Efficient Visible‐Light‐Driven CO2 Reduction Mediated by Defect‐Engineered BiOBr Atomic Layers

Solar CO₂ reduction efficiency is largely limited by poor photoabsorption, sluggish electron–hole separation, and a high CO₂ activation barrier. Defect engineering was employed to optimize these crucial processes. As a prototype, BiOBr atomic layers were fabricated and abundant oxygen vacancies were deliberately created on their surfaces. X‐ray absorption near‐edge structure and electron paramagnetic resonance spectra confirm the formation of oxygen vacancies. Theoretical calculations reveal the creation of new defect levels resulting from the oxygen vacancies, which extends the photoresponse into the visible‐light region. The charge delocalization around the oxygen vacancies contributes to CO₂ conversion into COOH* intermediate, which was confirmed by in situ Fourier‐transform infrared spectroscopy. Surface photovoltage spectra and time‐resolved fluorescence emission decay spectra indicate that the introduced oxygen vacancies promote the separation of carriers. As a result, the oxygen‐deficient BiOBr atomic layers achieve visible‐light‐driven CO₂ reduction with a CO formation rate of 87.4 μmol g^?1 h^?1, which was not only 20 and 24 times higher than that of BiOBr atomic layers and bulk BiOBr, respectively, but also outperformed most previously reported single photocatalysts under comparable conditions.     

Recent Advances in Photocatalytic CO2 Reduction Using Earth‐Abundant Metal Complexes‐Derived Photocatalysts

Photochemical reduction of CO₂ with H₂O into energy‐rich chemicals using inexhaustible solar energy is an appealing strategy to simultaneously address the global energy and environmental issues. Earth‐abundant metal complexes show promising application in this field due to their easy availability, rich redox valence and tunable property. Great progress has been seen on catalytic reduction of CO₂ under visible light illumination employing earth‐abundant metal complexes and their hybrids as key contributors, especially for producing CO and HCOOH via the two‐electron reduction process. In this minireview, we will summarize and update advances on earth‐abundant metal complex‐derived photocatalytic system for visible‐light driven CO₂ photoreduction over the last 5 years. Homogeneous earth‐abundant metal complex photocatalysts and earth‐abundant metal complex derived hybrid photocatalysts were both presented with focus on efficient improvement strategy.     

Visible‐Light Photocatalytic Radical Alkenylation of α‐Carbonyl Alkyl Bromides and Benzyl Bromides

Through the use of [Ru(bpy)₃Cl₂] (bpy=2,2′‐bipyridine) and [Ir(ppy)₃] (ppy=phenylpyridine) as photocatalysts, we have achieved the first example of visible‐light photocatalytic radical alkenylation of various α‐carbonyl alkyl bromides and benzyl bromides to furnish α‐vinyl carbonyls and allylbenzene derivatives, prominent structural elements of many bioactive molecules. Specifically, this transformation is regiospecific and can tolerate primary, secondary, and even tertiary alkyl halides that bear β‐hydrides, which can be challenging with traditional palladium‐catalyzed approaches. The key initiation step of this transformation is visible‐light‐induced single‐electron reduction of C? Br bonds to generate alkyl radical species promoted by photocatalysts. The following carbon? carbon bond‐forming step involves a radical addition step rather than a metal‐mediated process, thereby avoiding the undesired β‐hydride elimination side reaction. Moreover, we propose that the Ru and Ir photocatalysts play a dual role in the catalytic system: they absorb energy from the visible light to facilitate the reaction process and act as a medium of electron transfer to activate the alkyl halides more effectively. Overall, this photoredox catalysis method opens new synthetic opportunities for the efficient alkenylation of alkyl halides that contain β‐hydrides under mild conditions.     

