Selective and efficient photocatalytic CO2 reduction to CO using visible light and an iron-based homogeneous catalyst

Photocatalytic Conversion of CO2 to CO by a Copper(II) Quaterpyridine Complex

The invention of efficient systems for the photocatalytic reduction of CO₂ comprising earth-abundant metal catalysts is a promising approach for the production of solar fuels. One bottleneck is to design highly selective and robust molecular complexes that are able to transform the CO₂ gas. The Cu^II quaterpyridine complex [Cu(qpy)]²⁺ (1) is found to be a highly efficient and selective catalyst for visible-light driven CO₂ reduction in CH₃ CN using [Ru(bpy)₃ ]²⁺ (bpy: bipyridine) as photosensitizer and BIH/TEOA (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole/triethanolamine) as sacrificial reductant. The photocatalytic reaction is greatly enhanced by the presence of H₂ O (1-4 % v/v), and a turnover number of >12 400 for CO production can be achieved with 97 % selectivity, which is among the highest of molecular 3d CO₂ reduction catalysts. Results from Hg poisoning and dynamic light scattering experiments suggest that this photocatalyst is homogenous. To the best of our knowledge, 1 is the first example of molecular Cu-based catalyst for the photoreduction of CO₂ .

Synthesis and Photophysical Properties of Porphyrin Macrorings Composed of Free-Base Porphyrins and Slipped-Cofacial Zinc Porphyrin Dimers

The self-assembled macroring N-(Zn-Fb-Zn)₃ has been constructed by intermolecular complementary coordination among three trisporphyrin Zn-Fb-Zn molecules, each of which consists of a central free-base porphyrin and two imidazolyl-zinc-porphyrin ends. Thus, N-(Zn-Fb-Zn)₃ has three slipped-cofacial zinc porphyrin dimers ("special pair model") and three free-base porphyrins, alternately. The zinc porphyrin dimers in N-(Zn-Fb-Zn)₃ are covalently connected by a ring-closing olefin metathesis reaction between the allyl ether groups substituted on the zinc porphyrin dimers, giving a covalently linked macroring C-(Zn-Fb-Zn)₃. The fluorescence spectra of C-(Zn-Fb-Zn)₃ in several solvents show that the photoinduced energy transfer from one of the zinc porphyrin dimers to a free-base porphyrin occurs intramolecularly in toluene, whereas the photoinduced electron transfer predominantly occurs intramolecularly in N,N-dimethylformamide. Treatment of C-(Zn-Fb-Zn)₃ with copper(II) acetate gives a Cu-containing heteromultinuclear porphyrin macroring C-(Zn-Cu-Zn)₃, demonstrating that C-(Zn-Fb-Zn)₃ could be a good precursor to construct various heteromultinuclear porphyrin macrorings.

Powering a CO2 Reduction Catalyst with Visible Light through Multiple Sub-picosecond Electron Transfers from a Quantum Dot

Photosensitization of molecular catalysts to reduce CO₂ to CO is a sustainable route to storable solar fuels. Crucial to the sensitization process is highly efficient transfer of redox equivalents from sensitizer to catalyst; in systems with molecular sensitizers, this transfer is often slow because it is gated by diffusion-limited collisions between sensitizer and catalyst. This article describes the photosensitization of a meso-tetraphenylporphyrin iron(III) chloride (FeTPP) catalyst by colloidal, heavy metal-free CuInS₂/ZnS quantum dots (QDs) to reduce CO₂ to CO using 450 nm light. The sensitization efficiency (turnover number per absorbed unit of photon energy) of the QD system is a factor of 18 greater than that of an analogous system with a fac-tris(2-phenylpyridine)iridium sensitizer. This high efficiency originates in ultrafast electron transfer between the QD and FeTPP, enabled by formation of QD/FeTPP complexes. Optical spectroscopy reveals that the electron-transfer processes primarily responsible for the first two sensitization steps (Fe^IIITPP → Fe^IITPP, and Fe^IITPP → Fe^ITPP) both occur in <200 fs.

Perspectives on metal–organic frameworks with intrinsic electrocatalytic activity

This highlight article focuses on the rapidly emerging area of electrocatalytic metal–organic frameworks (MOFs) with a particular emphasis on those systems displaying intrinsic activity.

