Photogeneration of Carbon Monoxide and of Hydrogenvia Simultaneous Photochemical Reduction of Carbon Dioxide and Water by Visible-Light Irradiation of Organic Solutions Containing Tris(2,2?-bipyridine)ruthenium(II) and Cobalt(II) Species as Homogeneous Catalysts

Manipulating the Coordination Structure of Molecular Cobalt Sites in Periodic Mesoporous Organosilica for CO2 Photoreduction

Photocatalytic CO2 reduction, including reaction rate, product selectivity, and longevity, is highly sensitive to the coordination structure of the catalytic active sites, and the precise design of the active site remains a challenge in heterogeneous catalysts. Herein, we report on the modulation of the coordination structure of MN x -type active sites (M = Co or Ni; x = 4 or 5) anchored on a periodic mesoporous organosilica (PMO) support to improve photocatalytic CO2 reduction. The PMO was functionalized with pendant 3,6-di(2'-pyridyl)pyridazine (dppz) groups to allow immobilization of molecular Co and Ni complexes with polypyridine ligands. A comparative analysis of CO2 photoreduction in the presence of an organic photosensitizer (4CzIPN, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene) and a conventional [Ru(bpy)3]Cl2 sensitizer revealed strong influence of the coordination environment on the catalytic performance. CoN5-PMO demonstrated a superior CO2 photoreduction activity than the other materials and displayed a cobalt-based turnover number (TONCO) of 92 for CO evolution at ∼75% selectivity after 3 h irradiation in the presence of 4CzIPN. The hybrid CoN5-PMO catalyst exhibited better activity than its homogeneous [CoN5] counterpart, indicating that the heterogenization promotes the formation of isolated active sites with improved longevity and faster catalytic rate.

Application of bioorganometallic B12 in green organic synthesis

Bioorganometallic structure found in coenzyme B₁₂ is a key component in B₁₂-dependent enzymatic reactions in natural enzymes. Cleavage of a cobalt-carbon bond in organometallic B₁₂ compound provide reactive intermediate for molecular transformations. Application of the bioorganometallic B₁₂ in organic synthesis have been developed using natural vitamin B₁₂ as well as synthetic vitamin B₁₂ derivatives as a bioinspired catalyst in organic solvent. Vitamin B₁₂ derivatives composed of corrinoid structure should form stable organometallic compound having a cobalt-carbon bond. Using the unique property of the organometallic vitamin B₁₂ derivatives, various catalytic reactions have been developed in synthetic organic chemistry. The dual catalytic system of vitamin B₁₂ derivatives and photocatalyst, such as Ru(II) polypyridyl complex or titanium oxide, could construct light-driven molecular transformations. The B₁₂-dependent enzymes mimic reactions, such as the dechlorination of organic halides and the radical mediated isomerization reactions, catalytically proceed in the dual catalyst system. Electroorganic syntheses mediated by the vitamin B₁₂ derivatives have been developed as green molecular transformations. The redox active vitamin B₁₂ derivatives shows a unique catalysis in the electroorganic synthesis, such as alkene and alkyne reductions.

Photoredox Chemistry with Organic Catalysts: Role of Computational Methods

Organic catalysts have the potential to carry out a wide range of otherwise thermally inaccessible reactions via photoredox routes. Early demonstrated successes of organic photoredox catalysts include one-electron CO₂ reduction and H₂ generation via water splitting. Photoredox systems are challenging to study and design owing to the sheer number and diversity of phenomena involved, including light absorption, emission, intersystem crossing, partial or complete charge transfer, and bond breaking or formation. Designing a viable photoredox route therefore requires consideration of a host of factors such as absorption wavelength, solvent, choice of electron donor or acceptor, and so on. Quantum chemistry methods can play a critical role in demystifying photoredox phenomena. Using one-electron CO₂ reduction with phenylene-based chromophores as an illustrative example, this perspective highlights recent developments in quantum chemistry that can advance our understanding of photoredox processes and proposes a way forward for driving the design and discovery of organic catalysts.

The influence of inorganic anions on photocatalytic CO2 reduction

The influence of common inorganic anions on photocatalytic CO₂ reduction has been investigated in a semiconductor/complex (CdS/Co(bpy)₃Cl₂) system.

