In a comparative study of the electrocatalytic CO2 reduction, cobalt meso‐tetraphenylporphyrin (CoTPP) is used as a model molecular catalyst under both homogeneous and heterogeneous conditions. In ...the former case, employing N,N‐dimethylformamide as solvent, CoTPP performs poorly as an electrocatalyst giving low product selectivity in a slow reaction at a high overpotential. However, upon straightforward immobilization of CoTPP onto carbon nanotubes, a remarkable enhancement of the electrocatalytic abilities is seen with CO2 becoming selectively reduced to CO (>90 %) at a low overpotential in aqueous medium. This effect is ascribed to the particular environment created by the aqueous medium at the catalytic site of the immobilized catalyst that facilitates the adsorption and further reaction of CO2. This work highlights the significance of assessing an immobilized molecular catalyst from more than homogeneous measurements alone.
Heterogeneous vs. homogeneous: When cobalt meso‐tetraphenylporphyrin (CoTPP) is immobilized on carbon nanotubes, a remarkably enhanced catalytic activity in CO2 electroreduction is observed, with CoITPP− serving as the active species. The simple approach for heterogenization enables facile screening and evaluation of molecular catalysts under heterogeneous conditions.
The selective and efficient reduction of carbon dioxide represents a key solution to producing non‐fossil‐fuel‐based feedstocks for the chemical industry, while alleviating the increasing atmospheric ...concentration of this greenhouse gas. A variety of catalytic methods for the CO2 reduction reaction (CO2RR) have been developed, including hydrogenations and electrochemical or photochemical reductions. For many of the most significant breakthroughs reported in the last decade, we realized that amines or closely related functional groups play a critical role for such transformations, and in several cases, are directly associated with the catalyst as a pendant group. Amines play multiple roles, such as CO2 trapping agents, proton shuttles, electron donors, or facilitators of CO2 reductions through formamide derivatives. In this Viewpoint, we compile some of these recent findings, and discuss their significance in a broader context in an attempt to provide guidelines for the design of new catalysts with enhanced activity and selectivity.
Within the last decade, several of the most significant breakthroughs in the homogeneous electrochemical or photochemical reduction and hydrogenation of carbon dioxide have been driven by the introduction of amines or amine‐derived moieties in the reaction mixture. These amines play multiple roles, which are discussed herein to provide guidelines for the design of new catalysts with enhanced activity and selectivity.
Earth-abundant transition metal (Fe, Co, or Ni) and nitrogen-doped porous carbon electrocatalysts (M-N-C, where M denotes the metal) were synthesized from cheap precursors via silica-templated ...pyrolysis. The effect of the material composition and structure (i.e., porosity, nitrogen doping, metal identity, and oxygen functionalization) on the activity for the electrochemical CO2 reduction reaction (CO2RR) was investigated. The metal-free N-C exhibits a high selectivity but low activity for CO2RR. Incorporation of the Fe and Ni, but not Co, sites in the N-C material is able to significantly enhance the activity. The general selectivity order for CO2-to-CO conversion in water is found to be Ni > Fe ≫ Co with respect to the metal in M-N-C, while the activity follows Ni, Fe ≫ Co. Notably, the Ni-doped carbon exhibits a high selectivity with a faradaic efficiency of 93% for CO production. Tafel analysis shows a change of the rate-determining step as the metal overtakes the role of the nitrogen as the most active site. Recording the X-ray photoelectron spectra and extended X-ray absorption fine structure demonstrates that the metals are atomically dispersed in the carbon matrix, most likely coordinated to four nitrogen atoms and with carbon atoms serving as a second coordination shell. Presumably, the carbon atoms in the second coordination shell of the metal sites in M-N-C significantly affect the CO2RR activity because the opposite reactivity order is found for carbon supported metal meso-tetraphenylporphyrin complexes. From a better understanding of the relationship between the CO2RR activity and the material structure, it becomes possible to rationally design high-performance porous carbon electrocatalysts involving earth-abundant metals for CO2 valorization.
