Sacrificial Electron Acceptor Research Articles

A Cobalt Complex from Terpyridine‐Based Peptoid as an Efficient Catalyst for Visible Light Driven Water Oxidation

AbstractWe report on the first peptoid‐based catalyst for visible light driven water oxidation, the complex TPT2Co. Peptoids are N‐substituted glycine oligomers, and TPT is a peptoid trimer incorporating terpyridine ligand, an ethanolic group as a proton acceptor and a non‐catalytic benzyl group. We explored the ability of TPT2Co to perform as a catalyst for water oxidation in phosphate buffer at pH 9.5, in the presence of the photosensitizer [Ru(bpy)3]2+ and the sacrificial electron acceptor Na2S2O8. We demonstrated that TPT2Co can catalyze water oxidation by a blue LED with an intensity of only 1.5 mW/cm2 towards oxygen production at pH 9.5 with a maximal TON of 28.1 in 45 min. The stability of TPT2Co during the light‐driven water oxidation as well as its ability to stabilize the photosensitizer were confirmed by a combination of UV‐Vis, IR and ESI‐MS analysis.

Proximity-Enabled Photochemical C-H Functionalization using a Covalent Organic Framework-Confined Fe2IV-μ-oxo Species in Water.

Water has been recognized as an excellent solvent for maneuvering both the catalytic activity and selectivity, especially in the case of heterogeneous catalysis. However, maintaining the active catalytic species in their higher oxidation states (IV/V) while retaining the catalytic activity and recyclability in water is an enormous challenge. Herein, we have developed a solution to this problem using covalent organic frameworks (COFs) to immobilize the (Et4N)2[FeIII(Cl)bTAML] molecules, taking advantage of the COF's morphology and surface charge. By using the visible light and [CoIII(NH3)5Cl]Cl2 as a sacrificial electron acceptor within the COF, we have successfully generated and stabilized the [(bTAML)FeIV-O-FeIV(bTAML)]- species in water. The COF backbone simultaneously acts as a porous host and a photosensitizer. This is the first time that the photochemically generated Fe2IV-μ-oxo radical cation species has demonstrated high catalytic activity with moderate to high yield for the selective oxidation of the unactivated C-H bonds, even in water. To enhance the catalytic activity and achieve good recyclability, we have developed a TpDPP COF film by transforming the TpDPP COF nanospheres. We have achieved the regio- and stereoselective functionalization of unactivated C-H bonds of alkanes and alkenes (3°:2° = 102:1 for adamantane with the COF film), which is improbable in homogeneous conditions. The film exhibits C-H bond oxidation with higher catalytic yield (32-98%) and a higher degree of selectivity (cis/trans = 74:1; 3°:2° = 100:1 for cis-decalin).

Photocurrent Generation Properties of Half-Photocell Consisting of Electropolymerized Hierarchical Polythiophene Thin Films as Photoexciting Electron Donor and C60 as Electron Acceptor

Polybithiophene/poly(3,4-ethylenedioxythiophene)/indium-tin-oxide coated transparent glass electrodes were prepared by sequential electrochemical polymerization with cyclic voltammetry. The thickness of polybithiophene films was varied by the scan rate of applied potentials during polymerization. Obtained hierarchical polythiophene films were characterized by absorption spectra, fluorescence spectra, scanning electron microscopy, laser microscopy, and atomic force microscopy. Half-photocell properties of the hierarchical polythiophene thin films were evaluated by a three-electrode electrochemical cell in CH2Cl2 solution of electrolyte and C60 as a sacrificial electron acceptor under irradiation of monochromatic light. The thickness and absorbance of hierarchical polythiophene thin films increased by decreasing of scan rate of applied potentials during the polymerization of polybithiophene. While, maximum peaks of fluorescence intensity and internal quantum yield of cathodic photocurrent were observed at 20 nm thickness of polythiophene film under irradiation from the glass side of the transparent electrode. Almost maximum external quantum efficiencies of cathodic photocurrent were observed from 20 nm to 1.5 μm thicknesses of polythiophene films. These tendencies suggest that CH2Cl2 solution of C60 and electrolyte penetrate into the polybithiophene layer up to 1.5 μm.

