Highly Efficient and Robust Photocatalytic Systems for CO₂ Reduction Consisting of a Cu(I) Photosensitizer and Mn(I) Catalysts

Abstract

The development of highly efficient, selective, and durable photocatalytic CO2 reduction systems that only use earth-abundant elements is key for both solving global warming and tackling the shortage of energy and carbon resources. Here, we successfully developed CO2 reduction photocatalysts using [Cu2(P2bph)2]2+ (CuPS) (P2bph = 4,7-diphenyl-2,9-di­(diphenylphosphinotetramethylene)-1,10-phenanthroline) as a redox photosensitizer and fac-Mn­(X2bpy)­(CO)3Br (Mn­(4X)) (X2bpy = 4,4′-X2-2,2′-bipyridine (X = −H and −OMe) or Mn­(6mes) (6mes = 6,6′-(mesityl)2-2,2′-bipyridne)) as the catalyst. The most efficient photocatalysis was achieved by Mn­(4OMe): The total quantum yield of CO2 reduction products was 57%, the turnover number based on the Mn catalyst was over 1300, and the selectivity of CO2 reduction was 95%. Electronic and steric effects of the substituents (X) in the Mn complexes largely affected both the photocatalytic efficiency and the product selectivity. For example, the highest selectivity of CO formation was achieved by using Mn­(6mes) (selectivity S CO = 96.6%), whereas the photocatalytic system using Mn­(4H) yielded HCOOH as the main product (S HCOOH = 74.6%) with CO and H2 as minor products (S CO = 23.7%, S H2 = 1.7%). In these photocatalytic reactions, CuPS played its role as an efficient and very durable redox photosensitizer, while remaining stable in the reaction solution even after a turnover number of 200 had been reached (the catalyst used had a turnover number of over 1000).

Development of Visible-Light Driven Cu(I) Complex Photosensitizers for Photocatalytic CO2 Reduction

“…The excited states of these Cu I complexes were effectively quenched in the presence of BIH, except for that of Cu(NO 2 ph) , which means that all of these complexes might generate the corresponding one-electron reduced state through the reductive quenching of their excited states by BIH during photoirradiation (Tamaki et al, 2013; Takeda et al, 2018). The quenching rate constants ( k q ) were estimated to be ~10 10 M −1 s −1 , corresponding to quantitative quenching (η q ) under photocatalytic conditions with 10 mM of BIH (Table 3).…”

“…Thus, the resulting Cu I dimer complexes were much more stable, not only against thermal ligand exchanges in the ground state (Kaeser et al, 2013; Lennox et al, 2016) but also against “exciplex” deactivation of the excited state, even when CH 3 CN was used as a coordinating solvent (McMillin et al, 1985) and a Cu 0 metal particle formed via ligand dissociation in the one-electron reduced state (Eggleston et al, 1997). Thus, utilizing this Cu I complex as a redox photosensitizer in the photocatalytic CO 2 reduction, we could obtain the best photocatalytic performances for CO 2 reduction and clarified the photosensitizing scheme, which forms the corresponding one-electron reduced species through reductive quenching by a reductant such as BIH (1,3-dimethyl-2-phenyl-2,3-dihydro-1 H -benzo[d]imidazole) in the excited state, donating the added electrons to CO 2 reduction catalyst (Takeda et al, 2018).…”

Direct and indirect Z-scheme heterostructure-coupled photosystem enabling cooperation of CO2 reduction and H2O oxidation

“…Even using hole scavenger, the photocatalysts only exhibit a formation rate of HCOOH with tens of micromoles, typically such as some metal-organic framework (MOF) materials (NH 2 -MIL-125(Ti), MIL-101(Fe)) 17,18 , inorganic-organic hybrid materials (a binuclear ruthenium(II) complex coupled with Ag/C 3 N 4 (ref. 19,20 ), Cu(I) complex photosensitized Mn(I) complex catalysts 21 ), and metal sulfide semiconductors ((Mo−Bi)S x /CdS) 22 . The C and Fe co-doped LaCoO 3 was reported to display an HCOOH yield up to 128 μmol g −1 h −1 without sacrificial reagent, but the oxidation product O 2 was not analysed 23 .…”

Charge-Transfer Regulated Visible Light Driven Photocatalytic H2 Production and CO2 Reduction in Tetrathiafulvalene Based Coordination Polymer Gel

“…20 Only a handful of metal [21][22][23] , metal oxide [24][25][26] , and chalcogenides 27,28 based heterogeneous catalysts reported for photocatalytic CO 2 reduction to CH 4 , but most of them suffer from a low conversion e ciency and poor selectivity. 29 CH 4 formation is thermodynamically favourable (E 0 = -0.24 V versus RHE at pH=7) than CO formation (E 0 = -0.53 V versus RHE at pH=7) 30,31 as the former reaction takes place at a lower potential. Nevertheless, from a kinetic point of view, the eight-electron reduction of CO 2 to CH 4 is more di cult especially under photochemical condition than the two-electron reduction of CO 2 to CO. 32 To address challenges associated with photochemical H 2 production and CO 2 reduction, a novel photocatalytic system needs to be developed by the innovative design of photosensitizer and catalytic moiety.…”