ES2702082T3 - An electrochemical process for preparing a reaction product of a metal or metalloid element - Google Patents

Abstract

An electrochemical process for recovering reaction products of a metallic element or a metalloid element or a mixture of two or more of these from at least one water-soluble precursor compound comprising the metallic element or a metalloid element or two or more of these, in the form of nanoparticles, particularly nano crystals of at least one reaction product, wherein the process comprises the steps of - supplying a solution of the water-soluble precursor compound to a water-based catholyte from a cathodic compartment of a cell electrochemistry, equipped with a cathode comprising a gas diffusion electrode, wherein the gas diffusion electrode comprises an electrochemically active porous material having a BET surface of at least 50 m2 / g, - adjust the pH of the catholyte to a pH that is less than the pKa of the water-soluble precursor compound, - supplying at least one oxidizing gas to the gas diffusion electrode, - subjecting the cathode to a p Electrochemical potential for reduction of at least one oxidizing gas to one or more of the corresponding peroxides, ionic reactive species and / or radicals capable of reacting with a cation comprising the metallic element, or the metalloid element or a mixture of two or more of these, to nano particles of at least one reaction product with an average particle size, particularly nano crystals with an average crystallite size, equal to or less than 30.0 nm.

Description

DESCRIPTION

An electrochemical process for preparing a reaction product of a metal or metalloid element

The present invention relates to an electrochemical process for recovering reaction products of a metal element or a metalloid element or a mixture of two or more of these from at least one water-soluble precursor compound comprising the metal element or a metalloid element or two. or more of these, in the form of nanoparticles of at least one reaction product, according to the preamble of the first claim.

The present invention also relates to nanoparticles, particularly nano-crystals obtained with that process and with a device for carrying out the process of the invention.

BACKGROUND OF THE INVENTION

The nanoparticles and their compounds exhibit electronic, optical, magnetic and non-conventional chemical properties with respect to the massive phase particles and the macroscopic crystals. Therefore, they offer new or improved properties for their application in a wide variety of fields ranging from catalysis, cosmetics, textiles, nanoelectronics, high-tech components and protective devices, to pharmaceuticals, medical uses, sensors and diagnostics. In the smaller sizes (for example <20-50 nm), the properties of the nanoparticles vary typically irregularly and are specific for each size (in Rao C.N.R., Thomas P.J., Kulkami G.U., Nano crystals: Synthesis, Properties and Applications). Regardless of the method used for its preparation, there are many challenges to overcome, such as the control of particle growth, crystallinity, stability and reproducibility. A high quality synthesis process should conveniently produce nanoparticles with a controlled size distribution, often aspiring to a narrow size distribution. The narrower the size distribution, the more attractive the synthesis procedure will be. The best synthesis procedures currently available produce nanocrystals with a size distribution of approximately 5%. The control of the form is also an important feature. Synthesis methods that provide crystalline nanoparticles are preferred, as well as methods that provide shape stabilization. Particularly, synthesis methods that do not use hazardous solvents are preferred, with the idea of environmental sustainability. Modern methods for synthesizing amorphous or crystalline nanoparticles can include chemical reaction stages, as well as biological and physical treatment stages. Chemical methods for producing crystalline nanoparticles offer the advantage over physical methods that milder reaction conditions can be used. Better control can be achieved, compared to purely biological methods. Chemical methods typically employ the steps of planting crystals, allowing particle growth to take place and finalizing particle growth once the desired particle size has been obtained. Since these steps are often inseparable, synthesis is often initiated by providing a precursor of nanocrystals, a solvent and terminating agents. Electrochemical synthesis is frequently used for the production of zero-valent metal nanocrystals, through the stages of oxidative dissolution of an anode, migration of metal ions to the cathode and reduction to the zero-valence state, nucleation followed by particle growth, addition of agents of finishing (typically quaternary ammonium salts containing long chain alkanes) to inhibit the growth, and precipitation of the nanocrystals. The size of the nanocrystals can be adjusted, inter alia, by altering the current density, the variation of the distance between the electrodes, the control of the reaction time, the temperature and the polarity of the solvent. Classical chemical and electrochemical methods typically result in the formation of nanocrystals having an average particle size of about 100 nm. In some cases, the formation of nanocrystals with an average particle size of less than 100 nm has been observed, however, they present highly polydispersed (non-uniform) size and shape distributions.

US20060068026 discloses a thermal electrochemical method for preparing a stable colloidal suspension of bare metal nanocrystals or alloys with zero valent metals. The method comprises the steps of, at least partially, submerging in water essentially free of contaminants, a sacrificial metal anode that includes a metal reactant essentially free of contaminants for the nanocrystals and a cathode; and applying a voltage potential across the anode and the cathode to form a colloidally stable suspension of bare metal nanocrystals composed essentially of metallic sacrificial metal anode.

In analyzing the existing methods for synthesizing nanoparticles, the inventors realized that existing techniques can be considered from a different perspective. The chemical precursors of the nanoparticles are usually present in the dissolved state in the solution to be treated, for example, the chemical precursors are dissolved in an aqueous matrix. The formation of the nanoparticles and their conversion into a stable solid precipitate has the consequence that the water-soluble ions are removed from the aqueous matrix. Therefore, the nanoparticle synthesis method can also be considered as a method for removing water-soluble compounds from a solution and recovering them, for example, as a solid precipitate.

This is of special interest in the field of recovery of critical mineral raw materials, especially those with a high technological interest, such as rare earth elements (REE) that are used in the manufacture of electronic and telecommunications devices and high-performance applications. technology, clean and strategic energy technologies and defense instruments, to name a few examples. REEs are classified as critical raw materials not only because of their broad applicability, but mainly because of the risk of interruption of supply and probably also because of their high economic value. A key measure to anticipate the vulnerabilities of the REE supply is the recycling of products at the end of their useful life; however, recycling possibilities are far from sufficient to meet the demand of REE. As the risk of supply disruption and the value of REE increases, other matrices not yet explored begin to make economic sense for recovery.

WO 2012115273 A1 discloses a method for the extraction and separation of lanthanoid and actinoid elements by contacting a solution of these elements with a nanostructure carrying an adsorbent compound with metals, capable of functioning as an adsorbent for the target metal. To recover the target metal, the adsorbent compound with the metal adsorbed thereto is contacted with a back-extraction solution.

Another method for removing ionic species from fluids, for example from decomposed supplies of water, which makes use of capacitive deionization is described in US2011042219. The method described in US2011042219 employs an electrodialysis system and / or an electrodialysis reversal system using high surface, porous, non-Faraday electrodes, which do not perform electron transfer reactions. The system contains a membrane stack that includes transfer membranes with the cathode and alternating transfer membranes with the anode, as well as a porous cathode and a porous anode. As the direct current energy passes through the electrodes, the cations and anions migrate to opposite electrodes, thus causing a separation of the salt water in concentrated and diluted streamlines. A double-layer capacitor with a high apparent capacitance can thus be formed at each electrode. The method is typically applicable in industries where liquids may require the removal of ionic species, which includes the water, pharmaceutical, and food and beverage industries.

However, the methods described above do not provide true and economically viable recovery rates, when possible in a way that allows the metal to be reused. The existing extraction methods for the extraction of REE or other critical metals from aqueous matrices, for example, to meet the regulatory requirements, are insufficient and need to be adapted to provide a commercially interesting product.

US2015 / 0200082 A1 discloses a method of manufacturing metal hydroxide particles, such as indium, tin, copper, gallium, zinc, aluminum, iron, nickel, manganese and lithium, or an alloy containing at least one type selected from These metals, where the metal particles have a uniform particle diameter of 100 nm. A gas diffusion electrode is immersed in an electrolyte, for example ammonium nitrate, the anode is defined by the metal or the conductive metal oxide. The oxygen supplied to the cathode is reduced at the gas-liquid interface, thus generating hydroxide ions (O2 + 2H2O + 4e- ^ 4OH-), which causes the pH to increase as the process continues. The standard potential of the electrode (+0.40 V) of the oxygen reduction reaction is higher than the standard potential of the electrode (+0.01 V) of the nitrate ion reduction reaction, the reduction of nitrate ions occurs rarely. The metal ions that elute from the anode during electrolysis react with the hydroxide ions in the electrolytic solution to form metal hydroxide particles. The alkaline conditions in which the reaction is carried out cause the diameters of the particles to increase, and obtaining metal hydroxides as particles with a uniform desired particle diameter is difficult to achieve. If desired, the pH and the temperature of the electrolyte can be stabilized. When analyzing the morphology of the particles, it appeared that each particle consisted of a plurality of smaller particles added together to form a larger particle.

US-A-3,073,763 discloses a method for the electrolytic production of iron oxides or mixtures of metal oxides containing predominantly iron oxide, by the use of an iron anode and one or more metallic anodes and a cathode which consists of a conductive material, for example, metal or carbon. As an electrolyte, an alkaline saline solution is used, whose anions do not form insoluble salts with the metals of the anodes and have a greater affinity with hydrogen than with iron. In the course of the process, the metals at the anode will dissolve and come into contact with the alkaline medium that has formed adjacent to the cathode. If oxygen is supplied, the metal ions are converted into the corresponding oxides in the electrolyte solution, which precipitate in solution as particles having a large average particle size of the order of 100 nm or more. In the process of US-A-3,073,763, no oxygen reduction takes place.

US-A-4,067,788 discloses a method for producing finely divided metal powders, particularly powders of nickel, silver, gold and metals of the platinum group, having particles ranging in size from about 0.5 to about 10 microns. The anode is made of metal that is to be finely divided as oxide, hydroxide or metal itself. The anode is immersed in an alkaline solution of a salt, which has a typical pH between 8 and 12, the anion of the salt which is soluble when combined with said metal. The cathode is a catalyzed air electrode. Since the metal ions produced at the anode surface do not react immediately with the hydroxyl ions in the solution, and since the insoluble oxides or hydroxides are formed in the solution at a perceptible distance from the anode surface, the particle size can controlled by recirculation of the electrolyte. To further increase the particle size of the oxide or hydroxide, a portion of the solid material is returned to the cell in which the process is carried out to serve as a core on which additional oxide or hydroxide is deposited. The final step in the process is the chemical reduction of the metal oxide or metal hydroxide in the finely divided metal, for example by the use of formaldehyde or invert sugar.

UA-S-6,235,185 discloses a method and apparatus for producing particles of a metal compound, such as a metal oxide or a metal hydroxide, with particle diameters of not more than 1 micron, particularly from 1 to 1000 nm , preferably from 5 to 100 nm. The term "particle diameter" in the present description means the volume average particle diameter of 200 or more particles observed by a transmission electron microscope and a clear particle size distribution, from a less expensive raw material and with the use of a simplified production facility. For this purpose, an electrolytic solution containing metal ions is provided as raw material for the particles of ultrathin metal compounds in the anode chamber, an alkaline electrolytic solution is provided in the cathode chamber. A voltage is applied between the anode and the cathode to induce the transfer of the metal ions from the anodic side chamber to the cathode side chamber through an ion exchange layer. The particles of ultrafine metal compounds are precipitated in the alkaline electrolytic solution of the cathodic chamber. Examples of suitable metals for the method of US-A-6,235,185 include lithium, magnesium, aluminum, calcium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, rubidium , strontium, yttrium, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, cadmium, indium, tin, antimony, tellurium, cesium, barium, tantalum, tungsten, osmium, platinum, gold, mercury, lead, bismuth, cerium, neodymium , samarium and europium, or their combinations.

Document US2004 / 0108220 A1 describes a process for the production of mixed amorphous and / or crystalline oxides of metals with diameters of average particles ranging from 1 to 500 nm, more particularly metals from the third to the fifth main group or from the secondary groups of the periodic system. The ions dissolved in an organic electrolyte of those metals whose oxide (mixed) must be produced in the electrolyte of the cathode compartment are electrochemically reduced at the cathode in the presence of air as an oxidizing agent. The cathode compartment is separated from the anode compartment by a porous partition diaphragm which is permeable to cations and anions, but prevents the passage of elemental halogen from the anode to the cathode compartment. Preferred organic electrolytes include alcohols, polyalcohols or their mixtures and derivatives, ketones, ethers, nitrites, organic carbonates and aromatics, tetrahydrofuran, acetone, acetonitrile, toluene. The organic electrolyte may contain small amounts of water, for example, from about 0.01 to about 2% by weight. The air that is introduced in the form of small bubbles, on the one hand, provides a fine distribution of the oxidizing agent in the cathode compartment and, on the other hand, counteracts the formation of solid metal oxide at the cathode by providing a constant mixture of the electrolyte and, instead, it ensures that the metal oxide particles are discharged from the cathode and dispersed in the electrolyte.

Although in US2004 / 0108220 A1 it is said to produce mixed amorphous and / or crystalline oxide particles of metals with average particle diameters ranging from 1 to 500 nm, the examples do not reveal the actual particle size obtained. In addition, the process involves the use of an organic electrolyte and, therefore, is not suitable, for example, for recovering metal ions from aqueous solutions.

WO2013 / 179553 describes a method of manufacturing an ITO target and a metal hydroxide. While the use of the cathode is made of gas diffusion electrode. An anode is made of the metallic material to be converted into hydroxide (page 2, par. 0019 last sentence) or an electrically conductive metal hydroxide, immersed in an electrolytic solution. Examples of electrolyte solutions used include ammonium nitrate, ammonium chloride, etc.

US-A-3,073,763 describes an electrolytic process for preparing iron oxides or mixtures of metal oxides containing more than 80 mole percent iron oxide. The anode used is made of metals whose oxides must precipitate and / or anodes of a conductive material that, during electrolysis, do not dissolve, but are surrounded by one or more oxides of the metals that must precipitate. Care is taken that the metals will dissolve in the desired proportion and will precipitate in the form of their oxides by the alkali formed during electrolysis, before the metal ions reach the cathode. The cathode is arranged so that a strong alkaline medium is formed around the cathode to achieve precipitation of the metal ions in the form of their oxides before they can reach the cathode. The cathode is constructed in the form of a tube that is drilled below the level of the electrolyte. The anodes are separated by insulating partitions that do not extend completely to the cathode. As an electrolyte, an alkaline saline solution with a higher affinity with hydrogen than with iron is used, by dissolving the material from which the anode is made and adding an oxidizing gas in the bath. US-A-4,067,788 discloses a method for producing finely divided metal powders, metal hydroxides and metal oxides, particularly powders of nickel, silver, gold and the group of platinum metals, which have particles of a varying size. from about 0.5 microns to about 10 microns. The anode, which is made of the metal that must be converted into a finely divided powder, is consumed in the course of the process. The anode is immersed in an alkaline solution of a salt, with a typical pH between 8 and 12. The cathode is a catalyzed air electrode. Air is circulated through the cathode chamber, oxygen is converted to hydroxyl ions and hydrogen is also produced.

