Single-atom Pd anchored in the porphyrin-center of ultrathin 2D-MOFs as the active center to enhance photocatalytic hydrogen-evolution and NO-removal

Single-atom catalysts were widely used to treat atmospheric pollution and alleviate energy crises through photocatalysis. However, how to prevent the aggregation of single atoms during the preparation and catalytic processes remained a great challenge. Herein, a novel ultrathin two-dimensional porphyrin-based single-atom photocatalyst Ti-MOF (abbreviated as TMPd) obtained through a simple hydrothermal synthesis strategy was used for photocatalytic hydrogen evolution and NO removal, in which the single-atom Pd tightly anchored in the center of porphyrin to ensure single-atom Pd stable existence. Compared with most reported MOFs-based photocatalysts, the TMPd showed an excellent hydrogen evolution rate (1.32 mmol g−1 h−1) and the NO removal efficiency (62%) under visible light irradiation. Aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and synchrotron-radiation-based X-ray absorption fine-structure spectroscopy (XAFS) proved that pd in TMPd existed in an isolated state, and the atomic force microscope (AFM) proved the ultrathin morphology of TMPd. DFT calculations had demonstrated that single-atom Pd could serve as the active center and more effectively achieve electron transfer, indicating that single-atom Pd played a vital role in photocatalytic hydrogen evolution. In addition, a possible photocatalytic pathway of NO removal was proposed based on ESR and in-situ infrared spectra, in which the catalysts anchored with single-atom Pd could produce more active substances and more effectively oxidize NO to NO2− or NO3−. The results suggested that coordinating single-atom metal species as the active site in the center of porphyrin could be a feasible strategy to obtain various ultrathin porphyrin-based single-atom photocatalysts to acquire excellent photocatalytic performance further.