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

A robust Fe-based heterogeneous photocatalyst for the visible-light-mediated selective reduction of an impure CO2 stream

The transformation of CO2 into value-added products from an impure CO2 stream, such as flue gas or exhaust gas, directly contributes to the principle of carbon capture and utilization (CCU). Thus, we have developed a robust iron-based heterogeneous photocatalyst that can convert the exhaust gas from the car into CO with an exceptional production rate of 145 μmol g-1 h-1. We characterized this photocatalyst by PXRD, XPS, ssNMR, EXAFS, XANES, HR-TEM, and further provided mechanistic experiments, and multi-scale/level computational studies. We have reached a clear understanding of its properties and performance that indicates that this highly robust photocatalyst could be used to design an efficient visible-light-mediated reduction strategy for the transformation of impure CO2 streams into value-added products.

Efficient Visible-Light-Driven Carbon Dioxide Reduction using a Bioinspired Nickel Molecular Catalyst

Inspired by natural enzymes, this study presents a nickel-based molecular catalyst, [Ni‖(N2S2)]Cl2 (NiN2S2, N2S2=2,11-dithia[3,3](2,6)pyridinophane), for the photochemical catalytic reduction of CO2 under visible light. The catalyst was synthesized and characterized using various techniques, including liquid chromatography-high resolution mass spectrometry (LC-HRMS), UV-Visible spectroscopy, and X-ray crystallography. The crystallographic analysis revealed a slightly distorted octahedral coordination geometry with a mononuclear Ni2+ cation, two nitrogen atoms and two sulfur atoms. Photocatalytic CO2 reduction experiments were performed in homogeneous conditions using the catalyst in combination with [Ru(bpy)3]Cl2 (bpy=2,2'-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor. The catalyst achieved a high selectivity of 89 % towards CO and a remarkable turnover number (TON) of 7991 during 8 h of visible light irradiation under CO2 in the presence of phenol as a co-substrate. The turnover frequency (TOF) in the initial 6 h was 1079 h-1, with an apparent quantum yield (AQY) of 1.08 %. Controlled experiments confirmed the dependency on the catalyst, light, and sacrificial electron donor for the CO2 reduction process. These findings demonstrate this bioinspired nickel molecular catalyst could be effective for fast and efficient photochemical catalytic reduction of CO2 to CO.

Photocatalytic CO2 reduction with iron porphyrin catalysts and anthraquinone dyes

Herein we studied visible-light-driven CO2 reduction using a series of tetra-phenylporphyrin iron catalysts and inexpensive anthraquinone dyes. Varying the functional groups on the phenyl moieties of the catalysts significantly enhances the photocatalytic activity, achieving an optimal turnover number (TON) of 10 476 and a selectivity of 100% in the noble-metal-free systems. The highest activity found in a bromo-substituted catalyst is attributed to favorable electron transfer from the photosensitizer to the iron porphyrin.

Charge Transport Approaches in Photocatalytic Supramolecular Systems Composing of Semiconductor and Molecular Metal Complex for CO2 Reduction

The design of photocatalytic supramolecular systems composing of semiconductors and molecular metal complexes for CO2 reduction has attracted increasing attention. The supramolecular system combines the structural merits of semiconductors and metal complexes, where the semiconductor harvests light and undertakes the oxidative site, while the metal complex provides activity for CO2 reduction. The intermolecular charge transfer plays crucial role in ensuring photocatalytic performance. Here, we review the progress of photocatalytic supramolecular systems in reduction of CO2 and highlight the interfacial charge transfer pathways, as well as their state-of-the-art characterization methods. The remaining challenges and prospects for further design of supramolecular photocatalysts are also presented.

Photocatalytic CO2 Reduction Using Mixed Catalytic Systems Comprising an Iron Cation with Bulky Phenanthroline Ligands

This study reports on efficient photocatalytic CO2 reduction reactions using mixed catalytic systems of an Fe ion source and various 1,10-phenanthroline derivatives (R1R2p) as ligands in the presence of triethanolamine (TEOA). As the relatively bulky substituents at positions 2 and 9 of R1R2p weakened the ability to coordinate to the Fe ion, the Fe ion formed TEOA complexes. The free R1R2p accepted an electron from the reduced photosensitizer through proton-coupled electron transfer (PCET) using protons of TEOA dissolved in a CH3CN solution in a CO2 atmosphere as the initial step of the catalytic cycle. Although the mixed system of the nonsubstituted 1,10-phenanthroline generates a stable tris(phenanthroline)-Fe(II) complex in solution, this complex could not function as a CO2 reduction catalyst. The mechanism in which R1R2p interacts with the Fe ion after PCET was proposed for this efficient photocatalytic CO2 reduction. The proposed photocatalytic system using the 2,9-di-sec-butyl-phenanthroline ligand could produce CO with high efficiency (quantum yield of 8.2%) combined with a dinuclear Cu(I) complex as a photosensitizer.

Stabilizing electron-rich Ni single-atoms on black phosphorus nanosheets boosts photocatalytic carbon dioxide reduction

The development of unique single-atom catalysts with electron-rich feature is essential to promoting the photocatalytic CO2 reduction, yet remains a big challenge. Here, a conceptionally new single-atom catalyst constructed from atomically dispersed Ni-P3 species on black phosphorus (BP) nanosheets (BP-Ni) is synthesized for realizing highly efficient visible-light-driven CO2 reduction when trapping photogenerated electrons from homogeneous light absorbers in the presence of triethanolamine as the sacrificial agent. Both the experimental and theoretical calculation data reveal that the Ni-P3 species on BP nanosheets own the electron-rich feature that can improve the photogenerated charge separation efficiency and lower the activation barrier of CO2 conversion. This unique feature makes BP-Ni exhibit the much higher activity as cocatalyst in the photocatalytic CO2 reduction than BP nanosheets. The BP-Ni can also be applied as a cocatalyst for enhanced photocatalytic CO2 reduction after combining with CdSe/S colloidal crystal photocatalyst. The present study offers valuable inspirations for the design and construction of effective catalytic sites toward photocatalytic CO2 reduction reactions.