In situ preparation of polymeric cobalt phthalocyanine–decorated TiO2 nanorods for efficient photocatalytic CO2 reduction

The integration of photosensitizers with low-cost and non-toxic metal oxides is a promising strategy to design heterogeneous photocatalysts for CO₂ reduction. Herein, p–n heterojunction photocatalysts (T-CoPPcs) consisting of p-type polymeric cobalt phthalocyanines (CoPPcs) as a photosensitizer coupled with n-type TiO₂ nanorods were fabricated through a facile, eco-friendly, one-pot hydrothermal reaction. In this process, CoPPcs were grown on n-type TiO₂ nanorods, whereas protonated titanate nanorods began converting to the highly crystalline anatase phase with small crystals on the TiO₂ surfaces. The introduction of CoPPcs not only improved the solar light utilization but also accelerated the separation and migration of charge carriers via the p–n heterojunction with the strong interfacial contact Ti–O–Co bond. The increases in crystallinity and surface area of TiO₂ nanorods also contributed to the enhanced photoactivities of T-CoPPcs. The CO₂ photoreduction of the synthesized materials was evaluated in CO₂-saturated MeCN/water using [Co(bpy)₃]²⁺ as a cocatalyst and triethanolamine as a hole scavenger. The optimized nanocomposite exhibited a remarkable CO generation rate of 4.42 mmol/h/g with a high selectivity of 85.3% and outstanding catalytic stability. The influences of cocatalyst concentration, water content, catalyst loading, and hole scavenger concentration were optimized for efficient CO₂ reduction. The photocatalytic CO₂ conversion efficiency of the present system is found to be higher than that of TiO₂-based materials reported in the literature. We believe that this research into a heterostructural design strategy and photocatalytic system may be an inspiration for the development of photocatalytic CO₂-to-CO conversion.     

Copper-Sulfur-Nitrogen Cluster Providing a Local Proton for Efficient Carbon Dioxide Photoreduction

Atomically precise Cu clusters are highly desirable as catalysts for CO₂ reduction reaction (CO₂RR), and they provide an appropriate model platform for elaborating their structure–activity relationship. However, an efficient overall photocatalytic CO₂RR with H₂O using assembled Cu-cluster aggregates as single component photocatalyst has not been reported. Herein, we report a stable crystalline Cu−S−N cluster photocatalyst with local protonated N−H groups (denoted as Cu₆−NH ). The catalyst exhibits suitable photocatalytic redox potentials, high structural stability, active catalytic species, and a narrow band gap, which account for its outstanding photocatalytic CO₂RR performance under visible light, with ≈100 % selectivity for CO evolution. Remarkably, systematic isostructural Cu-cluster control experiments, in situ infrared spectroscopy, and density functional theory calculations revealed that the protonated pyrimidine N atoms in the Cu₆−NH cluster act as a proton relay station, providing a local proton during the photocatalytic CO₂RR. This efficiently lowers the energy barrier for the formation of the *COOH intermediate, which is the rate-limiting step, efficiently enhancing the photocatalytic performance. This work lays the foundation for the development of atomically precise metal-cluster-based photocatalysts.     

Electrochemical CO2 reduction with power generation in SOFCs with Cu-added LSCF–GDC cathode

Solid oxide fuel cell (SOFC) unit was constructed with Ni–GDC (gadolinia-doped ceria) as the anode, YSZ as the electrolyte, and Cu-added La_0.58Sr_0.4Co_0.2Fe_0.8O_3–δ–GDC as the cathode. Electrochemical CO₂ reduction occurs. The CO formation rate, the CO₂ conversion and the generated current density increase with increasing CO₂ concentration and temperature. The CO₂ conversion rate equals exactly the CO formation rate. No carbon deposition occurs. The activation energy is 2.72 kcal mol^?1. The electrochemical CO₂ reduction (dissociation) can have much lower activation barrier than the catalytic one. Simultaneous CO₂ reduction with power generation in SOFCs can be feasible.     

Thermal expansion-quench of nickel metal-organic framework into nanosheets for efficient visible light CO2 reduction

Metal-organic framework nanosheets (MOF NNs) offer potential opportunities for many applications,but an efficient strategy for the scalable preparation of few-layered two-dimensional (2D) MOF NNs are still a major challenge.Herein,we present an efficient top-down method for the synthesis of the Ni-BDC(Ni₂(OH)₂(1,4-BDC);1,4-BDC=1,4-benzenedicarboxylate) nanosheets utilizing a novel thermal expansionquench method of the flowerlike bulky MOFs in liquid N₂.The obtain...     

