Visible-light-driven CO2 reduction on a hybrid photocatalyst consisting of a Ru(ii) binuclear complex and a Ag-loaded TaON in aqueous solutions

A hybrid photocatalyst consisting of a Ru(ii) binuclear complex and a Ag-loaded TaON reduced CO2 by visible light even in aqueous solution. The distribution of the reduction products was strongly affected by the pH of the reaction solution. HCOOH was selectively produced in neutral conditions, whereas the formation of HCOOH competed with H2 evolution in acidic conditions. Detailed mechanistic studies revealed that the photocatalytic CO2 reduction proceeded via 'Z-schematic' electron transfer with step-by-step photoexcitation of TaON and the photosensitizer unit in the Ru(ii) binuclear complex. The maximum turnover number for HCOOH formation was 750 based on the Ru(ii) binuclear complex under visible-light irradiation, and the optimum external quantum efficiency of the HCOOH formation was 0.48% using 400 nm monochromic light with ethylenediaminetetraacetic acid disodium salt as a sacrificial reductant. Even in aqueous solution, the hybrid could also convert visible-light energy into chemical energy (ΔG0 = +83 kJ mol-1) by the reduction of CO2 to HCOOH with methanol oxidation.

Unique Solvent Effects on Visible-Light CO2 Reduction over Ruthenium(II)-Complex/Carbon Nitride Hybrid Photocatalysts

Photocatalytic CO2 reduction using hybrids of carbon nitride (C3N4) and a Ru(II) complex under visible light was studied with respect to reaction solvent. Three different Ru(II) complexes, trans(Cl)-[Ru(X2bpy) (CO)2Cl2] (X2bpy = 2,2'-bipyridine with substituents X in the 4-positions, X = COOH, PO3H2, or CH2PO3H2), were employed as promoters and will be abbreviated as RuC (X = COOH), RuP (X = PO3H2), and RuCP (X = CH2PO3H2). When C3N4 modified with a larger amount of RuCP (>7.8 μmol g(-1)) was employed as a photocatalyst in a solvent having a relatively high donor number (e.g., N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO)) with the aid of triethanolamine (TEOA) as an electron donor, the hybrid photocatalyst exhibited high performance for CO2 reduction, producing CO and HCOOH with relatively high CO selectivity (40-70%). On the other hand, HCOOH was the major product when RuC/C3N4 or RuP/C3N4 was employed regardless of the loading amount of the Ru(II) complex and the reaction solvent. Results of photocatalytic reactions and UV-visible diffuse reflectance spectroscopy indicated that polymeric Ru species, which were formed in situ from RuCP on C3N4 under irradiation in a solvent having a high donor number, were active catalysts for CO formation. Nonsacrificial CO2 reduction using RuP/C3N4 was accomplished in a DMA solution containing methanol as an electron donor, which means that visible light energy was stored as chemical energy in the form of CO and formaldehyde (ΔG° = +67.6 kJ mol(-1)). This study demonstrated the first successful example of an energy conversion scheme using carbon nitride through photocatalytic CO2 reduction.

A search for the ground state structure and the phase stability of tantalum pentoxide

Tantalum pentoxide (Ta2O5) is a wide-gap semiconductor that presents good catalytic and dielectric properties, conferring to this compound promising prospective use in a variety of technological applications. However, there is a lack of understanding regarding the relations among its crystalline phases, as some of them are not even completely characterized and there is currently no agreement about which models better explain the crystallographic data. Additionally, its phase diagram is unknown. In this work we performed first-principles density functional theory calculations to study the structural properties of the different phases and models of Ta2O5, the equation of state and the zone-centered vibrational frequencies. From our results, we conclude that the phases that are built up from only distorted octahedra instead of combinations with pentagonal and/or hexagonal bipyramids are energetically more favorable and dynamically stable. More importantly, this study establishes that, given the pressure range considered, the B-phase is the most favorable structure and there is no a crystallographic phase transition to another phase at high-pressure. Additionally, for the equilibrium volume of the B-phase and the λ-model, the description of the electronic structure and optical properties were performed using semi-local and hybrid functionals.

CO2 reduction with Re(i)–NHC compounds: driving selective catalysis with a silicon nanowire photoelectrode

Excellent selectivity was observed in CO₂ reduction using Re(i)–NHC catalysts on a silicon nanowire photoelectrode.

Conjugated microporous polycarbazole containing tris(2-phenylpyridine)iridium(iii) complexes: phosphorescence, porosity, and heterogeneous organic photocatalysis

Porous polycarbazole containing tris(2-phenylpyridine)iridium(iii) complexes exhibit intense phosphorescence, moderate porosities and photocatalytic activities for the aza-Henry reaction.

Visible-light-induced water oxidation by a hybrid photocatalyst consisting of bismuth vanadate and copper(ii) meso-tetra(4-carboxyphenyl)porphyrin

Copper(ii) meso-tetra(4-carboxyphenyl)porphyrin surface-modified monoclinic scheelite bismuth vanadate (CuTCPP/ms-BiVO₄) exhibits a high level of activity for the water oxidation to oxygen (O₂) under visible-light irradiation (λ > 430 nm).