Identification of Crucial Photosensitizing Factors to Promote CO2 -to-CO Conversion

The sensitizing ability of a catalytic system is closely related to the visible-light absorption ability, excited-state lifetime, redox potential, and electron-transfer rate of photosensitizers (PSs), however it remains a great challenge to concurrently mediate these factors to boost CO2 photoreduction. Herein, a series of Ir(III)-based PSs (Ir-1-Ir-6) were prepared as molecular platforms to understand the interplay of these factors and identify the primary factors for efficient CO2 photoreduction. Among them, less efficient visible-light absorption capacity results in lower CO yields of Ir-1, Ir-2 or Ir-4. Ir-3 shows the most efficient photocatalytic activity among these mononuclear PSs due to some comprehensive parameters. Although the Kobs of Ir-3 is ≈10 times higher than that of Ir-5, the CO yield of Ir-3 is slightly higher than that of Ir-5 due to the compensation of Ir-5's strong visible-light-absorbing ability. Ir-6 exhibits excellent photocatalytic performance due to the strong visible-light absorption ability, comparable thermodynamic driving force, and electron transfer rate among these PSs. Remarkably, the CO2 photoreduction to CO with Ir-6 can achieve 91.5 μmol, over 54 times higher than Ir-1, and the optimized TONC-1 can reach up to 28160. Various photophysical properties of the PSs were concurrently adjusted by fine ligand modification to promote CO2 photoreduction.

Contemporary advances in photocatalytic CO2 reduction using single-atom catalysts supported on carbon-based materials

The persistent issue of CO2 emissions and their subsequent impact on the Earth's atmosphere can be effectively addressed through the utilization of efficient photocatalysts. Employing a sustainable carbon cycle via photocatalysis presents a promising technology for simultaneously managing the greenhouse effect and the energy dilemma. However, the efficiency of energy conversion encounters limitations due to inadequate carrier utilization and a deficiency of reactive sites. Single-atom catalysts (SACs) have demonstrated exceptional performance in efficiently addressing the aforementioned challenges. This review article commences with an overview of SAC types, structures, fundamentals, synthesis strategies, and characterizations, providing a logical foundation for the design and properties of SACs based on the correlation between their structure and efficiency. Additionally, we delve into the general mechanism and the role of SACs in photocatalytic CO2 reduction. Furthermore, we furnish a comprehensive survey of the latest advancements in SACs concerning their capacity to enhance efficiency, long-term stability, and selectivity in CO2 reduction. Carbon-structured support materials such as covalent organic frameworks (COFs), graphitic carbon nitride (g-C3N4), metal-organic frameworks (MOFs), covalent triazine frameworks (CTFs), and graphene-based photocatalysts have garnered significant attention due to their substantial surface area, superior conductivity, and chemical stability. These carbon-based materials are frequently chosen as support matrices for anchoring single metal atoms, thereby enhancing catalytic activity and selectivity. The motivation behind this review article lies in evaluating recent developments in photocatalytic CO2 reduction employing SACs supported on carbon substrates. In conclusion, we highlight critical issues associated with SACs, potential prospects in photocatalytic CO2 reduction, and existing challenges. This review article is dedicated to providing a comprehensive and organized compilation of recent research findings on carbon support materials for SACs in photocatalytic CO2 reduction, with a specific focus on materials that are environmentally friendly, readily accessible, cost-effective, and exceptionally efficient. This work offers a critical assessment and serves as a systematic reference for the development of SACs supported on MOFs, COFs, g-C3N4, graphene, and CTFs support materials to enhance photocatalytic CO2 conversion.

Theoretical prediction of superatom WSi12-based catalysts for CO oxidation by N2O

Catalytic conversion of N2O and CO into nonharmful gases is of great significance to reduce their adverse impact on the environment. The potential of the WSi12 superatom to serve as a new cluster catalyst for CO oxidation by N2O is examined for the first time. It is found that WSi12 prefers to adsorb the N2O molecule rather than the CO molecule, and the charge transfer from WSi12 to N2O results in the full activation of N2O into a physically absorbed N2 molecule and an activated oxygen atom that is attached to an edge of the hexagonal prism structure of WSi12. After the release of N2, the remaining oxygen atom can oxidize one CO molecule via overcoming a rate-limiting barrier of 28.19 kcal mol-1. By replacing the central W atom with Cr and Mo, the resulting MSi12 (M = Cr and Mo) superatoms exhibit catalytic performance for CO oxidation comparable to the parent WSi12. In particular, the catalytic ability of WSi12 for CO oxidation is well maintained when it is extended into tube-like WnSi6(n+1) (n = 2, 4, and 6) clusters with energy barriers of 25.63-29.50 kcal mol-1. Moreover, all these studied MSi12 (M = Cr, Mo, and W) and WnSi6(n+1) (n = 2, 4, and 6) species have high structural stability and can absorb sunlight to drive the catalytic process. This study not only opens a new door for the atomically precise design of new silicon-based nanoscale catalysts for various chemical reactions but also provides useful atomic-scale insights into the size effect of such catalysts in heterogeneous catalysis.

Introducing proton/electron mediators enhances the catalytic ability of an iron porphyrin complex for photochemical CO2 reduction

A novel iron porphyrin complex with hydroquinone moieties as proton/electron mediators at meso positions was designed and synthesised. The complex serves as an efficient catalyst for photochemical CO2 reduction, and its turnover frequency (TOF = 1.3 × 104 h-1) was the highest among those of comparable systems with sufficient durability.

