Non-linear temperature dependence of nitrogen adsorption and decomposition on Fe(111) surface

Abstract

The cleavage of the N2 triple bond on the Fe(111) surface is believed to be the rate limiting step of the famed Haber-Bosch ammonia catalysis. Using a combination of machine learning potentials and advanced simulation techniques, we study this important catalytic step as a function of temperature. We find that at low temperatures our results agree with the well-established picture. However, if we increase the temperature to reach operando conditions the surface undergoes a global dynamical change and the step structure of the Fe(111) surface is destroyed. The catalytic sites, traditionally associated with the Fe(111) surface appear and disappear continuously. Our simulations illuminate the danger of extrapolating low-temperature results to operando conditions and indicate that the catalytic activity can only be inferred from calculations that take dynamics fully into account. More than that, they show that it is the transition to this highly fluctuating interfacial environment that drives the catalytic process.

How poisoning is avoided in a step of relevance to the Haber-Bosch catalysis

“…As found in Ref. [1] for the pristine surface, we discover that the catalyst behaves rather differently in these two regimes. At low temperature, poisoning inevitably sets in.…”

“…To answer this question, we study the properties of the Fe(111) surface at different N * coverage both at room and operando temperature. Using state-of-the-art simulations, we have already found that, at high temperatures, the surface atoms are highly mobile and that the catalytic centers normally associated with the surface activity acquire a finite lifetime [1]. Here, we discover that the N * surface atoms are highly mobile and that coverage reduces but does not eliminate iron mobility.…”

“…The (111) surface is indeed the most active one, and the cleavage of the di-nitrogen triple bond has been argued to be its rate-limiting step [4,5,7,27]. In this first investigation, we have discovered a great difference in behavior between low and high temperatures, and have highlighted the dramatic role played by surface dynamics [1,40,41]. In particular, already at T ≈ 600 K, the Fe surface atoms become highly mobile while, at the same time, the long-range order is preserved.…”

“…Encouraged by recent progress in ab initio-grade simulations [31][32][33][34][35][36][37][38][39], we have undertaken a long-term project on the Haber-Bosch catalysis. In the first study [1], we simulated the dissociation of N 2 on a clean BCC Fe(111). The (111) surface is indeed the most active one, and the cleavage of the di-nitrogen triple bond has been argued to be its rate-limiting step [4,5,7,27].…”

“…Using the computational workflow developed for the clean surface [1], we construct an ab initio-quality machine learning potential optimized to reproduce a wide range of density functional theory (DFT) energy and force calculations [32,42,43]. For this procedure to be successful, it is crucial to combine advanced sampling methods [34][35][36][37] with active learning procedures to collect all relevant configurations for the entire reaction path under the different conditions studied [1,[44][45][46][47]. The synergistic combination of these state-of-the-art methods allows performing large-scale and long-timescale molecular dynamics (MD) simulations of the Fe(111) surface at different N * atom coverages at room and operando temperatures.…”