Electron-transfer sensitization of H ₂ oxidation and CO ₂ reduction catalysts using a single chromophore

Abstract

Energy-storing artificial-photosynthetic systems for CO 2 reduction must derive the reducing equivalents from a renewable source rather than from sacrificial donors. To this end, a homogeneous, integrated chromophore/two-catalyst system is described that is thermodynamically capable of photochemically driving the energy-storing reverse water-gas shift reaction (CO 2 + H 2 → CO + H 2 O), where the reducing equivalents are provided by renewable H 2 . The system consists of the chromophore zinc tetraphenylporphyrin (ZnTPP), H 2 oxidation catalysts of the form [Cp R Cr(CO) 3 ] -, and CO 2 reduction catalysts of the type Re(bpy-4,4′-R 2 )(CO) 3 Cl. Using time-resolved spectroscopic methods, a comprehensive mechanistic and kinetic picture of the photoinitiated reactions of mixtures of these compounds has been developed. It has been found that absorption of a single photon by broadly absorbing ZnTPP sensitizes intercatalyst electron transfer to produce the substrate-active forms of each. The initial photochemical step is the heretofore unobserved reductive quenching of the low-energy T 1 state of ZnTPP. Under the experimental conditions, the catalytically competent state decays with a second-order half-life of ∼15 μs, which is of the right magnitude for substrate trapping of sensitized catalyst intermediates.artificial photosynthesis | catalysis | dual catalysis | photoredox | photocatalysis T he inexorable growth of global energy consumption and concern over its attendant environmental consequences has spurred considerable research into developing means to store solar energy in the form of renewable chemical fuels (1, 2). Among the potential feedstocks for these solar fuels, CO 2 is a desirable target because it is the end product of the combustion of fossil fuels. Thus, developing solar-driven mechanisms for chemically reducing CO 2 to energy-rich products holds the potential to recycle conventional fuels and mitigate their carbon impact (3-7).Numerous studies over the past 30 y have investigated homogeneous artificial-photosynthetic systems for CO 2 reduction, in which a photoexcited chromophore accomplishes the transfer of electrons from a source of reducing equivalents to a CO 2 reduction catalyst (8-14). With very rare exceptions (15), the reducing equivalents consumed in these photochemical CO 2 reduction reactions have been supplied by sacrificial electron donors. These reagents are used because their prompt decomposition following photoinitiated oxidation suppresses unproductive back-electron-transfer pathways, which are generally fast compared with substrate transformation, and because their decomposition products can provide additional reducing equivalents needed for some CO 2 reduction reactions, thus circumventing the one-photon/one-electron limit of molecular photosensitizers. Offsetting these practical advantages, however, is the fact that the stoichiometric consumption of conventional sacrificial donors in these reactions negates their energy-storing potential. In order for homogeneous systems to drive ...

Electronic state dependence of heterogeneous electron transfer: injection from the S1 and S2 state of phlorin into TiO2

“…The crude product was subjected to a second chromatographic step using a silica column with a mobile phase comprised of hexanes and ethyl acetate (1 : 1) to deliver 47 mg of the title compound as a green solid in 50% yield. 1 H NMR (400 MHz, CD 3 CN) d/ppm: 8.17…”

“…Porphyrins have attracted much attention as sensitizers for dye-sensitized solar cells (DSSCs) as well as for photocatalytic applications. [1][2][3][4][5][6] Recently, a fluorinated free-base porphyrinoid has been synthesized that showed multi-electron redox reaction capability 7,8 and enhanced absorption in the red spectral region as a potential photosensitizer in a DSSC. Many porphyrinoids show interesting photophysics due to a strong S 0 -S 2 transition dipole moment followed by a very rapid S 2 -S 1 internal conversion (IC).…”

2‐Electron Reduction of CO2 by Graphene Supported Ru Complexes – on the Role of Electron Donation.

“…The next step of the analysis, is the role of the possibility of the charge transfer from the graphene sheet. The first electron transfer to the CO 2 molecule is the most difficult one,,, several strategies have been discussed on how to overcome this issue. In homogeneous catalysis the presence of weak Lewis acid/base sites on the CO 2 molecule is exploited by making use of the strong Lewis base such as ethylene‐3‐amine (Et 3 N) or 1,8‐Diazabicyclo[5.4.0]undec‐7‐ene (DBU) to activate the CO 2 molecule.…”

“…The first step of the CO 2 reduction is the most difficult, as the reduction potential of CO 2 to CO 2 −. amounts to −1.9 V with respect to the standard hydrogen electrode ,. This step is considered as the one that seals the fate of the process, because the subsequent reduction steps are much easier.…”

Solar cell driven electrochemical process for the reduction of CO 2 to HCOOH on Zn and Sn electrocatalysts

“…Studies were done to develop a best possible kinetics and selection of catalyst for H 2 O oxidation and CO 2 reduction using solar energy (La Porte et al, 2014;Hull et al, 2012;Brendelberger and Sattler, 2015). The review toward the nano-materials and their structural properties for reduction of CO 2 to methane by catalytic and photo catalytic processes were reported (Fechete and Vedrine, 2015).…”