EP4634435A1

EP4634435A1 - Layered double-hydroxide catalytic coatings containing: cu, mg e al, achievable electro¬ chemically, for uses such as electrochemical reduction of carbon dioxide

Info

Publication number: EP4634435A1
Application number: EP23855814.2A
Authority: EP
Inventors: Domenica Tonelli; Erika Scavetta; Federica MARIANI; Martina SERAFINI; Andrea FASOLINI; Francesco Basile
Original assignee: Universita di Bologna
Current assignee: Universita di Bologna
Priority date: 2022-12-16
Filing date: 2023-12-14
Publication date: 2025-10-22
Also published as: WO2024127296A1; IT202200025860A1

Abstract

The present invention pertains to a new nanostructured composite material with a layered hydrotalcite-type ternary structure of CuMgAI LDH (layered double hydroxides) and comprising particles of metal copper Cu° and particles containing cuprous ion Cu⁺ (such as cuprous oxide), as a redox couple Cu°/Cu⁺, preferably placed in intimate contact with the layered hydrotalcite-type structure.

Description

“Layered double-hydroxide catalytic coatings containing: Cu, Mg e Al, achievable electrochemically, for uses such as electrochemical reduction of carbon dioxide”

DESCRIPTION

Technical field of the invention

The present invention pertains to a new nanostructured composite material with a layered hydrotalcite-type ternary structure of CuMgAI LDH (layered double hydroxide) and comprising particles of metal copper Cu° and particles containing cuprous ion Cu⁺ (such as cuprous oxide), as a redox couple Cu°/Cu⁺, preferably placed in intimate contact with the layered hydrotalcite-type structure. In particular, the present invention pertains to a new nanostructured composite material containing layered double hydroxides (LDHs) with a layered hydrotalcite-type ternary structure comprising three metals: Cu, Mg and Al, such as Cu²⁺, Mg²⁺ and Al³⁺, which are interleaved with or overlapped on or interconnected to such later, i.e. placed into direct contact with such layers (bru- citic layers of said hydrotalcite-type structure): metal copper, Cu°, preferably in the form of micro- /nano-particles and cuprous ion Cu⁺, preferably in the form of micro7nano-particles containing cuprous ion Cu⁺, more preferably in the form of micro7nano-particles of CU2O, said composite material having catalytic/electro-catalytic properties.

In particular, the composite material forming the subject of the present invention continuously or discontinuously coats, or is laid, in the form of layer or particle/s or of layer and particle/s, on a support made of a chemically inert material compared with the composite nanostructured material according to the present invention and/or thermally and/or electrically insulating or inert, again compared with the composite nanostructured material according to the present invention and/or thermally and/or electrically conductive, preferably as a gas diffusion membrane made of carbon, more preferably a gas diffusion membrane made of carbon fiber, to obtain a working electrode for redox electrochemical reactions, wherein the composite material according to the present invention performs a catalytic or electrocatalytic action; particularly suited for the electrochemical CO2 reduction into C2 compounds.

The composite material of the present invention is obtained through an electrochemical process, more preferably a potentiodynamic electrodeposition process, such as cyclic voltammetry.

State of the art

Carbon dioxide is the second greenhouse gas as per quantity in the atmosphere, following only water vapour¹. In the last 200 years, the average levels of CO2 have increased by 280 to 420 ppm and they are still growing despite the mitigation policies applied by industrialized countries²’³. Now days, most industrial processes still require an intensive use of fossil fuels, therefore making the full independence from such energy sources unlikely in the short period. However, it is more and more essential to limit carbon dioxide and therefore to reduce the concentration thereof in the atmosphere⁴. Due to the low effectiveness of the strategies pursued so far to limit CO2 emissions, the idea of using it as a possible feedstock is becoming more and more likely⁵. A remarkable quantity of products can be obtained from carbon dioxide, sorted into long-term products such as cement, durable polymers or insulating materials for buildings, and short-term products, which include chemical products and fuels. Although long-term products can reduce carbon dioxide emissions for a very long time, the development of new material capable of increasing the conversion into the second class of compounds is of scientific interest. Among those, a large number of chemical synthesis are based on two-carbon atom products and these have the highest industrial values⁶. Therefore, more and more resource and energy are being invested to obtain these products though carbon dioxide direct hydrogenation at ambient temperature and pressure, using readily available and low cost raw materials, from the perspective of a circular economy where the decarbonization process occurs by a green carbon recycling³⁷. Carbon dioxide conversion can be achieved by several methods using biochemical⁸’⁹, radiochemical¹⁰, thermochemical¹¹ , photochemical¹²’¹³ or electrochemical¹⁴’¹⁵ reactions. Among these, the electro-chemical reduction is becoming more and more interesting, since it provides the opportunity to work in light working conditions and to easily modify the reaction selectivity^16-18. Furthermore, the required energy for activating the carbon dioxide molecule can be obtained by generating electrical current by renewable sources, an eco- friendly alternative which becomes part of the promising chemistry field which uses renewable energy source as an alternative to fossil fuels¹⁵’^19-21. However, to take advantage of the electrochemical approach and make it attractive from an industrial point of view²², the electro-catalyst design is really important. Today, sustainable materials are drawing interest and among them the optimal candidates are represented by the carbon-based electrodes¹⁷²³. In fact, these materials, due to their large availability in nature, their low cost and their electro-chemical properties, have been frequently used in the electro-catalysis field^24-26. Among these, the gas diffusive carbonaceous membranes (GDE) are an attractive alternative in respect to their metal electrodes, since they aid the interaction with a gas reactant, locally generating a gas-liquid-solid interface, reducing the resistances to mass transport and granting high current densities²⁷’²⁸. However, the carbonaceous materials show themselves to be a poor catalytic activity to the carbon oxide reduction since the neutral carbon atoms cannot activate a thermodinamically highly stable molecule (activation energy of bond 0=0: 750 kJ mol^-1). Furthermore, in order to obtain the radical CC ’-, essential to start the reduction reaction, a very negative potential is needed (-1 .3 V vs Ag/AgCI in water)²⁵.

For these reasons it is essential to change the characteristics of the carbonaceous material in order to improve the electro-chemical performance thereof, increase the catalytic activity and the selectivity. In order to do that, two different approaches are available: adding an heteroatom, e.g. nitrogen, boron or sulphur, which serves as “dopant” and provides the carbonaceous material with functional groups capable of converting the carbon dioxide into carbon monoxide and formic acid ^29-31 or coating it with metals or metal oxides through physical or chemical processes. Thereby a material capable of changing the selectivity and the throughput/productivity to reduction products into one or more carbon atoms ²³³² is made. Today, it is widely shown that copper is the only metal capable of catalyzing the CO2 reduction reaction into complex hydrocarbons and alcohols with high faradic efficiency (FE)³³. The feature that makes copper unique is its ability to bond the anion radical *CO₂’-. Starting from the reaction investigations carried out in 1985 by Hori et al.³⁴, who were the first to use a copper sheet, it was shown that the copper materials are capable of producing both products as CO and HCOOH, and methane and ethylene³⁵ in a significant manner. Recent studies have also shown the importance of the active redox pair morphology that takes part in the reaction. On one side, by varying the micro- and nano-metric structures of copper, the reaction into the desired products production can be influenced, increasing selectivity and faradic efficiency¹⁴’^36- ⁴⁰. On the other side, although the debate on the reaction mechanism is still open, several research groups have validated the idea that catalysts containing sites of adjacent Cu° and Cu⁺ can aid the process of coupling C-C, preferentially leading therefore to the formation of products with two or more carbon atoms ¹⁵’^41-44. While it has been reported that the production of CO or HCOOH has already achieved faraday efficiency near 100%⁶, the compounds > C2 are usually achieved at highly cathodic potentials, resulting in severe efficiency losses and low faradic efficiencies⁴⁵. However, the possibility to obtain acetic acid at lower cathodic potentials and with a high selectivity using copperbased nanostructured electro-catalysts (comprising species of Cu° and CU2O) held on carbonaceous materials¹⁵’³⁷’⁴⁶’⁴⁷ has been recently shown. Acetic acid production is of particular industrial interest due to its multiple applications⁴⁸. Today, the acetic acid requirement is met by fermentative or chemical processes. The Monsanto and Cativa methods represent the two more common processes for producing such a large-scale compound which, however, use rhodium or iridium based catalysts in critical working conditions⁴⁹. In such a context the Cu electrocatalysts supported on sustainable materials, such as carbonaceous, have the right potentialities in order to outclass the traditional industrial approaches, reducing the number of process steps and converting directly carbon dioxide into cleaner and milder conditions⁵⁰. As already said, although the use of gas diffusive 3D membranes helps the mass transport events and therefore helps the development of current high densities, the throughput/productivity of such systems suffers the poor solubility of CO2 in an aqueous environment (~34 mM)⁴⁰’⁵¹. For this reason, with the object of increasing the availability of the reactant on the electrode surface and therefore increasing the copper gas diffusive electrodes, Perry et al.⁵² have verified the positive effects of adding polymers with inherent microporosity (PIM), which caused an increase in ethylene selectivity and throughput/productivity, reducing bubbling on the electrode surface thanks to their porosity ⁵³. Similarly, the organo-metallic lattice (defined as MOFs) represent a further class of nanomaterials with a well-defined porosity and their absorption capacity of the carbon dioxide with resulting conversion into methane⁵⁴ or products >Ci ⁵⁵ was widely shownHowever, there is a third class of compounds which, by virtue of alkaline air and high affinity for the carbonates and the CO2, can have a great potential in this scope: the layered double hydroxides (LDHs). Also known as hydrotalcite-type compounds, the layered double hydroxides are inorganic solids with the molecular form [M(ll)i-xM(ll l)x(OH)2]^x+(A^n-x/n) ■ IT1H2O, derived from the natural form of hydrotalcite MgeAl2(OH)i6(CO3) ■ 4H2O. These are composed of a layered structure made of mixed hydroxides of divalent and trivalent cations having octahedral coordination, interleaved with anions useful for balancing the excess positive charge of the layers, given by the partial substitution of the divalent cations for the trivalent cations^56-58. The likelihood of being synthesized starting from different cations and of being capable of easily exchange the interleaved anion, makes the LDHs a valid alternative to be applied in catalysis^5659-63, photocatalysis^64-66, oxygen electrochemical evolution⁶⁷’⁶⁸, sensoristic electrochemistry^69-71 , biology⁷² and in the cleavage processes⁷³. As to their uses in catalysis, the layered double hydroxides are mainly used as catalyst precursors for the methane reforming reaction to the synthesis gas production⁷⁴’⁷⁵ and CO2 hydrogenation processes into methane or methanolo⁷⁶’⁷⁷. For this reason, the interest towards the compounds, to be employed as electrocatalysts for electrochemical conversion of carbon dioxide, is growing. However, since these are usually used in electrochemical oxidation showing a low electrical conductivity when compared to metals, just a few studies have been conducted about the use of layered double hydroxides as such, when used as a material for reduction processes. Neverthe- less, compounds such as H2, CO and HCOOH could be achieved. The panel of Li et al.⁷⁸ reported the synthesis and the use of Cu/MgAI LDH and Au/MgAI LDH, both obtained by a co-precipitation method. The respective active phases (Cu and Au) have been introduced through two approaches: copper was inserted in MgAI LDH by adding preliminarily EDTA, while gold was added by an ion exchange reaction. Once obtained, the inks were deposited, though physical methods, on a glassy carbon electrode with a geometric area of 1 .8 cm². Their catalytic activity to the CO2 electrochemical reduction was tested in two different supporting electrolytes (KHCO3 and an amine solution comprising ethanolamine and diethanolamine). By changing the potential applied from -0.3 V to - 0.5 V vs RHE (reversible hydrogen electrode) for the Cu/MgAI LDH and from -0.45 V to -0.7 V vs RHE for the Au/MgAI LDH in KHCO3, the Authors have reported the production of H2, CO, CH4, and HCOOH. On the contrary, by applying the same potential range in the amino solution, the only products detected were H2 and CO. Iwase et al.⁷⁹ disclosed the use of bimetallic layered double hydroxides (CuAI LDH), obtained also in this case by co-precipitation and deposited on a 1 .89 cm² gas diffusive carbonaceous membrane. The catalyst, which directly contacts the gas CO2 under galvanostatic conditions (50 mA), led to the production of CO, HCOOH and H2 as prevailing products and ethylene and ethanol as minor products. These studies have shown the potential of the ternary or bimetallic layered double hydroxides to the CO2 electroreduction reaction. However, the reported preparation methods provide for long and complex processes which require high temperature and the production of inks having a good adherence to the support. Further, they have shown a higher throughput/productivity in compounds Ci , associated with a poor selectivity.

Therefore there was an extreme need to develop new materials and new methods to improve the finalized procedures to CO2 conversion into chemical products and fuels, such as C2 products (two- carbon atom products are the base of a large number of chemical synthesis and they have the highest industrial values) for example acetic acid, with higher selectivity and production yield.

Summary of the invention

When carrying out the research on the technical field, the Applicant surprisingly and unexpectedly, realized the following as the object of the present invention: a new nanostructured composite material with a layered hydrotalcite-type ternary structure of CuMgAI LDH and comprising particles of metal copper Cu° and particles containing cuprous ion Cu+ (such as cuprous oxide), as a redox couple Cu°/Cu⁺, preferably placed in intimate contact with the layered hydrotalcite-type structure.

In particular, nanostructured composite material according to the present invention, wherein metal copper, Cu°, preferably as micro/nano particles and cuprous ion Cu⁺, preferably as micro/nano particles containing cuprous ion Cu⁺, more preferably as micro/nanoparticles of CU2O, are interleaved with or overlapped on or interconnected to such layers, in other words placed into direct contact with such layers.

More particularly, and as a further object of the present invention, the new nanostructured composite material according to the present invention comprising layered double hydroxides (LDHs) with a layered hydrotalcite-type ternary structure containing three metals: Cu, Mg and Al, such as Cu²⁺, Mg²⁺ and Al³⁺, which are interleaved with or overlapped on or interconnected to such later, i.e. placed into direct contact with such layers (brucitic layers of said hydrotalcite-type structure): metal copper, Cu°, preferably in the form of micro7nano-particles and cuprous ion Cu⁺, preferably as mi- cro-/nano-particles containing cuprous ion Cu⁺, more preferably in the form of micro-/nano- particles of CU2O, said composite material having catalytic/electro-catalytic properties.

Even more particularly, and as further object of the present invention, the nanostructured composite material forming the subject of the present invention continuously or discontinuously coats, or is laid, in the form of layer or particle/s or of layer and particle/s, on a support made of a chemically inert material compared with the nanostructured composite material according to the present invention and/or thermally and/or electrically insulating or inert, again compared with the nanostructured composite material according to the present invention and/or thermally and/or electrically conductive, preferably as a gas diffusion membrane made of carbon, most preferably a gas diffusion membrane made of carbon fiber, to obtain a working electrode for redox electrochemical reactions, wherein the composite material according to the present invention performs a catalytic or electrocatalytic activity.

The nanostructured composite material object of the present invention is preferably achieved by an electrochemical process, more preferably a process comprising potentiodynamic electrodeposition. The nanostructured composite material according to the present invention having catalytic and/or electrocatalytic properties, comprises therefore copper in three different forms:

(i) metal copper: Cu°, preferably in the form of micro-/nano-particle/s,

(ii) cuprous ion: Cu⁺, preferably in the form of micro-/nano-particles containing cuprous ion Cu⁺, most preferably as micro7nano-particles of CU2O, wherein said metal copper Cu° and cuprous ion Cu⁺ are interleaved in or overlapped on or interconnected to, i.e. direct contact with, a ternary layered hydrotalcite-type structure of CuMgAI LDH, and

(iii) cupric ion: Cu²⁺, within the layered hydrotalcite-type structure of the double-layer hydroxide (LDH) containing three metals Mg, Cu and Al, such as: Mg²⁺, Al³⁺ and Cu²⁺.

