Structural flexibility dictates reactivity of single-atom catalysts

Abstract

Unravelling the origins of single-atom catalyst reactivity is a central challenge in heterogeneous catalysis research. A key question is whether the activity arises solely from atomic isolation or from distinct structural and electronic configurations of the single atoms. Here, we use precisely defined Fe-N${}_3$ and Fe-N${}_4$ model catalyst sites synthesized on an inert support to quantify the effect of coordination geometry on chemical reactivity. Both the Fe-N${}_3$ and Fe-N${}_4$ models have the same electronic configuration (high-spin Fe${}^{2+}$ with S=2), and even their d-orbital occupancies and positions with respect to Fermi level are almost identical. Despite this electronic similarity, the adsorption energy of CO differs by more than 0.6 eV between the Fe-N${}_3$ and Fe-N${}_4$ sites, as indicated by density functional theory computations and confirmed by atomically-resolved scanning tunneling microscopy experiments. We trace this reactivity difference to the structural flexibility of the Fe-N${}_3$ sites, which allows strengthening of the Fe 3d${}_{xz/yz}$-CO 2$\ddot{I}€$* back-bonding by lifting the Fe atom from the -N${}_3$ plane. These results demonstrate that coordination geometry plays a crucial role in defining the reactivity of single-atom catalysts, and that such effects cannot be predicted by analysis of the sites' electronic structures alone.