EP3042981A1

EP3042981A1 - An electrochemical process for preparing a compound comprising a metal or metalloid and a peroxide, ionic or radical species

Info

Publication number: EP3042981A1
Application number: EP15150649.0A
Authority: EP
Inventors: Xochitl Dominguez Benetton; Yolanda Alvarez Gallego; Christof Porto-Carrero; Katrijn Gijbels; Rajamani Sunita
Original assignee: Vito NV
Current assignee: Vito NV
Priority date: 2015-01-09
Filing date: 2015-01-09
Publication date: 2016-07-13
Also published as: DK3242963T3; MX2017009005A; US20180023201A1; EP3242963B1; CN107532309A; JP2018508659A; CN107532309B; ES2702082T3; EP3242963A1; CA2973289A1; WO2016110597A1

Abstract

The present invention relates to an electrochemical process for isolating from at least one water soluble precursor compound comprising a metal or a metalloid element or two or more thereof having a positive valence, a reaction product of the metal or metalloid element or two or more thereof, wherein the water soluble precursor compound is supplied to a water based catholyte contained in a cathode compartment of an electrochemical cell containing a cathode with an electrochemically active surface in contact with the catholyte, wherein the cathode is subjected to an electric potential, which is chosen such as to cause reduction of an oxidant gas present at the cathode to one or more corresponding peroxide, ionic and/or radical species capable of reacting with the metal or metalloid element, and to cause conversion of the reduced oxidant gas into a reaction product comprising a compound of the metal or metalloid element or two or more thereof and the peroxide, ionic and/or radical species, in particular into nano particles of the reaction product.

Description

The present invention relates to an electrochemical process for isolating from at least one water soluble precursor compound comprising a metal or a metalloid element or two or more thereof having a positive valence, a reaction product of the metal or metalloid element or two or more thereof, according to the preamble of the first claim.
The present invention further relates to a device for carrying out the process of the invention.

Background of the invention.

Nano particles and their composites exhibit unconventional electronic, optical, magnetic and chemical properties with respect to bulk phase particles and macroscopic crystals. Hence, they offer new or improved properties for application in a wide variety of fields ranging from catalysis, cosmetics, textiles, nano-electronics, high-tech components and defense gadgets, to pharmaceuticals, medical uses, sensors and diagnostics. At the smallest sizes (e.g. < 20-50 nm), nano particle properties typically vary irregularly and are specific to 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 their preparation many challenges have to be to overcome, amongst which controlling particle growth, crystallinity, stability and reproducibility. A high quality synthesis procedure should desirably produce nano particles with a narrow size distribution. The narrower the size distribution, the more attractive the synthesis procedure. The best synthesis procedures available today produce nano crystals with a size distribution of about 5%. Shape control is also an important feature. Synthesis methods that provide crystalline nano particles are preferred, as well as the methods that provide shape stabilization. Particularly preferred are synthesis methods that do not employ hazardous solvents, thinking of environmental sustainability.
Modern methods for synthesizing amorphous or crystalline nano particles may include chemical reaction steps, as well as physical treatment and biological steps. Chemical methods for producing crystalline nano particles offer the advantage over physical methods that milder reaction conditions may be used. In comparison with purely biological methods, an improved control may be achieved. Chemical methods typically employ the steps of crystal seeding, permitting particle growth to take place and terminating particle growth once the desired particle size has been obtained. Since these steps are often inseparable, synthesis is often initiated by providing a nano crystal precursor, a solvent and termination (capping) agents. Electrochemical synthesis is often employed for the production of zero-valent, metal nano crystals, by the steps of oxidative dissolution of an anode, migration of metal ions to the cathode and reduction to the zero valent state, nucleation followed by particle growth, addition of capping agents (typically quaternary ammonium salts containing long-chain alkanes) to inhibit growth, and precipitation of the nano crystals. The size of nano crystals may be tuned a.o. by altering current density, varying the distance between the electrodes, controlling the reaction time, temperature and the polarity of the solvent. Chemical and classical electrochemical methods typically result in the formation of nano crystals having an average particle size in the range of 2-100 nm.
US20060068026 discloses a method for preparing a colloidal stable suspension of naked metal nano crystals. The method comprises the steps of at least partly immersing into essentially contaminant-free water, a metallic sacrificial anode that includes an essentially contaminant-free metal starting material for the nano crystals and a cathode; and applying a voltage potential across the anode and the cathode to form a colloidally stable suspension of naked metal nano crystals composed essentially of metal from the metallic sacrificial anode.
When analyzing existing methods for synthesizing nano particles, the inventors realized that the existing techniques can be regarded from a different perspective. The chemical precursors for the nano particles are usually contained in the solution that is being treated in a dissolved state, for example dissolved in an aqueous matrix. Formation of the nano particles and their conversion into a stable solid precipitate, has the consequence that the water soluble ions are removed from the aqueous matrix. The method for synthesizing nano particles can therefore also be regarded 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 raw mineral materials, especially those with high technological interest such as the rare earth elements (REE) which are used in the manufacturing of electronic and telecommunication devices and high-tech applications, strategic and clean energy technologies and defense instruments to name a few examples. The REE are ranked as critical raw materials not only due to their wide applicability, but primarily due to the risk of supply interruption, but probably also to their economic value. A key measure to anticipate REE supply vulnerabilities is recycling from end-of-life products; yet this is far from sufficient to meet the REE demand. As the risk of supply interruption and the value of REE rise, other matrices not yet prospected start to make economic sense for recovery.
WO 2012115273 A1 discloses a method for the extraction and separation of lanthanoid elements and actinoid elements by contacting a solution of these elements with a nanostructure carrying a metal-adsorbent compound, capable of functioning as an adsorbent for the target metal. The adsorbent compound with the metal adsorbed to it is contacted with a back-extraction solution to extract the metal.
Another method for removing ionic species from fluids, for example impaired water supplies, which makes use of capacitive deionization is disclosed in US2011042219 . The method disclosed in US2011042219 employs an electrodialysis and/or an electrodialysis reversal system that utilizes high-surface area, porous, non-Faraday electrodes. The system contains a membrane stack which includes alternating cation-transfer membranes and anion-transfer membranes, as well as a porous cathode and a porous anode. As direct current power is passed through the electrodes, cations and anions migrate to opposing electrodes, thereby causing a separation of the saline water into concentrate and dilute stream lines. A double layer capacitor with a high apparent capacitance may be thus formed on each electrode. The method is typically applicable in industries in which liquids may require ionic species removal including water, pharmaceuticals and food and beverage industries.
However, the above described methods do not provide true, economically feasible recovery rates, where possible in a form which permits re-use of the metal. The existing extraction methods for extracting REE or other critical metals from aqueous matrixes, e.g. to meet regulatory requirements, are insufficient and need to be adapted to provide a commercially interesting product.
The present invention therefore also aims at providing an economically feasible method for isolating from a matrix, in particular an aqueous or water based matrix, at least one water soluble precursor compound comprising a metal or a metalloid element or two or more thereof having a positive valence.
This is achieved by the present invention with a method which shows the technical features of the characterising portion of the first claim.
Thereto, in the electrochemical process electrochemical process for isolating from at least one water soluble precursor compound comprising a metal or a metalloid element or two or more thereof having a positive valence, a reaction product of the metal or metalloid element or two or more thereof, the water soluble precursor compound is supplied to a water based catholyte contained in a cathode compartment of an electrochemical cell containing a cathode with an electrochemically active surface in contact with the catholyte. The cathode is subjected to an electric potential, which is chosen such as to cause reduction of an oxidant gas present at the cathode to one or more corresponding peroxide, ionic and/or radical species capable of reacting with the metal or metalloid element, and to cause conversion into a reaction product comprising a compound which consists of the metal or metalloid element or two or more thereof on the one hand and the peroxide, ionic and/or radical species on the other hand, in particular into nano particles of the reaction product.
The inventors have observed that subjecting the cathode to an electric potential which is chosen such that it is capable of causing reduction of the oxidant gas, a redox transformation of the metal and/or metalloid ion present at the electrochemically active surface of the cathode changes to a higher electrochemical oxidation state. Thereby the metal and/or metalloid cations dissolved in the catholyte get oxidized and form an interface with the electrolyte, which adheres at least temporarily to the electrochemically active surface of the cathode.
The inventors have observed that the oxidized metal and/or metalloid cations may accumulate at that interface, in a physical state which is different from the physical state of the surrounding electrolyte, so that they may be separated therefrom. Depending on the nature of the metal or metalloid cation, the reaction product may for example accumulate on the interface in the form of crystalline or amorphous nano particles, which may grow with time to take a larger size as the reaction proceeds, to form a different physical state which is different from the physical state of the electrolyte and permits isolation of the reaction product from the cathode and the electrolyte. The reaction product may be released in a variety of physical forms, for example in the form of a precipitate, or in the form of colloidal nano particles, for example in the form of a colloidal dispersion. After having been released into the electrolyte, the particles may further aggregate to form a stable solid phase, a separable precipitate or gel phase.
Metal and metalloid ions may take various oxidation states and form with the species which result from the reduction of the oxidant gas, reaction products or compounds which contain one or more polyatomic ions, in an oxidation state which leads to a phase that may be separated from the catholyte and from the cathode. The skilled person will be capable of identifying those oxidized compounds which form a separable phase in a water based electrolyte, and select the appropriate electric potential and pH. Pourbaix "Atlas of electrochemical equilibria in aqueous solutions", second edition 1974 discloses the solubility and stability as ions or solid compounds of several metals and their oxides as a function of the voltage potential and the pH. Diagrams for a wide variety of species can be constructed based on the premises provided therein. The skilled person is capable of identifying the electric potential at which electrochemical reduction of the oxidant gas, and the corresponding oxidation of the metal cation or metalloid cation may occur. The inventors have further observed that varying of the electrochemical potential at the cathode, permits to control the chemical composition of the reaction product.
Without wanting to be bound by this theory, the inventors assume that the reduction of the oxidant gas present in the cathode compartment may give rise to the formation of one or more peroxide, ionic and/or radical species, usually polyatomic species, which are adsorbed to the electrochemically active cathode surface.
The inventors further believe that the water soluble precursor compound is dissolved in the electrolyte, in particular in the catholyte, in an at least partly dissociated state :

