Photocatalytic CO2 Reduction Using Homogeneous Carbon Dots with a Molecular Cobalt Catalyst.

A simple and precious-metal free photosystem for the reduction of aqueous CO2 to syngas (CO and H2) is reported consisting of carbon dots (CDs) as the sole light harvester together with a molecular cobalt bis(terpyridine) CO2 reduction co-catalyst. This homogeneous photocatalytic system operates in the presence of a sacrificial electron donor (triethanolamine) in DMSO/H2O solution at ambient temperature. The photocatalytic system exhibits an activity of 7.7 ± 0.2 mmolsyngas gCDs -1 (3.6 ± 0.2 mmolCO gCDs -1 and 4.1 ± 0.1 mmolH2 gCDs -1) after 24 hours of full solar spectrum irradiation (AM 1.5G). Spectroscopic and electrochemical characterization supports that this photocatalytic performance is attributed to a favorable association between CDs and the molecular cobalt catalyst, which results in improved interfacial photoelectron transfer and catalytic mechanism. This work provides a scalable and inexpensive platform for the development of CO2 photoreduction systems using CDs.

Engineering Electro- and Photocatalytic Carbon Materials for CO2 Reduction by Formate Dehydrogenase.

Semiartificial approaches to renewable fuel synthesis exploit the integration of enzymes with synthetic materials for kinetically efficient fuel production. Here, a CO2 reductase, formate dehydrogenase (FDH) from Desulfovibrio vulgaris Hildenborough, is interfaced with carbon nanotubes (CNTs) and amorphous carbon dots (a-CDs). Each carbon substrate, tailored for electro- and photocatalysis, is functionalized with positive (-NHMe2+) and negative (-COO-) chemical surface groups to understand and optimize the electrostatic effect of protein association and orientation on CO2 reduction. Immobilization of FDH on positively charged CNT electrodes results in efficient and reversible electrochemical CO2 reduction via direct electron transfer with >90% Faradaic efficiency and -250 µA cm-2 at -0.6 V vs SHE (pH 6.7 and 25 °C) for formate production. In contrast, negatively charged CNTs only result in marginal currents with immobilized FDH. Quartz crystal microbalance analysis and attenuated total reflection infrared spectroscopy confirm the high binding affinity of active FDH to CNTs. FDH has subsequently been coupled to a-CDs, where the benefits of the positive charge (-NHMe2+-terminated a-CDs) were translated to a functional CD-FDH hybrid photocatalyst. High rates of photocatalytic CO2 reduction (turnover frequency: 3.5 × 103 h-1; AM 1.5G) with dl-dithiothreitol as the sacrificial electron donor were obtained after 6 h, providing benchmark rates for homogeneous photocatalytic CO2 reduction with metal-free light absorbers. This work provides a rational basis to understand interfacial surface/enzyme interactions at electrodes and photosensitizers to guide improvements with catalytic biohybrid materials.

Roadmap towards solar fuel synthesis at the water interface of liposome membranes.

Artificial photosynthesis has experienced rapid developments aimed at producing photocatalytic systems for the synthesis of chemical energy carriers. Conceptual advances of solar fuel systems have been inspired by improved understanding of natural photosynthesis and its key operational principles: (a) light harvesting, (b) charge separation, (c) directional proton and electron transport between reaction centres and across membranes, (d) water oxidation and (e) proton or CO2 reduction catalysis. Recently, there has been a surge of bio-inspired photosynthetic assemblies that use liposomes as nanocompartments to confine reaction spaces and enable vectorial charge transport across membranes. This approach, already investigated in the 1980s, offers in principle a promising platform for solar fuel synthesis. However, the fundamental principles governing the supramolecular assemblies of lipids and photoactive surfactant-like molecules in membranes, are intricate, and mastering membrane-supported photochemistry requires thorough understanding of the science behind liposomes. In this review, we provide an overview of approaches and considerations to construct a (semi)artificial liposome for solar fuel production. Key features to consider for the use of liposomes in solar fuel synthesis are highlighted, including the understanding of the orientation and binding of different components along the membrane, the controlled electron transport between the reaction centres, and the generation of proton gradients as driving force. Together with a list of experimental techniques for the characterisation of photoactive liposomes, this article provides the reader with a roadmap towards photocatalytic fuel production at the interface of lipid membranes and aqueous media.

