The present invention relates to the field of electrochemistry and, in general, to a method of conversion of CO₂ into hydrocarbons, namely C2 hydrocarbons (ethylene and ethane).

The electrochemical conversion of CO₂ to hydrocarbons has been intensively studied for more than 20 years (Hori et al., 1988; Cook et al., 1990). Cook et al. (1990) patented the electrochemical reduction of CO₂ to methane and ethylene and reported Faradaic efficiencies of 73% and 25%, respectively, at the first 15 minutes of reaction. Those were the highest Faradaic efficiencies yet reported for the CO₂ reduction reaction. They emphasize the importance of providing in situ deposited uniformly granular copper over the entire cathode surface to obtain such high Faradaic yields. However, it was not achieved a CH₄-free hydrocarbons mixture.

Copper is the only metallic electrode materials that yields hydrocarbons as major products (Gattrell et al., 2006, Jitaru, 2007). According to the literature, the mixture of hydrocarbons produced by the electroreduction of CO₂ is composed mostly of methane and ethylene. In general, the faradaic efficiency of CH₄ is commonly higher than that for C₂H₄. Kyriacou and Anagnostopoulos (1993) reported the following formation efficiencies for the electrochemical reduction of CO₂ on copper in a 0.5 M KHCO₃ solution at 298 K: 16% for methane and 14% for ethylene. Azuma et al. (1990) investigated the CO₂ reduction in a 0.05 M KHCO₃ aqueous solution at 293 K and obtained methane, ethylene and ethane with faradaic efficiencies of 17.8, 12.7 and 0.039%, respectively.

The product distribution and faradaic efficiencies are strongly sensitive to the electrode surface structure and local conditions such as pH, KHCO₃ and CO₂ concentration (Gattrell et al., 2006). Formation of ethylene is favored in dilute KHCO₃ solutions (high-pH electrolytes), whereas methane is preferentially produced in relatively concentrated KHCO₃ solutions (low-pH electrolytes). This product selectivity is derived from the electrogenerated OH- in the cathodic reaction which is instantaneously neutralized by HCO₃ when this specie is available. (Hori et al., 1988, 1989, 1997). More recently, Takahashi et al. (2002) reported the effect of copper crystal structure on the CO₂ electroreduction selectivity. They reported that the Cu(100) crystal faces yield mainly ethylene and the Cu(111) crystal faces favor methane production. Furthermore, they revealed that the increase of atomic density on Cu(110)crystallographic plane activates the formation of substances with more than two carbons (C2+) and suppress CH₄.

Molecular reaction pathways suggested by many authors were controversy. Hori and co-workers (1997) revised the mechanism proposed in 1989 and proposed that pathways to methane and ethylene may differ at first electron transfer to the carbon monoxide. To get insights of CO₂ reaction paths and products selectivity, Hori et al. (1997) investigated the electroreduction of carbon monoxide at copper, nickel and iron electrodes. They show that a surface-roughened Cu electrode yielded a small amount of ethane (1%) as well nickel and iron. However, ethane was not detected at smooth electropolished Cu electrodes or etched with dilute HCl solution. The authors suggested that adsorption of hydrogen species is structure sensitive. They also propose that hydrogen species composed of two hydrogen atoms are involved with the CO reduction on smooth copper electrodes, whereas atomically adsorbed hydrogen may be involved on roughened copper surfaces as well nickel or iron electrodes. The surface roughening provides surface defects such as steps and vacancies favorable for reaction of adsorbed hydrogen atoms. They conclude that ethylene formation is naturally more advantageous than ethane due to the limited supply of adsorbed hydrogen.

Usually, ethane is not detected at copper simply based electrodes. In previous works the employment of modified electrocatalysts to improve the conversion of CO2 to hydrocarbons was limited. For instance, Yano et al. (2004) used copper(I) halide-confined copper mesh electrodes at a three-phase interface (gas-liquid-solid) and the faradaic efficiency of ethylene was considerably enhanced with CuBr. Nevertheless, ethane faradaic efficiency was low (1.2 %) and methane production was not suppressed. Preliminary studies by Gonçalves et al. (2012) revealed that the formation of a C2 hydrocarbons mixture without methane is achievable at specific high surface area electrodeposits (nature and morphology/structure not indicated) in copper cathodes. Nevertheless, Tang et al., 2012 used a copper cathode covered with copper nanoparticles and CO₂ was preferentially converted to C₂H₄ with the faradaic efficiency of 36% and only 1% to CH₄, and ethane production was not reported.