The need remains for an economically viable electrochemical process that allows recovering from a water-soluble precursor compound containing a metal or metalloid element or two or more of these, a reaction product containing the metal or metalloid element or two or more of these . There remains a particular need for an economically viable process that allows recovering such metal or metalloid elements in the form of crystalline nanoparticles of a compound or of a mixture of two or more compounds containing at least one metal or metalloid element, wherein the nanoparticles crystallites have an average particle diameter of 50 nm or less, preferably 30 nm or less, more preferably 20 nm or less, most preferably 10 nm or less, or even 5 nm or less. More particularly, there remains a need for an electrochemical process that allows to produce such crystalline particles with a controlled particle size distribution, preferably a narrow particle size distribution or a particle size distribution called monodisperse. This process must be suitable for the recovery of the metal or metalloid elements commonly used, as well as those that are commonly used less.

The present invention therefore aims to provide an economically viable method for isolating at least one water soluble precursor compound comprising a metal or a metalloid element or two or more of these, nanoparticles, particularly crystalline nanoparticles, of at least one reaction product comprising a metal or a metalloid element or two or more thereof.

This is achieved by the present invention with a method that shows the technical characteristics of the characterizing portion of the first claim.

For this the electrochemical process of this invention comprises the steps of

- supplying a solution of the water-soluble precursor compound to a water-based catholyte of a cathodic compartment of an electrochemical cell, equipped with a cathode comprising a gas diffusion electrode, wherein the gas diffusion electrode comprises a material electrochemically active porous having a BET surface area of at least 50 m2 / g,

- adjust the pH of the catholyte to a pH that is lower than the pKa of the water soluble precursor compound,

- supplying at least one oxidizing gas to the gas diffusion electrode,

- subjecting the cathode to an electrochemical potential to cause reduction of at least one oxidizing gas in one or more of the peroxides, ionic reactive species and / or corresponding radicals capable of reacting with a cation comprising the metal element, or the metalloid element or a mixture of two or more of these, to form nanoparticles, particularly nano crystals, of at least one reaction product with an average particle size, particularly, with an average crystallite size of 30.0 nm or less.

The inventors have observed that subjecting the cathode to an electrochemical potential which is chosen to be capable of causing the reduction of an oxidizing gas supplied to it, allows to induce a redox transformation, on the electrochemically active surface of the gas diffusion electrode of the cathode, in other words, on the electrochemically active surface of the cathode. This redox transformation involves, on the one hand, a reduction of the oxidizing gas and, on the other hand, an in situ oxidation of the metal and / or metalloid element contained in the water-soluble precursor compound, to a higher oxidation state. This is surprising, since the skilled person would typically expect a reduction reaction of the metal and / or metalloid element to occur at the cathode. The assumption that in situ oxidation of at least one metallic and / or metalloid element takes place is supported by the observation that the conductivity of the catholyte decreases with an increasing degree of recovery of metal or metalloid ions from the aqueous solution, i.e. , a growing formation of nanoparticles, particularly of nano crystals of the reaction product. The inventors have further observed that the at least one metal and / or oxidized metalloid element having a positive valence is adsorbed at least temporarily to the electrochemically active surface of the cathode, and thereby form an interface with the catholyte.

The electrochemical potential or range of electrochemical potentials in which oxidative gas reduction can occur is well known to the experienced person and is represented by the Nernst equation. The Nernst equation is an equation that relates the reduction potential of a half cell or the total voltage at any time with the potential of the standard electrode, the temperature, the activity and the reaction ratio of the underlying reactions and the species used. The Nernst equation can be written as follows:

^{E = E ° + -} zF ^{In ^} ( R ^z ed or ^{E = E ° -} z ¥ ^biQ

where

- E is the reduction potential of the half cell at the temperature of interest

- E0 is the standard potential for reducing the half cell

- R is the universal gas constant, R = 8.314472 (15) J K-1 mol-1

- T is the absolute temperature

- a is the chemical activity for the species in question, where aRed is the reducing agent and aox is the oxidizing agent. ax = yxcx, where yx is the activity coefficient of species X. (Since the activity coefficients tend to unity at low concentrations, the activities in the Nernst equation are often replaced by simple concentrations.)

- F is the Faraday constant, the number of coulombs per mole of electrons: F = 9.64853399 (24) * 104 C mol-1 - z is the number of moles of electrons transferred in the cell reaction or half reaction

- Qr is the reaction quotient.

At room temperature (25 ° C), RT / F can be treated as a constant and replaced by 25,693 mV for the cells.

The Nernst equation is often expressed in terms of base 10 logarithms ( ie common logarithms) instead of natural logarithms, in which case it is written as follows for a cell at 25 ° C:

This equation can also be written according to the pH, as follows:

Where m represents the stoichiometric coefficient according to the reaction below:

(See Pourbaix M (1976) Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE Cebelcor, Houston, Tx.

The inventors believe that the redox transformation takes place on the active surface of the electrochemically active layer of the gas diffusion electrode, ie the outer surface of the porous material forming the electrochemically active layer, as well as on the inner surface of the material electrochemically active or, in other words, on the active surface inside the pores of the electrochemically active porous material. The fact that the process also occurs within the pores of the electrochemically active material has been observed when the electrical polarization is stopped, by the release of most of the nanoparticles, particularly the nano crystals in the electrolyte when the electric polarization is stopped .

Without wishing to be circumscribed to this theory, the inventors assume that the reduction of oxidizing gas at the cathode can result in the formation of one or more peroxide, ionic and / or radical species, usually polyatomic species, which adsorb to the surface electrochemically active cathode and create there a local environment confined within the pores of electrochemically active material. Within the pores of the electrochemically active material, a characteristic pH and a redox potential can develop, which are different from those of the solution and the outer surface of the electrode. Supersaturation of the local environment with these species may occur.

The inventors further believe that at least one water soluble precursor compound is dissolved in the electrolyte, particularly in the catholyte, in an at least partially dissociated state:

with M + representing a cation of the metal element or metalloid element or a compound having a positive valence comprising at least one metal element or metalloid element, or a mixture of two or more of the species mentioned above. It is noteworthy that at least one metal or metalloid ion or compound comprising at least one metal element or metalloid element can have a positive valence of 1, 2, 3 or any other valence that can be formed for that species. The cations of the soluble water-soluble precursor compound contained in the aqueous electrolyte can migrate or diffuse from the electrolyte solution to the cathode, adhere or adsorb to the external surface of the electrochemically active material and adsorb within the pores of the electrochemically active material, particularly at the active sites of this. Adhesion of cations positively charged to the electrochemically active surface of the gas diffusion electrode It can take place through several mechanisms, and can involve adsorption forces, mainly Van der Waals forces. In addition, a double electric layer can be formed on the active surface that involves the cations, thus allowing electrostatic adsorption or capacitive adsorption in the double layer, but reversible adsorption by ion exchange, complexation or chelation can also take place. any other form of adhesion or adsorption.

The inventors further believe that at least part of the functional groups present on the surface of the electrochemically active material will be present in an at least partially dissociated state (C * -R), especially when an electrochemical potential is applied to the electrode. These dissociated C * -R sites carrying an electrical charge can form ion exchange sites by the positively charged metal or metalloid ion. The surface of the electrochemically active layer can, for example, comprise sites of weak protonic acid in the form (C * -RH), which can be dissociated into C * -R and H +, where C * represents an active site in the electrochemically active layer of the cathode.

In the presence of an oxidizing gas such as oxygen or any other oxidizing gas, the availability of the active sites in the dissociated state at the electrochemically active surface can be accelerated and a local environment can be created with extreme redox and pH conditions, which are different from those found in most of the electrolyte solution and on the bare surface of the electrode:

One or more positively charged ions, i.e., metal or metalloid ions or a compound comprising one or more of the metal or metalloid elements, can then be adsorbed directly to a C * -R site or a reduced species of the oxidizing gas, e.g. , to a peroxide radical, to an ionic species or to another radical species, the peroxide radical is the most active species. Thus, a polymetallic ion (polyoxy radical of polymetallic ion) or a polyoxy radical of polymetaloid ion can be formed, which once adsorbed to an electrochemically active site and its supersaturation of such a site can on the surface of the electrochemically active material of the cathode , act as a nucleation site for the formation of an oxidized compound comprising at least one metal ion and / or at least one metalloid ion.

For example, in the case of the cerium ion (Ce3 +), this can lead to the following reactions:

The electrochemically active surface of the gas diffusion electrode and the cathode include the external surface as well as the active surface present in the pores of the electrochemically active material. In conventional gas diffusion electrodes using an electrochemically active porous material, particularly an electrochemically active porous material based on carbon, the active surface present in the pores provides by far the largest part of the active surface, since the porous material usually it will have a high BET surface area of at least 50 m2 / g. Thus, the electrochemical process of this invention, ie the formation of nanoparticles, particularly nano crystals of the reaction product, will take place mainly in the pores of the electrochemically active material, in which conditions of supersaturation of the polyatomic or radical ions may exist which are formed during the reduction of the oxidizing gas. As a result, the formation and growth of particles of the reaction product in the pores of the electrochemically active porous material will be limited by the dimensions of those pores. Depending on the nature of the porous material, the average diameter of the nanoparticles, particularly of the nano crystals, will be limited to a maximum of 5 nm. A specific choice of the electrochemically active material, particularly the pore size distribution of the electrochemically active material represented by the BET surface of the porous material, thus makes it possible to control the size of the nanoparticles, particularly the nano crystals of the reaction product, and thus the particle size of the reaction product.

The electrochemical potential to which the gas diffusion cathode is subjected is a reducing potential in relation to a reference electrode, preferably below the pH-potential thermodynamic equilibrium region of the stability of the oxidant gas in water, more preferably below the region of thermodynamic stability of water, but preferably not within the region of thermodynamic stability of hydrogen. In this way, the risk of electrolysis of water to form hydrogen is minimized. However, in certain embodiments, hydrogen formation may be preferred, for example, if the specific application of the nano formed particles, particularly nano crystals, is related to the capture or in-situ storage of hydrogen.

The inventors have observed that the oxidized metal and / or the metalloid cations formed in the process of this invention and schematized in the reaction schemes above, accumulate at the interface in the pores of the electrochemically active material, in a physical state that is different of the physical state of the surrounding liquid electrolyte, so that they can be separated from it. The reaction product can, for example, accumulate as particles of a product that forms a solid or colloidal phase. Depending on the nature of the metal or metalloid cation, the reaction product can, for example, accumulate at the interface in the form of crystalline or amorphous nanoparticles, which can grow over time to take up a larger size as the process progresses and forming a physical state different from the physical state of the electrolyte, so that the nanoparticles of the reaction product can be isolated from the cathode and the electrolyte. The reaction product can be released in a variety of physical forms, for example in the form of a precipitate, or in the form of colloidal nanoparticles, for example in the form of a colloidal dispersion. After being released into the electrolyte, the particles can be added further to form a stable solid phase, a separable precipitate, a gelatinous, frothy phase or a similar one to a Pickering emulsion.

Since the nature of the forces with which the reaction product comprising at least one metal element and / or metalloid element in the oxidized state adhere or adsorb to the electrochemically active surface of the gas diffusion electrode may vary with nature of that electrochemically active material and the nature of the oxidized compound, the release of the reaction product in the form of nanoparticles, particularly nano crystals, in the electrolyte can occur as such, or it may be necessary to force it.

Metal and metalloid elements can take various oxidation states and can form, with the resulting species of oxidizing gas reduction, reaction products or compounds containing one or more monoatomic or polyatomic ions, in an oxidation state that results in a phase that can be separated from the catholyte and the cathode. The person skilled in the art will be able to identify those oxidized compounds that form a separable phase in a water-based electrolyte, and select the appropriate electrical potential (or redox) and pH for the catholyte to allow the formation of a separable phase. Pourbaix "Atlas of Electrochemical Equilibria in Aqueous Solutions", second edition 1974 describes the solubility and stability as ions or solid compounds of various metals and their oxides as a function of voltage potential and pH, under standard conditions. Theoretical diagrams for a wide variety of species can be constructed without undue difficulty based on the premises provided in them, including those relating to oxidizing gases other than oxygen. The person with experience is able to identify the electrochemical potential in which the electrochemical reduction of the oxidizing gas can occur, and the corresponding oxidation of the metal cation or metalloid cation. This is illustrated in Figure 18, which shows the electrochemical series of some oxidizing gases.

It has been observed that the variation of the electrical or electrochemical potential at the cathode, the chemical composition of the reaction product can be controlled. Without wishing to be limited to this theory, the inventors assume that the ionic or radical species of the oxidizing gas can diffuse onto the charged electrochemically active surface and be grouped with other similar species, for example, peroxide radicals, by adhesion to the active surface of the cathode.

This grouping can lead to local supersaturation at the active surface of the electrochemically active material, which includes the active surface within the pores of the electrochemically active material, and the growth of the ionic or radical species present on that electrochemically active surface (e.g. , peroxide on the surface) in critical cores, unlike the formation of crystals in the electrolytic solution known from the prior art.

A dedicated selection of the pore size distribution of the electrochemically active material makes it possible to obtain the nano crystals of the reaction product in a selected crystalline form, and to affect its framework parameters. Particularly, when an electrochemically active material with a fairly uniform pore size distribution is used, nano crystals of the reaction product can be obtained which are monodisperse, or which at least have a small particle size distribution curve. Inside of the Scope of the present invention with monodisperses are denoted, particles where 75 % of the particles have a particle size that differs by 5% maximum, preferably 3% maximum, most preferably 1% maximum.