An Integrated Carbon Nitride-Nickel Photocatalyst for the Amination of Aryl Halides Using Sodium Azide.

The synthesis of primary anilines via sustainable methods remains a challenge in organic synthesis. We report a photocatalytic protocol for the selective synthesis of primary anilines via cross-coupling of a wide range of aryl/heteroaryl halides with sodium azide using a photocatalyst powder consisting of nickel(II) deposited on mesoporous carbon nitride (Ni-mpg-CNx ). This heterogeneous photocatalyst contains a high surface area with a visible light-absorbing and adaptive "built-in" solid-state ligand for the integrated catalytic Ni site. The method displays a high functional group tolerance, requires mild reaction conditions, and benefits from easy recovery and reuse of the photocatalyst powder. Thereby, it overcomes the need of complex ligand scaffolds required in homogeneous catalysis, precious metals and elevated temperatures/pressures in existing protocols of primary anilines synthesis. The reported heterogeneous Ni-mpg-CNx holds potential for applications in the academic and industrial synthesis of anilines and exploration of other photocatalytic transformations.

Photocatalytic Removal of the Greenhouse Gas Nitrous Oxide by Liposomal Microreactors.

Nitrous oxide (N2 O) is a potent greenhouse and ozone-reactive gas for which emissions are growing rapidly due to increasingly intensive agriculture. Synthetic catalysts for N2 O decomposition typically contain precious metals and/or operate at elevated temperatures driving a desire for more sustainable alternatives. Here we demonstrate self-assembly of liposomal microreactors enabling catalytic reduction of N2 O to the climate neutral product N2 . Photoexcitation of graphitic N-doped carbon dots delivers electrons to encapsulated N2 O Reductase enzymes via a lipid-soluble biomolecular wire provided by the MtrCAB protein complex. Within the microreactor, electron transfer from MtrCAB to N2 O Reductase is facilitated by the general redox mediator methyl viologen. The liposomal microreactors use only earth-abundant elements to catalyze N2 O removal in ambient, aqueous conditions.

Visible-Light Promoted C-O Bond Formation with an Integrated Carbon Nitride-Nickel Heterogeneous Photocatalyst.

Ni-deposited mesoporous graphitic carbon nitride (Ni-mpg-CNx ) is introduced as an inexpensive, robust, easily synthesizable and recyclable material that functions as an integrated dual photocatalytic system. This material overcomes the need of expensive photosensitizers, organic ligands and additives as well as limitations of catalyst deactivation in the existing photo/Ni dual catalytic cross-coupling reactions. The dual catalytic Ni-mpg-CNx is demonstrated for C-O coupling between aryl halides and aliphatic alcohols under mild condition. The reaction affords the ether product in good-to-excellent yields (60-92 %) with broad substrate scope, including heteroaryl and aryl halides bearing electron-withdrawing, -donating and neutral groups. The heterogeneous Ni-mpg-CNx can be easily recovered from the reaction mixture and reused over multiple cycles without loss of activity. The findings highlight exciting opportunities for dual catalysis promoted by a fully heterogeneous system.

A Unified Electro- and Photocatalytic CO2 to CO Reduction Mechanism with Aminopyridine Cobalt Complexes.

