Highly selective and stable electro-catalytic system with ionic liquids for the reduction of carbon dioxide to carbon monoxide

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Reversible Proton-Coupled Reduction of an Iron Nitrosyl Porphyrin within [DBU-H]+-Based Protic Ionic Liquid Nanodomains

The reduction of [Fe(OEP)(NO)] has been studied in the presence of aprotic room-temperature ionic liquids (RTIL) and protic (PIL) ionic liquids dissolved within a molecular solvent (MS). The cyclic voltammetric results showed the formation of RTIL nanodomains at low concentrations of the RTIL/PIL solutions. The pK_a values of the two PILs studied (i.e., trialkylammonium and [DBU-H]⁺-based ionic liquids) differed by four units in THF. While voltammetry in solutions containing all three RTILs showed similar potential shifts of the first reduction of [Fe(OEP)(NO)] to [Fe(OEP)(NO)]^- at low concentrations, significant differences were observed at higher concentrations for the ammonium PIL. The trialkylammonium cation had previously been shown to protonate the {FeNO}⁸ species at room temperature. Visible and infrared spectroelectrochemistry revealed that the [DBU-H]⁺-based PIL formed hydrogen bonds with [Fe(OEP)(NO)]^- rather than formally protonating it. Despite these differences, both PILs were able to efficiently reduce the nitrosyl species to the hydroxylamine complex, which could be further reduced to ammonia. On the voltammetric time scale and when the switching potential was positive of the Fe(II)/Fe(I) potential, the hydroxylamine complex was re-oxidized back to the NO complex via direct oxidation of the coordinated hydroxylamine at low scan rates or initial oxidation of the ferrous porphyrin at high scan rates. The results of this work show that, while [DBU-H]⁺ does not protonate electrochemically generated [Fe(OEP)(NO)]^-, it still plays an important role in efficiently reducing the nitroxyl ligand via a series of proton-coupled electron transfer steps to generate hydroxylamine and eventually ammonia. The overall reaction rates were independent of the PIL concentration, consistent with the nanodomain formation being important to the reduction process.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

This timely and comprehensive review mainly summarizes advances in heterogeneous electroreduction of CO₂: from fundamentals to value-added products.

Carbon dioxide reduction to multicarbon hydrocarbons and oxygenates on plant moss-derived, metal-free, in situ nitrogen-doped biochar

Electrochemical reduction of carbon dioxide (CO₂) is considered a promising renewable energy conversion technology, but it remains challenging to find active, stable, low-cost, and highly efficient electrocatalysts for the CO₂ conversion. Here, we develop an in situ nitrogen-doped, metal-free, porous biochar from plant moss to catalyze the electrochemical reduction of CO₂ into methane (CH₄), methanol (CH₃OH) and ethanol (C₂H₅OH) at high current densities and low overpotentials. Using this metal-free biochar electrocatalyst, production rates of approximately 36.1, 32.1, and 18.1 μg h^-1 cm^-2 towards CH₄, C₂H₅OH, and CH₃OH are obtained with Faradaic efficiencies of 56.0%, 26.0% and 10.5%, respectively. In addition, the total faradaic efficiency reaches 92.6% at -1.2 V (vs. Ag/AgCl) with good stability. A favorable pathway for the electrochemical reduction of CO₂ over the metal-free biochar is also provided. This study presents a new approach to produce cost-effective, in situ nitrogen-doped porous biochars with excellent efficiency and durability for the electrochemical reduction of CO₂.

Sustainable and efficient electrosynthesis of naproxen using carbon dioxide and ionic liquids

The use of CO₂ as a C1 carbon source for synthesis is raising increasing attention both as a strategy to bring value to carbon dioxide capture technologies and a sustainable approach towards chemicals and energy. The presented results focus on the application of electrochemical methods to incorporate CO₂ into organic compounds using ionic liquids as electrolytes, which provides a green alternative to the formation of C-C bonds. In this sense, the current manuscript shows that Naproxen (6-Methoxy-α-methyl-2-naphthaleneacetic acid) can be synthetizing in high yield (89%) and conversion rates (90%) through an electrocarboxylation process using CO₂ and ionic liquids. The role of the cathode and solvent, which can potentially enhance the synthesis, is also discussed. The "green" route described in the current work would open a new sustainable strategy for the electrochemical production of pharmaceutical compounds.

Understanding the Influence of [EMIM]Cl on the Suppression of the Hydrogen Evolution Reaction on Transition Metal Electrodes

We have studied the influence of low concentrations (0.1 M) of the ionic liquid 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) on suppressing the hydrogen evolution reaction (HER) using polycrystalline Ag, Cu, and Fe electrodes in aqueous acidic and basic media. HER suppression is generally desired when aiming to catalyze other reactions of interests, e.g., CO₂ electro-reduction. Cyclic voltammetry and chronoamperometry measurements were performed at potentials between -0.2 and -0.8 V versus the reversible hydrogen electrode (RHE) to investigate HER activity in a simulated CO₂ electrolysis environment without the CO₂. In an acidic electrolyte, a decrease in HER activity was observed for all three electrodes with the largest effect being that of Fe, where the HER activity was suppressed by 75% at -0.5 V versus RHE. In contrast to the effect of [EMIM]Cl in an acidic electrolyte, no HER suppression was observed in basic media. Using ¹H nuclear magnetic resonance spectroscopy on the electrolyte before and after electrolysis, it was determined that [EMIM]Cl breaks down at both the working and counter electrodes under reaction conditions under both acidic and basic conditions. These results underscore the challenges in employing ionic liquids for electrochemical reactions such as CO₂ reduction.

Highly efficient electrochemical reduction of CO2 to CH4 in an ionic liquid using a metal-organic framework cathode

Highly efficient electrochemical reduction of CO2 to CH4 is of great importance, but is challenging. Herein, Zn-1,3,5-benzenetricarboxylic acid metal-organic frameworks (Zn-BTC MOFs) deposited on carbon paper (CP) were used as cathodes in electrochemical reduction of CO2 using ionic liquids (ILs) as the electrolytes, which was the first work on combination of a MOF electrode and an pure IL electrolyte in the electrochemical reduction of CO2. It was found that the efficiency of the reaction depended strongly on the morphology of the Zn-MOFs. Compared with the commonly used metal electrodes, the electrochemical reaction showed much higher selectivity to CH4 and current density, and the overpotentials for CH4 is much lower. The excellent combination of the MOF cathodes and ILs opens a way for reduction of CO2 to CH4 effectively.