MA ↔ M⁺ + A^-

and that the metal ion or the metalloid ion or a mixture of two or more hereof, may migrate from the solution towards the cathode and adhere to the electrochemically active surface of the cathode, whereby an electric double layer may be formed. Adhesion of the metal or metalloid ion may take place through various mechanisms, for example capacitive adsorption or reversible ion exchange adsorption, complexating or chelation, but any other forms of adhesion may take place as well.
The inventors further believe that at least part of the functional groups present on the surface of the electrochemically active layer will be present in an at least partially dissociated state (C*-R ^-), especially when an electric potential is applied to the electrode. These dissociated charged sites C*-R ^- may form ion exchange sites for the positively charged metal or metalloid ion. The surface of the electrochemically active layer may for example comprise weak protonic acid sites in the form (C*-RH), where C* represents an active site on the electrochemically active layer of the cathode.
In the presence of an oxidant gas such as oxygen or any other oxidant gas, the availability of the active sites in a dissociated state may accelerate:

C*-R^- + O ² _(g)+ e^- → C*-RO_2ad ^•-

C*-RO_2ad ^•- + H₂O + e- → C*-RO_2ad•H_ad + OH

C*-RO_2ad•H_ad + e^- → C*-R^- + HO₂ ^- _aq
A positively charged metal or metalloid ion may be adsorbed either directly to a C* - R^- site or to a reduced species of the oxidant gas, for example a peroxide radical, an ionic or other radical species, the peroxide radical being the most active situation, thereby forming a polymetal ion polyoxy radical, which may act as a nucleation site for the formation of the oxidized compound on the surface of the electrochemically active material of the cathode. For the case of the cerium ion (Ce³⁺), this may lead to the following reactions :

C*-RO_2ad •- + 2Ce ³⁺ + 2e- → C*-RO_2ad2Ce_ad

C*-RO_2ad2Ce_ad + 2O _{2 (g)} + 2e- → C*-RO_2ad2Ce_ad2O_2ad ^•

C*-RO_2ad2Ce_ad2O2_ad ^• + H₂O + H⁺ + 2e- → C*-RO_2ad2Ce_ad2O_2ad2H_ad+OH^-
It is further believed that the ionic or radical species of the oxidant gas may diffuse over the charged electrochemically active surface and cluster with other similar species, for example peroxide radicals, adhering to the active surface of the cathode.
This may lead to local super-saturation and the growth of the surface peroxide into critical nuclei.
The inventors assume that the electrochemical process of this invention is capable of catalyzing an in situ oxidation of a metal or metalloid ion dissolved in the aqueous electrolyte to a higher oxidation state, whereas at the cathode typically a reduction reaction would be expected. This assumption is supported by the observation that the conductivity of the electrolyte decreases with an increasing degree of separation of metal or metalloid ion from the aqueous solution.
Since adhesion forces with which the oxidized compound adheres to the electrochemically active surface may vary with the nature of the electrochemically active surface of the cathode and the nature of the oxidized compound, release of the oxidized compound particles into the electrolyte may occur as such or may need to be forced.
Preferably, the electric potential to which the cathode is subjected, is a reducing potential relative to a reference electrode, preferably below the thermodynamic pH-potential equilibrium region of 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. This way the risk to the occurrence of water electrolysis to form hydrogen may be minimized.
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 will usually not be enough to ensure full recovery of all metal or metalloid ions dissolved in the electrolyte. Supply of an oxidant gas to the cathode may therefore be preferred in order to ensure maximum recovery of the metal ions dissolved in the water based electrolyte and optimize the reaction rate. Preferably, the oxidant gas is supplied through a hydrophobic gas-diffusion layer of a gas diffusion electrode towards the electrochemically active material.
Examples of oxidant gases suitable for use with this invention include organic as well as inorganic oxidant gases. Example of inorganic gases suitable for use with this invention include ozone, oxygen, carbon oxide gases for example CO₂, nitrogen oxides for example NO, N₂O₃, halogen gases,halogen oxide gases, sulfur oxide gases, air, biogas, flue gas, acid gas and combustion exhaust gas and mixtures or two or more of the afore mentioned gases. Preferably however, use is made of air. Other oxidant gases suitable for use with this invention include those capable of forming oxidant mono-atomic radicals and/or oxidant polyatomic radicals.

Particularly preferred oxidant gases are those which may be reduced so as to generate polyatomic ions, polyatomic radicals or polyatomic peroxides, for example those are summarized in the table below:

perchlorate	ClO₄ ^-1	hydrogen sulfate	HSO₄ ^-1	hydrogen phosphate	HPO₄ ^-2
chlorate	ClO₃ ^-1	dihydrogen phosphate	H₂PO₄ ^-1	peroxide	O₂ ^-2
chlorite	ClO₂ ^-1	permanganate	MnO₄ ^-1	tetraborate	B₄O₇ ^-2
hypochlorite	ClO^-1	periodate	IO₄ ^-1	borate	BO₃ ^-3
nitrate	NO₃ ^-1	hydrogen carbonate	HCO₃ ^-1
nitrite	NO₂ ^-1	sulfate	SO₄ ^-2
bromate	BrO₃ ^-1	sulfite	SO₃ ^-2
iodate	IO₃ ^-1	carbonate	CO₃ ^-2

