Homogeneous Photocatalyst Research Articles

Triazine Functionalized Porous Covalent Organic Framework for Photo-organocatalytic E- Z Isomerization of Olefins.

Visible light-mediated photocatalytic organic transformation has drawn significant attention as an alternative process for replacing thermal reactions. Although precious metal/organic dyes based homogeneous photocatalysts have been developed, their toxic and nonreusable nature makes them inappropriate for large-scale production. Therefore, we have synthesized a triazine and a keto functionalized nonmetal based covalent organic framework (TpTt) for heterogeneous photocatalysis. As the catalyst shows significant absorption of visible light, it has been applied for the photocatalytic uphill conversion of trans-stilbene to cis-stilbene in the presence of blue light-emitting diodes with broad substrate scope via an energy transfer process.

Porphyrinic Metal–Organic Frameworks Installed with Brønsted Acid Sites for Efficient Tandem Semisynthesis of Artemisinin

Artemisinin, an essential antimalarial drug, requires a synthetic pathway that has a high environmental and financial cost. Conventional homogeneous photocatalysts and acid catalysts usually suffer...

Polyoxometalates and Their Composites as Photocatalysts for Organic Pollutants Degradation in Aqueous Media—A Review

Polyoxometalates (POMs), eco‐friendly solid acids, are polyatomic anions containing transition metal (group 5 or group 6) oxyanions linked together through shared oxygen atoms. First, the structure, classification, synthesis, non‐photocatalytic applications, and their photocatalytic properties and applications are presented. POMs are very interesting due to their unique and remarkable characteristics. One of the most attractive characteristics of POMs is their activity as an electron relay, that is, they can undergo stepwise multi‐electron redox reactions while their construction remains intact. Based on this characteristic, POMs have been widely used as photooxidation catalysts for organic and inorganic contaminants removal. According to the literature, POMs are useful homogeneous photocatalysts for the mineralization of dyes, phenol and its derivatives, chloroacetic acids, and pesticides and wood pulp bleaching. Applications of POMs have been limited because of their high solubility in water. Immobilization and solidification of POMs using different methods and various supports for making them reusable are studied in this review. Varieties of composites of POMs are synthesized to make them photorespond to the visible light region.

Cover Feature: Iron Aquo Complex as an Efficient and Selective Homogeneous Photocatalyst for Organic Synthetic Reactions (ChemCatChem 20/2018)

Cover Feature: Iron Aquo Complex as an Efficient and Selective Homogeneous Photocatalyst for Organic Synthetic Reactions (ChemCatChem 20/2018)

Iron Aquo Complex as an Efficient and Selective Homogeneous Photocatalyst for Organic Synthetic Reactions

AbstractOwing to the inexpensiveness and non‐toxicity of iron, development of highly active and simple homogeneous iron photocatalysts is a crucially important, while challenging, for practical organic synthetic reactions. Here we report that iron(III) nitrate, which is one of the most typical iron salts and can form iron aquo complexes in aqueous and organic solutions, can be used as a highly efficient and selective homogeneous photocatalyst for fine chemical synthesis. We demonstrated the iron aquo complexes catalyzed the oxidation of cyclohexane into cyclohexanol and cyclohexanone with a turnover number (TON) of 88 and a nearly 100 % selectivity using O2 under irradiation of solar light, whereas existing TiO2 and α‐Fe2O3 heterogeneous photocatalysts showed low TON and no activity. The iron aquo complexes impregnated within a mesoporous silica showed the improved TON of up to 100 while maintaining nearly 100 % selectivity.

A Mononuclear Tungsten Photocatalyst for H2 Production

We report herein a mononuclear, homogeneous photocatalyst for H2 production with sunlight. The synthesis and characterization of a (pyridyl)-N-heterocyclic carbene tungsten tetracarbonyl complex W(pyNHC)(CO)4 is described, and its application as a precatalyst for photocatalytic generation of H2 is evaluated. Electrochemical and photophysical studies were used to characterize and evaluate the precatalyst and in situ generated catalyst [W(pyNHC)(CO)3] for the visible-light-driven production of H2 in the presence of triflic acid and decamethylferrocene without an additional photosensitizer. Under irradiation with a solar-simulated spectrum, a catalyst turnover number (TON) of >17 in 3 h of reaction time is observed for the production of H2 with this system, which compares favorably to a prior reported (multinuclear) homogeneous photocatalyst using visible light (4 TON). Photonic energy was found to be necessary to access the active catalysts from the precatalyst and in the catalytic cycle. A mechanism is detailed on the basis of a combined photophysical and computational approach.

