“Deactivation of copper electrode” in electrochemical reduction of CO2

Metallic Cu electrode can electrochemically reduce CO₂ to CH₄, C₂H₄ and alcohols with high yields as revealed by the present authors. Many workers reported that formation of CH₄ and C₂H₄ rapidly diminishes during electrolysis of CO₂ reduction. This paper shows that such deactivation of Cu electrode is reproduced with electrolyte solutions prepared from reagents used by these workers. Deactivated Cu electrodes recovered the electrocatalytic activity for CO₂ reduction by anodic polarization at −0.05 V versus she in agreement with the previous reports. Features of the deactivation depend greatly on the individual chemical reagents. Purification of the electrolyte solution by preelectrolysis with a Pt black electrode effectively prevents the deactivation of Cu electrode. Anode stripping voltammetry of Cu electrodes, which were deactivated during electrolysis of CO₂ reduction, showed anodic oxidation peaks at ca. −0.1 or −0.56 V versus she. The severer the deactivation of the Cu electrode was, the higher electric charge of the anodic peak was observed. It is presumed that some impurity heavy metal, originally contained in the electrolyte, is deposited on the Cu electrode during the CO₂ reduction, poisoning the electrocatalytic activity. On the basis of the potential of the anodic peaks, Fe²⁺ and Zn²⁺ are assumed to be the major contaminants, which cause the deactivation of the Cu electrode. Deliberate addition of Fe²⁺ or Zn²⁺ to the electrolyte solutions purified by preelectrolysis exactly reproduced the deactivation of a Cu electrode in CO₂ reduction. The amount of the deposited Fe or Zn on the electrode was below the monolayer coverage. Electrothermal atomic absorption spectrometry (etaas) showed that Fe originally contained in the electrolyte solution is effectively removed by the preelectrolysis of the solution. Mechanistic difference is discussed between Fe and Zn in the deterioration of the electrocatalytic property of Cu electrode in the CO₂ reduction. The concentration of the impurity substances originally contained in the chemical reagents as Fe or Zn is estimated to be far below the standard of the impurity levels guaranteed by the manufacturers. Presence of trimethylamine in the electrolyte solution also severely poisons a Cu electrode in the CO₂ reduction. It was concluded that the deactivation of Cu electrode in CO₂ reduction is not caused by adsorption of the products or the intermediates produced in CO₂ reduction.     

Electrochemical impedance spectroscopic investigation of CO2 reduction on polyaniline in methanol

The ac response of polyaniline thin films on platinum electrodes was measured at different dc potentials during the CO₂ reduction in methanol/LiClO₄ electrolyte with a small amount of 0.5 M H₂SO₄. The complex capacitance curves were simulated and the data obtained were used to calculate kinetic parameters, based on the assumption that the thermodynamic potential E₀ is in the region of −0.2-−0.1 V versus saturated calomel electrode (SCE). With E₀=−0.2 V versus SCE and β=0.6, a j₀ value of ca. 10⁻⁴ A cm⁻² was found for the electroreduction of CO₂ on the polyaniline electrode.     

Electrochemical reduction of carbon dioxide to ethylene with high Faradaic efficiency at a Cu electrode in CsOH/methanol

The electrochemical reduction of CO₂ with a Cu electrode in CsOH/methanol-based electrolyte was investigated. The main products from CO₂ were methane, ethylene, ethane, carbon monoxide and formic acid. A maximum Faradaic efficiency of ethylene was 32.3% at −3.5 V vs. Ag/AgCl saturated KCl. The best methane formation efficiency was 8.3% at −4.0 V. The ethylene/methane current efficiency ratio was in the range 2.9–7.9. In the CsOH/methanol, the efficiency of hydrogen formation, being a competitive reaction against CO₂ reduction, was depressed to below 23%.     

