1 research outputs found
Time-Resolved IR Spectroscopy Reveals a Mechanism with TiO<sub>2</sub> as a Reversible Electron Acceptor in a TiO<sub>2</sub>–Re Catalyst System for CO<sub>2</sub> Photoreduction
Attaching
the phosphonated molecular catalyst [Re<sup>I</sup>BrÂ(bpy)Â(CO)<sub>3</sub>]<sup>0</sup> to the wide-bandgap semiconductor TiO<sub>2</sub> strongly enhances the rate of visible-light-driven reduction of
CO<sub>2</sub> to CO in dimethylformamide with triethanolamine (TEOA)
as sacrificial electron donor. Herein, we show by transient mid-IR
spectroscopy that the mechanism of catalyst photoreduction is initiated
by ultrafast electron injection into TiO<sub>2</sub>, followed by
rapid (ps-ns) and sequential two-electron oxidation of TEOA that is
coordinated to the Re center. The injected electrons can be stored
in the conduction band of TiO<sub>2</sub> on an ms-s time scale, and
we propose that they lead to further reduction of the Re catalyst
and completion of the catalytic cycle. Thus, the excited Re catalyst
gives away one electron and would eventually get three electrons back.
The function of an electron reservoir would represent a role for TiO<sub>2</sub> in photocatalytic CO<sub>2</sub> reduction that has previously
not been considered. We propose that the increase in photocatalytic
activity upon heterogenization of the catalyst to TiO<sub>2</sub> is
due to the slow charge recombination and the high oxidative power
of the Re<sup>II</sup> species after electron injection as compared
to the excited MLCT state of the unbound Re catalyst or when immobilized
on ZrO<sub>2</sub>, which results in a more efficient reaction with
TEOA