Improving the Photocatalytic Reduction of CO 2 to CO through Immobilisation of a Molecular Re Catalyst on TiO 2

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 TiO 2 nanoparticles is reported. The heterogenised Re catalyst on the semiconductor, ReP–TiO 2 hybrid, displays an improvement in CO 2 reduction photocatalysis. A high turnover number (TON) of 48 mol CO  mol Re −1 is observed in DMF with the electron donor triethanolamine at λ >420 nm. ReP–TiO 2 compares favourably to previously reported homogeneous systems and is the highest TON reported to date for a CO 2 ‐reducing Re photocatalyst under visible light irradiation. Photocatalytic CO 2 reduction is even observed with ReP–TiO 2 at wavelengths of λ >495 nm. Infrared and X‐ray photoelectron spectroscopies confirm that an intact ReP catalyst is present on the TiO 2 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–TiO 2 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.