Speaker
Description
Understanding what controls the reactivity of single-atom catalyst sites remains a central challenge in heterogeneous catalysis. In this contribution, I will address this question from the computational perspective using atomically defined Fe–N$_{3}$ and Fe–N$_{4}$ model sites embedded in on-surface prepared two-dimensional Fe-DCA and Fe-TCNQ metal–organic frameworks on graphene.
The Fe–N$_{3}$ and Fe–N$_{4}$ sites in Fe-DCA and in Fe-TCNQ are electronically very similar prior to adsorption: both exhibit a high-spin Fe$^{2+}$ configuration with S = 2, and their d-orbital occupancies and energetic positions relative to the Fermi level are nearly identical. Nevertheless, density functional theory predicts a difference in CO adsorption energy of more than 0.6 eV, in agreement with atomically resolved scanning tunneling microscopy experiments.
I will show that this reactivity difference originates from the different structural response of the two coordination environments upon adsorption. The more flexible Fe–N$_{3}$ site can relax out of the N$_{3}$ plane, enhancing Fe 3d$_{xz/yz}$–CO 2π* back-donation and stabilizing CO binding. These results demonstrate that coordination geometry controls reactivity not only through electronic structure, but also through structural adaptability, highlighting a limitation of purely electronic descriptors.