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Carbon dioxide (CO2) is currently considered as a waste material due to its negative impact on the environment. However, it is possible to create value from CO2 by capturing and utilizing it as a b...

Electrochemical processes are a promising technology for industrial production of chemicals. One of the major drawbacks of electrochemical systems is the low mass transfer of reactants toward the a...

Recently, deep eutectic solvents (DES) have been considered as possible electrolytes for the electrochemical reduction of CO

Recently, deep eutectic solvents (DES) have been considered as possible electrolytes for the electrochemical reduction of CO2 to value-added products such as formic and oxalic acids. The applicability of pure DES as electrolytes is hindered by high viscosities. Mixtures of DES with organic solvents can be a promising way of designing superior electrolytes by exploiting the advantages of each solvent type. In this study, densities, viscosities, diffusivities, and ionic conductivities of mixed solvents comprising DES (i.e., reline and ethaline), methanol, and propylene carbonate were computed using molecular simulations. To provide a quantitative assessment of the affinity and mass transport of CO2 and oxalic and formic acids in the mixed solvents, the solubilities and self-diffusivities of these solutes were also computed. Our results show that the addition of DES to the organic solvents enhances the solubilities of oxalic and formic acids, while the solubility of CO2 in the ethaline-containing mixtures are in the same order of magnitude with the respective pure organic components. A monotonic increase in the densities and viscosities of the mixed solvents is observed as the mole fraction of DES in the mixture increases, with the exception of the density of ethaline-propylene carbonate which shows the opposite behavior due to the high viscosity of the pure organic component. The self-diffusivities of all species in the mixtures significantly decrease as the mole fraction of DES approaches unity. Similarly, the self-diffusivities of the dissolved CO2 and the oxalic and formic acids also decrease by at least 1 order of magnitude as the composition of the mixture shifts from the pure organic component to pure DES. The computed ionic conductivities of all mixed solvents show a maximum value for mole fractions of DES in the range from 0.2 to 0.6 and decrease as more DES is added to the mixtures. Since for most mixtures studied here no prior experimental measurements exist, our findings can serve as a first data set based on which further investigation of DES-containing electrolyte solutions can be performed for the electrochemical reduction of CO2 to useful chemicals.

Efficient and selective electrosynthesis of propylene carbonate can be performed by the reaction of carbon dioxide with propylene oxide at copper electrodes. In this paper, we investigate this electrochemical reaction by using cyclic voltammetry, Fourier transform infrared spectroscopy and high-performance liquid chromatography in order to unravel details of the catalytic mechanism of the reaction. The combination of the results obtained by these different techniques allows the exclusion of different reduced forms of CO2, such as CO and (bi)carbonates, as possible carboxylation agents. Moreover, the results also indicate that electrochemical activation of the propylene oxide by ring opening is not the initial step for this reaction, as no product was detected when a current was not applied in presence of “activated propylene oxide” and CO2. Our results show that the reaction is initiated by the activation of CO2 to CO2.−, which then attacks the epoxide to form the cyclic carbonate. This work also gives evidence for the non-catalytic nature of the synthesis of the cyclic carbonate because its formation also occurs on other metals such as gold and platinum in the same range of applied currents. This result clearly indicates the potential of in situ electrochemical techniques in the mechanistic investigation of electrosynthesis reactions.

Carbon dioxide (CO2) is currently considered as a waste material due to its negative impact on the environment. However, it is possible to create value from CO2 by capturing and utilizing it as a building block for commodity chemicals. Electrochemical conversion of CO2 has excellent potential for reducing greenhouse gas emissions and reaching zero net emissions by 2050. To date, Carbon Capture and Utilization (CCU) technologies have been studied independently. We report a novel methodology based on the integration of CO2 capture and conversion by the direct utilization of a CO2 capture media as electrolyte for electrochemical CO2 conversion. This has a high potential for reducing capital and operational cost when compared to traditional methodologies. A novel mixture of chemical and physical absorption solvents allowed for the captured CO2 to be converted to formic acid with faradaic efficiencies up to 50 % and with carbon conversion of ca. 30 %. By increasing the temperature in the electrochemical reactor from 20 °C to 75 °C, the productivity towards formic acid increased by a factor of 10, reaching up to 0.7 mmol∙m-2·s-1. The direct conversion of captured CO2 was also demonstrated for carbon monoxide formation with faradaic efficiencies up 45 %.

