title: Time-Resolved IR Spectroscopy Reveals a Mechanism with TiO2as a Reversible Electron Acceptor in a TiO2-Re Catalyst System for CO2Photoreduction.
abstract: Attaching the phosphonated molecular catalyst [ReIBr(bpy)(CO)3]0to the wide-bandgap semiconductor TiO2strongly enhances the rate of visible-light-driven reduction of CO2to CO in dimethylformamide with triethanolamine (TEOA) as sacrificial electron donor. Herein, we show by transient mid-IR spectroscopy that the mechanism of catalyst photoreduction is initiated by ultrafast electron injection into TiO2, followed by rapid (ps-ns) and sequential two-electron oxidation of TEOA that is coordinated to the Re center. The injected electrons can be stored in the conduction band of TiO2on an ms-s time scale, and we propose that they lead to further reduction of the Re catalyst and completion of the catalytic cycle. Thus, the excited Re catalyst gives away one electron and would eventually get three electrons back. The function of an electron reservoir would represent a role for TiO2in photocatalytic CO2reduction that has previously not been considered. We propose that the increase in photocatalytic activity upon heterogenization of the catalyst to TiO2is due to the slow charge recombination and the high oxidative power of the ReIIspecies after electron injection as compared to the excited MLCT state of the unbound Re catalyst or when immobilized on ZrO2, which results in a more efficient reaction with TEOA.
authors.affiliation: Ångström Laboratory, Department of Chemistry, Uppsala University , Box 523, 75120 Uppsala, Sweden.|Department of Chemistry, Qena Faculty of Science, South Valley University , 83523 Qena, Egypt.
authors.forename: Mohamed
authors.initials: M
authors.lastname: Abdellah
authors.affiliation: Ångström Laboratory, Department of Chemistry, Uppsala University , Box 523, 75120 Uppsala, Sweden.
authors.forename: Ahmed M
authors.initials: AM
authors.lastname: El-Zohry
authors.affiliation: Ångström Laboratory, Department of Chemistry, Uppsala University , Box 523, 75120 Uppsala, Sweden.
authors.forename: Liisa J
authors.initials: LJ
authors.lastname: Antila
authors.affiliation: Christian Doppler Laboratory for Sustainable SynGas Chemistry, Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Windle
authors.affiliation: Christian Doppler Laboratory for Sustainable SynGas Chemistry, Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Erwin
authors.identifier: 0000-0002-7781-1616
authors.initials: E
authors.lastname: Reisner
authors.affiliation: Ångström Laboratory, Department of Chemistry, Uppsala University , Box 523, 75120 Uppsala, Sweden.
authors.forename: Leif
authors.identifier: 0000-0002-9933-9084
authors.initials: L
authors.lastname: Hammarström