Biochar modified Co–Al LDH for enhancing photocatalytic reduction CO2 performance and mechanism insight

This study prepared a biochar-based photocatalyst (Co–Al LDH–C) via facile ultrasonic-assisted solvent treatment. The Co–Al LDH–C photocatalyst shows better photocatalytic activity in CO₂ reduciton than the pure Co–Al LDH without biochar modification. The Co–Al LDH–C affords a CO generation rate of 29.2 µmol g^?1. The enhanced CO₂ reduction activity is attributed to the biochar in Co–Al LDH enhanced the light absorption property and separation efficiency of the charge carriers. Additionally, a mechanism insight of Co–Al LDH reduction CO₂ is also investigated by a series of characterizations and experiments results. This work offers a new insight for CO₂ reduction by waste utilization of biomass and improved the performance of Co–Al LDH, and extends the broad potential application of biochar-based photocatalyst in the photocatalytic conversion from solar to carbon resource.

     

Supramolecular photocatalysts fixed on the inside of the polypyrrole layer in dye sensitized molecular photocathodes: application to photocatalytic CO2 reduction coupled with water oxidation

The development of systems for photocatalytic CO₂ reduction with water as a reductant and solar light as an energy source is one of the most important milestones on the way to artificial photosynthesis. Although such reduction can be performed using dye-sensitized molecular photocathodes comprising metal complexes as redox photosensitizers and catalyst units fixed on a p-type semiconductor electrode, the performance of the corresponding photoelectrochemical cells remains low, e.g., their highest incident photon-to-current conversion efficiency (IPCE) equals 1.2%. Herein, we report a novel dye-sensitized molecular photocathode for photocatalytic CO₂ reduction in water featuring a polypyrrole layer, [Ru(diimine)₃]²⁺ as a redox photosensitizer unit, and Ru(diimine)(CO)₂Cl₂ as the catalyst unit and reveal that the incorporation of the polypyrrole network significantly improves reactivity and durability relative to those of previously reported dye-sensitized molecular photocathodes. The irradiation of the novel photocathode with visible light under low applied bias stably induces the photocatalytic reduction of CO₂ to CO and HCOOH with high faradaic efficiency and selectivity (even in aqueous solution), and the highest IPCE is determined as 4.7%. The novel photocathode is coupled with n-type semiconductor photoanodes (CoO_x/BiVO₄ and RhO_x/TaON) to construct full cells that photocatalytically reduce CO₂ using water as the reductant upon visible light irradiation as the only energy input at zero bias. The artificial Z-scheme photoelectrochemical cell with the dye-sensitized molecular photocathode achieves the highest energy conversion efficiency of 8.3 × 10⁻²% under the irradiation of both electrodes with visible light, while a solar to chemical conversion efficiency of 4.2 × 10⁻²% is achieved for a tandem-type cell using a solar light simulator (AM 1.5, 100 mW cm⁻²).

A novel dye-sensitized molecular photocathode with polypyrrole networks exhibits high efficiency and durability for photocatalytic CO₂ reduction by using water as reductant and visible light as energy.     

Bi‐, Y‐Codoped TiO2 for Carbon Dioxide Photocatalytic Reduction to Formic Acid under Visible Light Irradiation

Bi‐ and Y‐codoped TiO₂ photocatalysts were synthesized through a sol‐gel method, and they were applied in the photocatalytic reduction of CO₂ to formic acid under visible light irradiation. The results revealed that, after doping Bi and Y, the surface area of TiO₂ was increased from 5.4 to 93.1 m²/g when the mole fractions of doping Bi and Y were 1.0% and 0.5%, respectively, and the lattice structures of the photocatalysts changed and the oxygen vacancies on the surface of the photocatalysts formed, which would act as the electron capture centers and slow down the recombination of photo‐induced electron and hole. The photocurrent spectra also proved that the photocatalysts had better electronic transmission capacities. The HCOOH yield in CO₂ photocatalytic reduction was 747.82 μmol/g_cat by using 1% Bi‐0.5% Y‐TiO₂ as a photocatalyst. The HCOOH yield was 1.17 times higher than that by using 1% Bi‐TiO₂, and 2.23 times higher than that by using pure TiO₂. Furthermore, the 1% Bi‐0.5% Y‐TiO₂ showed the highest apparent quantum efficiency (AQE) of 4.45%.     