Homogeneous photocatalytic Fe3+/Fe2+ redox cycle for simultaneous Cr(VI) reduction and organic pollutant oxidation: Roles of hydroxyl radical and degradation intermediates.

The sustained oxidation of aqueous organic pollutants using hydroxyl radicals (HO) generated in the UV-irradiated solution of ferric ions was investigated in the presence of Cr(VI). The synergistic effect of simultaneous 4-chlorophenol (4-CP) oxidation and Cr(VI) reduction is explained in terms of the various roles of OH radical, degradation intermediates, and Fe3+/Fe2+ redox cycle. The photolysis of FeIII(OH)2+ generates OH radical which degrades the organic substrate. The reduction of Cr(VI) was inhibited by the OH radical-induced re-oxidation of Cr(III) in the absence of 4-CP. The complete removal of Cr(VI) was achieved only in the presence of phenolic substrates which not only reacts with OH radical (hence inhibiting the reoxidation of Cr(III)) but also generates reducing intermediates which effectively reduce Cr(VI). Fe2+ also converted Cr(VI) to Cr(III) with regenerating Fe3+, which makes the overall process photocatalytic. The photocatalytic activity for the simultaneous removal of 4-CP and Cr(VI) was largely maintained up to five cycles. Such simultaneous and synergic photoactivity was also observed for other phenolic compounds (4-bromophenol, 4-nitrophenol, phenol). The simultaneous and synergic removal of phenolic compounds and Cr(VI) can be enabled through the redox couple of Fe3+/Fe2+ working as a homogeneous photocatalyst.

Heterogeneous Single-Atom Catalyst for Visible-Light-Driven High-Turnover CO2 Reduction: The Role of Electron Transfer.

Visible-light-driven conversion of CO2 into chemical fuels is an intriguing approach to address the energy and environmental challenges. In principle, light harvesting and catalytic reactions can be both optimized by combining the merits of homogeneous and heterogeneous photocatalysts; however, the efficiency of charge transfer between light absorbers and catalytic sites is often too low to limit the overall photocatalytic performance. In this communication, it is reported that the single-atom Co sites coordinated on the partially oxidized graphene nanosheets can serve as a highly active and durable heterogeneous catalyst for CO2 conversion, wherein the graphene bridges homogeneous light absorbers with single-atom catalytic sites for the efficient transfer of photoexcited electrons. As a result, the turnover number for CO production reaches a high value of 678 with an unprecedented turnover frequency of 3.77 min-1 , superior to those obtained with the state-of-the-art heterogeneous photocatalysts. This work provides fresh insights into the design of catalytic sites toward photocatalytic CO2 conversion from the angle of single-atom catalysis and highlights the role of charge kinetics in bridging the gap between heterogeneous and homogeneous photocatalysts.

Encapsulating [Mo3S13]2- clusters in cationic covalent organic frameworks: enhancing stability and recyclability by converting a homogeneous photocatalyst to a heterogeneous photocatalyst.

We encapsulated anionic [Mo3S13]2- clusters in cationic COFs (EB-COF) to obtain the novel composite photocatalytic material Mo3S13@EB-COF. Comprehensive studies indicated that the Mo3S13@EB-COF was the product of a successful conversion of a homogeneous to a heterogeneous catalyst, and exhibited excellent stability and recyclability as well as a remarkable photocatalytic hydrogen evolution rate of 13 215 μmol g-1 h-1 under visible-light irradiation over the course of 18 hours.

High throughput photo-oxidations in a packed bed reactor system

The efficiency gains produced by continuous-flow systems in conducting photochemical transformations have been extensively demonstrated. Recently, these systems have been used in developing safe and efficient methods for photo-oxidations using singlet oxygen generated by photosensitizers. Much of the previous work has focused on the use of homogeneous photocatalysts. The development of a unique, packed-bed photoreactor system using immobilized rose bengal expands these capabilities as this robust photocatalyst allows access to and elaboration from these highly useful building blocks without the need for further purification. With this platform we were able to demonstrate a wide scope of singlet oxygen ene, [4+2] cycloadditions and heteroatom oxidations. Furthermore, we applied this method as a strategic element in the synthesis of the high-volume antimalarial artemisinin.

