Structural flexibility dictates reactivity of single-atom catalysts

TL;DR

This study shows that the structural flexibility of single-atom catalyst sites, specifically Fe-N$_3$ versus Fe-N$_4$, significantly influences their reactivity, beyond what electronic structure analysis alone can predict.

Contribution

It provides experimental and computational evidence that coordination geometry and structural flexibility are key determinants of single-atom catalyst reactivity, challenging the focus on electronic configuration alone.

Findings

Fe-N$_3$ and Fe-N$_4$ have similar electronic configurations.

Reactivity difference of over 0.6 eV in CO adsorption energy.

Structural flexibility enhances back-bonding and reactivity.

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${\pi}$* 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.