Improving the photocatalytic reduction of CO2 to CO through immobilisation of a molecular Re catalyst on TiO2

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.

Keywords: CO2 reduction; heterogeneous catalysis; immobilisation; photocatalysis; time-resolved spectroscopy.

Figure 1

A) Structure of ReP catalysts. ^EtReP^Br: R=Et, L=Br, n=0; ^EtReP^pic: R=Et, L=3-picoline, n=1 (PF₆⁻ counter-ion not shown); ReP^Br: R=H, L=Br, n=0; ReP^pic: R=H (isolated as neutral, mono-deprotonated species), L=3-picoline, n=0; B) ORTEP view of the molecular structure of ^EtReP^Br determined by X-ray diffraction with thermal ellipsoids set at 50 % probability. H atoms and atom-labelling for C atoms omitted for clarity.

Figure 2

A) ATR-IR spectrum of ReP^pic–TiO₂ before and after 24 h of λ>420 nm irradiation in DMF/TEOA 5:1 under CO₂. Spectrum after 24 h normalised to the maximum of the 0 h spectrum.; B) ReP^Br–TiO₂ XPS signals for Re before (circles, 1.08 %) and after (squares, 0.82 %) and for N before (circles, 2.74 %) and after (squares, 4.19 %) catalysis (2 h, λ>420 nm in 5:1 DMF/TEOA, under CO₂).

Figure 3

Photocatalytic CO production from CO₂ under λ>420 nm irradiation with the ReP–TiO₂ hybrid system compared to homogeneous [ReCl(bpy)(CO)₃] with and without TiO₂ and ReP^Br in solution (ReP^pic is insoluble in DMF). 0.1 μmol Re catalyst in 4.5 mL DMF/TEOA 5:1 was used with 5 mg TiO₂. TON_CO=mol_CO mol_Re⁻¹.

Figure 4

Transient absorption decays of the reduced intermediate ReP⁻ probed at 500 nm after photoexcitation of the catalyst with 415 nm light (ca. 300 μJ cm⁻², 0.5 Hz repetition rate) in the presence of a sacrificial electron donor TEOA (1 m). A) ReP^pic in solution and anchored onto TiO₂ under N₂; B) ReP^pic in solution and anchored to TiO₂ under CO₂ with an inset showing a second normalisation of the kinetics of the slow phase for both systems.

Scheme 1

Proposed catalytic mechanism of a Re-based photocatalyst (ReP) immobilised on TiO₂ for CO₂ reduction under visible light illumination (solid lines). The deactivation pathways are represented with dotted lines. The second reduction step (dashed grey lines) was not observed by transient absorption spectroscopy measurements.