Photocatalytic Transformation Research Articles

Harnessing Photo‐to‐Thermal Conversion in Sulfur‐Vulcanized Mxene for High‐Efficiency Solar‐to‐Carbon‐Fuel Synthesis

AbstractHarnessing solar energy for the conversion of CO2 into value‐added chemicals and fuels represents a promising strategy for sustainable development. Photo‐to‐thermal (PTT) conversion, an often‐underestimated factor, offers a remarkable approach to enhance the photocatalytic transformation of CO2, by reducing the activation energy of catalytic reactions and accelerating reaction kinetics. In order to achieve a higher energy return on investment (EROI), in this study, a sulfur‐vulcanized, multi‐layer Ti3C2 MXene is unveiled, capable of efficient sunlight‐driven CO2 photoreduction, by capitalizing on PTT conversion across the full visible‐to‐near‐infrared (NIR) spectrum. The vulcanization strategy is pivotal here, as it not only introduces an abundance of reactive sites but also extends the NIR response (peaking at 1095 nm) of MXene. The resulting rapid PTT and synergistic photo‐thermal‐catalytic CO2 reduction constitute a significant advance in this area, where CH4 (12.03 mmol g−1 h−1) and C2H4 (3.55 mmol g−1 h−1) yields are achieved with a C2+ selectivity of 29.76% under concentrated natural sunlight. This work sets a new benchmark for EROI with an average solar‐to‐carbon‐fuel (STF) conversion efficiency greater than 0.045%.

Sunlight sparks spectacular C(sp3)─H bond arylation: Rod shape ACC@MeF composite shines bright in 2,4‐dinitrophenylhydrazine reaction

AbstractThe solar light responsive selective C(sp3)─H bond activation by photocatalytic pathway using graphene platform as photocatalyst is a green and sustainable route to achieve value‐added chemicals on gram scale synthesis. Graphene has low activity due to fast recombination charges, band gap suitability problems, and inefficient solar light absorption ability. Therefore, activated carbon cloth‐based nitrogen‐rich phenolic resin was developed from melem and formaldehyde. The ACC@MeF light‐harvesting composite demonstrates outstanding light‐harvesting capability, leading to enhanced photocatalytic performance in C(sp3)─H bond formation, suitability of band gap, high molar extinction coefficient and high thermal stability. The development of metal‐free activated carbon cloth‐based melem formaldehyde resin for the solar light photocatalytic transformation of organic compounds is anticipated to have broad‐reaching applications.

Donor‐Acceptor Pyrene‐Based Covalent Organic Framework for Blue Light Photocatalytic Oxidative Coupling of Amines

Covalent organic frameworks (COFs) with a donor‐acceptor (D‐A) architecture have been investigated as prospective semiconducting materials, particularly for photocatalytic processes. The incorporation of electron donating and accepting moieties into the COF backbone creates nano heterojunctions, enhancing efficient light absorption, charge separation, and charge transport. Herein, three pyrene‐based COFs were well designed for the aerobic coupling of amines under air atmosphere with blue‐LED irradiation. Among the three COFs, the Py‐Por‐COF with D‐A structure exhibits the highest photocatalytic performance and demonstrates a remarkable apparent quantum efficiency of 11.3 % at 420 nm.  In addition, the Py‐Por‐COF maintains a good level of crystallinity and high yield even undergone four cycles, showcasing the exceptional potential of D‐A structure COFs in the field of photocatalytic organic transformation.

