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 remains elusive.
To address this challenge, a collaborative team led by Prof. ZHANG Tao and Associate Prof. YANG Bing from the Dalian Institute of Chemical Physics (DICP) of Chinese Academy of Sciences (CAS), Prof. LU Junling and Prof. WU Xiaojun from the University of Science and Technology of China (USTC) innovatively introduced the Frontier Molecular Orbital (FMO) theory into SAC design. The study was published in Nature on April 2.
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 tuning 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 (MOx). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS) further demonstrated enhanced Pd-MOx electronic interactions.
In the reaction of semi-hydrogenation of acetylene, Pd SACs supported on nanoscale ZnO and CoOx 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/MOx 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/MOx.
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 amend the LUMO of anchored Pd1 atoms to enhance Pd1–adsorbate interactions for high activity. These findings consist 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.