Making Hydrogen from Water Using a Homogeneous System Without Noble Metals

Abstract

A photocatalytic noble metal-free system for the generation of hydrogen has been constructed using Eosin Y (1) as a photosensitizer, the complex [Co(dmgH)(2)pyCl](2+) (5, dmgH = dimethylglyoximate, py = pyridine) as a molecular catalyst, and triethanolamine (TEOA) as a sacrificial reducing agent. The system produces H(2) with an initial rate of approximately 100 turnovers per hour upon irradiation with visible light (lambda > 450 nm). Addition of free dmgH(2) greatly increases the durability of the system addition of 12 equiv of dmgH(2) (vs cobalt) to the system produces approximately 900 turnovers of H(2) after 14 h of irradiation. The rate of H(2) evolution is maximum at pH = 7 and decreases sharply at more acidic or basic pH. Spectroscopic study of photolysis solutions suggests that hydrogen production occurs through protonation of a Co(I) species to give a Co(III) hydride, which then reacts further by reduction and protolysis to give Co(II) and molecular hydrogen.

Photocatalytic Hydrogen Production Using Models of the Iron–Iron Hydrogenase Active Site Dispersed in Micellar Solution

“…[52] The reversible potentials for the couples EY À / 3 *EY 2À and RB À / 3 *RB 2À were estimated to be À0.87 and À0.78 V versus SHE at pH 7. [53] These potentials are both more negative than the reduction potential of complex 1 in aqueous SDS solutions (E 1/2 = À0.74 V vs. SHE at pH 7), [43] indicating that the electron transfer from the transient species 3 *EY 2À and 3 *RB 2À to complex 1 is thermodynami- cally favorable. The system 1/RB 2À /Et 3 N produced H 2 , but at a slower initial rate and with a TON that was half that of the system 1/EY 2À /Et 3 N ( Table 1).…”

Theoretical Modeling of Low‐Energy Electronic Absorption Bands in Reduced Cobaloximes

“…These compounds are known to be powerful nucleophiles in their reduced Co(I) state. It is accepted that the catalytic cycle for hydrogen evolution proceeds via protonation of the Co(I) species, yielding a Co(III)-H hydridocobaloxime intermediate that, after further reduction to the Co(II)-H state, can evolve dihydrogen through either protonation of the hydride moiety or bimolecular reductive elimination [1,2,3,4,5,10,11,6,13,14,15,16,17,18].…”

“…These compounds are known to be powerful nucleophiles in their reduced Co(I) state. It is accepted that the catalytic cycle for hydrogen evolution proceeds via protonation of the Co(I) species, yielding a Co(III)-H hydridocobaloxime intermediate that, after further reduction to the Co(II)-H state, can evolve dihydrogen through either protonation of the hydride moiety or bimolecular reductive elimination [1,2,3,4,5,10,11,6,13,14,15,16,17,18].Recent reports from the groups of Muckerman [19], and Jiang [21] have addressed aspects of the catalytic activity of these compounds using quantum chemical approaches and confirm the role of the Co(I) species. Experimentally, the spectroscopic signatures of Co(I) intermediates have been observed during the course of electro-and photo-catalytic experiments [4,5,22], and the Co(I) species [14,23,24,9], and its protonated Co(III)-H form [25,26,27] Figure 1).…”

Cobalt-dithiolene complexes for the photocatalytic and electrocatalytic reduction of protons in aqueous solutions

“…One of these electrocatalysts, CoðdmgHÞ 2 ðpyrÞCl (dmgH ¼ dimethylglyoximate monoanion) has also been found to be active as a hydrogen generating catalyst when combined with a suitable photosensitizer and sacrificial electron donor (6,10,20). In recently reported photochemical H 2 generation, systems containing cobaloxime catalysts have shown considerable activity (21)(22)(23)(24)(25) with turnover numbers (TONs) as high as 9,000 with respect to chromophore (25)(26)(27). However, one limitation is that the TONs are modest based on catalyst (<300, with TOF <100∕h), and hydrogen production ceases after 6 h of irradiation.…”