The performance of heterogeneous catalysts is closely related to the local structure of their active sites. Conventional catalyst design mainly focuses on nanoscale structural features, such as crystal facets, edges and surface defects. The development of single-atom catalysts has further enabled active sites to be engineered at the atomic scale by controlling coordination number, heteroatom identity, symmetry, or oxidation/spin state. However, even within an apparently identical single-atom coordination structure, subangstrom-level structural variations remain rarely explored.
Among these structural parameters, subangstrom-level variations, particularly changes in metal-ligand bond length, have rarely been explored. Even subtle changes in bond distance can affect the electronic structure of the active center and thereby influence reaction pathways and kinetics. However, precise control of metal-ligand bond lengths at the subangstrom scale remains challenging. Recently, a research team reported a strategy for precisely regulating the Fe-N bond distance in structurally uniform Fe1-N4single-atom catalysts. By combining thermal treatment with the curved surface of a nanodiamond support, the Fe-N bond was progressively compressed from 2.03 to 2.02, 1.96 and 1.93 Å. The results were published in Chinese Journal of Catalysis ( 10.1016/S1872-2067(26)65096-6 ) under the title Subangstrom spatial regulation of Fe1-N4coordination structure for remarkably efficient C-H bond oxidation.
A key feature of this work is that the Fe1-N4active structure remains essentially unchanged during bond compression. Mössbauer spectroscopy and X-ray absorption measurements showed that the Fe centers retained a four-coordinate, low-spin Fe1-N4configuration throughout the thermal treatment. Therefore, the variation in catalytic behavior could be directly associated with the subtle change in Fe-N bond distance rather than changes in coordination number or the formation of different Fe species.
The origin of this structural compression was further examined using ab initio molecular dynamics and theoretical models with different carbon curvatures. Increasing temperature promoted local structural distortion of Fe1-N4sites, while the curved carbon surface imposed geometric confinement on the coordination structure. Higher surface curvature resulted in greater Fe-N bond compression, and the theoretically predicted compression ratios were consistent with those obtained experimentally. These results indicate that the combined effects of surface curvature and thermal treatment give rise to the experimentally observed Fe-N bond compression.
Although the structural change was only about 0.1 Å, it produced a pronounced change in the electronic structure of the Fe center. Theoretical calculations showed that Fe-N bond compression promoted electron accumulation in the Fe 3d orbitals. Consistently, X-ray absorption spectroscopy revealed that the average oxidation state of Fe gradually decreased from approximately +2.58 to +2.44 as the Fe-N bond became shorter.
The change in electronic structure was also reflected in catalytic performance. In the solvent-free oxidation of ethylbenzene using tert-butyl hydroperoxide, the catalytic activity continuously increased with increasing Fe-N bond compression. The most compressed Fe1-N4/ND-600 catalyst achieved a turnover frequency of 4163 h-1, compared with 180.8 h-1 for Fe1-N4/ND-RT, corresponding to a nearly 23-fold enhancement. Meanwhile, the selectivity toward acetophenone remained above 99%. The optimized catalyst also showed good activity toward several other C-H and C-O bond oxidation reactions.
Density functional theory calculations further showed that the compressed Fe1-N4structure strengthens the enhances peroxide group activationand facilitates electron transfer from Fe to oxygen. The higher electron density at the Fe center makes peroxide activation thermodynamically more favorable, explaining the substantially enhanced oxidation activity.
By correlating Fe-N bond compression, Fe oxidation state and catalytic turnover frequency, the study establishes a clear relationship between bond distance, electronic structure and catalytic activity. The results demonstrate that the catalytic performanceof a structurally uniform single-atom site can be finely tuned simply by modifying its metal-ligand bond length.
This work demonstrates subangstrom regulation in a structurally uniform single-atom catalyst and establishes a direct correlation between Fe-N bond distance, electronic structure and catalytic kinetics.It expands the paradigm of "precise chemistry" from the nano- and atomic scales to the subangstrom scale.
About the Journal
Chinese Journal of Catalysis is co-sponsored by Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Chinese Chemical Society, and it is currently published by Elsevier group. This monthly journal publishes in English timely contributions of original and rigorously reviewed manuscripts covering all areas of catalysis. The journal publishes Reviews, Accounts, Communications, Articles, Highlights, Perspectives, and Viewpoints of highly scientific values that help understanding and defining of new concepts in both fundamental issues and practical applications of catalysis. Chinese Journal of Catalysis ranks among the top one journals in Applied Chemistry with a current SCI impact factor of 17.2. The Editors-in-Chief are Profs. Can Li and Tao Zhang.
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