(Invited) Hybrid Photocatalysts Composed of Titania Modified with Plasmonic Nanoparticles and Ruthenium Complexes for Photocatalytic Decomposition of Organic Compounds

Wide-bandgap semiconductors modified with nanoparticles (NPs) of noble metals have been extensively studied for more than forty years owing to possible enhanced photocatalytic activity under UV irradiation since NPs of noble metals work as an electron sink hindering the recombination of charge carriers. The research has intensified over the last decade for photocatalytic activity under visible-light irradiation due to localized surface-plasmon resonance (LSPR) of noble metals, and those materials have been named plasmonic photocatalysts. The mechanism under visible-light irradiation (electron and/or energy transfer) as well as the enhancement of photocatalytic performance (activity and stability) have been intensively examined. Although the photocatalytic activities of plasmonic photocatalysts have been confirmed for various environmental applications such as environmental purification and solar-energy conversion, the photocatalytic activities under visible-light irradiation are much lower (ca. 1-2 orders in magnitude) than that under UV. Therefore, the present study focuses on improvement of photocatalytic activity for possible commercial applications. In our studies, plasmonic photocatalysts composed of commercial or self-synthesized titania and NPs of silver, gold and copper have been examined. In general, the broader the LSPR peak was, the higher was the photocatalytic activity under visible-light irradiation. Various strategies for improvement of the photocatalytic performance have been applied, e.g., (i) enlargement of the interface between titania and noble-metal NPs, (ii) preparation of bimetallic plasmonic photocatalysts, (iii) the use of faceted anatase NPs (octahedral and decahedral), and (iv) preparation of hybrid photocatalysts composed of heterogeneous and homogeneous photocatalysts, i.e., plasmonic photocatalysts and ruthenium complexes. Hybrid photocatalysts were prepared by deposition of gold/silver NPs on commercial titania particles with different structural properties. Ruthenium(II) (Ru(II)) complexes with carboxylic and phosphoric acid-binding groups were synthesized and adsorbed on bare titania and noble metal-modified titania particles. The structural properties of the samples were characterized by diffuse-reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), thin layer chromatography (TLC), proton nuclear magnetic-resonance spectroscopy (1H-NMR) and two-dimensional homonuclear correlation spectroscopy (2D-COSY). Large surface area, small crystallite sizes, low pH value, nature of the deprotected phosphonate binding groups, and pre-deposited noble-metal NPs enhanced the adsorption yield. Modification caused titania activation toward visible light (> 450 nm) for 2-propanol oxidation and for methanol dehydrogenation under UV/vis irradiation. The modified samples exhibited enhanced activity under UV/vis irradiation for acetic-acid oxidation depending on the kind of modifiers and properties of titania, i.e., 2–6, 3–9 and 1–3-fold enhancement was observed after modification with gold NPs and silver NPs with Ru(II) complex, respectively. The time-resolved microwave conductivity (TRMC) method proved that higher photocatalytic activity of modified titania under UV irradiation was caused by scavenging of mobile electrons by noble-metal NPs, and therefore decreasing the recombination of charge carriers. The photocatalytic activity of hybrid photocatalysts under UV/vis irradiation was influenced by the nature of the plasmonic metal and structural properties of the metal and titania particles, e.g., crystallite size and polymorphic form. Different kinds of action were observed for two plasmonic metals in two reaction systems under UV irradiation, e.g., modification with an Ru(II) complex caused 12-times faster dehydrogenation of methanol for silver-modified large anatase titania (ST41) and hardly changed the activity of gold-modified samples, while during acetic acid oxidation, only hybrid photocatalysts composed of gold NPs and Ru(II) complex exhibited a slight increase of photocatalytic activity (1.1-1.3 times). The difference between gold and silver hybrid photocatalysts might have been caused by differences in surface charges of metallic deposits, i.e., the surface of silver NPs was mainly positively charged, while gold was zero-valent. It was proposed that Ru(II) complex bound also to the surface of positively charged silver. Therefore, the sequence of modification (Ru(II) complex adsorption or gold/silver deposition) was investigated to check its influence on the resultant properties and thus photocatalytic performance. Different distributions of gold particle sizes and chemical compositions were obtained for the hybrid photocatalysts prepared with opposite sequence. It was found that photocatalytic activities depended on the range of used irradiation (UV/vis or vis) and the kind of modifiers in different ways. Under visible-light irradiation, fine titania modified with Ru(II) complex exhibited the highest level of photocatalytic activity. The presence of Ru(II) complex highly enhanced photocatalytic activity of titania modified with plasmonic NPs. However, NPs of plasmonic metals hindered the photocatalytic activity of Ru(II)-titania.

