Mechanisms of catalytic reduction of CO2 with heme and nonheme metal complexes

Enhanced catalytic reaction at an air-liquid-solid triphase interface

Gaseous reactant involved heterogeneous catalysis is critical to the development of clean energy, environmental management, health monitoring, and chemical synthesis. However, in traditional heterogeneous catalysis with liquid–solid diphase reaction interfaces, the low solubility and slow transport of gaseous reactants strongly restrict the reaction efficiency. In this minireview, we summarize recent advances in tackling these drawbacks by designing catalytic systems with an air–liquid–solid triphase joint interface. At the triphase interface, abundant gaseous reactants can directly transport from the air phase to the reaction centre to overcome the limitations of low solubility and slow transport of the dissolved gas in liquid–solid diphase reaction systems. By constructing a triphase interface, the efficiency and/or selectivity of photocatalytic reactions, enzymatic reactions, and (photo)electrochemical reactions with consumption of gaseous reactants oxygen, carbon dioxide, and nitrogen are significantly improved.

Gaseous reactant involved liquid–solid diphase interface reactions can be significantly enhanced using rationally designed and constructed air–liquid–solid triphase systems.

Simple photoreduction of carbon dioxide to formic acid and true quantum yield

There is a need to develop techniques for conversion of carbon dioxide to useful products such as formaldehyde, formic acid, methanol, and hydrocarbons. Carbon dioxide can be converted into these products using either photochemically, electrochemically, thermochemical or hydrogenation by bacteria. Formate is of interest due to the possibility of being used in liquid fuel cells, as an additive in pyrolysis vapors and as a precursor for biological fuels. In this work, conversion of carbon dioxide to formic acid under acidic conditions and formate under basic or neutral conditions was accomplished through photoreduction using an inexpensive setup consisting of titanium dioxide, metal phthalocyanines and inexpensive incandescent sources. The yield of formic acid based on anion chromatography was 1.54%. This work also discusses and presents a true quantum yield determined using chemical actinometry which was near 2.0%. Detailed studies of the photoreduction process showed that the amount of sensitizer, light intensity and pH affect the amount of formate generated.

Electrocatalytic CO2 Reduction with a Half-Sandwich Cobalt Catalyst: Selectivity towards CO

We present herein a Cp*Co(III)-half-sandwich catalyst system for electrocatalytic CO₂ reduction in aqueous acetonitrile solution. In addition to an electron-donating Cp* ligand (Cp*=pentamethylcyclopentadienyl), the catalyst featured a proton-responsive pyridyl-benzimidazole-based N,N-bidentate ligand. Owing to the presence of a relatively electron-rich Co center, the reduced Co(I)-state was made prone to activate the electrophilic carbon center of CO₂ . At the same time, the proton-responsive benzimidazole scaffold was susceptible to facilitate proton-transfer during the subsequent reduction of CO₂ . The above factors rendered the present catalyst active toward producing CO as the major product over the other potential 2e/2H⁺ reduced product HCOOH, in contrast to the only known similar half-sandwich CpCo(III)-based CO₂ -reduction catalysts which produced HCOOH selectively. The system exhibited a Faradaic efficiency (FE) of about 70% while the overpotential for CO production was found to be 0.78 V, as determined by controlled-potential electrolysis.

Photocatalytic redox reactions with metalloporphyrins

Metalloporphyrinoids are utilized as efficient sensitizers and catalysts in photosynthesis and the reverse reaction that is respiration. Because metalloporphyrinoids show strong absorption in the visible region and redox active, metalloporphyrinoids are also suited as photoredox catalysts for photo-driven redox reactions using solar energy. In particular, metalloporphyrins are utilized as pivotal components to mimic the structure and function of the photosynthetic reaction center. Metalloporphyrins are used as photoredox catalysts for hydrogen evolution from electron and proton sources combining hydrogen evolution catalysts. Metalloporphyrins also act as thermal redox catalysts for photocatalytic reduction of CO2 with photoredox catalysts. Metalloporphyrins are also used as dual catalysts for a photoredox catalyst for oxygenation of substrates with H2O and a redox catalyst for O2 reduction when dioxygen is used as a two-electron oxidant and H2O as an oxygen source, both of which are the greenest reactants. Free base porphyrins can also be employed as promising photoredox catalysts for C–C bond formation reactions.