Multimodale Analyse lichtinduzierter Wasseroxidation in nanoporösen Blockcopolymer‐Membranen**

AbstractDie heterogene lichtgetriebene Katalyse ist ein Eckpfeiler nachhaltiger Energieumwandlung. Die meisten katalytischen Studien konzentrieren sich auf die Analyse von Wasserstoff und Sauerstoff, wobei in der Regel über die Gesamtantwort gemittelt wird, und somit eine Korrelation von Heterogenitäten der Matrix mit unterschiedlichen molekularen Aktivitäten limitiert ist. Hier berichten wir von Untersuchungen eines heterogenisierten Katalysator‐Photosensibilisator‐Systems, bei dem ein Polyoxometalat‐Katalysator zur Wasseroxidation und ein molekularer Modell‐Photosensibilisator gemeinsam in einer nanoporösen Blockcopolymer‐Membran immobilisiert wurden. Mittels elektrochemischer Rastersondenmikroskopie (SECM) wurde die lichtinduzierte Sauerstoffentwicklung mit Natriumperoxodisulfat (Na2S2O8) als Opferelektronenakzeptor an den modifizierten Blockcopolymer‐Membranen lokal bestimmt. Ex situ elementspezifische Analysen erzielten räumlich Informationen über die lokale Konzentration und Verteilung der molekularen Komponenten. Untersuchungen der modifizierten Membranen mittels abgeschwächter Infrarot‐Totalreflexion (IR‐ATR) zeigten keinen Abbau des Wasseroxidationskatalysators unter den beschriebenen lichtinduzierten Bedingungen.

Multimodal Analysis of Light-Driven Water Oxidation in Nanoporous Block Copolymer Membranes.

Heterogeneous light-driven catalysis is a cornerstone of sustainable energy conversion. Most catalytic studies focus on bulk analyses of the hydrogen and oxygen evolved, which impede the correlation of matrix heterogeneities, molecular features, and bulk reactivity. Here, we report studies of a heterogenized catalyst/photosensitizer system using a polyoxometalate water oxidation catalyst and a model, molecular photosensitizer that were co-immobilized within a nanoporous block copolymer membrane. Via operando scanning electrochemical microscopy (SECM), light-induced oxygen evolution was determined using sodium peroxodisulfate (Na2 S2 O8 ) as sacrificial electron acceptor. Ex situ element analyses provided spatially resolved information on the local concentration and distribution of the molecular components. Infrared attenuated total reflection (IR-ATR) studies of the modified membranes showed no degradation of the water oxidation catalyst under the reported light-driven conditions.

Copper-catalyzed electrochemical C–H fluorination

We report the systematic development of an electrooxidative methodology that translates stoichiometric C-H fluorination reactivity of an isolable CuIII fluoride complex into a catalytic process. The critical challenges of electrocatalysis with a highly reactive CuIII species were addressed by the judicious selection of electrolyte, F- source, and sacrificial electron acceptor. Catalyst-controlled C-H fluorination occurs with a preference for hydridic C-H bonds with high bond dissociation energies over weaker but less hydridic C-H bonds. The selectivity is driven by an oxidative asynchronous proton-coupled elelctron transfer (PCET) at an electrophilic CuIII-F complex. We further demonstrate that the asynchronicity factor of hydrogen atom transfer η can be used as a guideline to rationalize the selectivity of C-H fluorination.

Mechanistic Insights into the Charge Transfer Dynamics of Photocatalytic Water Oxidation at the Lipid Bilayer-Water Interface.