A mechanistic understanding of electro- and photocatalytic CO2 reduction is crucial to develop strategies to overcome catalytic bottlenecks. In this regard, for a new CO2-to-CO reduction cobalt aminopyridine catalyst, a detailed experimental and theoretical mechanistic study is herein presented toward the identification of bottlenecks and potential strategies to alleviate them. The combination of electrochemistry and in situ spectroelectrochemistry together with spectroscopic techniques led us to identify elusive key electrocatalytic intermediates derived from complex [LN4Co(OTf)2] (1) (LN4 = 1-[2-pyridylmethyl]-4,7-dimethyl-1,4,7-triazacyclononane) such as a highly reactive cobalt(I) (1(I)) and a cobalt(I) carbonyl (1(I)-CO) species. The combination of spectroelectrochemical studies under CO2, 13CO2, and CO with DFT disclosed that 1(I) reacts with CO2 to form the pivotal 1(I)-CO intermediate at the 1(II/I) redox potential. However, at this reduction potential, the formation of 1(I)-CO restricts the electrocatalysis due to the endergonicity of the CO release step. In agreement with the experimentally observed CO2-to-CO electrocatalysis at the CoI/0 redox potential, computational studies suggested that the electrocatalytic cycle involves striking metal carbonyls. In contrast, under photochemical conditions, the catalysis smoothly proceeds at the 1(II/I) redox potential. Under the latter conditions, it is proposed that the electron transfer to form 1(I)-CO from 1(II)-CO is under diffusion control. Then, the CO release from 1(II)-CO is kinetically favored, facilitating the catalysis. Finally, we have found that visible-light irradiation has a positive impact under electrocatalytic conditions. We envision that light irradiation can serve as an effective strategy to circumvent the CO poisoning and improve the performance of CO2 reduction molecular catalysts.

Design of Iron Coordination Complexes as Highly Active Homogenous Water Oxidation Catalysts by Deuteration of Oxidation-Sensitive Sites.

The nature of the oxidizing species in water oxidation reactions with chemical oxidants catalyzed by α-[Fe(OTf)2(mcp)] (1α; mcp = N, N'-dimethyl- N, N'-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine, OTf = trifluoromethanesulfonate anion) and ß-[Fe(OTf)2(mcp)] (1ß) has been investigated. Mössbauer spectroscopy provides definitive evidence that 1α and 1ß generate oxoiron(IV) species as the resting state. Decomposition paths of the catalysts have been investigated by identifying and quantifying ligand fragments that form upon degradation. This analysis correlates the water oxidation activity of 1α and 1ß with stability against oxidative damage of the ligand via aliphatic C-H oxidation. The site of degradation and the relative stability against oxidative degradation are shown to be dependent on the topology of the catalyst. Furthermore, the mechanisms of catalyst degradation have been rationalized by computational analyses, which also explain why the topology of the catalyst enforces different oxidation-sensitive sites. This information has served in creating catalysts where sensitive C-H bonds have been replaced by C-D bonds. The deuterated analogues D4-α-[Fe(OTf)2(mcp)] (D4-1α), D4-ß-[Fe(OTf)2(mcp)] (D4-1ß), and D6-ß-[Fe(OTf)2(mcp)] (D6-1ß) were prepared, and their catalytic activity has been studied. D4-1α proves to be an extraordinarily active and efficient catalyst (up to 91% of O2 yield); it exhibits initial reaction rates identical with those of its protio analogue, but it is substantially more robust toward oxidative degradation and yields more than 3400 TON ( n(O2)/ n(Fe)). Altogether this evidences that the water oxidation catalytic activity is performed by a well-defined coordination complex and not by iron oxides formed after oxidative degradation of the ligands.

Spectroscopic, Electrochemical and Computational Characterisation of Ru Species Involved in Catalytic Water Oxidation: Evidence for a [Ru(V) (O)(Py2 (Me) tacn)] Intermediate.