The development and characterization of structures with copper electrodeposits were studied by Nikolić et al. (2007), namely the honeycomb-like structures. According to these authors, the main characteristics of honeycomb-like structures are the existence of two groups of craters or holes of distinct nature. They pointed out that a group of holes or craters is formed by the connection of hydrogen bubbles that resulted from an intense hydrogen evolution reaction during the deposition. The other group is originated from the agglomerates of copper grains formed in the initial stage of electrodeposition. They also referred that for longer times of deposition a third class of holes could be formed through the combination of the holes of the first two groups. Nevertheless, these authors did not refer any application of this type of electrodeposits on the electroreduction of CO₂ or any data that allowed the prediction of the behavior of this type of deposits on the referred reaction.

One of the most important issues, in the point of view of a continuous electrolytic process, is the steadiness and durability of the gases production. Unfortunately, all the faradaic efficiencies reported in the literature fall suddenly after short periods of electrochemical reduction, and the copper electrode loses its high catalytic activity toward the conversion of CO₂ (Cook, 1990; Kyriacou and Anagnostopoulos, 1992).

The process of this invention concerns the electrochemical conversion of carbon dioxide in aqueous solutions into hydrocarbons and, more specifically to C2 hydrocarbons (ethylene and ethane) without the formation of methane. The achievement of mixtures with a higher C2 hydrocarbons content represents an advantage in relation to the mixtures described in the prior art, since higher energetic densities are attained considering the same content of hydrogen or carbon monoxide. This advantage makes them more appropriated, for instance, to be utilized as fuels.

The process is carried out in an electrochemical cell wherein the cathode is a modified electrode with copper electrodeposits that increases the activity and selectivity of the method under stable CO₂ conversion. The conversion of CO₂ is performed in a one step, easily scalable and can operate at ambient temperature and atmospheric pressure. The number of carbon atoms in the generated hydrocarbon molecules can be controlled by the increase of copper active surface area available for CO₂ electroreduction. The hydrocarbons produced can be subsequently used as industrial feedstock.

It is an object of the present invention a process for the selective electrochemical conversion of CO2 into C2 hydrocarbons, at room temperature and atmospheric pressure, comprising a two-phase liquid / solid system consisting of an electrochemical cell with an anode and a cathode, said cathode being modified by copper electrodeposits having either:

1. a. a monolayer porous structure / morphology with craters (1) ;
2. b. a monolayer dendritic structure morphology or
3. c. a multilayer tridimensional pore structure with different porous sizes (2).

• Figure 1 depicts the SEM image of the electrode surface with electrodeposits having a dendritic crystal structure. The deposits were obtained by an ex-situ electrodeposition of copper on a copper mesh as referred in example I.
• Figure 2 depicts the SEM image of the electrode surface with electrodeposits having a honeycomb-like structure. The deposits were obtained by an ex-situ electrodeposition of copper on a copper mesh as referred in example II. Number (1) indicates two distinct craters.
• Figure 3 depicts the SEM image of the electrode surface with electrodeposits having a porous 3D sponge structure. It was obtained by electrodeposition ex-situ of copper on a copper foil as mentioned in example IV. Number (2) indicates two pores in distinct layers.
• Figure 4 depicts a schematic drawing of an example of a laboratory-size electrochemical cell in which the process of the invention can be carried out. The numbers indicate: (3) cathodic compartment, (4) anodic compartment, (5) membrane compartment (optional), (6) gas inlet, (7) reference electrode inlet, (8) gas outlet, (9) pH sensor inlet.
• Figure 5 - Graphical representation of current efficiency (%) vs cathode potential (V vs Ag/AgCl) obtained in example III.

The process of this invention for the conversion of carbon dioxide into hydrocarbons, and more specifically into hydrocarbons with two carbon atoms (C2 hydrocarbons) is performed in an electrochemical cell. The cell is preferentially a two compartment cell in which the cathode and the anode are separated preferentially by an ion exchange membrane.

