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.