Photocatalytic Oxidation of Sulfinates to Vinyl Sulfones with Cyanamide‐Functionalised Carbon Nitride

The cyanamide‐functionalised carbon nitride (NCN‐CNx) can be employed as a photo‐redox catalyst for the light‐induced sulfonylation of alkenes with sulfinate salts, attaining product yields twice those obtained with the non‐functionalised counterpart and comparable to those achieved with the benchmark homogeneous photocatalyst Eosin Y. NCN‐CNx as a heterogeneous photocatalyst is easily isolatable and can be re‐used for this reaction for over three cycles. This photocatalytic reaction can proceed with various sulfinates (aryl‐, alkyl‐ and thiophene‐based) and alkenes with electron‐donating and ‐withdrawing groups, obtaining good to excellent product yields. A reductive quenching route was determined to be the likely mechanism for this photocatalyst.

Cover Picture: A Dinuclear Cobalt Cryptate as a Homogeneous Photocatalyst for Highly Selective and Efficient Visible‐Light Driven CO2 Reduction to CO in CH3CN/H2O Solution (Angew. Chem. Int. Ed. 3/2017)

Cover Picture: A Dinuclear Cobalt Cryptate as a Homogeneous Photocatalyst for Highly Selective and Efficient Visible‐Light Driven CO₂ Reduction to CO in CH₃CN/H₂O Solution (Angew. Chem. Int. Ed. 3/2017)

A Dinuclear Cobalt Cryptate as a Homogeneous Photocatalyst for Highly Selective and Efficient Visible‐Light Driven CO2 Reduction to CO in CH3CN/H2O Solution

AbstractA dinuclear cobalt complex [Co2(OH)L1](ClO4)3 (1, L1=N[(CH2)2NHCH2(m‐C6H4)CH2NH(CH2)2]3N) displays high selectivity and efficiency for the photocatalytic reduction of CO2 to CO in CH3CN/H2O (v/v=4:1) under a 450 nm LED light irradiation, with a light intensity of 100 mW cm−2. The selectivity reaches as high as 98 %, and the turnover numbers (TON) and turnover frequencies (TOF) reach as high as 16896 and 0.47 s−1, respectively, with the calculated quantum yield of 0.04 %. Such high activity can be attributed to the synergistic catalysis effect between two CoII ions within 1, which is strongly supported by the results of control experiments and DFT calculations.

A Dinuclear Cobalt Cryptate as a Homogeneous Photocatalyst for Highly Selective and Efficient Visible-Light Driven CO2 Reduction to CO in CH3 CN/H2 O Solution.

A dinuclear cobalt complex [Co2 (OH)L1 ](ClO4 )3 (1, L1 =N[(CH2 )2 NHCH2 (m-C6 H4 )CH2 NH(CH2 )2 ]3 N) displays high selectivity and efficiency for the photocatalytic reduction of CO2 to CO in CH3 CN/H2 O (v/v=4:1) under a 450 nm LED light irradiation, with a light intensity of 100 mW cm-2 . The selectivity reaches as high as 98 %, and the turnover numbers (TON) and turnover frequencies (TOF) reach as high as 16896 and 0.47 s-1 , respectively, with the calculated quantum yield of 0.04 %. Such high activity can be attributed to the synergistic catalysis effect between two CoII ions within 1, which is strongly supported by the results of control experiments and DFT calculations.

Disentangling the Photocatalytic Hydrogen Evolution Mechanism of One Homogeneous Cobalt-Coordinated Polymer

One novel cobalt-coordinated graphitic carbon nitride-type polymer (Co-g-CN) integrating the advantages of both molecular catalytic efficiency and nano semiconductor stability was fabricated, which served as homogeneous photocatalyst exhibiting superior hydrogen evolution efficiency (ca. ∼ 12.3 mmol g–1 h–1) under visible light irradiation in the absence of noble metal cocatalyst. Various techniques including laser photolysis and electron paramagnetic resonance were combined to disentangle the underlying photocatalytic mechanism, which suggested that, unlike nano semiconducting catalysis, the multivalent Co metal center of the polymer mediated the electron transfer process, directly got involved in the proton reduction by sequentially exchanging electrons in a way similar to those molecular coordinated catalysts. These findings provide useful insight into the photocatalytic mechanism of the metal center-mediated water splitting process, and the employment of an economical non-noble metal-coordinated polym...