The metal Cu and CU2O, preferably in the form of micro-/nano-particles directly contact the layered structure of the double layer hydroxide (LDH) containing three metals Mg, Cu and Al, such as: Mg²⁺, Al³⁺ and Cu²⁺. Said layered structure, regardless of the existence of micro-/nano-particles, in turn is made of a more complex structure wherein mixed hydroxide layers of divalent and trivalent cations (Cu²⁺, Mg²⁺ and Al³⁺) are present.

In particular, said nanostructured material is a composite material comprising Cu, Mg and Al-based layered double hydroxides (LDH) with layered hydrotalcite-type structure, such as mixed hydroxide layers of divalent and trivalent cations (Cu²⁺, Mg²⁺ e Al³⁺), said mixed hydroxide layers being in direct contact with species of Cu° and Cu⁺, preferably in the form of micro-/nano-particles of metal Cu and CU2O, respectively.

More particularly, said nanostructured composite material according to the present invention continuously or discontinuously coats in the form of layer or particle/s or of layer and particle/s, a support made of a chemically inert material compared with the nanostructured composite material according to the present invention and/or thermally and/or electrically insulating or inert, again compared with the nanostructured composite material according to the present invention and/or thermally and/or electrically conductive, to give a working electrode, to be preferably used as a cata- lyst/electro-catalyst for the carbon dioxide reduction; more preferably when said nanostructured composite material according to the present invention coats, continuously or discontinuously, in the form of layer or particle/s or of layer and particle/s, a gas diffusion carbon membrane support (CP) or gas diffusive membrane(GDL), even more preferably said membrane is made of carbon fiber.

A further object of the present invention is the process for the carbon dioxide reduction in C2 products, such as acetic acid, electrochemically carried out using, as a working electrode, nanostructured composite material according to the present invention, said material having catalyt- ic/electrocatalytic activity; said nanostructured composite material according to the present invention preferably present as a, partial or full, coating, on a support made of a chemically inert material, in respect of the nanostructured composite material according to the present invention, and/or thermally and or electrically insulating or inert, still in respect of the nanostructured composite material according to the present invention and/or thermally and or electrically conductive, more preferably on a support of carbon (CP), or gas diffusive (GDL) gas diffusion membrane/s, even more preferably said membrane made of carbon fibers.

In fact, by combining the alkaline nature of layered double hydroxides (LDH) with a layered hydrotalcite-type structure, containing Cu, Mg and Al, in the form of Mg²⁺, Al³⁺ and Cu²⁺, and their affinity regarding CO2 with the species Cu°/Cu⁺, preferably in the form of metal Cu and CU2O, a composite material was obtained capable of improving the availability of a carbon source, i.e. CO2, in the case of its electro reduction process, on the outer surface of the nanostructured composite material according to the present invention, which serves as catalyst/electrocatalyst on the electrode finalized to such a process, as when the working electrode support, as defined herein, is preferably a support for the carbon (CP) or gas diffusive (GDL) gas diffusion membrane/s, even more preferably said membrane made of carbon fibers, and to lead the CO2 reduction reaction towards a C2 product, such as acetic acid, due to the existence of an active redox pair Cu°/Cu⁺. Electrocatalytic activity of the nanostructured composite material according to the present invention which however exists regardless of the origin/nature of the suppor.

In the nanostructured composite material according to the present invention, copper can be found both as metal copper Cu° and as cuprous ion Cu⁺, preferably in the form of nano-/micro- particles of Cu° and CU2O, respectively, interleaved in/overlapped on or interconnected to the LDH layered hydrotalcite-type structure, both as ion Cu²⁺ and, with Mg²⁺ e Al³⁺, gives rise to the brucite layers of said layered hydrotalcite-type structure. Therefore, a ternary LDH structure composed of Cu, Mg and Al, present in the form of divalent and trivalent cations is present, which has the nitrate (NO3 ) as interleaved anion.

The nanostructured composite material according to the present invention is obtained according to a process comprising a single electro-deposition step, performed at room temperature and ambient pressure, such as the standard conditions STP (25° C and 1 atm), wherein on a substrate serving as a support, cations Cu²⁺, Mg²⁺ and Al³⁺ are deposited, from a solution where the relative salts are solubilized, to form exactly the nanostructured composite material comprising layered double hydroxides (LDH) with a layered hydrotalcite-type ternary structure containing three metals: Cu, Mg and Al, such as Cu²⁺, Mg²⁺ and Al³⁺, which are interleaved with or overlapped on or interconnected to such later, i.e. placed into direct contact with such layers (brucitic layers of said hydrotalcite-type structure): metal copper, Cu°, preferably in the form of micro7nano-particles and cuprous ion Cu⁺, preferably as micro7nano-particles containing cuprous ion Cu⁺, most preferably in the form of mi- cro-/nano-particles of CU2O, said composite material having catalytic/electro-catalytic properties. In particular, said preparation process for nanostructured composite material according to the present invention preferably comprises potentiodynamic electrodeposition, such as cyclic voltammetry (CV), wherein a triangular waveform potential is applied to a stationary electrode dipped in a, preferably aqueous, non-stirred deposition solution of soluble salts, preferably nitrates, of cations Cu²⁺, Mg²⁺ and Al³⁺, making use of a conventional cell having three electrodes: a working electrode on which the electrodeposition of the nanostructured composite material according to the present invention is caused, a counter-electrode, for example made of platinum, and a reference electrode, for example with saturated calomel (SCE). The deposition solution has a fixed total concentration of all the solubilized salts, such as, for example, 0.03 M. A procedure of potentiodynamic electrodeposition, making use of the cyclic voltammetry, wherein the potential is changed from 0.0 to - 1 .4 V vs SCE, with a scanning rate that ranges from 5 and 50 mVs^-1, preferably from 10 to 40 mVs^-1, more preferably from 15 and 30 mVs^-1, most preferred 30 mVs^-1.

Due to the co-existence of a ternary hydrotalcite-type structure of CuMgAI LDH containing ions Cu²⁺, Mg²⁺ and Al³⁺, and of copper in other forms/oxidation states, i.e. metal copper Cu° and cuprous ion Cu⁺, i.e. the redox pair Cu°/Cu⁺, a nanostructured composite material with high electrocatalytic activity and improved selectivity about the formation of C2 compounds (i.e. acetic acid) and high throughput/productivity is obtained.

In particular, the composite material according to the present invention shows a direct contact between the hydrotalcite-type structure CuMgAI LDH in the form of layered veils or cauliflowershaped particles and the species Cu°/Cu2O, in the form of “coral-like particles”.

Furthermore all the nanostructured composite materials according to the present invention, as catalysts in the CO2 electrochemical reduction reaction to give C2 compounds, have shown selectivity regarding the acetic acid production, whereas such catalytic activity in its highest yield form at the same time and amount of nanostructured composite material used, was obtained using the nanostructured composite material according to the present invention wherein in the hydrotalcitetype structure the theoretical stoichiometric ratio between divalent and trivalent cations M(ll)/M(lll) is 3:1 and the theoretical stoichiometric ratio CuMgAI is of CuMgAI is 2:1 :1 , identified as working electrode CuMgAI 2:1 :1 LDH/CP, i.e. when the support is a carbon (CP) or gas diffusive (GDL) gas diffusion membrane preferably made of carbon fiber.

This results in said preferred stoichiometric ratio representing a plus, since a technical effect which is always present in respect to what is disclosed in the state of the art (Li et al.), is the selectivity in the production of acetic acid, regardless of the stoichiometric ratio CuMgAI in the hydrotalcite-type structure, due to the presence of the redox pair Cu°/Cu⁺.

This is also regardless of the amount of nanostructured composite material according to the present invention used as catalyst/electrocatalyst in the process for CO2 electrochemical reduction in C2 compounds.

For nanostructured composite material, according to the present invention, we mean a material from the differentiated chemical composition, i.e. comprising more atoms having a different chemical nature and a different oxidation status and with a particular aggregation status to each other, to give nanometric structures of said material comprising the atoms according to the present invention. Said structures in the form of particles, clusters or crystals having a nanometric size (< 100 nm), in turn aggregated to each other, give rise to an end solid which does not need to have also a nanometric size.

A further object of the present invention is a process of preparing a nanostructured composite material according to any of the embodiments of the present invention, as disclosed above, said process comprising a potentiodynamic electrodeposition, by cyclic voltammetry, in which a triangular wave potential is applied to a stationary electrode dipped in an unstirred electrodeposition solution of soluble salts of cations Cu²⁺, Mg²⁺, and Al³⁺.

In a further embodiment of the present invention, in the process of preparing the nanostructured composite material according to any of the embodiments of the present invention, preferably the potential is caused to change from 0.0 to -1 .4 V vs SCE, with a scan rate within the range from 5 and 50 mVs^-1 , preferably from 10 to 40 mVs^-1 , more preferably from 150 to 30 mVs^-1 , most preferred 30 mVs’¹.

In a further preferred embodiment of the present invention of the process of preparing of the nanostructured composite material according to any of the embodiments of the present invention, in the electrodeposition solution, the molar ratio between the sum of the divalent cations (Cu²⁺ e Mg²⁺) M(l I) and the trivalent cation (Al³⁺) M(lll), i.e. the ratio M(l l)/M(l II), is 3:1 and/or the molar ratio among the different cations (Cu²⁺: Mg²⁺: Al³⁺) is 2:1 :1 .

The nanostructured composite materials according to the present invention are all the nanostructured composite materials having the technical characteristics as disclosed herein and more generally claimed both in product claims 1 and 2, and material obtained according to the procedures as disclosed herein and generally claimed in process claims 3, 4 and 5; i.e. those nanostructured composite materials according to the present invention having 100% selectivity in the CO2 reduction reaction into C2 compounds: the main advantage of the nanostructured composite material according to the present invention. The nanostructured composite materials according to the present invention obtained according to the procedures as disclosed herein and claimed in process claims 3, 4 and 5, are even more preferred, i.e. those nanostructured composite materials which lead also to an increase in productivity as mmol g_caf¹ IT¹ , that is millimoles of C2, acetic acid, per gram of catalyst, i.e. gram of nanostructured composite material, per working hour, in the catalyt- ic/electrocatalytic CO2 reduction.

As further object of the present invention is a catalytic system comprising the nanostructured composite material according to any of the embodiments of the present invention, as above described, as a continuous or discontinuous coating in the form of layer or particle/s or of layer and particle/s, on a support made of a material which is: chemically inert, compared to the nanostructured composite material according to the present invention, and/or thermally and/or electrically insulating or inert, still compared to the nanostructured composite material according to the present invention.

According to the present invention, support made of “a chemically inert material, compared to the nanostructured composite material according to the present invention, and/or thermally and/or electrically isolating or inert, still compared to the nanostructured composite material according to the present invention”, means a support that, during the deposition of the composite material, does not chemically and/or thermally and/or electrically “react/interact” directly with the nanostructured composite material formed according to any of the embodiments of the present invention, as described above. Specifically, the support serves as a “current vector”, using its electrical conductivity to form the (solid) catalytic film starting from the ion liquid solution, according to the present invention. A further object of the present invention is a catalytic system comprising the nanostructured composite material according to any of the embodiments of the present invention, as above described, as a continuous or discontinuous coating in the form of layer or particle/s or of layer and particle/s, on a support made of a thermally and/or electrically conductive material.

A further object of the present invention is a working electrode comprising the nanostructured composite material according to any of the embodiments of the present invention, as above described, as a continuous or discontinuous coating in the form of layer or particle/s or of layer and particle/s, on a support made of an electrically conductive material.

A further object of the present invention is a working electrode comprising the nanostructured composite material according to any of the embodiments of the present invention, as above described, as a continuous or discontinuous coating in the form of layer or particle/s or of layer and particle/s, on a support made of an electrically conductive material, wherein the support made of electrically conductive material is a gas diffusion membrane made of carbon, preferably of carbon fiber.

A further object of the present invention is a process for the electrochemical CO2 reduction into C2 compounds, preferably acetic acid, where the working electrode on which the CO2 is reduced is a working electrode comprising the nanostructured composite material according to any of the embodiments of the present invention, as above described, as a continuous or discontinuous coating in the form of layer or particle/s or of layer and particle/s, on a support made of an electrically conductive material, preferably wherein the support made of electrically conductive material is a gas diffusion membrane made of carbon, more preferably of carbon fiber.

Brief description of drawings

Fig. 1 : (a) Diagram of the processes that occur to the working electrode during the deposition; (b) current density registered during the potentiodynamic deposition of the LDH films above the gas diffusive membrane CP; (c) X-Ray diffraction analysis and (d) SEM images of the electrodeposited film of nanostructured composite material according to the present invention, i.e. of the electrocata- lyst/working electrode CuMgAI 2:1 :1 LDH/CP.

Fig. 2: shows the deposition curves comparing the first (a) and the second (b) voltammetric segments registered during the electrodeposition of MgAI LDH/CP and the CuMgAI LDH/CP nanostructured composite material according to the present invention.

Fig. 3: shows the characterization CV of the electro-catalysts/working electrode CuMgAI 2:1 :1 LDH/CP in NaOH 1 M, scan rate: 50 mV s^-1, under N2. The peaks have been identified in accordance with a work reported in literature⁸⁰.

Fig. 4: the outcome of the further analysis on the optimized electro-catalyst/working electrode (CuMgAI 2:1 :1 LDH/CP) by means of EDS and SEM-FEG assessments such as (a), (b), (c) SEM- FEG images of the layered deposit on the carbon fibers; of the deposit layered over the carbon fibers; SEM-FEG images of the cauliflower- (d) and coral-shaped particles (e, f).

Fig. 5: the EDS analysis of CuMgAI 2:1 :1 LDH/CP are reported: (a) layered deposit and (b) coralshaped particle.

Fig. 6: the Raman analysis on the nanostructured composite material film, according to the present invention in the case of electro-catalyst/working electrode of CuMgAI 2:1 :1 LDH/CP are reported: (a) along the carbon fibers and (b) on a coral-shaped particle.

Fig. 7: EDS mapping for distributing cations in the electro-deposited film of the nanostructured composite material, according to the present invention in the case of the electro-catalyst/working electrode of CuMgAI 2:1 :1 LDH/CP.

Fig. 8: SEM images of CuMgAI LDH, nanostructured composite material according to the present invention, synthesized from solutions having a different mole ratio of the divalent (Cu²⁺, Mg²⁺) and trivalent (Al³⁺) cations.

Fig. 9: SEM-FEG images of the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP (a) and of the electrocatalyst/working electrode CuMgAI 3:3:1 LDH/CP (b).

Fig. 10: (a) X-Ray diffraction analysis of the CuMgAI LDH/CP nanostructured composite material according to the present invention, with a different cation ratio; (b) Estimate of the M(ll)/M(l II) ratio provided by the EDS analysis; (c) distribution of the reduction products to -0.4 V vs RHE (reaction time: 1 h) for the two compositions.

Fig. 11 : XRD spectrum of the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP (solid line), of the electrocatalyst/working electrode CuMgAI 3:3:1 LDH/CP (broken line-dots), and carbonaceous support - CP (broken line-dash)

Fig. 12: (a) Density of the registered current during the electrodeposition process of the cata- lyst/nanostructured composite material according to the present invention, in accordance with the number of cycles in CV; (b) distribution of the products obtained at -0.4V vs RHE (reaction time: 1 h) for the different amounts of catalyst/nanostructured composite material deposited according to the present invention.

Fig. 13: (a) X-Ray diffraction analysis of CuMgAI LDH having different molar ratio between each cation; (b) Estimate of the ratio between cations obtained by EDS analysis.