Although the electrochemically active surface of the cathode may contain adsorbed reactive radicals and / or adsorbed oxidant gas, and although the water-based electrolyte may contain some dissolved oxidant gas, this usually will not be sufficient to ensure complete recovery of all metal ions. or metalloids present in the electrolyte. The supply of an oxidizing gas to the cathode, particularly to the gas diffusion electrode, may therefore be preferable to ensure maximum recovery of the reaction product comprising at least one metal or metalloid element in the oxidized state, from the soluble precursor compound in water dissolved in the water-based electrolyte and optimize the reaction rate. In practice, an oxidizing gas supplied to the gas diffusion electrode will migrate from the hydrophobic side of the gas diffusion electrode, through the gas chamber of the gas diffusion electrode, into, into and through the electrochemically active material of the gas diffusion electrode. gas and cathode diffusion electrode.

Examples of oxidizing gases suitable for use with this invention include inorganic oxidizing gases. Example of inorganic gases suitable for use with this invention include ozone, oxygen, carbon dioxide gases for example CO2, nitrogen oxides for example NO, N2O3, halogen gases, halogen oxide gases, sulfur oxide gases, air , biogas, combustion gas, acid gas and combustion exhaust gases and mixtures or two or more of the gases mentioned above.

Preferably, however, air is used due to economic reasons. Other oxidizing gases particularly suitable for use with this invention include those capable of forming oxidizing monoatomic radicals and / or polyatomic oxidizing radicals, for example oxygen, ozone, carbon dioxide etc.

Particularly preferred oxidizing gases are those that can be reduced to generate polyatomic ions, polyatomic radicals or polyatomic peroxides, for example those summarized in the table below:

perchlorate ClO4-1 Hydrosulfate HSO4-1 Hydrophosphate HPO4-2 Chlorate ClO3-1 Dihydrofosphate H2PO4-1 Peroxide O2-2 Chloride ClO2-1 Permanganate MnO4-1 Tetraborate B4O7-2 Hypochlorite ClO-1 Periodate IO4-1 Borate BO3-3 Nitrate NO3 -1 HCO3-1 hydrocarbonate

Nitrite NO2-1 sulfate SO4-2

bromate BrO3-1 sulphite SO3-2

Iodine IO3-1 carbonate CO3-2

The at least one oxidizing gas is preferably selected so as to generate one or more of the preferred polyatomic ions, particularly one or more of the polyatomic ions selected from the group of acetate (CH3COO-), acetylide (C22-), carbonate (CO32) -), peroxide (O22-), phosphate (PO43-), sulfate (SO42-), nitrate (NO3-).

The at least one oxidizing gas can be used as such or in a mixture with one or more inert gases, for example N2, Ar or He or a mixture of two or more of these gases.

The partial pressure of at least one oxidizing gas within the gas mixture is not critical to the invention and can vary within wide ranges. Even so, the variation of the partial pressure of the oxidizing gas or the rate of supply of oxidizing gas will make it possible to control the size of the crystals of the reaction product containing at least one metallic and / or metalloid element. In addition, the variation of the partial pressure of the oxidizing gas, in particular the increase or decrease in the partial pressure of at least one oxidizing gas, will also make it possible to control, in particular, the increase or reduction of the average parameter of the lattice of the crystals of the reaction product. , as measured by X-ray diffraction measured in a given crystallographic plane or by the use of transmission electron microscopy imaging. The variation of the partial pressure of the oxidizing gas or the rate of supply of the oxidizing gas will especially affect the amount of oxidizing gas that can accumulate on the electrochemically active surface, particularly by the amount of oxidizing gas present on the active surface within the pores. of the electrochemically active porous material. This quantity of gas can in turn be controlled by controlling the rate of gas supply to the gas chamber of the gas diffusion electrode, and controlling the concentration of the oxidizing gas in the gas supplied to the gas chamber. gases of the gas diffusion electrode. When air is used as the oxidizing gas, the flow rate will usually vary between 100 and 400 ml / min, preferably between 200 and 400 ml / min.

The concentration of the water-soluble precursor compound in the electrolyte is not critical to the invention. However, the concentration of the water-soluble compound is preferably at least 100 ppm. Usually, the concentration of the water soluble compound will not be higher than 10 g / liter, since the efficiency of the process can hardly be improved at higher concentrations. Therefore, the concentration of the water-soluble compound in the catholyte is preferably between 100 and 500 ppm. The concentration of the water-soluble precursor in the catholyte can, for example, be varied by varying the rate at which the water-soluble precursor is supplied to the catholyte. This variation will be carried out particularly when the variation of the particle size, particularly the size of the crystals of the reaction product, is foreseen, since the variation of the concentration of the water-soluble precursor makes it possible to vary the particle size, particularly the size of the particles. crystals of the reaction product. The inventors have observed that the particle size, particularly the size of the crystals, may increase as the concentration of the precursor increases or that the particle size, particularly the size of the crystals, may decrease as the concentration of the precursor decreases.

The nature of the compound that can be recovered from at least one water soluble precursor compound can be varied by selection of the appropriate oxidant gas. When O2 or a gas containing O2 is supplied as the oxidizing gas, the compound will usually take the form of an oxide or a mixed oxide of the metal or metalloid ion. When CO2 or a nitrogen oxide gas is supplied as the oxidizing gas, the compound can take the form of a carbonate, a nitrite or a nitrate. In other words, the nature of the anion of the reaction product can be varied by a suitable selection of the oxidizing gas.

The person skilled in the art will be able to adapt the amount of oxidizing gas supplied and the gas flow rate to the concentration of the water soluble precursor compound that needs to be isolated from the electrolyte. In particular, it may be convenient to vary the rate of gas supply, particularly in case the process develops continuously and a continuous supply of water soluble precursor compound to be removed takes place. In addition, the gas supply can create a convective mass transfer in the catholyte and not only promote the diffusion of the metal and / or metalloid ions of the water-soluble precursor compound to the electrochemically active surface, but can also facilitate diffusion in the surface of the oxidizing gas reduction products, ie the ionic and / or peroxide radicals, as well as the surface diffusion of the metal ions and / or metalloids adhered, or of any intermediate reaction products, and therefore, increase the reaction speed. Other suitable ways to create a convective mass transfer include those known to the experienced person, for example, the use of an agitator, the supply of gases or the presence of a spacer material capable of creating turbulent flow conditions.

In a preferred embodiment, the electrochemically active porous material for the gas diffusion electrode and the cathode are chosen so as to have an active surface comprising a plurality of active sites provided by surface functional groups, wherein the functional groups preferably contain one or more portions selected from the group of a nitrogen-containing portion, an oxygen-containing portion, a chlorine-containing portion or a sulfur-containing portion.

To ensure maximum recovery, in a preferred embodiment, before supplying the water-soluble cationic compound, the pH of the electrolyte in the cathode compartment is adjusted to a pH <7.0, preferably a pH under acidic conditions, in which the person with experience could not wait for the formation of a solid reaction product. More preferably, before supplying the water soluble precursor compound, the pH of the electrolyte is adjusted to a pH which is below the dissociation constant of the acid or water soluble precursor compound salt, more preferably below 5.0 . Usually, the pH will be at least 1.5, since below this value the process slows down too much. The inventors have observed that the pH of the catholyte progresses gradually towards alkalinity in the course of the reaction, which is often above the dissociation constant of the acid or salt of the ionic metal or the metalloid compound. Particularly, the final pH of the catholyte can be increased to a value greater than 4, often higher than 6 or 7, more preferably above 9, most preferably above 11.

In a preferred embodiment of the method of this invention, an amount of a weak protonic electrolyte is supplied to the catholyte. The inventors have discovered that the oxidation rate of metals can be accelerated. Without wishing to be limited to this theory, the inventors believe that the weak protonic electrolyte acts as a catalyst or cocatalyst in the formation of reactive species of radicals, ionic and / or peroxides of the oxidizing gas at the cathode and in the electrochemical reactions in which the water-soluble compound becomes a reaction product that can be separated from the cathode and catholyte. The cocatalyst has been found capable of accelerating the oxidation of the metal cation or metalloid ion to the separable compound, by accelerating the availability of reactive species. The inventors have further found that the addition of the weak protonic electrolyte can not only increase the conductivity of the catholyte, but can also increase the current density on the cathode.

In addition, the presence of the weak protonic electrolyte has the effect that variations in the pH of the catholyte in the course of the oxidation reaction can be minimized. This helps to minimize the risk of undesirable side reactions that could lead to the formation of compounds that could not be easily separated from the cathode and / or the catholyte and which, for example, would be soluble in water. This separability provides an important advantage, since such a process may be suitable for use in or for direct coupling to isolate reaction products from processes employing biological material.

The amount of weak protonic electrolyte can vary within wide ranges but is preferably not less than a solution of 10 mM and preferably not more than a 1.5 M solution, more preferably the weak electrolyte concentration varies between 10 and 500 mM, with the highest preference around 100 mM.

The weak protonic electrolyte can be the same as a weak protonic acid as a weak protonic base, depending on the pH range at which the separable compound can be formed. Particularly, the weak protonic electrolyte may be a weak polyprotonic acid or a weak polypropionic base.

A weak protonic acid is a protonic acid that only partially dissociates in water:

HA {aq) h H + jatj) A (a (¡)

A weak polyprotonic acid is a weak acid that has more than one ionizable proton per molecule. The dissociation constant of a weak monoprotic acid can be represented by the following formula:

Preferred weak protonic acids have a pKa between 2.0 and 8.0, preferably between 3.0 and 7.0, more preferably about 7.0. Examples of weak protonic acids suitable for use with the present invention include those selected from the group of weak organic acids and weak inorganic acids, particularly acetic acid, citric acid, oxalic acid, lactic acid, gluconic acid, ascorbic acid, formic acid, glycolic acid , potassium monohydrogen phosphate, potassium dihydrogen phosphate, ammonium chloride, boric acid, sodium sulfate, sodium hydrogen carbonate, ammonium chloride, and mixtures of two or more of these. Preferred weak protonic acids are those having a pKa that is at least one unit higher than the pH of the catholyte.

Preferred weak proton bases have a pKa of between 6.0 and 12.0, preferably between 7.0 and 11.0. Examples of weak protonic bases suitable for use with this invention include those selected from the group of ammonia, trimethylammonium, ammonium hydroxide, pyridine, the conjugated bases of acetic acid, citric acid, oxalic acid, lactic acid, gluconic acid, ascorbic acid, formic acid, glycolic acid, potassium monohydrogen phosphate, potassium dihydrogen phosphate, ammonium chloride, boric acid, sodium hydrosulfate, sodium hydrocarbonate, or a mixture of two or more of the compounds mentioned above.

To ensure maximum recovery as a solid material, in a preferred embodiment, the pH of the electrolyte in the cathode compartment is adjusted to acidic conditions, in which initially the formation of a solid phase is not anticipated. Subsequently, the pH becomes progressively more basic as the reaction progresses, where colloidal particles in suspension can become evident.

In another preferred embodiment, a water-soluble ionic salt is supplied to the catholyte, in order to control, particularly to increase the ionic strength of the catholyte. Chloride salts with an alkali metal ion are preferred, NaCl is particularly preferred. However, other electrolytes can also be used. An amount of NaCl greater than 1 g.L-1 is preferred, preferably the amount added will be greater than 10 g.L-1, more preferably at least 30 g.L-1.

The process of the present invention shows the advantage that the overall conductivity of the electrolyte in the cathode compartment will vary to a minimal degree only in the course of the process. Particularly, virtually no or only a minor decrease in overall conductivity has been observed. This is probably due to the adhesion or adsorption of cations containing at least one metallic element and / or a metalloid element of the water soluble precursor compound, to the electrochemically active surface of the cathode, which is generally expected to reach an almost stable level when all the positive valence ions to be isolated have been oxidized and transformed into a separable phase, especially in a batch process.

However, any undesired variation in the conductivity can be compensated by supplying additional electrolyte or by incorporating a binary electrolyte into the catholyte. This may be of particular importance when the process of this invention is executed in a continuous manner, and the continuous supply of metal and / or metalloid ions to be recovered takes place. Due to the presence of the binary electrolyte, the electrolytic conductivity can be increased to at least 5 mS.cirr1, more preferably between 20 and 80 mS.cm-1 and even more preferably between 20 and 50 mS.cirr1 'and thus the risk of a variable conductivity as a result of the elimination of metal ions and / or metalloids.

To facilitate the release of the nanoparticles, particularly the nano crystals, from the electrochemically active material and facilitate the recovery of the precipitate, the cathode can be subjected to the inversion of the Polarization. The polarization inversion can also be used to clean the cathode of any unwanted residue adhering to it. This will allow to recover from the solution at least 10% of the amount of metal or metalloid ion that had been supplied to the cathode, with a greater preference to recover at least 40% of this and even more preferably to recover at least 80% of it.

The electrochemical process of the present invention as described in the present application makes it possible to recover metal or metalloid ions or a cation containing such metal or metalloid element contained in the water soluble precursor compound. Therefore, recovery rates can be substantially complete, and amount to more than 99.0% by weight of this. The recovery rates are usually at least 20% by weight of the initial concentration of at least one metallic or metalloid element present in the parent compound, preferably at least 50% by weight, more preferably at least 80% and with the maximum preference more than 90% by weight or even more than 99% of this.

The electrochemical process of the present invention is suitable for recovering components of a wide variety of water soluble precursor compounds, in a wide variety of concentrations.

The present invention is suitable for isolating a wide variety of compounds from an aqueous solution of the corresponding water soluble precursor compound. The precursor compound can be, for example, a compound of an ion of an element selected from the group of elements of groups II, III and IV of the periodic table of elements, excluding C and Si, most of the transition metal elements , actinides and lanthanides. The water-soluble compound may also be a compound of an ion of an element selected from the group I group of elements when it is in a compound that also contains P or S. The water soluble compound may also be a metal organic compound or complex , or an organic compound.