Mixed-Blocks-Driving Nickel-Porphyrin Covalent Organic Polymers for Photocatalytic Syngas Production from CO2 with Fine Selectivity

CO2 photoconversion to syngas with superb selectivity is a splendid and bright option to achieve environmental improvement, energy substitution, and industrial needs. Herein, a series of Ni-porphyrin covalent organic polymers (COPs) interspersed with furan and thiophene using a mixed-blocks-engineering strategy, named as OXSY-Ni COPs (X and Y refer to the relative amounts of furan and thiophene blocks, respectively), are synthesized for photocatalytic CO2-to-syngas. Ni-coordinated porphyrin cores prefer to act as mediators of CO2-to-CO photoconversion because of the higher adsorption capacity of CO2. Ni-free porphyrins work mainly as active sites of H2 photoevolution. Furthermore, introducing different amounts of furan and thiophene modulates jointly the electronic structure of Ni-porphyrin COPs and optimizes the conduction band alignment. The above controllable variables achieve a wonderful syngas (CO/H2) ratio range from 2:1.06 to 1:1.04 for the Fischer-Tropsch process within common industrial reactions. Notably, the COP of the O1S3-Ni COPs exhibits excellent photocatalytic CO2-to-syngas activity under visible light, with a syngas yield of 8442.5 μmol g-1 h-1 (CO/H2 = 1:1.02) and an apparent quantum efficiency (AQE) of 1.92% at 450 nm. This strategy would provide a significance path to design functional and efficient organic semiconductors.

Incorporating Catalytic Units into Nanomaterials: Rational Design of Multipurpose Catalysts for CO2 Valorization

ConspectusCO2 conversion to valuable chemicals is effective at reducing CO2 emissions. We previously proposed valorization strategies and developed efficient catalysts to address thermodynamic stability and kinetic inertness issues related to CO2 conversion. Earlier, we developed molecular capture reagents and catalysts to integrate CO2 capture and conversion, i.e., in situ transformation. Based on the mechanistic understanding of CO2 capture, activation, and transformation at a molecular level, we set out to develop heterogeneous catalysts by incorporating catalytic units into nanomaterials via the immobilization of active molecular catalysts onto nanomaterials and designing nanomaterials with intrinsic catalytic sites.In thermocatalytic CO2 conversion, carbonaceous and metal-organic framework (MOF)-based catalysts were developed for nonreductive and reductive CO2 conversion. Novel Cu- and Zn-based MOFs and carbon-supported Cu catalysts were prepared and successfully applied to the cycloaddition, carboxylation, and carboxylative cyclization reactions with CO2, generating cyclic carbonates, carboxyl acids, and oxazolidinones as respective target products. Reductive conversion of CO2, especially reductive functionalization with CO2, is a promising transformation strategy to produce valuable chemicals, alleviating chemical production that relies on petrochemistry. We explored the hierarchical reductive functionalization of CO2 using organocatalysts and proposed strategies to regulate the CO2 reduction level, triggering heterogeneous catalyst investigation. Introducing multiple active sites into nanomaterials opens possibilities to develop novel CO2 transformation strategies. CO2 capture and in situ conversion were realized with an N-doped carbon-supported Zn complex and MOF materials as CO2 adsorbents and catalysts. These nanomaterial-based catalysts feature high stability and excellent efficiency and act as shape-selective catalysts in some cases due to their unique pore structure.Nanomaterial-based catalysts are also appealing candidates for photocatalytic CO2 reduction (PCO2RR) and electrocatalytic CO2 reduction (ECO2RR), so we developed a series of hybrid photo-/electrocatalysts by incorporating active metal complexes into different matrixes such as porous organic polymers (POPs), metal-organic layers (MOLs), micelles, and conducting polymers. By introducing Re-bipyridine and Fe-porphyrin complexes into POPs and regulating the structure of the polymer chain, catalyst stability and efficiency increased in PCO2RR. PCO2RR in aqueous solution was realized by designing the Re-bipyridine-containing amphiphilic polymer to form micelles in aqueous solution and act as nanoreactors. We prepared MOLs with two different metallic centers, i.e., the Ni-bipyridine site and Ni-O node, to improve the efficiency for PCO2RR due to the synergistic effect of these metal centers. Sulfylphenoxy-decorated cobalt phthalocyanine (CoPc) cross-linked polypyrrole was prepared and used as a cathode, achieving the electrocatalytic transformation of diluted CO2 benefiting from the CO2 adsorption capability of polypyrrole. We fabricated immobilized 4-(t-butyl)-phenoxy cobalt phthalocyanine and Bi-MOF as cathodes to promote the paired electrolysis of CO2 and 5-hydroxymethylfurfural (HMF) and obtained CO2 reductive products and 2,5-furandicarboxylic acid (FDCA) efficiently.

Artificial Photosynthesis: Current Advancements and Future Prospects

Artificial photosynthesis is a technology with immense potential that aims to emulate the natural photosynthetic process. The process of natural photosynthesis involves the conversion of solar energy into chemical energy, which is stored in organic compounds. Catalysis is an essential aspect of artificial photosynthesis, as it facilitates the reactions that convert solar energy into chemical energy. In this review, we aim to provide an extensive overview of recent developments in the field of artificial photosynthesis by catalysis. We will discuss the various catalyst types used in artificial photosynthesis, including homogeneous catalysts, heterogeneous catalysts, and biocatalysts. Additionally, we will explore the different strategies employed to enhance the efficiency and selectivity of catalytic reactions, such as the utilization of nanomaterials, photoelectrochemical cells, and molecular engineering. Lastly, we will examine the challenges and opportunities of this technology as well as its potential applications in areas such as renewable energy, carbon capture and utilization, and sustainable agriculture. This review aims to provide a comprehensive and critical analysis of state-of-the-art methods in artificial photosynthesis by catalysis, as well as to identify key research directions for future advancements in this field.