Isolated Iridium Sites on Potassium‐Doped Carbon‐nitride wrapped Tellurium Nanostructures for Enhanced Glycerol Photooxidation

AbstractMany industrial processes such transesterification of fatty acid for biodiesel production, soap manufacturing and biosynthesis of ethanol generate glycerol as a major by‐product that can be used to produce commodity chemicals. Photocatalytic transformation of glycerol is an enticing approach that can exclude the need of harsh oxidants and extraneous thermal energy. However, the product yield and selectivity remain poor due to low absorption and unsymmetrical site distribution on the catalyst surface. Herein, tellurium (Te) nanorods/nanosheets (TeNRs/NSs) wrapped potassium‐doped carbon nitride (KCN) van der Waal (vdW) heterojunction (TeKCN) is designed to enhance charge separation and visible‐NIR absorption. The iridium (Ir) single atom sites decoration on the TeKCN core‐shell structure (TeKCNIr) promotes selective oxidation of glycerol to glyceraldehyde with a conversion of 45.6% and selectivity of 61.6% under AM1.5G irradiation. The catalytic selectivity can reach up to 88% under 450 nm monochromatic light. X‐ray absorption spectroscopy (XAS) demonstrates the presence of undercoordinated IrN2O2 sites which improved catalytic selectivity for glycol oxidation. Band energies and computational calculations reveal faile charge transfer in the TeKCNIr heterostructure. EPR and scavenger tests discern that superoxide (O2•−) and hydroxyl (•OH) radicals are prime components driving glycerol oxidation.

The synthesis of CdS hierarchical micro-nanostructures with different pore structure and their influence on the photocatalytic ethanol transformation

The synthesis of CdS hierarchical micro-nanostructures with different pore structure and their influence on the photocatalytic ethanol transformation

Recyclable and Stable Porphyrin-Based Self-Assemblies by Electrostatic Force for Efficient Photocatalytic Organic Transformation.

Development of efficient, stable, and recyclable photocatalysts for organic synthesis is vital for transformation of traditional thermal organic chemistry into green sustainable organic chemistry. In this work, the study reports an electrostatic approach to assemble meso-tetra (4-sulfonate phenyl) porphyrin (TPPS)tetra (4-sulfonate phenyl) porphyrin (TPPS) as a donor and benzyl viologen (BV) as an acceptor into stable and recyclable photocatalyst for an efficient organic transformation reaction - aryl sulfide oxidation. By use of the electrostatic TPPS-BV photocatalysts, 0.1mmol aryl sulfide with electron-donating group can be completely transformed into aryl sulfoxide in 60min without overoxidation into sulfone, rendering near 100% yield and selectivity. The photocatalyst can be recycled up to 95% when 10mg amount is used. Mechanistic study reveals that efficient charge separation between TPPS and BV results in sufficient formation of superoxide which further reacts with the oxidized sulfide by the photocatalyst to produce the sulfoxide. This mechanistic pathway differs significantly from the previously proposed singlet oxygen-dominated process in homogeneous TPPS photocatalysis.

Polyarene Oxides with Tunable Quinone Units for Photocatalytic CO2 Reduction: A Simple Strategy toward Effective and Selective Catalysts.

The photocatalytic transformation of carbon dioxide (CO2) into valuable chemicals is a challenging process that requires effective and selective catalysts. However, most polymer-based photocatalysts with electron donor-acceptor (D-A) structures are synthesized with a fixed D-A ratio by using expensive monomers. Herein, we report a simple strategy to prepare polyarene oxides (PAOs) with quinone structural units via oxidation treatment of polyarene (PA). The resultant PAOs show tunable D-A structures and electronic band positions depending on the degree of oxidation, which can catalyze the photoreduction of CO2 with water under visible light irradiation, generating CO as the sole carbonaceous product without H2 generation. Especially, the PAO with an oxygen content of 17.6% afforded the highest CO production rate of 161.9 μmol g-1 h-1. It is verified that the redox transformation between quinone and phenolic hydroxyl in PAOs achieves CO2 photoreduction coupled with water oxidation. This study provides a facile way to access conjugated polymers with a tunable D-A structure and demonstrates that the resultant PAOs are promising photocatalysts for CO2 reduction.

Phenalenyl-Based Photocatalyst for Bioinspired Oxidative Dehydrogenation of N-Heterocycles and Benzyl Alcohols.