The oxidant gas is preferably selected such that one or more of the preferred polyatomic ions is generated, in particular one or more of the polyatomic ions selected from the group of acetate (CH₃COO^-), acetylide (C₂ ^2-), carbonate (CO₃ ^2-), peroxide (O₂ ^2-), phosphate (PO₄ ^3-), sulfate (SO₄ ^2-), nitrate (NO₃ ^-).
According to another preferred embodiment, the at least one oxidant gas is selected from the group of organic gases, including ethers (e.g. ethylene oxide, propylene oxide), alkenes (e.g. ethylene, propylene), alkynes (e.g. acetylene), or conjugated dienes (e.g. butadiene) or mixtures of two or more of these gases.
The oxidant gas may 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 the oxidant gas within the gas mixture is not critical to the invention and may vary within wide ranges. Varying the oxidant gas partial pressure will permit to control the size of the metal or metalloid particles isolated from the composition containing the precursor compound. Varying the oxidant gas partial pressure, in particular increasing or decreasing the partial pressure, will also permit to control, in particular to increase or reduce the lattice parameter of the crystalline particles of the reaction product, as measured by X-ray diffraction measured over a given crystallographic plane or transmission electron microscopy imaging.
The nature of the compound which may be isolated from the solution may be varied by selecting the appropriate oxidant gas. When O₂ or an O₂ containing gas is supplied as the oxidant 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 oxidant gas, the compound may take the form of a carbonate, a nitrite or a nitrate. In other words, the nature of the anion of the reaction product may be varied by a proper selection of the oxidant gas.
The skilled person will be capable of adapting the amount of oxidant gas supplied and the gas flow rate, to the concentration of water soluble precursor compound that needs to be isolated from the electrolyte. In particular, it may be desirable to vary the gas supply rate, in particular in case the process is operated in a continuous mode where a continuous supply of water soluble precursor compound to be removed takes place. Moreover, gas supply may create convective mass transfer in the catholyte and not only promote diffusion of the metal and/or metalloid ions from the water soluble precursor compound to the electrochemically active surface, but may also facilitate surface diffusion of reduction products of the oxidant gas, i.e. peroxide ionic and/or radical species, as well as surface diffusion of adhered metal and/or metalloid ions, or any intermediate reaction products, and thereby increase the reaction rate. Other suitable ways to create convective mass transfer comprise those known to the skilled person, for example the use of a stirrer, gas supply, the presence of a spacer material capable of creating turbulent flow conditions.
In a preferred embodiment, the electrochemically active surface of the cathode comprises a plurality of active sites provided by surface functional groups, wherein the functional groups preferably contain one or more moieties selected from the group of a nitrogen containing moiety, an oxygen containing moiety, a chlorine containing moiety or a sulfur containing moiety..
To ensure maximum recovery, in a preferred embodiment, before supplying the cationic water soluble compound, the pH of the electrolyte is adjusted to a pH ≤ 7.0, preferably a pH in acidic conditions, in which the formation of a solid reaction product would not be expected by the skilled person. 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 salt of the a cationic water soluble compound, more preferably below 5.0. The inventors have observed the pH of the catholyte gradually progresses towards alkalinity that in the course of the reaction, often above the dissociation constant of the acid or salt of the ionic metal or metalloid compound. In particular the final pH of the catholyte may raise to a value of above 4, often above 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 found that the metal oxidation rate may thereby be accelerated. Without wanting to be bound to this theory, the inventors believe that the weak protonic electrolyte acts as a catalyst or co-catalyst in the formation of reactive peroxide, ionic and/or radical species from the oxidant gas at the cathode and in the electrochemical reactions in which the water soluble compound is converted into a reaction product that may be separated from the cathode and the catholyte. The co-catalyst has been found capable of accelerating the oxidation of the metal cation or metalloid ion towards the separable compound, by accelerating the availability of reactive species. The inventors have further found that addition of the weak protonic electrolyte may not only increase the conductivity of the catholyte, but that it may also increase the current density over the cathode.
Moreover, 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, may be reduced to a minimum. This contributes to minimizing the risk to the occurrence of unwanted side reactions which would lead to the formation of compounds which could not easily be separated from the cathode and/or the catholyte and for example be water soluble. This separability provides an important advantage, as 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 may vary within wide ranges but is preferably not less than a 10 mM solution and preferably not more than a 1.5 M solution, more preferably the concentration of the weak electrolyte varies between 10 and 500 mM, most preferably around 100 mM.
The weak protonic electrolyte may either be a weak protonic acid or a weak protonic base, depending on the pH range at which the separable compound may be formed. In particular, the weak protonic electrolyte may be a weak polyprotonic acid or a weak polyprotonic base.
A weak protonic acid is a protonic acid which only partially dissociates in water :