The first organocatalyzed trapping of CO2 through CC and CO bond formation is reported. Alkynyl indoles together with catalytic amounts of an organic base and five equivalents of CO2 resulted in ...the formation new heterocyclic structures. These tricyclic indole‐containing products were successfully prepared under mild reaction conditions from aromatic, heteroaromatic, and aliphatic alkynyl indoles with complete regioselectivity. Further investigations suggest that CC bond formation is the initial intermolecular step, followed by lactone‐forming CO bond formation.
Caught in a trap: The first trapping of CO2 through organocatalyzed CC and CO bond formation is reported. By using alkynyl indoles, this method generates novel indole lactone derivatives by using as little as 5 mol % of the simple organic base 1,5,7‐triazabicyclo‐4.4.0dec‐5‐ene as an organocatalyst. The transformation shows excellent atom economy and a broad substrate scope, including aromatic, heteroaromatic, and aliphatic 2‐alkynyl indoles.
Electrocatalysis is a promising tool for utilizing carbon dioxide as a feedstock in the chemical industry. However, controlling the selectivity for different CO2 reduction products remains a major ...challenge. We report a series of manganese carbonyl complexes with elaborated bipyridine or phenanthroline ligands that can reduce CO2 to either formic acid, if the ligand structure contains strategically positioned tertiary amines, or CO, if the amine groups are absent in the ligand or are placed far from the metal center. The amine-modified complexes are benchmarked to be among the most active catalysts for reducing CO2 to formic acid, with a maximum turnover frequency of up to 5500 s–1 at an overpotential of 630 mV. The conversion even works at overpotentials as low as 300 mV, although through an alternative mechanism. Mechanistically, the formation of a Mn–hydride species aided by in situ protonated amine groups was determined to be a key intermediate by cyclic voltammetry, 1H NMR, DFT calculations, and infrared spectroelectrochemistry.
We report the use of electrogenerated anthraquinone radical anion (AQ•−) to trigger fast catalytic depolymerization of polymers derived from poly(dithiothreitol) (pDTT)—a self-immolative polymer ...(SIP) with a backbone of dithiothreitols connected with disulfide bonds and end-capped via disulfide bonds to pyridyl groups. The pDTT derivatives studied include polymers with simple thiohexyl end-caps or modified with AQ or methyl groups by Steglich esterification. All polymers were shown to be depolymerized using catalytic amounts of electrons delivered by AQ•−. For pDTT, as little as 0.2 electrons per polymer chain was needed to achieve complete depolymerization. We hypothesize that the reaction proceeds with AQ•− as an electron carrier (either molecularly or as a pendant group), which transfers an electron to a disulfide bond in the polymer in a dissociative manner, generating a thiyl radical and a thiolate. The rapid and catalytic depolymerization is driven by thiyl radicals attacking other disulfide bonds internally or between pDTT chains in a chain reaction. Electrochemical triggering works as a general method for initiating depolymerization of pDTT derivatives and may likely also be used for depolymerization of other disulfide polymers.
Copper offers unique capability as catalyst for multicarbon compounds production in the electrochemical carbon dioxide reduction reaction. In lieu of conventional catalysis alloying with other ...elements, copper can be modified with organic molecules to regulate product distribution. Here, we systematically study to which extent the carbon dioxide reduction is affected by film thickness and porosity. On a polycrystalline copper electrode, immobilization of porous bipyridine-based films of varying thicknesses is shown to result in almost an order of magnitude enhancement of the intrinsic current density pertaining to ethylene formation while multicarbon products selectivity increases from 9.7 to 61.9%. In contrast, the total current density remains mostly unaffected by the modification once it is normalized with respect to the electrochemical active surface area. Supported by a microkinetic model, we propose that porous and thick films increase both local carbon monoxide partial pressure and the carbon monoxide surface coverage by retaining in situ generated carbon monoxide. This reroutes the reaction pathway toward multicarbon products by enhancing carbon-carbon coupling. Our study highlights the significance of customizing the molecular film structure to improve the selectivity of copper catalysts for carbon dioxide reduction reaction.
