Determining the Magnetic Status of Active Sites on Nanocatalysts

Identifying the atomic structure and chemical composition of active sites on nanocatalysts has been a long pursuit in heterogeneous catalysis. Yet, determining the magnetic structure of a well-defined active site is even more challenging. However, explicit morphology and reaction temperature have not been considered in identifying the magnetic behaviors of the nanocatalysts, especially in theoretical studies. Herein, we determined the magnetic status of nanoscale catalysts at finite temperatures by using atomistic spin models. The size dependence of the Curie point and the magnetic premelting have been discussed, indicating that the magnetic properties over a localized active center can greatly differ from the bulk. Therefore, the magnetic phase transitions and its concomitant magneto-catalytic effect can be induced at a considerably low temperature. Our analysis demonstrated that an 8 nm cobalt-based core-shell nanoparticle can achieve the optimal magnetization with Sabatier optimal activity for ammonia synthesis at 523 K, which is in accord with the reaction condition of the Haber-Bosch process. We believe our findings elucidate the importance of determining the localized magnetic configuration for active sites. Furthermore, including this unexcavated dimension in the dynamical simulations of the catalytic process can provide us with a more complete and comprehensive understanding of the reaction mechanism under working conditions.