Fig. 14: SEM images of CuMgAI LDH with different molar ratio between cations.

Fig. 15: Histograms of: (a) Production of CH3COOH and (b) evolution of hydrogen to -0.4V vs RHE (reaction time: 1 h).

Fig. 16: Histogram of distribution of (a) liquid and (b) gas products obtained during the carbon dioxide electrochemical reduction at different potentials for 1 h reactions, using the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP.

Fig. 17: XRD spectrum of the film of nanostructured composite material according to the present invention of the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP after 1 h reaction at -0.4 V vs RHE.

Fig. 18: Current density registered for a 5 hour reaction using the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP at -0.4 vs RHE.

Fig. 19: Schematic depiction of H-cell wherein the CO2 electrochemical reduction in liquid phase is carried out.

Detailed description of the invention and embodiments thereof

Therefore, it is an object of the present invention: a new nanostructured composite material with a layered hydrotalcite-type ternary structure of CuMgAI LDH and comprising particles of metal copper Cu° and particles containing cuprous ion Cu⁺ (such as cuprous oxide), as a redox couple Cu°/Cu⁺, preferably placed in intimate contact with the layered hydrotalcite-type structure.

In particular, nanostructured composite material according to the present invention, wherein metal copper, Cu°, preferably as micro/nano particles and cuprous ion Cu⁺, preferably as micro/nano par- tides containing cuprous ion Cu⁺, even more preferably as micro/nanoparticles of CU2O, are interleaved with or overlapped on or interconnected to such layers, in other words placed into direct contact with such layers.

More particularly, and as a further object of the present invention, the new nanostructured composite material according to the present invention comprising layered double hydroxides (LDHs) with a layered hydrotalcite-type ternary structure containing three metals: Cu, Mg and Al, such as Cu²⁺, Mg²⁺ and Al³⁺, which are interleaved with or overlapped on or interconnected to such later, i.e. placed into direct contact with such layers (brucitic layers of said hydrotalcite-type structure): metal copper, Cu°, preferably in the form of micro-/nano-particles and cuprous ion Cu⁺, preferably as mi- cro-/nano-particles containing cuprous ion Cu⁺, even more preferably in the form of micro-/nano- particles of CU2O, said composite material having catalytic/electro-catalytic properties.

Even more particularly, and as further object of the present invention, the nanostructured composite material according to the present invention continuously or discontinuously coats, or is laid, in the form of layer or particle/s or of layer and particle/s, on a support made of a chemically inert material, compared with the nanostructured composite material according to the present invention and/or thermally and/or electrically insulating or inert, again compared with the nanostructured composite material according to the present invention and/or thermally and/or electrically conductive, preferably as a gas diffusion membrane made of carbon, even more preferably a gas diffusion membrane made of carbon fiber, to obtain a working electrode for redox electrochemical reactions, wherein the composite material according to the present invention performs an electrocatalytic activity.

The nanostructured composite material object of the present invention is preferably achieved by an electrochemical process, most preferably a process comprising potentiodynamic electrodeposition, by the cyclic voltammetry.

The nanostructured composite material according to the present invention having catalytic and/or electrocatalytic properties, comprises therefore copper in the different forms:

(i) metal copper: Cu°, preferably in the form of micro-/nano-particle/s,

(iii) cupric ion: Cu²⁺, within the layered hydrotalcite-type structure of the double-layer hydroxide (LDH) containing three metals Mg, Cu and Al, such as: Mg²⁺, Al³⁺ and Cu²⁺ indeed.

The metal Cu and CU2O, preferably in the form of micro-/nano-particles, directly contact the layered structure of the double layer hydroxide (LDH) containing three metals Mg, Cu and Al, such as: Mg²⁺, Al³⁺ and Cu²⁺. Said layered structure, regardless of the existence of micro-/nano-particles, in turn is made of a more complex structure wherein mixed hydroxide layers of divalent and trivalent cations (Cu²⁺, Mg²⁺ and Al³⁺) can be found.

In particular, said nanostructured material is a composite material comprising Cu, Mg and Al-based layered double hydroxides (LDH) with layered hydrotalcite-type structure, such as mixed hydroxide layers of divalent and trivalent cations (Cu²⁺, Mg²⁺ e Al³⁺), said mixed hydroxide layers being in di- rect contact with species of Cu° and Cu⁺, preferably in the form of micro7nano-particles of metal Cu and C112O, respectively.

More particularly, said nanostructured composite material according to the present invention continuously or discontinuously coats in the form of layer or particle/s or of layer and particle/s, a support made of a chemically inert material compared with the nanostructured composite material according to the present invention and/or thermally and/or electrically insulating or inert, again compared with the nanostructured composite material according to the present invention and/or thermally and/or electrically conductive, to give a working electrode, to be preferably used as a cata- lyst/electro-catalyst for the carbon dioxide reduction; more preferably when said nanostructured composite material according to the present invention coats, continuously or discontinuously, in the form of layer or particle/s or of layer and particle/s, a support of carbon (CP), or gas diffusive (GDL) gas diffusion membrane/s, even more preferably said membrane made of carbon fiber.

A further object of the present invention is the process for the carbon dioxide reduction in C2 products, such as acetic acid, electrochemically carried out using, as a working electrode, nanostructured composite material according to the present invention, said material having catalyt- ic/electrocatalytic activity; said nanostructured composite material according to the present invention preferably present as a, partial or full, coating, on a support made of in chemically inert material, in respect to the nanostructured composite material according to the present invention, and/or thermally and or electrically insulating or inert, still in respect to the nanostructured composite material according to the present invention and/or thermally and or electrically conductive, more preferably on a support of carbon (CP), or gas diffusive (GDL) gas diffusion membrane/s, even more preferably said membrane made of carbon fibers.

In fact, by combining the alkaline nature of layered double hydroxides (LDH) with a layered hydrotalcite-type structure, containing Cu, Mg and Al, in the form of Mg²⁺, Al³⁺ and Cu²⁺, and their affinity regarding CO2 with the species Cu°/Cu⁺, preferably in the form of metal Cu and CU2O, a composite material was achieved capable of improving the availability of a carbon source, i.e. CO2, in the case of its electroreduction process, on the outer surface of the nanostructured composite material according to the present invention, which serves as catalyst/electrocatalyst on the electrode finalized to such a process, also when the support is present, as defined herein, preferably a support for the carbon (CP) or gas diffusive (GDL) gas diffusion membrane/s, even more preferably said membrane made of carbon fibers, and to lead the CO2 reduction reaction towards a C2 product, such as acetic acid, due to the existence of an active redox pair Cu°/Cu⁺. Electrocatalytic activity of the nanostructured composite material according to the present invention that however exists regardless of the nature/origin of the support.

In the nanostructured composite material according to the present invention, copper can be found both as metal copper Cu° and as cuprous ion Cu⁺, preferably in the form of nano-/micro- particles of Cu° and CU2O, respectively, interleaved in/overlapped on or interconnected to the LDH layered hydrotalcite-type structure, both as ion Cu²⁺ and, with Mg²⁺ e Al³⁺, gives rise to the brucite layers of said layered hydrotalcite-type structure. Therefore, a ternary LDH structure composed of Cu, Mg and Al, present in the form of binary and ternary cations is present, which has the nitrate (NO3 ) as interleaved anion.

The nanostructured composite material according to the present invention is obtained according to a process comprising a single electro-deposition step, performed at room temperature and ambient pressure, such as the standard conditions STP (25° C and 1 atm), wherein on a substrate serving as a support, cations Cu²⁺, Mg²⁺ and Al³⁺, are deposited, from a solution were the relative salts are solubilized, to form exactly the nanostructured composite material comprising layered double hydroxides (LDH) with a layered hydrotalcite-type ternary structure containing three metals: Cu, Mg and Al, such as Cu²⁺, Mg²⁺ and Al³⁺, which are interleaved with or overlapped on or interconnected to such later, i.e. placed into direct contact with such layers (brucitic layers of said hydrotalcite-type structure): metal copper, Cu°, preferably in the form of micro7nano-particles and cuprous ion Cu⁺, preferably as micro7nano-particles containing cuprous ion Cu⁺, most preferably in the form of mi- cro-/nano-particles of CU2O, said composite material having catalytic/electro-catalytic properties.

In particular, said preparation process for nanostructured composite material according to the present invention preferably comprises potentiodynamic electrodeposition by cyclic voltammetry wherein preferably a triangular waveform potential is applied to a stationary electrode dipped in a, preferably aqueous (pH of the electrodeposition solution being dependent on the chemical nature of the soluble salts of the three cations used, in particular lower than 7), non-stirred deposition solution, of soluble salts, preferably nitrates, of cations Cu²⁺, Mg²⁺ and Al³⁺, making use of a conventional cell having three electrodes: a working electrode on which the electrodeposition of the nanostructured composite material according to the present invention is caused/performed, a counter-electrode, for example made of platinum, and a reference electrode, for example with saturated calomel (SCE). The deposition solution having a fixed total concentration of all the solubilized salts, such as, for example, 0.03 M. A procedure of potentiodynamic electrodeposition, making use of the cyclic voltammetry, wherein the potential is changed from 0.0 to - 1 .4 V vs SCE, with a scanning rate that ranges from 5 and 50 mVs ¹ , preferably from 10 to 40 mVs ¹ , more preferably from 15 and 30 rnVs^-1 , most preferred 30 mVs^-1.

Due to the co-existence of a ternary hydrotalcite-type structure of CuMgAI LDH containing ions Cu²⁺, Mg²⁺ and Al³⁺, and of copper in other forms/oxidation states, i.e. metal copper Cu° and cuprous ion Cu⁺, i.e. the redox pair Cu°/Cu⁺, a nanostructured composite material with high catalytic activity and improved selectivity about the formation of C2 compounds (i.e. acetic acid) and high throughput/productivity is obtained.

In particular, the composite material according to the present invention shows a direct contact between the hydrotalcite-type structure CuMgAI LDH in the form of layered veils or cauliflowershaped particles and the species Cu°/Cu2O, in the form of coral-like particles.

Furthermore all the nanostructured composite materials according to the present invention, as catalysts in the CO2 electrochemical reduction reaction to give C2 compounds, have shown selectivity regarding the acetic acid production, whereas such catalytic activity in its highest yield form, at the same time and amount of nanostructured composite material used, was obtained using the nanostructured composite material according to the present invention wherein in the hydrotalcitetype structure the theoretical stoichiometric ratio between divalent and trivalent cations M(ll)/M(lll) is 3:1 and the theoretical stoichiometric ratio CuMgAI is of CuMgAI is 2:1 :1 , identified as working electrode CuMgAI 2:1 :1 LDH/CP, i.e. when the support is a carbon (CP) or gas diffusive (GDL) gas diffusion membrane, preferably made of carbon fiber.

This results in said preferred stoichiometric ratio representing a plus, since a technical effect which is always present in respect to what was disclosed in the state of the art (Li et al.), is the selectivity in the production of acetic acid, regardless of the stoichiometric ratio CuMgAI in the hydrotalcitetype structure, due to the present of the redox pair Cu°/Cu⁺.

This is also regardless of the amount of nanostructured composite material according to the present invention used as catalyst/electrocatalyst in the process for CO2 electrochemical reduction into C2 compounds.

For nanostructured composite material according to the present invention a material from the differentiated chemical composition, i.e. comprising more atoms having a different chemical nature and a different oxidation status and with a particular aggregation status to each other, to give nanometric structures of said material comprising the atoms according to the present invention. Said structures in the form of particles, clusters or crystals having a nanometric size (< 100 nm), in turn aggregated to each other, give rise to an end solid which does not need to have also a nanometric size.

In a further embodiment of the present invention, in the process for preparing the nanostructured composite material according to any of embodiments of the present invention, preferably the potential is caused to change from 0.0 to -1 .4 V vs SCE, with a scan rate comprised between 5 and 50 mVs^-1 , preferably between 10 and 40 mVs^-1 , more preferably between 15 and 30 mVs^-1 , most preferred 30 mVs ¹.

In a further preferred embodiment of the present invention of the process of preparing of the nanostructured composite material according to any of the embodiments of the present invention, in the electrodeposition solution, the molar ratio between the sum of the divalent cations (Cu²⁺ e Mg²⁺) M(ll) and the trivalent cation (Al³⁺) M(lll), i.e. the ratio M(l l)/M(ll I) is 3:1 and/or the molar ratio among the different cations (Cu²⁺: Mg²⁺: Al³⁺) is 2:1 :1 .

The nanostructured composite materials according to the present invention are all the nanostructured composite materials having the technical characteristics as disclosed herein and more generally claimed both in product claims 1 and 2, and material obtained according to the procedures as disclosed herein and generally claimed in process claims 3, 4 and 5; i.e. those nanostructured composite materials according to the present invention having 100% selectivity in the CO2 reduction reaction into C2 compounds: main advantage of the nanostructured composite material according to the present invention. The nanostructured composite materials according to the present invention obtained according to the procedures as disclosed herein and claimed in process claims 3, 4 and 5, are even more preferred, i.e. those nanostructured composite materials which lead also to an increase in productivity as mmol g_caf¹ IT¹ , that is millimoles of C2, acetic acid, per gram of catalyst, i.e. gram of nanostructured composite material, per working hour, in the catalyt- ic/electrocatalytic CO2 reduction..

A further object of the present invention is a catalytic system comprising the nanostructured composite material according to any of the embodiments of the present invention, as above described, as a continuous or discontinuous coating in the form of layer or particle/s or of layer and particle/s, on a support made of a material which is: chemically inert, compared to the nanostructured composite material according to the present invention, and/or thermally and/or electrically insulating or inert, still compared to the nanostructured composite material according to the present invention.

According to the present invention, support made of “a chemically inert material, compared to the nanostructured composite material according to the present invention, and/or thermally and/or electrically isolating or inert, still compared to the nanostructured composite material according to the present invention”, means a support that, during the deposition of the composite material, does not chemically and/or thermally and/or electrically “react/interact” directly with the nanostructured composite material formed according to any of the embodiments of the present invention, as described above. Specifically, the support serves as a “current vector”, using its electrical conductivity to form the (solid) catalytic film starting from the ion liquid solution, according to the present invention.

A further object of the present invention is a catalytic system comprising the nanostructured composite material according to any of the embodiments of the present invention, as above described, as a continuous or discontinuous coating in the form of layer or particle/s or of layer and particle/s, on a support made of an thermally and/or electrically conductive material.

A further object of the present invention is a process for the electrochemical CO2 reduction into C2 compounds, preferably acetic acid, wherein the working electrode on which the CO2 is reduced is a working electrode comprising the nanostructured composite material according to any of the embodiments of the present invention, as above described, as a continuous or discontinuous coating in the form of layer or particle/s or of layer and particle/s, on a support made of an electrically conductive material, preferably wherein the support made of electrically conductive material is a gas diffusion membrane made of carbon, more preferably of carbon fiber.

In accordance with one of the particularly preferred embodiments of the nanostructured composite material, according to the present invention, CuMgAI LDH nanostructured materials were synthesized for carbon dioxide electrochemical reduction, directly formed on the surface of a gas diffusion membrane composed of carbon fibers (CP), a carbonaceous support, 4 cm² in size, by a simple potentiodynamic, electrochemical process, as indicated above. The morphology of each material was investigated by means of SEM-EDS techniques, X-Ray diffraction and Raman spectroscopy. From the morphological studies it was highlighted that the selected synthesis process results in obtaining a composite material where CuMgAI LDH are in direct contact with the species of Cu°/Cu2O. During the optimization of the process or preparing the composite material according to the present invention, several parameters were assessed based on the catalytic/electrocatalytic ac- tivity shown in the process for CO2 electrocatalytic reduction to give C2 compounds. Such parameters are: (i) the different molar ratios between the sum of divalent Cu²⁺ and Mg²⁺ in respect of the trivalent Al³⁺ cations available in the reducing solution, (ii) the different amount of nanostructured composite material according to the present invention/catalyst deposited on the carbonaceous support CP and (iii) the different molar ratio between individual cations: Cu²⁺, Mg²⁺ and Al³⁺ available in the electrodeposition solution.