Co3 O4 Hexagonal Platelets with Controllable Facets Enabling Highly Efficient Visible-Light Photocatalytic Reduction of CO2

A heterogeneous catalyst made of well-defined Co3 O4 hexagonal platelets with varied exposed facets is coupled with [Ru(bpy)3 ]Cl2 photosensitizers to effectively and efficiently reduce CO2 under visible-light irradiation. Systematic investigation based on both experiment and theory discloses that the exposed {112} facets are crucial for activating CO2 molecules, giving rise to significant enhancement of photocatalytic CO2 reduction efficiency.

Photochemical and electrochemical catalytic reduction of CO2 with NHC-containing dicarbonyl rhenium(i) bipyridine complexes

The electrochemical and photochemical catalytic reductions of CO2 using N,O and N,S-NHC-containing dicarbonyl rhenium(i) bipyridine complexes have been investigated. By replacing the carbonyl ligand in tricarbonyl rhenium(i) complexes with a weaker π-accepting ligand, the characteristic MLCT transitions shifted to lower energy. This makes photocatalysts capable of harvesting low-energy visible light for catalyzing CO2 reduction. A detailed study revealed that these dicarbonyl rhenium(i) complexes are also highly selective for photocatalysis of CO2 to CO with a good quantum efficiency (10%), similar to that of the tricarbonyl rhenium(i) complex analogues. From the electrochemical study, it was observed that the catalysts efficiently produce CO from CO2 with high turnover frequency and good stability over time.

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.

Iron-catalyzed photoreduction of carbon dioxide to synthesis gas

Photocatalytic processes to convert CO₂ to useful products including CO and HCOOH are of particular interest as a means to harvest the power of the sun for sustainable energy applications. Herein, we report the photocatalytic reduction of CO₂ using iron-based catalysts and visible light generating varying ratios of synthesis gas.

Highly active and selective photochemical reduction of CO2 to CO using molecular-defined cyclopentadienone iron complexes

Highly active and selective visible-light-driven CO₂ reduction to CO catalyzed by well-defined cyclopentadienone iron complexes.

Photocatalytic reduction of CO2 and H2O to CO and H2 with a cobalt chlorin complex adsorbed on multi-walled carbon nanotubes

The photocatalytic reduction of CO₂ and H₂O with triethylamine occurred using a cobalt(ii) chlorin complex adsorbed on multi-walled carbon nanotubes as a catalyst to yield CO and H₂ with a high turnover number of 710.

The effects of chelating N4 ligand coordination on Co(ii)-catalysed photochemical conversion of CO2 to CO: reaction mechanism and DFT calculations

cis-[Co(PDP)Cl₂] complex mediated reduction conversion of CO₂ to CO under photocatalytic or electrocatalytic conditions with high turnovers or Faraday efficiency.

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.

Photocatalytic Reduction of CO2 with Re-Pyridyl-NHCs

A series of Re(I) pyridyl N-heterocyclic carbene (NHC) complexes have been synthesized and examined in the photocatalytic reduction of CO2 using a simulated solar spectrum. The catalysts were characterized through NMR, UV-vis, cyclic voltammetry under nitrogen, and cyclic voltammetry under carbon dioxide. The complexes were compared directly with a known benchmark catalyst, Re(bpy) (CO)3Br. An electron-deficient NHC substituent (PhCF3) was found to promote catalytic activity when compared with electron-neutral and -rich substituents. Re(PyNHC-PhCF3) (CO)3Br was found to exceed the CO production of the benchmark Re(bpy) (CO)3Br catalyst (51 vs 33 TON) in the presence of electron donor BIH and photosensitizer fac-Ir(ppy)3. Importantly, Re(PyNHC-PhCF3) (CO)3Br was found to function without a photosensitizer (32 TON) at substantially higher turnovers than the benchmark catalyst Re(bpy) (CO)3Br (14 TON) under a solar simulated spectrum.

Highly effective sites and selectivity of nitrogen-doped graphene/CNT catalysts for CO2 electrochemical reduction

The selectivity of CO₂ electrochemical reduction can be tuned for N-doped graphene/CNT catalysts after active sites are determined.

Metal-free catalysts, such as graphene/carbon nanostructures, are highly cost-effective to replace expensive noble metals for CO₂ reduction if fundamental issues, such as active sites and selectivity, are clearly understood. Using both density functional theory (DFT) and ab initio molecular dynamic calculations, we show that the interplay of N-doping and curvature can effectively tune the activity and selectivity of graphene/carbon-nanotube (CNT) catalysts. The CO₂ activation barrier can be optimized to 0.58 eV for graphitic-N doped graphene edges, compared with 1.3 eV in the un-doped counterpart. The graphene catalyst without curvature shows strong selectivity for CO/HCOOH production, whereas the (6, 0) CNT with a high degree of curvature is effective for both CH₃OH and HCHO production. Curvature is also very influential to tune the overpotential for a given product, e.g. from 1.5 to 0.02 V for CO production and from 1.29 to 0.49 V for CH₃OH production. Hence, the graphene/CNT nanostructures offer great scope and flexibility for effective tunning of catalyst efficiency and selectivity, as shown here for CO₂ reduction.

