No abstract

The photocatalytic activity of phosphonated Re complexes, [Re(2,2′-bipyridine-4,4′-bisphosphonic acid) (CO)3(L)] (ReP; L=3-picoline or bromide) immobilised on TiO2 nanoparticles is reported. The heterogenised Re catalyst on the semiconductor, ReP–TiO2 hybrid, displays an improvement in CO2 reduction photocatalysis. A high turnover number (TON) of 48 molCO molRe−1 is observed in DMF with the electron donor triethanolamine at λ>420 nm. ReP–TiO2 compares favourably to previously reported homogeneous systems and is the highest TON reported to date for a CO2-reducing Re photocatalyst under visible light irradiation. Photocatalytic CO2 reduction is even observed with ReP–TiO2 at wavelengths of λ>495 nm. Infrared and X-ray photoelectron spectroscopies confirm that an intact ReP catalyst is present on the TiO2 surface before and during catalysis. Transient absorption spectroscopy suggests that the high activity upon heterogenisation is due to an increase in the lifetime of the immobilised anionic Re intermediate (t50 %>1 s for ReP–TiO2 compared with t50 %=60 ms for ReP in solution) and immobilisation might also reduce the formation of inactive Re dimers. This study demonstrates that the activity of a homogeneous photocatalyst can be improved through immobilisation on a metal oxide surface by favourably modifying its photochemical kinetics.

Electrocatalytic CO2 reduction to CO was achieved with a novel Mn complex, fac‐[MnBr(4,4′‐bis(phosphonic acid)‐2,2′‐bipyridine)(CO)3] (MnP), immobilized on a mesoporous TiO2 electrode. A benchmark turnover number of 112±17 was attained with these TiO2|MnP electrodes after 2 h electrolysis. Post‐catalysis IR spectroscopy demonstrated that the molecular structure of the MnP catalyst was retained. UV/vis spectroscopy confirmed that an active Mn–Mn dimer was formed during catalysis on the TiO2 electrode, showing the dynamic formation of a catalytically active dimer on an electrode surface. Finally, we combined the light‐protected TiO2|MnP cathode with a CdS‐sensitized photoanode to enable solar‐light‐driven CO2 reduction with the light‐sensitive MnP catalyst.

Attaching the phosphonated molecular catalyst [ReBr(bpy)(CO)] to the wide-bandgap semiconductor TiO strongly enhances the rate of visible-light-driven reduction of CO 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, 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 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 in photocatalytic CO reduction that has previously not been considered. We propose that the increase in photocatalytic activity upon heterogenization of the catalyst to TiO is due to the slow charge recombination and the high oxidative power of the Re species after electron injection as compared to the excited MLCT state of the unbound Re catalyst or when immobilized on ZrO, which results in a more efficient reaction with TEOA.