Recent Innovation of Metal-Organic Frameworks for Carbon Dioxide Photocatalytic Reduction

The accumulation of carbon dioxide (CO2) pollutants in the atmosphere begets global warming, forcing us to face tangible catastrophes worldwide. Environmental affability, affordability, and efficient CO2 metamorphotic capacity are critical factors for photocatalysts; metal-organic frameworks (MOFs) are one of the best candidates. MOFs, as hybrid organic ligand and inorganic nodal metal with tailorable morphological texture and adaptable electronic structure, are contemporary artificial photocatalysts. The semiconducting nature and porous topology of MOFs, respectively, assists with photogenerated multi-exciton injection and adsorption of substrate proximate to void cavities, thereby converting CO2. The vitality of the employment of MOFs in CO2 photolytic reaction has emerged from the fact that they are not only an inherently eco-friendly weapon for pollutant extermination, but also a potential tool for alleviating foreseeable fuel crises. The excellent synergistic interaction between the central metal and organic linker allows decisive implementation for the design, integration, and application of the catalytic bundle. In this review, we presented recent MOF headway focusing on reports of the last three years, exhaustively categorized based on central metal-type, and novel discussion, from material preparation to photocatalytic, simulated performance recordings of respective as-synthesized materials. The selective CO2 reduction capacities into syngas or formate of standalone or composite MOFs with definite photocatalytic reaction conditions was considered and compared.

Conjugated Electron Donor⁻Acceptor Hybrid Polymeric Carbon Nitride as a Photocatalyst for CO2 Reduction

This work incorporates a variety of conjugated donor-acceptor (DA) co-monomers such as 2,6-diaminopurine (DP) into the structure of a polymeric carbon nitride (PCN) backbone using a unique nanostructure co-polymerization strategy and examines its photocatalytic activity performance in the field of photocatalytic CO2 reduction to CO and H2 under visible light irradiation. The as-synthesized samples were successfully analyzed using different characterization methods to explain their electronic and optical properties, crystal phase, microstructure, and their morphology that influenced the performance due to the interactions between the PCN and the DPco-monomer. Based on the density functional theory (DFT) calculation result, pure PCN and CNU-DP15.0 trimers (interpreted as incorporation of the co-monomer at two different positions) were extensively evaluated and exhibited remarkable structural optimization without the inclusion of any symmetry constraints (the non-modified sample derived from urea, named as CNU), and their optical and electronic properties were also manipulated to control occupation of their respective highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Also, co-polymerization of the donor-acceptor 2,6-diamino-purine co-monomer with PCN influenced the chemical affinities, polarities, and acid-base functions of the PCN, remarkably enhancing the photocatalytic activity for the production of CO and H2 from CO2 by 15.02-fold compared than that of the parental CNU, while also improving the selectivity.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Molecular polypyridine-based metal complexes as catalysts for the reduction of CO2

Polypyridyl transition metal complexes represent one of the more thoroughly studied classes of molecular catalysts towards CO₂ reduction to date. Initial reports in the 1980s began with an emphasis on 2nd and 3rd row late transition metals, but more recently the focus has shifted towards earlier metals and base metals. Polypyridyl platforms have proven quite versatile and amenable to studying various parameters that govern product distribution for CO₂ reduction. However, open questions remain regarding the key mechanistic steps that govern product selectivity and efficiency. Polypyridyl complexes have also been immobilized through a variety of methods to afford active catalytic materials for CO₂ reductions. While still an emerging field, materials incorporating molecular catalysts represent a promising strategy for electrochemical and photoelectrochemical devices capable of CO₂ reduction. In general, this class of compounds remains the most promising for the continued development of molecular systems for CO₂ reduction and an inspiration for the design of related non-polypyridyl catalysts.

Photochemical CO2 reduction using structurally controlled g-C3N4

Graphitic carbon nitride (g-C₃N₄) synthesised from a urea precursor is an excellent CO₂ reduction photocatalyst using [Co(bpy)_n]²⁺ as a co-catalyst. A five-fold increase in activity for the highly polymerised urea derived g-C₃N₄ is achieved compared to alternative precursors. Transient absorption, time-resolved and steady-state emission studies indicate that the enhanced activity is related to both an increased driving force for photoelectron transfer and a greater availability of photogenerated charges.

A perovskite oxide LaCoO3 cocatalyst for efficient photocatalytic reduction of CO2 with visible light

As a unique class of functional materials, perovskite oxides have shown great opportunities in various energy storage and conversion applications.

TiO2 modified with a Ru(ii)–N′NN′ 8-hydroxyquinolyl complex for efficient gaseous photoreduction of CO2

A rare example of a Ru(ii) pincer complex for the efficient photoreduction of CO₂ to CO/CH₄ in a gaseous system was developed.

