EP3042981A1 - Procédé électrochimique pour preparer un composé d'un métal ou métalloide et d'un peroxyde ou des especes ioniques ou radicalaires - Google Patents

Procédé électrochimique pour preparer un composé d'un métal ou métalloide et d'un peroxyde ou des especes ioniques ou radicalaires Download PDF

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EP3042981A1
EP3042981A1 EP15150649.0A EP15150649A EP3042981A1 EP 3042981 A1 EP3042981 A1 EP 3042981A1 EP 15150649 A EP15150649 A EP 15150649A EP 3042981 A1 EP3042981 A1 EP 3042981A1
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Prior art keywords
cathode
metal
gas
catholyte
compound
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EP15150649.0A
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German (de)
English (en)
Inventor
Xochitl Dominguez Benetton
Yolanda Alvarez Gallego
Christof Porto-Carrero
Katrijn Gijbels
Rajamani Sunita
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Vito NV
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Vito NV
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Priority to EP15150649.0A priority Critical patent/EP3042981A1/fr
Priority to US15/542,375 priority patent/US20180023201A1/en
Priority to PCT/EP2016/050379 priority patent/WO2016110597A1/fr
Priority to EP16700274.0A priority patent/EP3242963B1/fr
Priority to DK16700274.0T priority patent/DK3242963T3/en
Priority to JP2017554650A priority patent/JP2018508659A/ja
Priority to CN201680014658.5A priority patent/CN107532309B/zh
Priority to ES16700274T priority patent/ES2702082T3/es
Priority to MX2017009005A priority patent/MX2017009005A/es
Priority to CA2973289A priority patent/CA2973289A1/fr
Publication of EP3042981A1 publication Critical patent/EP3042981A1/fr
Withdrawn legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00—Electrolytic production of inorganic compounds or non-metals
    • C25B1/01—Products
    • C25B1/28—Per-compounds
    • C25B1/30—Peroxides
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00—Electrolytic production of inorganic compounds or non-metals
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for

