Thermodynamic Considerations for Optimizing Selective CO2 Reduction by Molecular Catalysts

Kinetic and mechanistic analysis of a synthetic reversible CO2/HCO2- electrocatalyst

[Pt(depe)2](PF6)2 electrocatalyzes the reversible conversion between CO2 and HCO2- with high selectivity and low overpotential but low rates. A comprehensive kinetic analysis indicates the rate determining step for CO2 reduction is the reactivity of a Pt hydride intermediate to produce HCO2-. To accelerate catalysis, the use of cationic and hydrogen-bond donor additives are explored.

Enhanced Molecular CO2 Electroreduction Enabled by a Flexible Hydrophilic Channel for Relay Proton Shuttling

The effects of primary and second coordination spheres on molecular electrocatalysis have been extensively studied, yet investigations of third functional spheres are rarely reported. Here, an electrocatalyst (ZnPEG8T) was developed with a hydrophilic channel as a third functional sphere that facilitates relay proton shuttling to the primary and second coordination spheres for enhanced catalytic CO₂ reduction. Using foot-of-the-wave analysis, the ZnPEG8T catalyst displayed CO₂ -to-CO activity (TOF_max ) thirty times greater than that of the benchmark catalyst without a third functional sphere. A kinetic isotopic effect (KIE) study, in conjunction with voltammetry and UV/Vis spectroscopy, uncovered that the rate-limiting step was not the protonation step of the metallocarboxylate intermediate, as observed in many other molecular CO₂ reduction electrocatalysts, but rather the replenishment of protons in the proton-shuttling channel. Controlled-potential electrolysis using ZnPEG8T displayed a faradaic efficiency of 100 % for CO₂ -to-CO conversion at -2.4 V vs. Fc/Fc⁺ . A Tafel plot was also generated for a comparison to other reported molecular catalysts. This report validates a strategy for incorporating higher functional spheres for enhanced catalytic efficiency in proton-coupled electron-transfer reactions.

Homogeneous Electrochemical Reduction of CO2 to CO by a Cobalt Pyridine Thiolate Complex

The chemical and electrochemical reduction of CO₂ to value added chemicals entails the development of efficient and selective catalysts. Synthesis, characterization and electrochemical CO₂ reduction activity of a air-stable cobalt(III) diphenylphosphenethano-bis(2-pyridinethiolate)chloride [{Co(dppe)(2-PyS)₂}Cl, 1-Cl] complex is divulged. The complex reduces CO₂ under homogeneous electrocatalytic conditions to produce CO with high Faradaic efficiency (FE > 92%) and selectivity in the presence of water. Through detailed electrochemical investigations, product analysis, and mechanistic investigations supported by theoretical calculations, it is established that complex 1-Cl reduces CO₂ in its Co(I) state. A reductive cleavage leads to a dangling protonated pyridine arm which enables facile CO₂ binding through a H-bond donation and facilitates the C-O bond cleavage via a directed protonation. A systematic benchmarking of this catalyst indicates that it has a modest overpotential (∼180 mV) and a TOF of ∼20 s^-1 for selective reduction of CO₂ to CO with H₂O as a proton source.

Tracking Mechanistic Pathway of Photocatalytic CO2 Reaction at Ni Sites Using Operando, Time-Resolved Spectroscopy

Harvesting solar energy for catalytic conversion of CO₂ into valuable chemical fuels/feedstocks is an attractive yet challenging strategy to realize a sustainable carbon-cycle utilization. Homogeneous catalysts typically exhibit higher activity and selectivity as compared with heterogeneous counterparts, benefiting from their atomically dispersed catalytic sites and versatile coordination structures. However, it is still a "black box" how the coordination and electronic structures of catalysts dynamically evolve during the reaction, forming the bottleneck for understanding their reaction pathways. Herein, we demonstrate to track the mechanistic pathway of photocatalytic CO₂ reduction using a terpyridine nickel(II) complex as a catalyst model. Integrated with a typical homogeneous photosensitizer, the catalytic system offers a high selectivity of 99% for CO₂-to-CO conversion with turnover number and turnover frequency as high as 2.36 × 10⁷ and 385.6 s^-1, respectively. We employ operando and time-resolved X-ray absorption spectroscopy, in combination with other in situ spectroscopic techniques and theoretical computations, to track the intermediate species of Ni catalyst in the photocatalytic CO₂ reduction reaction for the first time. Taken together with the charge dynamics resolved by optical transient absorption spectroscopy, the investigation elucidates the full mechanistic reaction pathway including some key factors that have been often overlooked. This work opens the "black box" for CO₂ reduction in the system of homogeneous catalysts and provides key information for developing efficient catalysts toward artificial photosynthesis.

