Search

The development of multicomponent molecular systems for the photocatalytic reduction of water to hydrogen has experienced considerable growth since the end of the 1970s. Recently, with the aim of improving the efficiency of the catalysis, single-component photocatalysts have been developed in which the photosensitizer is chemically coupled to the hydrogen-evolving catalyst in the same molecule through a bridging ligand. Until now, none of these photocatalysts has operated efficiently in pure aqueous solution: a highly desirable medium for energy-conversion applications. Herein, we introduce a new ruthenium-rhodium polypyridyl complex as the first efficient homogeneous photocatalyst for H2 production in water with turnover numbers of several hundred. This study also demonstrates unambiguously that the catalytic performance of such systems linked through a nonconjugated bridge is significantly improved as compared to that of a mixture of the separate components. [ABSTRACT FROM AUTHOR]

We report a very efficient homogeneous system for the visible-light-driven hydrogen production in pure aqueous solution at room temperature. This comprises [RhIII(dmbpy)2Cl2]Cl ( 1) as catalyst, [Ru(bpy)3]Cl2 ( PS1) as photosensitizer, and ascorbate as sacrificial electron donor. Comparative studies in aqueous solutions also performed with other known rhodium catalysts, or with an iridium photosensitizer, show that 1) the PS1/ 1/ascorbate/ascorbic acid system is by far the most active rhodium-based homogeneous photocatalytic system for hydrogen production in a purely aqueous medium when compared to the previously reported rhodium catalysts, Na3[RhI(dpm)3Cl] and [RhIII(bpy)Cp*(H2O)]SO4 and 2) the system is less efficient when [IrIII(ppy)2(bpy)]Cl ( PS2) is used as photosensitizer. Because catalyst 1 is the most efficient rhodium-based H2-evolving catalyst in water, the performance limits of this complex were further investigated by varying the PS1/ 1 ratio at pH 4.0. Under optimal conditions, the system gives up to 1010 turnovers versus the catalyst with an initial turnover frequency as high as 857 TON h−1. Nanosecond transient absorption spectroscopy measurements show that the initial step of the photocatalytic H2-evolution mechanism is a reductive quenching of the PS1 excited state by ascorbate, leading to the reduced form of PS1, which is then able to reduce [RhIII(dmbpy)2Cl2]+ to [RhI(dmbpy)2]+. This reduced species can react with protons to yield the hydride [RhIII(H)(dmbpy)2(H2O)]2+, which is the key intermediate for the H2 production. [ABSTRACT FROM AUTHOR]

We recently reported a very efficient homogeneous system for visible-light driven hydrogen production in water based on the cobalt(III) tetraaza-macrocyclic complex [Co(CR)Cl 2 ] + ( 1 ) (CR = 2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) as a noble metal-free catalyst, with [Ru II (bpy) 3 ] 2+ ( Ru ) as photosensitizer and ascorbate/ascorbic acid (HA − /H 2 A) as a sacrificial electron donor and buffer ( PhysChemChemPhys 2013, 15, 17544). This catalyst presents the particularity to achieve very high turnover numbers (TONs) (up to 1000) at pH 4.0 at a relative high concentration (0.1 mM) generating a large amount of hydrogen and having a long term stability. A similar activity was observed for the aquo derivative [Co III (CR)(H 2 O) 2 ] 3+ ( 2 ) due to substitution of chloro ligands by water molecule in water. In this work, the geometry and electronic structures of 2 and its analog [Zn II (CR)Cl] + ( 3 ) derivative containing the redox innocent Zn(II) metal ion have been investigated by DFT calculations under various oxidation states. We also further studied the photocatalytic activity of this system and evaluated the influence of varying the relative concentration of the different components on the H 2 -evolving activity. Turnover numbers versus catalyst (TON Cat ) were found to be dependent on the catalyst concentration with the highest value of 1130 obtained at 0.05 mM. Interestingly, the analogous nickel derivative, [Ni II (CR)Cl 2 ] ( 4 ), when tested under the same experimental conditions was found to be fully inactive for H 2 production. Nanosecond transient absorption spectroscopy measurements have revealed that the first electron-transfer steps of the photocatalytic H 2 -evolution mechanism with the Ru /cobalt tetraaza/HA − /H 2 A system involve a reductive quenching of the excited state of the photosensitizer by ascorbate ( k q = 2.5 × 10 7 M − 1 s − 1 ) followed by an electron transfer from the reduced photosensitizer to the catalyst ( k et = 1.4 × 10 9 M − 1 s − 1 ). The reduced catalyst can then enter into the cycle of hydrogen evolution. [ABSTRACT FROM AUTHOR]