Artificial photosynthesis for high‐value‐added chemicals: Old material, new opportunity

Abstract

Solar energy utilization has drawn attention due to ever‐increasing environmental and energy issues. Photoelectrochemical (PEC) and photocatalytic (PC) water splitting for hydrogen production, which is the most popular and well‐established solar‐to‐chemical conversion process, has been studied thoroughly to date but is now facing limitations related to low conversion efficiency. To resolve this issue, research in PEC cells or photocatalysts has recently aimed to produce alternative value‐added chemicals by modifying their redox reactions, which potentially enables high economic reward to compensate for the low efficiency. Here, various kinds of redox reactions that decouple classic water splitting reactions to produce value‐added chemicals via PEC and PC processes are introduced. Successful coupling of CO2 reduction, O2 reduction and organic synthesis with either water oxidation or water reduction is comprehensively discussed from the perspective of basic fundamental and product selectivity in terms of the band structure of materials, cocatalyst design, and thermodynamics and kinetics of the reactions. Throughout the review, future challenges and opportunities are suggested with respect to the redesigned artificial synthesis, which might be an alternative development for the commercialization of PEC or PC value‐added chemical production technologies in the near future.

Enhanced Selective Photooxidation of Toluene to Benzaldehyde over Co3O4‐Modified BiOBr/AgBr S‐Scheme Heterojunction

“…Moreover, the reactants with electron‐donating substituent (Entries 3 and 4) at the p‐site of toluene exhibit higher reactivity than those with electron‐withdrawing substituent (Entries 5 and 6), which may be attributed to the easily formed carbon‐centered radicals of toluene derivatives with electron‐donating substituent. [ 12,52 ]…”

“…Quenching experiments were carried out to ascertain the major active species responsible for the selective toluene oxidation (Figure 5b). 2,2,6,6‐Tetramethyl‐1‐piperidinyloxy (TEMPO), [ 7 ] butylated hydroxytoluene (BHT), benzoquinone (BQ), [ 7,52 ] ammonium oxalate (AO), [ 12 ] potassium persulfate (Potass), [ 7,53 ] and t ‐BuOH [ 54 ] were employed to eliminate the all radicals, carbon‐centered radicals, •O 2 − radical, photogenerated holes (h + ), photogenerated electrons (e − ), and •OH radical, respectively. Apparently, benzaldehyde yield is completely quenched with the addition of TEMPO, which strongly confirms the radical‐initiated pathway.…”

“…[ 5–9 ] In a typical thermal catalytic procedure, harsh conditions, including high temperature and pressure, as well as the addition of toxic and aggressive oxidation reagents, are required, thus causing poor product selectivity. [ 10–12 ] In contrast, semiconductor‐based photocatalysis shows great potential to supersede the conversional thermocatalysis for breaking CH bonds under mild conditions, which is a sustainable way by utilizing inexhaustible solar energy and green oxygen. [ 6,13–18 ]…”

En route to artificial photosynthesis: the role of polyoxometalate based photocatalysts

“…or methane, the highest value CO 2 reduction process is to produce carbon and hydrogen-rich C 2+ products (i.e., long-chain hydrocarbons, alcohols, and carboxylates). 35,36 However, in carrying out such chemistry, there are several challenges in effecting extensive carbon-carbon bond formation with selectivity and high solar energy efficiency. 37 To construct an ideal articial photosynthesis system, three components are vital, namely photosensitizer, water oxidation catalyst (WOC), and CO 2 reduction catalyst (CO 2 RC) (Scheme 1).…”

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

“…To circumvent these issues, researchers have recently started to investigate the possibility of coupling proton reduction with other chemical reactions. More in detail, two main processes have been performed to achieve this goal: waste photoreforming and photoredox catalysis on pure organic substrates to obtain value-added products ( Reisner, 2019 ; Toe et al, 2021 ; Kim et al, 2022 ). In photoreforming, the proton reduction reaction is coupled with the oxidation of organic substrates such as waste, plastics, and pollutants, with the formation of either CO 2 or value-added compounds ( Cui et al, 2021 ; Luo et al, 2021 ).…”