Photocatalytic hydrogen evolution enabled by integrating stable C^N cyclometalated [Pt(C^N)(O^O)] motifs into covalent organic frameworks

Abstract

Metal-covalent organic frameworks (MCOFs) are ideal candidates for constructing single-atom photocatalysts due to their high crystallinity, inherent porosity, and uniform distribution of active metal centers. They have gained significant attention as effective photocatalysts for photocatalytic water splitting to produce hydrogen (H₂). However, developing single-atom-based MCOF photocatalysts with well-defined structures and long-term durability for cocatalyst-free photocatalytic hydrogen evolution (PHE) remains a significant challenge because of the structural limitations of metalated building blocks and inefficient photoinduced charge separation. Herein, by directly employing a novel organometallic Pt(II) complex [Pt(fpnd)(acac)] as a building block, two MCOFs (Pt-TAPT-COF and Pt-TPB-COF) with well-defined structures and high crystallinity were successfully synthesized. Notably, aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption spectroscopy demonstrated that these MCOFs feature atomically dispersed Pt even at an exceptionally high loading of >10 wt%. Among the two MCOFs, Pt-TAPT-COF produced a higher cocatalyst-free PHE rate (η_H2) of 670 µmol g⁻¹ h⁻¹ compared to Pt-TPB-COF (18 µmol g⁻¹ h⁻¹). The enhanced PHE performance can be attributed to the introduction of nitrogen-rich triazine cores, which facilitate an accelerated electron transfer rate from photoexcited Pt-TAPT-COF to protons and subsequently their reduction to H₂. Remarkably, Pt-TAPT-COF also exhibited excellent long-term photostability for PHE, with its H₂ evolution activity continuously increasing over 40 h of light irradiation. Furthermore, in situ X-ray photoelectron spectroscopy and density functional theory calculations reveal that the introduced Pt–C motifs serve as the most favorable sites for H₂ adsorption and desorption, confirming the cocatalyst-free PHE process. This work provides strategic insights into developing stable single-atom-based MCOF photocatalysts for efficient PHE through the structural engineering of building blocks.