Fe-bcc(111). In addition, Co was found to prefer the subsurface for Fe-bcc(211)­R, leading also to improved rates . In each case, the presence of the dopant was found to significantly decrease the important reaction barriers for the relevant surface reaction mechanism.…”

Controlling the Shapes of Nanoparticles by Dopant-Induced Enhancement of Chemisorption and Catalytic Activity: Application to Fe-Based Ammonia Synthesis

“…Then, based on the derived reaction network, we developed a hierarchical high-throughput catalyst screening (HHTCS) approach to enable very rapid predictions to identify promising dopants that would dramatically improve reaction rates. ,− For Fe-bcc(111), screening 34 transition metal elements and 18 nontransition metal elements, we identified several candidates (Rh, Pt, Pd, Cu, Ni, Co, and Si) that could increase the overall reaction rates by factors of 4 to ∼40. Here we found that the best case is subsurface Si that would be 42 times faster at HB conditions, allowing a substantial reduction in the extreme HB industrial conditions (500 °C, 200 atm) to much milder conditions of either 500 °C at 20 atm or 400 °C at 60 atm while maintaining the same NH 3 production rates as HB on pure Fe-bcc(111) …”