Efficient photocatalytic hydrogen production in water using a cobalt(iii) tetraaza-macrocyclic catalyst: electrochemical generation of the low-valent Co(i) species and its reactivity toward proton reduction

Abstract

A very efficient homogeneous system for visible-light driven hydrogen production in water is reported. This comprises the [Co(CR)Cl2](+) cobalt(III) tetraaza-macrocyclic complex (Cat1) as a noble metal-free catalyst, [Ru(bpy)3]Cl2 as a photosensitizer and ascorbate/ascorbic acid as a sacrificial electron donor and buffer. This system gives up to 1000 turnovers at pH 4.0 versus the catalyst with a relatively low photosensitizer/catalyst ratio (10/1) and a high concentration of catalyst (1 × 10(−4) M), thus producing a significant amount of H2 (12.3 mL for 5 mL of solution). It also exhibits long-term stability (more than 20 hours). The efficiency of Cat1 has been compared under the same experimental conditions to those of three other H2-evolving catalysts, which are known to operate in water, [Co{(DO)(DOH)pn}Br2] (Cat2), [Co(dmbpy)3]Cl2 (Cat3) and [Rh(dmbpy)2Cl2]Cl (Cat4). These comparative studies show that Cat4, although based on a noble metal, is about four times less active, while Cat2 and Cat3 produce more than one hundred times less hydrogen than Cat1. The low-valent CoI form of Cat1 has been successfully electrogenerated in CH3CN. Its high stability can be related to the high catalytic performance of the Cat1 system. We have also shown that in acidic aqueous solution (photocatalytic conditions) reduction at a slightly more negative potential than the Co(II)/Co(I) couple is needed to ensure efficient catalysis; this reduction is performed by the photogenerated [Ru(II)(bpy)2(bpy(˙−))](+) species.

Dye-sensitized PS- b -P2VP-templated nickel oxide films for photoelectrochemical applications

“…The stable photocurrents obtained in pH 4.5 acetate buffer electrolyte, i.e. conditions where cobalt-based catalysts have proved to be stable and active for H 2 evolution [53,54], hold promise for the development of functional photoelectrodes. This study resulting from the combined efforts of four distinct groups from France and Germany demonstrates the relevance of a Global Artificial Photosynthesis Project promoting interactions at the interface between various fields such as materials science, molecular chemistry, theoretical chemistry and physical chemistry.…”

Photo-induced redox catalysis for proton reduction to hydrogen with homogeneous molecular systems using rhodium-based catalysts

“…Some of these homogeneous photocatalytic systems can operate very efficiently (in terms of number of catalytic cycles or turnover number (TON)) in organic or mixed aqueous-organic solvents. Those reaching a turnover number versus catalyst (TON Cat ) above 100 in fully aqueous solution were rare and limited to rhodium [36][37][38][39] and platinum [40] but, since two years, several examples with cobalt [41][42][43][44][45][46][47][48][49][50][51][52][53][54], iron [55][56][57][58] nickel [59] were reported. Developing H 2 -evolving photocatalytic systems functioning in pure water is essential for their coupling with water oxidation systems in photoelectrochemical water-splitting devices.…”

Tracking the Structural and Electronic Configurations of a Cobalt Proton Reduction Catalyst in Water

“…[4][5][6][7][8][9] The catalyst precursor [LCo III Cl 2 ] + (L = macrocyclic ligand) in Scheme 1 belongs to this family of HEC and has been used in electrochemical as well as photochemical catalysis showing excellent results. [10][11][12][13][14] In contrast to many other molecular HEC that are active only in organic solvents, complex [LCo III Cl 2 ] + works in pure aqueous conditions showing remarkable stability over a period of several hours. Although mechanistic studies based on electrochemical and spectroscopic methods such as electron paramagnetic resonance or UV-Vis spectroscopy have been reported, 12 a full understanding of the reaction pathway for the hydrogen evolution reaction catalyzed by [LCo III Cl 2 ] + has not been described.…”

“…[10][11][12][13][14] In contrast to many other molecular HEC that are active only in organic solvents, complex [LCo III Cl 2 ] + works in pure aqueous conditions showing remarkable stability over a period of several hours. Although mechanistic studies based on electrochemical and spectroscopic methods such as electron paramagnetic resonance or UV-Vis spectroscopy have been reported, 12 a full understanding of the reaction pathway for the hydrogen evolution reaction catalyzed by [LCo III Cl 2 ] + has not been described. The main reason for this information gap is the high reactivity of the intermediate species that are involved in the catalysis, with lifetimes in the pico to microsecond timescales.…”