The electrocatalytic activity of the nanostructured composite material according to the present invention, as electrodeposited on a carbonaceous support, as disclosed above to give the relative electrocatalyst/working electrode, in the reaction of electrochemical CO2 reduction into C2 compounds, was tested by a potentiostatic technique at -0,4 V vs RHE in 0.3 M KHCO3 solution. The nanostructured composite materials and the corresponding electrocatalysts/working electrodes obtained, as according to the present invention, are designated by the molar ratio of the cations present in the electrodeposition solution wherein the nanostructured composite materials and the relative electrocatalysts/working electrodes were obtained. The nanostructured composite material according to the present invention which, as the catalyst/electrocatalyst in the electrochemical CO2 reduction reaction into C2 compounds, has shown the highest throughput/productivity of acetic acid equal to 2.0 mmol g_caf¹ IT¹ identified as the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP, i.e. the nanostructured composite material according to the present invention, wherein the theoretical stoichiometric ratio between the sum of the divalent cations (Cu²⁺ e Mg²⁺) M(l I) and the trivalent cation (Al³⁺) M(lll), i.e. the ratio M(l l)/M(l II) is 3:1 and the ratio among the different cations (Cu²⁺: Mg²⁺: Al³⁺) is 2:1 :1 . In fact it is the nanostructured composite material according to the present invention obtained by potentiodynamic electrodeposition, by cyclic voltammetry, wherein, in the electrodeposition solution, the molar ratio between the sum of divalent cations (Cu²⁺ and Mg²⁺) M(ll) and the trivalent cation (Al³⁺) M(lll), defined as the ratio M(l I)/ M(lll), is 3:1 and the molar ratio among different cations (Cu²⁺ : Mg²+ : Al³⁺) is 2:1 :1 .

This unprecedented result showed the positive effects of the combination between layered double hydroxides CuMgAI and species of copper Cu⁺/Cu° dispersed therein, i.e. the morphological, structural and physical-chemical characteristics of the nanostructured composite material according to the present invention. This allowed to obtain a product C2 at a relatively low cathodic potential, with 100% selectivity as to the liquid products, with a little amount of gas phase hydrogen, and a higher throughput/productivity compared to that obtained by means of the copper nanostructured composite materials containing species of Cu°/Cu2O supported on a carbonaceous materials¹⁵. In such composite materials, in combination with the species Cu°/Cu2O, the hydrotalcite-type ternary structure of CuMgAI LDH containing ions Cu²⁺, Mg²⁺ and Al³⁺ are missing, unlike the nanostructured composite material according to the present invention.

The electrodeposition reactions of the nanostructured composite materials with ternary hydrotalcite-type structure of layered double hydroxides LDH according to the present invention, were realized in a conventional three electrode cell connected to a potentiostat (CH Instrument 660 C). A “Toray” piece of paper with 4 cm² surface, obtained from a sheet of 19 X 19 cm in size, was used as a working electrode. Those working electrodes undergo a suitable pre-processing. Each of those paper portions, carbon fiber membranes, forming the carbonaceous support (CP), on which the nanostructured composite material according to the present invention is electrodeposited, was dipped in 1 M H2SO4 solution for 2h, followed by a 1 h treatment in pure ethanol. Finally, the membranes were rinsed with distilled H2O and dried until a constant weight of about 360 g is reached. The carbonaceous support is also defined as a gas diffusion membrane (CP) or gas diffusive (GDL). A platinum net was used as counter-electrode and a saturated calomel electrode (SCE) was used as the reference electrode. All the potentials are provided in reference to the saturated calomel electrode except the catalytic tests, where the value was converted in reference to the reversible hydrogen electrode (RHE).

In the electrodeposition of the nanostructured composite material according to the present invention, comprising the layered double hydroxides LDH CuMgAI, on the carbon electrode, the three- metal LDH films deposit on clean CP supports through a process comprising a methodology of po- tentiodynamic electrodeposition, through cyclic voltammetry, as known in literature^81-83. Initially the electrodeposition solution was prepared containing nitrates of the three cations Cu²⁺, Mg²⁺ e Al³⁺, while maintaining a total part concentration at 0.03 M and suitably changing the molar ratios between the cations, in accordance with the parameter to be investigated in the optimization step. Once connected to the potentiostat cell, a variable potential from 0.0 to -1 .4 V was applied with a scan rate of 30 mV s ¹. At the end of the electrodeposition, the electrocatalysts/working electrodes obtained thereby were rinsed in distilled water and dried until constant weight was reached. Each nanostructured composite material and the related electrocatalyst/working electrode obtained is designated by the cation moral ratio Cu²⁺, Mg²⁺ e Al³⁺, used in the electrodeposition solution.

The morphology and the structure of the electrocatalysts/working electrodes was investigated through standard scanning electron microscopy (SEM), through the instrument E-SEM Zeiss EVO 50 Series, and through field-emission scanning electron microscopy (FEG-SEM) by means of the instrument LEO 1530 ZEISS provided by a Schottky emitter and a “In-lens” detector. The EDS spectroscopy analysis was carried out though the Oxford INCA system, equipped with a silicon drift (30 mm) and a Brucker Quantax 200 detector. The X-Ray diffraction (XRD) analysis was performed by two different instruments: a diffractometer PW1050/81 (Philips/Malvern, Roystonm UK), equipped with a graphite monochromator and controlled by a PW1710 unit (Cu Ka, A = 0.15418 nm), and a diffractometer PANalytical X’Pert PRO equipped with a detector X’ Celerator. For the analysis a X-Ray tube was used with Cu, Ka anode (A = 0.15418 nm), at 40 mA and 40 kV and an angular range 20 between 3,5° and 80° was investigated with a step of 0,066° and time intervals for each angular step of 300 s. The Raman spectra were registered with a micro-spectrometer Raman RM1000 (Renishaw/Thermo Fisher, New Mills, Wotton-under-Edge, Gloucestershire, UK), equipped with an optical microscope Leica DMLM and a CCD detector. The excitation wavelength was generated from an Ar⁺ laser (A = 514.5 nm) with 25 mW output power. This power was lowered in accordance with the requirements by means of neutral-density filters to prevent damage to the nanostructured composite material according to the present invention, electrodeposited on the carbonaceous membrane CP. The process for CO2 electrochemical reduction to give C2, in accordance with the process according to the present invention, and further object thereof, were carried out in a two-compartment electrochemical cell (H-cell), wherein 0.3 M KHCO3 solution was employed as the support electrolyte (pH = 8.30) in both cell compartments (Pine Research Instrumentation, Inc.).

The electrocatalysts, i.e. the carbon membranes (CP) on which the nanostructured composite ma- terials/catalysts were electrodeposited, according to the present invention, were used as working electrodes on which to cause the CO2 reduction to occur to give C2 compounds, such as acetic acid, whereas an electrode Ag/AgCI (sat. KCI) and a platinum net were used, respectively, as the reference electrode and counter-electrode. The gas products were analyzed by a Thermo Focus GC (TCD detector) with a carbon molecular sieve column (CARBOSPHERE 80/100 6' x 1/8"). The ¹H- NMR spectra were registered by means of a Inova 600 spectrometer (600 MHz) couplet to a triple resonance probe.

The volume of the electrolyte present in the working electrode compartment was pre-saturated with a constant flow of carbon dioxide 20 mL min^-1) for 30 min and held at 5 mL min^-1 while reacting. All the potentials applied are expressed referring to the reversible hydrogen electrode (RHE). The liquid products were analyzed by means of quantitative analysis H¹-NMR, adding phenol as inner standard and deuterated water to provide a fixed inner signal to the instrument (defines “lock” signal) which helped maintain the magnetic field intensity constant. The gas products developed during the reaction are collected in gas sampling bags and analyzed through gas chromatography.

As said above, the electrodeposition of the layered double hydroxides LDH CuMgAI, containing Cu in the form of metal copper Cu° and cuprous cation rameoso Cu⁺, was achieved by cyclic voltammetric, which is a well-known methodology, even though applied to different systems⁸¹. When optimizing the nanostructured composite material according to the present invention, both the cation ratios in the electrolytic solution and the number of electrodeposition cycles were assessed. The carbon-based membrane was pre-treated before each deposition¹⁵. The outcome reported in this paragraph refers to the optimized material CuMgAI 2:1 :1 LDH/CP, selected as representative of all the samples obtained.

The material and the reactants used for that end were: the Nation N-1 15 membrane (0.125 mm thick, exchange capacity > 0.90 meq/g), “Toray” paper (TGP-H-60), ammonium nitrate (NH4NO3), hexahydrate magnesium nitrate (Mg(NOs)2 ■ 6H2O) and the copper strip purchased from Alfa Ae- sar. The copper nitrate trihydrate (Cu(NOs)2 ■ 3H2O), the aluminum nitrate nonahydrate (AI(NOs)3 ■ 9H2O), sulfuric acid (H2SO4 96% - 98%), ethanol (96.0 - 97.2%), potassium bicarbonate and phenol were purchased from Sigma-Aldrich. Deuterated water (99.96%) was purchased from Eurisotop. Pure carbon dioxide (> 99,9%) was purchased from Rivoira S.r.L The gas sampling bags were purchased from Supelco. All the reagents are of analytical or superior grade.

During the electrodeposition process, schematically depicted in Fig. 1 a, several reactions occur, starting from the nitrate and water reduction which cause ions H⁺ to be consumed and/or ions OH- to be produced + ~ ^u

Once produced, ions OH- diffuse towards the electrolytic solution wherein the electrode is dipped, whereas the cations there diffuse towards the electrode surface forming the LDH nuclei, according to the present invention. When the amount of OH- products equals the amount removed by cation precipitation, nucleation and consequent crystal growth occur on the electrode surface. The precipitation rate of LDH according to the present invention depends on the degree of saturation of the solution and the existence of flaws on the electrode surface, which serve as nucleation centers⁸¹.

In Fig. 1 b the current density registered during the electrodeposition process is reported, where two cathode peaks are evident: one at -0,40 V, slightly advanced -0,28 V in the second cycle, and one at -1 ,03 V. By comparing the deposition curves, shown in Figure 2, obtained with and without copper within the deposition solution, the first peak can be attributed to the copper ion reduction, since it is not visible in the curve where only Mg and Al are present, while in both cases the second cathode peak is observed to be related to the nitrate reduction. Furthermore, as can be seen from Fig. 1 b, the current related to the copper reduction peak diminishes in the second cycle whereas the peak current increases at -1 .03 V. Therefore, it can be assumed that, upon application of cathodic potential, a partial copper reduction occurs, in order to hinder the nitrate reduction, which is aided during the second cycle. The existence of Cu, as metal copper Cu° and cuprous ion Cu⁺, as CU2O, was further confirmed by the electrochemical characterization obtained in basic solution (Fig. 3) which shows the common redox peaks associated with the different species¹⁵’⁸⁰. The least cathodic peak, registered during the second scan, is associated with the copper deposition on an existing film on the conductive support, in accordance with the evidence reported in the literature for platinum and stainless steel. In order to assess the morphology and the chemical nature of the electrodeposited material, XRD, SEM-EDS and Raman were performed.

The analysis of the crystal structure by X-Ray diffraction (Fig. 1 c) shows the first confirmation of the occurrence of layered double hydroxides. In the spectrum the common reflections of the hydrotalcite phase are visible, containing ions NOs- as the interleaved anion, at 10.4° and 21 ,2° (20) respectively related to platens (003) and (006)⁵⁶’⁶¹. The Bragg angle associated with the reflection (003) was used to compute the perpendicular size of the crystallites⁸⁵ and the related value of the interlayer (d-spacing). All the values are reported below, where the investigation about several nanostructured composite materials produced according to the present invention is disclosed. However, only from the X-Ray diffraction analysis, the ternary nature of the electrodeposited LDH could not be confirmed. Furthermore, the SEM image reported in Fig: 1 d highlights that, following the deposition, the carbon fiber are evenly coated by a micro- and nano-structured coating featuring several morphological characteristics. Therefore, further analysis was carried out on the optimized electrocatalyst/working electrode (CuMgAI 2:1 :1 LDH/CP) by EDS and SEM-FEG analysis and the results are reported in Fig. 4.

Fig. 4a shows the existence of a thin veil which coats the superficial fibers of the carbonaceous support CP, which was not evident from the SEM image, shown in Fig. 1 d. In images 2b and c, a portion of such ripped film is reported, which highlights the veil morphology, with an estimated thickness of some tens of nanometers. The layered nature is highlighted in Fig. 5a, typical characteristic of hydrotalcites, whereas their ternary nature is confirmed by the EDS analysis carried out in position 1 (Figure 5a). Here, the molar ratio among the cations is 1 .4 for Cu/Mg, 1 .8 for Cu/AI and 1.3 for Mg/AI, whereas the total molar ratio among divalent and trivalent cations is 3.0, following quite accurately the CuMgAI ratio 2:1 :1 existing in the electrodeposition solution. However, in addition to the hydrotalcite coating, Figs. 4c-f show the existence of particles having different morphologies, located both on the carbon fibers of the carbonaceous support CP, combined within the veil structure of LDH. In particular, the EDS analysis showed how the smaller round particles which decorate the fiber in Fig. 4d are formed by metal copper Cu°, whereas the bigger coral-shaped particles in Fig. 4e are formed by species of Cu° and Cu oxide CU2O (atomic percentage 68% Cu and 16% O, Fig. 5b). In order to define more precisely the chemical nature of coral-shaped particles, a Raman analysis was carried out both along the carbon fiber coating of the carbonaceous support CP and on one of the above mentioned particles (Fig. 6). From the spectrum, common peaks related to cuprite (CU2O) at 142, 214 and 626 cm-^{1 15}’⁸⁶ can be recognized. On the contrary, the analysis carried out on the carbon fiber coating of the carbonaceous support CP, once again confirms the existence of LDH, showing a peak related to the stretching of nitrate ions which are then interleaved inside the structure⁸³’⁸⁷. The remaining peaks can be attributed to the bands D and G of the carbon fiber (1354 cm^-1 e 1581 cm’¹, respectively⁸⁸).

Together with the round and coral-shaped particles, also some cauliflower-shaped particles can be observed (Fig. 4d), which morphology can be attributed once more to the LDH⁸⁹. Here, the accurate EDS analysis, along with an EDS mapping (Fig. 7), showed again the existence of copper, magnesium and aluminum. However, in this case, the molar ratio of the cations in the solution 2:1 :1 are not followed (0.60 for Cu/Mg, 1 .5 for Cu/AI and 2.5 Mg/AI), suggesting the existence of a hydrotalcite-type structure with a number of flaws. Finally, the coral-shaped particles are found also in the pores in the veil of LDH and the EDS analysis confirmed the mixed nature of copper and copper oxide thereof, similar to those dispersed on the fibers (atomic percentage 57% Cu and 8% O).

These results confirmed that the obtained material for the electrodeposition on the carbon fibers of the carbonaceous support CP is composed of ternary hydrotalcites containing Cu, Mg and Al, directly in contact with metal copper Cu° and copper oxide CU2O particles.