HA_(aq) ↔ H⁺ _(aq) + A^- _(aq)
A weak polyprotonic acid is a weak acid which has more than one ionisable proton per molecule. The dissociation constant of a weak monoprotonic acid may be represented by the formula below : $K_{a} = \frac{[H^{+}] [A^{-}]}{[HA]}$
Preferred weak protonic acids have a pKa of 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 and weak inorganic acids, in particular 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 hydrogen sulphate, sodium hydrogen carbonate, ammonium chloride, and mixtures of two or more hereof. Particularly preferred weak protonic acids are those having a pKa which is at least one unit higher than the pH of the catholyte.
Preferred weak protonic bases haves 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, trimethylammonia, ammoniumhydroxide, 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 hydrogen sulphate, sodium hydrogen carbonate, or a mixture of two or more of the afore mentioned compounds.
To ensure maximum recovery as solid material, in a preferred embodiment the pH of the electrolyte is adjusted to acidic conditions, in which the formation of a solid phase is initially not anticipated. Later, the pH progressively turns more basic as the reaction progresses, wherein colloidal particles in suspension may become apparent.
In another preferred embodiment, an ionic water soluble salt is supplied to the catholyte, with the purpose of controlling, in particular of increasing the ionic strength of the catholyte. Salts of chloride with an alkali metal ion are preferred, NaCl being particularly preferred. However other electrolytes may be used as well. An amount of NaCl higher than 1 g.L-1 is preferred, more preferably the amount added will be higher than 10 g.L^-1, most 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 minimum extent only in the course of the process. In particular, virtually no or only a minor decrease of the overall conductivity has been observed. This is probably due to adhesion of cations to the electrochemically active surface of the cathode, which will in general attain a quasi stable level when all of the positive valenced elements to be isolated have been oxidized and transformed into a separable phase, especially in a batch-wise operated process. Nevertheless, any unwanted variations in the conductivity may be compensated by supplying additional electrolyte, or by incorporating into the catholyte a binary electrolyte. This may be of particular importance when the process of this invention is operated in a continuous manner, and continuous supply of metal and/or metalloid ions to be recovered takes place. By the presence of the binary electrolyte, the electrolytic conductivity may be increased to at least 5 mS.cm^-1, more preferably between 20 and 80 mS.cm^-1 and even more preferably between 20 and 50 mS.cm^-1, and thereby the risk to a varying conductivity as a result of the removal of metal and/or metalloid ions may be minimised.
In order to facilitate release of the particles from the cathode and facilitate recovering of the precipitate, the cathode may be subjected to polarization reversal. Polarisation reversal may also be used to clean the cathode from any unwanted remainders adhering thereto. This will permit to recover from the solution at least 10% of the amount of metal or metalloid ion that had been supplied to the cathode, more preferably to recover at least 40% thereof and even more preferably to recover at least 80% thereof.
The electrochemical process of the present invention as described in the present application makes it possible to remove metal or metalloid ions or any other elements with a positive valence contained in the precursor compound, in a concentration which corresponds to at least 20 wt. % of the initial concentration of the element with a positive valence present in the precursor compound, preferably at least 50 wt. %, more preferably at least 80% and most preferably more than 90 wt. % of even more than 99% thereof.
The electrochemical process of the present invention is suitable for isolating a wide variety of precursor water soluble compounds from the water based electrolyte, in a wide variety of concentrations. The concentration of the water soluble precursor may be varied to vary the size of the particles formed on the electrochemically active surface. The inventors have observed that the crystal size of the nano particles may increase with increasing precursor concentration, or that the crystal size may decrease with decreasing precursor concentration.
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 may for example be a compound of an ion of an element selected from the group of group II, III and IV elements of the periodic table of elements, C and Si excluded, the majority of the transition metal elements, the actinides and the lanthanides. The water soluble compound may also be a compound of an ion of an element selected from the group of group I elements when in a compound also containing 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 precursor water soluble metal compound is selected from the group of precursor compounds containing one ore 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 precursor water soluble ionic metal 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 at metal ion contained in the least one precursor water soluble ionic metal compound is selected from the group of transition metals, preferably one ore more of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, 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 ions. In a fourth preferred embodiment, the at least one metal ion is selected from the group of post-transition metals, in particular one or more of Al, Ga, In, Sn, Tl, Bi ions. In a fifth preferred embodiment, the at least one precursor water soluble ionic metalloid compound is selected from the group of B, Si, Ge, As, Sb, Te, Se or C ions or mixtures of two or more hereof. In a sixth preferred embodiment, the at least one precursor water soluble ionic metalloid 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 of wherein the monoatomic 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 hereof.
The water soluble precursor compound may be supplied as a precursor compound comprising one single type of metal or metalloid ion or element with a positive valence, but it is within the scope of this invention that a composition comprising a mixture of two or more metal ions or metalloid ions or elements with a positive valence may be supplied as well. In case a single metal or metalloid ion is supplied, the reaction product that may be separated from the aqueous precursor solution is preferably a compound comprising one single metal or metalloid in an oxidized state. In case the electrolyte comprises a mixture of two or more metal or metalloid ions or elements with a positive valence, the reaction product may comprise a mixture of compounds of the in an oxidized state, all reaction products for example responding to the formula MxOy, but it may also comprise mixed metal or metalloid compounds for example MxNzOy. It is however also within the scope of this invention that a matrix comprising one a precursor compounds is supplied 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 precursor compound may contain crystalline oxide nano particles, for example, but not limited to CeO₂, La₂O₃, Co₂O₃, Al₂O₃, Cs₂O, Li₂O, CoFe₂O₄, FeAsO₄, or non-stoichiometric forms thereof or hydrated forms thereof. In a second embodiment, the reaction product may contain crystalline carbonate nano particles, 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 contains a mixture of amorphous or crystalline metal oxide nano particles or mixed oxides.
The particles may be released in a variety of physical forms, for example in the form of colloidal nano particles, for example in the form of a colloidal dispersion, a separable precipitate of particles or a gel phase. Usually a stable dispersion or gel will be obtained. In order to improve the stability, the a dispersion or suspension of the particles may be subjected to sonication or ultrasonication. According to another variant, one or more additives may added to the precursor, the suspension or dispersion, selected from the group of dispersants, stabilizers, surfactants, polymers, copolymers, emulsifiers, cross-linking agents, capping agents and free flow agents or mixtures thereof.
In a preferred embodiment of this invention, the electrochemically active surface of the cathode preferably comprises a plurality of active sites having a weak protonic acid functionality, i.e. active sites which only partially dissociate in water. Various electrochemically active materials may be used to achieve this. Preferred are those materials which have a surface comprising protonic acid functional groups. Particularly preferred are those materials which comprise electrically conductive particles of carbonaceous origin, more preferably those comprising electrically conductive particles of carbonaceous origin with a catalytically active surface comprising a plurality of protonic acid groups. It is believed that the protonic acidic functional groups present on the catalytically active surface, in particular acidic functional groups of the type R-H, may partly dissociate at a corresponding pH. The inventors also believe that the thus dissociated surface groups C-R*^- have a high oxygen affinity and thus intervene in the oxidation of the metal ion or the metalloid ion.
As electrochemically active material, a wide variety of conductive materials may be used, but preferred are porous materials, in particular those which contain weak protonic acid functional groups. 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 in the electrochemically active surface is preferred, because of its catalytic activity in combination with a reasonable cost and abundant availability in comparison to other materials. Examples of suitable materials include graphite, carbon nanotubes, graphene, carbon black, acetylene black, activated carbon or synthetic carbons such as vulcan. Other electrochemically active materials suitable for use with this invention include carbonaceous materials the surface of which has been chemically modified to adapt its catalytic activity and compatibility with the reaction medium. Without wanting to be bound by this theory, it is believed that the presence of oxygen-containing functional groups support the oxidation reaction. Particularly preferred carbon materials have a surface with quinone-type functional groups.
Suitable porous material for use as the electrochemically active layer preferably have a high specific surface area as measured by the BET method described in ASTM D5665, in particular 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 area larger than 750 or 1000 m²/g or even more may be particularly preferred. Porous materials particularly suitable for use as the electrochemically active layer include particles of carbonaceous origin, also those having a small BET surface area, but preferred are those with a high specific surface area as measured by the BET method, in particular carbonaceous particles selected from the group of graphite, carbon nanotubes, graphene, carbon black, activated carbon or synthetic carbons. Preferred conductive carbonaceous 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 area larger than 750 or 1000 m²/g or even more may be particularly preferred.
The activated carbon preferably has a particle size in the range of 75 to 300 microns, preferably from 100 to 250 microns.
Suitable porous material for use as the electrochemically active layer preferably form a continuous layer on the cathode. Thereto, use can be made of a polymer material which functions as a support for the electrochemically active material.
According to another preferred embodiment, the electrochemically active porous material is a solid which is dispersable or flowable in the water based electrolyte. Hereby, the solid may be made of one or more of the above described materials.
In the method of the present invention, preferably use is made of a cathode comprising a porous gas diffusion electrode, wherein one side of the gas diffusion electrode comprises a layer of at least one electrochemically active material active for or capable of catalyzing the reduction of oxygen to hydrogen peroxide. Preferred active materials have been described above. In order to increase the reaction rate, convective mass transfer may also be created at least in the cathodic gas compartment.
A device suitable for carrying out the process of the present invention is shown in fig. 11. The device shown in fig. 1 comprises an electrochemical cell, comprising at least one anodic compartment 5 and at least one cathode compartment 15. If so desired a plurality of anodic and cathode compartments may be present as well. If a plurality of anode and cathode compartments is provided, they are preferably arranged in a unipolar arrangement, with a plurality of alternating positive and negative electrodes forming a stack separated by ion permeable membranes. In a unipolar design, electrochemical cells forming the stack are externally connected, the cathodes are electrically connected in parallel as well as the anodes.
The anode or anodes 1 are immersed in an anode compartment comprising an aqueous anolyte fluid 2. The cathode or cathodes 10 are immersed in a cathode compartment comprising an aqueous catholyte fluid 12. The anodic compartment and cathodic compartment are in fluid communication to allow transport of cations, in particular transport of protons from the anodic compartment to the catholyte compartment, and transport of anions from the cathodic compartment to the anodic compartment. As anolyte fluid, any anolyte considered suitable by the skilled person may be used. In particular any aqueous electrolyte, conventionally used in electrochemical reduction reactions may be used. The anolyte may for example comprise an aqueous solution of an electrolyte selected from the group of sulphates, phosphates, chlorides and mixtures of two or more of these compounds. The anolyte chamber may comprise a supply member for feeding anolyte fluid. The catholyte chamber may comprise a supply member for feeding catholyte fluid. The catholyte may be different from the anolyte, but anolyte and catholyte may also be the same. Suitable catholyte materials include those well known to the skilled person, such as an aqueous solution of an electrolyte selected from the group of sulphates, phosphates, chlorides and mixtures of two or more of these compounds
The anode and cathode compartment 5, 15 may be made of any material considered suitable by the skilled person, but are preferably made of a polymeric material. Suitable materials include polyvinylidene difluoride (PVDF), polytetrafluorethylene (PTFE), ethylene tetrafluoroethylene (EFTE), polyvinylchloride (PVC), chlorinated polyvinyl chloride (CPVC), polyacrylate, polymethylmethacrylate (PMMA), polypropylene (PP), high density polytethylene, polycarbonate and blends or composites of two or more of these compounds.
The at least one anode and the at least one cathode compartment 5, 15 are preferably separated from each other by an ion permeable membrane 11 to control exchange of cations and anions between both compartments. Preferred ion permeable membranes comprise synthetic polymer materials. The ion permeable membrane on the one hand ensures that cations, in particular protons, may migrate from the anode to the cathode compartment, and on the other hand serves as a gas barrier and therewith counteracts the occurrence of so-called chemical short cuts. The ion permeable membrane also counteracts the occurrence of a pH reduction of the catholyte in the cathodic compartment. Suitable materials for use as ion permeable membrane include polyvinyldifluoride (PVDF), polytetra-fluoroethylene (PTFE or Teflon), poly(ethene-co-tetrafluoroethene (EFTE), polyesters, aromatic polyamides, polyhenylenesulfide, polyolefin resins, polysulphone resins, perfltiolorovinyl ether (PFVE), tripropylene glycol, poly-1,3-butanediol or blends of two or more of these compounds, or composites containing one or more of these compounds and being 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 comprise an ion exchange material if so desired.
To improve structural integrity, the ion-permeable membrane 11 separating the anode and cathode compartment 5, 15 may be reinforced with a rigid support, for example a rigid support made of a sheet, a fleece, which may be woven or non-woven or otherwise made of a porous polymer or a web or a mesh of metal fibres or metal fibres 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 oxidant gas to the electrochemically active surface present at the cathode, and a sufficiently high reaction yield, taking into account the limited solubility of oxygen in water. The gas diffusion electrode is preferably a multilayered electrode comprising a current density distributor 3 for supplying electric current to an electrochemically active surface 4 deposited on top of the current distributor.
The electrochemically active material 4 is preferably a material which has a higher electric conductivity than the current density distributor. This permits the electrochemically active material to take away or bring the electron from and to the current density distributor.
The electrochemically active surface may be formed of any conductive materials or composites with a high surface area. Examples of such electrode materials include carbon, carbon nanotubes, graphite, carbon fiber, carbon cloth, carbon aerogel, metallic powders, for example nickel, metal oxides, for example ruthenium oxide, conductive polymers, and any mixtures of any of the above. It should be appreciated that the entire electrodes may be porous and conductive enough so that a substrate is not needed. It should also 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 enables the voltage to be minimized. By contacting the porous portion with the ionic electrolyte, the apparent capacitance of the electrodes can be very high when charged.
The gas diffusion electrode that is used as the cathode 10 in the device of this invention preferably comprises a current density distributor 3, which may be made of any material and form considered suitable by the skilled person. Preferably however, use is made of a mesh type current density distributor, having a mesh received in a circumferential electrically conductive frame or an array of several meshes. The current density distributor is connected to a source of electric energy along a current feeder, for supplying electrical energy to the current density distributor. The mesh comprises a plurality of electrically conductive paths. The mesh may be formed of any suitable metallic structure, such as, for example, a plate, a mesh, a foil, or a sheet having a plurality of perforations or holes. Furthermore, the mesh may be formed of suitable conductive materials, such as, for example, stainless steel, graphite, titanium, platinum, iridium, rhodium, or conductive plastic. In addition, the metals may be uncoated or coated. One such example is a platinum coated stainless steel mesh. In one embodiment, the mesh is a titanium mesh. In other embodiments, use is made of a stainless steel mesh, a graphite plate, or a titanium plate. The wording "mesh" is meant to include a square meshes with a substantially rectangular shape and orientation of the conductive wires and insulating threads, but the mesh may also be tubular, or a coil film, or a otherwise shaped three-dimensional materials. Still other types of meshes suitable for use with this invention include perforated sheets, plates or foils made of a non-conductive material, having a plurality of wires or threads of a conductive material interlaced in the direction parallel to the current flow. A further type of mesh suitable for use with the present invention includes lines/wires of a conductive material, which extend parallel to the current flow direction, printed on a perforated sheet, foil or plate.
One side of the current density distributor 3 may be coated with an electrochemically active surface 4 capable of catalyzing the reduction of the oxidant gas. The layer of electrochemically active material 4, i.e. the layer which is catalytically active in the reduction of the oxidant gas as described above, is preferably applied to 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 repellant (hydrophobic gas diffusion) layer 13 on the other side.
The device preferably comprises a supply member for supplying an oxidant gas to the side of the cathode comprising the electrochemically active layer.
The cathode compartment may comprise, preferably on a side opposite the side of the cathode comprising the electrochemically active layer, an inlet for supplying at least one weak protonic electrolyte, preferably an aqueous electrolyte. Preferably the flow rate with which the weak protonic electrolyte is variable.
The electrochemically active surface 4 may be coated on the side facing the gas phase 13, with a water repellant layer 13 or a hydrophobic gas diffusion layer to minimize the risk of water leaking through the electrode into the gas phase. This hydrophobic layer or water repellant layer 13 may also be deposited on top of the electrochemically active surface 4. Suitable materials for use as the water repellant layer include polyvinyldifluoride (PVDF), polytetrafluoroethylene (PTFE or Teflon), PSU, but other materials considered suitable by the skilled person may be used as well.
The anode 1 used in the device of this invention may be a conventional electrode, or may be a gas diffusion electrode similar to the cathode. The pH of the anolyte is preferably acidic, preferably below 5, more preferably below 3.
The invention is further illustrated in the examples below.

