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).
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