A new family of ruthenium complexes based on the N-pentadentate ligand Py2 (Me) tacn (N-methyl-N',N''-bis(2-picolyl)-1,4,7-triazacyclononane) has been synthesised and its catalytic activity has been studied in the water-oxidation (WO) reaction. We have used chemical oxidants (ceric ammonium nitrate and NaIO4 ) to generate the WO intermediates [Ru(II) (OH2 )(Py2 (Me) tacn)](2+) , [Ru(III) (OH2 )(Py2 (Me) tacn)](3+) , [Ru(III) (OH)(Py2 (Me) tacn)](2+) and [Ru(IV) (O)(Py2 (Me) tacn)](2+) , which have been characterised spectroscopically. Their relative redox and pH stability in water has been studied by using UV/Vis and NMR spectroscopies, HRMS and spectroelectrochemistry. [Ru(IV) (O)(Py2 (Me) tacn)](2+) has a long half-life (>48 h) in water. The catalytic cycle of WO has been elucidated by using kinetic, spectroscopic, (18) O-labelling and theoretical studies, and the conclusion is that the rate-determining step is a single-site water nucleophilic attack on a metal-oxo species. Moreover, [Ru(IV) (O)(Py2 (Me) tacn)](2+) is proposed to be the resting state under catalytic conditions. By monitoring Ce(IV) consumption, we found that the O2 evolution rate is redox-controlled and independent of the initial concentration of Ce(IV) . Based on these facts, we propose herein that [Ru(IV) (O)(Py2 (Me) tacn)](2+) is oxidised to [Ru(V) (O)(Py2 (Me) tacn)](2+) prior to attack by a water molecule to give [Ru(III) (OOH)(Py2 (Me) tacn)](2+) . Finally, it is shown that the difference in WO reactivity between the homologous iron and ruthenium [M(OH2 )(Py2 (Me) tacn)](2+) (M=Ru, Fe) complexes is due to the difference in the redox stability of the key M(V) (O) intermediate. These results contribute to a better understanding of the WO mechanism and the differences between iron and ruthenium complexes in WO reactions.

Bioinspired photocatalytic systems towards compartmentalized artificial photosynthesis.

Artificial photosynthesis aims to produce fuels and chemicals from simple building blocks (i.e. water and carbon dioxide) using sunlight as energy source. Achieving effective photocatalytic systems necessitates a comprehensive understanding of the underlying mechanisms and factors that control the reactivity. This review underscores the growing interest in utilizing bioinspired artificial vesicles to develop compartmentalized photocatalytic systems. Herein, we summarize different scaffolds employed to develop artificial vesicles, and discuss recent examples where such systems are used to study pivotal processes of artificial photosynthesis, including light harvesting, charge transfer, and fuel production. These systems offer valuable lessons regarding the appropriate choice of membrane scaffolds, reaction partners and spatial arrangement to enhance photocatalytic activity, selectivity and efficiency. These studies highlight the pivotal role of the membrane to increase the stability of the immobilized reaction partners, generate a suitable local environment, and force proximity between electron donor and acceptor molecules (or catalysts and photosensitizers) to increase electron transfer rates. Overall, these findings pave the way for further development of bioinspired photocatalytic systems for compartmentalized artificial photosynthesis.

Low-Volume Reaction Monitoring of Carbon Dot Light Absorbers in Optofluidic Microreactors.

Optical monitoring and screening of photocatalytic batch reactions using cuvettes ex situ is time-consuming, requires substantial amounts of samples, and does not allow the analysis of species with low extinction coefficients. Hollow-core photonic crystal fibers (HC-PCFs) provide an innovative approach for in situ reaction detection using ultraviolet-visible absorption spectroscopy, with the potential for high-throughput automation using extremely low sample volumes with high sensitivity for monitoring of the analyte. HC-PCFs use interference effects to guide light at the center of a microfluidic channel and use this to enhance detection sensitivity. They open the possibility of comprehensively studying photocatalysts to extract structure-activity relationships, which is unfeasible with similar reaction volume, time, and sensitivity in cuvettes. Here, we demonstrate the use of HC-PCF microreactors for the screening of the electron transfer properties of carbon dots (CDs), a nanometer-sized material that is emerging as a homogeneous light absorber in photocatalysis. The CD-driven photoreduction reaction of viologens (XV2+) to the corresponding radical monocation XV•+ is monitored in situ as a model reaction, using a sample volume of 1 µL per measurement and with a detectability of <1 µM. A range of different reaction conditions have been systematically studied, including different types of CDs (i.e., amorphous, graphitic, and graphitic nitrogen-doped CDs), surface chemistry, viologens, and electron donors. Furthermore, the excitation irradiance was varied to study its effect on the photoreduction rate. The findings are correlated with the electron transfer properties of CDs based on their electronic structure characterized by soft X-ray absorption spectroscopy. Optofluidic microreactors with real-time optical detection provide unique insight into the reaction dynamics of photocatalytic systems and could form the basis of future automated catalyst screening platforms, where samples are only available on small scales or at a high cost.