Optimization of the electrocatalyst/working electrode and catalytic activity in the CO₂ reduction reaction in C₂ compounds

The optimization of the composition and the catalyst/nanostructured composite material properties according to the present invention was carried out by changing the following parameters: (i) different molar ratios among the sum of the divalent cations Cu²⁺ e Mg²⁺ in respect to the trivalent cations Al³⁺ present in the electrolytic solution, (ii) different quantity of catalyst/nanostructured composite material according to the present invention deposited on the carbonaceous support CP and (iii) different molar ratio among individual cations: Cu²⁺, Mg²⁺ and Al³⁺ in the electrolytic solution. Each nanostructured composite material/catalyst electrodeposited according to the present invention, was tested in order to check the electrocatalytic properties thereof during the carbon dioxide electrochemical reduction in liquid phase, at constant potential (-0,4 V) for 1 h. The optimization level achieved for the various nanostructured composite materials according to the present invention was assessed based on the throughput/productivity parameter of acetic acid, when used as elec- trocatalysts/working electrodes in the CO2 reduction reaction into C2 compounds, such acetic acid.

The selection of such parameters, along with the reaction condition, was made in accordance with the results obtained in the previous work, whereby electrocatalysts/working electrodes wherein on a carbonaceous support made of carbon fibers CP a nanostructured composite material was electrodeposited comprising the only redox pair Cu°/Cu⁺ as the pair Cu°/Cu2O, the acetic acid resulted in the prevailing CO2 reduction product¹⁵.

Effect of the total molar ratio between divalent M(ll) and trivalent M(lll) cations in the electrodeposi- tion solution

The first optimized parameter was the molar ratio in the electrodeposition solution between the sum of divalent cations M(ll) (Cu²⁺ e Mg²⁺) and the trivalent cation M(lll) (Al³⁺). In fact, this ratio dramatically influences the catalytic activity of LDH, or the electrocatalysts/working electrodes defined therefrom, for several processes^90-92. A set of electrocatalysts was synthesized starting from two different deposition solutions, however keeping the total concentration of 0.03 M of the cations:

I. M(ll): M(lll) = 6: 1 , corresponding to a theoretical ratio in the nanostructured composite material according to the present invention: Cu: Mg: Al = 3: 3: 1 LDH as electrodeposited on a carbonaceous substrate CP (CuMgAI 3:3:1 LDH/CP)

II. M(ll): M(lll) = 3: 1 , corresponding to a theoretical ratio in the nanostructured composite material according to the present invention: Cu: Mg: Al = 1 .5: 1 .5: 1 LDH as electrodeposited on a carbonaceous substrate CP (CuMgAI 1 .5:1 .5:1 LDH/CP)

The materials obtained were analyzed through SEM analysis (Fig. 8). In both cases, the entire area of the carbonaceous membrane was completely covered by the catalyst/nanostructured composite material according to the present invention, but the most homogeneous coating was obtained using the second composition of the electrodeposition solution. In fact, in the case of electrodeposition solutions characterized by the ratio M(l I) : M(lll) = 6:1 , the LDH film of the nanostructured composite material according to the present invention was interrupted in several locations and adhered poorly to carbon-based fiber, whereas the nanostructured composite material according to the present invention obtained by electrodeposition on a carbonaceous support CP, when the electrodeposition solutions are characterized by the ratio M(ll): M(lll) = 3:1 , seemed much more uniform and adhering. Later, more thorough investigations were carried out though SEM-FEG analysis on the LDH layer of the nanostructured composite material according to the present invention, highlighting the existence of pores inside the structure (Fig. 9). The composite CuMgAI 3:3:1 LDH material according to the present invention, features a higher number of coral-shaped particles embodied in the pores along with a plain structure near the hydrotalcite layers.

Fig. 10 and Fig. 1 1 show the spectra acquired through XRD analysis for both samples of the nanostructured composite materials according to the present invention and they confirm the existence of layered double hydroxides LDH, which can be observed by reflection (003). In both cases the medium size of crystallites was computed equal to 19.5 mm, by Scherrer equation, considering the mid-way width of the reflection (003) with a shape factor K = 1⁹³. Furthermore, from the angular position of the same reflection, the interlayer size could be obtained, which is slightly smaller for the sample of nanostructured composite material CuMgAI 3:3:1 LDH CP, 8,1 A, in respect of 8,8 A of the sample CuMgA1 1 .5:1 .5:1 LDH/CP, with both remaining common for the anions NOs^{- 56}. The reason behind this difference was shown by the investigation carried out in 1983 by Miyata⁹⁴ and subsequently Marappa et al.⁹⁵. In fact, both analyzed the increase in the interlayer in the layered double hydroxide MgAI with interleaved nitrates, following the increase in the component x in the formula [(M^ll)i-_x(M^ll (OH)2]^x+(A^m- _x/_m) ■ nH2O] and, therefore, the increase in the positive charge of the brucite layers. In the articles it is disclosed that for a value of x = 0.3, the basal spacing increases until 8.8 A, whereas the lower values of x (i.e. x = 0.2) the basal spacing reaches values around 8.0 A. generally, the positive charge is high, the nitrate ions interleaf with their molecular plane inclined at an angle of about 70° compared to the hydroxide layer, thereby helping the en- trance of a higher number of anions, while, when x is small, the plane formed by NOs- is parallel to the brucite layer. Therefore, as to the two samples of nanostructured composite material LDH according to the present invention studied, the increase in the interlayer space is in accordance with the increase in the molar fraction of Al(lll), which causes an increase in the amount of negative ions (NO₃- o CO3² ) needed to balance the excess positive charge. Furthermore, the EDS analysis (Fig. 10b) shows that the film composition of the nanostructured composite material according to the present invention CuMgAI 1 .5:1 .5:1 LDH reflects as best the electrodeposition solution, confirming also for the ternary nanostructured composite material LDH according to the present invention that the best molar ratio between the total amount of M(l I) e M(lll) is 3:1 , as found for the binary LDH systems reported in the literature⁹².

On the contrary, in the sample of the nanostructured composite material according to the present invention CuMgAI 3:3:1 LDH, several mismatches were found. Notwithstanding the X-Ray diffraction, analysis showed once again the existence of the hydrotalcite-type structure, the EDS analysis carried out in position 3 (Fig. 9) revealed the existence of several species of Cu/AI oxides, near the LDH structure. Furthermore, the estimated molar ratio among the three cations at the hydrotalcitetype structure did not match the ratio of the electrodeposition electrolytic solution. So, such evidence suggests the existence of a layered double hydroxides LDH with a number of flaws on the carbon fibers, where the cation ratio is 4:1 or 5:1 instead of 6:1 . Therefore, it can be stated that the use of a higher molar ratio among divalent and trivalent cations in the electrodeposition solution led to the formation of (i) a lower interlayer space, (ii) a LDH structure with flaws and (Hi) the deposition of undesired oxides. Finally, in order to study the effect of the total molar ratio of cations on the catalytic/electrocatalytic activity, the samples of two nanostructured composite materials were tested in the process of electrochemical CO2 reduction into C2 compounds, such as acetic acid (Fig. 10c). Both led to the production of acetic acid by application of a constant potential of -0.4 V vs RHE for 1 hour, as a main liquid product. However, the higher throughput/productivity was achieved with the sample of nanostructured composite material in the form of electrocata- lyst/working electrode CuMgAI 1 ,5:1 ,5:1 LDH/CP, exceeding the throughput/productivity of the other nanostructured composite material by 5 times (1 .5 mmolcHscooH g_caf¹ hr¹ vs 0.3 mmolcHscooH g_caf ¹ hr¹). This result can be attributed to two main characteristics, the final molar ratio and the interlayer distance. On one side, the molar ratio 3:1 led to the formation of a more controlled LDH structure, since the ratios in the nanostructured composite material coating the fibers reflected those present in the electrodeposition solution. On the other side, a higher interlayer space common for the nanostructured composite material LDH obtained from the electrodeposition solution having the ratio M(ll): M(lll) = 3:1 , might have helped the diffusion of the carbon dioxide inside the structure, leading to a higher concentration of carbon sources contacting the active phases, with higher production of acetic acid as a consequence. The existence of carbonates inside the structure LDH was confirmed by the X-Ray diffraction analysis, performed after the reaction and will be discussed later on. Due to these reasons, further optimizations of the material were carried out, keeping a molar ratio between divalent and trivalent cations of: M(l I) : M(lll) = 3:1 .

Effect of the amount of catalyst/ nanostructured composite material according to the present invention deposited on the substrate

Once the best molar ratio between M(ll) and M(lll) was found, the effect of several amounts of catalyst/nanostructured composite material according to the present invention deposited on the carbonaceous membrane CP was investigated. In this case, a composition of the material was investigated equal to CuMgAI 1 .5:1 .5:1 , changing the number of deposition cycles.

By increasing the number of voltammetric cycles (FIG. 12a), the deposited amount of catalyst/nanostructured composite material, according to the present invention, increases proportionally with the current density. A mass increase of 94% was observed between the second and the fourth deposition cycle (1 .14 mg vs 2.21 mg).

Comparing the results of the catalytic test, it can be noted that a higher productivity per gram of catalyst/nanostructured composite material according to the present invention was obtained with the working electrode on which a minor amount of nanostructured composite material according to the present invention was deposited (Fig: 12b).

The chosen potential has again led to the formation of acetic acid as the main product, with throughput/productivity values as:

- 1 .5 mmol g_caf¹ IT¹ for the nanostructured composite material sample, according to the present invention, achieved with 2 deposition cycles and

0.4 mmol g_caf¹ IT¹ for the nanostructured composite material sample, according to the present invention, achieved with 4 deposition cycles. Such a result suggests that the existence of a thicker LDH layer, and therefore containing a larger amount of non-redox active species (Mg and Al), resulted in a less conductive material wherein the charge transfer from the support to the active redox species (Cu°/Cu⁺) is probably found to be more hindered. Furthermore, also the diffusion of ions H⁺ to the electroactive surface of the electrode, resulting in a decreased catalytic activity. In fact, after the second deposition cycle, the peak related to the nitrate reduction started to increase, at the expense of the related peak upon the deposition of copper Cu° which, on the other side, has slowly disappeared, thereby suggesting the prevailing formation of ternary hydrotalcites. Thereby, the coral-shaped particles formed by Cu°/Cu2O are found to be more covered by unactive species (Mg and Al), changing the electrode/electrolyte interface and negatively affecting the activation processes⁹⁶’⁹⁷. Therefore, while carrying out the material optimization process, only the nanostructured composite materials and the related electrocatalysts/working electrodes according to the present invention were selected, obtained with only the cycles of electrodeposition.

Effect of the molar ratio among Cu, Mg and Al

Finally, several samples were made of nanostructured composite material CuMgAI LDH and related electrocatalysts/electrocatalysts with molar ratio M(ll)/M(lll) being 3:1 and with only 2 electrodeposition cycles, changing the molar ratio among the individual cations. To do so, three different electrodeposition solutions were prepared:

I. Cu²⁺: Mg²⁺: Al³⁺ = 1 :2:1 , corresponding to a theoretical ratio in the nanostructured composite material according to the present invention of CuMgAI 1 :2:1 LDH as electrodeposited on the carbonaceous substrate CP (CuMgAI 1 :2:1 LDH/CP)

II. Cu²⁺: Mg²⁺: Al³⁺ = 1 .5:1 .5:1 , corresponding to a theoretical ratio in the nanostructured composite material according to the present invention of CuMgAI 1 .5:1 .5:1 LDH as electrodeposited on the carbonaceous substrate CP (CuMgAI 1 .5:1 .5:1 LDH/CP)

III. Cu²⁺: Mg²⁺: Al³⁺ = 1 :2:1 , corresponding to a theoretical ratio in the nanostructured composite ma- terial according to the present invention of CuMgAI 1 :2:1 LDH as electrodeposited on the carbonaceous substrate CP (CuMgAI 1 :2:1 LDH/CP)

From the XRD analysis carried out (FIG. 13a) it can be noted that the existence of the reflection at 10.4° (20), which does not change in all the three samples, this characteristic being attributed to a similar ion beam of Mg²⁺ (0.72 A) and Cu²⁺ (0.73 A)⁹⁸. However it is worth noting that the highest intensity of the reflection (003) was registered for the sample of the nanostructured composite material according to the present invention of CuMgAI 2:1 :1 LDH which, in respect to the other nanostructured composite materials according to the present invention, is the only one wherein it can be observed also the second reflection at (006), thereby suggesting a higher crystallinity. Since the amount of aluminum is constant and the different ratio of divalent cations does not change the interlayer space value, such a value, computed for all the three materials, is always 8.5 A. A significant difference among the deposited films of nanostructured composite materials according to the present invention is given by the sizes of crystallites and the estimated molar ratios obtained from the EDS analysis. The average dimensions of the crystallites calculated taking into account the reflection peak at 10.4° (20), are 120.9, 19.5 and 9.9 nm for the respective nanostructured composite materials/catalysts CuMgAI 1 :2:1 LDH, CuMgAI 1 ,5:1 , 5:1 LDH and CuMgAI 2:1 :1 LDH. This trend was also confirmed by the SEM acquired images (Fig. 14), which, not only showed the existence of a homogeneous film with particles well dispersed along the fibers, but they also clearly highlighted the decrease in the size of particles upon the increase of the amount of copper.

Further, from the EDS analysis on the molar ratio among cations and the ratio between the sum of divalent and trivalent cations (Fig. 13b), it was confirmed that for the nanostructured composite material and the related electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP, the stoichiometric ratio among the cations reflect very accurately the electrodeposition solution. Only by ensuring the double amount of copper, the molar ratio could be kept stable. Finally, all the three materials were tested as electrocatalysts in CO2 reduction reactions to give C2, (Fig. 15) in order to assess which catalyst/nanostructured composite material according to the present invention was the best.

Again, the reduction tests at -0.4 V led to the formation of acetic acid and with significant results in terms of throughput/productivity. The highest throughput/productivity in acetic acid was obtained by using the most crystalline material and with the smallest size of the crystallites. In particular, it has increased from 1.1 mmolcHscooH g_caf¹ hr¹ obtained by means of the electrocatalyst/working electrode CuMgAI 1 :2:1 LDH/CP up to 2.0 mmolcHscooH g_caf¹ IT¹ obtained by means of the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP.

Instead, as the only gas compound, also in these instances, only hydrogen was detected, of which, however, a smooth trend could not be observed. As to the production of acetic acid, it can be stated that a higher Cu molar ratio than the other active non-redox species (Mg and Al) in the hydrotalcite-type structure LDH led to the formation of the product. Table 1 shows the current registered during the reactions using three materials containing several molar ratios of cations. Again, the highest current density was registered for the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH.

Table 1. Current densities [J (mA mg_caf¹)] registered during the CO2 reduction reaction for the three catalysts Electrocatalyst/working electrode J (mA mg_Cat'¹)

CuMgAI 1 :2:1 LDH/CP 0,84

CuMgAI 1 ,5:1 ,5:1 LDH/CP 1 ,00

CuMgAI 2:1 :1 LDH/CP 1 ,10

This characteristic promoted both the production of CH3COOH and the hydrogen evolution, thereby showing that both processes are catalyzed at their best by the same LDH, i.e. the one containing the largest quantity of Cu.

As a whole, the obtained results highlight the following:

I. The direct contact among the ternary hydrotalcites of CuMgAI and the CuO/Cu20 promoted the increase of the production of acetic acid, suggesting that the existence of LDH structure increases the availability of carbon sources inside and on the electrode surface.

II. The largest interlayer space obtained with a molar ratio M(l l)/M(l 11) of 3:1 helped the gas reactant to enter the layered porous structure, thereby increasing the carbonate species and, accordingly, the acetic acid production.

III. The higher existence of particles and nanometric crystallites, along with the higher crystallinity shown by the catalyst/nanostructured composite material according to the present invention available in the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP, led to the formation of a more active nanostructured composite material.

