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

TL;DR

This paper shows how adding catalytic centers and ascorbate improves the efficiency of a 2D material for producing hydrogen through photocatalysis.

Contribution

The study introduces a noble metal-free cobalt catalyst as a viable alternative to platinum for photocatalytic hydrogen evolution.

Findings

PtNPs achieve high hydrogen evolution rates of 106,000 μmol H2 g–1 h–1 with 5% Pt.

Ascorbic acid enhances TpDTz photoluminescence and charge extraction efficiency.

A cobalt-based catalyst achieves 10,400 μmol H2 g–1 h–1, three times slower than the Pt system but without noble metals.

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.

Figures