Ruthenium polypyridyl complexes are good candidates for photoactivated chemotherapy (PACT) provided that they are stable in the dark but efficiently photosubstitute one of their ligands. Here the use of the natural amino acid l-proline as a protecting ligand for ruthenium-based PACT compounds is investigated in the series of complexes Λ-[Ru(bpy)2(l-prol)]PF6 ([1a]PF6; bpy = 2,2′-bipyridine and l-prol = l-proline), Λ-[Ru(bpy)(dmbpy)(l-prol)]PF6 ([2a]PF6 and [2b]PF6; dmbpy = 6,6′-dimethyl-2,2′-bipyridine), and Λ-[Ru(dmbpy)2(l-prol)]PF6 ([3a]PF6). The synthesis of the tris-heteroleptic complex bearing the dissymmetric proline ligand yielded only two of the four possible regioisomers, called [2a]PF6 and [2b]PF6. Both isomers were isolated and characterized by a combination of spectroscopy and density functional theory calculations. The photoreactivity of all four complexes [1a]PF6, [2a]PF6, [2b]PF6, and [3a]PF6 was studied in water (H2O) and acetonitrile (MeCN) using UV–vis spectroscopy, circular dichroism spectroscopy, mass spectrometry, and 1H NMR spectroscopy. In H2O, upon visible-light irradiation in the presence of oxygen, no photosubstitution took place, but the amine of complex [1a]PF6 was photooxidized to an imine. Contrary to expectations, enhancing the steric strain by the addition of two ([2b]PF6) or four ([3a]PF6) methyl substituents did not lead, in phosphate-buffered saline (PBS), to ligand photosubstitution. However, it prevented photoxidation, probably as a consequence of the electron-donating effect of the methyl substituents. In addition, whereas [2b]PF6 was photostable in PBS, [2a]PF6 quantitatively isomerized to [2b]PF6 upon light irradiation. In pure MeCN, [2a]PF6 and [3a]PF6 showed non-selective photosubstitution of both the l-proline and dmbpy ligands, whereas the non-strained complex [1a]PF6 was photostable. Finally, in H2O–MeCN mixtures, [3a]PF6 showed selective photosubstitution of l-proline, thus demonstrating the active role played by the solvent on the photoreactivity of this series of complexes. The role of the solvent polarity and coordination properties on the photochemical properties of polypyridyl complexes is discussed., Three ruthenium polypyridyl l-proline complexes with increasing strain (R, R′ = H or Me) were synthesized and their photoreactivities studied in phosphate-buffered saline, pure acetonitrile (MeCN), and water−MeCN mixtures. Depending on the number of methyl groups, on the presence of air, and on the nature of the solvent, either photoisomerization, photooxidation of l-proline, selective photosubstitution, or nonselective photosubstitution was observed.

The electrocatalytic reduction of carbon dioxide is a promising approach for storing (excess) renewable electricity as chemical energy in fuels. Here, we review recent advances and challenges in the understanding of electrochemical CO2 reduction. We discuss existing models for the initial activation of CO2 on the electrocatalyst and their importance for understanding selectivity. Carbon–carbon bond formation is also a key mechanistic step in CO2 electroreduction to high-density and high-value fuels. We show that both the initial CO2 activation and C–C bond formation are influenced by an intricate interplay between surface structure (both on the nano- and on the mesoscale), electrolyte effects (pH, buffer strength, ion effects) and mass transport conditions. This complex interplay is currently still far from being completely understood. In addition, we discuss recent progress in in situ spectroscopic techniques and computational techniques for mechanistic work. Finally, we identify some challenges in furthering our understanding of these themes. Electrocatalytic reduction of CO2 to fuels could be used as an approach to store renewable energy in the form of chemical energy. Here, Birdja et al. review current understanding of electrocatalytic systems and reaction pathways for these conversions.

The complexity of the electrocatalytic reduction of CO to CH4 and C2H4 on copper electrodes prevents a straightforward elucidation of the reaction mechanism and the design of new and better catalysts. Although structural and electrolyte effects have been separately studied, there are no reports on structure-sensitive cation effects on the catalyst’s selectivity over a wide potential range. Therefore, we investigated CO reduction on Cu(100), Cu(111), and Cu(polycrystalline) electrodes in 0.1 M alkaline hydroxide electrolytes (LiOH, NaOH, KOH, RbOH, CsOH) between 0 and −1.5 V vs RHE. We used online electrochemical mass spectrometry and high-performance liquid chromatography to determine the product distribution as a function of electrode structure, cation size, and applied potential. First, cation effects are potential dependent, as larger cations increase the selectivity of all electrodes toward ethylene at E > −0.45 V vs RHE, but methane is favored at more negative potentials. Second, cation effects are structure-sensitive, as the onset potential for C2H4 formation depends on the electrode structure and cation size, whereas that for CH4 does not. Fourier Transform infrared spectroscopy (FTIR) and density functional theory help to understand how cations favor ethylene over methane at low overpotentials on Cu(100). The rate-determining step to methane and ethylene formation is CO hydrogenation, which is considerably easier in the presence of alkaline cations for a CO dimer compared to a CO monomer. For Li+ and Na+, the stabilization is such that hydrogenated dimers are observable with FTIR at low overpotentials. Thus, potential-dependent, structure-sensitive cation effects help steer the selectivity toward specific products.

