Improving the photocatalytic reduction of CO₂to CO through immobilisation of a molecular Re catalyst on TiO₂

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 mol_COmol_Re^-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 (t_50%>1 s for ReP-TiO₂ compared with t_50%=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.