Mechanistic Insights into Photochemical CO2 Reduction to CH4 by a Molecular Iron-Porphyrin Catalyst

Iron tetraphenylporphyrin complex modified with four trimethylammonium groups (Fe-p-TMA) is found to be capable of catalyzing the eight-electron eight-proton reduction of CO₂ to CH₄ photochemically in acetonitrile. In the present work, density functional theory (DFT) calculations have been performed to investigate the reaction mechanism and to rationalize the product selectivity. Our results revealed that the initial catalyst Fe-p-TMA ([Cl-Fe(III)-LR₄]⁴⁺, where L = tetraphenylporphyrin ligand with a total charge of -2, and R₄ = four trimethylammonium groups with a total charge of +4) undergoes three reduction steps, accompanied by the dissociation of the chloride ion to form [Fe(II)-L^••2-R₄]²⁺. [Fe(II)-L^••2-R₄]²⁺, bearing a Fe(II) center ferromagnetically coupled with a tetraphenylporphyrin diradical, performs a nucleophilic attack on CO₂ to produce the ¹η-CO₂ adduct [CO₂^•--Fe(II)-L^•-R₄]²⁺. Two intermolecular proton transfer steps then take place at the CO₂ moiety of [CO₂^•--Fe(II)-L^•-R₄]²⁺, resulting in the cleavage of the C-O bond and the formation of the critical intermediate [Fe(II)-CO]⁴⁺ after releasing a water molecule. Subsequently, [Fe(II)-CO]⁴⁺ accepts three electrons and one proton to generate [CHO-Fe(II)-L^•-R₄]²⁺, which finally undergoes a successive four-electron-five-proton reduction to produce methane without forming formaldehyde, methanol, or formate. Notably, the redox non-innocent tetraphenylporphyrin ligand was found to play an important role in CO₂ reduction since it could accept and transfer electron(s) during catalysis, thus keeping the ferrous ion at a relatively high oxidation state. Hydrogen evolution reaction via the formation of Fe-hydride ([Fe(II)-H]³⁺) turns out to endure a higher total barrier than the CO₂ reduction reaction, therefore providing a reasonable explanation for the origin of the product selectivity.

Graphene Oxide-Mediated Synthesis of Ultrathin Co-MOL for CO2 Photoreduction

Metal-organic framework (MOF) materials have broad application prospects in catalysis because of their ordered structure and molecular adjustability. However, the large volume of bulky MOF usually leads to insufficient exposure of the active sites and the obstruction of charge/mass transfer, which greatly limits their catalytic performance. Herein, we developed a simple graphene oxide (GO) template method to fabricate ultrathin Co-metal-organic layer (2.0 nm) on reduced GO (Co-MOL@r-GO). The as-synthesized hybrid material Co-MOL@r-GO-2 exhibits highly efficient photocatalytic performance for CO₂ reduction, and the CO yield can reach as high as 25,442 μmol/g_Co-MOL, which is over 20 times higher than that of the bulky Co-MOF. Systematic investigations demonstrate that GO can act as a template for the synthesis of the ultrathin Co-MOL with more active sites and can be used as the electron transport medium between the photosensitizer and the Co-MOL to enhance the catalytic activity for CO₂ photoreduction.

Electronic effects promoted the catalytic activities of binuclear Co(II) complexes for visible-light-driven CO2 reduction in a water-containing system

Under the action of a catalyst, the photoinduced reduction of CO₂ to chemicals and fuels is one of the greenest and environment-friendly approaches for decreasing atmospheric CO₂ emissions. Since the environment was affected by the greenhouse effect, scientists have never stopped exploring efficient photoinduced CO₂ reduction systems, particularly the highly desired non-noble metal complexes. Most of the currently reported complexes based on non-noble metals exhibit low catalytic activity, selectivity, and stability in aqueous systems under the irradiation of visible light. Herein, we report a new binuclear cobalt complex [Co₂(L¹)(OAc)₂](OAc) (Co2L1, HL¹ = 2,6-bis((bis(pyridin-2-ylmethyl)amino)methyl)-4-methoxyphenol), which accelerates the visible-light-driven conversion of CO₂ to CO in acetonitrile/water (4/1, v/v) nearly 40% more than that for the previously reported [Co₂(L²)(OAc)₂](OAc) (Co2L2, HL² = 2, 6-bis((bis(pyridin-2-ylmethyl)amino)methyl)-4-(tert-butyl)phenol) by our research group. It has an excellent CO selectivity of 98%, and the TON_CO is as high as 5920. Experimental results and DFT calculations showed that the enhanced catalytic performance of Co2L1 is due to the electron-donating effect of a methoxy group (-OCH₃) in Co2L1 compared to a tertiary butyl group (-C(CH₃)₃) in Co2L2, which reduces the energy barrier of the rate-limiting CO₂ coordination step in the visible-light-driven CO₂ reduction process.

Photocatalytic CO2 reduction with aminoanthraquinone organic dyes

The direct utilization of solar energy to convert CO₂ into renewable chemicals remains a challenge. One essential difficulty is the development of efficient and inexpensive light-absorbers. Here we show a series of aminoanthraquinone organic dyes to promote the efficiency for visible light-driven CO₂ reduction to CO when coupled with an Fe porphyrin catalyst. Importantly, high turnover numbers can be obtained for both the photosensitizer and the catalyst, which has not been achieved in current light-driven systems. Structure-function study performed with substituents having distinct electronic effects reveals that the built-in donor-acceptor property of the photosensitizer significantly promotes the photocatalytic activity. We anticipate this study gives insight into the continued development of advanced photocatalysts for solar energy conversion.