EXAMPLE 1

Materials and Methods

Chemicals

Activated carbon employed was Norit^® SX1G from Norit Americas Inc. Fluorinated ethylene propylene resin (Teflon® FEP 8000) was obtained from Dupont. Crystalline ultradry CeCl₃ 99.9% (REO) ampouled under argon was received from Alfa Aesar. K₂HPO₄ was procured from Merck. HCl at 35%, CeN₃O₉·6H₂O 99.99% trace metal basis, and analytical grade KI were purchased from Aldrich. 50% NaOH, analytical grade potassium hydrogen phthalate (KHP), and analytical grade (NH₄)₆Mo₇O₂₄·4H₂O were acquired from Merck.

Electrochemical cell setup

Experiments were performed in a half-cell electrochemical reactor (Figure 1). The cathode half-cell consisted of a cathode, a reference electrode and a counter-electrode. Ag/AgCl 3 M KCl (+200 mV vs SHE) was used as a reference electrode (Koslow Scientific), whereas a Pt disk fixed by laser welding over a titanium (Ti) plate was used as a counter-electrode. All potentials here reported stay true for the Ag/AgCl 3 M KCl reference electrode. Cathode and counter electrode were separated by liquid electrolyte and the separating membrane, Zirfon^® (AGFA). Working and counter-electrode were separated from each other by a distance of 4 cm, whereas the membrane was accommodated right in the middle (at 2 cm from each electrode). The principal function of Zirfon^® was to prevent oxygen eventually evolved at the counter-electrode from reaching the working-electrode. The electrodes and separator had a projected electrode surface area of 10 cm². Inert or reactant gas flows (N₂ or air, respectively) were fed through the cathode gas compartment on each individual experiment. Gas flow rate was set at 400 mL min^-1 (excess) in all cases and an overpressure of 10 mbar was applied. Electrolyte feeds were independently circulated through the cathode and counter-electrode compartments with a dual-head peristaltic pump, at a flow rate of approximately 100 mL min^-1 (Watson-Marlow). Both liquid and gas streams under these conditions were consistent with a laminar flow profile.
A schematic representation of the experimental electrochemical half-cell reactor is shown in Figure 1.

Gas diffusion electrodes

A multilayered VITO CORE™ electrode was used which consists of a current collector (metal gauze), an active layer made of activated carbon embedded in a porous polymer matrix, and a hydrophobic gas-diffusion layer. PVDF was used as polymer binder, both for the active layer and the hydrophobic gas-diffusion layer (GDL). The hydrophobic particles in the hydrophobic backing were FEP 8000. A typical GDL is composed of 50 wt% PVDF and 50 wt% FEP 8000. The composition of the active layer for the uncatalyzed cathode was 20% PTFE with 80 wt% activated carbon, whereas for the catalyzed electrode it was 20 wt% PTFE with 76 wt% activated carbon and 4 wt% CeO₂.

Electrolyte composition

Independent electrodes were tested as gas-diffusion cathodes, in presence of air or N₂ respectively, at the cathodic gas compartment. CeN₃O₉ ·6H₂O was added to the cathodic electrolyte, composed of 30 g/L NaCl and 10 mM sodium acetate dissolved in demineralized water and adjusted at pH 2.7 with HCl. Different concentrations of CeN₃O₉ ·6H₂O were independently tested, as follows: 0 ppm, 100 ppm, 500 ppm, 1000 ppm, 2000 ppm, 3000 ppm, 5000 ppm and 10000 ppm. The electrolyte at the counter-electrode (anode) compartment remained the same, but without the addition of Ce. The experiments were carried out at room temperature (18±2 °C).

Electrochemical operation and characterization

A Bio-Logic VMP3 potentiostat/galvanostat and frequency response analyzer was used in order to perform the electrochemical measurements. EC-Lab v.10.23 software was used for data acquisition. Chronoamperometric experiments were carried out at -0.350 V vs the reference electrode during a period of 120 min. Within that period steady state was achieved. Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) were registered before and after the polarization, in order to indirectly evaluate the effectiveness of the metal recovery process.
During the CA experiments at -0.350 V the production of H₂O₂ after O₂ electrochemical reduction takes place with the electrodes and electrolyte here proposed. At these conditions, electrodeposition of metallic Ce is not expected, as the thermodynamic condition for Ce reduction in aqueous medium within the pH range here studied (as shown in the Pourbaix diagrams) would happen only at potentials lower than -2.7 V [Pourbaix, 1974]. Still, transport of dissociated Ce³⁺ ions towards the cathode is expected, with possible subsequent adsorption in the porous electrode active sites. Otherwise, water electrolysis to form hydrogen is not expected at the conditions of the study.
Electrochemical Impedance Spectroscopy (EIS) was recorded at the steady state polarization potential (-0.350 V) a frequency range from 3 kHz to 3 mHz, with 6 points per logarithmic decade, using an amplitude of 10 mV. Careful attention was paid to guarantee stability, linearity, causality and finiteness, so that reliable and valid impedance data were obtained. The EIS response was only recorded when the variation of current was detected to be < ±10 µA during a period of at least one hour. The time of a whole impedance scan was of about 19 minutes. Linearity was verified by real time monitoring of non-distortions in Lissajous plots, which were observed via an on-line connected oscilloscope. Causality was ensured as spurious (noisy) data were not observed while recording EIS. Validity of the data was verified by using the Kramers-Kronig transforms. After the corresponding EIS measurements, CVs were recorded in 2 cycles at 1 mV s-1, in a potential range from -0.450 to 0.450 V us Ag/AgCl. Only the second cycle of the CV is here reported. No IR drop correction was established for the experiments here performed.

