The Power of Catalytic Centers and Ascorbate in Boosting the Photocatalytic Hydrogen Evolution Performance of TpDTz 2D-COF

The photocatalytic hydrogen evolution activity of a model 2D covalent organic framework (TpDTz) containing a thiazolo[5,4-d]thiazole (DTz) electron acceptor and triformylphloroglucinol (Tp) electron donor groups is enhanced by combining it with well-defined catalytic centers and suitable sacrificial electron donors. Platinum nanoparticles (PtNPs) with an average diameter of 2.7 ± 0.4 nm achieve rates up to 106 000 μmol H g h (5% Pt w/w). The best system requires the use of ascorbic acid/ascorbate buffer, which has been demonstrated to enhance the photoluminescence of TpDTz by forming aggregates while efficiently extracting charges from the excited TpDTz (TpDTz*). The productive charge extraction by the PtNPs from TpDTz* is also supported by steady state and time-resolved photoluminescence studies. All these factors combined with the high catalytic activity of PtNPs catalytic centers lead to the high performance of the overall system. In addition, a noble metal-free molecular catalyst based on a tetraazamacrocyclic cobalt complex has been identified as a good alternative catalyst candidate, efficiently quenching TpDTz photoluminescence. Under optimal conditions, the cobalt-based system achieves catalytic rates of 10 400 μmol H g h (1% Co w/w) which is only three times slower than the noble metal-based PtNPs system (1% Pt w/w, 28 300 μmol H g h). By using controlled catalytic centers, it was possible to identify the factors limiting the hydrogen evolution photocatalytic activity of TpDTz allowing one to minimize undesired pathways and enhancing its performance by 2 orders of magnitude.