Definitions

  • 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.
  • 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 ).
  • 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.
  • 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.
  • REE rare earth elements
  • 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.
  • US2011042219 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.
  • 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.
  • 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.
  • 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 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.
  • 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.
  • 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 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*-R H), where C* represents an active site on the electrochemically active layer of the cathode.
  • 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.
  • a peroxide radical an ionic or other radical species
  • 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 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.
  • 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.
  • 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.
  • 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.
  • the oxidant gas is supplied through a hydrophobic gas-diffusion layer of a gas diffusion electrode towards the electrochemically active material.
  • oxidant gases suitable for use with this invention include organic as well as inorganic oxidant gases.
  • inorganic gases suitable for use with this invention include ozone, oxygen, carbon oxide gases for example CO 2 , nitrogen oxides for example NO, N 2 O 3 , 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.
  • oxidant gases suitable for use with this invention include those capable of forming oxidant mono-atomic radicals and/or oxidant polyatomic radicals.
  • 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 ClO 4 -1 hydrogen sulfate HSO 4 -1 hydrogen phosphate HPO 4 -2 chlorate ClO 3 -1 dihydrogen phosphate H 2 PO 4 -1 peroxide O 2 -2 chlorite ClO 2 -1 permanganate MnO 4 -1 tetraborate B 4 O 7 -2 hypochlorite ClO -1 periodate IO 4 -1 borate BO 3 -3 nitrate NO 3 -1 hydrogen carbonate HCO 3 -1 nitrite NO 2 -1 sulfate SO 4 -2 bromate BrO 3 -1 sulfite SO 3 -2 iodate IO 3 -1 carbonate CO 3 -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 (CH 3 COO - ), acetylide (C 2 2- ), carbonate (CO 3 2- ), peroxide (O 2 2- ), phosphate (PO 4 3- ), sulfate (SO 4 2- ), nitrate (NO 3 - ).
  • 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.
  • ethers e.g. ethylene oxide, propylene oxide
  • alkenes e.g. ethylene, propylene
  • alkynes e.g. acetylene
  • conjugated dienes e.g. butadiene
  • the oxidant gas may be used as such or in a mixture with one or more inert gases, for example N 2 , 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.
  • O 2 or an O 2 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.
  • CO 2 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 + , Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Al 3+ , Ag + , Zn 2+ , Fe 2+ , Fe 3+ , Cu 2+ , 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.
  • 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.
  • 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.
  • the reaction product that is formed from the precursor compound may contain crystalline oxide nano particles, for example, but not limited to CeO 2 , La 2 O 3 , Co 2 O 3 , Al 2 O 3 , Cs 2 O, Li 2 O, CoFe 2 O 4 , FeAsO 4 , or non-stoichiometric forms thereof or hydrated forms thereof.
  • the reaction product may contain crystalline carbonate nano particles, preferably but not limited to Na 3 La 3 (CO 3 ) 5 , NaHCO 3 , or non-stoichiometric forms thereof or hydrated forms thereof.
  • 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.
  • 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.
  • porous materials in particular those which contain weak protonic acid functional groups.
  • 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.
  • 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.
  • 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 m 2 /g, preferably at least 100 m 2 /g, more preferably at least 200 or 250 m 2 /g, most preferably at least 400 or 500 m 2 /g, but those having a surface area larger than 750 or 1000 m 2 /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 m 2 /g, preferably at least 100 m 2 /g, more preferably at least 200 or 250 m 2 /g, most preferably at least 400 or 500 m 2 /g, but those having a surface area larger than 750 or 1000 m 2 /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.
  • use can be made of a polymer material which functions as a support for the electrochemically active material.
  • the electrochemically active porous material is a solid which is dispersable or flowable in the water based electrolyte.
  • the solid may be made of one or more of the above described materials.
  • 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.
  • electrochemically active material for or capable of catalyzing the reduction of oxygen to hydrogen peroxide.
  • Preferred active materials have been described above.
  • 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.
  • anolyte fluid any anolyte considered suitable by the skilled person may be used.
  • 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.
  • PVDF polyvinylidene difluoride
  • PTFE polytetrafluorethylene
  • EFTE ethylene tetrafluoroethylene
  • PVC polyvinylchloride
  • CPVC chlorinated polyvinyl chloride
  • PMMA polymethylmethacrylate
  • PP polypropylene
  • high density polytethylene polycarbonate and blends or composites of two or more of
  • 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.
  • 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.
  • 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.
  • 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.
  • the entire electrodes may be porous and conductive enough so that a substrate is not needed.
  • 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.
  • a current density distributor 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.