Synergistic Porosity and Charge Effects in a Supramolecular Porphyrin Cage Promote Efficient Photocatalytic CO2 Reduction

We present a supramolecular approach to catalyzing photochemical CO2 reduction through second-sphere porosity and charge effects. An iron porphyrin box (PB) bearing 24 cationic groups, FePB-2(P), was made via post-synthetic modification of an alkyne-functionalized supramolecular synthon. FePB-2(P) promotes the photochemical CO2 reduction reaction (CO2 RR) with 97 % selectivity for CO product, achieving turnover numbers (TON) exceeding 7000 and initial turnover frequencies (TOFmax ) reaching 1400 min-1 . The cooperativity between porosity and charge results in a 41-fold increase in activity relative to the parent Fe tetraphenylporphyrin (FeTPP) catalyst, which is far greater than analogs that augment catalysis through porosity (FePB-3(N), 4-fold increase) or charge (Fe p-tetramethylanilinium porphyrin (Fe-p-TMA), 6-fold increase) alone. This work establishes that synergistic pendants in the secondary coordination sphere can be leveraged as a design element to augment catalysis at primary active sites within confined spaces.

Homoleptic Al(III) Photosensitizers for Durable CO2 Photoreduction

Exploiting noble-metal-free systems for high-performance photocatalytic CO₂ reduction still presents a key challenge, partially due to the long-standing difficulties in developing potent and durable earth-abundant photosensitizers. Therefore, based on the very cheap aluminum metal, we have deployed a systematic series of homoleptic Al(III) photosensitizers featuring 2-pyridylpyrrolide ligands for CO₂ photoreduction. The combined studies of steady-state and time-resolved spectroscopy as well as quantum chemical calculations demonstrate that in anerobic CH₃CN solutions at room temperature, visible-light excitation of the Al(III) photosensitizers leads to an efficient population of singlet excited states with nanosecond-scale lifetimes and notable emission quantum yields (10-40%). The results of transient absorption spectroscopy further identified the presence of emissive singlet and unexpectedly nonemissive triplet excited states. More importantly, the introduction of methyl groups at the pyrrolide rings can greatly improve the visible-light absorption, reducing power, and durability of the Al(III) photosensitizers. With triethanolamine, BIH (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole), and an Fe(II)-quaterpyridine catalyst, the most methylated Al(III) photosensitizer achieves an apparent quantum efficiency of 2.8% at 450 nm for selective (>99%) CO₂-to-CO conversion, which is nearly 28 times that of the unmethylated one (0.1%) under identical conditions. The optimal system realizes a maximum turnover number of 10250 and higher robustness than the systems with Ru(II) and Cu(I) benchmark photosensitizers. Quenching experiments using fluorescence spectroscopy elucidate that the photoinduced electron transfer in the Al(III)-sensitized system follows a reductive quenching pathway. The remarkable tunability and cost efficiency of these Al(III) photosensitizers should allow them as promising components in noble-metal-free systems for solar fuel conversion.

Deep transfer learning for predicting frontier orbital energies of organic materials using small data and its application to porphyrin photocatalysts

A deep transfer learning approach is used to predict HOMO/LUMO energies of organic materials with a small amount of training data.

Polymeric tungsten carbide nanoclusters as potential non-noble metal catalysts for CO oxidation

The discovery of tungsten carbide (WC) as an analog of the noble metal Pt atom is of great significance toward designing novel highly-active catalysts from the viewpoint of the superatom concept. The potential of such a superatom to serve as building blocks of replacement catalysts for Pt has been evaluated in this work. The electronic properties, adsorption behaviors, and catalytic mechanisms towards the CO oxidation of (WC)_n and Pt_n (n = 1, 2, 4, and 6) were compared. Counterintuitively, these studied (WC)_n clusters exhibit quite different electronic properties and adsorption behaviours from the corresponding Pt_n species. For instance, (WC)_n preferentially adsorbs O₂, whereas Pt_n tends to first combine with CO. Even so, it is interesting to find that the catalytic performances of (WC)_n are always superior to the corresponding Pt_n, and especially, the largest (WC)₆ cluster exhibits the best catalytic ability towards CO oxidation. Therefore, assembling superatomic WC clusters into larger polymeric clusters can be regarded as a novel strategy to develop efficient superatom-assembled catalysts for CO oxidation. It is highly expected to see the realization of non-noble metal catalysts for various reactions in the near future experiments by using superatoms as building blocks.

Highly Efficient Visible-Light-Driven CO2-to-CO Conversion by Coordinatively Unsaturated Co-Salen Complexes in a Water-Containing System

The development of cost-effective catalysts for CO₂ reduction is highly desired but remains a significant challenge. The unsaturated coordination metal center in a catalyst is favorable for the process of catalytic CO₂ reduction. In this paper, two asymmetric salen ligands were used to synthesize two coordinatively unsaturated Co-salen complexes. The two Co-salen complexes exhibit an unsaturated coordination pattern and display high activity and CO selectivity for visible-light-driven CO₂ reduction in a water-containing system. The photocatalytic performance of 2 is higher than that of 1 because the reduction potential of the catalytic Co^II center and the energy barrier of the catalytic transition states of 2 are lower than those of 1, with turnover numbers (TON_CO), turnover frequencies (TOF), and CO selectivity values of 8640, 0.24 s^-1, and 97% for 2, respectively. The photocatalytic reduction of CO₂ to CO for 2 is well supported by control experiments and density functional theory (DFT) calculations.