Analysis of the concentration of H ₂ O ₂

A spectrophotometric method was employed to determine the concentration of H ₂ O ₂ in solution as disclosed by Aryal & Liakopoulou-Kyriakides 2013, 3:117. Reagent A was prepared by mixing 33 g KI, 1 g NaOH and 0.1 g (NH₄)₆Mo₇O₂₄·4H₂O into 500 mL deionized water. This solution was kept in dark conditions to inhibit oxidation of I. Reagent B was prepared with 10 g KHP dissolved into 500 mL deionized water. The standard calibration curve (not shown) was prepared from known H₂O₂ concentrations from 0 to 3 mg L^-1, dissolved into the same electrolyte used for the experiments, without cerium. Further analysis was carried out by pipetting 3.0 mL of Reagent A, 3.0 mL of Reagent B and 3.0 of 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 concentrations calculated of H₂O₂ are the average of the quantitative results obtained with 5 averaged calibration curves, described by the following equation : $A_{351} = 0.3687 C_{H 2 O 2} R^{2} = 0.9991$
Where C_H2O2 refers to the concentration of hydrogen peroxide (mg L^-1) and A₃₅₁ denotes the absorbance registered at 351 nm.
Beside the known concentrations, problem samples were obtained after the electrochemical characterization experiments and were analyzed through the same procedure as the standards.

X-ray diffraction

X-ray powder diffraction (XRD) experiments were carried out using diffractometer PANalytical X'Pert Pro with CuKα (X = 1.5405Å) at 40kV. The conditions were: 4sec/step; step = 0.04° and continuous scan. 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 concerning the dry samples are reported here.
The identification of the crystalline phases was done by comparison with the database. The crystallite size (D) was calculated using Scherrer's equation (Eq. 8): $D = \frac{Bλ}{β}$
where B is the Scherrer constant (0.89), λ is the wavelength of the X-ray beam (1.5405Å), β _1/2 is the full width at half maximum of the diffraction peak and θ is the diffraction angle.
Independent aqueous solutions with fixed concentrations of NaCl and sodium acetate (CH₃COONa) were supplemented with varying concentrations of Ce(NO₃)₃ ·6H₂O (namely 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 fixed at 2.7, with HCl. A colourless solution was formed in all cases. A constant potential of -0.350 V vs Ag/AgCl (3M KCl) was applied to the said cathodes. At the gas-compartment, N₂ or air were supplied for each independent experiment, at a constant flow rate (∼400 mL.min^-1). Under such operational conditions water electrolysis is avoided; however, when air is supplied through the GDE, O₂ electrochemically reduces to H₂O₂, upon availability of protons and electrons [Yang et al., 2000; Zhimin et al., 2001[RS1]].
The overall solution was considered to be electroneutral before the electrochemical polarization was applied. Given the high concentration of NaCl, ion transport by migration is unlikely to occur.
As soon as the electrical polarization was applied to the cathode, a gradient of electrochemical potential developed across the half-cell. Since the concentration gradients were initially absent, the transport of some positively charged ions was likely steered from the solution in equilibrium towards the surface of the porous cathode, which were thus captured by potential-modulated electrosorption and stored capacitively in the diffuse part of the electric double layer.
Figure 2 shows the extent of transport of the Ce³⁺ ions (removal efficiency in %) from the bulk solution in the presence of N₂ supplied through the gas-diffusion cathode and in the absence of oxidant gas. The removal efficiency (%) was calculated as a function of the initial content of Ce³⁺ in solution (Ce_T,i / mg): ${REM}_{eff} = \frac{C_{T, i} - C_{T, f}}{C_{T, i}} \times 100 %$
Ce_T,f (mg) stands for the final content of Ce³⁺ in solution.
When no Ce³⁺ was supplied in the aqueous matrix (0 ppm), as a consequence of the starting concentration gradient established, the transport of Na⁺ within the porous electrode microstructure may have been prolonged by diffusion to the rest of the electrode porosity. Yet, Na⁺ was available at its highest concentration in the bulk. Altogether this establishes diffusion from the bulk to the EDL in the overall porosity of the GDE as the rate limiting step for Na⁺ transport, until a dynamic equilibrium was reached.
Figure 3 shows the electrochemical response obtained for the experiments where no oxidant gas was supplied through the gas-diffusion electrode and only N₂ was provided. Fig. 3a : Frequency response obtained by Electrochemical Impedance Spectroscopy (EIS) recorded at 20 mV amplitude, in the frequency range from 100 kHz to 3 mHz. Fig. 3b shows the cyclic voltammetry response obtained at a scan rate of 1 mV.s^-1. Fig. 3c and d shows typical EIS responses for diffusional limitation across a film of infinite thickness (left) and limitations by finite diffusion through a film with fixed amount of electroactive substance, which once consumed is not replenished at the electrode or is only replenished very slowly (right). Fig. 3e shows a typical capacitive and pseudo-capacitive CV responses.
In figure 3, the symbols given below relate to the indicated experiments:

○ t_i, C _Ce 3+ _,i = 0 mg.L-1 Ce³⁺.
● t_f, C _Ce 3+_,i = 0 mg.L-1 Ce³⁺.
Δ t₀, C _Ce 3+ _,i = 10 g.L-1 Ce³⁺.
▲ t_f, C _Ce 3+_,i = 10 g.L-1 Ce³⁺.

The frequency response for this case, obtained by electrochemical impedance spectroscopy (EIS) was found to be typical of semi-infinite linear diffusion (see Fig. 3a), this is, unrestricted diffusion to the large porous cathode. In the high frequency range, EIS presented a shift from a typical constant phase element behaviour (at the beginning of the experiment) to a pseudo-transfer resistance behaviour (at the end of the experiment) which is characteristic of the occluded porosity [Kaiser et al (1976) Electrochim. Acta, 21, 539]. The response in cyclic voltammetry (see Fig. 3b) is characteristic of porous electrodes with pseudo-capacitive behaviour (see Fig. 3c), which confirms the capacitive storage of Na⁺ [Yang et al., 2003, J Electroanal Chem 540:159]; yet, the overall process is limited by diffusion. Although some Na⁺ is indeed considered to be electrostatically adsorbed, virtually no changes were observed on its bulk concentration (seen as conductivity) due to the proportion between the small quantity of ions that can be actually electrosorbed at the EDL and those exceedingly available in the aqueous matrix.
Conversely, for the cases supplemented with Ce³⁺ (4 mg to 403 mg of Ce³⁺, corresponding to the aforementioned concentrations of Ce(NO₃)₃ from 100 ppm to 10000 ppm) the frequency response was observed to be distinctive of limitations by finite diffusion through a film with fixed amount of electroactive substance, which once consumed is not replenished at the electrode or is only replenished very slowly (see Fig. 3a.
Although Fig. 3 only presents the EIS and CV data obtained for the systems without Ce(No₃)₃ ·6H₂O or those supplemented with 10 g.L-1 of Ce(NO₃)₃ ·6H₂O, the electrochemical behaviour is representative of all cases where N₂-flows at the cathodic gas compartment and where the electrolyte is supplemented with Ce³⁺ even at concentrations as low as 100 mg.L-1 of Ce(NO₃)₃·6H₂O.
The pH and conductivity were monitored at the catholyte, at the start and end of the experiments. For the cases where N₂ was supplied (Finding 1) the starting pH of 2.7 for each individual experiment increased in about 0.3±0.18 by the end of the experiments, whereas it slightly decreased as a function of concentration (in no case it decreased below 2.8±0.2). The starting conductivity for the case without Ce³⁺ (i.e. 30 g.L^-1 NaCl + 10 mM sodium acetate) was 49.7±0.6 and it remained quasi-stable by the end of the experiments (50.1±0.3). This shows that practically no variation in the concentration of NaCl could be achieved at such high NaCl concentrations. For the cases with Ce³⁺, an ordinary increase of the conductivity was observed as a function of concentration, before polarization. In this case, the conductivity decreased slightly after the polarization treatment was applied, in good agreement with the removal efficiencies observed in Figure 2; this is, by the end of each experiment the conductivity approximately corresponded to that of the 30 mg.L^-1 NaCl alone.
For the system where N₂ was passed through the GDE the average Ce³⁺ removal efficiency was 25.42 ±12.14% (see Fig. 2). In the absence of oxygen or other oxidant gases the removed amount of metal ions (Ce³⁺) is believed to be captured at the porous electrode structure mostly by ion-exchange at the surface functionalities which contained Cl, S and O groups as characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy.