  • the mesh may be formed of suitable conductive materials, such as, for example, stainless steel, graphite, titanium, platinum, iridium, rhodium, or conductive plastic.
  • the metals may be uncoated or coated.
  • One such example is a platinum coated stainless steel mesh.
  • the mesh is a titanium mesh.
  • 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.
  • an inlet for supplying at least one weak protonic electrolyte preferably an aqueous electrolyte.
  • 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.
  • Activated carbon employed was Norit ® SX1G from Norit Americas Inc.
  • Fluorinated ethylene propylene resin (Teflon® FEP 8000) was obtained from Dupont. Crystalline ultradry CeCl 3 99.9% (REO) ampouled under argon was received from Alfa Aesar.
  • K 2 HPO 4 was procured from Merck. HCl at 35%, CeN 3 O 9 ⁇ 6H 2 O 99.99% trace metal basis, and analytical grade KI were purchased from Aldrich. 50% NaOH, analytical grade potassium hydrogen phthalate (KHP), and analytical grade (NH 4 ) 6 Mo 7 O 24 ⁇ 4H 2 O were acquired from Merck.
  • 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).
  • 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.
  • FIG. 1 A schematic representation of the experimental electrochemical half-cell reactor is shown in Figure 1 .
  • a multilayered VITO CORETM electrode 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.
  • 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% CeO 2 .
  • 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.
  • Electrochemical Impedance Spectroscopy 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 ⁇ A 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.
  • Reagent A was prepared by mixing 33 g KI, 1 g NaOH and 0.1 g (NH 4 ) 6 Mo 7 O 24 ⁇ 4H 2 O 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 H 2 O 2 concentrations from 0 to 3 mg L -1 , dissolved into the same electrolyte used for the experiments, without cerium.
  • C H2O2 refers to the concentration of hydrogen peroxide (mg L -1 ) and A 351 denotes the absorbance registered at 351 nm.
  • the identification of the crystalline phases was done by comparison with the database.
  • N 2 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, O 2 electrochemically reduces to H 2 O 2 , 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.
  • Figure 2 shows the extent of transport of the Ce 3+ ions (removal efficiency in %) from the bulk solution in the presence of N 2 supplied through the gas-diffusion cathode and in the absence of oxidant gas.
  • FIG. 3 shows the electrochemical response obtained for the experiments where no oxidant gas was supplied through the gas-diffusion electrode and only N 2 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.
  • 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.
  • 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 is characteristic of porous electrodes with pseudo-capacitive behaviour (see Fig.
  • Fig. 3 only presents the EIS and CV data obtained for the systems without Ce(No 3 ) 3 ⁇ 6H 2 O or those supplemented with 10 g.L-1 of Ce(NO 3 ) 3 ⁇ 6H 2 O, the electrochemical behaviour is representative of all cases where N 2 -flows at the cathodic gas compartment and where the electrolyte is supplemented with Ce 3+ even at concentrations as low as 100 mg.L-1 of Ce(NO 3 ) 3 ⁇ 6H 2 O.
  • VITO CORETM 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 m 2 .g -1 .
  • the active carbon layer typically has a specific surface area as measured according to the BET method of between 621 m 2 .g -1 to 745 m 2 .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 Ce 3+ ions (removal efficiency %) from the bulk solution in the presence of O 2 as the oxidant gas supplied through the gas-diffusion cathode and flowing through the gas compartment and diffusing through the gas-diffusion electrode.
  • O 2 the oxidant gas supplied through the gas-diffusion cathode and flowing through the gas compartment and diffusing through the gas-diffusion electrode.
  • 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).
  • Figure 5 shows the recovery efficiency (%) of the Ce 3+ ions transformed into a solid product recovered as precipitate after being released from the electrode and sedimented in solution, in the presence of O 2 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 CeO 2 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.
  • GDE porous activated carbon gas-diffusion electrodes
  • the intermediates, byproducts e.g. an adsorbed form of superoxide O 2 •- ( ads )
  • the electrosynthesized H 2 O 2 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 O 2 reduction to H 2 O 2 .
  • Figure 6 shows the electrochemical response obtained for the experiments where air was supplied through the gas-diffusion electrode :
  • Figure 7 shows the crystallite size and lattice parameter found for the different initial Ce 3+ concentrations studied.
  • Figure 7a shows the crystallite size (220) for CeO 2 and NaCl.
  • Figure 7b shows the lattice parameter CeO 2 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 Ce 3+ , but also proportionally to the concentration of H 2 O 2 found in solution ( Figure 7a ). At lower Ce 3+ concentrations the crystal size of CeO 2 is smaller whereas as the concentration increases the crystal size is larger.
  • the average crystal size for CeO 2 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 CeO 2 is still stable, e.g. pH >10 or by size exclusion (e.g. screening) after drying.
  • Figure 8 shows transmission electron micrographies evidencing the characteristic morphology of CeO 2 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.
  • 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.
  • 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.
  • 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.
  • 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(NO 3 ) 3 ⁇ 6H 2 O by the start of experimentation.
  • Concentration (ppm) 0 100 500 1000 5000 Catholyte pH 2.54 2.15 2.70 2.78 2.76 Conductivity (mS.cm -1 ) 51.0 51.6 50.4 50.3 52
  • the concentration of lanthanum was quantitatively analyzed by means of ICP-MS.
  • 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.

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