Photocatalytic hydrogen evolution by iridium(III)–rhodium(III) system: Effect of the free ligand addition

Rh(III) complexes of the general form {RhL4Cl2} with pyridine derivatives ligands (L) were used for the first time as water reduction catalysts in photocatalytic hydrogen generation process in pair with [Ir(ppy)2bpy]+ complex as a photosynthesizer. The efficiency of given Ir(III)–Rh(III) systems in the photocatalytic hydrogen generation process was found to be on the level of 20–30% which is corresponding to the highest currently achieved quantum efficiency values for homogeneous photocatalysts. Fast deactivation of the systems due to destruction of {RhL4Cl2} units was observed but it was suppressed via addition of extra amount of free L ligand in the reaction mixture. Quantum efficiency of modified in this manner [Ir(ppy)2bpy]+/{RhL4Cl2} systems achieved up to 80% value.

A novel hexanuclear titanium(iv)-oxo-iminodiacetate cluster with a Ti6O9 core: single-crystal structure and photocatalytic activities.

A new family of hexanuclear titanium(iv)-oxo-carboxylate cluster K7H[Ti6O9(ida)6]Cl2·13H2O {Ti6O9} has been synthesized via the H2O2-assisted reaction between TiCl4 and iminodiacetate ligands. This cluster was fully characterized by single-crystal X-ray diffraction and a wide range of analytical methods, including FT-IR, UV/vis spectroscopy as well as electrochemistry and thermogravimetric analysis. As a new type of carboxylate substituted Ti-oxo-cluster, the structural motif of the {Ti6O9} cluster consists of one symmetric {Ti6O6} hexagonal prism with two staggered triangular {Ti3O3} subunits linked by three μ2-O bridges. The {Ti6O9} polyanions are linked by K(+) cations to form a novel 3D architecture. The structural information and stability of the {Ti6O9} polyanion in aqueous solution were thoroughly investigated by solid-state/solution NMR, ESI-MS spectroscopy. Moreover, this Ti-oxo cluster exhibits remarkable potential as a visible-light homogeneous photocatalyst for degradation of rhodamine B (RhB). Finally, a proposed peroxotitanium(iv)-mediated photocatalytic pathway involved is illustrated by spectroscopic data.

Improving the photocatalytic reduction of CO2 to CO through immobilisation of a molecular Re catalyst on TiO2.

The photocatalytic activity of phosphonated Re complexes, [Re(2,2′-bipyridine-4,4′-bisphosphonic acid) (CO)3(L)] (ReP; L=3-picoline or bromide) immobilised on TiO2 nanoparticles is reported. The heterogenised Re catalyst on the semiconductor, ReP–TiO2 hybrid, displays an improvement in CO2 reduction photocatalysis. A high turnover number (TON) of 48 molCO molRe−1 is observed in DMF with the electron donor triethanolamine at λ>420 nm. ReP–TiO2 compares favourably to previously reported homogeneous systems and is the highest TON reported to date for a CO2-reducing Re photocatalyst under visible light irradiation. Photocatalytic CO2 reduction is even observed with ReP–TiO2 at wavelengths of λ>495 nm. Infrared and X-ray photoelectron spectroscopies confirm that an intact ReP catalyst is present on the TiO2 surface before and during catalysis. Transient absorption spectroscopy suggests that the high activity upon heterogenisation is due to an increase in the lifetime of the immobilised anionic Re intermediate (t50 %>1 s for ReP–TiO2 compared with t50 %=60 ms for ReP in solution) and immobilisation might also reduce the formation of inactive Re dimers. This study demonstrates that the activity of a homogeneous photocatalyst can be improved through immobilisation on a metal oxide surface by favourably modifying its photochemical kinetics.

Unraveling Side Reactions in the Photocatalytic Reduction of CO2: Evidence for Light‐Induced Deactivation Processes in Homogeneous Photocatalysis

AbstractPhotocatalytic reduction of CO2 with rhenium(I) bipyridine complexes has been studied for several decades. Nonetheless, important parameters affecting the catalytic performance remain elusive to date. By using the standard catalyst [Re(dmb)(CO)3Cl] (dmb=4,4′‐dimethyl‐2,2′‐bipyridine), the effect of catalyst concentration and irradiation intensity is studied in detail and important correlations are revealed. The decomposition of the catalyst is investigated, and two main deactivation pathways are proposed, both of which involve the one‐electron‐reduced species and are likely to be valid for other homogeneous photocatalysts as well. The rate of deactivation is linked to the relative concentration of 1) the catalyst in its electronic ground state, 2) the catalyst in its excited state, 3) the one‐electron‐reduced species, and 4) quencher radicals. Adequate tuning of catalyst concentration and irradiation intensity leads to the highest quantum yield (Φ=0.53) reported to date for a single‐molecule system.