Photosystem II, the natural water-oxidizing system, is a large protein complex embedded in a phospholipid membrane. A much simpler system for photocatalytic water oxidation consists of liposomes functionalized with amphiphilic ruthenium(II)-tris-bipyridine photosensitizer (PS) and 6,6'-dicarboxylato-2,2'-bipyridine-ruthenium(II) catalysts (Cat) with a water-soluble sacrificial electron acceptor (Na2S2O8). However, the effect of embedding this photocatalytic system in liposome membranes on the mechanism of photocatalytic water oxidation was not well understood. Here, several phenomena have been identified by spectroscopic tools, which explain the drastically different kinetics of water photo-oxidizing liposomes, compared with analogous homogeneous systems. First, the oxidative quenching of photoexcited PS* by S2O82- at the liposome surface occurs solely via static quenching, while dynamic quenching is observed for the homogeneous system. Moreover, the charge separation efficiency after the quenching reaction is much smaller than unity, in contrast to the quantitative generation of PS+ in homogeneous solution. In parallel, the high local concentration of the membrane-bound PS induces self-quenching at 10:1-40:1 molar lipid-PS ratios. Finally, while the hole transfer from PS+ to catalyst is rather fast in homogeneous solution (kobs > 1 × 104 s-1 at [catalyst] > 50 μM), in liposomes at pH = 4, the reaction is rather slow (kobs ≈ 17 s-1 for 5 μM catalyst in 100 μM DMPC lipid). Overall, the better understanding of these productive and unproductive pathways explains what limits the rate of photocatalytic water oxidation in liposomal vs homogeneous systems, which is required for future optimization of light-driven catalysis within self-assembled lipid interfaces.

Highly Chemoselective Catalytic Photooxidations by Using Solvent as a Sacrificial Electron Acceptor.

Catalyst recovery is an integral part of photoredox catalysis. It is often solved by adding another component-a sacrificial agent-whose role is to convert the catalyst back into its original oxidation state. However, an additive may cause a side reaction thus decreasing the selectivity and overall efficiency. Herein, we present a novel approach towards chemoselective photooxidation reactions based on suitable solvent-acetonitrile acting simultaneously as an electron acceptor for catalyst recovery, and on anaerobic conditions. This is allowed by the unique properties of the catalyst, 7,8-dimethoxy-3-methyl-5-phenyl-5-deazaflavinium chloride existing in both strongly oxidizing and reducing forms, whose strength is increased by excitation with visible light. Usefulness of this system is demonstrated in chemoselective dehydrogenations of 4-methoxy- and 4-chlorobenzyl alcohols to aldehydes without over-oxidation to benzoic acids achieving yields up to 70 %. 4-Substituted 1-phenylethanols were oxidized to ketones with yields 80-100 % and, moreover, with yields 31-98 % in the presence of benzylic methyl group, diphenylmethane or thioanisole which are readily oxidized in the presence of oxygen but these were untouched with our system. Mechanistic studies based on UV-Vis spectro-electrochemistry, EPR and time-resolved spectroscopy measurements showed that the process involving an electron release from an excited deazaflavin radical to acetonitrile under formation of solvated electron is crucial for the catalyst recovery.

Donor-acceptor carbazole-based conjugated microporous polymers as photocatalysts for visible-light-driven H2 and O2 evolution from water splitting

Although novel donor–acceptor conjugated microporous polymers (D-A CMPs) that absorb visible light broadly and allow the efficient isolation of photoinduced electrons and holes are being sought for water splitting photocatalysis, their elusive energy band structures have made it a challenge to produce both H2 and O2 simultaneously. In this paper, for the first time, we develop a series of semiconducting D-A CMPs with various sulfur and nitrogen contents and degrees of conjugation for effective photocatalytic H2 and O2 generation from water. Here, we used a carbazole (Cz) moiety as the donor, and pyrene (Py), triphenyltriazine (TPT), benzothiadiazole (BT), and thiazolylthiazole (TzTz) as the acceptors. The resulting D-A CMPs had large surface areas (up to 1530 m2 g–1) and high thermal stabilities (Td10: up to 623 °C; char yield: up to 83 wt%). Moreover, the Cz-TzTz CMP provided an excellent hydrogen evolution rate (HER), reaching 15.3 mmol g–1 h–1 in the presence of ascorbic acid as the sacrificial electron donor, but without the need for an additional Pt co-catalyst. Interestingly, the Cz-TPT CMP provided an excellent oxygen evolution rate (OER) of 3.38 mmol g–1 h–1 when using AgNO3 as a sacrificial electron acceptor; this value is competitive with those of most previously reported organic photocatalysts. These highly active CMP-based photocatalysts appear to be useful materials for solar energy harvesting and conversion.