O ₂ supplied as the oxidant gas through the gas-diffusion cathode

VITO CORE™ cold-rolled gas-diffusion electrodes (GDE), made of porous activated carbon (NORIT SX 1G), were employed as. The specific surface area for the powder of which the electrodes are made is of about 1000 m².g^-1. Once shaped in the form of the porous electrode, the active carbon layer typically has a specific surface area as measured according to the BET method of between 621 m².g^-1 to 745 m².g^-1 (Alvarez-Gallego et al 2012 Electrochim Acta 82:415, Sharma et al., 2014 Electrochimica Acta 140 191)
Figure 4 shows the extent of transport of the Ce³⁺ ions (removal efficiency %) from the bulk solution in the presence of O₂ as the oxidant gas supplied through the gas-diffusion cathode and flowing through the gas compartment and diffusing through the gas-diffusion electrode. In this case, about the entire amount of Ce³⁺ was removed from solution (average 99.47±0.53%), as shown in Fig. 4. Contrary to the previous case, the removal efficiency does not increase as a function of the concentration of metal in solution, indicating that adsorption by ion-exchange is not the prevailing phenomenon as in a classical electrosorption case (see finding 1).
The removal efficiency when O₂ was supplied through the gas-diffusion cathode was much more significant than in the case where only N₂ was supplied.
Not only removal of Ce³⁺ ions from solution took place but also the formation of a stable solid phase. Figure 5 shows the recovery efficiency (%) of the Ce³⁺ ions transformed into a solid product recovered as precipitate after being released from the electrode and sedimented in solution, in the presence of O₂ as the oxidant gas supplied through the gas-diffusion cathode, on the basis of dry weight of the recovered product.
The solid phase is composed of CeO₂ isotropic nanocrystals, as identified by XRD and microscopic evidence described later, which precipitated at the interface between the porous activated carbon gas-diffusion electrodes (GDE) and the adjacent aqueous electrolyte. These were initially identified as colloidal nano particles dispersed in solution, which aggregate and precipitate as the process keeps running. Some of these are released into the bulk electrolyte whereas others stay attached to the electrode and are only released after stopping or reverting the electric polarization.
Higher recovery percentages were obtained at lower Ce³⁺ concentrations. It should be noted that the low recovery efficiencies are not due to low conversion rates. The discharge of the crystalline nano particles was not done by other means than just reversing the flow. Those nano particles that could be collected within that reversal time are those which were quantified. In this case, polarization reversal increases recovery.
The intermediates, byproducts (e.g. an adsorbed form of superoxide O ₂ ^•- _(ads)) and the electrosynthesized H₂O₂ are believed to also play a role. The EIS behaviour was found to be typical of faradic reactions (charge transfer) coupled by adsorbed intermediates (Wu et al 2012 Chem Rev, 112:3959), as observed in Fig. 6a. The CV response (Fig. 6b) further indicated that the limiting process at the GDE at -0.350 V vs Ag/AgCl were not anymore capacitive ion-storage or electrosorption alone but an electrocatalytic reduction, presumably O₂ reduction to H₂O₂.
Figure 6 shows the electrochemical response obtained for the experiments where air was supplied through the gas-diffusion electrode :

Fig. 6a shows the frequency response obtained by Electrochemical Impedance Spectroscopy (EIS) recorded at 20 mV amplitude, in the frequency range from 100 kHz to 3 mHz.
Fig. 6b shows the cyclic voltammetry response obtained at a scan rate of 1 mV.s^-1.
Fig 6c and d show typical EIS responses for adsorption limited processes linked to charge transfer reactions.

The symbols in fig. 6 have the following meaning :

○ t_i, C _Ce 3+_,i = 0 mg.L-1 Ce³⁺.
● t_f, C _Ce 3+_,i = 0 mg.L-1 Ce³⁺.
Δ t₀, C _ce 3+ _,i = 10 g.L-1 Ce³⁺.
▲ t_f, C _Ce 3+ _,i = 10 g.L-1 Ce³⁺.

Figure 7 shows the crystallite size and lattice parameter found for the different initial Ce³⁺ concentrations studied.
Figure 7a shows the crystallite size (220) for CeO₂ and NaCl.
Figure 7b shows the lattice parameter CeO₂ and NaCl. There was a limit in detection for both parameters at Ce below 20 mg.
The crystal size of the crystalline product varied in gradient as a function of the initial concentration of Ce³⁺, but also proportionally to the concentration of H₂O₂ found in solution (Figure 7a). At lower Ce³⁺ concentrations the crystal size of CeO₂ is smaller whereas as the concentration increases the crystal size is larger. The average crystal size for CeO₂ was 3.5±0.337 nm, whereas for NaCl it was 45.1275±0.337. This makes possible further separation either by re-dissolution of NaCl with a pH where CeO₂ is still stable, e.g. pH >10 or by size exclusion (e.g. screening) after drying. The lattice parameters observed in Fig 7b, also varied as a function of initial Ce³⁺ ion concentration and proportionally to the concentration of H₂O₂ found in solution. It is possible that Ce³⁺ plays a co-catalytic role in the electrosynthesis of H₂O₂ itself.
Figure 8 shows transmission electron micrographies evidencing the characteristic morphology of CeO₂ nano particles with crystallite sizes matching those obtained by XRD.
Figure 9 shows transmission electron micrographies evidencing the aggregation of the small crystalline nano particles of Figure 8 into larger size nano particles.
Fig. 8 and 9 in fact show characteristic fingerprints of the materials formed by the method of this invention, whose properties can be tuned as per controlled variations in the physicochemical or electrochemical conditions provided.

EXAMPLE 2.

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

Table A: Composition of 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 hydrate	Dy(NO₃)₃·xH₂O	80
3	Erbium nitrate pentahydrate	Er(NO₃)₃·5H₂O	53
4	Europium nitrate pentahydate	Eu(NO₃)₃· 5H₂O	6
5	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 hydrate	Lu(NO₃)₃·xH₂O	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	Terbium nitrate hexahydrate	Tb(NO₃)₃· 6H₂O	12
13	Thulium nitrate pentahydrate	Tm(NO₃)₃· 5H₂O	8
14	Yttrium nitrate hexahydrate	Y(NO₃)₃·6H₂O	536
15	Ytterbium nitrate pentahydrate	Yb(NO₃)₃· 5H₂O	50

Additionally, 30 g/L NaCl were provided and dissolved. The operational volume of the catholyte in each experiment was 125 mL.

The concentration of the different metals was quantitatively analyzed by means of ICP-MS.
The process was applied at constant polarization at -0.350 mv vs the previously referred reference electrode for a period of 2 hours. After few minutes of processing (<20 min), the color of the electrolyte progressively shifted from transparent towards white in one appreciable turbid phase. The process showed a gradual change in pH up to 11. Current densities above 40 mA.cm^-2 were registered under the constant cathodic polarization conditions. After the process was stopped, the solid particles formed aggregated and sedimented leaving a clear liquid medium and a separable solid precipitate phase.
Most of the metal content was found to be removed from solution (Figure 10), this is, >99.9 for all metals together, as determined by ICP-MS.
Figure 10 shows the removal efficiency (%) of the different metal ions from 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 solid REE content were recuperated which correspond to about 25% of the total ionic (dissolved) REE content in the original aqueous matrix. The isolated products showed crystalline properties matching with crystallite sizes of 1.97 nm, 1.71 nm and 2.29 nm, respectively.

