USTC proposes frontier molecular orbital theory for single-atom catalyst design

Single-atom catalysts (SACs) comprise individual metal atoms—such as palladium (Pd), or platinum (Pt)—anchored on a high-surface-area solid support, which both isolates the metal and influences its behavior. SACs hold promise for a wide range of applications, especially in heterogeneous catalysis, where the catalysts and reactants are in different phases, as well as in energy conversion processes.

The performance of SACs is primarily measured by two metrics: activity—how effectively the SAC speeds up a chemical reaction—and stability—how long it remains functional. These properties are governed by two critical interactions: metal–adsorbate (i.e., reactant) interactions, which affect activity, and metal–support (i.e., semiconductor oxide supports) interactions, which influence stability. However, the precise ways in which these interactions shape the catalytic performance of SACs have not been well understood, and a unified theoretical framework that explains both activity and stability has remained an open question.

In a study published in Nature on April 2, a research team led by Prof. LU Junling from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), along with Prof. WU Xiaojun from USTC and YANG Bing from the Dalian Institute of Chemical Physics of CAS, innovatively introduced the Frontier Molecular Orbital (FMO) theory into the design of heterogeneous SACs. They achieved the first successful application of the FMO theory in heterogeneous catalysis and obtained a highly active and stable SAC for hydrogenation reactions.

The researchers constructed 34 Pd₁ SACs on 14 semiconductor oxide supports. By adjusting the size and composition of the supports, they were able to precisely tune the energy levels of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO). The relationship between particle size and the energy positions of LUMO and HOMO were experimentally determined using ultraviolet‒visible (UV‒Vis) spectroscopy and Mott–Schottky plots.

Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) confirmed the 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 as the oxide particle size decreased.

In the semi-hydrogenation of acetylene, the researchers found that Pd₁ SACs supported on nanoscale ZnO and CoO_x exhibited a 20-fold increase in activity compared to their bulk-oxide-supported counterparts, while maintaining high selectivity. Notably, the Pd₁ SAC on 1.9 nm ZnO achieved a remarkable turnover frequency (TOF) of 25.6 min^-1 at 80 °C, surpassing all previously reported Pd₁ SACs. This catalyst also demonstrated exceptional stability over 100 hours, with no visible coke formation or metal aggregation.

Furthermore, correlating the intrinsic activities with the properties of Pd₁ in these SACs revealed that the activities of Pd₁/MO_x catalysts did not show a clear relationship 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 supports in Pd₁/MO_x systems.

Through theoretical calculations, the researchers elucidated the underlying mechanism involving both metal–support and metal–adsorbate frontier-orbital interactions. They showed that reducing the size of ZnO raises its LUMO level and widens its bandgap. The elevated LUMO of the support decreases the energy gap with the HOMO of Pd₁ atoms, promoting Pd₁–support orbital hybridization and contributing to enhanced stability. Meanwhile, the variation in Pd₁–support orbital hybridization further adjusts the LUMO level of the anchored Pd₁ atoms, strengthening Pd₁–adsorbate interactions and thereby improving catalytic activity. These findings are consistent with the FMO theory, validating its application in heterogeneous catalysis.

This work represents the first direct experimental demonstration of the FMO theory in heterogeneous catalytic systems and provides a general descriptor for designing SACs with both high activity and stability. It also offers a promising strategy for high-throughput screening of proper metal-support combinations, particularly powered by artificial intelligence.