Carbon dioxide and carbon monoxide can be electrochemically reduced to useful products such as ethylene and ethanol on copper electrocatalysts. The process is yet to be optimized and the exact mechanism and the corresponding reaction intermediates are under debate or unknown. In particular, it has been hypothesized that the C-C bond formation proceeds via CO dimerization and further hydrogenation. Although computational support for this hypothesis exists, direct experimental evidence has been elusive. In this work, we detect a hydrogenated dimer intermediate (OCCOH) using Fourier transform infrared spectroscopy at low overpotentials in LiOH solutions. Density functional theory calculations support our assignment of the observed vibrational bands. The formation of this intermediate is structure sensitive, as it is observed only during CO reduction on Cu(100) and not on Cu(111), in agreement with previous experimental and computational observations.

The pH is an important parameter in the reaction mechanism of the electrochemical reduction of carbon dioxide and carbon monoxide to methane and ethylene on copper electrodes. We have investigated the influence of the pH on this reaction using Cu(1 1 1) and Cu(1 0 0) single crystal electrodes. The results support our recently proposed reaction mechanism, in which two different reaction pathways to ethylene can be distinguished: a first, pH-dependent pathway that has a common intermediate with the formation of methane that occurs mainly on Cu(1 1 1), and a second, pH-independent pathway via a carbon monoxide dimer. The latter pathway occurs on Cu(1 0 0) only.

The use of single-crystals in electrochemistry requires careful characterization of the surface structure. This paper addresses the characterization of Cu single-crystals using blank cyclic voltammetry in alkaline media. The adsorption and desorption of OH species in the underpotential region of Cu 2 O formation in alkaline media occur at different potentials on Cu(1 1 1) and Cu(1 0 0), whereas OH adsorption on Cu(1 1 0) is not observed in this potential region. This allows for a direct distinction of the Cu( hkl ) basal planes. The adsorption of OH on Cu(1 1 1) induces a reconstructed adlayer on the surface. On Cu(3 2 2), a stepped surface with 5 atom wide (1 1 1) terraces, OH adsorption is observed in the same potential range as on Cu(1 1 1), but on Cu(3 2 2) reconstruction does not seem to take place. This is explained by the fact that the unit cell of the reconstructed layer is much larger than the (1 1 1) terrace width of Cu(3 2 2) and, therefore, reconstruction cannot take place. Cu(9 1 1), having 5 atom wide (1 0 0) terraces, exhibits the same voltammetric features as Cu(1 0 0), but with a lower intensity. This is explained by the lower amount of (1 0 0) terraces present on this surface.

Nitrate reduction on Cu (100) and Cu (111) surfaces in alkaline and acidic solutions was studied by electrochemical methods (cyclic voltammetry, rotating disc electrode) coupled with online and in situ characterization techniques (mass spectrometry, ion chromatography and Fourier transformed infra-red spectroscopy) to evaluate the reaction mechanism and products on the different surfaces. Electrochemical results show that reduction of nitrate in alkaline media on Cu is structure sensitive. The onset potential on Cu (100) is +0.1 V vs. RHE, ca. 50 mV earlier than on Cu (111). The onset potentials for nitrate reduction on Cu (100) and Cu (111) in acidic media are rather similar. Analytical techniques show a diverse product distribution for both surfaces and for both electrolytes. Whereas in acidic media both Cu electrodes show the formation of NO and ammonia, in alkaline media Cu reduces nitrate to nitrite and further to hydroxylamine. In alkaline media, Cu (100) is a more active surface for the formation of hydroxylamine than Cu (111).

Ethanol is a highly desirable product of the electrochemical reduction of CO and/or CO 2 on copper. Although ethanol and ethylene share common intermediates at the early stages of CO/CO 2 reduction to C 2 species on copper, the pathways bifurcate and most copper surfaces favor the formation of ethylene. We present here a combined experimental-computational study of the electroreduction of acetaldehyde to ethanol on Cu(1 1 1), Cu(1 0 0) and Cu(3 2 2). The experiments show structure-sensitive onset potentials for acetaldehyde reduction such that lower overpotentials are observed for more open facets ( η 3 2 2 1 0 0 1 1 1 ). Our DFT calculations show that the electrochemical reduction of acetaldehyde proceeds via a CH 3 CH 2 O* intermediate on the three electrodes at high *H coverage, and that the stability of this weakly bound intermediate determines the onset potential. Our results suggest that during the late stages of CO/CO 2 reduction to C 2 species on copper, ethanol formation has higher energetic barriers than ethylene formation, and hence the selectivity is inclined toward the latter. Importantly, our results suggest that the barriers for ethanol formation can be lowered by making use of its structure sensitivity.

Carbon monoxide is a key intermediate in the electrochemical reduction of carbon dioxide to methane and ethylene on copper electrodes. We investigated the electrochemical reduction of CO on two single-crystal copper electrodes and observed two different reaction mechanisms for ethylene formation: one pathway has a common intermediate with the formation of methane and takes place preferentially at (111) facets or steps, and the other pathway involves selective reduction of CO to ethylene at relatively low overpotentials at (100) facets. The (100) facets seem to be the dominant crystal facets in polycrystalline copper, opening up new routes to affordable (photo)electrochemical production of hydrocarbons from CO(2).