The environmental benefits of molecular oxygen as the oxidizing agent in oxidation reactions that synthesize fine chemicals cannot be overstated. Increased interest in developing robust photocatalysts is stimulated by the fact that the current photocatalytic transformation boom has made previously inaccessible synthetic approaches possible. Motivated by enzymatic catalysis, employing a reusable phenalenyl-based photocatalyst, we have successfully developed oxidative dehydrogenation utilizing molecular oxygen as a greener oxidant. Under photoinduced oxidative dehydrogenation conditions, different types of saturated N-heterocycles and alcohols were successfully dehydrogenated. The versatility of this bioinspired protocol is demonstrated by the fact that a wide variety of N-heteroaromatics, such as quinoline, carbazole, quinoxaline, acridine, and indole derivatives, as well as aldehydes and ketones, were successfully synthesized. Detailed mechanistic studies validate the proposed mechanism. Fluorescence lifetime and CV experiments revealed the crucial role of water on the efficiency of the reaction. The present protocol also provides chemoselectivity and scalability, leading to superior results and allowing for the functionalization of bioactive molecules at a late stage in a sustainable manner.

Decarboxylative Radical Sulfilimination via Photoredox, Copper, and Brønsted Base Catalysis

AbstractSulfilimines, the aza‐variants of sulfoxides, are key structural motifs in natural products, pharmaceuticals, and agrochemicals; and sulfilimine synthesis is therefore important in organic chemistry. However, methods for radical sulfilimination remain elusive, and as a result, the structural diversity of currently available sulfilimines is limited. Herein, we report the first protocol for decarboxylative radical sulfilimination reactions between sulfenamides and N‐hydroxyphthalimide esters of primary, secondary, and tertiary alkyl carboxylic acids, which were achieved via a combination of photoredox, copper, and Brønsted base catalysis. This novel protocol provided a wide variety of sulfilimines, in addition to serving as an efficient route for the synthesis of S‐alkyl/S‐aryl homocysteine sulfilimines and S‐(4‐methylphenyl) homocysteine sulfoximine. Moreover, it could be used for late‐stage introduction of a sulfilimine group into structurally complex molecules, thereby avoiding the need to preserve labile organosulfur moieties through multistep synthetic sequences. A mechanism involving photocatalytic substrate transformation and copper‐mediated C(sp3)−S bond formation is proposed.

Decarboxylative Radical Sulfilimination via Photoredox, Copper, and Brønsted Base Catalysis.

Sulfilimines, the aza-variants of sulfoxides, are key structural motifs in natural products, pharmaceuticals, and agrochemicals; and sulfilimine synthesis is therefore important in organic chemistry. However, methods for radical sulfilimination remain elusive, and as a result, the structural diversity of currently available sulfilimines is limited. Herein, we report the first protocol for decarboxylative radical sulfilimination reactions between sulfenamides and N-hydroxyphthalimide esters of primary, secondary, and tertiary alkyl carboxylic acids, which were achieved via a combination of photoredox, copper, and Brønsted base catalysis. This novel protocol provided a wide variety of sulfilimines, in addition to serving as an efficient route for the synthesis of S-alkyl/S-aryl homocysteine sulfilimines and S-(4-methylphenyl) homocysteine sulfoximine. Moreover, it could be used for late-stage introduction of a sulfilimine group into structurally complex molecules, thereby avoiding the need to preserve labile organosulfur moieties through multistep synthetic sequences. A mechanism involving photocatalytic substrate transformation and copper-mediated C(sp3 )-S bond formation is proposed.

Optimizing the electronic configuration of h-BN for boosting the photocatalytic transformation of acid gases under visible light

The incorporation of heteroatomic carbon optimizes the electronic configuration of boron nitride (BN) to enhance its performance in photocatalytic elimination of acidic gases.