EXAMPLE 3.

The composition of the electrolyte was identical to that explained for 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 concentrations of the metal are disclosed in Table b. The operational volume of the catholyte in each experiment was 125 mL.

Table b: Measured pH and conductivity of the catholytes with different concentrations of lanthanum nitrate La(NO₃)₃·6H₂O by the start of experimentation.

	Concentration (ppm)	0	100	500	1000	5000
Catholyte	pH	2.54	2.15	2.70	2.78	2.76
Catholyte	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
Anolyte	Conductivity (mS.cm^-1 )	49.7	49.7	49.8	49.8	49.8

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

A colourless solution was formed when dissolving the chemicals. Air was supplied to the gas compartment. After 2 h of processing at constant polarization conditions of -0.350 V vs Ag/AgCl (3M KCl), the color of the electrolyte remained transparent throughout the experiment. However when stopping the polarization and reversing the flow, visible white turbidity was released into the medium. The amount of product released (or turbidity) corresponded to the initial concentration of lanthanum nitrate. After about an hour, all the turbid product had precipitated. The pH changes were similar to those observed in the catholyte in example 1, the pH of the catholyte significantly increased by the end of the experiments where air was supplied through the gas diffusion compartment. The conductivity and pH of catholyte and anolyte, remained almost the same. An overall slight decrease in catholyte conductivity could be debated.

Table c: pH and conductivity of the catholytes for different concentrations of La(NO₃)₃·6H₂O at the end of the experiment.

	Concentration (ppm)	0	100	500	1000	5000
Catholyte	pH	11.5	12.47	11.62	11.8	5.37
Catholyte	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
Anolyte	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. For all cases >99.9% of lanthanum had been removed from the solution. When analyzing the white precipitate by XRD, the produced solid showed characteristics of crystalline nano particles matching those of burbankite and more specifically lanthanum remondite, this is Na₃La₃(CO₃)₃. An amorphous phase was additionally 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 for all experiments (5 g.L^-1). The effect of the polarization potential was evaluated. The following potentials vs. the reference electrode were compared: -0.350 V, - 0.550 V, -0.750 V, -0.950 V. The operational volume of the catholyte in each experiment was 125 mL.

Table D: pH and conductivity of the catholyte at the start of experimentation at different cathode potentials.

	Applied potential (V vs Ag/AgCl 3 M KCl)	-0.150	-0.350	-0.550	-0.750	-0.950
Catholyte	pH	2.67	2.63	2.51	2.51	2.76
Catholyte	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
Anolyte	Conductivity (mS.cm^-1 )	49.8	49.8	49.7	49.7	49.7

From the changes in the pH and conductivity, especially of the catholyte, it can be observed that the same trend found in previous examples was observed; this is, the pH significantly increased throughout the experiment. However, only in the case at -0.950 V a visible colour change of the electrolyte towards yellow could be observed.

The shift in pH was directly correlated to the applied potential. The pH change took place during the first hour of the experiment and even increasing further the time of polarization (i.e. from 2 to 4 h) did not result in pH variations to higher magnitudes.

Table E: Measured pH and conductivity of the catholytes by the end of experimentation at different applied cathode potentials.

	Applied potential (V vs Ag/AgCl 3 M KC1)	-0.150	-0.350	-0.550	-0.750	-0.950
Catholyte	pH	5.37	6.55	6.92	8.45	8.5
Catholyte	Conductivity (mS.cm^-1)	45.2	46.6	46	44.1	45
Anolyte	pH	2.68	2.13	2.15	2.13	2.15
Anolyte	Conductivity (mS.cm^-1 )	47.5	51.0	50.3	50.5	50.8

After centrifugation and drying, a crystalline product matching the characteristics of sassolite could be recuperated.

Claims

An electrochemical process for isolating from at least one water soluble precursor compound comprising a metal or a metalloid element or two or more thereof having a positive valence, a reaction product of the metal or metalloid element or two or more thereof,
- wherein the water soluble precursor compound is supplied to a water based catholyte contained in a cathode compartment of an electrochemical cell containing a cathode with an electrochemically active surface in contact with the catholyte,

- wherein the cathode is subjected to an electric potential, which is chosen such as to cause reduction of an oxidant gas present at the cathode to one or more corresponding peroxide, ionic and/or radical species capable of reacting with the metal or metalloid element,

- and to cause conversion of the reduced oxidant gas into a reaction product comprising a compound of the metal or metalloid element or two or more thereof and the peroxide, ionic and/or radical species, in particular into nano particles of the reaction product.
A process as claimed in claim 1, wherein the metal or metalloid element or two or more thereof having a positive valence, is a cation of that element or a mixture of cations of two or more elements.
A process as claimed in claim 1 or 2, wherein as the cathode use is made of a gas diffusion electrode and wherein oxidant gas is supplied to a gas compartment of the gas diffusion electrode.
A process as claimed in any of the previous claims, wherein the oxidant gas is an organic oxidant gas or an inorganic oxidant gas or a mixture of two or more of such gases.
A process as claimed in claim 4, wherein the inorganic oxidant gas is selected from the group of ozone, oxygen, carbon oxides, nitrogen oxides, halogen oxides, sulfur oxides, halogens, air, biogas, flue gas, acid gas, combustion exhaust gas, or a mixture of two or more of the afore mentioned gases; and wherein the organic oxidant gas is selected from the group of ethers, in particular ethylene oxide and propylene oxide, alkenes, in particular ethylene or propylene, alkynes, in particular acethylene, conjugated dienes, in particular butadiene, or a mixture of two or more of the afore mentioned oxidant gases.
A process as claimed in any of the previous claims, wherein a first side of the electrochemically active surface of the-cathode encompasses a hydrophobic layer facing the gas phase, and an opposite side of the electrochemically active surface encompasses a plurality of functional groups which may be polarized upon application of the electric potential, wherein the functional groups preferably contain one or more moieties selected from the group of a nitrogen containing moiety, an oxygen containing moiety, a chlorine containing moiety or a sulfur containing moiety.
An electrochemical process as claimed in any of the previous claims, wherein in advance of supplying the precursor compound to the cathode compartment, the pH of the catholyte is adjusted to a pH ≤ 7, preferably to a pH ≤ 5, more preferably to a pH which is below the pKa of water soluble precursor compound.
An electrochemical process as claimed in any of the previous claims, wherein an aquous 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 polyprotonic base or a weak polyprotonic acid.
An electrochemical process as claimed in claim 8, wherein the weak protonic acid has a pKa which is at least one unit higher than the pH of the catholyte.
An electrochemical process as claimed in claim 8, wherein the weak protonic base has a pKa of between 6.0 and 12.0, preferably between 7.0 and 11.0.
An electrochemical process as claimed in any of the previous claims, wherein a binary electrolyte is supplied to the catholyte, preferably a water soluble ionic salt, more preferably sodiumchloride, with the purpose of raising the electric conductivity of the catholyte, preferably raising the electric conductivity of the catholyte to at least 5 mS.cm^-1, more preferably between 20 and 80 mS.cm^-1, most preferably between 20 and 50 mS.cm^-1.
An electrochemical process as claimed in any of the previous claims, wherein the electric potential to which the cathode is subjected is a reducing potential relative to a reference electrode, which is below the thermodynamic pH potential equilibrium region of stability of the oxidant gas in water, preferably below the region of thermodynamic stability of water, more preferably outside of the region of thermodynamic stability of hydrogen.
An electrochemical process as claimed in any of the previous claims, wherein the precursor compound is a compound of an ion of an element selected from the group of group I, II, III and IV elements of the periodic table of elements, the transition metal elements, the actinides and lanthanides ore a compound containing two or more of such elements.
An electrochemical process as claimed in any of the previous claims, wherein the electric potential to which the cathode is subjected is reversed with the purpose of recovering the nano particles from the cathode.
A composition obtainable by the process according to any one of claims 1-14 comprising a plurality of aggregate particles of nano crystals of a compound of a metal or a metalloid element or two or more thereof and a polyatomic anion, the nano particles having a crystal size of between 0.2 and 30.0 nm, a lattice parameter of between 1.0 and 18.0 nm, the particles having an average particle size of below 500 nm, preferably below 100 nm, more preferably below 30 nm.