Frontier molecular orbital theory aids single-atom catalyst design

Single-atom catalysts (SACs), with their excellent metal atom utilization and unique physicochemical properties, hold promise for broad applications, especially in heterogeneous catalysis and energy conversions. Essentially, the activity and stability of SACs are governed by the pair of metal-adsorbate and metal-support interactions. However, the rationale of these interactions with their catalytic performance of SACs in nature and a unified theoretical model to describe both activity and stability remain elusive.

To address this challenge, a team led by Prof. Lu Junling from the University of Science and Technology of China (USTC), along with Prof. Wu Xiaojun from USTC and Associate Researcher Yang Bing from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), innovatively introduced the frontier molecular orbital (FMO) theory into SAC design. The study was published in Nature.

In this work, the team constructed 34 palladium (Pd) SACs on 14 semiconductor supports. By adjusting support size and composition, they were able to precisely tune the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of the supports. The energy positions of LUMO and HOMO with particle size were experimentally determined by ultraviolet-visible (UV-Vis) spectroscopy and Mott–Schottky plots.

Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) confirmed atomic dispersion of Pd on metal oxide particles (MO_x). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS) further demonstrated enhanced Pd-MO_x electronic interactions.

In the reaction of semi-hydrogenation of acetylene, Pd SACs supported on nanoscale ZnO and CoO_x exhibited a 20-fold activity enhancement over bulk oxide-supported counterparts while maintaining high selectivity.

Notably, Pd₁ SACs on 1.9 nm ZnO achieved a remarkable turnover frequency (TOF) of 25.6 min⁻¹ at 80 °C, surpassing all the Pd₁ SACs reported in the literature. These catalysts also demonstrated exceptional stability over 100 hours without coke formation or metal aggregation.

Correlation of the intrinsic activities with the properties of Pd₁ discloses that the activities of Pd₁/MO_x SACs didn’t show a clear correlation with the Pd charge states. In contrast, their activities showed a linear scaling relationship with the LUMO positions of the n- and p-type oxide particle supports in Pd₁/MO_x.

Theoretical calculations elucidated the underlying mechanism: reducing ZnO size elevates its LUMO level and widens the bandgap. The elevated LUMO of support reduces its energy gap with the HOMO of Pd₁ atoms, which promotes Pd₁–support orbital hybridizations for high stability.

Meanwhile, the variation of Pd₁–support orbital hybridizations further amends the LUMO of anchored Pd₁ atoms to enhance Pd₁–adsorbate interactions for high activity. These findings are consistent with the FMO theory.

This work presents the first direct experimental substantiation of FMO theory in full view and provides a general descriptor for the design of a highly active and stable SAC for the first time. These findings open a new and operational approach for high-throughput screening of proper metal-support pairs to achieve high activity and stability, particularly powered by artificial intelligence.