Porphyrin-Based Two-Dimensional Metal-Organic framework nanosheets for efficient photocatalytic CO2 transformation

2D metal − organic framework (MOF) nanosheets have been demonstrated to exhibit excellent properties and prospects in various aspects, especially in photocatalysis. However, the design and direct preparation of 2D MOF photocatalyst is highly desirable but still challenging. Herein, a novel 2D porphyrin-based MOF Fe-DBP(Co), which is assembled by dicarboxylic porphyrin ligands and the unprecedented undecametallic ferric-oxo clusters, is designed and developed. The unique nanosheet morphology and assembled layered structure endows the photocatalyst with higher electron-hole separation efficiency and longer photogenerated carrier lifetime compared to the analogous MOFs. Consequently, Fe-DBP(Co) exhibits strikingly excellent and stable photocatalytic performance in CO2 cycloaddition, and the reaction rate of 103.2 mmol∙g−1∙h−1 is among the state-of-the-art cases for MOF-photocatalyzed CO2 cycloaddition. The underlying mechanism investigation suggested that photogenerated electrons transferred from CoDBP ligands to the Fe clusters, and the abundant electrons and holes promoted the generation of Fe(II) and Co(III) respectively, leading to excellent catalytic performance. This work proposes a new strategy to design and develop efficient MOF photocatalysts for photocatalytic transformation of CO2.

Switchable Decarboxylation by Energy- or Electron-Transfer Photocatalysis.

Kolbe dimerization and Hofer-Moest reactions are well-investigated carboxylic acid transformations, wherein new carbon-carbon and carbon-heteroatom bonds are constructed via electrochemical decarboxylation. These transformations can be switched by choosing an electrode that allows control of the reactive intermediate, such as carbon radical or carbocation. However, the requirement of a high current density diminishes the functional group compatibility with these electrochemical reactions. Here, we demonstrate the photocatalytic decarboxylative transformation of activated carboxylic acids in a switchable and functional group-compatible manner. We discovered that switching between Kolbe-type or Hofer-Moest-type reactions can be accomplished with suitable photocatalysts by controlling the reaction pathways: energy transfer (EnT) and single-electron transfer (SET). The EnT pathway promoted by an organo-photocatalyst yielded 1,2-diarylethane from arylacetic acids, whereas the ruthenium photoredox catalyst allows the construction of an ester scaffold with two arylmethyl moieties via the SET pathway. The resulting radical intermediates were coupled to olefins to realize multicomponent reactions. Consequently, four different products were selectively obtained from a simple carboxylic acid. This discovery offers new opportunities for selectively synthesizing multiple products via switchable reactions using identical substrates with minimal cost and effort.

Cooperative biomass-derived furfural synthesis and H2 evolution in one photoredox cycle over a MnxCd1-xS/Ti3C2 MXene Schottky junction structure

Selective and efficient photocatalytic transformation of biomass-derived platform molecules to high-value-added fuels or chemicals is a great challenge for green chemistry, especially utilizing visible light energy and earth-abundant catalytic materials. In this work, we report a 1D/2D MnxCd1-xS/Ti3C2 (MCSTC-X) Schottky junction structure to achieve efficient photocharge separation/transfer via in-situ interfacial self-assembly strategy. Under visible light illumination, the optimized MCSTC-7.5 catalyst enables the selective conversion of furfuryl alcohol into furfural with a production rate of 764.3 μmol·g−1·h−1, accompanied by water splitting into H2 with a generation rate of 756.5 μmol·g−1·h−1. Benefiting from the internal electric field in the Schottky junction structure, the efficient separation and transfer of photoinduced charge carriers are realized in one photoredox cycle. Moreover, a plausible reaction mechanism for the photocatalytic selective valorization of furfuryl alcohol to furfural and H2 evolution is proposed. Hopefully, this work would open a new avenue for sustainable and practical production of solar chemicals and fuels by rationally designing 1D/2D Schottky junction.

