A noble-metal-free system for photochemical hydrogen production is described, based on ascorbic acid as sacrificial donor, aluminium pyridylporphyrin as photosensitizer, and cobaloxime as catalyst. Although the aluminium porphyrin platform has docking sites for both the sacrificial donor and the catalyst, the resulting associated species are essentially inactive because of fast unimolecular reversible electron transfer quenching. Rather, the photochemically active species is the fraction of sensitizer present, in the aqueous/organic solvent used for hydrogen evolution, as free species. As shown by Nanosecond Laser Flash Photolysis experiments, its long-lived triplet state reacts bimolecularly with the ascorbate donor, and the reduced sensitizer thus formed subsequently reacts with the cobaloxime catalyst, triggering the hydrogen evolution process. The performance is good, particularly in terms of turnover frequency (TOF = 10.8 min-1 or 3.6 min-1, relative to sensitizer or catalyst, respectively) and quantum yield (Φ = 4.6%, i.e. 9.2% of maximum possible value). At high sacrificial donor concentration, the maximum turnover number (TON = 352 or 117, relative to sensitizer or catalyst, respectively) is eventually limited by hydrogenation of both sensitizer (chlorin formation) and catalyst.
Photocatalytic Hydrogen Evolution with a Self-Assembling Reductant-Sensitizer-Catalyst System
NATALI, Mirco;ARGAZZI, Roberto;CHIORBOLI, Claudio;SCANDOLA, Franco
2013
Abstract
A noble-metal-free system for photochemical hydrogen production is described, based on ascorbic acid as sacrificial donor, aluminium pyridylporphyrin as photosensitizer, and cobaloxime as catalyst. Although the aluminium porphyrin platform has docking sites for both the sacrificial donor and the catalyst, the resulting associated species are essentially inactive because of fast unimolecular reversible electron transfer quenching. Rather, the photochemically active species is the fraction of sensitizer present, in the aqueous/organic solvent used for hydrogen evolution, as free species. As shown by Nanosecond Laser Flash Photolysis experiments, its long-lived triplet state reacts bimolecularly with the ascorbate donor, and the reduced sensitizer thus formed subsequently reacts with the cobaloxime catalyst, triggering the hydrogen evolution process. The performance is good, particularly in terms of turnover frequency (TOF = 10.8 min-1 or 3.6 min-1, relative to sensitizer or catalyst, respectively) and quantum yield (Φ = 4.6%, i.e. 9.2% of maximum possible value). At high sacrificial donor concentration, the maximum turnover number (TON = 352 or 117, relative to sensitizer or catalyst, respectively) is eventually limited by hydrogenation of both sensitizer (chlorin formation) and catalyst.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