In a preferred embodiment, the at least one water-soluble metal precursor compound is selected from the group of precursor compounds containing one or more alkali metal ions, preferably one or more of Li, Na, K, Cs ions, more preferably Li and / or Na. In a second preferred embodiment, the at least one ion-soluble water-soluble precursor compound contains at least one metal ion selected from the group of alkaline earth metals, in particular preferably Ca and / or Mg. In a third preferred embodiment, the metal ion contained at least one water-soluble precursor metal ion compound of the transition metal group, preferably one or more of Sc, Ti, V, Cr, Mn, Fe, Co, Ni ions. , Cu, Zn, Y, Zr, Nb, Mo, Pd, Ag, Cd, Hf, Ta, Tu, Re, Ir, Pt, or Au ions, more preferably one or more of V, Mn, Co, Nb, Ag , Pt or Au. In a fourth preferred embodiment, the at least one metal ion is selected from the group of post-transition metals, particularly one or more ions of Al, Ga, In, Sn, Tl, Bi. In a fifth preferred embodiment, the at least one water-soluble ionic metalloid precursor compound is selected from the group of ions of B, Si, Ge, As, Sb, Te, Se or C or mixtures of two or more of these. In a sixth preferred embodiment, the at least one water-soluble ionic metalloid precursor compound is selected from the group of Li, Na, Ca, Fe, Mg, Al or Zr ions. In a particularly preferred embodiment, the metal and / or metalloid ion is selected from the group where the monatomic cation is selected from the group of H +, Li +, Na +, K +, Cs +, Mg2 +, Ca2 +, Sr2 +, Ba2 +, Al3 +, Ag +, Zn2 +, Fe2 +, Fe3 +, Cu2 +, Cu + and mixtures of two or more of these. Although most of the aforementioned metal and / or metalloid ions can be used as such within the process of this invention, for some of them, particularly Na and K may need to be used in a mixture with a metal and / or more distant metalloid ion.

The water soluble precursor compound can be supplied as a precursor compound comprising a single type of ion or metal and / or metalloid element with a positive valence, but it is also within the scope of this invention that a composition comprising a mixture of two or more ions or metallic or metalloid elements with a positive valence. In case a single metal or metalloid ion is supplied, the reaction product that can be separated from the aqueous solution of the precursor is preferably a compound comprising a single metal or metalloid in an oxidized state. In case the electrolyte comprises a mixture of two or more ions or metallic or metalloid elements with a positive valence, the reaction product may comprise a mixture of compounds in an oxidized state, all the reaction products, for example, which respond to the formula MxOy, but may also comprise mixed metal or metalloid compounds, for example MxNzOy. However, it is within the scope of this invention in addition to supplying a matrix comprising one of precursor compounds or a matrix containing a mixture of two or more precursor compounds.

In a first embodiment of this invention, the reaction product that is formed from the parent compound may contain nano crystalline oxide particles, for example, but not limited to CeO2, La2O3, Co2O3, A ^ O3, Cs2O, Li2O, CoFe2O4, FeAsO4 , or non-stoichiometric forms of these or hydrated forms thereof (for example CeO-i.75). In a second embodiment, the reaction product may contain crystalline carbonate nanoparticles, preferably but not limited to Na3La3 (CO3) 5, NaHCO3, or non-stoichiometric forms thereof or hydrated forms thereof. In a third embodiment of this invention, the separable compound may contain a mixture of amorphous or crystalline nanoparticles of mixed metal oxides or oxides. In a fourth embodiment of this invention the additional ionic compounds can be supplied to the catholyte as ionic liquids. As a result of the presence of such an ionic liquid, the structure of the reaction product may comprise the ionic liquid products of its decomposition, for example, but not limited to terbutylammonium chloride or chloride of terbutylammonium. Surprisingly it has been found, and this has been confirmed by the IR spectra of the reaction product formed - although the structure is not completely understood, that by supplying an ionic liquid a foaming reaction product is obtained.

The particles of the reaction product can be released in a variety of physical forms, for example in the form of colloidal nanoparticles, for example in the form of a colloidal dispersion, a separable precipitate of particles or a gelatinous, foaming or similar phase to a Pickering emulsion. Usually a stable dispersion or gel will be obtained. To improve the stability, the dispersion or suspension of the particles can be subjected to sonication or ultrasonication. According to another variant, one or more additives can be added to the water-soluble precursor compound, the electrolyte at the cathode, the suspension or dispersion at any convenient time. The additives can be selected from the group of dispersants, stabilizers, surfactants, polymers, copolymers, emulsifiers, crosslinking agents, finishing agents and free flowing agents or mixtures thereof. Such addition can be made to stop the growth of the nanoparticles, particularly of the nano crystals, to reduce the risk of agglomeration of the nanoparticles and to provide a stable dispersion.

In a preferred embodiment of this invention, the electrochemically active material of the gas diffusion electrode forming part of the cathode preferably comprises an active surface having a plurality of active sites with a weak protonic acid functionality, ie active sites which are only partially dissociate in water. Various electrochemically active materials can be used to achieve this. Those materials having a surface comprising protonic acid functional groups are preferred. Particularly preferred are those materials comprising electrically conductive particles of carbonic origin, more preferably those comprising electrically conductive particles of carbonic origin with a catalytically active surface comprising a plurality of protonic acid groups. It is believed that the protonic acid functional groups present on the catalytically active surface, particularly the acid functional groups of the R-H type, can be partially dissociated at a corresponding pH. The inventors further believe that the surface groups thus dissociated C-R * -have a high affinity for oxygen and thus intervene in the oxidation of the metal ion or the metalloid ion.

As the electrochemically active material, a wide variety of conductive materials can be used, but porous materials are preferred, particularly those containing functional groups of weak protonic acid. Examples of such materials are well known to the skilled person and include porous metals and metalloids, for example porous nickel or copper, porous carbon based materials, porous ion exchange resins, carbon aerogels, silicon, conductive polymers, conductive foams. or conductive gels, among others. The use of a porous carbon-based material as or on the electrochemically active surface is preferred, due to its catalytic activity in combination with a reasonable cost and abundant availability compared to other materials. Examples of suitable materials include graphite, carbon nanotubes, graphene, carbon black, acetylene black, activated carbon or synthetic coals such as vulcan. Other electrochemically active materials suitable for use with this invention include carbon materials whose surface has been chemically modified to adapt its catalytic activity and compatibility with the reaction medium. Without wishing to be limited to this theory, it is believed that the presence of oxygen-containing functional groups supports the oxidation reaction. Especially preferred carbon materials have a surface with quinone type functional groups.

The porous material suitable for use as an electrochemically active layer has a high specific surface area, measured by the BET method described in ASTM D5665, of at least 50 m2 / g, preferably at least 100 m2 / g, more preferably at least 200 or 250 m2 / g, most preferably at least 400 or 500 m2 / g, but those having a surface greater than 750 or 1000 m2 / g or even more can be particularly preferred. Porous materials particularly suitable for use as an electrochemically active layer include particles of carbon origin, particularly carbon particles selected from the group of graphite, carbon nanotubes, graphene, carbon black, activated carbon or synthetic carbons. The conductive carbon particles have a BET surface area of at least 50 m2 / g, preferably at least 100 m2 / g, more preferably at least 200 or 250 m2 / g, most preferably at least 400 or 500 m2 / g, but those having a surface greater than 750 or 1000 m2 / g or even more are particularly preferred.

The activated carbon preferably has a particle size in the range of 75 to 300 μm, preferably 100 to 250 μm.

The porous material suitable for use as an electrochemically active layer preferably forms a continuous layer at the cathode. For this, a polymeric material that functions as a support for the electrochemically active material can be used.

In accordance with another preferred embodiment, the electrochemically active porous material is a solid which is dispersible or fluid in the water-based electrolyte. Hereby, the solid can be made from one or more of the materials described above.

In the method of the present invention, preferably a cathode comprising a porous gas diffusion electrode is used, wherein one side of the gas diffusion electrode comprises a layer of at least one electrochemically active material for or capable of catalyzing the reduction from oxygen to hydrogen peroxide. The preferred active materials have been described above. To increase the reaction rate, convective mass transfer can also be created at least in the cathode gas compartment.

The process of the present invention is suitable for use in a wide variety of applications. The process of the present invention can for example be used to produce nano crystals of selected materials in a selected particle size distribution, with a selected crystal shape, with selected framework parameters. The process of the present invention can be further used to recover metal ions or metalloid ions from an aqueous solution, and provides a suitable method for recovering for example precious metals from aqueous solutions, particularly resulting from the destruction of electronic materials.

The reaction product obtained by the process according to the present invention, in the form of nano crystals, can have a wide variety of uses. For example, for reaction products containing cerium can be formed, CeO2o sub-oxides such as CeOus which are particularly suitable for the purification of oxidizing agents such as oxygen or hydrogen peroxide. Their use as oxygen scavengers may be of particular interest to the electronics industry as additives for screen sealing agents. Their use as hydrogen peroxide scavengers may be of particular interest to the cosmetic industry, for example, but not limited to the depuration of peroxides that occur naturally in the hair follicles to prevent or reverse white hair. Other compositions, for example, those that arise with the reaction with ionic liquids, can be used as hydrogen or CO2 scavengers among many other applications.

The present application further relates to a composition comprising at least one aggregate particle of nano crystals obtained by the process described above, wherein the nanoparticles, particularly the nanoparticles, have a particle size, particularly a crystal size of between 0.2 and 30.0 nm. Where a crystalline product is obtained, the nano crystals will usually have a network parameter of between 1.0 and 18.0 nm, and any aggregated particles formed will have an average particle size of <30 nm. The reaction product can take the form of a dispersion of the reaction product in an aqueous medium, with a solids content of between 1.0 and 30.0% by weight, preferably between 5.0 and 10.0% by weight. The aqueous medium may comprise the electrolyte, water or an aqueous solution.

The present invention further relates to a method for selectively isolating at least one metal element or a metalloid element or two or more thereof from an aqueous solution comprising a water-soluble precursor compound thereof, wherein the process is used as described above. described as described above. The present invention further relates to a device for recovering a metal element or a metalloid element or two or more of these from at least one water-soluble precursor compound comprising a metal element or a metalloid element or two or more thereof, in the nanoparticle form, particularly nano crystals of at least one reaction product, wherein the device comprises

- means for supplying the water-soluble precursor compound at least partially dissolved in a water-based catholyte in a cathodic compartment of an electrochemical cell, equipped with a cathode comprising a gas diffusion electrode, wherein the diffusion electrode of gases comprises an electrochemically active porous material having a BET surface area of at least 50 m2 / g,

- wherein the catholyte has a pH that is lower than the pKa of the water soluble precursor compound,

- means for supplying at least one oxidizing gas to the gas diffusion electrode,

- wherein the cathode has an electrochemical potential that is such as to be capable of causing the reduction of at least one oxidizing gas to one or more corresponding peroxides, reactive ionic species and / or radicals capable of reacting with the metallic element, the element metalloid or a cation comprising such metal or metalloid element or two or more of these, to form at least one nanoparticle, particularly, at least one nano crystal of at least one reaction product with an average crystallite size equal to or less than 30.0 nm.

The invention is further illustrated in the appended figures.

Figure 1a shows a schematic representation of the semi-cell experimental electrochemical reactor suitable for use with the present invention.

Figure 1b shows the process of the present invention that includes the reactions and the formation of nano crystals within the pores of the electrochemically active material.

Figure 2 shows the removal efficiency in% Ce3 + ions of the bulk solution in the presence of N2 which is supplied through the gas diffusion cathode, in the absence of oxidizing gas, of example 1.

Figure 3 shows the electrochemical response in case no oxidizing gas is supplied through the gas diffusion electrode, but only N2.

Figure 3a: Frequency response obtained in Example 1 by Electrochemical Impedance Spectroscopy (EIS) recorded at an amplitude of 20 mV, in the frequency range from 100 kHz to 3 mHz.

Figure 3b shows the cyclic voltammetry response obtained at a scanning speed of 1 mV.s-1 in Example 1.

Figures 3c and d show the typical EIS responses for diffusional limitation through a film of infinite thickness (left) and limitations by finite diffusion through a film with a fixed amount of electroactive substance, which once consumed is not replaced on the electrode or it is only replaced very slowly (right), in example 1.

Figure 3e shows typical responses of capacitive and pseudocapacitive CV, in example 1.

Figure 4 shows the degree of removal of the Ce3 + ions from the bulk solution in the presence of O2 as the oxidizing gas supplied to the gas diffusion electrode in example 1.

Figure 5 shows the recovery efficiency (%) of the Ce3 + ions transformed into a solid product recovered as a precipitate after being released from the electrode and sedimented in solution, in the presence of O2 as the oxidizing gas supplied through the gas diffusion cathode, on the basis of the dry weight of the recovered product, in example 1.

Figure 6 shows the electrochemical response obtained for the experiments where air was supplied through the gas diffusion electrode, in example 1:

Figure 6a shows the frequency response obtained by Electrochemical Impedance Spectroscopy (EIS) recorded at an amplitude of 20 mV, in the frequency range from 100 kHz to 3 mHz.

Figure 6b shows the cyclic voltammetry response obtained at a scanning speed of 1 mV.s-1.

Figures 6c and d show the typical EIS responses for the adsorption-limited processes linked to the charge transfer reactions.

Figure 7 shows the size of the crystallite and the network parameter found for the different initial concentrations of Ce3 + studied, in example 1.

Figure 7a shows the size of the crystallite (220) for CeO2 and NaCl.

Figure 7b shows the network parameter of CeO2 and NaCl. There was a limit of detection for both parameters in Ce below 20 mg.

Figure 8 shows micrographs of transmission electrons that show the characteristic morphology of the CeO2 nanoparticles with crystallite sizes that coincide with those obtained by XRD, in example 1.

Figure 9 shows transmission electron micrographs showing the aggregation of the small crystalline nanoparticles of Figure 8 into larger nanoparticles, of example 1.

Figure 10 shows the removal efficiency (%) of the different metal ions of the bulk solution in the presence of air supplied through the gas diffusion cathode.

Figure 11 shows a suitable device for carrying out the process of the present invention.

Figure 12 shows a high resolution transmission microscopy (HRTEM) of nanoparticles obtained with example 1.

Figure 13 shows IR spectra of the precipitates obtained under different experimental conditions with TBAB (Figure 13a) and IR spectra of the reagents initially used for the TBAB experiment (Figure 13b). The IR spectra for TBAC cases are shown in Figure 13c.

Figure 14 shows nanocrystals precipitated in the presence of excess IL in Example 7 and under in situ hydrogen electrosynthesis. Small bubbles in the 0.1 mm size range were visible, occupying more than 10% of the volume of the floating material.

Figure 15 shows the XRD pattern for the crystalline nano product obtained with TBAB 30.

Figure 16 shows the charge in Coulombs, consumed during the experiments carried out at different concentrations (molar ratios) of TBAB.

Figure 17 shows the TGA characteristic of the nanocrystalline product formed with TBAB in a molar ratio of 20.