Mass spectrometric monitoring of ligand-bridged hot electron transfer and anaerobic oxidization on auto renewable droplet-based plasmonic nanoreactors under visible light illumination

The light induced hot-electron on plasmonic nanostructures has been recognized as a breakthrough discovery for photovoltaic and photocatalytic applications. With mass spectrometry, we demonstrate the dynamics of hot electron transfers of anaerobic oxidization reactions on Au decorated TiO2 plasmonic nanoparticles, which were coated on the inner surface of a flask. Those nanoparticles were covered by continuously renewed liquid droplets of solvent and reactants that were transported through a Venturi jet mixer with auto-spray. In addition to intensive mass transfer in such droplet-based nanoreactors, as well as strong adsorption of reactants and rapid desorption of products on materials surfaces, the localized surface plasmon resonance (LSPR) excitation upon visible light illumination, by which accumulated energies of plasmons are transferred to electrons in the conduction band of the material, attributes to the efficient photocatalytic transformation. Mass spectrometric detection of intermediate radical anions and negative ions with stable isotope labeling unambiguously identifies that highly energetic hot electrons can escape from the plasmonic nanostructures, be collected by adsorbed molecules, and initiate bond cleavages. It was demonstrated that losses of two H atoms result in the anaerobic oxidization of each benzyl alcohol molecule to a benzyl aldehyde molecule in the absence of molecular oxygen with more than 90 % yields. The well recyclable plasmonic nanoreactors implicate the injection of transferred electrons eventually back to electronically depleted Au+ positive ions. Bridged by adsorbed molecules, electrons were repeatedly circulated back and forth in plasmonic nanoreactors, where the collected light was eventually converted into chemical energy.

Zn-Cr Layered Double Hydroxides for Photocatalytic Transformation of CO2 under Visible Light Irradiation: The Effect of the Metal Ratio and Interlayer Anion

Carbon dioxide is the main gas responsible for the greenhouse effect. Over the last few years, the research focus of many studies has been to transform CO2 into valuable products (CO, HCOOH, HCHO, CH3OH and CH4), since it would contribute to mitigating global warming and environmental pollution. Layered double hydroxides (LDHs) are two-dimensional materials with high CO2 adsorption capacity and compositional flexibility with potential catalytic properties to be applied in CO2 reduction processes. Herein, Zn-Cr LDH-based materials with different metal ratio and interlayer anions, i.e., chloride (Cl−), graphene quantum dots (GQDs), sodium dodecyl sulfate (SDS) and sodium deoxycholate (SDC), have been prepared by a co-precipitation method and characterized by different techniques. The influence of the interlayer inorganic and organic anions and the metal ratio on the application of Zn-Cr LDHs as catalysts for the photocatalytic CO2 reduction reaction under visible light irradiation is unprecedentedly reported. The catalytic tests have been carried out with Ru(bpy)32+ as photosensitizer (PS) and triethanolamine as sacrificial electron donor (ED) at λ = 450 nm. All LDHs materials exhibited good photocatalytic activity towards CO. Among them, LDH3-SDC showed the best catalytic performance, achieving 10,977 µmol CO g−1 at 24 h under visible light irradiation with a CO selectivity of 88%. This study provides pertinent findings about the modified physicochemical features of Zn-Cr LDHs, such as particle size, surface area and the nature of the interlayer anion, and how they influence the catalytic activity in CO2 photoreduction.

Value-added conversion of ethanol on morphologically controlled and defect-engineered titanium dioxide nanorods.

Developing an environmentally benign and highly effective strategy for the value-added conversion of biomass platform molecules such as ethanol has emerged as a significant challenge and opportunity. This challenge stems from the need to harness renewable solar energy and conduct thermodynamically unfavorable reactions at room temperature. To tackle this challenge, one-dimensional titanium dioxide photocatalysts gave been designed and fabricated to achieve a remarkable photocatalytic selectivity of almost 100% for transforming ethanol into value-added 1,1-diethoxyethane, contrasting the primary production of acetaldehyde in titanium dioxide nanoparticles. By incorporating a Pt co-catalyst and infusing oxygen vacancies into the one-dimensional catalyst, the ethanol transformation rate has doubled to be 128.78 mmol/g/h with respect to that of its unmodified counterpart, about 66.66 mmol/g/h. The underlying mechanism for this increased conversion and selectivity resides in the narrowed bandgap of the catalyst and the prolonged lifetime of the photo-generated carriers. This research offers a promising strategy that intertwines morphological control and defect engineering, offering a promising way for the photocatalytic transformation of essential biomass platform molecules.