Comparison of rhenium-porphyrin dyads for CO2 photoreduction: photocatalytic studies and charge separation dynamics studied by time-resolved IR spectroscopy

We report a study of the photocatalytic reduction of CO₂ to CO by zinc porphyrins covalently linked to [Re^I(2,2'-bipyridine)(CO)₃L]^+/0 moieties with visible light of wavelength >520 nm. Dyad 1 contains an amide C₆H₄NHC(O) link from porphyrin to bipyridine (Bpy), Dyad 2 contains an additional methoxybenzamide within the bridge C₆H₄NHC(O)C₆H₃(OMe)NHC(O), while Dyad 3 has a saturated bridge C₆H₄NHC(O)CH₂; each dyad is studied with either L = Br or 3-picoline. The syntheses, spectroscopic characterisation and cyclic voltammetry of Dyad 3 Br and [Dyad 3 pic]OTf are described. The photocatalytic performance of [Dyad 3 pic]OTf in DMF/triethanolamine (5 : 1) is approximately an order of magnitude better than [Dyad 1 pic]PF₆ or [Dyad 2 pic]OTf in turnover frequency and turnover number, reaching a turnover number of 360. The performance of the dyads with Re-Br units is very similar to that of the dyads with [Re-pic]⁺ units in spite of the adverse free energy of electron transfer. The dyads undergo reactions during photocatalysis: hydrogenation of the porphyrin to form chlorin and isobacteriochlorin units is detected by visible absorption spectroscopy, while IR spectroscopy reveals replacement of the axial ligand by a triethanolaminato group and insertion of CO₂ into the latter to form a carbonate. Time-resolved IR spectra of [Dyad 2 pic]OTf and [Dyad 3 pic]OTf (560 nm excitation in CH₂Cl₂) demonstrated electron transfer from porphyrin to Re(Bpy) units resulting in a shift of ν(CO) bands to low wavenumbers. The rise time of the charge-separated species for [Dyad 3 pic]OTf is longest at 8 (±1) ps and its lifetime is also the longest at 320 (±15) ps. The TRIR spectra of Dyad 1 Br and Dyad 2 Br are quite different showing a mixture of ³MLCT, IL and charge-separated excited states. In the case of Dyad 3 Br, the charge-separated state is absent altogether. The TRIR spectra emphasize the very different excited states of the bromide complexes and the picoline complexes. Thus, the similarity of the photocatalytic data for bromide and picoline dyads suggests that they share common intermediates. Most likely, these involve hydrogenation of the porphyrin and substitution of the axial ligand at rhenium.

Face the Edges: Catalytic Active Sites of Nanomaterials

Edges are special sites in nanomaterials. The atoms residing on the edges have different environments compared to those in other parts of a nanomaterial and, therefore, they may have different properties. Here, recent progress in nanomaterial fields is summarized from the viewpoint of the edges. Typically, edge sites in MoS2 or metals, other than surface atoms, can perform as active centers for catalytic reactions, so the method to enhance performance lies in the optimization of the edge structures. The edges of multicomponent interfaces present even more possibilities to enhance the activities of nanomaterials. Nanoframes and ultrathin nanowires have similarities to conventional edges of nanoparticles, the application of which as catalysts can help to reduce the use of costly materials. Looking beyond this, the edge structures of graphene are also essential for their properties. In short, the edge structure can influence many properties of materials.

Nitrogen-doped graphene-supported copper complex: a novel photocatalyst for CO2 reduction under visible light irradiation

Nitrogen-doped graphene immobilized with copper(ii) complex (GrN₇₀₀–CuC) is demonstrated to be an efficient photocatalyst for CO₂ reduction into methanol under visible light irradiation.

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.

Photochemical CO2-reduction catalyzed by mono- and dinuclear phenanthroline-extended tetramesityl porphyrin complexes

The conversion of CO₂ into CO is catalyzed by mono- and dinuclear phenanthroline-extended porphyrin complexes. The influence of the central metal center in the porphyrin cavity as well as of an attached ruthenium fragment at the phenanthroline moiety was investigated in wavelength-dependent photolysis experiments.