Photocatalytic hydrogen evolution is a key technology for clean energy conversion, in which platinum (Pt) is widely used as an effective cocatalyst. The anchoring and dispersion of Pt play a decisive role in catalytic performance. However, achieving precise control over metal-support interactions at the atomic level remains challenging due to the chemical heterogeneity of catalyst surfaces.

In a study published in Angewandte Chemie International Edition, Prof. ZHOU Xukai and his colleagues from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences introduced a conformational isomer strategy to precisely tune the nitrogen atom positions, enhancing the photocatalytic hydrogen evolution performance of covalent organic frameworks (COFs).

COFs is ideal for studying metal-support interactions at the atomic level due to their programmable topology and well-defined pore environments. Combining trisubstituted aldehydes with trisubstituted aromatic amines or aromatic methyl compounds, researchers designed four COFs: two olefin-linked COFs (COF-A1/A2) and two imine-linked COFs (COF-I1/I2).

These COFs shared the same hexagonal pore topology, and the positions of nitrogen anchoring sites could be precisely tuned at the angstrom-level. After in situ photodeposition of Pt, the spatial arrangement of nitrogen atoms was found to play a decisive role in determining the dispersion and coordination environment of Pt species.

Multiple characterizations revealed that the imine-linked COF-I2 could simultaneously stabilize both Pt²⁺ single atoms and metallic Pt clusters, forming dual active sites, while the olefin-linked COF-A2 primarily anchored Pt single atoms. This structural difference directly led to a great disparity in photocatalytic performance: the hydrogen evolution rate of COF-I2-Pt was 6.1 times higher than that of COF-A2-Pt. Under monochromatic light irradiation at 420 nm, COF-I2-Pt achieved an apparent quantum efficiency of 12.1%.

Mechanistic investigations showed that the superior performance of COF-I2-Pt was because of the synergistic effect between Pt clusters and single atoms. The charge redistribution between them effectively promoted the separation of photogenerated electron-hole pairs and optimized the kinetics of proton adsorption and reduction. Femtosecond transient absorption spectroscopy confirmed that a prolonged lifetime of the key charge-separated state in COF-I2-Pt was the primary reason for efficient hydrogen evolution.

"Our study provides a new way for the rational design of atomically precise photocatalysts," said Prof. ZHOU. "And the concept of 'nitrogen-shift engineering' can be extended to the design of other porous framework materials, providing guidance for developing efficient energy conversion materials."