Photocatalytic dehydrogenative coupling of silanes with alcohols triggered by light-induced sulfur vacancies on CdS nanosheets

Photocatalytic organosilicon transformation is a green and emerging synthetic strategy toward silyl ethers production. This approach takes advantage of the highly reactive photogenerated electron-hole pairs to trigger redox reactions for chemical transformations under mild conditions. However, the development of heterogenous photocatalysts for silyl ethers synthesis is greatly hampered by the lack of sufficient reactive sites on the catalyst surface for the adsorption and activation of silanes. Herein, we demonstrate that the light-induced sulfur vacancies (SVs) in-situ generated on ultrathin CdS nanosheet (CdS-NS) can function as typical Lewis acid sites to adsorb and activate silanes via the formation of SiH···Cd coordination species. In addition, these electron-deficient SVs can also trap the photogenerated electrons to promote charge separation. The CdS-NS with SVs exhibits an excellent photocatalytic performance toward silyl ethers synthesis while producing H2 simultaneously. This synthetic strategy based on in-situ creation of abundant Lewis acid sites on photocatalyst surface to activate silanes will have broad implications in the development of high-performance photocatalysts for organosilicon transformations.

Construction of graphene supported dual-phase perovskite quantum dots for efficient photocatalytic CO2 reduction

Lead halide perovskites (LHPs) quantum dots (QDs) has been deemed as a promising material for photocatalytic conversion (e.g., CO2 reduction or water splitting) due to their remarkable photoelectrical properties. However, the severe carrier recombination and low charge transfer efficiency largely restrain their photocatalytic performance. Herein, a series of graphene supported dual-phase perovskite QDs photocatalysts were designed by a facile hot-injection strategy for highly efficient photocatalytic CO2 reduction. Catalytic results showed that under a 5 W white LED irradiation, the CsPbBr3/Cs4PbBr6@G5% composite with 5 wt% graphene loading exhibited an optimum CO yield of 18.76 μmol g−1 with a selectivity of 52.99% towards the product of CO for photocatalytic CO2 reduction. The enhanced photocatalytic CO2 reduction performance over the composites compared with that of CsPbBr3, Cs4PbBr6 and CsPbBr3/Cs4PbBr6 QDs could be ascribed to the type-I heterojunction structure and graphene support as the electron acceptor in the composite, which could stepwise accelerate the charge separation, suppresses recombination process, and provide more active sites for CO2 adsorption and reaction. This work would bring new insight in designing novel efficient perovskite-based catalysts for photocatalytic CO2 transformation.

Enhanced Plasmonic Hot Electron Transfer on Aucore‐Agshell Nanoparticles under Visible‐Light Irradiation

AbstractPlasmonic photocatalysis under visible‐light irradiation has long been regarded as a very promising strategy for inducing chemical transformations. However, the efficient utilization of these hot electrons on monometallic nanoparticles to induce chemical reaction remains a challenging subject. Here, we study plasmonic hot electron activity of Aucore‐Agshell bimetallic nanoparticles towards the four‐electron reduction of 4‐nitrothiophenol to 4,4′‐dimercaptoazobenzene. Our results show that Aucore‐Agshell nanoparticles possess a higher catalytic activity than pure Au and Ag nanoparticles and the photocatalytic transformation is strongly dependent on the thickness of Ag shell. The plasmonic catalytic activity could be explained by a boosting hot‐electron‐hole separation driven by the contact potential at the bimetallic interface. This work provides new opportunities to enhance the efficient utilization of hot electron for plasmonic photocatalysis reaction.