Christopher D. Windle

Electrocatalytic and Solar‐Driven CO2 Reduction to CO with a Molecular Manganese Catalyst Immobilized on Mesoporous TiO2

Abstract Electrocatalytic CO2 reduction to CO was achieved with a novel Mn complex, fac‐[MnBr(4,4′‐bis(phosphonic acid)‐2,2′‐bipyridine)(CO)3] (MnP), immobilized on a mesoporous TiO2 electrode. A benchmark turnover number of 112±17 was attained with these TiO2|MnP electrodes after 2 h electrolysis. Post‐catalysis IR spectroscopy demonstrated that the molecular structure of the MnP catalyst was retained. UV/vis spectroscopy confirmed that an active Mn–Mn dimer was formed during catalysis on the TiO2 electrode, showing the dynamic formation of a catalytically active dimer on an electrode surface. Finally, we combined the light‐protected TiO2|MnP cathode with a CdS‐sensitized photoanode to enable solar‐light‐driven CO2 reduction with the light‐sensitive MnP catalyst.

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

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.

Time-Resolved IR Spectroscopy Reveals a Mechanism with TiO2 as a Reversible Electron Acceptor in a TiO2–Re Catalyst System for CO2 Photoreduction

Attaching the phosphonated molecular catalyst [ReIBr(bpy)(CO)3]0 to the wide-bandgap semiconductor TiO2 strongly enhances the rate of visible-light-driven reduction of CO2 to 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 TiO2 on 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 TiO2 in photocatalytic CO2 reduction that has previously not been considered. We propose that the increase in photocatalytic activity upon heterogenization of the catalyst to TiO2 is due to the slow charge recombination and the high oxidative power of the ReII species 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.

Heterogenised Molecular Catalysts for the Reduction of CO2 to Fuels.

CO2 conversion provides a possible solution to curtail the growing CO2 levels in our atmosphere and reduce dependence on fossil fuels. To this end, it is essential to develop efficient catalysts for the reduction of CO2. The structure and activity of molecular CO2 reduction catalysts can be tuned and they offer good selectivity with reasonable stability. Heterogenisation of these molecules reduces solvent restrictions, facilitates recyclability and can dramatically improve activity by preventing catalyst inactivation and perturbing the kinetics of intermediates. The nature and morphology of the solid-state material upon which the catalyst is immobilised can significantly influence the activity of the hybrid assembly. Although work in this area began forty years ago, it has only drawn substantial attention in recent years. This review article gives an overview of the historical development of the field.CO(2) conversion provides a possible solution to curtail the growing CO(2) levels in our atmosphere and reduce dependence on fossil fuels. To this end, it is essential to develop efficient catalysts for the reduction of CO(2). The structure and activity of molecular CO(2) reduction catalysts can be tuned and they offer good selectivity with reasonable stability. Heterogenisation of these molecules reduces solvent restrictions, facilitates recyclability and can dramatically improve activity by preventing catalyst inactivation and perturbing the kinetics of intermediates. The nature and morphology of the solid-state material upon which the catalyst is immobilised can significantly influence the activity of the hybrid assembly. Although work in this area began forty years ago, it has only drawn substantial attention in recent years. This review article gives an overview of the historical development of the field.

A noble metal-free photocatalytic system based on a novel cobalt tetrapyridyl catalyst for hydrogen production in fully aqueous medium

The new cobalt tetrapyridyl complex 1(BF4)2 was characterized and assessed for hydrogen production in fully aqueous solution. Mechanistic information was gained thanks to a fast screening method, using chemical reductants and a Clark microelectrode. Optimal hydrogen production (443 TONs – 3 mL of H2) was achieved under visible light-driven conditions, in the presence of a noble metal-free photosensitizer, the water soluble porphyrin 2Cl4, and the ascorbate/tris-(2-carboxyethyl)phosphine (TCEP) sacrificial electron donor system.

Molecular catalysts for artificial photosynthesis: general discussion

Mei Wang, Vincent Artero, Leif Hammarström, Jose Martinez, Joshua Karlsson, Devens Gust, Peter Summers, Charles Machan, Peter Brueggeller, Christopher D. Windle, Yosuke Kageshima, Richard Cogdell, Kristine Rodulfo Tolod, Alexander Kibler, Dogukan Hazar Apaydin, Etsuko Fujita, Johannes Ehrmaier, Seigo Shima, Elizabeth Gibson, Ferdi Karadas, Anthony Harriman, Haruo Inoue, Akihiko Kudo, Tomoaki Takayama, Michael Wasielewski, Flavia Cassiola, Masayuki Yagi, Hitoshi Ishida, Federico Franco, Sang Ook Kang, Daniel Nocera, Can Li, Fabio Di Fonzo, Hyunwoong Park, Licheng Sun, Tohru Setoyama, Young Soo Kang, Osamu Ishitani, Jian-Ren Shen, Ho-Jin Son and Shigeyuki Masaoka

Time-Resolved IR Spectroscopy Reveals a Mechanism with TiO2 as a Reversible Electron Acceptor in a TiO2 - Re Catalyst CO2 Photoreduction System

Knut and Alice Wallenberg Foundation, Swedish Energy Agency, Swedish Research Council, Austrian Christian Doppler Research Association, OMV Group

Inorganic assembly catalysts for artificial photosynthesis: general discussion

Hiromu Kumagai, Leif Hammarström, Dong Ryeol Whang, Yuki Shinohara, Jose Martinez, Joshua Karlsson, Peter Summers, Christopher D. Windle, Masanori Kodera, Richard Cogdell, Kristine Rodulfo Tolod, Dogukan Hazar Apaydin, Etsuko Fujita, Alexander Kibler, Fengtao Fan, Elizabeth A. Gibson, Hisanao Usami, Akihide Iwase, Haruo Inoue, Akihiko Kudo, Devens Gust, Kazunari Domen, Flavia Cassiola, Katsuhiko Takagi, Sang Ook Kang, Akira Yamakata, Can Li, Licheng Sun, Hyunwoong Park, Young Soo Kang, Rengui Li, Fabio Di Fonzo, Tohru Setoyama and Osamu Ishitani