Photo- and Electrochemical Valorization of Carbon Dioxide Using Earth-Abundant Molecular Catalysts

The dramatic increase in anthropogenic carbon dioxide emissions in recent decades has forced us to look for alternative carbon-neutral processes for the production of energy vectors and commodity chemicals. Photo- and electrochemical reduction of CO₂ are appealing strategies for the storage of sustainable and intermittent energies in the form of chemical bonds of synthetic fuels and value-added molecules. In these approaches, carbon dioxide is converted to products such as CO, HCOOH and MeOH through proton-coupled electron transfer reactions. The use of earth-abundant elements as components of the catalytic materials is crucial for the large-scale applicability of this technology. This review summarizes the most recent advances related to this issue, with particular focus on studies where molecular metal complexes are used as catalysts. In addition, with the aim of aiding in the design of more robust and efficient non-noble metal-based catalysts, we discuss the lessons learned from the corresponding mechanistic studies.

Temperature dependence of photocatalytic CO2reduction by trans(Cl)–Ru(bpy)(CO)2Cl2: activation energy difference between CO and formate production

The temperature dependence of photocatalytic CO₂reduction bytrans(Cl)–Ru(bpy)(CO)₂Cl₂(bpy: 2,2′-bipyridine) has been researched in ethanol (EtOH)/N,N-dimethylacetamide (DMA) solutions containing [Ru(bpy)₃]²⁺(a photosensitizer) and 1-benzyl-1,4-dihydronicotinamide (BNAH, an electron donor). The catalytic system efficiently reduces CO₂to carbon monoxide (CO) with formate (HCOO⁻) as a minor product. The mechanism of the catalysis consists of the electron-relay cycle and the catalytic cycle: in the former cycle the photochemically generated reduced species of the photosensitizer injects an electron to the catalyst, and in the latter the catalyst reduces CO₂. At a low concentration of the catalyst (5.0 μM), where the catalytic cycle is rate-determining, the temperature dependence of CO/HCOO⁻is also dependent on the EtOH contents: the selectivity of CO/HCOO⁻decreases in 20% and 40%-EtOH/DMA with increasing temperature, while it increases in 60%-EtOH/DMA. The temperature dependence of the CO/HCOO⁻selectivity indicates that the difference in activation energy (ΔΔG^‡) between CO and HCOO⁻production is estimated asca.3.06 kJ mol⁻¹in 40%-EtOH/DMA at 298 K.

Iridium(III) 1-Phenylisoquinoline Complexes as a Photosensitizer for Photocatalytic CO2 Reduction: A Mixed System with a Re(I) Catalyst and a Supramolecular Photocatalyst

An Ir(III) complex with 1-phenylisoquinoline (piq) ligands [Ir(piq)2(dmb)](+) (Ir, dmb = 4,4'-dimethyl-2,2'-bipyridine) exhibited strong absorption in the visible region, and the lifetime of its excited state was very long (τ = 2.8 μs). Photochemical reduction of Ir efficiently proceeded with 1-benzyl-1,4-dihydronicotinamide (BNAH) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as reductants, giving the one-electron-reduced species (OERS), which was stable in solution at ambient temperature. The OERS of the Ir complex possessed strong reductive power, sufficient to supply an electron to fac-Re(dmb)(CO)3Br (Re). The photocatalytic reduction of CO2 proceeded efficiently using a mixed system constructed with Ir as a redox photosensitizer and Re as a catalyst, selectively giving CO (ΦCO = 0.16 using BNAH at λex = 480 nm). Ir was a more suitable photosensitizer for evaluating the activity of the Re catalyst in the photocatalytic reaction compared to [Ru(dmb)3](2+) (Ru) because the Ir complex was more stable in the photocatalytic reaction, and its decomposition products did not function as catalysts for CO2 reduction while the decomposition products of the Ru complex functioned as catalysts for the reduction of CO2 to HCOOH, inducing a drastic perturbation of the product distribution. A supramolecular photocatalyst (Ir-Re), in which the Ir(III) photosensitizer and the Re(I) catalyst were connected by a bridging ligand, was newly synthesized. When using BNAH, Ir-Re possessed a greater photocatalytic ability (ΦCO = 0.21, TONCO = 130) than the corresponding mixed system of the Ir and Re mononuclear complexes. Using BIH as the reductant, both Ir-Re and the mixed system showed very high photocatalytic activity (ΦCO = 0.40-0.41, TONCO = 1700).