Studies on Photocatalytic CO2 Reduction over NH2‐Uio‐66(Zr) and Its Derivatives: Towards a Better Understanding of Photocatalysis on Metal–Organic Frameworks

Metal–organic framework (MOF) NH₂‐Uio‐66(Zr) exhibits photocatalytic activity for CO₂ reduction in the presence of triethanolamine as sacrificial agent under visible‐light irradiation. Photoinduced electron transfer from the excited 2‐aminoterephthalate (ATA) to Zr oxo clusters in NH₂‐Uio‐66(Zr) was for the first time revealed by photoluminescence studies. Generation of Zr^III and its involvement in photocatalytic CO₂ reduction was confirmed by ESR analysis. Moreover, NH₂‐Uio‐66(Zr) with mixed ATA and 2,5‐diaminoterephthalate (DTA) ligands was prepared and shown to exhibit higher performance for photocatalytic CO₂ reduction due to its enhanced light adsorption and increased adsorption of CO₂. This study provides a better understanding of photocatalytic CO₂ reduction over MOF‐based photocatalysts and also demonstrates the great potential of using MOFs as highly stable, molecularly tunable, and recyclable photocatalysts in CO₂ reduction.     

Striking Differences in Properties of Geometric Isomers of [Ir(tpy)(ppy)H]+: Experimental and Computational Studies of their Hydricities,Interaction with CO2, and Photochemistry

We prepared two geometric isomers of [Ir(tpy)(ppy)H]⁺, previously proposed as a key intermediate in the photochemical reduction of CO₂ to CO, and characterized their notably different ground‐ and excited‐state interactions with CO₂ and their hydricities using experimental and computational methods. Only one isomer, C‐trans‐[Ir(tpy)(ppy)H]⁺, reacts with CO₂ to generate the formato complex in the ground state, consistent with its calculated hydricity. Under photocatalytic conditions in CH₃CN/TEOA, a common reactive C‐trans‐[Ir(tpy)(ppy)]⁰ species, irrespective of the starting isomer or monodentate ligand (such as hydride or Cl), reacts with CO₂ and produces CO with the same catalytic efficiency.     

Striking Differences in Properties of Geometric Isomers of [Ir(tpy)(ppy)H]+: Experimental and Computational Studies of their Hydricities,Interaction with CO2, and Photochemistry

We prepared two geometric isomers of [Ir(tpy)(ppy)H]⁺, previously proposed as a key intermediate in the photochemical reduction of CO₂ to CO, and characterized their notably different ground‐ and excited‐state interactions with CO₂ and their hydricities using experimental and computational methods. Only one isomer, C‐trans‐[Ir(tpy)(ppy)H]⁺, reacts with CO₂ to generate the formato complex in the ground state, consistent with its calculated hydricity. Under photocatalytic conditions in CH₃CN/TEOA, a common reactive C‐trans‐[Ir(tpy)(ppy)]⁰ species, irrespective of the starting isomer or monodentate ligand (such as hydride or Cl), reacts with CO₂ and produces CO with the same catalytic efficiency.     