Visible-Light Augmented Lithium Storage Capacity in a Ruthenium(II) Photosensitizer Conjugated with a Dione-Catechol Redox Couple.

Controlling redox activity of judiciously appended redox units on a photo-sensitive molecular core is an effective strategy for visible light energy harvesting and storage. The first example of a photosensitizer - electron donor coordination compound in which the photoinduced electron transfer step is used for light to electrical energy conversion and storage is reported. A photo-responsive Ru-diimine module conjugated with redox-active catechol groups in [Ru(II)(phenanthroline-5,6-diolate)3 ]4- photosensitizer can mediate photoinduced catechol to dione oxidation in the presence of a sacrificial electron acceptor or at the surface of an electrode. Under potentiostatic condition, visible light triggered current density enhancement confirmed the light harvesting ability of this photosensitizer. Upon implementation in galvanostatic charge-discharge of a Li battery configuration, the storage capacity was found to be increased by 100 %, under 470 nm illumination with output power of 4.0 mW/cm-2 . This proof-of-concept molecular system marks an important milestone towards a new generation of molecular photo-rechargeable materials.

Electrochemical fabrication of hierarchical thin films consisting of different polythiophenes and change in photoelectric conversion properties with film thickness

Hierarchical polybithiophene/poly(3,4-ethylenedioxythiophene) films with polybithiophene layers of various thicknesses were prepared on an indium tin oxide glass transparent electrode by sequential electrochemical polymerization. In the presence of methyl viologen as a sacrificial electron acceptor, cathodic photocurrents were generated under monochromatic light irradiation. The irradiation light wavelength dependence of the incident photon-to-electron conversion efficiency changed drastically with the thickness of the polybithiophene layer. The main reason for the change in the photoelectric conversion properties was light absorption by the polybithiophene layer.

Square planar Ni(II) thiosemicarbazone complexes as functional models for carbon monoxide dehydrogenase

Two Ni(II) complexes, [Ni(dmoTSCH)Cl] (1) and [Ni(dmoPhTSCH)Cl] (2) of the tridentate thiosemicarbazone ligands diacetylmonooxime thiosemicarbazone (dmoTSCH2) and diacetylmonooxime (4-phenyl)thiosemicarbazone (dmoPhTSCH2) have been synthesized. X-ray crystal structure of [Ni(dmoTSCPhTSCH)Cl] (2) indicates that the Ni(II) assumes a square planar geometry in the complexes, with the ligand coordinated in a monoanionic N,N,S donor mode and the fourth coordination position of Ni(II) is occupied by a chloride ion. Cyclic and differential pulse voltammetric experiments suggest that the Ni(II) complexes can undergo a two electron reduction at about −1.0V. It is shown that the Ni(II) complexes in DMF or DMSO solutions can mimic CO-dehydrogenase activity by oxidizing CO to CO2 in presence of a base like NaOAc and a sacrificial electron acceptor like methyl viologen and the colour of the resultant MV.+ can be used to monitor the reaction.

Cobalt hexacyanoferrate as an effective cocatalyst boosting water oxidation on oxynitride TaON photocatalyst under visible light