Molecular quaterpyridine-based metal complexes for small molecule activation: water splitting and CO2 reduction

This tutorial describes recent developments in the use of metal quaterpyridine complexes as electrocatalysts and photocatalysts for water splitting and CO₂ reduction.

Transition metal-based catalysts for the electrochemical CO2 reduction: from atoms and molecules to nanostructured materials

An overview of the main strategies for the rational design of transition metal-based catalysts for the electrochemical conversion of CO₂, ranging from molecular systems to single-atom and nanostructured catalysts.

Secondary-Sphere Effects in Molecular Electrocatalytic CO2 Reduction

The generation of fuels and value-added chemicals from carbon dioxide (CO2) using electrocatalysis is a promising approach to the eventual large-scale utilization of intermittent renewable energy sources. To mediate kinetically and thermodynamically challenging transformations of CO2, early reports of molecular catalysts focused primarily on precious metal centers. However, through careful ligand design, earth-abundant first-row transition metals have also demonstrated activity and selectivity for electrocatalytic CO2 reduction. A particularly effective and promising approach for enhancement of reaction rates and efficiencies of molecular electrocatalysts for CO2 reduction is the modulation of the secondary coordination sphere of the active site. In practice, this has been achieved through the mimicry of enzyme structures: incorporating pendent Brønsted acid/base sites, charged residues, sterically hindered environments, and bimetallic active sites have all proved to be valid strategies for iterative optimization. Herein, the development of secondary-sphere strategies to facilitate rapid and selective CO2 reduction is reviewed with an in-depth examination of the classic [Fe(tetraphenylporphyrin)]+, [Ni(cyclam)]2+, Mn(bpy)(CO)3X, and Re(bpy)(CO)3X (X = solvent or halide) systems, including relevant highlights from other recently developed ligand platforms.

Anchoring CoII Ions into a Thiol-Laced Metal-Organic Framework for Efficient Visible-Light-Driven Conversion of CO2 into CO

Using solar energy to convert CO₂ into valuable fuels or chemicals offers a powerful solution to urgent energy and environmental problems. However, the development of efficient and selective catalysts remains a considerable scientific challenge. To address this, catalytically active Co^II centers can be anchored into the porous matrix of metal-organic frameworks (MOFs) by utilizing a robust Zr-based MOF (Zr-DMBD) functionalized with freestanding thiol groups to enable efficient post-synthetic metal insertion. The thus-prepared Zr-DMBD-Co MOF solids are modified by well-defined Co-thiolate units and have the capability of photocatalytically converting CO₂ into CO with high efficiency and selectivity under visible-light irradiation in a water-containing system. The turnover number and CO selectivity reach as high as 97 941 and 98 %, respectively.

Mini-review on an engineering approach towards the selection of transition metal complex-based catalysts for photocatalytic H2 production

Advances in transition-metal (Ru, Co, Cu, and Fe) complex-based catalysts since 2000 are briefly summarized in terms of catalyst selection and application for photocatalytic H₂ evolution.

A molecular noble metal-free system for efficient visible light-driven reduction of CO2 to CO

A new iron complex bearing a pentadentate quinoline–pyridine ligand exhibits excellent photocatalytic activity towards CO₂-to-CO conversion using the commercially available organic dye purpurin as the photosensitizer and BIH as the electron donor.

The regulation of reaction processes and rate-limiting steps for efficient photocatalytic CO2 reduction into methane over the tailored facets of TiO2

A design diagram for the complicated reaction processes of CO₂ reduction into CH₄ on tailored anatase TiO₂ facets.