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

Abstract

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 H2 g–1 h–1 (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 H2 g–1 h–1 (1% Co w/w), which is only three times slower than the noble metal-based PtNPs system (1% Pt w/w, 28 300 μmol H2 g–1 h–1). 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.