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.

Reversible and Selective CO2 to HCO2 - Electrocatalysis near the Thermodynamic Potential

Reversible catalysis is a hallmark of energy-efficient chemical transformations, but can only be achieved if the changes in free energy of intermediate steps are minimized and the catalytic cycle is devoid of high transition-state barriers. Using these criteria, we demonstrate reversible CO₂ /HCO₂ ^- conversion catalyzed by [Pt(depe)₂ ]²⁺ (depe=1,2-bis(diethylphosphino)ethane). Direct measurement of the free energies associated with each catalytic step correctly predicts a slight bias towards CO₂ reduction. We demonstrate how the experimentally measured free energy of each step directly contributes to the <50 mV overpotential. We also find that for CO₂ reduction, H₂ evolution is negligible and the Faradaic efficiency for HCO₂ ^- production is nearly quantitative. A free-energy analysis reveals H₂ evolution is endergonic, providing a thermodynamic basis for highly selective 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.

Remote Ligand Modifications Tune Electronic Distribution and Reactivity in Site-Differentiated, High-Spin Iron Clusters: Flipping Scaling Relationships

We report the synthesis, characterization, and reactivity of [LFe₃O(^RArIm)₃Fe][OTf]₂, the first Hammett series of a site-differentiated cluster. The cluster reduction potentials and CO stretching frequencies shift as expected on the basis of the electronic properties of the ligand: electron-donating substituents result in more reducing clusters and weaker C-O bonds. However, unusual trends in the energetics of their two sequential CO binding events with the substituent σ_p parameters are observed. Specifically, introduction of electron-donating substituents suppresses the first CO binding event (ΔΔH by as much as 7.9 kcal mol^-1) but enhances the second (ΔΔH by as much as 1.9 kcal mol^-1). X-ray crystallography, including multiple-wavelength anomalous diffraction, Mössbauer spectroscopy, and SQUID magnetometry, reveal that these substituent effects result from changes in the energetic penalty associated with electronic redistribution within the cluster, which occurs during the CO binding event.

Thermodynamic targeting of electrocatalytic CO2 reduction: advantages, limitations, and insights for catalyst design

Herein is reported the electrocatalytic reduction of CO₂ with the complex [Ni(bis-NHC)(dmpe)]²⁺ (1) (bis-NHC = 1,l':3,3'-bis(1,3-propanediyl)dibenzimidazolin-2,2'-diylidene; dmpe = 1,2-bis(dimethylphosphino)ethane). The hydricity of 1 was previously benchmarked to be , equating to a driving force of a minimum of ∼3.4 kcal mol^-1 for hydride transfer to CO₂. While hydride transfer to CO₂ is thermodynamically favorable, electrocatalytic and infrared spectroelectrochemical (IR-SEC) experiments reveal that hydride transfer is blocked by direct reactivity with CO₂ in the reduced, Ni(0) state of the catalyst, yielding CO via reductive disproportionation (2CO₂ + 2e^- = CO + CO₃^2-) and concomitant catalyst degradation. Although thermodynamic scaling relationships provide guidance in catalyst targeting, the findings herein illustrate the fundamental kinetic challenges in balancing substrate reactivity and selectivity in the design of CO₂ reduction electrocatalysts. Advantages and limitations of this scaling relationship as well as approaches by which divergence from it may be achieved are discussed, which provides insight on important parameters for future catalyst design.