Stable luminescent iridium(iii) complexes with bis(N-heterocyclic carbene) ligands: photo-stability, excited state properties, visible-light-driven radical cyclization and CO2 reduction, and cellular imaging

Excited state properties, photo-catalysis and cellular imaging of photo-stable bis-NHC Ir(iii) complexes are described.

A new class of cyclometalated Ir(iii) complexes supported by various bidentate C-deprotonated (C^N) and cis-chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been synthesized. These complexes display strong emission in deaerated solutions at room temperature with photoluminescence quantum yields up to 89% and emission lifetimes up to 96 μs. A photo-stable complex containing C-deprotonated fluorenyl-substituted C^N shows no significant decomposition even upon irradiation for over 120 h by blue LEDs (12 W). These, together with the strong absorption in the visible region and rich photo-redox properties, allow the bis-NHC Ir(iii) complexes to act as good photo-catalysts for reductive C–C bond formation from C(sp³/sp²)–Br bonds cleavage using visible-light irradiation (λ > 440 nm). A water-soluble complex with a glucose-functionalized bis-NHC ligand catalysed a visible-light-driven radical cyclization for the synthesis of pyrrolidine in aqueous media. Also, the bis-NHC Ir(iii) complex in combination with a cobalt catalyst can catalyse the visible-light-driven CO₂ reduction with excellent turnover numbers (>2400) and selectivity (CO over H₂ in gas phase: >95%). Additionally, this series of bis-NHC Ir(iii) complexes are found to localize in and stain endoplasmic reticulum (ER) of various cell lines with high selectivity, and exhibit high cytotoxicity towards cancer cells, revealing their potential uses as bioimaging and/or anti-cancer agents.

Metal–organic redox vehicles to encapsulate organic dyes for photocatalytic protons and carbon dioxide reduction

By encapsulation of an organic dye, a supramolecular nickel–organic macrocycle for the photocatalytic reduction of protons and CO₂ has been reported.

Reinforced photocatalytic reduction of CO2 to CO by a ternary metal oxide NiCo2O4

The work reported herein was the facile preparation of uniform urchin-like NiCo2O4 microspheres, and their use as an efficient and stable cocatalyst for photocatalytic CO2 reduction catalysis. A combined solvothermal-calcination strategy was applied to synthesize the NiCo2O4 material that was systematically characterized by physical and chemical measurements (e.g. SEM, TEM, XRD, XPS, EDX, elemental mapping and N2 physisorption analysis). By cooperation with a visible light photosensitizer, the NiCo2O4 material effectively promoted the deoxygenative reduction of CO2 to CO by more than twenty times under mild reaction conditions. The carbon origin of CO evolution was validated by (13)CO2 isotope tracer experiments. Various reaction parameters were examined and optimized, and a possible reaction mechanism was proposed. Furthermore, the stability and reusability of NiCo2O4 cocatalysts were firmly confirmed.

Unexpected effect of catalyst concentration on photochemical CO2 reduction by trans(Cl)-Ru(bpy)(CO)2Cl2: new mechanistic insight into the CO/HCOO- selectivity

Photochemical CO₂ reduction catalysed by trans(Cl)-Ru(bpy)(CO)₂Cl₂ (bpy = 2,2'-bipyridine) efficiently produces carbon monoxide (CO) and formate (HCOO^-) in N,N-dimethylacetamide (DMA)/water containing [Ru(bpy)₃]²⁺ as a photosensitizer and 1-benzyl-1,4-dihydronicotinamide (BNAH) as an electron donor. We have unexpectedly found catalyst concentration dependence of the product ratio (CO/HCOO^-) in the photochemical CO₂ reduction: the ratio of CO/HCOO^- decreases with increasing catalyst concentration. The result has led us to propose a new mechanism in which HCOO^- is selectively produced by the formation of a Ru(i)-Ru(i) dimer as the catalyst intermediate. This reaction mechanism predicts that the Ru-Ru bond dissociates in the reaction of the dimer with CO₂, and that the insufficient electron supply to the catalyst results in the dominant formation of HCOO^-. The proposed mechanism is supported by the result that the time-course profiles of CO and HCOO^- in the photochemical CO₂ reduction catalysed by [Ru(bpy)(CO)₂Cl]₂ (0.05 mM) are very similar to those of the reduction catalysed by trans(Cl)-Ru(bpy)(CO)₂Cl₂ (0.10 mM), and that HCOO^- formation becomes dominant under low-intensity light. The kinetic analyses based on the proposed mechanism could excellently reproduce the unusual catalyst concentration effect on the product ratio. The catalyst concentration effect observed in the photochemical CO₂ reduction using [Ru(4dmbpy)₃]²⁺ (4dmbpy = 4,4'-dimethyl-2,2'-bipyridine) instead of [Ru(bpy)₃]²⁺ as the photosensitizer is also explained with the kinetic analyses, reflecting the smaller quenching rate constant of excited [Ru(4dmbpy)₃]²⁺ by BNAH than that of excited [Ru(bpy)₃]²⁺. We have further synthesized trans(Cl)-Ru(6Mes-bpy)(CO)₂Cl₂ (6Mes-bpy = 6,6'-dimesityl-2,2'-bipyridine), which bears bulky substituents at the 6,6'-positions in the 2,2'-bipyridyl ligand, so that the ruthenium complex cannot form the dimer due to the steric hindrance. We have found that this ruthenium complex selectively produces CO, which strongly supports the catalytic mechanism proposed in this work.