Figure 18 shows electrochemical series of a few oxidizing gases, the distribution of current density over different materials of gas diffusion cathodes (different types of carbon), recorded at different steady state reduction potentials.

A suitable device for carrying out the process of the present invention is shown in Figure 11. The device shown in Figure 1 comprises an electrochemical cell, comprising at least one anodic compartment 5 and at least one cathodic compartment 15. If desired, a plurality of anode and cathode compartments may also be present. If a plurality of anode and cathode compartments are provided, they are preferably arranged in a unipolar arrangement, with a plurality of alternating positive and negative electrodes forming a stack separated by permeable ionic membranes. In a unipolar design, the electrochemical cells that form the battery are connected externally, the cathodes are electrically connected in parallel, as well as the anodes.

The anode or anodes 1 are immersed in an anodic compartment comprising an aqueous fluid of anolytes 2. The cathode or cathodes 10 are immersed in a cathodic compartment comprising an aqueous fluid of catholytes 12. The anodic compartment and the cathode compartment are in communication fluid to allow the transport of cations, particularly the transport of protons from the anodic compartment to the compartment of catholytes, and the transport of anions from the cathodic compartment to the anodic compartment. As anolyte fluid, any suitable anolyte that the experienced person considers can be used. Particularly, any aqueous electrolyte, conventionally used in electrochemical reduction reactions, can be used. The anolyte, for example, may comprise an aqueous solution of an electrolyte selected from the group of sulfates, phosphates, chlorides and mixtures of two or more of these compounds. The anolyte chamber may comprise a supply member for feeding the anolyte fluid. The catholyte chamber may comprise a supply member for feeding the catholyte fluid. The catholyte may be different from the anolyte, but the anolyte and catholyte may also be the same. Suitable catholytic materials include those well known to the skilled person, such as an aqueous solution of an electrolyte selected from the group of sulfates, phosphates, chlorides and mixtures of two or more of these compounds

The anode and cathode compartment 5, 15 can be made of any material that the experienced person considers suitable, but preferably they are made of a polymeric material. Suitable materials include polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene (EFTE), polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polyacrylate, polymethyl methacrylate (PMMA), polypropylene (PP), polyethylene high density, polycarbonate, and mixtures and complexes composed of two or more of these compounds.

At least one compartment of the anode and at least one of the cathode 5, 15 are preferably separated from each other by an ion-permeable membrane 11 to control the exchange of cations and anions between both compartments. Preferred ion-permeable membranes comprise synthetic polymeric materials. The ion-permeable membrane, on the one hand, ensures that cations, particularly protons, can migrate from the anode compartment to the cathode compartment and, on the other hand, serves as a barrier to gases and counteracts the appearance of so-called shortcuts chemical The ion-permeable membrane also counteracts the occurrence of a reduction in the pH of the catholyte in the cathode compartment. Materials suitable for use as an ion-permeable membrane include polyvinyl difluoride (PVDF), polytetrafluoroethylene (PTFE or Teflon), poly (ethylene-tetrafluoroethane) (EFTE), polyesters, aromatic polyamides, polyphenylene sulfite, resins polyolefins, polysulfone resins, perfluothiolorovinyl ether (PFVE), tripropylene glycol, poly-1,3-butanediol or mixtures of two or more of these compounds, or complexes containing one or more of these compounds and obtained by dispersion of a metal oxide and / or a metal hydroxide in a solution of the polymer to increase the ionic conductivity. The ion permeable membrane may also contain an ion exchange material if desired.

To improve the structural integrity, the ion-permeable membrane 11 which separates the anode and cathode compartment 5, 15 can be reinforced with a rigid support, for example, a rigid support which is made of a sheet, a fleece, which can be woven or non-woven or made of a porous polymer or of a weft or a mesh of metallic fibers or metal fibers arranged in a woven or non-woven structure.

The cathode 10 used in the device of this invention is preferably a gas diffusion electrode, to ensure a sufficiently high mass transfer of oxidizing gas to the electrochemically active surface present at the cathode, and a sufficiently high reaction yield, which has account for the limited solubility of oxygen in water. The gas diffusion electrode is preferably a multilayer electrode comprising a current density distributor 3 for supplying electric current to an electrochemically active surface 4 which is deposited on top of the current distributor.

The electrochemically active material 4 is preferably a material having a high electrical conductivity. This allows the electrochemically active material to carry or bring the electron to and from the current density distributor.

The electrochemically active surface can be formed of any conductive materials or composite materials with a large surface area. Examples of such electrode materials include carbon, carbon nanotubes, graphite, carbon fiber, carbon cloth, carbon airgel, graphene, metal powders, for example nickel, metal oxides, for example ruthenium oxide, conductive polymers, and any mixtures of any of the foregoing. It must be taken into account that all electrodes can be sufficiently porous and conductive so that a substrate is not needed. It should further be appreciated that the substrate may be formed of a non-conductive material that is coated with a conductive coating, such as, for example, platinum, rhodium (Rh), iridium (Ir) or alloys of any of the above metals. The high surface area allows to minimize the voltage. Upon contacting the porous portion with the ionic electrolyte, the apparent capacitance of the electrodes can be very high when they are charged.

The gas diffusion electrode that is used as the cathode 10 in the method of this invention preferably comprises a current density distributor 3, which can be made of any material and form that is considered suitable by the experienced person. Preferably, however, a mesh-type current density distributor is used, which has an accepted mesh in an electrically conductive circumferential frame or an array of several meshes. The current density distributor is connected to a source of electrical energy through a current feeder, to supply electrical power to the current density distributor. The mesh comprises a plurality of electrically conductive routes. The mesh can be formed of any suitable metallic structure, such as, for example, a plate, a mesh, a thin sheet or a sheet with a plurality of perforations or holes. In addition, the mesh can be formed of suitable conductive materials, such as, for example, stainless steel, graphite, titanium, platinum, iridium, rhodium or conductive plastic. In addition, metals can be coated or uncoated. An example of this is a stainless steel mesh coated with platinum. In one embodiment, the mesh is a titanium mesh. In other embodiments, a stainless steel mesh, a graphite plate or a titanium plate is used. The term "mesh" is intended to include square meshes with a shape and orientation of the conducting wires and insulating threads substantially rectangular, but the mesh may also be tubular, or a spiral film, or three-dimensional materials with another shape. Still other types of meshes suitable for use with this invention include perforated sheets, plates or sheets that are made of a non-conductive material, having a plurality of wires or wires of an interlaced conductive material in the direction parallel to the current flow. An additional type of mesh suitable for use with the present invention includes lines / cables of a conductive material, which extend in parallel to the current flow direction, printed on a perforated sheet, sheet or plate.

One side of the current density distributor 3 can be coated with an electrochemically active surface 4 capable of catalyzing the reduction of the oxidizing gas. The layer of electrochemically active material 4, ie, the layer which is catalytically active in the reduction of the oxidizing gas as described above, is preferably applied on the side of the current density distributor facing the gas phase. The electrochemically active surface usually has an interface with the electrolyte on one surface (i.e. the side facing the current distributor) and a water repellent layer 13 (hydrophobic diffusion gas) on the other side.

The device preferably comprises a delivery member for supplying an oxidizing gas to the side of the cathode comprising the electrochemically active layer.

The cathode compartment may preferably comprise, on an opposite side to the side of the cathode comprising the electrochemically active layer, an inlet for supplying at least a weak protonic electrolyte, preferably an aqueous electrolyte. Preferably the flow rate with which the weak protonic electrolyte is variable.

The electrochemically active surface 4 can be coated on the side facing the gas phase 13, with a water repellent layer 13 or a hydrophobic gas diffusion layer to minimize the risk of water leaking through the electrode into the gas phase . This hydrophobic or water repellent layer 13 can also be deposited on the electrochemically active surface 4. Materials suitable for use as a water repellent layer include polyvinyl difluoride (PVDF), polytetrafluoroethylene (PTFE or Teflon), PSU, but can also be use other materials that the experienced person considers appropriate.

The anode 1 used in the process of this invention can be a conventional electrode, or it can be a gas diffusion electrode similar to the cathode. The pH of the anolyte is preferably acidic, preferably <7.0, more preferably <5.0, most preferably <3.0 but not less than 1.5. The electrolyte will usually have an ionic conductivity of at least 1.0 mS / cm, but in practice it may amount to 70-100 mS / cm.

The invention is further illustrated in the examples below.

Example 1

Materials and methods

Chemical compounds

The activated carbon used was Norit® SX1G from Norit Americas Inc. The fluorinated ethylene propylene resin (Teflon® FEP 8000) was obtained from Dupont. The 99.9% ultra-dry crystalline CeCl3 (REO) with argon was received from Alfa Aesar. K2HPO4 was purchased from Merck. HCl at 35%, CeN3Og-6H2O base metals at traces at 99.99%, and analytical grade KI were purchased from Aldrich. 50% NaOH, analytical grade potassium hydrophalate (KHP) and (NH4) 6MoyO24-4H2O analytical grade were purchased from Merck.

Configuration of electrochemical cells

The experiments were carried out in a semi-cell electrochemical reactor (Figure 1). The half cell of the cathode consisted of a cathode, a reference electrode and a counter electrode. As reference electrode, 3 M Ag / AgCl of KCl (+200 mV against SHE) (Koslow Scientific) was used, while a Pt disk fixed by laser welding on a titanium plate (Ti) was used as the counter electrode. All the potentials reported here remain accurate for the 3 M Ag / AgCl KCl reference electrode. The cathode and the counter electrode were separated by the liquid electrolyte and the separating membrane, Zirfon® (AGFA). The electrode in operation and the counter electrode were separated from each other by a distance of 4 cm, while the membrane was accommodated just in the center (2 cm from each electrode). The main function of Zirfon® was to prevent the oxygen that finally emanated in the counter electrode from reaching the electrode in operation. The electrodes and the separator had an area 10 cm2 projected electrode surface The inert or reactive gas streams (N2 or air, respectively) were fed through the cathode gas compartment in each individual experiment. The gas flow rate was set at 400 mL min-1 (excess) in all cases and an overpressure of 10 mbar was applied. The electrolyte feeds circulated independently through the cathode and counter electrode compartments with a double head peristaltic pump, at a flow rate of approximately 100 mL min-1 (Watson-Marlow). Both the liquid and gaseous streams under these conditions were consistent with a laminar flow profile. Figure 1a shows a schematic representation of the semi-cell experimental electrochemical reactor. Gas diffusion electrodes

A VITO CORE ™ multi-layer electrode consisting of a current collector (metal gauze), an active layer of activated carbon embedded in a porous polymer matrix and a hydrophobic gas diffusion layer was used. PVDF was used as a polymeric binder, both for the active layer and for the hydrophobic gas diffusion layer (GDL). The hydrophobic particles in the hydrophobic support were FEP 8000. A typical GDL is composed of 50% by weight of PVD ^f and 50% by weight of FEP 8000. The composition of the active layer for the non-catalyzed cathode was 20% PTFE with 80% by weight of activated carbon. Otherwise, a catalysed electrode can be used, for example, this was 20% PTFE with 76% by weight of activated carbon and 4% by weight of CeO2, which can improve the efficiency of the process but is not essential to carry out the specific synthesis.

Electrolyte composition

The independent electrodes were tested as gas diffusion cathodes, in the presence of air or N2 respectively, in the cathode gas compartment. CeN3Og6H2O was added to the cathode electrolyte, composed of 30 g / L of NaCl and 10 mM of sodium acetate dissolved in demineralised water and adjusted to pH 2.7 with Hcl. Sodium acetate is not essential to carry out the specific synthesis, however, it can affect its rate. Different concentrations of CeN3Og6H2O were tested independently, as follows: 0 ppm, 100 ppm, 500 ppm, 1000 ppm, 2000 ppm, 3000 ppm, 5000 ppm and 10000 ppm. The electrolyte in the counter electrode compartment (anode) remained the same, but without the addition of Ce. The tests were carried out at room temperature (18 ± 2 ° C).

Operation and electrochemical characterization

A Bio-Logic VMP3 potentiostat / galvanostat and a frequency response analyzer were used to perform the electrochemical measurements. The EC-Lab v.10.23 software was used for data acquisition. Chronoamperometric experiments were carried out at -0.350 V against the reference electrode for a period of 120 min. Within this period a steady state was achieved. Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) before and after polarization were recorded to indirectly evaluate the effectiveness of the metal recovery process.

During the CA experiments at -0.350 V, the production of H2O2 takes place after the electrochemical reduction of O2 with the electrodes and electrolyte proposed here. Under these conditions, the electrodeposition of the metallic Ce is not expected, since the thermodynamic condition for the reduction of Ce in aqueous medium within the pH range studied here (as shown in the Pourbaix diagrams) would occur only in potentials below - 2.7 V [Pourbaix, 1974]. Even so, the transport of dissociated Ce3 + ions towards the cathode is expected mainly by diffusion, with possible subsequent adsorption in the active sites of the porous electrode. Otherwise, the electrolysis of water is not expected to form hydrogen under the conditions of this example.

Electrochemical Impedance Spectroscopy (EIS) was recorded in the steady-state polarization potential (-0.350 V) in a frequency range of 3 kHz to 3 mHz, with 6 points per logarithmic decade, with the use of an amplitude of 10 mV. Special attention was paid to guarantee stability, linearity, causality and finitude, in order to obtain reliable and valid impedance data. The EIS response was only recorded when it was detected that the variation of the current was <± 10 pA during a period of at least one hour. The time of a full impedance scan was approximately 19 minutes. The linearity was verified by real-time monitoring of the non-distortions in the Lissajous plots, which were observed by means of an oscilloscope connected in line. The causality was assured, since spurious (noisy) data were not recorded when recording the EIS. The validity of the data was verified with the use of the Kramers-Kronig transformations. After the corresponding EIS measurements, CVs were recorded in 2 cycles at 1 mV s-1, in a potential range of -0.450 to 0.450 V versus Ag / AgCl. Only the second cycle of the CV is reported here. No correction of the IR drop was established for the experiments performed here.