Multifunctional Charge and Hydrogen-Bond Effects of Second-Sphere Imidazolium Pendants Promote Capture and Electrochemical Reduction of CO2 in Water Catalyzed by Iron Porphyrins

Microenvironments tailored by multifunctional secondary coordination sphere groups can enhance catalytic performance at primary metal active sites in natural systems. Here, we capture this biological concept in synthetic systems by developing a family of iron porphyrins decorated with imidazolium (im) pendants for the electrochemical CO2 reduction reaction (CO2 RR), which promotes multiple synergistic effects to enhance CO2 RR and enables the disentangling of second-sphere contributions that stem from each type of interaction. Fe-ortho-im(H), which poises imidazolium units featuring both positive charge and hydrogen-bond capabilities proximal to the active iron center, increases CO2 binding affinity by 25-fold and CO2 RR activity by 2000-fold relative to the parent Fe tetraphenylporphyrin (Fe-TPP). Comparison with monofunctional analogs reveals that through-space charge effects have a greater impact on catalytic CO2 RR performance compared to hydrogen bonding in this context.

Dual electronic effects achieving a high-performance Ni(II) pincer catalyst for CO2 photoreduction in a noble-metal-free system

A carbazolide-bis(NHC) Ni^II catalyst (1; NHC, N-heterocyclic carbene) for selective CO₂ photoreduction was designed herein by a one-stone-two-birds strategy. The extended π-conjugation and the strong σ/π electron-donation characteristics (two birds) of the carbazolide fragment (one stone) lead to significantly enhanced activity for photoreduction of CO₂ to CO. The turnover number (TON) and turnover frequency (TOF) of 1 were ninefold and eightfold higher than those of the reported pyridinol-bis(NHC) Ni^II complex at the same catalyst concentration using an identical Ir photosensitizer, respectively, with a selectivity of ∼100%. More importantly, an organic dye was applied to displace the Ir photosensitizer to develop a noble-metal-free photocatalytic system, which maintained excellent performance and obtained an outstanding quantum yield of 11.2%. Detailed investigations combining experimental and computational studies revealed the catalytic mechanism, which highlights the potential of the one-stone-two-birds effect.

Highly Robust Rhenium(I) Bipyridyl Complexes Containing Dipyrromethene-BF2 Chromophores for Visible Light-Driven CO2 Reduction

New rhenium bipyridyl complexes with dipyrromethene-BF₂ chromophores (A-ReBDP-CZ, A-ReBDP₂ , ReBDP-CZ, and ReBDP₂ ) were developed for highly efficient photocatalytic carbon dioxide (CO₂ ) reduction to carbon monoxide (CO). These catalysts consisted of two moderate electron-deficient groups (dipyrromethene-BF₂ , BDP) as the visible-light-harvesting antenna as well as both electron donor (N-phenylcarbazole, CZ) and acceptor (BDP) on Re bipyridyl framework. Among ReBDP-CZ and ReBDP₂ complexes, the ReBDP₂ incorporating two electron-deficient BDP chromophores had a longer-lived photoexcited state (182.4 μs) and a twofold enhanced molar absorption coefficient (ϵ=157000 m^-1  cm^-1 ) compared with ReBDP-CZ. Thus, ReBDP₂ achieved the superior photocatalytic reactivity and stability with a CO turnover number (TON_CO ) value as high as 1323 and quantum yield (Φ_CO ) up to 55 %, which was the most excellent photocatalysis efficiency among the single-active-site Re catalysts without additional photosensitizer. Furthermore, the acetylene-bridged linker was detrimental to the photoactivity and durability of the catalyst. In brief, two BDP-based Re bipyridyl systems with outstanding catalytic performance and significant visible-light-harvesting capabilities in the solar spectrum offer a promising strategy for solar-to-fuel conversion schemes.

Co-facial π-π Interaction Expedites Sensitizer-to-Catalyst Electron Transfer for High-Performance CO2 Photoreduction

The sunlight-driven reduction of CO₂ into carbonaceous fuels can lower the atmospheric CO₂ concentration and provide renewable energy simultaneously, attracting scientists to design photocatalytic systems for facilitating this process. Significant progress has been made in designing high-performance photosensitizers and catalysts in this regard, and further improvement can be realized by installing additional interactions between the abovementioned two components, however, the design strategies and mechanistic investigations on such interactions remain challenging. Here, we present the construction of molecular models for intermolecular π-π interactions between the photosensitizer and the catalyst, via the introduction of pyrene groups into both molecular components. The presence, types, and strengths of diverse π-π interactions, as well as their roles in the photocatalytic mechanism, have been examined by ¹H NMR titration, fluorescence quenching measurements, transient absorption spectroscopy, and quantum chemical simulations. We have also explored the rare dual emission behavior of the pyrene-appended iridium photosensitizer, of which the excited state can deliver the photo-excited electron to the pyrene-decorated cobalt catalyst at a fast rate of 2.60 × 10⁶ s^-1 via co-facial π-π interaction, enabling a remarkable apparent quantum efficiency of 14.3 ± 0.8% at 425 nm and a high selectivity of 98% for the photocatalytic CO₂-to-CO conversion. This research demonstrates non-covalent interaction construction as an effective strategy to achieve rapid CO₂ photoreduction besides a conventional photosensitizer/catalyst design.

Exploiting consecutive photoinduced electron transfer (ConPET) in CO2 photoreduction

The consecutive photoinduced electron transfer (ConPET) process of 1,2,3,5-Tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) in CO₂ photoreduction to achieve powerful reducing species has been disclosed by activating a bis(terpyridine)ruthenium(II) complex bearing a high overpotential for selective light-driven reduction of CO₂ to CO in homogeneous solution.