IV. The increase in the amount of coral-shaped particles in the catalyst/nanostructured composite material according to the present invention available in the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP (Fig. 14) suggests the prevailing existence of the active redox pair Cu+/Cu0, capable of promoting the creation of bond C-C.

Screening of the potential applied to the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP during the CO₂ reduction reaction into C₂ compounds and stability over the time.

In summary, the nanostructured composite material, according to the present invention, obtained by 2 cycles of electrodeposition with a molar ratio between M(l I) and M(lll) of 3:1 and among different cations of 2:1 :1 (Cu: Mg: Al) was chosen as the optimized catalyst/nanostructured composite material according to the present invention. Therefore, the following was carried out: an investigation based on the variation in the applied potential upon the CO₂ reduction reaction to give C₂ compounds, such as acetic acid, and an assessment of the nanostructured composite material stability, according to the present invention, over the time. The screening of the potentials was carried out to assess the actual selectivity to acetic acid by applying -0,4 V vs RHE and the opportunity to obtain other reduction products, by applying both a less cathodic potential and a more cathodic one. In this regard, -0,2 V and -0,8 V vs RHE were chosen and the obtained results were compared with those at -0.4 V vs RHE.

Fig. 16 shows the distribution of the obtained products. As it can be noted, by applying the more cathodic potential (-0.8 V), the selectivity to acetic acid is decreased, promoting also the formation of formic acid. Furthermore, due to the more favorable potential, a larger amount of developed hydrogen, was highlighted which probably acted as an obstacle for the production of acetic acid and formic acid or other reduction products. The reaction at -0.2 V vs RHE, on the other side, produced only a little amount of hydrogen, without showing any products in the liquid phase or at least in amounts not detectable by the instrument. For these reasons, the potential -0,4 V vs RHE was again confirmed as the most selective for producing acetic acid in the experimental set-up used in this work. Further, comparing the best result obtained by means of the electrocatalyst CU2O- Cu°/CP, the acetic acid production was found to be higher: production of acetic acid of 0.31 mmol gcat^-1 hr¹ with the electrocatalyst/working electrode Cu2O-Cu°/CP (composite material comprising metal copper and copper oxide deposited on a carbonaceous substrate CP) vs production of acetic acid: 2.0 mmol g_caf¹ hr¹ with the electrocatalyst/working electrode according to the present invention.

The catalyst/nanostructured composite material according to the present invention based on layered double hydroxides containing Cu, CuO and Cu⁺, such as CU2O, showed catalytic activity for the CO2 reduction reaction in liquid phase, using both the existence of copper active species and the basicity of hydroxides Mg and Al, which likely promoted the interaction with the gas reactant". Furthermore, due to the 3D structure of the gas diffusion membrane with the lamellar nature of LDH, dioxide carbon diffusion could be maximized toward the electrode surface and, due to the better affinity with carbonates⁷⁸, the nanostructured composite material LDH could act also as concentrator during the reaction. To confirm the entry of carbonates in the lamellar structure and check the stability of the crystallinity, a further XRD analysis was registered just after the reaction (FIG. 17). The spectrum shows the existence of the reflection (003) confirming the stability of the structure, whereas the reflection (006) disappears. However, the intensity of such a reflection is lower than registered for the starting nanostructured composite material LDH, thereby suggesting a loss of crystallinity during the process for CO2 reduction into C2 compounds. However, the material kept the nanometric sizes of crystallites, undergoing just a little variation from 9.9 nm to 25.6 nm. This change can be due to a sintering process, which is widely disclosed in literature for nanoparticles lower than 25 nm in size¹⁰⁰’¹⁰¹. It is also worth noting that the reflection angle (003) shifts from 10.4° to 1 1 .7° (20) with a resulting decrease of the interlayer space from 8.5 A to 7.6 A. This last value confirms the exchange of the inner anion from NOs- to COs^2-, proving the effective absorption/entry of carbon dioxide inside the nanostructured composite structure LDH during the reduction reaction of CO2 into C2. Finally the electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP stability over the time is assessed, applying -0,4 V vs RHE. FIG. 18 shows the current density seen during a 5 hour reaction, from which it is inferred that no sample degradation processes are registered, with a standard relative offset of 5% against the average value of Jtot. All the electro-CO2 reduction into C2 compounds as disclosed above were carried out in an H-cell as depicted in Fig. 19.

BENEFITS

Due to the use of the new layered double hydroxide nanostructured composite materials CuMgAI LDH according to the present invention with a layered hydrotalcite-type ternary structure containing three metals: Cu, Mg e Al, such as Cu²⁺, Mg²⁺ e Al³⁺, wherein they are interleaved with or overlapped on or interconnected to such layers (brucitic layers of said hydrotalcite-type structure): metal copper, Cu°, preferably in the form of micro7nano-particles and cuprous ion Cu⁺, preferably in the form of micro7nano-particles containing cuprous ion Cu⁺, most preferably as CU2O micro-/nano- particles) such as catalysts for the process for the electrochemical CO2 reduction, remarkable performances have been achieved, particularly in reference to the selectivity about the C2 compounds and the related throughput/productivity. The Applicant has successfully proved the likelihood to use the layered double hydroxides, generally used as precursors and widely used in oxidation processes, also in a reduction reaction, due to the concurrent existence of a redox pair Cu°/Cu⁺, taking into account that for the layered double hydroxide composite materials CuMgAI LDH containing three metals: Cu, Mg e Al, such as Cu²⁺, Mg²⁺ e Al³⁺, as disclosed in Li et al., a catalytic activity of reducing CO2 only into Ci compounds was shown. Unlike the hydrotalcites as reported in literature ⁷⁸’⁷⁹, obtained by means of the traditional co-precipitation procedures, the material made was obtained directly by a simple electrochemical deposition on a diffusive carbon gas membrane or other support, also of the electro- or thermo-conductive type, such as metals, like gold. That procedure, carried out at room temperature and pressure, in the case of a carbonaceous gas-diffusive membrane, allowed a 4 cm² electrode to be obtained with its carbon fibers completely coated by homogeneous and tight films of the nanostructured composite material object of the present invention, as the electro-catalyst of the process for electrochemical CO2 reduction into C2 compounds. Furthermore, the morphology and the physical-chemical characteristics of each electrocatalyst have been thoroughly investigated, showing the existence of a nanostructured composite material featuring a direct contact between the layered double hydroxides CuMgAI LDH with a layered hydrotalcite-type ternary structure containing three metals: Cu, Mg e Al, such as Cu²⁺, Mg²⁺ e Al³⁺, in the form of, preferably, layered veils, or particles, e.g. in the form of cauliflower and the species Cu°, metal copper, and Cu⁺ cuprous ion, preferably cuprous oxide, CU2O, preferably in the form of micro- /nano-particles, for example similar to coral. Several parameters have been assessed which allowed the nanostructured composite materials of the present invention to be identified and obtained which, besides keeping the selectivity about the composites C2, such as acetic acid, has the most pronounced electrocatalytic activity in terms of output of C2 compounds, such as acetic acid, during the C2 reduction to constant potential at -0.4V vs RHE for 1 hour. All the nanostructured composite materials according to the present invention, such as catalysts/electrocatalysts, have shown selectivity for producing acetic acid, whereas the highest catalytic/electrocatalytic activity in terms of throughput/productivity was achieved by the composite material, according to the present invention, wherein the resulting stoichiometric ratio CuMgAI is 2:1 :1 , to give the working electrode indicated as electrocatalyst/working electrode CuMgAI 2:1 :1 LDH/CP, exceeding the previous results obtained by means of working electrodes, used in CO2 electrocatalytic reduction reactions to give C2 compounds, wherein only the composite material CU2O-CU⁰ coated a gas diffusive carbonaceous membrane (CP) to give a working electrode intended as the catalyst Cu2O-Cu°/CP ¹⁵. This result shows that, by virtue of their flexibility in preparation, when a large number of basic sites exist and with the increased affinity for carbonated anions, the layered doble hydroxides of the system CuMgAI (LHD), in combination with several active phases of copper: metal copper Cu°, and Cu⁺, such as CU2O, cuprous ion, for example in the form of particles, show to be promising useful materials for CO2 electrochemical reduction and are capable of producing high added value compounds with optimum throughput/productivity. In particular, as to the production of C2 compounds, it is carried out with relatively low cathodic potentials with a 100% selectivity for liquids, with a little amount of hydrogen in gas phase and an increased throughput/productivity per weight unit of the nanostructured composite materials according to the present invention per reduction time unit. Furthermore it is noted that the working electrode according to the present invention having as a sup- port the carbonaceous support CP, as defined herein, coated with the nanostructured composite material, according to any of the embodiments according to the present invention, for the CO2 reduction into acetic acid, with the highest yield in millimole of acetic acid per weight unit of the catalyst (gram) and reduction time unit (hour), is the working electrode, wherein the weight % of the nanostructured composite material according to the present invention is not above 0.6%, preferably within 0.4 and 0.2%, more preferably 0.3 wt. %, 100 being the overall weight of the working electrode, made of the nanostructured composite material according to the present invention and the carbonaceous support CP as defined herein.

Riferences

(1) Karl, T. R.; Trenberth, K. E. Modern Global Climate Change. Science (80-. ). 2003, 302 (5651 ), 1719-1723. https://doi.org/10.1126/science.1090228.

(2) CO2 Levels: Current and Historic Atmospheric Carbon Dioxide/Global Temperature Graph and Widget, https://www.co2levels.org.

(3) Wang, Q.; Warnan, J.; Rodrfguez-Jimenez, S.; Leung, J. J.; Kalathil, S.; Andrei, V.; Domen, K.; Reisner, E. Molecularly Engineered Photocatalyst Sheet for Scalable Solar Formate Production from Carbon Dioxide and Water. Nat. Energy 2020, 5 (9), 703-710. https://doi.Org/10.1038/S41560-020-0678-6.

(4) Pachauri, R. K.; Meyer, L. A.; Core Writing Team. IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change; Geneva, Switzerland, 2014.

(5) The Royal Society. Dealing with carbon dioxide at scale. Sackler Forum. http://www.nasonline.org/programs/scientific-forum/sackler-forum-2017-carbon.pdf.

(6) Xu, D.; Li, K.; Jia, B.; Sun, W.; Zhang, W.; Liu, X.; Ma, T. Electrocatalytic CO2 Reduction towards Industrial Applications. Carbon Energy 2022. https://doi.org/10.1002/cey2.230.

(7) Ampelli, C.; Genovese, C.; Marepally, B. C.; Papanikolaou, G.; Perathoner, S.; Centi, G.

Electrocatalytic Conversion of CO2 to Produce Solar Fuels in Electrolyte or Electrolyte-Less Configurations of PEC Cells. Faraday Discuss. 2015, 183, 125-145. https://doi.Org/10.1039/C5FD00069F.

(8) Shi, J.; Jiang, Y.; Jiang, Z.; Wang, X.; Wang, X.; Zhang, S.; Han, P.; Yang, C. Enzymatic Conversion of Carbon Dioxide. Chem. Soc. Rev. 2015, 44 (17), 5981-6000. https://doi.Org/10.1039/C5CS00182J.

(9) Sultana, S.; Chandra Sahoo, P.; Martha, S.; Parida, K. A Review of Harvesting Clean Fuels from Enzymatic CO2 Reduction. RSC Adv. 2016, 6 (50), 44170-44194. https://doi.Org/10.1039/C6RA05472B.

(10) Grodkowski, J.; Neta, P. Copper-Catalyzed Radiolytic Reduction of CO2 to CO in Aqueous Solutions. J. Phys. Chem. B 2001 , 105 (21 ), 4967-4972. https://doi.org/10.1021/jp004567d.

(11) Roy, S.; Cherevotan, A.; Peter, S. C. Thermochemical CO2 Hydrogenation to Single Carbon Products: Scientific and Technological Challenges. ACS Energy Lett. 2018, 3 (8), 1938-1966. https://doi.org/10.1021/acsenergylett.8b00740.

(12) Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K. Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors. Angew. Chemie Int. Ed. 2013, 52 (29), 7372-7408. https://d0i.0rg/l 0.1002/anie.201207199. (13) Rej, S.; Bisetto, M.; Naldoni, A.; Fornasiero, P. Well-Defined C112O Photocatalysts for Solar Fuels and Chemicals. J. Mater. Chem. A 2021 , 9 (10), 5915-5951. https://doi.Org/10.1039/D0TA10181 H.

(14) Genovese, C.; Ampelli, C.; Perathoner, S.; Centi, G. Electrocatalytic Conversion of CO2 on Carbon Nanotube-Based Electrodes for Producing Solar Fuels. J. Catal. 2013, 308, 237-249. https://d0i.0rg/l 0.1016/j .jcat.2013.08.026.

(15) Serafini, M.; Mariani, F.; Fasolini, A.; Scavetta, E.; Basile, F.; Tonelli, D. Nanostructured Copper-Based Electrodes Electrochemically Synthesized on a Carbonaceous Gas Diffusion Membrane with Catalytic Activity for the Electroreduction of CO2. ACS Appt. Mater. Interfaces 2021 , 13 (48), 57451-57461. https://doi.org/10.1021/acsami.1c18844.

(16) Bevilacqua, M.; Filippi, J.; Miller, H. A.; Vizza, F. Recent Technological Progress in CO2 Electroreduction to Fuels and Energy Carriers in Aqueous Environments. Energy Technol. 2015, 3 (3), 197-210. https://doi.org/10.1002/ente.201402166.

(17) Li, L.; Huang, Y.; Li, Y. Carbonaceous Materials for Electrochemical CO2 Reduction. En- ergyChem 2020, 2 (1 ), 100024. https://doi.Org/10.1016/j.enchem.2019.100024.

(18) Jin, S.; Hao, Z.; Zhang, K.; Yan, Z.; Chen, J. Advances and Challenges for the Electrochemical Reduction of CO2 to CO: From Fundamentals to Industrialization. Angew. Chemie Int. Ed. 2021 , 60 (38), 20627-20648. https://doi.org/10.1002/anie.202101818.

(19) Bensaid, S.; Centi, G.; Garrone, E.; Perathoner, S.; Saracco, G. Towards Artificial Leaves for Solar Hydrogen and Fuels from Carbon Dioxide. ChemSusChem 2012, 5 (3), 500-521. https://doi.Org/10.1002/cssc.201100661 .

(20) Pawar, A. U.; Kim, C. W.; Nguyen-Le, M.-T.; Kang, Y. S. General Review on the Components and Parameters of Photoelectrochemical System for CO2 Reduction with in Situ Analysis. ACS Sustain. Chem. Eng. 2019, 7 (8), 7431-7455. https://doi.org/10.1021/acssuschemeng.8b06303.

(21) Kumar, A.; Hasija, V.; Sudhaik, A.; Raizada, P.; Van Le, Q.; Singh, P.; Pham, T.-H.; Kim, T.; Ghotekar, S.; Nguyen, V.-H. Artificial Leaf for Light-Driven CO2 Reduction: Basic Concepts, Advanced Structures and Selective Solar-to-Chemical Products. Chem. Eng. J. 2022, 430, 133031. https://d0i.0rg/l 0.1016/j .cej .2021 .133031 .

(22) Sanchez, O. G.; Birdja, Y. Y.; Bulut, M.; Vaes, J.; Breugelmans, T.; Pant, D. Recent Advances in Industrial CO2 Electroreduction. Curr. Opin. Green Sustain. Chem. 2019, 16, 47-56. https://d0i.0rg/l 0.1016/j.cogsc.2019.01 .005.

(23) Jiwanti, P. K.; Sultana, S.; Wicaksono, W. P.; Einaga, Y. Metal Modified Carbon-Based Electrode for CO2 Electrochemical Reduction: A Review. J. Electroanal. Chem. 2021 , 898, 115634. https://doi.org/10.1016/jjelechem.2021 .115634.