Analysis of H2O2 concentration

A spectrophotometric method was used to determine the concentration of H2O2 in solution, as described by Aryal & Liakopoulou-Kyriakides 2013, 3: 117. Reagent A was prepared by mixing 33 g of KI, 1 g of NaOH and 0.1 g of (NH4) 6Mo7O24'4H2O in 500 mL of deionized water. This solution remained in conditions dark to inhibit oxidation of I. Reagent B was prepared with 10 g of KHP dissolved in 500 mL of deionized water. The standard calibration curve (not shown) was prepared from known concentrations of H2O2 from 0 to 3 mg L-1, dissolved in the same electrolyte used for the experiments, without cerium. Further analysis was performed by pipetting 3.0 mL of Reagent A, 3.0 mL of Reagent B and 3.0 of the standard sample into a beaker. The content of the mixture was allowed to react for 5 minutes, before reading the absorbance of the solution at 351 nm [GSI Scientific Report. (2009) Helmholtzzentrum für Schwerionenforschung, 2010-1].

The calculated concentrations of H2O2 are the average of the quantitative results obtained with 5 averaged calibration curves, described by the following equation:

Where Ch2O2 refers to the concentration of hydrogen peroxide (mg L-1) and A351 denotes the absorbance recorded at 351 nm.

In addition to the known concentrations, problematic samples were obtained after the electrochemical characterization experiments and analyzed by the same procedure as the standard ones.

X-ray diffraction

X-ray diffraction (XRD) experiments were performed with the PANalytical X'Pert Pro diffractometer with CuKa (A = 1.5405Á) at 40kV. The conditions were: 4sec / stage; stage = 0.04 ° and continuous sweep. The wet precipitates were placed on a monocrystal. The samples were measured both wet and dry. Since there were no important variations between them, only the values relative to the dry samples are reported here.

The identification of the crystalline phases was made by comparison with the database. The crystallite size (D) was calculated with the use of the Scherrer equation (Eq. 8):

where B is the Scherrer constant (0.89), A is the wavelength of the X-ray beam (1.5405Á), pv ² is the full width at half the maximum of the diffraction peak and 9 is the diffraction angle.

Independent aqueous solutions with fixed concentrations of NaCl and sodium acetate (CHsCOONa) were supplemented with varying concentrations of Ce (NOa) 3-6H2O (specifically 0 mg.L-1, 100 mg.L-1, 500 mg.L- 1, 1000 mg.L-1, 2000 mg.L-1, 3000 mg.L-1, 5000 mg.L-1, and 10000 mg.L-1, respectively). The pH of each electrolyte was set at 2.7, with HCl. In all cases, a colorless solution was formed. To these cathodes a constant potential of -0.350 V was applied to Ag / AgCl (3M of KCl). In the gas compartment, N2 or air was supplied for each independent experiment, at a constant flow rate (~ 400 mL.min-1). Under these operating conditions the electrolysis of water is avoided; However, when air is supplied through the GDE, the O2 is reduced electrochemically to H2O2, depending on the availability of protons and electrons.

The general solution was considered electroneutral before applying electrochemical polarization. Given the high concentration of NaCl, the transport of ions by migration is unlikely.

As soon as the electric polarization was applied to the cathode, a gradient of electrochemical potential was developed through the half cell. Since the concentration gradients were initially absent, the transport of some positively charged ions was probably conducted from the solution in equilibrium to the surface of the porous cathode, which was thus captured by the potential-modulated electrosorption and was capacitively stored in the diffuse of the double electric layer.

Figure 2 shows the degree of transport of the Ce3 + ions (removal efficiency in%) of the bulk solution in the presence of N2 that is supplied through the gas diffusion cathode and in the absence of oxidizing gas. The elimination efficiency (%) was calculated based on the initial content of Ce3 + in solution (Cej, i / mg):

remc / 7 - Ct í Ct j X 100% Ec .9

1 'C ^tá

Cejf (mg) represents the final content of Ce3 + in solution.

When Ce3 + was not supplied in the aqueous matrix (0 ppm), as a consequence of the established initial concentration gradient, the transport of Na + within the microstructure of the porous electrode may have been prolonged by diffusion to the rest of the porosity of the electrode. However, Na + was available at its highest concentration high in volume. Together, this establishes the diffusion from the volume to the EDL in the global porosity of the GDE as the limiting stage of the speed for Na + transport until a dynamic equilibrium was reached.

Figure 3 shows the electrochemical response obtained for the experiments where no oxidizing gas was supplied through the gas diffusion electrode and only N2 was provided. Figure 3a: Frequency response obtained by Electrochemical Impedance Spectroscopy (EIS) recorded at an amplitude of 20 mV, in the frequency range from 100 kHz to 3 mHz. Figure 3b shows the cyclic voltammetry response obtained at a scanning speed of 1 mV.s-1. Figures 3c and d show the typical EIS responses for diffusional limitation through a film of infinite thickness (left) and limitations by finite diffusion through a film with a fixed amount of electroactive substance, which once consumed is not replaced in the electrode or it is only replaced very slowly (right). Figure 3e shows typical responses of capacitive and pseudocapacitive CV.

In Figure 3, the symbols shown below are related to the indicated experiments:

° ti, C ^{ce 3 +, i} = 0 mg.L-1 Ce3 +.

• t ^f , C ^{ce 3 +, i} = 0 mg.L-1 Ce3 +.

A t ^o , C ^{ce 3 +, i} = 10 gL-1 Ce3 +.

▲ t, C ^{ce 3 +, i} = 10 gL-1 Ce3 +.

The frequency response for this case, obtained by electrochemical impedance spectroscopy (EIS), was found to be typical of the semiinfinite linear diffusion (see Fig. 3a), ie, unrestricted diffusion to the large porous cathode. In the high frequency range, EIS showed a change from a typical behavior of the constant phase element (at the beginning of the experiment) to a resistance pseudotransference behavior (at the end of the experiment) that is characteristic of the occluded porosity [Kaiser et al. (1976) Electrochim. Acta, 21, 539]. The response in cyclic voltammetry (see Figure 3b) is characteristic of porous electrodes with pseudo-capacitive behavior (see Fig. 3c), confirming the capacitive storage of Na + [Yang et al., 2003, J Electroanal Chem 540: 159]; however, the general process is limited by diffusion. Although some Na + is considered effectively electrostatically adsorbed, virtually no changes in its apparent concentration (seen as conductivity) were observed due to the ratio between the small amount of ions that can actually be electrosorbed in the EDL and those that are excessively available in the matrix watery

On the other hand, for cases supplemented with Ce3 + (4 mg to 403 mg of Ce3 +, corresponding to the aforementioned Ce (NO3) 3 concentrations of 100 ppm to 10000 ppm), it was observed that the frequency response is distinctive of the limitations by finite diffusion through a film with a fixed amount of electroactive substance, which once consumed does not replenish the electrode or only replenishes very slowly (see Fig. 3a). Although Figure 3 only presents the EIS and CV data obtained for systems without Ce (NO3) 3'6H2O or those supplemented with 10 gL-1 of Ce (NO3) 3-6H2O, the electrochemical behavior is representative of all cases in which the N2-flows in the cathodic gas compartment and in which the electrolyte is supplemented with Ce3 + even in concentrations as low as 100 mg.L-1 of Ce (NO3) 3-6H2O.

The pH and conductivity were monitored in the catholyte, at the beginning and end of the experiments. For the cases where N2 was supplied (Result 1), the initial pH of 2.7 for each individual experiment increased by approximately 0.3 ± 0.18 at the end of the experiments, while it decreased slightly as a function of concentration (in no case did it decrease below 2.8 ± 0.2). The initial conductivity for the case without Ce3 + (ie 30 g.L-1 NaCl 10 mM sodium acetate) was 49.7 ± 0.6 and remained almost stable at the end of the experiments (50.1 ± 0.3). This shows that practically no variation in NaCl concentration can be achieved at such high concentrations of NaCl. For cases with Ce3 +, an ordinary increase in conductivity was observed as a function of concentration, before polarization. In this case, the conductivity decreased slightly after applying the polarization treatment, in agreement with the elimination efficiencies observed in Figure 2; that is, at the end of each experiment the conductivity corresponded approximately to that of NaCl of 30 mg.L-1 alone.

For the system where N2 passed through the GDE, the average efficiency of elimination of Ce3 + was 25.42 ± 12.14% (see Fig. 2). In the absence of oxygen or other oxidizing gases, it is believed that the amount of metal ions (Ce3 +) removed is captured in the porous structure of the electrode, mainly by ion exchange in the surface functionalities containing groups of Cl, S and O, characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy.

O2 was supplied as an oxidizing gas through the gas diffusion cathode

We also used VITO CORE ™ cold rolled gas diffusion electrodes (GDE), manufactured with porous activated carbon (NORIT SX 1G). The specific surface area for the powder from which the electrodes are made is approximately 1000 m2.g_1. Once formed in the form of a porous electrode, the active layer of carbon typically has a specific surface area, measured according to the BET method, of between 621 m2.g_1 to 745 m2.g_ 1 (Alvarez-Gallego and others 2012 Electrochim Acta 82: 415, Sharma et al., 2014 Electrochimica Acta 140191) Figure 4 shows the degree of transport of the Ce3 + ions (removal efficiency in%) of the bulk solution in the presence of O2 as the oxidizing gas which is supplied through the gas diffusion cathode and which flows through the compartment. gases and diffuses through the gas diffusion electrode. In this case, approximately the total amount of Ce3 + was removed from the solution (average 99.47 ± 0.53%), as shown in Figure 4. Contrary to the previous case, the elimination efficiency does not increase as a function of the metal concentration in the solution, which indicates that ion exchange adsorption is not the predominant phenomenon as in a classic case of electrosorption (see result 1).

The removal efficiency when O2 is supplied through the gas diffusion cathode was much more significant than in the case where only N2 was supplied.

Not only did the removal of Ce3 + ions from the solution take place, but a stable solid phase was also formed. Figure 5 shows the recovery efficiency (%) of the Ce3 + ions transformed into a solid product recovered as a precipitate after being released from the electrode and sedimented in solution, in the presence of O2 as the oxidizing gas supplied through the gas diffusion cathode, on the basis of the dry weight of the recovered product.

The solid phase is composed of isotropic nanocrystals of CeO2 identified by the XRD and the microscopic evidence described below, which precipitated at the interface between the porous electrodes of activated carbon gas (GDE) diffusion and the adjacent aqueous electrolyte. Initially they were identified as nano colloidal particles dispersed in solution, which were added and precipitated as the process maintained its course. Some of these are released into the bulk electrolyte, while others remain attached to the electrode and are only released after stopping or reversing the electrical polarization.

Higher recovery percentages were obtained at lower concentrations of Ce3 +. It should be noted that the low recovery efficiencies are not due to the low conversion rates. The discharge of the crystalline nanoparticles was not carried out by other means than the inversion of the flow. Those nano particles that could be collected within that investment time are those that were quantified. In this case, polarization inversion increases recovery.

It is further believed that intermediates, by-products (for example, an adsorbed form of superoxide O ² m ' ( ads)) and electrosynthesized H2O2 play a role. It was found that the behavior of EIS is typical of faradic reactions (charge transfer) coupled by adsorbed intermediates (Wu and other 2012 Chem Rev, 112: 3959), as seen in Fig. 6a. The response of the CV (Fig. 6b) also indicated that the limiting process in the GDE at -0.350 V versus Ag / AgCl was no longer the capacitive storage of ions or electrosorption alone, but an electrocatalytic reduction, presumably a reduction of O2 to H2O2. This is part of the proof that the nano-crystal formation process does not occur in the solution as a result of changes in pH, but in the porosity of the electrode as a consequence of electrocatalytic reactions, as observed in the characteristic response of EIS speed limitation for adsorption that is linked to the formation of superoxide intermediates in carbon materials. This latter response is not specifically observed when incorporating non-porous metal catalysts in the gas diffusion electrode, since in that case the mechanism does not involve the adsorption of such superoxide radicals that trigger supersaturation conditions within the pores of the electrochemically active material of the electrode.

Figure 6 shows the electrochemical response obtained for the experiments in which air was supplied through the gas diffusion electrode:

Figure 6a shows the frequency response obtained by Electrochemical Impedance Spectroscopy (EIS) recorded at an amplitude of 20 mV, in the frequency range from 100 kHz to 3 mHz.

Figure 6b shows the cyclic voltammetry response obtained at a scanning speed of 1 mV.s-1.

Figures 6c and d show the typical EIS responses for the adsorption-limited processes linked to the charge transfer reactions.

The symbols in Figure 6 have the following meaning:

or you, Cce ³ +, i = 0 mg.L-1 Ce3 +.

• tf, Cce ³ +, i = 0 mg.L-1 Ce3 +.

A to, Cce ³ + i = 10 gL-1 Ce3 +.

▲ t, Cce ³ +, i = 10 gL-1 Ce3 +.

Figure 7 shows the size of the crystallite and the network parameter found for the different initial concentrations of Ce3 + studied.

Figure 7a shows the size of the crystallite (220) for CeO2 and NaCl.

Figure 7b shows the network parameter of CeO2 and NaCl. There was a limit of detection for both parameters in Ce below 20 mg.

Figure 8 shows the steady-state current density as a function of time for different oxygen-reducing gas diffusion cathodes, with and without CeO2 catalyst after 120 minutes of electrocatalytic production of hydrogen peroxide. N: Norit, V: Vulcan-Norit, AB: Acetylene Black-Norit. All the electrodes were composed of a mixture of 80% carbon and 20% polymer (PVDF).

The crystal size of the crystalline product varied in a gradient depending on the initial concentration of Ce3 +, but also proportionally to the concentration of H2O2 found in the solution (Figure 7a). At lower concentrations of Ce3 + the size of the CeO2 crystal is smaller, while as the concentration increases, the size of the crystal is larger. The average crystal size for CeO2 was 3.5 ± 0.337 nm, while for NaCl it was 45.1275 ± 0.337. This makes possible further separation, either by redissolution of NaCl with a pH where the CeO2 is still stable, for example, pH> 10, or by size exclusion (for example, by screening) after drying. The network parameters observed in Figure 7b also varied according to the initial concentration of the Ce3 + ion and proportionally to the concentration of H2O2 found in the solution. It is possible that Ce3 + plays a cocatalytic role in the electrosynthesis of H2O2 itself.

Figure 8 shows transmission electron micrographs showing the characteristic morphology of the CeO2 nanoparticles with crystallite sizes that coincide with those obtained by XRD.