Electrochemical reduction of carbon dioxide to ethylene at a copper electrode in methanol using potassium hydroxide and rubidium hydroxide supporting electrolytes

The electrochemical reduction of CO₂ with a Cu electrode was investigated in methanol using potassium hydroxide and rubidium hydroxide supporting salts. The main products from CO₂ were methane, ethylene, carbon monoxide and formic acid. The maximum current efficiency for ethylene was of 37.5%, at −4.0 V versus Ag/AgCl, saturated KCl in KOH/methanol. The typical ratios of current efficiency for ethylene/methane, r_f(C₂H₄)/r_f(CH₄), were 2.3 and 6.7, in KOH/methanol and RbOH/methanol-based electrolytes, respectively. In KOH/methanol, the efficiency of hydrogen formation, a competing reaction of CO₂ reduction, was depressed to below 3.3%. On the basis of this work, the high efficiency electrochemical CO₂-to-ethylene conversion method appears to be achieved. Future work to advance this technology may include the use of solar energy as the electric energy source. This research can contribute to the large-scale manufacturing of fuel gases from readily available and inexpensive raw materials, CO₂-saturated methanol from industrial absorbers (the Rectisol process).     

Gasification of charcoal using supercritical CO2 at high pressures

A novel one-shell high temperature and high pressure semi-continuous reactor has been developed for the study of the Boudouard reaction at temperatures up to 820 °C and pressures up to 32.5 MPa. Semicontinuous gasification of charcoal using supercritical CO₂ has been achieved at conversions up to 90.8% (w/w) at LSHV between 20 and 30 h⁻¹ after 5–9 h. A gasification model is proposed and validated. Effective rates of gasification (1.32 ± 0.12) × 10⁻⁶ to (6.10 ± 2.03) × 10⁻⁵ s⁻¹ were obtained. The results indicated that this method is technically feasible for the on-line production of high pressures streams of CO/CO₂ in the lab for carrying out further chemistries, avoiding the use of CO high pressure bottles.     

Effect of TiO2 particle size on the photocatalytic reduction of CO2

Pure TiO₂ anatase particles with a crystallite diameters ranging from 4.5 to 29 nm were prepared by precipitation and sol–gel method, characterized by X-ray diffraction (XRD), BET surface area measurement, UV–vis and scanning electron microscopy (SEM) and tested in CO₂ photocatalytic reduction. Methane and methanol were the main reduction products. The optimum particle size corresponding to the highest yields of both products was 14 nm. The observed optimum particle size is a result of competing effects of specific surface area, charge–carrier dynamics and light absorption efficiency.     

CO2 hydrogenation to methanol over Cu/ZnO/ZrO2 catalysts prepared via a route of solid-state reaction

Cu/ZnO/ZrO₂ catalysts were prepared by a route of solid-state reaction and tested for the synthesis of methanol from CO₂ hydrogenation. The effects of calcination temperature on the physicochemical properties of as-prepared catalysts were investigated by N₂ adsorption, XRD, TEM, N₂O titration and H₂-TPR techniques. The results show that the dispersion of copper species decreases with the increase in calcination temperature. Meanwhile, the phase transformation of zirconia from tetragonal to monoclinic was observed. The highest activity was achieved over the catalyst calcined at 400 °C. This method is a promising alternative for the preparation of highly efficient Cu/ZnO/ZrO₂ catalysts.     

Electrochemical conversion of carbon dioxide to methane in aqueous NaHCO3 solution at less than 273 K

The electrochemical reduction of CO₂ on a Cu electrode was investigated in aqueous NaHCO₃ solution, at low temperature. A divided H-type cell was employed, the catholyte was 0.65 mol dm⁻³ NaHCO₃ aqueous solution and the anolyte was 1.1 mol dm⁻³ KHCO₃ aqueous solution. The temperature during the electrolysis of CO₂ was decreased stepwise to 271 K. Methane and formic acid were obtained as the main products. The maximum Faradaic efficiency of methane was 46% at −2.0 V and 271 K. The efficiency of hydrogen formation, a competing reaction of CO₂ reduction, was significantly depressed with decreasing temperature. Based on the results of this work, the proposed electrochemical method appears to be a viable means for removing CO₂ from the atmosphere and converting it into more valuable chemicals. The synthesis of methane by the electrochemical method might be of practical interest for fuel production and the storage of solar energy.     