Hydrogen energy future with formic acid: a renewable chemical hydrogen storage system

Formic acid, the simplest carboxylic acid, could serve as one of the better fuels for portable devices, vehicles and other energy-related applications in the future.

Photocatalytic water oxidation via combination of BiVO4–RGO and molecular cobalt catalysts

Co₄O₄cubic complexes were found to be efficient cocatalysts for light-driven water oxidation in a system containing BiVO₄–RGO and AgNO₃.

Photocatalytic and photoelectrocatalytic reduction of CO2 using heterogeneous catalysts with controlled nanostructures

Recent advances in photocatalytic and photoelectrocatalytic reduction of CO₂ with H₂O using semiconductor-based catalysts have been highlighted.

A Z-scheme photocatalyst constructed with an yttrium–tantalum oxynitride and a binuclear Ru(ii) complex for visible-light CO2 reduction

A hybrid photocatalyst composed of an yttrium–tantalum oxynitride (with a 2.1 eV band gap) and a binuclear Ru(ii) complex containing both photosensitizing and catalytic units was capable of reducing CO₂ to HCOOH with very high selectivity (>99%) under visible light (>400 nm) irradiation.

Mechanism of the Multiple-Electron Oxygen Reduction Reaction in the Presence of the Binuclear Cu(acac)2 Complex

The potential of the electron for the serial oxygen reduction reaction is calculated by DFT in an aqueous solution in the presence and absence of Cu(acac)2 complex. The study provides interesting information about the rational design of complex-semiconductor hybrid photocatalysts and cathodes for polymer electrolyte membrane fuel cells.

Molecular Catalyst Immobilized Photocathodes for Water/Proton and Carbon Dioxide Reduction

As one of the components in a tandem photoelectrochemical cell for solar-fuel production, the photocathode carries out the reduction reaction to convert solar light and the corresponding substrate (e.g., proton and CO2) into target fuels. Immobilizing molecular catalysts onto the photocathode is a promising strategy to enhance the interfacial electron/hole-transfer process and to improve the stability of the catalysts. Furthermore, the molecular catalysts are beneficial in improving the selectivity of the reduction reaction, particularly for CO2 reduction. On the photocathode, the binding mode of the catalysts and the arrangement between the photosensitizer and the catalyst also play crucial roles in the performance and stability of the final device. How to firmly and effectively immobilize the catalyst on the photoelectrode is now becoming a scientific question. Recent publications on molecular catalyst immobilized photocathodes are therefore surveyed.

A novel Ru/TiO2 hybrid nanocomposite catalyzed photoreduction of CO2 to methanol under visible light

A novel in situ synthesized Ru(bpy)3/TiO2 hybrid nanocomposite is developed for the photoreduction of CO2 into methanol under visible light irradiation. The prepared composite was characterized by means of SEM, TEM, XRD, DT-TGA, XPS, UV-Vis and FT-IR techniques. The photocatalytic activity of the synthesized hybrid catalyst was tested for the photoreduction of CO2 under visible light using triethylamine as a sacrificial donor. The methanol yield for the Ru(bpy)3/TiO2 hybrid nanocomposite was found to be 1876 μmol g(-1) cat (ϕMeOH 0.024 mol Einstein(-1)) that was much higher in comparison with the in situ synthesized TiO2, 828 μmol g(-1) cat (ϕMeOH 0.010 mol Einstein(-1)) and the homogeneous Ru(bpy)3Cl2 complex, 385 μmol g(-1) cat (ϕMeOH 0.005 mol Einstein(-1)).

Iridium(III) Bis-Pyridine-2-Sulfonamide Complexes as Efficient and Durable Catalysts for Homogeneous Water Oxidation

A family of tetradentate bis(pyridine-2-sulfonamide) (bpsa) compounds was synthesized as a ligand platform for designing resilient and electronically tunable catalysts capable of performing water oxidation catalysis and other processes in highly oxidizing environments. These wrap-around ligands were coordinated to Ir(III) octahedrally, forming an anionic complex with chloride ions bound to the two remaining coordination sites. NMR spectroscopy documented that the more rigid ligand frameworks-[Ir(bpsa-Cy)Cl2](-) and [Ir(bpsa-Ph)Cl2](-)-produced C1-symmetric complexes, while the complex with the more flexible ethylene linker in [Ir(bpsa-en)Cl2](-) displays C2 symmetry. Their electronic structure was explored with DFT calculations and cyclic voltammetry in nonaqueous environments, which unveiled highly reversible Ir(III)/Ir(IV) redox processes and more complex, irreversible reduction chemistry. Addition of water to the electrolyte revealed the ability of these complexes to catalyze the water oxidation reaction efficiently. Electrochemical quartz crystal microbalance studies confirmed that a molecular species is responsible for the observed electrocatalytic behavior and ruled out the formation of active IrOx. The electrochemical studies were complemented by work on chemically driven water oxidation, where the catalytic activity of the iridium complexes was studied upon exposure to ceric ammonium nitrate, a strong, one-electron oxidant. Variation of the catalyst concentrations helped to illuminate the kinetics of these water oxidation processes and highlighted the robustness of these systems. Stable performance for over 10 days with thousands of catalyst turnovers was observed with the C1-symmetric catalysts. Dynamic light scattering experiments ascertained that a molecular species is responsible for the catalytic activity and excluded the formation of IrOx particles.