Figure 9 shows transmission electron micrographs showing the aggregation of the small crystalline nanoparticles of Figure 8 into larger nanoparticles.

Figures 8 and 9 in fact show characteristic fingerprints of the materials formed by the method of this invention, whose properties can be readjusted as controlled variations in physicochemical or electrochemical conditions are provided.

To release the nano crystals from the electrochemically active layer of the gas diffusion electrode, various techniques can be used. Suitable techniques include reversing the polarity of the electrode and accelerating the electrolyte flow rate. The fact that these techniques cause the release of the nanocrystals in the catholyte is further proof that the process takes place in the pores of the electrochemically active layer and not in the electrolyte solution. In fact, when comparing the size and morphology of the product obtained simply by changing the pH of the electrolyte, the difference is clear. Nano crystals are not obtained and instead an amorphous product appreciated by XRD is visible. In addition, the nanocrystal product obtained by this invention shows a characteristic monodisperse morphology similar to the foam (see Figure 12 and 14 HRTEM) while the product obtained by simple addition of a base until an alkaline pH is reached shows a high non-uniformity .

Example 2

The independent electrodes were tested as gas diffusion cathodes, in the presence of air in the gas compartment (to provide O2 for reduction to H2O2, its ions or polyatomic radicals). The reagents presented in Table A were dissolved in demineralized water and the pH of the solution adjusted to about 4.

Table A: Composition of the catholyte in demineralized water.

Chemical Name Chemical Formula Quantity (mg.L-1) 1 Cerium Nitrate hexahydrate Ce (NO3) 3'6H2O 350

2 Dysprosium Nitrate x hydrated Dy (NO3) 3xH2O 80

3 Erbium nitrate pentahydrate Er (NO3) 3'5H2O 53

4 Europium nitrate pentahydrate Eu (NO3) 3'5H2O 6

Gadolinium Nitrate hexahydrate Gd (NO3) 3'6H2O 59

6 Holmium nitrate pentahydrate Ho (NO3) 3'5H2O 18

7 Lanthanum nitrate hexahydrate La (NO3) 3'6H2O 159

8 Lutetium nitrate hydrated Lu (NO3) 3xH2O 6

9 Neodymium Nitrate hexahydrate Nd (NO3) 3'6H2O 206

10 Praseodymium nitrate hexahydrate Pr (NO3) 3'6H2O 51

11 Samarium nitrate hexahydrate Sm (NO3) 3-6H2O 53 12 Nitrate of terbium hexahydrate Tb (NO3) 3'6H2O 12 13 Nitrate of thulium pentahydrate Tm (NO3) 3'5H2O 8 14 Nitrate of yttrium hexahydrate Y (NO3) 3 ' 6H2O 536 15 Ytterbium nitrate pentahydrate Yb (NO3) 3'5H2O 50

In addition, 30 g / L NaCl was supplied and dissolved. The operating volume of the catholyte in each experiment was 125 ml.

The concentration of the different metals was analyzed quantitatively by ICP-MS.

The process was applied to constant polarization at -0.350 mV against the reference electrode referred to above for a period of 2 hours. After a few minutes of processing (<20 min), the color of the electrolyte progressively changed from clear to white in an appreciable cloudy phase. The process showed a gradual change in pH up to 11. Current densities greater than 40 mA.cirr2 were recorded under constant cathodic polarization conditions. After the process was stopped, the solid particles formed were added and sedimented, which left a clear liquid medium and a separable solid precipitate phase.

It was found that most of the metal content was removed from the solution (Figure 10), ie,> 99.9 for all metals together, as determined by ICP-MS.

Figure 10 shows the removal efficiency (%) of the different metal ions of the bulk solution in the presence of air supplied through the gas diffusion cathode.

A mixed crystalline concentrate was obtained. In total, 91 mg of REE solid content was recovered, which corresponds to approximately 25% of the total ionic (dissolved) REE content in the original aqueous matrix. The isolated products showed crystalline properties that coincide with crystallite sizes of 1.97 nm, 1.71 nm and 2.29 nm, respectively.

Example 3

The electrolyte composition was identical to that of Example 1, but lanthanum nitrate was used instead of cerium nitrate, in concentrations of 0 ppm, 100 ppm, 500 ppm, 1000 ppm and 5000 ppm.

The initial pH and conductivity of the catholytes containing the different metal concentrations are described in Table b. The operating volume of the catholyte in each experiment was 125 ml.

Table B: Measurement of the pH and conductivity of the catholytes with different concentrations of lanthanum nitrate (NO3) 3-6H2O at the beginning of the experiment.

Concentration (ppm) 0 100 500 1000 5000 Catolith PH 2.54 2.15 2.70 2.78 2.76

Conductivity (mS.cm-1) 51.0 51.6 50.4 50.3 52 Anolyte PH 2.8 2.8 2.74 2.74 2.74

Conductivity (mS.cm-1) 49.7 49.7 49.8 49.8 49.8

The concentration of lanthanum was analyzed quantitatively by ICP-MS.

A colorless solution was formed by dissolving the chemicals. Air was supplied to the gas compartment. After 2 h processing under constant polarization conditions of -0.350 V vs. Ag / AgCl (3M KCl), the color of the electrolyte remained clear during the experiment. However, by stopping the polarization and reversing the flow, a visible white haze was released in the medium. The amount of product released (or turbidity) corresponds to the initial concentration of lanthanum nitrate. After about one hour, all the cloudy product had precipitated. The changes in pH were similar to those observed in the catholyte in Example 1, the pH of the catholyte increased significantly at the end of the experiments in which air was supplied through the gas diffusion chamber. The conductivity and pH of the catholyte and the anolyte remained almost the same. A slight overall decrease in the conductivity of the catholytes could be debated.

Table c: pH and conductivity of the catholytes for different concentrations of La (NOa) 3-6H2O at the end of the experiment.

Concentration (ppm) 0 100 500 1000 5000 Catolith PH 11.5 12.47 11.62 11.8 5.37

Conductivity (mS.cm-1) 49.1 49.9 51.6 50.7 47.4 Anolyte PH 2.37 2.78 2.23 2.20 2.68

Conductivity (mS.cm-1) 49.7 49.9 50.7 49.5 47.5

The clear solution and the solid white precipitate were separated and analyzed. In all cases, more than 99.9% of lanthanum had been removed from the solution. When analyzing the white precipitate by XRD, the produced solid showed characteristics of crystalline nanoparticles that coincide with those of burbankite and more specifically with lanthanum remondite, this is Na3La3 (CO3) 3. In addition, an amorphous phase was detected.

Example 4

The composition of the electrolyte was identical to that explained, for Example 1, but instead of cerium nitrate, a boric acid was supplied in the catholyte. The concentration of boric acid was kept constant in all experiments (5 g.L-1). The effect of the polarization potential was evaluated. The following potentials were compared to the reference electrode: -0.350 V, -0.550 V, -0.750 V, -0.950 V. The operating volume of the catholyte in each experiment was 125 mL.

Table D: pH and conductivity of the catholyte at the beginning of the experiment at different cathode potentials.

Applied potential (V vs Ag / AgCl 3

_{M of KCl)} -0.150 -0.350 -0.550 -0.750 -0.950 Catolito pH 2.67 2.63 2.51 2.51 2.76

Conductivity (mS.cm-1) 47.1 47.4 47.2 47.4 46.9 Anolyte pH 2.74 2.74 2.8 2.8 2.8

Conductivity (mS.cm-1) 49.8 49.8 49.7 49.7 49.7

Of the changes in pH and conductivity, especially of the catholyte, it can be observed that the same tendency found in previous examples was observed; that is, the pH increased significantly throughout the experiment. However, only in the case of -0.950 V could a visible color change from the electrolyte to yellow be observed.

The change in pH was directly related to the applied potential. The pH change occurred during the first hour of the experiment and even when the polarization time increased more (ie, from 2 to 4 h), it did not result in pH variations in larger magnitudes.

Table E: Measurement of the pH and conductivity of the catholytes at the end of the experiment with different applied cathodic potentials.

Applied potential (V vs Ag / AgCl 3 M

_{of KCl)} -0.150 -0.350 -0.550 -0.750 -0.950 Catolito pH 5.37 6.55 6.92 8.45 8.5

Conductivity (mS.cm-1) 45.2 46.6 46 44.1 45 Anolyte pH 2.68 2.13 2.15 2.13 2.15

Conductivity (mS.cm-1) 47.5 51.0 50.3 50.5 50.8

After centrifugation and drying, a crystalline product could be recovered that fulfilled the characteristics of the sassolite, except that it formed at -0.950 V, had characteristics of nanocrisaline borax Na2B4O7-10H2O. This last product can have application as a dopant in flexible and fluorescent electronics. The following example shows the electrochemical series that shows the reduction potential of different oxidizing gases to peroxides or their ions or polyatomic radicals. The potentials reported in this table refer to the normal hydrogen electrode (NHE). These potentials are calculated by the Nernst equation.

Table F

Redox pair (half-cell reaction) Potential for thermodynamic reduction of the half-cell reaction at pH 1 (V vs NHE)

NO / NO3- 0.958

O2 / H2O2 0.547

O2 / HO2 ^ -0.13

CO2 / HOOCCOO- -0.43

CO2 / HCO2- -0.49

CO2 / HCOO- -0.61

Example 5

Eight different electrolytic compositions were tested, as described in Example 1, but instead of cerium nitrate, the following independent compounds were supplemented with the catholyte at a concentration of 500 ppm, respectively:

1. Co (NOa ^ 6H2O

2. Al (NOa) 3 ^ 9H2O

3. CsCl

4. LiCl

5. KNO3

6. Sm (NO3 ^ 6H2O

7. Er (NO3 ^ 5H2O

8. Lu (NO3 ^ xH2O

9. Fe (NO3) 3

The process was carried out at -0.350 V. The operating volume of the catholyte in each experiment was 125 mL. In all cases, a crystalline nano product could be recovered except for KNO3. In each case in which a precipitate was found, it had characteristics (color, morphology) associated with the specific precursor used, which shows that nano-crystalline materials can be formed with a wide variety of metals from different groups in the periodic table. In all cases, the crystallite size was <30 nm.

Example 6

A catholyte was prepared, identical to the one explained for Example 1, but instead of cerium nitrate, arsenic acid was used in a solution containing 72 g / L of arsenic and iron chloride with an Fe / As ratio of 1.5 and acid. sulfuric with 5.61 g / L of S. The process was carried out in the same experimental conditions as in example 1, but at an operating temperature> 80 ° C. After processing, stable nanocrystalline scorodite with a crystallite size <30 nm was obtained. This modality may be applicable to immobilization with arsenic.

Example 7

Experimental conditions

In the present example, the claimed process was studied in the presence of ionic liquids based on ammonium (IL). More specifically, tetrabutylammonium bromide (TBAB) and tetrabutylammonium chloride (TBAC) were used. There are few literatures that show the use of ILs for the synthesis of metallic organic structures (MOF) by ionothermic synthesis (Lei Liu, David S. Wragg, Hongyan Zhang, Ying Kong, Peter J. Byrne, Timothy J. Prior, John E Warren, Zhuojia Lin, Jinxiang Dong and Russell E. Morris, lonothermal synthesis, structure and characterization of three-dimensional zinc phosphates, Dalton Trans., 2009, 6715-6718. Few literatures also report that ILs stabilized with nanoparticles can behave like Pickering emulsions in water (Huan Ma and Lenore L. Dai .. Particle Self-Assembly in lonic Liquid-in-Water Pickering Emulsions, Langmuir, 2011, 27 (2), pp. 508-512) that is, they assemble themselves to form cage-like structures where some gas can be trapped. These emulsions are typically stable as long as the medium is not disturbed. Otherwise, ILs can be used as electrolytes due to their conductive properties (Chiappe and Rajamani 2012, Chiappe and others 2011 and Andrzej Lewandowski, Agnieszka Swiderska-Mocek), and liquids as electrolytes for Li-ion batteries-An overview of electrochemical studies. Journal of Power Sources. Volume 194, issue 2, December 1, 2009, Pages 601-609.). All reagents were used as purchased without further purification.

Demineralized water was used throughout the study. The activated carbon used was Norit® SX1G from Norit Americas Inc. The fluorinated ethylene propylene resin (Teflon® FEP 8000) was obtained from Dupont. Two ILs based on ammonium base were purchased, ie TBAB (Sigma-Aldrich> 98.0%) and TBAC (Aldrich> 97.0% (NT)). Lanthanum nitrate hexahydrate (La (NO3) g.6H2O) was purchased from Aldrich® (99.99% trace metals). All reagents were stored according to the recommendations of the corresponding provider. The handling of all the reagents was carried out in accordance with the recommendations of the corresponding Material Safety Data Sheets (MSDS). The experiments were performed in semi-cell electrochemical reactors equivalent to those described in example 1, unless otherwise specified. The main functions of Zirfon® here were to prevent the oxygen that finally emanated in the counter electrode from reaching the electrode in operation, to serve as an ion-permeable separator, and also as a barrier to the transport of fluids to maintain the desired pH in the compartment. the catholites. The electrodes and the separator had a projected electrode surface area of 10 cm 2. The reactant gas (oxidant) streams (air or N2, respectively) were fed through the cathode gas compartment in each individual experiment. The gas flow rate was set at 200 mL / min (excess) in all cases and an overpressure of 30 mbar was applied. The electrolyte feeds circulated independently through the cathode and counter electrode compartments with a double head peristaltic pump, at a flow rate of approximately 20 rpm (Watson-Marlow). In all the experiments related to this example an acidic basal electrolyte was used, consisting of demineralized water supplemented with HCl to achieve an initial pH of 2.7. The effect of two independent oxidizing gases, also known as air (the composition is Nitrogen: 78%, Oxygen: 20%, CO2: 0.03%, inert gases mainly argon: 0.97%, water vapor: 1%) and CO2 was investigated. / N280% / 20%, respectively. La (NO3) 3'6H2O was added to the cathode electrolyte at a concentration of 500 ppm. Different molar ratios between IL and La (NO3) 3-6H2O were investigated as follows: 0, 0.1, 1, 10, 20, 30 and 86.5. The last molar ratio corresponds to a concentration of 0.1 M for ILs. The electrolyte in the counter electrode compartment (anodic) was identical to that of the catholyte (acid basal electrolyte), but without the addition of lanthanum nitrate or IL.

