How Poisoning Is Avoided in a Step of Relevance to the Haber–Bosch Catalysis

For a catalyst to be efficient and durable, it is crucial that the reaction products do not poison the catalyst. In the case of the Haber-Bosch process, the rate-limiting step is believed to be the decomposition of nitrogen molecules on the Fe(111) surface. This step leads to the production on the surface of atomic nitrogen (N*), which, unless hydrogenated and eventually released as ammonia, remains adsorbed and occupies the active sites. Thus, it is important to ascertain how a high N* coverage affects the nitrogen dissociative chemisorption. To answer this question, we study the properties of the Fe(111) surface at different N* coverage both at room and operando temperature. In the latter regime, we have already found that Fe surface atoms exhibit a high mobility, promoting the formation of adatoms and vacancies, and causing the catalytic centers to acquire a finite lifetime [Bonati et al. Proceedings of the National Academy of Sciences 2023, 120 (50), e2313023120 ]. We discover that the N* coverage reduces but does not eliminate the iron mobility. Remarkably, the N* atoms stabilize triangular surface structures associated with the formation of vacancies, which are a sign of a frustrated drive toward a more stable Fe4N phase. As a consequence, nitrogen atoms tend to cluster, reducing their poisoning effect. At the same time, the reduction in the number of catalytic centers is counteracted by an increase in their lifetime. The combined effect is that the dissociation barrier is not significantly altered in the range of coverages studied. These results bring to light the complex role that dynamics plays in catalytic reactivity under operando conditions.