Ru(ii)-Re(i) binuclear photocatalysts connected by -CH2XCH2- (X = O, S, CH2) for CO2 reduction

We developed Ru(ii)-Re(i) supramolecular photocatalysts in which each metal complex unit is connected by a -CH₂XCH₂- (X = O, S, CH₂) chain. The photocatalyst with X = O exhibited the best photocatalytic efficiency for CO₂ reduction in the reported systems using a NAD(P)H model compound as an electron donor because the introduced oxygen atom strengthened the oxidation power of the Ru photosensitizer unit in the excited state and accelerated electron transfer from the one-electron-reduced Ru photosensitizer unit to the Re catalyst unit. In contrast, the catalytic ability of the photocatalyst with X = S rapidly decreased during irradiation because the supramolecular structure split into mononuclear complexes. A detailed mechanism for the efficient photocatalytic reaction involving these supramolecular photocatalysts was investigated for the first time.

Development of a stable MnCo2O4 cocatalyst for photocatalytic CO2 reduction with visible light

The synthesis of uniform MnCo2O4 microspheres and their cooperation with a visible light harvester to achieve efficient photocatalytic CO2 reduction under ambient conditions are reported here. The MnCo2O4 materials were prepared by a facile two-step solvothermal-calcination method and were characterized by XRD, SEM, TEM, EDX, XPS, elemental mapping, and N2 adsorption measurements. By using the MnCo2O4 microspheres as a heterogeneous cocatalyst, the photocatalytic performance of the CO2-to-CO conversion catalysis was remarkably enhanced, and no decrease in the promotional effect of the cocatalyst was observed after repeatedly operating the reaction for six cycles. (13)CO2 isotope tracer experiments verified that the CO product originated from the CO2 reactant. The effect of synthetic conditions and various reaction parameters on the photocatalytic activity of the system were investigated and optimized. The stability of the MnCo2O4 cocatalyst in the CO2 reduction system was confirmed by several techniques. Moreover, a possible mechanism for MnCo2O4-cocatalyzed CO2 photoreduction catalysis is proposed.

A stable ZnCo2O4cocatalyst for photocatalytic CO2reduction

Spinel ZnCo₂O₄nanorods were developed as stable cocatalysts cooperative with a light harvester for CO₂photoreduction.

Direct electrochemical capture and release of carbon dioxide using an industrial organic pigment: quinacridone

Limiting anthropogenic carbon dioxide emissions constitutes a major issue faced by scientists today. Herein we report an efficient way of controlled capture and release of carbon dioxide using nature inspired, cheap, abundant and non-toxic, industrial pigment namely, quinacridone. An electrochemically reduced electrode consisting of a quinacridone thin film (ca. 100 nm thick) on an ITO support forms a quinacridone carbonate salt. The captured CO2 can be released by electrochemical oxidation. The amount of captured CO2 was quantified by FT-IR. The uptake value for electrochemical release process was 4.61 mmol g(-1). This value is among the highest reported uptake efficiencies for electrochemical CO2 capture. For comparison, the state-of-the-art aqueous amine industrial capture process has an uptake efficiency of ca. 8 mmol g(-1).

Terpyridine complexes of first row transition metals and electrochemical reduction of CO2 to CO

Homoleptic terpyridine complexes of 3d transition metals are found to electrocatalytically reduce CO₂ to either CO or tuneable CO–H₂ mixtures.