Photocatalytic oxidation of organic compounds in a hybrid system composed of a molecular catalyst and visible light-absorbing semiconductor

Photocatalytic oxidation of organic compounds proceeded efficiently in a hybrid system with ruthenium aqua complexes as catalysts, BiVO4 as a light absorber, [Co(NH3)5Cl](2+) as a sacrificial electron acceptor and water as an oxygen source. The photogenerated holes in the semiconductor are used to oxidize molecular catalysts into the high-valent Ru(IV)=O intermediates for 2e(-) oxidation.

Selective Formic Acid Production via CO2 Reduction with Visible Light Using a Hybrid of a Perovskite Tantalum Oxynitride and a Binuclear Ruthenium(II) Complex

A hybrid material consisting of CaTaO2N (a perovskite oxynitride semiconductor having a band gap of 2.5 eV) and a binuclear Ru(II) complex photocatalytically produced HCOOH via CO2 reduction with high selectivity (>99%) under visible light (λ>400 nm). Results of photocatalytic reactions, spectroscopic measurements, and electron microscopy observations indicated that the reaction was driven according to a two-step photoexcitation of CaTaO2N and the Ru photosensitizer unit, where Ag nanoparticles loaded on CaTaO2N with optimal distribution mediated interfacial electron transfer due to reductive quenching.

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.

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.

Photocatalytic carbon dioxide reduction by copper oxide nanocluster-grafted niobate nanosheets

Amorphous copper oxide (Cu(II)) nanoclusters function as efficient electrocatalysts for the reduction of carbon dioxide (CO2) to carbon monoxide (CO). In addition to promoting electrocatalytic activity, Cu(II) nanoclusters act as efficient cocatalyts for CO2 photoreduction when grafted onto the surface of a semiconductor (light harvester), such as niobate (Nb3O8(-)) nanosheets. Here, the photocatalytic activity and reaction pathway of Cu(II)-grafted Nb3O8(-) nanosheets was investigated using electron spin resonance (ESR) analysis and isotope-labeled molecules (H2(18)O and (13)CO2). The results of the labeling experiments demonstrated that under UV irradiation, electrons are extracted from water to produce oxygen ((18)O2) and then reduce CO2 to produce (13)CO. ESR analysis confirmed that excited holes in the valence band of Nb3O8(-) nanosheets react with water, and that excited electrons in the conduction band of Nb3O8(-) nanosheets are injected into the Cu(II) nanoclusters through the interface and are involved in the reduction of CO2 into CO. The Cu(II) nanocluster-grafted Nb3O8(-) nanosheets are composed of nontoxic and abundant elements and can be facilely synthesized by a wet chemical method. The nanocluster grafting technique described here can be applied for the surface activation of various semiconductor light harvesters, such as metal oxide and/or metal chalcogenides, and is expected to aid in the development of efficient CO2 photoreduction systems.

A functionalised nickel cyclam catalyst for CO2 reduction: electrocatalysis, semiconductor surface immobilisation and light-driven electron transfer

Covalent immobilisation of a low cost electrocatalyst leads to an enhanced rate of photoelectron transfer from a light absorbing semiconductor.

Heterostructured nanocomposite tin phthalocyanine@mesoporous ceria (SnPc@CeO2) for photoreduction of CO2 in visible light

Heterostructured tin phthalocyanine supported to mesoporous ceria was synthesized and used a photocatalyst for CO₂ reduction under visible light.

Photoreduction of carbon dioxide to formic acid in aqueous suspension: a comparison between phthalocyanine/TiO2 and porphyrin/TiO2 catalysed processes

Composite materials prepared by loading polycrystalline TiO₂ powders with lipophilic highly branched Cu(II)- and metal-free phthalocyanines or porphyrins, which have been used in the past as photocatalysts for photodegradative processes, have been successfully tested for the efficient photoreduction of carbon dioxide in aqueous suspension affording significant amounts of formic acid. The results indicated that the presence of the sensitizers is beneficial for the photoactivity, confirming the important role of Cu(II) co-ordinated in the middle of the macrocycles. A comparison between Cu(II) phthalocyanines and Cu(II) porphyrins indicated that the Cu(II)- phthalocyanine sensitizer was more efficient in the photoreduction of CO₂ to formic acid, probably due to its favorable reduction potential.