Simultaneous easy CO2 recovery and drastic reduction of SOx and NOx in O2/CO2 coal combustion with heat recirculation

Coal combustion with O₂/CO₂ is promising because of its easy CO₂ recovery, extremely low NO_x emission and high desulfurization efficiency. Based on our own fundamental experimental data combined with a sophisticated data analysis, its characteristics were investigated. It was revealed that the conversion ratio from fuel-N to exhausted NO in O₂/CO₂ pulverized coal combustion was only about one fourth of conventional pulverized coal combustion. To decrease exhausted NO further and realize simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x, a new scheme, i.e. O₂/CO₂ coal combustion with heat recirculation, was proposed. It was clarified that in O₂/CO₂ coal combustion, with about 40% of heat recirculation, the same coal combustion intensity as that of coal combustion in air could be realized even at an O₂ concentration of as low as 15%. Thus exhausted NO could be decreased further into only one seventh of conventional coal combustion. Simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x could be realized with this new scheme.     

Investigation on electrochemical reduction process of Nb2O5 powder in molten CaCl2 with metallic cavity electrode

The direct electrochemical reduction process of Nb₂O₅ powder was investigated by cyclic voltammetry and constant potential electrolysis with a novel metallic cavity electrode in molten calcium chloride at 850 °C. The products of both constant potential and constant voltage electrolysis were characterized by XRD, SEM and EDX. CaNb₂O₆ was formed upon addition of solid Nb₂O₅ into molten CaCl₂ when CaO was present. During the electrolysis solid Nb₂O₅ was reduced to various niobium oxides of lower oxidation states, including some composite oxides, and then was converted completely to metallic niobium near −0.35 V (vs. Ag/AgCl), which was more positive than the reduction potential of Ca²⁺. Constant potential electrolysis was applied at the potentials near the reduction current peaks derived from the cyclic voltammetry curves, and cell voltages were monitored. The voltage was near 2.4 V when the oxide was metallized at −0.35 V (vs. Ag/AgCl). Nb₂O₅ pellet could be used to prepared metallic niobium at cell voltage 2.4 V in a larger electrolysis bath filled with calcium chloride at 850 °C. The experiment results further demonstrated the direct electrochemical reduction mechanism of Nb₂O₅ powder in a molten system.     

Electrochemical promotion of the CO2 hydrogenation reaction using thin Rh, Pt and Cu films in a monolithic reactor at atmospheric pressure

A monolithic electropromoted reactor (MEPR) with up to 22 thin Rh/YSZ/Pt or Cu/TiO₂/YSZ/Au plate cells was used to investigate the hydrogenation of CO₂ at atmospheric pressure and temperatures 220–380 °C. The Rh/YSZ/Pt cells lead to CO and CH₄ formation and the open-circuit selectivity to CH₄ is less than 5%. Both positive and negative applied potentials enhance significantly the total hydrogenation rate but the selectivity to CH₄ remains below 12%. The Cu/TiO₂/YSZ/Au cells produce CO, CH₄ and C₂H₄ with selectivities to CH₄ and C₂H₄ up to 80% and 2%. Both positive and negative applied potential significantly enhance the hydrogenation rate and the selectivity to C₂H₄. It was found that the addition of small (0.5 kPa) amounts of CH₃OH in the feed has a pronounced promotional effect on the reaction rate and selectivity of the Cu/TiO₂/YSZ/Au cells. The selective reduction of CO₂ to CH₄ starts at 220 °C (vs 320 °C in absence of CH₃OH) with near 100% CH₄ selectivity at open-circuit and under polarization conditions at temperatures 220–380 °C. The results show the possibility of direct CO₂ conversion to useful products in a MEPR via electrochemical promotion at atmospheric pressure.     