(24) McCreery, R. L. Advanced Carbon Electrode Materials for Molecular Electrochemistry. Chem. Rev. 2008, 108 (7), 2646-2687. https://doi.org/10.1021/cr068076m.

(25) Yang, N.; Waldvogel, S. R.; Jiang, X. Electrochemistry of Carbon Dioxide on Carbon

Electrodes. ACS Appl. Mater. Interfaces 2016, 8 (42), 28357-28371. https://d0i.0rg/l 0.1021 /acsami.5b09825.

(26) Zhai, Q.; Pan, Y.; Dai, L. Carbon-Based Metal-Free Electrocatalysts: Past, Present, and

Future. Accounts Mater. Res. 2021 , 2 (12), 1239-1250. https://doi.org/10.1021 /accountsmr.1 c00190.

(27) De Gregorio, G. L.; Burdyny, T.; Loiudice, A.; Iyengar, P.; Smith, W. A.; Buonsanti, R.

Facet-Dependent Selectivity of Cu Catalysts in Electrochemical CO2 Reduction at Commercially Viable Current Densities. ACS Catal. 2020, 10 (9), 4854-4862. https://d0i.0rg/l 0.1021 /acscatal.0c00297.

(28) Xing, Z.; Hu, L.; Ripatti, D. S.; Hu, X.; Feng, X. Enhancing Carbon Dioxide Gas-Diffusion Electrolysis by Creating a Hydrophobic Catalyst Microenvironment. Nat. Commun. 2021 , 12 (1 ), 136. https://doi.Org/10.1038/S41467-020-20397-5.

(29) Cui, H.; Guo, Y.; Guo, L.; Wang, L.; Zhou, Z.; Peng, Z. Heteroatom-Doped Carbon Materials and Their Composites as Electrocatalysts for CO2 Reduction. J. Mater. Chem. A 2018, 6 (39), 18782-18793. https://doi.Org/10.1039/C8TA07430E.

(30) Duan, X.; Xu, J.; Wei, Z.; Ma, J.; Guo, S.; Wang, S.; Liu, H.; Dou, S. Metal-Free Carbon Materials for CO2 Electrochemical Reduction. Adv. Mater. 2017, 29 (41 ), 1701784. https://d0i.0rg/l 0.1002/adma.201701784.

(31) Perez-Sequera, A. C.; Diaz-Perez, M. A.; Serrano-Ruiz, J. C. Recent Advances in the Electroreduction of CO2 over Heteroatom-Doped Carbon Materials. Catalysts 2020, 10 (10), 1179. https://d0i.0rg/l 0.3390/catal10101179.

(32) Baturina, O. A.; Lu, Q.; Padilla, M. A.; Xin, L.; Li, W.; Serov, A.; Artyushkova, K.; At- anassov, P.; Xu, F.; Epshteyn, A.; Brintlinger, T.; Schuette, M.; Collins, G. E. CO2 Electroreduction to Hydrocarbons on Carbon-Supported Cu Nanoparticles. ACS Catal. 2014, 4 (10), 3682-3695. https://doi.Org/10.1021 /cs500537y.

(33) Nitopi, S.; Bertheussen, E.; Scott, S. B.; Liu, X.; Engstfeld, A. K.; Horch, S.; Seger, B.; Stephens, I. E. L.; Chan, K.; Hahn, C.; Norskov, J. K.; Jaramillo, T. F.; Chorkendorff, I. Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte. Chem. Rev. 2019, / /9 (12), 7610-7672. https://doi.org/10.1021/acs.chemrev.8b00705.

(34) Hori, Y.; Kikuchi, K.; Suzuki, S. Production of CO and CH4 in Electrochemical Reduction of CO2 at Metal Electrodes in Aqueous Hydrogencarbonate Solution. Chem. Lett. 1985, 14 (11 ), 1695-1698. https://doi.Org/10.1246/CI.1985.1695.

(35) Hori, Y. Electrochemical CO2 Reduction on Metal Electrodes. In Modem Aspects of Electrochemistry, Springer New York: New York, NY, 2008; pp 89-189. https://doi.org/10.1007/978-0- 387-49489-0_3.

(36) Chi, D.; Yang, H.; Du, Y.; Lv, T.; Sui, G.; Wang, H.; Lu, J. Morphology-Controlled CuO Nanoparticles for Electroreduction of CO2 to Ethanol. RSC Adv. 2014, 4 (70), 37329-37332. https://doi.Org/10.1039/C4RA05415F.

(37) Genovese, C.; Ampelli, C.; Perathoner, S.; Centi, G. Mechanism of C-C Bond Formation in the Electrocatalytic Reduction of CO2 to Acetic Acid. A Challenging Reaction to Use Renewable Energy with Chemistry. Green Chem. 2017, 19 (10), 2406-2415. https://doi.Org/10.1039/C6GC03422E.

(38) Marepally, B. C.; Ampelli, C.; Genovese, C.; Tavella, F.; Veyre, L.; Quadrelli, E. A.; Perathoner, S.; Centi, G. Role of Small Cu Nanoparticles in the Behaviour of Nanocarbon-Based Electrodes for the Electrocatalytic Reduction of CO2. J. CO2 Util. 2017, 21, 534-542. https://doi.Org/10.1016/j.jcou.2017.08.008. (39) de Lucas-Consuegra, A.; Serrano-Ruiz, J.; Gutierrez-Guerra, N.; Valverde, J. Low- Temperature Electrocatalytic Conversion of CO2 to Liquid Fuels: Effect of the Cu Particle Size. Catalysts 2018, 8 (8), 340. https://doi.org/10.3390/catal8080340.

(40) Perry, S. C.; Leung, P.; Wang, L.; Ponce de Leon, C. Developments on Carbon Dioxide Reduction: Their Promise, Achievements, and Challenges. Corr. Opin. Electrochem. 2020, 20, 88- 98. https://d0i.0rg/l 0.1016/j . coelec.2020.04.014.

(41) Kim, D.; Lee, S.; Ocon, J. D.; Jeong, B.; Lee, J. K.; Lee, J. Insights into an Autonomously Formed Oxygen-Evacuated CU2O Electrode for the Selective Production of C2H4 from CO2. Phys. Chem. Chem. Phys. 2015, 17(2), 824-830. https://doi.org/10.1039/C4CP03172E.

(42) Xiao, FL; Goddard, W. A.; Cheng, T.; Liu, Y. Cu Metal Embedded in Oxidized Matrix Catalyst to Promote CO2 Activation and CO Dimerization for Electrochemical Reduction of CO2. Proc. Natl. Acad. Sci. 2017, 201702405. https://doi.org/10.1073/pnas.1702405114.

(43) Aran-Ais, R. M.; Scholten, F.; Kunze, S.; Rizo, R.; Roldan Cuenya, B. The Role of in Situ Generated Morphological Motifs and Cu(i) Species in C2+ Product Selectivity during CO2 Pulsed Electroreduction. Nat. Energy 2020, 5 (4), 317-325. https://doi.org/10.1038/s41560-020-0594-9.

(44) Aran-Ais, R. M.; Rizo, R.; Grosse, P.; Algara-Siller, G.; Dembele, K.; Plodinec, M.; Lunkenbein, T.; Chee, S. W.; Cuenya, B. R. Imaging Electrochemically Synthesized CU2O Cubes and Their Morphological Evolution under Conditions Relevant to CO2 Electroreduction. Nat. Common. 2020, 11 (1 ), 3489. https://doi.org/10.1038/s41467-020-17220-6.

(45) Varela, A. S.; Ju, W.; Reier, T.; Strasser, P. Tuning the Catalytic Activity and Selectivity of Cu for CO2 Electroreduction in the Presence of Halides. ACS Catal. 2016, 6 (4), 2136-2144. https://d0i.0rg/l 0.1021 /acscatal.5b02550.

(46) de Brito, J. F.; Genovese, C.; Tavella, F.; Ampelli, C.; Boldrin Zanoni, M. V.; Centi, G.; Perathoner, S. CO2 Reduction of Hybrid Cu2O-Cu/Gas Diffusion Layer Electrodes and Their Integration in a Cu-based Photoelectrocatalytic Cell. ChemSusChem 2019, 12 (18), 4274-4284. https://doi.Org/10.1002/cssc.201901352.

(47) Zhu, Q.; Sun, X.; Yang, D.; Ma, J.; Kang, X.; Zheng, L.; Zhang, J.; Wu, Z.; Han, B. Carbon Dioxide Electroreduction to C2 Products over Copper-Cuprous Oxide Derived from Electrosynthesized Copper Complex. Nat. Common. 2019, 10 (1 ), 3851. https://doi.org/10.1038/s41467-019- 11599-7.

(48) Manyar, H.; Deshmukh, G. Production Pathways of Acetic Acid and Versatile Applications in Food Industry. Intech Open.

(49) Jones, J. H. The Cativa™ Process for the Manufacture of Acetic Acid. Platin. Met. Rev. 2000, 44 (2), 94.

(50) Senocrate, A.; Battaglia, C. Electrochemical CO2 Reduction at Room Temperature: Status and Perspectives. J. Energy Storage 2021 , 36, 102373. https://d0i.0rg/l 0.1016/j .est.2021 .102373.

(51) Durst, J.; Rudnev, A.; Dutta, A.; Fu, Y.; Herranz, J.; Kaliginedi, V.; Kuzume, A.; Permyakova, A. A.; Paratcha, Y.; Broekmann, P.; Schmidt, T. J. Electrochemical CO2 Reduction - A Critical View on Fundamentals, Materials and Applications. Chimia (Aarao). 2015, 69 (12), 769- 776. https://doi.Org/10.2533/chimia.2015.769.

(52) Perry, S. C.; Gateman, S. M.; Malpass-Evans, R.; McKeown, N.; Wegener, M.; Nazarovs, P.; Mauzeroll, J.; Wang, L.; Ponce de Leon, C. Polymers with Intrinsic Microporosity (PIMs) for Targeted CO2 Reduction to Ethylene. Chemosphere 2020, 248, 125993. https://d0i.0rg/l 0.1016/j.chemosphere.2020.125993.

(53) Marken, F.; Madrid, E.; Zhao, Y.; Carta, M.; McKeown, N. B. Polymers of Intrinsic Microporosity in Triphasic Electrochemistry: Perspectives. ChemElectroChem 2019, 6 (17), 4332- 4342. https://doi.Org/10.1002/celc.201900717.

(54) Tan, X.; Yu, C.; Zhao, C.; Huang, H.; Yao, X.; Han, X.; Guo, W.; Cui, S.; Huang, H.; Qiu,

J. Restructuring of CU2O to Cu2O@Cu-Metal-Organic Frameworks for Selective Electrochemical Reduction of CO2. ACS Appl. Mater. Interfaces 2019, 11 (10), 9904-9910. https://doi.org/10.1021/acsami.8b19111 .

(55) Marepally, B. C.; Ampelli, C.; Genovese, C.; Saboo, T.; Perathoner, S.; Wisser, F. M.; Veyre, L.; Canivet, J.; Quadrelli, E. A.; Centi, G. Enhanced Formation of >Ci Products in Electroreduction of CO2 by Adding a CO2 Adsorption Component to a Gas-Diffusion Layer-Type Catalytic Electrode. ChemSusChem 2017 , 10 (22), 4442-4446. https://doi.org/10.1002/cssc.201701506.

(56) Cavani, F.; Trifird, F.; Vaccari, A. Hydrotalcite-Type Anionic Clays: Preparation, Properties and Applications. Catal. Today 1991 , 11 (2), 173-301. https://doi.Org/https://doi.org/10.1016/0920-5861 (91 )80068-K.

(57) Forano, C.; Hibino, T.; Leroux, F.; Taviot-Gueho, C. Chapter 13.1 Layered Double Hydroxides; 2006; pp 1021-1095. https://doi.Org/10.1016/S1572-4352(05)01039-1 .

(58) Tonelli, D.; Gualandi, I.; Musella, E.; Scavetta, E. Synthesis and Characterization of Layered Double Hydroxides as Materials for Electrocatalytic Applications. Nanomaterials 2021 , 11 (3), 725. https://doi.Org/10.3390/nano11030725.

(59) Basile, F.; Basini, L.; Fornasari, G.; Gazzano, M.; Trifird, F.; Vaccari, A. Anionic Clays as Precursors of Noble Metal Based Catalysts for Methane Activation. In Preparation of Catalysts VII; 1998; Vol. 118, pp 31-40.

(60) Basile, F.; Fornasari, G.; Gazzano, M.; Vaccari, A. Rh, Ru and Ir Catalysts Obtained by HT Precursors: Effect of the Thermal Evolution and Composition on the Material Structure and Use. J. Mater. Chem. 2002, 12 (11 ), 3296-3303. https://doi.org/10.1039/B205146J.

(61) Monti, M.; Benito, P.; Basile, F.; Fornasari, G.; Gazzano, M.; Scavetta, E.; Tonelli, D.; Vaccari, A. Electrosynthesis of Ni/AI and Mg/AI Layered Double Hydroxides on Pt and FeCrAlloy Supports: Study and Control of the pH near the Electrode Surface. Electrochim. Acta 2013, 108, 596-604. https://d0i.0rg/l 0.1016/j.electacta.2O13.06.143.

(62) Tabanelli, T.; Cocchi, S.; Gumina, B.; Izzo, L.; Melia, M.; Passeri, S.; Cavani, F.; Lucarelli, C.; Schutz, J.; Bonrath, W.; Netscher, T. Mg/Ga Mixed-Oxide Catalysts for Phenol Methylation: Outstanding Performance in 2,4,6-Trimethylphenol Synthesis with Co-Feeding of Water. Appl. Catal. A Gen. 2018, 552, 86-97. https://doi.Org/10.1016/j.apcata.2018.01.001.

(63) De Maron, J.; Eberle, M.; Cavani, F.; Basile, F.; Dimitratos, N.; Maireles-Torres, P. J.; Rodriguez-Castelldn, E.; Tabanelli, T. Continuous-Flow Methyl Methacrylate Synthesis over Gallium-Based Bifunctional Catalysts. ACS Sustain. Chem. Eng. 2021 , 9 (4), 1790-1803. https://doi.org/10.1021/acssuschemeng.0c07932.

(64) Mohapatra, L.; Parida, K. A Review on the Recent Progress, Challenges and Perspective of Layered Double Hydroxides as Promising Photocatalysts. J. Mater. Chem. A 2016, 4 (28), 10744-10766. https://doi.Org/10.1039/C6TA01668E.

(65) Khalil, M.; Gunlazuardi, J.; Ivandini, T. A.; Umar, A. Photocatalytic Conversion of CO2 Using Earth-Abundant Catalysts: A Review on Mechanism and Catalytic Performance. Renew. Sustain. Energy Rev. 2019, 113, 109246. https://doi.Org/10.1016/j.rser.2019.109246.

(66) Fasolini, A.; Sangiorgi, N.; Tosi Brandi, E.; Sangiorgi, A.; Mariani, F.; Scavetta, E.; Sanson, A.; Basile, F. Increased Efficiency and Stability of Dye-Sensitized Solar Cells (DSSC) Photoanode by Intercalation of Eosin Y into Zn/AI Layered Double Hydroxide. Appl. Clay Sci. 2021 , 212 (May), 106219. https://doi.Org/10.1016/j.clay.2021.106219.

(67) Viamidis, Y.; Scavetta, E.; Gazzano, M.; Tonelli, D. Iron vs Aluminum Based Layered Double Hydroxides as Water Splitting Catalysts. Electrochim. Acta 2016, 188, 653-660. https://d0i.0rg/l 0.1016/j.electacta.2O15.12.059.