Tin(II)-Based Metal–Organic Frameworks Enabling Efficient,Selective Reduction of CO2 to Formate under Visible Light

Certain metal complexes are known as high-performance CO₂ reduction photocatalysts driven by visible light. However, most of them rely on rare, precious metals as principal components, and integrating the functions of light absorption and catalysis into a single molecular unit based on abundant metals remains a challenge. Metal-organic frameworks (MOFs), which can be regarded as intermediate compounds between molecules and inorganic solids, are potential platforms for the construction of a simple photocatalytic system composed only of Earth-abundant nontoxic elements. In this work, we report that a tin-based MOF enables the conversion of CO₂ into formic acid with a record high apparent quantum yield (9.8 % at 400 nm) and >99 % selectivity without the need for any additional photosensitizer or catalyst. This work highlights a new MOF with strong potential for photocatalytic CO₂ reduction driven by solar energy.     

A high-yield synthetic approach to trans-[Ir(ER3)2(CO)X] (ER3=PMe3, PEt3, PPhMe2, PPh2Me,P(OMe)3, AsMe3, AsEt3, AsPh3, SbPh3; X=Cl,Br)

The complex [Ir(CO)₂X₂][NBu₄] (X=Cl, Br) forms Vaska-type complexes, trans-[Ir(ER₃)₂(CO)X], when treated with two equivalents of aryl- or alkyl-phosphines, arsines, or stibines under a CO atmosphere. The synthesis is general for a wide range of phosphines, arsines, or stibines irrespective of the cone angle. For small cone-angle ligands, the initial addition of ligand to [Ir(CO)₂X₂][NBu₄] is performed at low temperature. The synthesis and characterisation of three new Vaska-type complexes trans-[Ir(P(OMe)₃)₂(CO)Cl], trans-[Ir(AsMe₃)₂(CO)Cl], and trans-[Ir(AsEt₃)₂(CO)Cl] is also reported.     

Iridium(III) Photocatalysis: A Visible‐Light‐Induced Dearomatizative Tandem [4+2] Cyclization to Furnish Benzindolizidines

A photocatalytic dearomatizative tandem [4+2] cyclization between N‐(2‐iodoethyl)indoles and a variety of alkenes leads to tri‐ and tetracyclic benzindolizidines with high diastereoselectivity and yield. The intermolecular annulation reaction is performed under visible‐light irradiation and employs [Ir(ppy)₃] or [Ir(dtbbpy)(ppy)₂] PF₆ as photocatalysts, in combination with tertiary amines as electron and hydrogen atom donors.     

Rapid green and blue light‐induced CO release from bromazepam Mn(I) and Ru(II) carbonyls: synthesis,density functional theory and biological activity evaluation

Two new photoactivatable carbon monoxide‐releasing molecules (CORMs), fac ‐[Mn(CO)₃(BZM)Br] ( 1 ) and [RuCl₂(BZM)(CO)₂] ( 2 ), derived from the anti‐anxiety drug bromazepam (BZM) and capable of rapid release of CO upon the illumination with light‐emitting diode (LED) source light (470–525 nm) have been synthesized and characterized. The photo‐delivery of CO to myoglobin solution could be achieved via the illumination of 1 at 525 nm for 10 min or at 470 nm for 1 min. The addition of water to a dimethylsulfoxide (DMSO) solution of 1 increases its stability, but it decreases the rate of the CO‐releasing process. CORM 2 behaves as a good visible photoCORM, whereby its two CO molecules are released within 10 min upon exposure to LED light source at 470 nm in DMSO. The illumination profile of 2 was also examined using solution infrared spectroscopy. The cis /trans stereochemistry around the Ru(II) ion was determined using quantum chemical calculations. The formation of photoCORMs leads to an improvement in the toxicity of BZM against tested microbes.     

Amide-bridged conjugated organic polymers: efficient metal-free catalysts for visible-light-driven CO2 reduction with H2O to CO

The visible-light-driven photoreduction of CO₂ to value-added chemicals over metal-free photocatalysts without sacrificial reagents is very interesting, but challenging. Herein, we present amide-bridged conjugated organic polymers (amide-COPs) prepared via self-condensation of amino nitriles in combination with hydrolysis, for the photoreduction of CO₂ with H₂O without any photosensitizers or sacrificial reagents under visible light irradiation. These catalysts can afford CO as the sole carbonaceous product without H₂ generation. Especially, amide-DAMN derived from diaminomaleonitrile exhibited the highest activity for the photoreduction of CO₂ to CO with a generation rate of 20.6 μmol g⁻¹ h⁻¹. Experiments and DFT calculations confirmed cyano/amide groups as active sites for CO₂ reduction and second amine groups for H₂O oxidation, and suggested that superior selectivity towards CO may be attributed to the adjacent redox sites. This work presents a new insight into designing photocatalysts for artificial photosynthesis.

