Abstract
Catalytic reactions taking place in industrial processes are often performed under extreme conditions of temperatures and pressures. A typical example is the Haber–Bosch process to industrially synthesize ammonia from nitrogen and hydrogen which operates under reaction pressures from 10 to 15 MPa and temperatures higher than 400 °C (1). Following the dynamics of heterogeneous catalysts under such extreme conditions is highly challenging both from experimental and theoretical point of view. In their paper, Bonati et al. give unique molecular insights into the dynamics, adsorption, and bond breakage of the N2 molecule when interacting with the (111) iron surface at high temperatures relevant for the Haber–Bosch catalytic system (2). The simulations reveal that the surface is much more dynamic than anticipated from low-temperature experiments or simulations. Active sites are continuously formed and disrupted, and this behavior is instrumental for driving the catalytic process. To follow the weakening of the nitrogen–nitrogen bond, the degree of charge transfer from the metallic surface to the triple bond was followed during various steps of the catalytic process.
The study of Bonati et al. is an important proof-of-concept study, showcasing that reaction mechanisms may be highly dependent on the reaction conditions and that an evaluation of the dynamics of the system at industrially relevant conditions is of utmost importance to obtain molecular insights. Such insights can not be obtained from low-temperature investigations. It is notoriously difficult to simulate the dynamics of industrially relevant catalytic reactions under operating conditions. Hence, Bonati et al. had to combine various innovations in the field of machine learning and enhanced sampling molecular dynamics to follow the adsorption and reaction on the fly at operating conditions during sufficiently long time scales. A summary of some essential ingredients of their workflow is schematically shown in Fig. 1 and discussed further below. The impact of the methodological advances presented in their study is of much greater importance than the specific case study discussed in the PNAS paper and opens perspectives to follow industrially relevant catalytic reactions on the fly at the conditions where the catalyst does the work.