Porphyrins and phthalocyanines as biomimetic tools for photocatalytic H2 production and CO2 reduction

The increasing energy demand and environmental issues caused by the over-exploitation of fossil fuels render the need for renewable, clean, and environmentally benign energy sources unquestionably urgent. The zero-emission energy carrier, H₂ is an ideal alternative to carbon-based fuels especially when it is generated photocatalytically from water. Additionally, the photocatalytic conversion of CO₂ into chemical fuels can reduce the CO₂ emissions and have a positive environmental and economic impact. Inspired by natural photosynthesis, plenty of artificial photocatalytic schemes based on porphyrinoids have been investigated. This review covers the recent advances in photocatalytic H₂ production and CO₂ reduction systems containing porphyrin or phthalocyanine derivatives. The unique properties of porphyrinoids enable their utilization both as chromophores and as catalysts. The homogeneous photocatalytic systems are initially described, presenting the various approaches for the improvement of photosensitizing activity and the enhancement of catalytic performance at the molecular level. On the other hand, for the development of the heterogeneous systems, numerous methods were employed such as self-assembled supramolecular porphyrinoid nanostructures, construction of organic frameworks, combination with 2D materials and adsorption onto semiconductors. The dye sensitization on semiconductors opened the way for molecular-based dye-sensitized photoelectrochemical cells (DSPECs) devices based on porphyrins and phthalocyanines. The research in photocatalytic systems as discussed herein remains challenging since there are still many limitations making them unfeasible to be used at a large scale application before finding a large-scale application.

Dissection of Light-Induced Charge Accumulation at a Highly Active Iron Porphyrin: Insights in the Photocatalytic CO2 Reduction

Iron porphyrins are among the best molecular catalysts for the electrocatalytic CO₂ reduction reaction. Powering these catalysts with the help of photosensitizers comes along with a couple of unsolved challenges that need to be addressed with much vigor. We have designed an iron porphyrin catalyst decorated with urea functions (UrFe) acting as a multipoint hydrogen bonding scaffold towards the CO₂ substrate. We found a spectacular photocatalytic activity reaching unreported TONs and TOFs as high as 7270 and 3720 h^-1 , respectively. While the Fe⁰ redox state has been widely accepted as the catalytically active species, we show here that the Fe^I species is already involved in the CO₂ activation, which represents the rate-determining step in the photocatalytic cycle. The urea functions help to dock the CO₂ upon photocatalysis. DFT calculations bring support to our experimental findings that constitute a new paradigm in the catalytic reduction of CO₂ .

Ultrathin Nanosheet Assembled Multishelled Superstructures for Photocatalytic CO2 Reduction

Solar-driven conversion of CO₂ is considered an efficient way to tackle the energy and environmental crisis. However, the photocatalytic performance is severely restricted due to the insufficient accessible active sites and inhibited electron transfer efficiency. This work demonstrates a general in situ topological transformation strategy for the integration of uniform Co-based species to fabricate a series of multishelled superstructures (MSSs) for CO₂ photocatalytic conversion. Thorough characterizations reveal the obtained MSSs feature ultrathin Co-based nanosheet assembled polyhedral structures with tunable shell numbers, inner cavity sizes, and compositions. The superstructures increase the spatial density of Co-based active sites while maintaining their high accessibility. Further, the ultrathin nanosheets also facilitate the transfer of photogenerated electrons. As a result, the ZnCo bimetallic hydroxide featuring an ultrathin nanosheet assembled quadruple-shell hollow structure (ZnCo-OH QUNH) exhibits high photocatalytic efficiency toward CO₂ reduction with a CO evolution rate of 134.2 μmol h^-1 and an apparent quantum yield of 6.76% at 450 nm. The quasi in situ spectra and theoretical calculations disclose that Co sites in ZnCo-OH QUNH act as highly active centers to stabilize the COOH* intermediate, while Zn species play the role of adsorption sites for the [Ru(bpy)₃]²⁺ molecules.

Understanding the Role of Inter- and Intramolecular Promoters in Electro- and Photochemical CO2 Reduction Using Mn, Re, and Ru Catalysts

Recycling of carbon dioxide to fuels and chemicals is a promising strategy for renewable energy storage. Carbon dioxide conversion can be achieved by (i) artificial photosynthesis using photoinduced electrons; (ii) electrolysis using electricity produced by photovoltaics; and (iii) thermal CO₂ hydrogenation using renewable H₂. The focus of our group's research is on molecular catalysts, in particular coordination complexes of transition metals (e.g., Mn, Re, and Ru), which offer versatile platforms for mechanistic studies of photo- and electrochemical CO₂ reduction. The interactions of catalytic intermediates with Lewis or Brønsted acids, hydrogen-bonding moieties, solvents, cations, etc., that function as promoters or cofactors have become increasingly important for efficient catalysis. These interactions may have dramatic effects on selectivity and rates by stabilizing intermediates or lowering transition state barriers, but they are difficult to elucidate and challenging to predict. We have been carrying out experimental and theoretical studies of CO₂ reduction using molecular catalysts toward addressing mechanisms of efficient CO₂ reduction systems with emphasis on those containing intramolecular (or pendent) and intermolecular (solution phase) additives. This Account describes the identification of reaction intermediates produced during CO₂ reduction in the presence of triethanolamine or ionic liquids, the benefits of hydrogen-bonding interactions among intermediates or cofactors, and the complications of pendent phenolic donors/phenoxide bases under electrochemical conditions.Triethanolamine (TEOA) is a common sacrificial electron donor for photosensitizer excited state reductive quenching and has a long history of use in photocatalytic CO₂ reduction. It also functions as a Brønsted base in conjunction with more potent sacrificial electron donors, such as 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). Deprotonation of the BIH^•+ cation radical promotes irreversible photoinduced electron transfer by preventing charge recombination. Despite its wide use, most research to date has not considered the broader reactions of TEOA, including its direct interaction with CO₂ or its influence on catalytic intermediates. We found that in acetonitrile, TEOA captures CO₂ in the form of a zwitterionic adduct without any metal catalyst. In the presence of ruthenium carbonyl catalysts bearing α-diimine ligands, it participates in metal hydride formation, accelerates hydride transfer to CO₂ to form the bound formate intermediate, and assists in the dissociation of formate anion from the catalyst ( J. Am. Chem. Soc. 2020, 142, 2413-2428).Hydrogen bonding and acid/base promoters are understood to interact with key catalytic intermediates, such as the metallocarboxylate or metallocarboxylic acid during CO₂ reduction. The former is a high energy species, and hydrogen-bonding or Lewis acid-stabilization are beneficial. We have found that imidazolium-based ionic liquid cations can stabilize the doubly reduced form of the [ReCl(bpy)(CO)₃] (bpy = 2,2'-bipyridine) electrocatalyst through both hydrogen-bonding and π-π interactions, resulting in CO₂ reduction occurring at a more positive potential with a higher catalytic current ( J. Phys. Chem. Lett. 2014, 5, 2033-2038). Hydrogen bonding interactions between Lewis basic methoxy groups in the second coordination sphere of a Mn-based catalyst and the OH group of the Mn-COOH intermediate in the presence of a Brønsted acid were also found to promote C-(OH) bond cleavage, enabling access to a low-energy protonation-first pathway for CO₂ reduction ( J. Am. Chem. Soc. 2017, 139, 2604-2618).The kinetics of forming the metallocarboxylic acid can be enhanced by internal acids, and its proton-induced C-OH bond cleavage to the metallocarbonyl and H₂O is often the rate-limiting step. Therefore, proton movement organized by pendent hydrogen-bonding networks may also accelerate this step. In contrast, during electrolysis, OH groups in the second coordination sphere are deprotonated to the oxyanions, which deter catalytic CO₂ reduction by directly binding CO₂ to form the carbonate or by making an M-O bond in competition with CO₂ binding ( Inorg. Chem. 2016, 55, 4582-4594). Our results emphasize that detailed mechanistic research is critical in discovering the design principles for improved catalysts.

