Water‐Soluble Organic Dyes as Molecular Photocatalysts for H₂O₂ Evolution

Abstract

cousin H 2 . To its advantage, H 2 O 2 forms stable aqueous solutions, [8] and does not require compression and storage as gaseous H 2 does. Photocatalytic evolution of peroxide has proven to be more selective than CO 2 reduction, [9] giving only one product. What is more, the energy accumulated in H 2 O 2 can be utilized directly for electricity generation in a membranefree hydrogen peroxide fuel cell (theoretical V oc = 1.1 V, comparable with the 1.23 V of H 2 fuel cells), which can be made as a single-compartment device based on abundant and inexpensive components. [10,11] Taken together, hydrogen peroxide offers the promise of a carbonfree liquid fuel with high energy density, which can be directly used in fuel cells. The cornerstone in the peroxide energy cycle is photogeneration via a catalyzed reaction of solar irradiation with oxygen and water. Several promising catalytic systems for oxygen reduction to peroxide have been successfully demonstrated. Photocatalysts are used in the form of suspensions of particles varying in size from tens of micrometers to a few nanometers. This convenient strategy allows for using many different materials for this purpose, such as metal oxides, [12] carbon nitride materials, [3,13] or organic semiconductors. [14] Carbon nitrides can be tailored with different organic molecular components that modulate their H 2 O 2 generation efficiency. [3,15] Molecular organic semiconductors, in addition, can be used to produce photocathodes offering high faradaicyield peroxide-generating photoelectrochemical cells. [16][17][18] On the other hand, a fundamental obstacle in all these examples is related to the fact that only the surface of the particle, exposed to the electrolyte, can actively participate in the photo(electro) catalytic process. The part of the material making up the particle core is inactive. The larger the average particle size, the higher the resulting inefficiency of the catalyst utilization. This issue can be considered more acute for the situation of oxygen reduction in aqueous medium since the low solubility of O 2 in water enforces a strict diffusion limit on the efficiency of photochemical oxygen reduction. A thought experiment therefore is: if the photocatalytic material is broken up to into smaller units, what happens when the material is divided into molecular form and dissolved in aqueous media? This is the question central to beginning the present work. Can the constituent molecules of the organic semiconductors reported to date for catalyzing peroxide evolution be photocatalytically active? It can be hypothesized that the molecule is intrinsically Photochemical generation of hydrogen peroxide via oxygen reduction is a critical component of emerging sustainable energy conversion concepts. Light-absorbing semiconductors as well as electrodes modified with sensitizers typically catalyze oxygen photoreduction to hydrogen peroxide. Here, it is reported that, in contrast to these heterogeneous systems, a homogeneous solution of a metal-free organic dye can perfor...

Photochemical evolution of hydrogen peroxide on lignins

“…2b and c). Based on our previously published results on peroxide-evolving dyes 16 in solution we would maintain that it is most probable that both for lignin and LS the reaction mechanism proceeds via single-electron reduction of oxygen to superoxide, which subsequently disproportionates to form stable hydrogen peroxide. Such a mechanism is kinetically most facile, however a contribution from concerted two-electron/two-proton reduction of oxygen to peroxide cannot be ruled out.…”

“…1 These demands motivated us to explore the possibility of peroxide evolution catalyzed by abundant biomaterials. The present study was guided by recent findings that organic carbonyl dyes 16 and pigments 17 as well as the structurally-related biopolymer eumelanin 18 are photocatalysts for selective reduction of oxygen to peroxide. Lignin shares critical structural features with these proven catalytic speciesnamely aromatic conjugated units with quinone/ hydroquinone redox moieties.…”

Visible-light photooxidation in water by 1O2-generating supramolecular hydrogels

“…[1][2][3][4][5][6][7][8][9] In this context, organic photocatalysts that operate in aqueous solvents have been the focus of considerable recent interest as sustainable alternatives to conventional photocatalysts. [10][11][12][13][14] To date, these photocatalysts/photosensitizers are primarily porphyrin-based, and their photocatalytic applications are mostly limited to water-soluble substrates.…”

Untangling Photofaradaic and Photocapacitive Effects in Organic Optoelectronic Stimulation Devices

“…Oxygen reduction reactions, on the other hand, have been discovered to be highly favored on organic semiconductors (Bellani et al, 2015). The single-electron reduction of O 2 to superoxide (Suppes et al, 2013;Gryszel et al, 2019) or the two-electron reduction to produce hydrogen peroxide (Jakešová et al, 2016;Węcławski et al, 2017;Gryszel et al, 2018b), H 2 O 2 , were demonstrated to proceed efficiently for several organic semiconductors. Both oxygen reduction reactions are thermodynamically more favorable than hydrogen evolution, with the two-electron peroxide reaction being 700 mV lower than H 2 production.…”