The anode may be any suitable electrically conducting material appropriate for effective operation in an electrolytic cell, for example, platinum, graphite and glassy carbon. The cathode material may be any suitable electrically conducting material such as copper or glassy carbon. The cathode substrate may have any suitable configuration appropriate for electro-deposition, including mesh and foil configurations.

The cathode surface is modified by in-situ or ex-situ copper electrodeposition. The ex-situ deposition is preferable to provide a stable and exclusive conversion of carbon dioxide into C2 hydrocarbons and can be performed using preferably copper sulphate as the source of copper cations and sulphuric acid to increase the acidity of the deposition bath. Modified copper electrodes with good mechanical resistance and an extremely large useable surface area were used as obtained.

As regards in-situ electrodeposition, it consists of in-situ electroplating which comprises reduction of carbon dioxide through the addition of copper ions to the electrolyte at a concentration higher than 0,005 M.

In an embodiment of the process of the present invention, it further comprises addition and contact with the surface of the cathode of a carbon dioxide flow and a current supply between the anode and cathode for the selective reduction of carbon dioxide into ethane to ethylene.

Three types of structures for the copper electrodeposits prepared by ex-situ electrodeposition are presented in Figures 1, 2 and 3.

The presence of the electrodeposits at the electrode surface strongly modifies the catalytic behavior of the electrodes for the conversion of carbon dioxide, consequently changing the composition of the gaseous hydrocarbon products created.

The catholyte may be any inorganic salt aqueous solution in which the carbon dioxide is soluble, such as KHCO₃, NaHCO₃ and KCl, preferably KHCO₃, in concentrations of around 0.03 to 0.5 M, at a pH preferably of around 4 to 9. The anolyte may be preferably the same as the catholyte.

A suitable ion exchange membrane that can be used may be any ion exchange membrane that allows the passage of protons, such as Nafion 117, or Nafion 417.

The electrochemical reduction of CO₂ to C2 hydrocarbons is achievable by using an electrochemical cell that possesses one cathode with copper electrodeposits submerged in an electrolyte in which the CO₂ is soluble. Regarding the physical configuration of the electrochemical cell, any suitable shape and disposition of an electrochemical cell can be used.

A voltage, provided by an external power supply, is applied between the cathode and the anode. The electrochemical reduction process can be carried out in a continuous mode (galvanostatic or potentiostatic) or in a pulsed electrolytic mode. Preferably, in said galvanostatic or potentiostatic mode, the applied voltage is in the range between -0.5 V to -4 V in relation to the reference electrode of silver / silver chloride or by an electrolytic pulsed mode. The modified cathodes presented in Figures 1 to 3 are used in the electrochemical cell for the conversion of carbon dioxide. The electrodeposit at the electrode surface strongly modifies the catalytic behavior of the electrodes used for the carbon dioxide conversion, modifying subsequently the composition of the gaseous hydrocarbons (reaction products).

The structure shown in Fig. 1 consists of dendritic copper deposits. It was discovered that this type of electrodeposits promotes ethylene production in detriment of methane. Additionally, it was verified that the bigger the coverage of the surface electrode with dendritic copper electrodeposits, the bigger was the selectivity for ethylene.

The cathode modified with copper honeycomb-like structures shown in Fig. 2 promotes only the production of C2 hydrocarbons (ethylene and ethane), not being detected methane. This result is innovative and is due to a high surface area of the electrodeposits, its morphology and structure. The honeycomb-like structure of electrodeposits has a higher quantity of copper grains per volume. This configuration enhances the proximity of the active electro-catalytic sites, which results in an increased likelihood for the formations of hydrocarbons with longer chains.

The modified cathode illustrated in Fig. 3 is a three-dimensional copper open structure which is extremely porous. This configuration allows rapid transport of gas and liquid, and its high surface area and mechanically well-supported structure is desirable for electrochemical reactions. Using this cathode for the conversion of carbon dioxide the products are C2 hydrocarbons without formation of methane. This result confirms that the availability of the copper active surface and the spatial distribution of the electro-catalytic zones are the key parameters for the conversion of carbon dioxide in hydrocarbons with two carbon atoms. Additionally it was verified that the copper electrodeposits obtained by ex-situ electrodeposition lead to the stability of the carbon dioxide reduction process, this is, to the stability of the hydrocarbon production due to the weak deactivation of the cathode.

