Pressurized CO2 Electrochemical Conversion to Formic Acid: From Theoretical Model to Experimental Results

To curb the severely rising levels of carbon dioxide in the atmosphere, new approaches to capture and utilize this greenhouse gas are currently being investigated. In the last few years, many researches have focused on the electrochemical conversion of CO2 to added-value products in aqueous electrolyte solutions. In this backdrop, the pressurized electroreduction of CO2 can be assumed an up-and-coming alternative process for the production of valuable organic chemicals [1-3]. In this work, the process was studied in an undivided cell with tin cathode in order to produce formic acid and develop a theoretical model, predicting the effect of several operative parameters. The model is based on the cathodic conversion of pressurized CO2 to HCOOH and it also accounts for its anodic oxidation. In particular, the electrochemical reduction of CO2 to formic acid was performed in pressurized filter press cell with a continuous recirculation of electrolytic solution (0.9 L) at a tin cathode (9 cm2) for a long time (charge passed 67’000 C). It was shown that it is possible to scale-up the process by maintaining good results in terms of faradaic efficiency and generating significantly high concentrations of HCOOH (about 0.4 M) [4]. It was also demonstrated that, for pressurized systems, the process is under the mixed kinetic control of mass transfer of CO2 and the reduction of adsorbed CO2 (described by the Langmuir equation), following our proposed reaction mechanism [5]. Moreover, the theoretical model is in good agreement with the experimental results collected and well describes the effect of several operating parameters, including current density, pressure, and the type of reactor used. 1. Ma, S., & Kenis, P. J. (2013). Electrochemical conversion of CO2 to useful chemicals: current status, remaining challenges, and future opportunities. Current Opinion in Chemical Engineering, 2(2), 191-199. 2. Endrődi, B., Bencsik, G., Darvas, F., Jones, R., Rajeshwar, K., & Janáky, C. (2017). Continuous-flow electroreduction of carbon dioxide. Progress in Energy and Combustion Science, 62, 133-154. 3. Dufek, E. J., Lister, T. E., Stone, S. G., & McIlwain, M. E. (2012). Operation of a pressurized system for continuous reduction of CO2. Journal of The Electrochemical Society, 159(9), F514-F517. 4. Proietto, F., Schiavo, B., Galia, A., & Scialdone, O. (2018). Electrochemical conversion of CO2 to HCOOH at tin cathode in a pressurized undivided filter-press cell. Electrochimica Acta, 277, 30-40. 5. Proietto, F., Galia, A., & Scialdone, O. (2019) Electrochemical conversion of CO2 to HCOOH at tin cathode: development of a theoretical model and comparison with experimental results. ChemElectroChem, 6, 162-172

Innovative chemical processes for the treatment of water polluted by recalcitrant organic substances

In the last years, many research groups have focused their attention on the innovative chemical processes adopted for the treatment of water effluents polluted by recalcitrant organic substances, i.e., substances resistant to biological treatment. The electrochemical oxidation is one of the most studied technologies because it presents high versatility and low cost, it is realized under mild conditions of pressure and temperature and generally it does not involve the use of toxic substances [1]. In this work, the comparative performance of different electrochemical approaches such as direct oxidation processes, oxidation by means of electrogenerated chlorine and electro-Fenton was investigated. The influence of numerous parameters, such as the nature of the electrodic material and of the organic pollutant, the pH, the flow dynamic regime, the current density, the pollutant concentration and the temperature, on the electrochemical incineration of some carboxylic acids and aliphatic chlorides, chosen as model organic compounds, was studied in detail. Two very different anodes were used: Ti/IrO2-Ta2O5, which presents a quite low oxygen overpotential, and boron-doped diamond (BDD), which certainly is one of the most promising materials for the electrochemical incineration [2,3]. Incineration of carboxylic acids was favored by high flow rate and low current density, i.e., when the oxidation process was mainly under kinetic control. Moreover, the process resulted to be favored by high initial concentrations of the organic substrate and low pH but it did not depend, under the adopted operative conditions, on the nature of the supporting electrolyte. The effect of sodium chloride on the electrochemical oxidation of oxalic acid was observed to depend on the nature of the anode as well as on the pH. The best results were achieved by using IrO2-Ta2O5 electrode, with addition of sodium chloride at acid pH. Furthermore, in the presence of high amounts of sodium chloride, a higher abatement of oxalic acid was obtained when high current densities and low flow rates were imposed. Comparing the performance of electro-Fenton process coupled with anodic oxidation at BDD anode and that of simple anodic oxidation, higher abatements of 1,2-dichloroethane and 1,1,2,2-tetrachloroethane were obtained in the first case thanks to the presence of Fe2+ in the solution. Higher applied currents led to a faster electrogeneration of H2O2 and regeneration of Fe2+, thus giving rise to a faster degradation of the starting compounds. The time course of the concentration of the main intermediates accumulated in the electrochemical cell during the treatments, particularly short-chain carboxylic acids and chlorinated ions, was also reported. [1] Jüttner K. et al., Electrochimica Acta, 45 (2000) 2575–2594. [2] Panizza M. and Cerisola G., Electrochimica Acta, 51 (2005) 191–199. [3] Martinez-Huitle C. A. and Ferro S., Chem. Soc. Rev., 35 (2006) 1324-1340