Developing efficient cocatalysts that improve water oxidation on photocatalyst particles is highly desirable because water oxidation is the key process in artificial photosynthesis systems and represents the rate-determining step. Prussian blue analogues (PBAs) have recently attracted considerable attention as functional materials, including water oxidation (electro)catalysts, owing to their high controllability in various properties via cation and/or ligand substitutions. However, there are still only a few successful studies on the enhancement of water oxidation on particulate photocatalysts by modification with PBAs. Herein, we report the use of cobalt hexacyanoferrate (KxCo[Fe(CN)6]y·nH2O, denoted as CoHCF) as an effective cocatalyst on a visible-light-responsive oxynitride (TaON) photocatalyst for improving O2 evolution in the presence of various electron acceptors. The loading of CoHCF nanoparticles on the TaON surface via a simple impregnation method clearly enhanced the O2 evolution rate from a basic (pH 8) aqueous solution containing Ag+ as a sacrificial electron donor; the enhancement was comparable to that of the widely-studied cobalt oxyhydroxide (CoOx(OH)y). Importantly, it was revealed that the CoHCF nanoparticles loaded on TaON are chemically stable even under acidic conditions with pH below 3, whereas conventional Co-based cocatalysts such as oxide (CoOx) and oxyhydroxide (CoOx(OH)y) are unstable in these conditions due to their dissolution and thereby lose their functions. Thus, the TaON modified with CoHCF cocatalysts exhibited higher O2 evolution rates than unloaded one in acidic aqueous solutions in the presence of not only sacrificial electron acceptor (Ag+) but also reversible ones (Fe3+, IO3–) under visible light irradiation.

Visible-Light-Driven Water Oxidation on Self-Assembled Metal-Free Organic@Carbon Junctions at Neutral pH.

Sustainable water oxidation requires low-cost, stable, and efficient redox couples, photosensitizers, and catalysts. Here, we introduce the in situ self-assembly of metal-atom-free organic-based semiconductive structures on the surface of carbon supports. The resulting TTF/TTF•+@carbon junction (TTF = tetrathiafulvalene) acts as an all-in-one highly stable redox-shuttle/photosensitizer/molecular-catalyst triad for the visible-light-driven water oxidation reaction (WOR) at neutral pH, eliminating the need for metallic or organometallic catalysts and sacrificial electron acceptors. A water/butyronitrile emulsion was used to physically separate the photoproducts of the WOR, H+ and TTF, allowing the extraction and subsequent reduction of protons in water, and the in situ electrochemical oxidation of TTF to TTF•+ on carbon in butyronitrile by constant anode potential electrolysis. During 100 h, no decomposition of TTF was observed and O2 was generated from the emulsion while H2 was constantly produced in the aqueous phase. This work opens new perspectives for a new generation of metal-atom-free, low-cost, redox-driven water-splitting strategies.

Light-driven oxygen evolution from water oxidation with immobilised TiO2 engineered for high performance

Calcination treatments in the range of 500–900 °C of TiO2 synthesised by the sol–gel resulted in materials with variable physicochemical (i.e., optical, specific surface area, crystallite size and crystalline phase) and morphological properties. The photocatalytic performance of the prepared materials was evaluated in the oxygen evolution reaction (OER) following UV-LED irradiation of aqueous solutions containing iron ions as sacrificial electron acceptors. The highest activity for water oxidation was obtained with the photocatalyst thermally treated at 700 °C (TiO2-700). Photocatalysts with larger anatase to rutile ratio of the crystalline phases and higher surface density of oxygen vacancies (defects) displayed the best performance in OER. The oxygen defects at the photocatalyst surface have proven to be responsible for the enhanced photoactivity, acting as important active adsorption sites for water oxidation. Seeking technological application, water oxidation was accomplished by immobilising the photocatalyst with the highest OER rate measured under the established batch conditions (TiO2-700). Experiments operating under continuous mode revealed a remarkable efficiency for oxygen production, exceeding 12% of the apparent quantum efficiency (AQE) at 384 nm (UV-LED system) compared to the batch operation mode.

(Invited) Particulate Photocatalysts with Perovskite-Type Structures and Their Application in Water Splitting