Hybrid photocathode based on a Ni molecular catalyst and Sb2Se3 for solar H2 production.

We report a H2 evolving hybrid photocathode based on Sb2Se3 and a precious metal free molecular catalyst. Through the use of a high surface area TiO2 scaffold, we successfully increased the Ni molecular catalyst loading from 7.08 ± 0.43 to 45.76 ± 0.81 nmol cm-2, achieving photocurrents of 1.3 mA cm-2 at 0 V vs. RHE, which is 81-fold higher than the device without the TiO2 mesoporous layer.

Size-dependent activity of carbon dots for photocatalytic H2 generation in combination with a molecular Ni cocatalyst.

Carbon dots (CDs) are low-cost light-absorbers in photocatalytic multicomponent systems, but their wide size distribution has hampered rational design and the identification of the factors that lead to their best performance. To address this challenge, we report herein the use of gel filtration size exclusion chromatography to separate amorphous, graphitic, and graphitic N-doped CDs depending on their lateral size to study the effect of their size on photocatalytic H2 evolution with a DuBois-type Ni cocatalyst. Transmission electron microscopy and dynamic light scattering confirm the size-dependent separation of the CDs, whereas UV-vis and fluorescence spectroscopy of the more monodisperse fractions show a distinct response which computational modelling attributes to a complex interplay between CD size and optical properties. A size-dependent effect on the photocatalytic H2 evolution performance of the CDs in combination with a molecular Ni cocatalyst is demonstrated with a maximum activity at approximately 2-3 nm CD diameter. Overall, size separation leads to a two-fold increase in the specific photocatalytic activity for H2 evolution using the monodisperse CDs compared to the as synthesized polydisperse samples, highlighting the size-dependent effect on photocatalytic performance.

Synthesis and characterization of highly diluted polyanionic iron(II) spin crossover systems.

Spin crossover (SCO) complexes, through their reversible spin transition under external stimuli, can work as switchable memory materials. Here, we present a protocol for the synthesis and characterization of a specific polyanionic iron SCO complex and its diluted systems. We describe steps for its synthesis and the determination of crystallographic structure of the SCO complex in diluted systems. We then detail a range of spectroscopic and magnetic techniques employed to monitor the spin state of the SCO complex in both diluted solid- and liquid-state systems. For complete details on the use and execution of this protocol, please refer to Galán-Mascaros et al.1.

Light-driven reduction of aromatic olefins in aqueous media catalysed by aminopyridine cobalt complexes.

A catalytic system based on earth-abundant elements that efficiently hydrogenates aryl olefins using visible light as the driving-force and H2O as the sole hydrogen atom source is reported. The catalytic system involves a robust and well-defined aminopyridine cobalt complex and a heteroleptic Cu photoredox catalyst. The system shows the reduction of styrene in aqueous media with a remarkable selectivity (>20 000) versus water reduction (WR). Reactivity and mechanistic studies support the formation of a [Co-H] intermediate, which reacts with the olefin via a hydrogen atom transfer (HAT). Synthetically useful deuterium-labelled compounds can be straightforwardly obtained by replacing H2O with D2O. Moreover, the dual photocatalytic system and the photocatalytic conditions can be rationally designed to tune the selectivity for aryl olefin vs. aryl ketone reduction; not only by changing the structural and electronic properties of the cobalt catalysts, but also by modifying the reduction properties of the photoredox catalyst.

XAS and EPR in Situ Observation of Ru(V) Oxo Intermediate in a Ru Water Oxidation Complex.

In this study, we combine in situ spectroelectrochemistry coupled with electron paramagnetic resonance (EPR) and X-ray absorption spectroscopies (XAS) to investigate a molecular Ru-based water oxidation catalyst bearing a polypyridinic backbone [RuII(OH2)(Py2Metacn)]2+ . Although high valent key intermediate species arising in catalytic cycles of this family of compounds have remain elusive due to the lack of additional anionic ligands that could potentially stabilize them, mechanistic studies performed on this system proposed a water nucleophilic attack (WNA) mechanism for the O-O bond formation. Employing in situ experimental conditions and complementary spectroscopic techniques allowed to observe intermediates that provide support for a WNA mechanism, including for the first time a Ru(V) oxo intermediate based on the Py2Metacn ligand, in agreement with the previously proposed mechanism.