Oxidation of carboxylic acids in water at IrO2-Ta2O5 and Boron Doped Diamond anodes

The electrochemical oxidation of different carboxylic acids (namely, oxalic, formic and maleic) in water at boron doped diamond (BDD) and IrO2-Ta2O5 (DSA-O2) anodes was performed to study the influence of the operative parameters and of the nature of the acid on the performances of the process. Higher abatements were obtained at BDD with respect to DSA anodes for all the selected carboxylic compounds. The rate of abatement decreased in the order oxalic > formic >> maleic at iridium anodes while an opposite trend was observed at diamond anodes (formic maleic > oxalic), thus indicating that different oxidant agents are involved at these two electrodes. Also the effect of the temperature depends on both the nature of the acid and of the anode. Higher current efficiencies were obtained when most part of the process was under the kinetic control of the oxidation reaction, i.e., when low current densities and high flow rates were imposed. High concentrations of carboxylic acids enhanced the current efficiency at all kinetic regimes

Valorization of CO2-Riched Gaseous to Formic Acid via Electrochemical Routes: Current Status and Perspectives

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Electrochemical conversion of pressurized CO2 at simple silver-based cathodes in undivided cells: study of the effect of pressure and other operative parameters

Electrochemical reduction of pressurized CO2 is proposed as an interesting approach to overcome the main hurdle of the CO2 electrochemical conversion in aqueous solution, its low solubility (ca. 0.033 M), and to achieve good faradaic efficiency in CO using simple sheet silver cathodes and undivided cells, thus lowering the overall costs of the process. The effect on the process of CO2 pressure (1–30 bar), current density, nature of the supporting electrolyte and other operative conditions, such as the surface of the cathode or the mixing rate, was studied to enhance the production of CO. It was shown that pressurized conditions allow to improve drastically the current efficiency of CO (CECO). Furthermore, at relatively high pressure (20 bars), the utilization of simple sheet silver cathodes and silver electrodes with high surfaces gave similar CECO. The stability of the system was monitored for 10 h; it was shown that at a relatively high pressure (15 bar) in aqueous electrolyte of KOH using a simple plate silver cathode a constant current efficiency of CO close to 70% was obtained

Loading carbonaceous materials with silver for the treatment of chloro-organic compounds in aqueous phase

Many electrochemical technologies, either based on novel concepts (such as microbial fuel cells), experimental setups (such as photoelectrochemical or solar photoelectro-Fenton reactors) or materials (mainly focused on the use of large O2-overpotential anodes like BDD) have been devised in recent years for water remediation. Special attention has been paid to highly toxic, biorefractory organic pollutants such as the chlorinated hydrocarbons, which conjugate toxicity with chemical stability, bioaccumulation and long-range diffusivity [1]. Electroreduction at silver cathodes becomes an interesting alternative to degrade chloro-organic compounds, but it may lead to the accumulation of reaction by-products, even upon coupling with electro-oxidation at BDD [2]. On the other hand, some Fenton-based processes have proven very effective for the destruction of organic matter due to the action of •OH formed when cathodically electrogenerated H2O2 reacts with added Fe2+ [3]. Based on this, we have envisaged a potential strategy for the enhanced removal of chloro-organic pollutants and their by-products: electro-Fenton process in the bulk upon H2O2 electrogeneration at a carbonaceous cathode, which can simultaneously act as the substrate for electroreduction at loaded Ag nanoparticles. To achieve this goal, a highly efficient material for H2O2 production, i.e., a gas diffusion electrode (GDE), has been chosen for Ag-loading experiments. Several authors have reported the preparation of Ag-loaded carbonaceous materials based on a simple electroless deposition (ELD) process from Ag+ solutions. Some of them have addressed the full preparation of GDEs with Ag catalysts [4,5]. Here, we report the use of a commercial GDE as a suitable substrate to obtain conveniently dispersed Ag nanoparticles. The effect of several ELD parameters (e.g., nature of reductant, mode of application and deposition time) on the surface morphology has been mainly studied by SEM-EDX. Bulk electrolyses in 50 mM Na2SO4 at various pH were subsequently performed with the best materials to assess their ability to electrogenerate H2O2. For comparison, carbon paper was used as an alternative substrate. An important objective of the research was to find the optimum conditions to load the substrate so as to keep the balance between covered and uncovered area, in order to favor both H2O2 production and pollutant electroreduction. The performances of these electrodes for the electrogeneration of H2O2 and the abatement of chloro-organic pollutants is currently being investigated. [1] S. Rondinini, A. Vertova, in Electrochemistry for the environment, 2010, pp. 279–306. [2] O. Scialdone, A. Galia, L. Gurreri, S. Randazzo, Electrochim. Acta 55 (2010) 701–708. [3] E. Brillas, I. Sirés, M.A. Oturan, Chem. Rev. 109 (2009) 6570–6631. [4] E. Gülzow, N. Wagner, M. Schulze, Fuel Cells 3 (2003) 67–72. [5] S. Rondinini, G. Aricci, Z. Krpetic, C. Locatelli, A. Minguzzi, F. Porta, A. Vertova, Fuel Cells 3 (2009) 253–263

