Mechanism of Photocatalytic Hydrogen Generation by a Polypyridyl-Based Cobalt Catalyst in Aqueous Solution

Abstract

The mechanism of photocatalytic hydrogen production was studied with a three-component system consisting of fac-[Re(py)(CO)3bipy](+) (py = pyridine, bipy = 2,2'-bipyridine) as photosensitizer, [Co(TPY-OH)(OH2)](2+) (TPY-OH = 2-bis(2-pyridyl)(hydroxy)methyl-6-pyridylpyridine), a polypyridyl-based cobalt complex, as water reduction catalyst (WRC), and triethanolamine (TEOA) as sacrificial electron donor in aqueous solution. A detailed mechanistic picture is provided, which covers all processes from excited state quenching on the time scale of a few nanoseconds to hydrogen release taking place between seconds and minutes at moderately basic reaction conditions. Altogether these processes span 9 orders of magnitude in time. The following reaction sequence was found to be the dominant pathway for hydrogen generation: After reductive quenching by TEOA, the reduced photosensitizer (PS) transfers an electron to the Co(II)-WRC. Protonation of Co(I) yields Co(III)H which is reduced in the presence of excess Co(I). Co(II)H releases hydrogen after a second protonation step, which is detected time-resolved by a clark-type hydrogen electrode. Aside from these productive steps, the role of side and back reactions involving TEOA-derived species is assessed, which is particularly relevant in laser flash photolysis measurements with significantly larger transient concentrations of reactive species as compared to continuous photolysis experiments. Most notable is an equilibrium reaction involving Co(I), which is explained by a nucleophilic addition of Co(I) to the oxidation product of TEOA, an electrophilic iminium ion. Quantum chemical calculations indicate that the reaction is energetically feasible. The calculated spectra of the adduct are consistent with the spectroscopic observations.

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“…The μs time constants for formation of the Co( i ) species in both biohybrids are similar to what has been observed in the literature for Co( i ) species in multimolecular systems with Ru and Re PS. 30,48,49 …”

Microsecond X‐ray Absorption Spectroscopy Identification of CoI Intermediates in Cobaloxime‐Catalyzed Hydrogen Evolution

“…[34,35] Alternatively, the initial Co(I) intermediate may react with an iminium species resulting from the decomposition of TEOA and generate a Co(III)-alkyl species. [36] In both cases, the resulting Co(III) species can be Smolentsev et …”

The Hydrogen Grand Challenge

“…The investigation of real-time motions of (electro-) chemical reactions on the atomic level have recently become possible for model systems [H2-desorption from Ru (Frischkorn, 2008), water dynamics (Tielrooij et al, 2010), electron-transfer reactions (Hsu et al, 2014), and solar water splitting (Zhang et al, 2014;Rodenberg et al, 2015)]. The implementation of femto-second time resolution with nanometer spatial resolution is the ultimate goal, and will open new perspectives for functional nanomaterials.…”