The present invention and the knowledgment of the state-of-the-art indicate that the selective reduction of the carbon dioxide into C2 hydrocarbons may follow the following reaction mechanism:

The modified electrodes have a distinct catalytic behavior from the substrate behavior modifying the distribution of the hydrocarbons resulting from the reaction. This catalytic activity is mainly dependent on the characteristics of the electrodeposits, more specifically on the active surface area, on the morphology, and on the crystalinity (this is the structure of the copper crystals).

The following examples of experiments performed by the inventors with specific conditions and materials are only intended to exemplify the invention and not to limit it in any way.

A copper mesh cathode was modified with copper electrodeposits by ex-situ electrodeposition, having the configuration of Fig.1.

Potentiostatic reduction of carbon dioxide was performed in a flat cell (similar to the one depicted in Figure 4) at room temperature and atmospheric pressure, under conditions of continuous carbon dioxide flow. The electrolytic solution used was of potassium bicarbonate (Merck, p.a.) with a concentration of 0.1 M. A cationic exchange membrane separated the catholyte and anolyte compartments. The anode was a platinum mesh. The applied electrode potential was - 1.9 V, measured against a silver/silver chloride reference electrode.

The outlet gas composition was analyzed on-line by gas chromatography using a Restek ShinCarbon ST micropacked column (L = 2.0 m, U = 1/8 in., ID = 1 mm, 100/200 mesh) and an Agilent 4890D GC equipped with a thermal conductivity detector (TCD) and a 6-port gas sampling valve with a 0.250 mL loop. The faradaic efficiencies of the products were calculated on the basis of the number of electrons required for the formation of one molecule of the products from carbon dioxide and water; eight for methane, twelve for ethylene, fourteen for ethane, two for carbon monoxide and two for molecular hydrogen. Only gaseous products were analyzed.

The utilization of electrodes with a dendritic morphology resulted in a selective production of ethylene instead of methane. The electrode promotes an almost selective production of ethylene (33.3 %), being the methane production of 3.6 %.

Use of an electrolysis cell in accordance with that described in Example I, except that the cathode was ex-situ modified by electrodeposition to obtain a final configuration according to Figure 2. With this system higher currents are obtained during the potentiostatic reduction of CO₂ at -1.9 V in relation to Ag / AgCl. This indicates that the presence of copper electrodeposits with a honeycomb structure increases the active surface area available. This copper based honeycomb structure does not favor the production of methane, but gives rise to a mixture containing only C2 hydrocarbons (C₂H₄ (10.7%) and C₂H₆ (3.7%)). Probably, this type of electrodeposits that has a larger quantity of copper grains and a configuration that increases the closeness and spatial distribution of the electrocatalytic active sites favors the formation of higher hydrocarbons.

The release of hydrogen was higher on this type of electrode (Fig. 2). In aqueous solution, the production of hydrogen competes with the electrocatalytic reduction of CO₂ due to the fact that this reaction is very sensitive to the relative concentration of protons and CO₂.

To confirm that the products obtained in Example II were an inherent characteristic of the electrode, different potentials and different concentrations of electrolyte were tested. We used an electrolysis cell in accordance with that described in Example I, except that the cathode was a copper foil modified ex-situ by deposition in order to obtain an electrodeposits configuration according to Fig. 2. The CO₂ electroreduction was carried out at various cathode potentials and electrolyte concentrations. The results for the potentiostatic reduction in the range -1.5 to -1.9 V in relation to Ag / AgCl are shown in Fig. 5. A high selectivity to C2 hydrocarbons without production of methane was observed for all conditions tested. Only the C2 hydrocarbons composition was modified.

From these studies it can be concluded that the selectivity changes for the less negative potential or for a lower electrolyte concentration: the production of hydrogen decreases but the system produces more ethane than ethylene. The formation of CO is promoted at less negative potentials and lower electrolyte concentrations, i.e. when the formation is C₂H₆ increases, a fact that from the point of view of the CO₂ conversion mechanism merits being underlined. It is also important to highlight that methane was not detected and ethane was always present on all the tests performed on the high specific surface electrode (Fig. 2). These tests confirmed that the product composition, without modification of the C2 hydrocarbons distribution changes on this electrode.