Bridging Plastic Recycling and Organic Catalysis: Photocatalytic Deconstruction of Polystyrene via a C-H Oxidation Pathway.

Chemical recycling of synthetic polymers represents a promising strategy to deconstruct plastic waste and make valuable products. Inspired by small-molecule C-H bond activation, a visible-light-driven reaction is developed to deconstruct polystyrene (PS) into ∼40% benzoic acid as well as ∼20% other monomeric aromatic products at 50 °C and ambient pressure. The practicality of this strategy is demonstrated by deconstruction of real-world PS foam on a gram scale. The reaction is proposed to proceed via a C-H bond oxidation pathway, which is supported by theoretical calculations and experimental results. Fluorescence quenching experiments also support efficient electron transfer between the photocatalyst and the polymer substrate, providing further evidence for the proposed mechanism. This study introduces concepts from small-molecule catalysis to polymer deconstruction and provides a promising method to tackle the global crisis of plastic pollution.

Photocatalytic Removal of the Greenhouse Gas Nitrous Oxide by Liposomal Microreactors.

Nitrous oxide (N2O) is a potent greenhouse and ozone-reactive gas for which emissions are growing rapidly due to increasingly intensive agriculture. Synthetic catalysts for N2O decomposition typically contain precious metals and/or operate at elevated temperatures driving a desire for more sustainable alternatives. Here we demonstrate self-assembly of liposomal microreactors enabling catalytic reduction of N2O to the climate neutral product N2. Photoexcitation of graphitic N-doped carbon dots delivers electrons to encapsulated N2O Reductase enzymes via a lipid-soluble biomolecular wire provided by the MtrCAB protein complex. Within the microreactor, electron transfer from MtrCAB to N2O Reductase is facilitated by the general redox mediator methyl viologen. The liposomal microreactors use only earth-abundant elements to catalyze N2O removal in ambient, aqueous conditions.

Visible-Light Promoted C-O Bond Formation with an Integrated Carbon Nitride-Nickel Heterogeneous Photocatalyst.

Ni-deposited mesoporous graphitic carbon nitride (Ni-mpg-CNx) is introduced as an inexpensive, robust, easily synthesizable and recyclable material that functions as an integrated dual photocatalytic system. This material overcomes the need of expensive photosensitizers, organic ligands and additives as well as limitations of catalyst deactivation in the existing photo/Ni dual catalytic cross-coupling reactions. The dual catalytic Ni-mpg-CNx is demonstrated for C-O coupling between aryl halides and aliphatic alcohols under mild condition. The reaction affords the ether product in good-to-excellent yields (60-92 %) with broad substrate scope, including heteroaryl and aryl halides bearing electron-withdrawing, -donating and neutral groups. The heterogeneous Ni-mpg-CNx can be easily recovered from the reaction mixture and reused over multiple cycles without loss of activity. The findings highlight exciting opportunities for dual catalysis promoted by a fully heterogeneous system.

Photocatalytic C-H Azolation of Arenes Using Heterogeneous Carbon Nitride in Batch and Flow.

The functionalization of aryl C(sp2 )-H bonds is a useful strategy for the late-stage modification of biologically active molecules, especially for the regioselective introduction of azole heterocycles to prepare medicinally-relevant compounds. Herein, we describe a practical photocatalytic transformation using a mesoporous carbon nitride (mpg-CNx ) photocatalyst, which enables the efficient azolation of various arenes through direct oxidation. The method exhibits a broad substrate scope and is amenable to the late-stage functionalization of several pharmaceuticals. Due to the heterogeneous nature and high photocatalytic stability of mpg-CNx , the catalyst can be easily recovered and reused leading to greener and more sustainable routes, using either batch or flow processing, to prepare these important compounds of interest in pharmaceutical and agrochemical research.