HIGHLY SELECTIVE PHOTOCATALYTIC CO2 REDUCTION by IMMOBILIZED Mn(Tpy)(CO)3 + CATALYST AND

HIGHLY SELECTIVE PHOTOCATALYTIC CO2 REDUCTION BY IMMOBILIZED [Mn(tpy)(CO)3]+ CATALYST AND [Ru(bpy)3]2+ PHOTOSENSITIZER ON TiO2 NANOPARTICLES

L. Le-Quanga,b, M. Stanburya , V. Maurelb, S. Chardona and J. Chauvina

a Laboratoire de Chimie Inorganique Rédox, Département de Chimie Moléculaire, Université Grenoble-Alpes, UMR CNRS 5250, BP 53, 38041 Grenoble, France

b Laboratoire de Résonance Magnétique, Institut Nanosciences et Cryogénie, CEA, 17 rue des Martyrs, 38054 Grenoble, France

The quest for sustainable energy sources and diminution of CO2 greenhouse gas has become a great challenge for human beings especially in recent years. Attempts to capture and convert CO2 to energy-rich compounds such as formic acid have been made by numerous researchers to tackle the issue. Recent advances in the field of photocatalytic CO2 reduction have been achieved via different approaches: novel molecular homogeneous catalysts, supramolecular chemistry incorporating a visible-light photosensitizer (PS) and a molecular catalyst (C), and hybrid photocatalyst using a semiconductor.1,2 Fixing both C and PS on nanoparticles (NPs) has shown promising efficiency and selectivity where the NPs can act as an inert platform3 or an electron relay4 between PS and C. The use of NPs may allow the components in close proximity, thus facilitating electron transfer processes.

Herein we report the co-immobilization of [Mn(ttpy)(CO)3]+ (ttpy = 4’-tolyl-2,2’:6’-2’’-terpyridine) and [Ru(bpy)3]2+ (bpy = 2,2’-bipyridine) as C and PS, respectively, onto anatase TiO2 NPs via phosphonic anchoring groups to form RuII/TiO2/MnI NPs. The ratio of Mn/Ru content was chosen at 1/10 as it is the optimized ratio of a mixture of the homogeneous complexes for photocatalytic CO2 reduction in DMF/TEOA (CTEOA = 1 M) in the presence of BNAH (0.1 M) as sacrificial electron donor. After 20 hours of irradiation at 450 nm, the homogeneous system produced a mixture of CO and HCOOH with relatively high turnover number (TON): TONCO = 19, TONHCOOH = 31. In the same conditions, RuII/TiO2/MnI NPs selectively produced HCOOH (390 µmol.g-1, TON = 26) with no traces of CO or H2.

Mechanistic study of RuII/TiO2 colloid reveals fast photo-induced electron injection from RuII* to TiO2 in nanosecond time scale while the back electron transfer occurs in millisecond one. Long-lived charge separated state RuIII/TiO2(e-) is also directly observed with electron paramagnetic resonance (EPR) spectroscopy. Adsorption of both species on TiO2 is evidenced by FT-IR. X-ray photoelectron spectroscopy of the hybrid system before and after photocatalysis is under investigation to confirm the Mn/Ru ratio and their stability after prolonged irradiation.

[1] Y. Yamazaki et al., J. Photochem. Photobiol. C: Photochem. Rev., 25, 106 (2015)

[2] G. Sahara and O. Ishitani, Inorg. Chem., 54, 5096 (2015)

[3] G. Neri et al., Chem. Commun., 52, 14200 (2016)

[4] D.-I. Won et al., J. Am. Chem. Soc., 137, 13679 (2015)