Sunlight-driven water splitting is studied actively for production of renewable solar hydrogen as a storable and transportable energy carrier. Both the efficiency and scalability of water- splitting systems are essential factors for practical application of renewable solar hydrogen because of the low areal density of solar energy [1]. It is desirable to develop particulate photocatalysts and their reaction systems that efficiently split water, because such systems can be spread over wide areas by inexpensive processes potentially. The author’s group has studied various oxide and oxynitride semiconductors as photocatalysts for water splitting. In my talk, recent progress in photocatalytic materials and reactors will be presented with focus on perovskite-type materials.SrTiO3 loaded with proper cocatalysts has been known to be active in overall water splitting under ultraviolet light irradiation since 1980s. Recently, the apparent quantum yield (AQY) of this photocatalyst in overall water splitting has been improved drastically [2]. The author's group has found that doping Al3+ into the titanium site of SrTiO3 boosts the water splitting activity by two orders of magnitude [3]. By refining the preparation conditions of the Al-doped SrTiO3 (SrTiO3:Al) photocatalyst and the loading conditions of cocatalysts working as hydrogen and oxygen evolution sites, the AQY of overall water splitting has reached more than 90% at 365 nm [2], equivalent to an internal quantum efficiency of almost unity. This quantum efficiency is the highest yet reported and indicate that a particulate photocatalyst can drive the greatly endergonic overall water splitting reaction at a quantum efficiency comparable to values obtained from photon-to-chemical or photon-to-current conversion in photosynthesis or photovoltaic systems, respectively.Photocatalyst sheets based on SrTiO3:Al contained in a panel-type reactor split water into hydrogen and oxygen and release gas bubbles at a rate corresponding to a solar-to-hydrogen energy conversion efficiency of 10% under intense ultrviolet illumination even when the water depth is merely 1 mm [4]. Moreover, the photocatalyst can maintain 80% of its initial activity during 1300 h of constant simulated sunlight irradiation at ambient pressure with appropriate surface modifications [5]. A prototype 1-m2-sized panel reactor containing SrTiO3:Al photocatalyst sheets splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. A solar hydrogen production system with a greater size (100 m2) was recently built, and its performance and system characteristics are under investigation.SrTiO3:Al harvests only the ultraviolet light of the sunlight. It is essential to develop photocatalysts active under visible light irradiation. BaTaO2N is a visible light that can evolve hydrogen and oxygen from aqueous solutions containing sacrificial electron donors and acceptors, respectively. Recently, the AQYs of BaTaO2N in these sacrificial reactions have been improved significantly. Highly-crystalline BaTaO2N can be synthesized by directly nitriding a mixture of BaCO3 in the presence of RbCl flux [6]. The resultant BaTaO2N shows decent hydrogen evolution activity when a platinum cocatalyst is loaded by impregnation. By optimizing the cocatalyst loading conditions, the electron transfer from the BaTaO2N photocatalyst to the platinum cocatalyst, and thus the hydrogen evolution activity, was enhanced. The AQY of the resultant Pt-loaded BaTaO2N reached 6.8% at 420 nm in the sacrificial hydrogen evolution reaction. This is exceptionally high among nitride-type photocatalysts with visible light absorption of up to 650 nm.In conventional synthesis, BaTaO2N is nitrided from a mixture of BaCO3, Ta2O5, Ba- and Ta-containing complex oxides, often in the presence of flux. However, these starting materials have different crystal structures from the product BaTaO2N, and so the crystal structures have to be change during the nitridation. This is thought to induce defects in the product material. Therefore, a perovskite-type oxide with a Ba/Ta molar ratio of unity was designed, namely a complex oxide nominally denoted as Na1/4Ba3/4Zn1/4Ta3/4O3 [7]. This oxide can be nitrided into BaTaO2N because Na and Zn components are volatile at high temperature. The resultant BaTaO2N exhibited an AQY of 11.9% at 420 nm in the sacrificial oxygen evolution reaction after loading of a cobalt oxide cocatalyst. Hisatomi et al. Catal. 2019, 2, 387.Takata et al., Nature 2020, 581, 411.Ham et al., Mater. Chem. A 2016, 4, 3027.Goto et al., Joule 2018, 2, 509.Lyu et al. Sci. 2019, 10, 3196.Luo et al., Growth Des. 2020, 20, 255.Jadhav et al. J. Mater. Chem. A. 2020, 8, 1127.

Ligand Controls the Activity of Light-Driven Water Oxidation Catalyzed by Nickel(II) Porphyrin Complexes in Neutral Homogeneous Aqueous Solutions.