Self-immolative polymers (SIPs) are promising members of the emerging class of recyclable polymers with the ability to end-to-end depolymerize to their monomers. Unfortunately, SIPs are often ...synthesized by cumbersome procedures at low temperatures in protected atmosphere. In this study, a SIP with a novel poly(disulfide) backbone is introduced, using dl-dithiothreitol (DTT) as the monomer. Remarkably, poly(DTT) can be produced by solid-state polymerization in a robust and easily scalable process by mechanically mixing DTT with 2,2′-dithiodipyridine as the end-capping agent. The new polymer possesses good thermal and chemical stabilities, but once its depolymerization is triggered, this proceeds smoothly within minutes to afford cyclic DTT because of a favorable intramolecular back-biting thiol–disulfide exchange reaction in the polymer backbone. As a proof of concept, the cyclic DTT waste was recovered, reduced to DTT monomer, and repolymerized in a closed-loop approach.
The reduction of an aryl iodide is generally believed to involve a clean-cut two-electron reduction to produce an aryl anion and iodide. This is in contradiction to what is observed if a highly ...efficient grafting agent, such as an aryldiazonium salt, is employed. The difference in behavior is explained by the much more extreme potentials required for reducing an aryl iodide, which facilitates the further reduction of the aryl radical formed as an intermediate. However, in this study we disclose that electrografting of aryl iodides is indeed possible upon extended voltammetric cycling. This implies that even if the number of aryl radicals left unreduced at the electrode surface is exceedingly small, a functionalization of the surface may still be promoted. In fact, the grafting efficiency is found to increase during the grafting process, which may be explained by the inhibiting effect the growing film exerts on the competing reduction of the aryl radical. The slow buildup of the organic film results in a well-ordered structure as shown by the well-defined electrochemical response from a grafted film containing ferrocenylmethyl groups. Hence, the reduction of aryl iodides allows a precisely controlled, albeit slow, growth of thin organic films.
The electrical properties of pristine fluoropolymers are inferior due to their low polar crystalline phase content and rigid dipoles that tend to retain their fixed moment and orientation. Several ...strategies, such as electrospinning, electrohydrodynamic pulling, and template‐assisted growing, have been proven to enhance the electrical properties of fluoropolymers; however, these techniques are mostly very hard to scale‐up and expensive. Here, a facile interfacial engineering approach based on amine‐functionalized graphene oxide (AGO) is proposed to manipulate the intermolecular interactions in poly(vinylidenefluoride‐trifluoroethylene) (PVDF‐TrFE) to induce β‐phase formation, enlarge the lamellae dimensions, and align the micro‐dipoles. The coexistence of primary amine and hydroxyl groups on AGO nanosheets offers strong hydrogen bonding with fluorine atoms, which facilitates domain alignment, resulting in an exceptional remnant polarization of 11.3 µC cm−2. PVDF‐TrFE films with 0.1 wt.% AGO demonstrate voltage coefficient, energy density, and energy‐harvesting figure of merit values of 0.30 Vm N−1, 4.75 J cm−3, and 14 pm3 J−1, respectively, making it outstanding compared with state‐of‐the‐art ceramic‐free ferroelectric films. It is believed that this work can open‐up new insights toward structural and morphological tailoring of fluoropolymers to enhance their electrical and electromechanical performance and pave the way for their industrial deployment in next‐generation wearables and human‐machine interfaces.
In this research, a facile interfacial engineering approach based on amine‐functionalized graphene oxide (AGO) is presented to improve the electrical properties of fluoropolymers. It is shown that AGO incorporation can lead to a higher β/α ratio and larger lamellae dimensions, which is translated into higher remnant polarization, and larger longitudinal piezoelectric coefficient in PVDF‐TrFE.