Use of an electrolysis cell in accordance with that described in Example I, except that the cathode was ex-situ modified by electrodeposition to obtain a final configuration according to Fig. 3. The 1h 15min potentiostatic reduction results at -1.9 V in relation to Ag/AgCl are: 0% CH₄, 9.3% C₂H₄ and 5.7% C₂H₆. This electrode provides a mixture of C2 hydrocarbons (C₂H₄, and C₂H₆) without production of methane. This result confirms that the availability of a copper active surface and the spatial distribution of the active electro-catalytic sites is the key for the conversion of CO₂ into C2 hydrocarbons.

The present invention was described and based on certain aspects, but it will be apparent to an expert in the state of the art that additional results could be included, that other experimental conditions may be varied, always in the scope of the invention. Moreover, detailed descriptions of well-known processes and devices were omitted to not overload the description of the present invention with unnecessary details.

R. L. Cook, R. C. MacDuff and A. F. Sammells. Electrochemical reduction of CO₂ to CH₄ and C₂H₄. (1990) Patent US4897167.

• G. Kyriacou, A. Anagnostopoulos, Influence of CO₂ partial pressure and the supporting electrolyte cation on the product distribution in CO₂ electroreduction. Journal of Applied Electrochemistry 23 (1993) 483-486.
• G. Kyriacou, A. Anagnostopoulos. Electroreduction of CO₂ on differently prepared copper electrodes. The influence of electrode treatment on the current efficiencies. Journal of Electroanalytical Chemistry 322 (1992) 233-246.
• H. Yano, T. Tanaka, M. Nakayama. Selective electrochemical reduction of CO2 to ethylene at a three-phase interface on copper(I) halide-confined Cu-mesh electrodes in acidic solutions of potassium halides. Journal of Electroanalytical Chemistry 565 (2004) 287.
• I. Takahashi, O. Koga, N. Hoshi and Y. Hori. Electrochemical reduction of carbon dioxide at copper single crystal electrodes Cu(S)-[n(111)x(111)] and Cu(S)-[n(110)x(100)] electrodes. Journal of Electroanalytical Chemistry 533 (2002) 135-143.
• M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata. Electrochemical reduction of carbon dioxide on various metal electrodes in low-temperature aqueous KHCO₃ media. Journal of the Electrochemical Society 137 (1990) 1772-1778.
• M. Gattrell, N. Gupta and A. Co. A review of the aqueous electrochemical reduction of CO₂ to hydrocarbons at copper. Journal of Electroanalytical Chemistry 594 (2006) 1-19.
• M. Jitaru. Electrochemical carbon dioxide reduction - Fundamental and applied topics (review). Journal of the University of Chemical Technology and Metallurgy 42 (2007) 333-344.
• M. R. Gonçalves, A. Gomes, J. Condeço, R. Fernandes, T. Pardal, C.A.C. Sequeira, J. B. Branco. Selective electrochemical conversion of CO₂ to C2 hydrocarbons. Energy Conversion and Management. 51 (2010) 30
• N. D. Nikolić, K. I. Popov, Lj. J. Pavlović and M. G. Pavlović. Phenomenology of a formation of a honeycomb-like structure during copper electrodeposition Journal of Solid State Electrochemistry 11 (2007) 667-675.
• W. Tang, A. A. Peterson, A. S. Varela, Z. P. Jovanov, L. Bech, W. J. Durand, S. Dahl, J. K. Nørskov, I. Chorkendorff. The importance of surface morphology in controlling the selectivity of polycrystalline copper for CO2 electroreduction. Physical Chemistry Chemical Physics, (2012) 14, 76-81.
• Y. Hori, A. Murata and R. Takahashi. Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper electrode in aqueous solution. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 85 (1989) 2309-2326.
• Y. Hori, A. Murata, R. Takahashi and S. Suzuki. Enhanced formation of ethylene and alcohols at ambient temperature and pressure in electrochemical reduction of carbon dioxide at copper electrode. Journal of the Chemical Society, Chemical Communications (1988) 17-19.
• Y. Hori, R. Takahashi, Y. Yoshinami and A. Murata. Electrochemical reduction of CO at a copper electrode. Journal of Physical Chemistry B 101 (1997) 7075-7081.