Finding photostable, first‐row transition metal‐based molecular systems for photocatalytic water oxidation is a step towards sustainable solar fuel production. Herein, we discovered that nickel(II) hydrophilic porphyrins are molecular catalysts for photocatalytic water oxidation in neutral to acidic aqueous solutions using [Ru(bpy)3]2+ as photosensitizer and [S2O8]2− as sacrificial electron acceptor. Electron‐poorer Ni‐porphyrins bearing 8 fluorine or 4 methylpyridinium substituents as electron‐poorer porphyrins afforded 6‐fold higher turnover frequencies (TOFs; ca. 0.65 min−1) than electron‐richer analogues. However, the electron‐poorest Ni‐porphyrin bearing 16 fluorine substituents was photocatalytically inactive under such conditions, because the potential at which catalytic O2 evolution starts was too high (+1.23 V vs. NHE) to be driven by the photochemically generated [Ru(bpy)3]3+. Critically, these Ni‐porphyrin catalysts showed excellent stability in photocatalytic conditions, as a second photocatalytic run replenished with a new dose of photosensitizer, afforded only 1–3 % less O2 than during the first photocatalytic run.

Ligand Controls the Activity of Light‐Driven Water Oxidation Catalyzed by Nickel(II) Porphyrin Complexes in Neutral Homogeneous Aqueous Solutions

AbstractFinding photostable, first‐row transition metal‐based molecular systems for photocatalytic water oxidation is a step towards sustainable solar fuel production. Herein, we discovered that nickel(II) hydrophilic porphyrins are molecular catalysts for photocatalytic water oxidation in neutral to acidic aqueous solutions using [Ru(bpy)3]2+ as photosensitizer and [S2O8]2− as sacrificial electron acceptor. Electron‐poorer Ni‐porphyrins bearing 8 fluorine or 4 methylpyridinium substituents as electron‐poorer porphyrins afforded 6‐fold higher turnover frequencies (TOFs; ca. 0.65 min−1) than electron‐richer analogues. However, the electron‐poorest Ni‐porphyrin bearing 16 fluorine substituents was photocatalytically inactive under such conditions, because the potential at which catalytic O2 evolution starts was too high (+1.23 V vs. NHE) to be driven by the photochemically generated [Ru(bpy)3]3+. Critically, these Ni‐porphyrin catalysts showed excellent stability in photocatalytic conditions, as a second photocatalytic run replenished with a new dose of photosensitizer, afforded only 1–3 % less O2 than during the first photocatalytic run.

Revisiting Dinuclear Ruthenium Water Oxidation Catalysts: Effect of Bridging Ligand Architecture on Catalytic Activity.

An attractive catalytic pathway for the conversion of water to oxygen would involve two metal oxide centers combining in a constructive sense to make O═O. This prospect makes the study of certain dinuclear transition metal complexes particularly attractive. In this work, we describe the design and synthesis of two symmetrical bis-tridentate polypyridine ligands 6 and 12 that bind two RuII centers at a separation of 3.6 Å in 7 and 5.7 Å in 13. In the presence of CeIV at pH = 1, these systems oxidize water with the system having the more proximal metals being more reactive. In the case of the more proximal metal centers, the bridging ligand is a 3,6-disubstituted pyridazine which, under the influence of CeIV, cleaves into two [Ru(bpc)(pic)2CH3CN]+ fragments (14) which then function as the actual catalyst (bpc = 2,2'-bipyridine-6-carboxylate, pic = 4-methylpyridine). The second dinuclear catalyst contains a central pyrimidine ring which is less sensitive to oxidative decay and hence less reactive. Caution is advised in the use of CeIV as a sacrificial electron acceptor due to unexpected oxidative decay of the catalyst.

Probing P25 TiO2 photocatalysis using photoinduced absorption spectroscopy (PIAS)

Photoinduced absorption spectroscopy, PIAS, is used for the first time to probe the kinetics exhibited by the most commonly employed powder photocatalyst, P25 TiO2, in mesoporous film form, in the photocatalysed oxidation of a commonly used test organic pollutant, 4-chlorophenol, 4CP. The results show that PIAS can provide previously unobtainable, invaluable, direct kinetic, and mechanistic, information concerning photogenerated holes in powdered photocatalysts.