Amino nitrile-derived conjugated organic polymers can realize the photoreduction of CO₂ with water to CO without H₂ generation efficiently.     

Imidazolium-modification enhances photocatalytic CO2 reduction on ZnSe quantum dots

Colloidal photocatalysts can utilize solar light for the conversion of CO₂ to carbon-based fuels, but controlling the product selectivity for CO₂ reduction remains challenging, in particular in aqueous solution. Here, we present an organic surface modification strategy to tune the product selectivity of colloidal ZnSe quantum dots (QDs) towards photocatalytic CO₂ reduction even in the absence of transition metal co-catalysts. Besides H₂, imidazolium-modified ZnSe QDs evolve up to 2.4 mmol_CO g_ZnSe⁻¹ (TON_QD > 370) after 10 h of visible light irradiation (AM 1.5G, λ > 400 nm) in aqueous ascorbate solution with a CO-selectivity of up to 20%. This represents a four-fold increase in CO-formation yield and 13-fold increase in CO-selectivity compared to non-functionalized ZnSe QDs. The binding of the thiolated imidazolium ligand to the QD surface is characterized quantitatively using ¹H-NMR spectroscopy and isothermal titration calorimetry, revealing that a subset of 12 to 17 ligands interacts strongly with the QDs. Transient absorption spectroscopy reveals an influence of the ligand on the intrinsic charge carrier dynamics through passivating Zn surface sites. Density functional theory calculations indicate that the imidazolium capping ligand plays a key role in stabilizing the surface-bound *CO₂⁻ intermediate, increasing the yield and selectivity toward CO production. Overall, this work unveils a powerful tool of using organic capping ligands to modify the chemical environment on colloids, thus enabling control over the product selectivity within photocatalyzed CO₂ reduction.

A photocatalyst system consisting of ZnSe quantum dots modified with a thiolated imidazolium capping ligand for visible light-driven reduction of aqueous CO₂ to CO is reported without the need for a metal complex co-catalyst.     

Extending the π-System in MnI Diimine Tricarbonyl Complexes: Impacts on Photochemistry,Electrochemistry, and CO2 Catalytic Reduction Activity

The complex [Mn(bpy)(CO)₃Br], has been previously studied as both an electrocatalyst and a photocatalyst, in conjugation with a photosensitizer, for CO₂ reduction to CO. This study considers the relationship between this catalytic activity and the steric and electronic nature of the aromatic diimine ligand. To this end, the π-system in the bidentate ligand is increased step-wise from 2,2′-bipyridine ( bpy ) to 2-(2-pyridyl)quinoline ( pq ) to 2,2′-biquinoline ( bqn ) in a series of three fac-[Mn(α-diimine)(CO)₃Br] complexes. It is found that the propensity of these complexes to photochemically dimerize trends with the energy of the α-diimine π* energy. Electrochemically, it is observed that the second reduction event in these systems becomes increasingly thermodynamically favorable and approaches the potential of the first reduction event as the π-system expands. In fac-[Mn(bqn)(CO)₃Br], the second reduction is more favorable than the first reduction, precluding the formation of a dimer intermediate; even though, chemical reduction of fac-[Mn(bqn)(CO)₃Br] confirms that the dimer, [Mn(bqn)(CO)₃Br]₂ is able to form and not prevented by steric considerations. Though the second reduction potential is more positive for bqn and pq than for bpy , the CO₂ reduction mechanism changes such that the overpotential for carbon dioxide reduction occurs at more negative potentials, leading to a decrease in overall catalytic activity.