Selective CO Production by Photoelectrochemical CO2 Reduction in an Aqueous Solution with Cobalt-Based Molecular Redox Catalysts

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.

Enzyme-Inspired Coordination Polymers for Selective Oxidization of C(sp3)-H Bonds via Multiphoton Excitation

Nature's blueprint provides the fundamental principles for expanding the use of abundant metals in catalysis; however, mimicking both the structure and function of copper enzymes simultaneously in one artificial system for selective C-H bond oxidation faces marked challenges. Herein, we report a new approach to the assembly of artificial monooxygenases utilizing a binuclear Cu₂S₂Cl₂ cluster to duplicate the identical structure and catalysis of the Cu_A enzyme. The designed monooxygenase Cu-Cl-bpyc facilitates well-defined redox potential that initially activated O₂via photoinduced electron transfer, and generated an active chlorine radical via a ligand-to-metal charge transfer (LMCT) process from the consecutive excitation of the in situ formed copper(II) center. The chlorine radical abstracts a hydrogen atom selectively from C(sp³)-H bonds to generate the radical intermediate; meanwhile, the O₂^•- species interacted with the mimic to form mixed-valence species, giving the desired oxidization products with inherent product selectivity of copper monooxygenases and recovering the catalyst directly. This enzymatic protocol exhibits excellent recyclability, good functional group tolerance, and broad substrate scope, including some biological and pharmacologically relevant targets. Mechanistic studies indicate that the C-H bond cleavage was the rate-determining step and the cuprous interactions were essential to stabilize the active oxygen species. The well-defined structural characters and the fine-modified catalytic properties open a new avenue to develop robust artificial enzymes with uniform and precise active sites and high catalytic performances.

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.

Cuprous Cluster-Based Coordination Sheets as Photocatalytic Regulators to Activate Oxygen, Benzoquinone, and Thianthrenium Salts

Cuprous clusters are well known for their important fluorescent properties and tunable redox behavior, but the coordinated protecting groups restrict their application in photocatalysis, in particular, the inner-sphere activation of substrates. By modifying fluorescent cuprous clusters with terminal iodides into two-dimensional coordination sheets, we report a photocatalytic regulator to synergistically combine electron transfer and energy transfer for the oxidative coupling of benzoquinone and terminal alkynes. Under visible light irradiation, the well-modified excited state of the cuprous clusters in the coordination sheets reduces benzoquinones to generate aoxy radicals through electron transfer and activates oxygen through energy transfer. The aoxy radicals interact with copper-coordinated phenylacetylene to form an active intermediate, which is further oxidized by the in situ formed active oxygen species and aryl ketones are obtained. The warranted potential of the excited coordination sheets enables the reductive activation of thianthrenium salts as radical precursors, facilitating radical capture and further C-N coupling via an inner-sphere activation mechanism. The new catalytic approach optimizes the redox properties and excited-state lifetime, shortens the electron transfer steps, and promotes the potential collision of a low concentration of active species in tandem catalytic cycles, thus paving a new way to develop ecologically benign, cost-effective, multipurpose, and flexible catalytic systems.

Electron-Rich Pincer Ligand-Coupled Cobalt Porphyrin Polymer with Single-Atom Sites for Efficient (Photo)Electrocatalytic CO2 Reduction at Ultralow Overpotential

Porphyrin and phthalocyanine complexes bearing single-atom catalytic sites (M-N₄ ) have been explored as promising electrocatalysts for CO₂ reduction reaction (CO₂ RR), whose activity can be improved by regulating the ligands and/or the metal centers. Moreover, their photosensitive features also provide the possibility for highly efficient photoelectrocatalytic CO₂ RR. Herein, a novel N'NN'-pincer-ligand (N₃ )-coupled cobalt porphyrin (CoPor-N₃ ) polymer is developed for realizing efficient (photo)electrocatalytic CO₂ RR. The unraveled electronic structure and (photo)electrocatalytic features suggest that a synergistic effect between the electron-rich N₃ ligands and the Co-N₄ single-atom sites in the CoPor-N₃ polymer results in the Co centers attaining more electrons, which is beneficial to facilitating the electron transfer to CO₂ for the activation and reduction processes. As expected, the resultant CoPor-N₃ polymer delivers a good long-term durability and high CO faradaic efficiency (96%) at an ultralow overpotential (0.39 V), which outperforms the CoPor alone and most porphyrin-/phthalocyanine-based electrocatalysts reported so far. Moreover, the photosensitivity of CoPor units can further reduce the overpotential to 0.34 V with a CO faradaic efficiency over 90% under light illumination. The present findings offer a new approach to constructing porphyrin-based photosensitive electrocatalysts with high-efficiency photoelectrocatalytic CO₂ RR.

Accelerating Electron-Transfer and Tuning Product Selectivity Through Surficial Vacancy Engineering on CZTS/CdS for Photoelectrochemical CO2 Reduction

Copper-based chalcogenides have been considered as potential photocathode materials for photoelectrochemical (PEC) CO₂ reduction due to their excellent photovoltaic performance and favorable conduction band alignment with the CO₂ reduction potential. However, they suffer from low PEC efficiency due to the sluggish charge transfer kinetics and poor selectivity, resulting from random CO₂ reduction reaction pathways. Herein, a facile heat treatment (HT) of a Cu₂ ZnSnS₄ (CZTS)/CdS photocathode is demonstrated to enable significant improvement in the photocurrent density (-0.75 mA cm^-2 at -0.6 V vs RHE), tripling that of pristine CZTS, as a result of the enhanced charge transfer and promoted band alignment originating from the elemental inter-diffusion at the CZTS/CdS interface. In addition, rationally regulated CO₂ reduction selectivity toward CO or alcohols can be obtained by tailoring the surficial sulfur vacancies by HT in different atmospheres (air and nitrogen). Sulfur vacancies replenished by O-doping is shown to favor CO adsorption and the CC coupling pathway, and thereby produce methanol and ethanol, whilst the CdS surface with more S vacancies promotes CO desorption capability with higher selectivity toward CO. The strategy in this work rationalizes the interface charge transfer optimization and surface vacancy engineering simultaneously, providing a new insight into PEC CO₂ reduction photocathode design.