CO oxidation and CO2 reduction on carbon supported PtWO3 catalyst

The activity of a carbon supported PtWO₃ (PtWO₃/C) catalyst in the CO oxidation and CO₂ reduction reactions was evaluated in sulfuric acid solution at room temperature.Cyclic voltammetry combined with on-line mass spectrometry shows that the oxidation of both saturated CO adlayer and dissolved CO on PtWO₃/C material commences at rather low potentials, ca. 0.18 and 0.12 V vs. RHE, respectively. However, the low-potential process seems to involve only a minor fraction of the CO adlayer, the major part of the adsorbed CO layer being oxidised at the potentials as high as those for pure Pt catalysts—ca. 0.7 V vs. RHE. PtWO₃/C material was found to reversibly de-activate upon a prolonged exposure to the CO-saturated solution due to the inhibition of the hydrogen tungsten bronze formation.The reduction of CO₂ on PtWO₃/C leads to the formation of an adsorbate - presumably CO - on the Pt sites of the catalyst. Although the rate of the adsorbate build-up on PtWO₃/C at 0.1 V is lower than that on pure Pt/C, our results indicate that upon a prolonged exposure of the PtWO₃/C electrode to a CO₂-saturated solution a complete poisoning of the Pt sites with the adsorbate is likely to occur at room temperature.     

Electrochemical window of molten LiCl-KCl-CaCl2 and the Ag/Ag reference electrode

The electrochemical window of an LiCl-KCl-CaCl₂ eutectic melt (52.3:11.6:36.1 mol%) was determined by cyclic voltammetry and open-circuit potentiometry at 723-873 K. The reaction at the anodic limit was confirmed to be Cl₂ gas evolution. The reaction at the cathodic limit was found to be a liquid Ca-Li alloy formation on the basis of ICP analysis of the deposits. An Ag⁺/Ag reference electrode separated with a Pyrex membrane showed good stability for more than 1 week. The standard electrode potential of Ag⁺/Ag was determined in the temperature range of 723-823 K by measuring the potential of a silver electrode in different concentrations of Ag⁺ ions.     

Production of syngas plus oxygen from CO2 in a gas-diffusion electrode-based electrolytic cell

Most of the research published in electrochemical CO₂ reduction has been reported for half-cells, with little consideration of the overall system. However, it is necessary to consider the eventual involvement of full cells. We conducted CO₂ reduction and water oxidation in a CO₂-reducing full cell with larger geometric surface area (2×2 cm²) and with a relatively small inter-electrode gap (1-2 mm) in order to minimize ohmic losses. The result was an ca. 1:1 CO/H₂ (v/v) gas ratio at a current density of 10 mA cm⁻² and a cell voltage of 3.05 V, producing O₂ at the counter electrode. Based on an enthalpic voltage of 1.36 V, this constitutes an overall energy efficiency of 44.6%.     

Cetirizine solubility in supercritical CO2 at different pressures and temperatures

A simple static technique was used to obtain the solubility of cetirizine in supercritical carbon dioxide. The solubility measurements were performed at temperatures and pressures ranging from 308.15 to 338.15 K and 160 to 400 bar, respectively; resulting in mole fractions in the 1.05 × 10⁻⁵ to 4.92 × 10⁻³ range. The Chrastil, Bartle, Kumar & Johnston and the Mendez-Santiago and Teja (MST) models were used to correlate the experimental data. The calculated solubilities showed good agreement with the experimental data in the temperature and pressure ranges studied.     

Deactivation mechanism of Pd supported on ordered and non-ordered mesoporous silica in the direct H2O2 synthesis using CO2-expanded methanol