The tests were carried out at room temperature (18 ± 2 ° C). Parameters such as H2O2 concentration, conductivity and pH were measured before and after each experiment in both sets. Three more cases were carried out in which the catholyte was only acidic basal electrolyte, La (NO3) 3.6H2O in basal acid, TBAC (0.1 M). At higher concentrations of TBAB (especially 0.1 M) precipitates or yellow by-products were obtained in the anolyte chamber which may be some bromide or nitrate compounds. The use of TBAb (0.1 M) without the metallic nitrate in the acid basal electrolyte showed no positive results since the potential did not reach -0.95 V versus Ag / AgCl. At the highest concentration for both TBAB and TBAC, the experiments were performed at -0.35 V versus Ag / AgCl. A Bio-Logic VMP3 potentiostat / galvanostat and a frequency response analyzer were used to perform the electrochemical measurements. The EC-Lab v.10.40 software was used for data acquisition. A linear sweep voltammetry (LSV) from 0 to -0.950 V followed by chronoamperometry (CA) experiments at -0.950 V versus Ag / AgCl until reaching a pH of 11. The concentration of La was analyzed quantitatively in samples of catholyte and anolyte solutions, respectively, by means of ICP-MS. First, the sample solutions were pipetted from the supernatants. Each sample was diluted with a factor x and acidified before analysis. For the case of 10x dilution, 1 mL of sample solution was mixed with 1 mL of HNO3 and the result was diluted with 10 mL with Milli-Q water. For the 100x dilution case, a dilution was used up to a final volume of 100 mL. The calibration of the observed ion intensities was carried out against a series of calibration standards prepared on the basis of a 1000 mg / l solution of Certipur, Merck no. of Cat 1.70311.0100, solution of Ce (NO3) 3 in 2-3% HNO3. Calibration standards of 0, 1, 2, 5, 10, 20 ppb, 100, 200, 500, 1000 and 2000 ppb were prepared. All measurements were made with a Varian 820 ICP-MS. A mass calibration was performed daily; ion lenses and gas flows were optimized to obtain a maximum signal-to-noise ratio with a multi-element standard of 10 ppb provided by Varian.

The conductivities of catholytes measured before the different individual experiments performed are presented in Table 1. At the end of all the experiments, the conductivity decreased considerably, which was related to a decrease in both the amount of IL present in the electrolyte and the concentration of La in the electrolyte (Table G2). The IL present in the electrolyte was removed from the electrolyte by 60-75% in all cases and was partially found in the precipitates. However, after the precipitates were washed with distilled water, most of the IL present was washed.

Table G. Initial and final conductivity of the catholytes used for the different experiments.

Experimental condition Initial conductivity (mS / cm) Final conductivity (mS / cm)

Without adding IL 0.853 No change

TBAB 10 1,885 0.095

TBAB 20 2.72 0.190

TBAB 30 3.57 0.280

TBAB 86.5 7.57 0.300

TBAC 10 1.873 0.116

TBAC 20 2.78 0.160

TBAC 30 3.38 0.375

TBAC 86.5 7.56 0.378

In addition to IL, mg / L amounts of formic and acetic acid were found, without having a particular correlation with the amount of IL added. The inventors believe that the quantified formic acid and acetic acid are the result of the reduction of CO2 or the partial degradation of IL. The inventors further believe that these products can serve as linkers between the IL or its degradation products and the nanocrystalline products formed within the process, similar to what occurs in the more classical electrochemical formation of the MOF. The product formed at -0.350 V has the appearance of the classic precipitates. The formation of the product at -0.350 V was compared with the formation at -0.950 V. As shown by the IR spectra of Figures 13a, b and c, the characteristic response and, therefore, the characteristic links of the product are identical and differ only in intensity in correlation with the concentration of IL used and not by the type of IL. Furthermore, it is shown that only a part of the characteristic bonds of the IL remains in the precipitate, which suggests that at least a part of the IL remains part of the structure of the precipitated product. The IR spectra of the precipitate samples were compared before (P2) and after (P3) the washing, which shows the elimination of an excess of IL but still after washing the structural effect of part of the IL remains. Thus, it is believed that the product formed is a metal-organic compound with characteristics of both IL and nanocrystals.

Of the IR spectra of Figure 13a, b and c respectively, the inventors could interpret that the peak a is related to the binding of lanthanum and nitrogen in the case of the nitrate of the original. This peak becomes wider when the lanthanum binds to the TBA nitrogen. Peak b is related to the N-CH2 bond. This peak, however, appears in the first figure (part not washed), but disappears in the second figure, which indicates that some TBAB molecules remain as surfactants in these compounds. Peak c (many peaks) are seen in the case of original TBAB (lower blue curve in Figure 13b). These refer to the C-N bonds in the aliphatic amines. In the figure corresponding to the washed samples (P3) it can be seen that some of these peaks are lost, although not all indicate that there are some changes in the union between the nitrogen and methylene groups. It must be taken into account that under the condition of polarization applied to - 0.950 V, it is also possible the formation of hydrogen. Under this condition, the precipitates were still initially seen as discrete precipitates, however, it was noted that they allowed the sequestration of the hydrogen formed in situ and finally they formed a product similar to foam that floats on a column of the catholyte due to the hydrogen that they transported (see Figure 14). The inventors believe that this product behaves similarly to a Pickering emulsion where the nanocrystals formed stabilize the remaining IL by self-assembly in solution, which forms the characteristic structures of the cage where the gas can be sequestered. The product also showed the characteristic of the stability Pickering emulsions until the medium is disturbed by simple agitation. Under these conditions, hydrogen bubbles are released back into the air. The amount of hydrogen sequestered in the nanocrystalline product formed was estimated to be above 10% by volume with respect to the solid precipitate.

Figure 14 shows nanocrystals precipitated in the presence of excess IL and under hydrogen electrosynthesis in situ. Small bubbles in the 0.1 mm size range were visible, occupying more than 10% of the volume of the floating material.

The XRD patterns of the washed precipitated product were recorded. Although the XRD pattern did not correspond to any correspondence in the available databases, the product exhibited characteristics of high crystallinity and of a nanomaterial with a small crystallite size (for example, <10-5 nm). Figure 15 below shows a characteristic XRD spectrum for TBAB 30.

The characterization by scanning electron microscopy (SEM) showed N, C, La and O and in the elemental analysis of the precipitate, with a strong correlation in position, which is a second indicator of the structural incorporation of at least part of the IL in the nanocrystals. In addition, Bry Cl from IL was found, but they were lost when the precipitate was washed.

The elemental analysis of the products formed showed an approximate 1: 1 ratio of hydrogen and carbon. Based on the electrochemical response obtained, the load consumed for each experiment was quantified and the results are shown in Figure 12. An exponential correlation was found between the load consumed and the IL concentration used, which indicates that the IL participates effectively in the electrochemical process.

The characterization of TGA (see Figure 12) of the obtained precipitates indicates that the nanocrystalline product does not contain the complete ionic liquid but only fractions thereof, which can be incorporated after the partial degradation of the IL during the process. It is believed that the polyatomic ion radicals emitted by the reduction of the oxidizing gas contribute to this degradation process of the IL and, ultimately, to the linking of the fractions of the IL with the final structure of the precipitate. Although the structure of the nanocrystalline product can not be fully established at this time, the authors support the idea that at least a part of the IL is found in the final nanocrystals, which are a metal-organic product.

More than 83% of the initial La precursor could be eliminated when CO2 was used only as an oxidizing gas, while more than 98% was removed when the oxidizing gas comprised air. The use of pure CO2 led to the formation of lanthanum carbonates, while the use of air led to the formation of the metal-organic nanocrystalline product described above.

Claims (26)

Claims An electrochemical process for recovering reaction products of a metal element or a metalloid element or a mixture of two or more of these from at least one water-soluble precursor compound comprising the metal element or a metalloid element or two or more of these, in the form of nanoparticles, particularly nano crystals of at least one reaction product, wherein the process comprises the steps of - supplying a solution of the water-soluble precursor compound to a water-based catholyte of a cathodic compartment of an electrochemical cell, equipped with a cathode comprising a gas diffusion electrode, wherein the gas diffusion electrode comprises a material electrochemically active porous having a BET surface area of at least 50 m2 / g, - adjust the pH of the catholyte to a pH that is lower than the pKa of the water soluble precursor compound, - supply at least one oxidizing gas to the gas diffusion electrode, - subjecting the cathode to an electrochemical potential for reducing at least one oxidizing gas to one or more of the peroxides, ionic reactive species and / or corresponding radicals capable of reacting with a cation comprising the metal element, or the metalloid element or a mixing two or more of these, to nano particles of at least one reaction product with an average particle size, particularly nano crystals with an average crystallite size, equal to or less than 30.0 nm. 2. A process, as claimed in claim 1, wherein at least one water-soluble precursor compound comprises the metal element or the metalloid element in the form of a cation of that metal or metalloid element, or at least one precursor compound soluble in it. Water comprises a cation comprising the metal element or the metalloid element or a mixture of two or more of these, or at least one water-soluble precursor compound comprises a mixture of two or more of the aforementioned compounds. 3. A process, as claimed in claim 1 or 2, wherein the oxidizing gas is an inorganic oxidizing gas or a mixture of two or more such gases. 4. A process, as claimed in claim 3, wherein the inorganic oxidizing gas is selected from the group of ozone, oxygen, carbon oxides, nitrogen oxides, halogenated oxides, sulfur oxides, halogens, air, biogas, gases of combustion, exhaust gases, acid gases or a mixture of two or more of the gases mentioned above. 5. A process according to any of the preceding claims, wherein a rate of supply with which the oxidizing gas is supplied to the gas diffusion electrode is variable. 6. A process according to any of the preceding claims, wherein a partial pressure of the oxidizing gas that is supplied to the gas diffusion electrode is variable. A process as claimed in any of the preceding claims, wherein at least one water-soluble precursor compound is supplied to the water-based catholyte as a solution of the water-soluble precursor. A process as claimed in any of the preceding claims, wherein at least one water-soluble precursor compound is supplied to the water-based catholyte as a solid, and is at least partially dissolved in the catholyte. 9. A process as claimed in any of the preceding claims, wherein the concentration of at least one water-soluble precursor compound that is supplied to the water-based catholyte is variable. 10. A process as claimed in any of the preceding claims, wherein the catholyte has an ionic conductivity of at least 1.0 mS / cm, A process as claimed in any of the preceding claims, wherein the electrochemically active material comprises an active surface having at least one weak acid or a weak basic functional group or a mixture of at least one weak acid and a basic group weak, preferably the electrochemically active material comprises at least a weak protonic acid or a weak basic protonic functional group or a mixture thereof, at least a weak protonic acid and a weak basic protonic group. 12. A process as claimed in any of the preceding claims, wherein a hydrophilic side of the electrochemically active surface of the gas diffusion electrode encompasses a plurality of functional groups that can be polarized or deprotonated upon application of the electrochemical potential, wherein the functional groups preferably contain one or more portions selected from the group of a portion containing nitrogen, a portion containing oxygen, a portion containing chlorine or a portion containing sulfur. 13. An electrochemical process as claimed in any of the preceding claims, wherein before supplying the water-soluble precursor compound to the cathode compartment, the pH of the catholyte is adjusted to a pH of <7, preferably at a pH of < 5. An electrochemical process as claimed in any of the preceding claims, wherein an aqueous solution of a weak protonic electrolyte is supplied to the catholyte, preferably a weak protonic base or a weak protonic acid, more preferably a weak polypropionic base or a weak polyprotonic acid. 15. An electrochemical process as claimed in claim 14, wherein the weak protonic acid has a pKa that is at least one unit higher than the pH of the catholyte. 16. An electrochemical process as claimed in claim 14, wherein wherein the weak protonic base has a pKa of between 6.0 and 12.0, preferably between 7.0 and 11.0. 17. An electrochemical process as claimed in any of the preceding claims, wherein the electrochemically active material has a BET surface area of at least 100 m2 / g, preferably at least 200 or 250 m2 / g, more preferably at least 400 or 500 m2 / g, most preferably greater than 750 or 1000 m2 / g. 18. An electrochemical process as claimed in any of the preceding claims, wherein a binary electrolyte is supplied to the catholyte in order to raise the electrical conductivity of the catholyte, preferably to raise the electrical conductivity of the catholyte by at least 5 mS.cm- 1, more preferably between 20 and 80 mS.cm-1, most preferably between 20 and 50 mS.cm-1. 19. An electrochemical process as claimed in claim 18, wherein the binary electrolyte is a water-soluble ionic salt or a mixture of two or more such salts, but preferably the binary electrolyte is sodium chloride, An electrochemical process as claimed in any of the preceding claims, wherein an ionic liquid is supplied to the catholyte, in a molar ratio of at least 10: 1 with respect to the water soluble precursor compound, in order to produce nanocrystals containing the ionic liquid or one or more of its decomposition products. 21. An electrochemical process as claimed in claim 20, wherein the ionic liquid is a hydrophilic ionic liquid, preferably tetrabutylammonium chloride or tetrabutylammonium bromide. 22. An electrochemical process as claimed in any of the preceding claims, wherein the electrochemical potential to which the cathode is subjected is a reduction potential in relation to a reference electrode, which is below the thermodynamic equilibrium region of the electrode. pH potential of the stability of the oxidant gas in the water, preferably below the thermodynamic stability region of the water, preferably outside the region of thermodynamic stability of the hydrogen. An electrochemical process as claimed in any of the preceding claims, wherein the precursor compound is a compound of an ion of an element selected from the group of elements of groups I, II, III and IV of the periodic table of elements , transition metal elements, actinides and lanthanides or a compound containing two or more such elements. 24. An electrochemical process as claimed in any of the preceding claims, wherein the electrochemical potential to which the cathode is subjected is reversed in order to recover the nano crystals of the electrochemical active porous material at the gas diffusion cathode. 25. An electrochemical process as claimed in any of the preceding claims, wherein one or more additives are supplied to the catholyte, the one or more additives that are selected from the dispersants, stabilizers, surfactants, polymers, copolymers, emulsifiers, crosslinking agents , finishing agents and free flowing agents or mixtures of two or more of these compounds. 26. A method for selectively isolating a reaction product from at least one metal element or metalloid element or two or more thereof from an aqueous solution containing a water soluble precursor compound, wherein the process is used as claimed in any of claims 1-24.