Highly Selective and Efficient Solar-Light-Driven CO2 Conversion with an Ambient-Stable 2D/2D Co2 P@BP/g-C3 N4 Heterojunction

Renewable solar-driven carbon dioxide (CO₂ ) conversion to highly valuable fuels is an economical and prospective strategy for both the energy crisis and ecological environment disorder. However, the selectivity and activity of current photocatalysts have great room for improvement due to the diversification and complexity of products. Here, an ambient-stable 2D/2D Co₂ P@BP/g-C₃ N₄ heterojunction is designed for highly selective and efficient photocatalytic CO₂ reduction reaction. The resulting Co₂ P@BP/g-C₃ N₄ material has a remarkable conversion of CO₂ to carbon monoxide (CO) with an ≈96% selectivity, coupled with a dramatically increased CO generation rate of 16.21 µmol g^-1 h^-1 , which is 5.4 times higher than pristine graphitic carbon nitride (g-C₃ N₄ ). In addition, this photocatalyst exhibits good ambient stability of black phosphorus (BP) without oxidation even over 180 days. The excellent photocatalytic selectivity and activity of Co₂ P@BP/g-C₃ N₄ heterojunction are attributed to its lower energy barriers of *COOH, *CO, and *+CO in the process of CO₂ reduction, coupled with rapid charge transfer at the heterointerfaces of BP/g-C₃ N₄ and Co₂ P/BP. This is solidly verified by both density functional theory calculation and mechanism experiments. Therefore, this work holds great promise for an ambient-stable efficient and high selectivity photocatalyst in solar-driven CO₂ conversion.

Mechanistic Insights Into Iron(II) Bis(pyridyl)amine-Bipyridine Skeleton for Selective CO2 Photoreduction

A bis(pyridyl)amine-bipyridine-iron(II) framework (Fe(BPAbipy)) of complexes 1-3 is reported to shed light on the multistep nature of CO₂ reduction. Herein, photocatalytic conversion of CO₂ to CO even at low CO₂ concentration (1 %), together with detailed mechanistic study and DFT calculations, reveal that 1 first undergoes two sequential one-electron transfer affording an intermediate with electron density on both Fe and ligand for CO₂ binding over proton. The following 2 H⁺ -assisted Fe-CO formation is rate-determining for selective CO₂ -to-CO reduction. A pendant, proton-shuttling α-OH group (2) initiates PCET for predominant H₂ evolution, while an α-OMe group (3) cancels the selectivity control for either CO or H₂ . The near-unity selectivity of 1 and 2 enables self-sorting syngas production at flexible CO/H₂ ratios. The unprecedented results from one kind of molecular catalyst skeleton encourage insight into the beauty of advanced multi-electron and multi-proton transfer processes for robust CO₂ RR by photocatalysis.

Metalloporphyrin-Decorated Titanium Dioxide Nanosheets for Efficient Photocatalytic Carbon Dioxide Reduction

In photocatalysis, the most efficient way to separate photogenerated electron-hole pairs has been extensively studied. However, the methods to increase the quantities of free electrons are neglected. Herein, we used a self-assembly method to fabricate MTCPP/TiO₂ composite materials with a series of metalloporphyrins (MTCPPs, M = Fe, Co, Zn) as sensitizers to modify TiO₂ nanosheets. First, abundant carboxyl and hydroxyl on porphyrin were adsorbed by metal ions. Then, the remaining carboxyl and hydroxyl on porphyrin were anchored on the surface of TiO₂ nanosheets. Finally, MTCPP/TiO₂ was obtained by a layer-by-layer self-assembly process. MTCPP broadens the light response of TiO₂ from ultraviolet light to visible light and enhances the CO₂ adsorption ability. Moreover, metal ions coordinating with porphyrin regulate the electron density of the porphyrin ring and provide a stronger π feedback bond, which promote charge separation. Consequently, by optimizing the type of metal ion, the yield of ZnTCPP/TiO₂ composites reached 109.33 μmol/(g h) of CO and 9.94 μmol/(g h) of CH₄, which was more than 50 times that of pure TiO₂. This study proposes a possible visible-light-induced CO₂ reduction mechanism of metal-ion-based photocatalysis, which provides great insights into optimizing the designation of efficient photocatalysis.