(68) Dionigi, F.; Zeng, Z.; Sinev, I.; Merzdorf, T.; Deshpande, S.; Lopez, M. B.; Kunze, S.; Ze- gkinoglou, I.; Sarodnik, H.; Fan, D.; Bergmann, A.; Drnec, J.; Araujo, J. F. de; Gliech, M.; Teschner, D.; Zhu, J.; Li, W.-X.; Greeley, J.; Cuenya, B. R.; Strasser, P. In-Situ Structure and Catalytic Mechanism of NiFe and CoFe Layered Double Hydroxides during Oxygen Evolution. Nat. Commun. 2020, 11 (1 ), 2522. https://doi.org/10.1038/s41467-020-16237-1.

(69) Ballarin, B.; Seeber, R.; Tonelli, D.; Vaccari, A. Electrocatalytic Properties of Nickel(ll) Hydrotalcite-Type Anionic Clay: Application to Methanol and Ethanol Oxidation. J. Electroanal. Chem. 1999, 463 (1), 123-127. https://doi.Org/10.1016/S0022-0728(98)00436-7.

(70) Scavetta, E.; Tonelli, D. Amperometric Sensors Based on Synthetic Hydrotalcites and Their Application for Ethanol Detection in Beer. Electroanalysis 2005, 17 (4), 363-370. https://d0i.0rg/l 0.1002/elan.200403103.

(71) Gualandi, I.; Tessarolo, M.; Mariani, F.; Arcangeli, D.; Possanzini, L.; Tonelli, D.; Fraboni, B.; Scavetta, E. Layered Double Hydroxide-Modified Organic Electrochemical Transistor for Glucose and Lactate Biosensing. Sensors 2020, 20 (12), 3453. https://doi.org/10.3390/s20123453.

(72) Choy, J.; Choi, S.; Oh, J.; Park, T. Clay Minerals and Layered Double Hydroxides for

Novel Biological Applications. Appl. Clay Sci. 2007, 36 (1-3), 122-132. https://d0i.0rg/l 0.1016/j.clay.2006.07.007.

(73) Millange, F.; Walton, R. I.; Lei, L.; 0‘Hare, D. Efficient Separation of Terephthalate and

Phthalate Anions by Selective Ion-Exchange Intercalation in the Layered Double Hydroxide Ca₂AI(OH)₆ -NO₃ -2H₂O. Chem. Mater. 2000, 12 (7), 1990-1994. https://doi.Org/10.1021 /cm0002057.

(74) Fasolini, A.; Abate, S.; Barbera, D.; Centi, G.; Basile, F. Pure H2 Production by Methane Oxy-Reforming over Rh-Mg-AI Hydrotalcite-Derived Catalysts Coupled with a Pd Membrane. Appl. Catal. A Gen. 2019, 581 (May), 91-102. https://doi.Org/10.1016/j.apcata.2019.05.024.

(75) Kim, K. Y.; Lee, J. H.; Lee, H.; Noh, W. Y.; Kim, E. H.; Ra, E. C.; Kim, S. K.; An, K.; Lee, J. S. Layered Double Hydroxide-Derived Intermetallic Ni3GaCo.₂5 Catalysts for Dry Reforming of Methane. ACS Catal. 2021 , 11 (17), 11091-11102. https://doi.org/10.1021/acscatal.1c02200.

(76) Dewangan, N.; Hui, W. M.; Jayaprakash, S.; Bawah, A.-R.; Poerjoto, A. J.; Jie, T.; Jan- gam, A.; Hidajat, K.; Kawi, S. Recent Progress on Layered Double Hydroxide (LDH) Derived Metal- Based Catalysts for CO₂ Conversion to Valuable Chemicals. Catal. Today 2020, 356, 490-513. https://d0i.0rg/l 0.1016/j.cattod.2020.06.020. (77) Li, M. M.-J.; Chen, C.; Ayvali, T.; Sue, H.; Zheng, J.; Teixeira, I. F.; Ye, L.; Zou, H.; O’Hare, D.; Tsang, S. C. E. CO2 Hydrogenation to Methanol over Catalysts Derived from Single Cationic Layer CuZnGa LDH Precursors. ACS Catal. 2018, 8 (5), 4390-4401. https://d0i.0rg/l 0.1021 /acscatal.8b00474.

(78) Li, L.; Yang, J.; Li, L.; Huang, Y.; Zhao, J. Electrolytic Reduction of CO2 in KHCO3 and Alkanolamine Solutions with Layered Double Hydroxides Intercalated with Gold or Copper. Electrochim. Acta 2022, 402, 139523. https://doi.Org/10.1016/j.electacta.2021.139523.

(79) Iwase, K.; Hirano, T.; Honma, I. Copper Aluminum Layered Double Hydroxides with Different Compositions and Morphologies as Electrocatalysts for the Carbon Dioxide Reduction Reaction. ChemSusChem 2022, 15 (2). https://doi.org/10.1002/cssc.202102340.

(80) Caballero-Briones, F.; Artes, J. M.; Diez-Perez, I.; Gorostiza, P.; Sanz, F. Direct Observation of the Valence Band Edge by in Situ ECSTM-ECTS in p-Type CU2O Layers Prepared by Copper Anodization. J. Phys. Chem. C 2009, 1/3 (3), 1028-1036. https://doi.org/10.1021/jp805915a.

(81 ) Gualandi, I.; Viamidis, Y.; Mazzei, L.; Musella, E.; Giorgetti, M.; Christian, M.; Morandi, V.;

Scavetta, E.; Tonelli, D. Ni/AI Layered Double Hydroxide and Carbon Nanomaterial Composites for Glucose Sensing. ACS Appl. Nano Mater. 2019, 2 (1 ), 143-155. https://d0i.0rg/l 0.1021 /acsanm.8b01765.

(82) Musella, E.; Gualandi, I.; Scavetta, E.; Rivalta, A.; Venuti, E.; Christian, M.; Morandi, V.; Mullaliu, A.; Giorgetti, M.; Tonelli, D. Newly Developed Electrochemical Synthesis of Co-Based Layered Double Hydroxides: Toward Noble Metal-Free Electro-Catalysis. J. Mater. Chem. A 2019, 7(18), 11241-11249. https://doi.org/10.1039/C8TA11812D.

(83) Musella, E.; Gualandi, I.; Giorgetti, M.; Scavetta, E.; Basile, F.; Rivalta, A.; Venuti, E.; Corticelli, F.; Christian, M.; Morandi, V.; Tonelli, D. Electrosynthesis and Characterization of Layered Double Hydroxides on Different Supports. Appl. Clay Sci. 2021 , 202, 105949. https://d0i.0rg/l 0.1016/j .clay.2020.105949.

(84) Coetzee, C.; Tadie, M.; Dorfling, C. Evaluating the Effect of Molecular Properties of Polyacrylamide Reagents on Deposit Growth in Copper Electrowinning. Hydrometallurgy 2020, 195, 105407. https://d0i.0rg/l 0.1016/j. hydromet.2020.105407.

(85) Gevers, B. R.; Naseem, S.; Leuteritz, A.; Labuschagne, F. J. W. J. Comparison of Nano- Structured Transition Metal Modified Tri-Metal MgMAI-LDHs (M = Fe, Zn, Cu, Ni, Co) Prepared Using Co-Precipitation. RSC Adv. 2019, 9 (48), 28262-28275. https://doi.org/10.1039/C9RA05452A.

(86) Huang, Y.; Han, Y.; Sun, J.; Zhang, Y.; Han, L. Dual Nanocatalysts Co-Decorated Three- Dimensional, Laser-Induced Graphene Hybrid Nanomaterials Integrated with a Smartphone Portable Electrochemical System for Point-of-Care Non-Enzymatic Glucose Diagnosis. Mater. Today Chem. 2022, 24, 100895. https://doi.Org/10.1016/j.mtchem.2022.100895.

(87) Cosano, D.; Esquivel, D.; Romero-Salguero, F. J.; Jimenez-Sanchidrian, C.; Ruiz, J. R. Use of Raman Spectroscopy to Assess Nitrate Uptake by Calcined LDH Phases. Colloids Surfaces A Physicochem. Eng. Asp. 2020, 602, 125066. https://doi.Org/10.1016/j.colsurfa.2020.125066.

(88) Liu, J.; Tian, Y.; Chen, Y.; Liang, J.; Zhang, L.; Fong, H. A Surface Treatment Technique of Electrochemical Oxidation to Simultaneously Improve the Interfacial Bonding Strength and the Tensile Strength of PAN-Based Carbon Fibers. Mater. Chem. Phys. 2010, 122 (2-3), 548-555. https://d0i.0rg/l 0.1016/j.matchemphys.2O10.03.045. (89) Amini, R.; Asadpour-Zeynali, K. Cauliflower-like NiCo2C>4 -Zn/AI Layered Double Hydroxide Nanocomposite as an Efficient Electrochemical Sensing Platform for Selective Pyridoxine Detection. Electroanalysis 2020, 32 (6), 1160-1169. https://doi.org/10.1002/elan.201900600.

(90) Scavetta, E.; Mignani, A.; Prandstraller, D.; Tonelli, D. Electrosynthesis of Thin Films of Ni, Al Hydrotalcite Like Compounds. Chem. Mater. 2007, 19 (18), 4523-4529. https://doi.org/10.1021/cm071132v.

(91) Basile, F.; Benito, P.; Fornasari, G.; Monti, M.; Scavetta, E.; Tonelli, D.; Vaccari, A. Novel Rh-Based Structured Catalysts for the Catalytic Partial Oxidation of Methane. Catal. Today 2010, 157 (1-4), 183-190. https://doi.Org/10.1016/j.cattod.2010.04.039.

(92) Musella, E.; Gualandi, I.; Scavetta, E.; Gazzano, M.; Rivalta, A.; Venuti, E.; Christian, M.; Morandi, V.; Tonelli, D. Electrochemical Approach for the Production of Layered Double Hydroxides with a Well-Defined Co/Me¹" Ratio. Chem. - A Eur. J. 2019, 25 (71 ), 16301-16310. https://d0i.0rg/l 0.1002/chem.201903288.

(93) Klug, H. P.; Alexander, L. E. X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials. 2^nd Ed. John Wiley Sons 1974, 618-708.

(94) Miyata, S. Anion-Exchange Properties of Hydrotalcite-Like Compounds. Clays Clay Miner. 1983, 31 (4), 305-311. https://doi.org/10.1346/CCMN.1983.0310409.

(95) Marappa, S.; Radha, S.; Kamath, P. V. Nitrate-Intercalated Layered Double Hydroxides - Structure Model, Order, and Disorder. Eur. J. Inorg. Chem. 2013, 2013 (12), 2122-2128. https://doi.org/10.1002/ejic.201201405.

(96) Monteiro, M. C. O.; Dattila, F.; Hagedoorn, B.; Garcfa-Muelas, R.; Lopez, N.; Koper, M. T. M. Absence of CO2 Electroreduction on Copper, Gold and Silver Electrodes without Metal Cations in Solution. Nat. Catal. 2021 , 4 (8), 654-662. https://doi.org/10.1038/s41929-021 -00655-5.

(97) Lu, S.; Wang, Y.; Xiang, H.; Lei, H.; Xu, B. Bin; Xing, L.; Yu, E. H.; Liu, T. X. Mass Transfer Effect to Electrochemical Reduction of CO2: Electrode, Electrocatalyst and Electrolyte. J. Energy Storage 2022, 52, 104764. https://doi.Org/10.1016/j.est.2022.104764.

(98) Jia, M.; Zhang, Y.; Bao, Y.; Wang, J.; Xu, A. Recyclable CuMgAI Hydrotalcite for Oxidative Esterification of Aldehydes with Alkylbenzenes. Green Chem. Lett. Rev. 2018, 11 (3), 230-236. https://doi.Org/10.1080/17518253.2018.1470681 .

(99) Hadj-Abdelkader, N. E. H.; Beltrao-Nunes, A.-P.; Belkhadem, F.; Benselka, N.; Roy, R.; Azzouz, A. New Insights in MgAI and MgFe-LDH Affinity towards Carbon Dioxide - Role of the Hydrophilic Character on CO2 Retention Strength. Appl. Clay Sci. 2020, 198, 105829. https://d0i.0rg/l 0.1016/j .clay.2020.105829.

(100) Goodman, E. D.; Schwalbe, J. A.; Cargnello, M. Mechanistic Understanding and the Rational Design of Sinter-Resistant Heterogeneous Catalysts. ACS Catal. 2017, 7 (10), 7156-7173. https://d0i.0rg/l 0.1021 /acscatal.7b01975.

(101) Wu, L.; Kolmeijer, K. E.; Zhang, Y.; An, H.; Arnouts, S.; Bals, S.; Altantzis, T.; Hofmann, J. P.; Costa Figueiredo, M.; Hensen, E. J. M.; Weckhuysen, B. M.; van der Stam, W. Stabilization Effects in Binary Colloidal Cu and Ag Nanoparticle Electrodes under Electrochemical CO2 Reduction Conditions. Nanoscale 2021 , 13 (9), 4835-4844. https://doi.org/10.1039/D0NR09040A.

Claims

1 . Nanostructured composite material with a layered hydrotalcite-type ternary structure of CuM- gAI LDH (layered double hydroxide) comprising particles of metal copper Cu° and particles containing cuprous ion Cu⁺ (such as cuprous oxide), as a redox couple Cu°/Cu⁺, preferably placed in intimate contact with the layered hydrotalcite-type structure.

2. Nanostructured composite material according to claim 1 , wherein metal copper, Cu°, preferably as micro/nano particles and cuprous ion Cu⁺, preferably as micro/nano particles containing cuprous ion Cu⁺, more preferably as micro/nanoparticles of CU2O, are interleaved with or overlapped on or interconnected to such layers, in other words placed into direct contact with such layers.

3. Process of preparing a nanostructured composite material according to claim 1 , said process comprising a potentiodynamic electrodeposition, such as cyclic voltammetry, in which a triangular wave potential is applied to a stationary electrode dipped in an unstirred electrodeposition solution of soluble salts of cations Cu²⁺, Mg²⁺, and Al³⁺.

4 Process according to claim 3, wherein the potential is varied from 0.0 V to -1 .4 V vs SCE (Saturated Calomel Electrode), with a scan rate comprised between 5 and 50 mVs ¹ , preferably between 10 and 40 mVs^-1 , more preferably between 15 and 30 mVs^-1 , the most preferred being 30 rnVs^-1.

5. Process according to claim 3, wherein the molar ratio between the sum of divalent cations (Cu²⁺ and Mg²⁺) M(ll) and the trivalent cation (Al³⁺) M(lll), defined as the ratio M(ll)/ M(lll), is 3:1 and/or where the molar ratio among different cations (Cu²⁺ : Mg²⁺ : Al³⁺) is 2:1 :1 , in the electrodeposition solution.

6. Catalytic system comprising the composite nanostructured material according to claim 1 , as a continuous or discontinuous coating in the form of layer or particle/s or of layer and particle/s, on a support made of a chemically inert material compared with the composite nanostructured material itself, and/or thermally and/or electrically inert, or insulating compared with the composite nanostructured material itself.

7. Catalytic system comprising the composite nanostructured material according to claim 1 as a continuous or discontinuous coating in the form of layer or particle/s or layer and particle/s, on a support made of a thermally and/or electrically conductive material.

8. Working electrode comprising the composite nanostructured material according to claim 1 as continuous or discontinuous coating, in the form of layer or particle/s or layer and particle/s, on a support made of an electrically conductive material.

9. Working electrode according to claim 8, wherein the electrically conductive support is a gas diffusion membrane made of carbon, preferably of carbon fiber.

10. Process for the electrochemical CO2 reduction into C2 compounds, preferably into acetic acid, wherein the working electrode, on which CO2 is reduced, is the working electrode according to claims 8 and 9.