Beyond Water Oxidation: Hybrid, Molecular-Based Photoanodes for the Production of Value-Added Organics

Abstract

The political and environmental problems related to the massive use of fossil fuels prompted researchers to develop alternative strategies to obtain green and renewable fuels such as hydrogen. The light-driven water splitting process (i.e., the photochemical decomposition of water into hydrogen and oxygen) is one of the most investigated strategies to achieve this goal. However, the water oxidation reaction still constitutes a formidable challenge because of its kinetic and thermodynamic requirements. Recent research efforts have been focused on the exploration of alternative and more favorable oxidation processes, such as the oxidation of organic substrates, to obtain value-added products in addition to solar fuels. In this mini-review, some of the most intriguing and recent results are presented. In particular, attention is directed on hybrid photoanodes comprising molecular light-absorbing moieties (sensitizers) and catalysts grafted onto either mesoporous semiconductors or conductors. Such systems have been exploited so far for the photoelectrochemical oxidation of alcohols to aldehydes in the presence of suitable co-catalysts. Challenges and future perspectives are also briefly discussed, with special focus on the application of such hybrid molecular-based systems to more challenging reactions, such as the activation of C–H bonds.

Photoelectrochemical C−H Activation Through a Quinacridone Dye Enabling Proton‐Coupled Electron Transfer

“…[54] State of the art dye-sensitized photoelectrodes provide photocurrent densities in the range 18-400 μAcm À 2 for the oxidation of TEMPO, [1,11,55] retaining interest in mediating the oxidation of alcohols (see Ref. [10] for a recent benchmarking collection of these systems). Analysis of the incident photon to current efficiency (IPCE) reveals a profile that matches well the absorption spectrum of the QNC dye, peaking at 0.35 % at 510 nm (Figure 5B; a ca.…”

“…More recently, oxidation of organic compounds has emerged as an alternative anodic process, [8,9] with the double advantage of i) bypassing the significant thermodynamic and kinetic hurdles of the water oxidation reaction, while ii) targeting valuable reaction products instead of oxygen. So far, the applications of dye‐sensitized photoanodes in this field have mainly considered the oxidation of alcohols, exploiting both ruthenium polypyridine and organic dyes, by integrating aminoxyl radicals as catalytic mediators [1,10–12] . Given the milder operative conditions with respect to water oxidation and the endless arsenal of organic chemistry, this field is expected to explode in the next years by proposing new dye families [13,14] and by targeting more challenging transformations [15] .…”

“…So far, the applications of dye-sensitized photoanodes in this field have mainly considered the oxidation of alcohols, exploiting both ruthenium polypyridine and organic dyes, by integrating aminoxyl radicals as catalytic mediators. [1,[10][11][12] Given the milder operative conditions with respect to water oxidation and the endless arsenal of organic chemistry, this field is expected to explode in the next years by proposing new dye families [13,14] and by targeting more challenging transformations. [15] Among these processes, the oxidative activation of CÀ H bonds is considered a Holy Grail in organic reactivity.…”

Heterojunction WO3–BiVO4 Photoanodes for TEMPO-Mediated Benzyl Alcohol Dehydrogenation in Organic Media

“…The production of value-added chemicals represents an important area for development and study in the field of PECs. − Only a handful of reports have studied the photoelectrosynthetic oxidation of primary alcohols in organic media using a WO 3 –BiVO 4 photoanode as outlined in Table . Regarding this body of work, Tateno et al used WO 3 –BiVO 4 heterojunctions for the oxidation of benzyl alcohol derivatives in organic media without a base or mediator present in solution .…”

A breath of sunshine: oxygenic photosynthesis by functional molecular architectures

“…To this end, the oxidation of alcohols is one of the most investigated strategies, 141 that is often accomplished by integrating a photoanodic process with an aminoxyl radical catalyst, 142 added in solution or chemically anchored to the photoelectrode. Primary and benzylic alcohols are generally oxidised between 0.7 and 1.5 V vs. NHE 143,144 to afford the corresponding aldehyde as a two-electron oxidation product, eventually undergoing a sequential oxidation step to the corresponding carboxylic acid.…”