Electrochemical treatment of phenol in water in the presence of active chlorine

Active chlorine is generally generated during Electrochemical Advanced Oxidation Processes (EAOPs) used for the treatment of wastewater which contains a large amount of chlorides [1]. In the presence of polluting organics, the production of chlorinated by-products occurs and this could represent one of the worst drawbacks of these processes. In this study, we have evaluated for the first time the development of an EAOP method in a microfluidic electrochemical reactor in the presence of chlorides. Indeed, a comparative study was carried out to evaluate the performances of a conventional cell in comparison with a microfluidic reactor (using a very small inter-electrode distance of 145 μm), both equipped with a BDD anode and a Ni cathode, for the abatement of phenol 2 mM in the presence of NaCl 0.5 M. The oxidation of phenol was considered and the overall organics concentration was monitored by TOC analyses. Even the potential production of chloroacetic acids, chlorophenols, carboxylic acids, chlorate and perchlorate was carefully evaluated. Particular attention was devoted to monitoring the presence of perchlorate, produced by oxidation of chlorate, highly soluble in water and quite stable, very difficult to be removed [2]. Moreover, the treatment of phenol in the presence of chloride was extended to the chemical oxidation process by addition of sodium hypochlorite solution 10-13% in a batch stirred reactor for a more comprehensive study on the action of the active chlorine, both via chemical and electrochemical oxidation, where, in the latter case, it is generated in situ. As expected, it was demonstrated that the electrogenerated active-chlorine promoted a more quickly oxidation of phenol and by-products with respect to the chemical process. In addition, it was shown that the use of the microfluidic device, operating under a continuous mode, let to achieve higher current efficiencies and a lower generation of some important by-products such as chlorate and perchlorate. For sake of completeness, the effect of various parameters (namely, flow rate, current density and nature of cathode) was also investigated in the case of electrochemical oxidation [3]

Electrochemical synthesis of C-glycosides as non-natural mimetics of biologically active oligosaccharides

Natural oligosaccharides inhibitors of heparanase and selectins are emerging as promising drugs for cancer therapy. As an alternative tool to the natural ones, sulfated tri maltose C-C-linked dimers (alfa,alfa alfa,beta and beta,beta STMCs) were prepared by bromo-maltotriose electroreduction on silver cathode,1 followed by sulfation. The presence of an interglycosidic C-C bond makes STMCs less vulnerable to metabolic processing then their O-analogues. For this reason, STMCs have been studied as drug candidates and inhibitors of carbohydrate processing enzymes. Their activity as inhibitor of Pselectin in vivo and in the attenuation of metastasis both on B16-BL6 melanoma cells and on MC- 38 carcinoma cells2 prompted to the optimization of their synthetic process. Therefore, the electrochemical process for the C-C coupling of the model molecule acetobromoglucose has been investigated by changing various reaction conditions such as solvent and arrangement of the electrolytic cell, aiming at the final scale-up of the reaction

Study of electrochemical remediation of clay spiked with C12-C18 alkanes

To date, the management of polluted soils and sediments is challenging because they can be characterized by heterogeneous conditions, miscellaneous contaminants (organic and inorganic ones), fine grains and low-hydraulic permeability. In these cases, the current treatment methods are poorly effective. ElectroChemical Remediation Technologies (ECRTs) are considered some of the main appealing strategies for the remediation of such complex sites. The ECRTs are based on the application of a relatively low cell potential value, between two or more electrodes, inducing an electric field (E) through the polluted media, which prompts the remediation of the contaminated site. This work was focused on the study of the electrochemical remediation of kaolin artificially spiked with a miscellaneous of five alkanes (C12H26, C13H28, C14H30, C16H34, C18H38), namely C12-C18. Kaolin was selected as a model reproducible, low-buffering, and low-permeability clay and the mixture C12-C18 as a hazardous model of petrol hydrocarbon compounds. The effect of several operative conditions, including the E intensity, type of technology, presence of supporting electrolyte, was investigated. It was found that adopted low E values can simultaneously mobilize and degrade in situ the C12-C18 mixture and that the shorten the chain compound, the easier the remediation efficiency, R

Conversion of CO2 to formic acid in a microfluidic electrochemical cell with and without supporting electrolyte

Electrochemical reduction of carbon dioxide to formic acid or formate (FA) is considered an interesting route to valorize CO2 effluents. Here, we have performed the conversion of CO2 to FA in an undivided microchannel electrochemical reactor characterized by very small inter-electrode distances (75–250 μm) using Na2SO4 as supporting electrolyte (SE). It was found that the use of the microfluidic cell allows to work both in the presence and in the absence of SE with lower cell potentials with respect to conventional cells and to obtain significant conversions per pass of CO2 to FA. The effect of many parameters, such as distance between electrodes, flow rate, current density, concentration of Na2SO4 and pH, was studied. In particular, it was shown that the production of FA increases by reducing the concentrations of Na2SO4 and it presents the maximum value in the absence of it