The deactivation mechanism of Pd supported on silica and mesoporous silica (SBA-15) using CO₂-expanded methanol as solvent was studied in the direct synthesis of H₂O₂ in batch and semi-continuous batch reactor tests as well as its hydrogenolysis. Fresh and used catalysts were characterized by TPR and CO chemisorption. The results evidence the presence of deactivation, which can be correlated to the loss of accessible active metal surface area due to sintering of Pd, but there is also an effect of the presence of the ordered mesoporous structure and of the reaction conditions. The higher concentration of H₂ in solution in semi-continuous batch reactor tests with respect to batch reactor tests leads to a more relevant deactivation in Pd-SiO₂ with respect to Pd-SBA-15, but a higher initial activity, due to the fact that H₂ accelerates the reduction of the Pd species which are less reducible in Pd-SiO₂ than in Pd-SBA-15. Pd-SBA-15 shows a higher H₂O₂ selectivity and productivity with respect to Pd-SiO₂ in batch reactor tests, related to the presence of easier reducible Pd species. Another difference is related to the different mechanism of sintering. On the SBA-15 support, due to the presence of the ordered mesoporosity, the Pd particles migrate into the SBA-15 channels forming elongated 1D-type particles. In Pd-SiO₂ catalyst, instead, the sintering of the Pd particles leads to large aggregates of Pd particles in the range of 20-25 nm.     

CO2 reforming of CH4 over stabilized mesoporous Ni-CaO-ZrO2 composites

Mesoporous Ni-CaO-ZrO₂ nanocomposites with high thermal stability were designed and employed in the CO₂/CH₄ reforming. The nanocomposites with appropriate Ni/Ca/Zr molar ratios exhibited excellent activity and prominent coking resistivity. The Ni crystallites were effectively controlled under the critical size for coke formation in such nanocomposites. It was found that low Ni content resulted in high metal dispersion and good catalytic performance. Moreover, the basicity of the matrices improved the chemisorption of CO₂ and promoted the gasification of deposited coke on the catalyst.     

Production of bioethanol under high pressure of CO2: The effect of process conditions

The aim of this work is to investigate the possibility of producing ethanol by glucose fermentation under high pressure of carbon dioxide up to 48 bar, in order to exploit both ethanol and denser CO₂ as a by-product of the process. The fermentation is carried out using Ethanol Red™-Lesaffre Saccharomyces cerevisiae yeast strain, which is commonly applied in industrial bioethanol production.The experiments were performed in six small reactors (2 mL of volume each) connected in parallel, to investigate the effect of the process variables at the same conditions of temperature and pressure, and in one pilot reactor (1 L of volume) to confirm the results obtained at the lower scale.The influence of operative variables, such as carbon dioxide pressure (0–48 bar), temperature (32 and 36 °C), glucose (150–250 g/L), inositol (0–400 mg/L) and biomass concentration (OD 2 and 3.5), was measured in terms of ethanol concentration (by gas chromatography) and ethanol productivity (expressed as grams of ethanol per CFU of yeast). Both of these parameters were found to be strongly dependent on glucose concentration and CO₂ pressure, which negatively affects the fermentation. Nevertheless, also at 50 bar it is possible to produce appreciable amounts of ethanol.     

CO2 attraction by specifically adsorbed anions and subsequent accelerated electrochemical reduction

The electrochemical reduction of CO₂ was studied on a copper mesh electrode in aqueous solutions containing 3 M solutions of KCl, KBr and KI as the electrolytes in a two and three phase configurations. Electrochemical experiments were carried out in a laboratory-made, divided H-type cell. The working electrode was a copper mesh, while the counter and reference electrodes were Pt wire and Ag/AgCl electrode, respectively. Results of our work suggest a reaction mechanism for the electrochemical reduction of CO₂ in the two phase configuration where the presence of Cu-X as the catalytic layer facilitates the electron transfer from the electrode to CO₂. Electron-transfer to CO₂ may occur via the X_ad⁻(Br⁻, Cl⁻, I⁻)-C bond, which is formed by the electron flow from the specifically adsorbed halide anion to the vacant orbital of CO₂. The stronger the adsorption of the halide anion to the electrode, the more strongly CO₂ is restrained, resulting in higher CO₂ reduction current. Furthermore, it is suggested that specifically adsorbed halide anions could suppress the adsorption of protons, leading to a higher hydrogen overvoltage. These effects may synergistically mitigate the overpotential necessary for CO₂ reduction, and thus increase the rate of electrochemical CO₂ reduction.