Note: Descriptions are shown in the official language in which they were submitted.
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AN ELECTROCHEMICAL PROCESS FOR PREPARING A COMPOUND COMPRISING A METAL OR
METALLOID AND A
PEROXIDE, IONIC OR RADICAL SPECIES
The present invention relates to electrochemical process for recovering
a metal element or a metalloid element or a mixture of two or more thereof
from at least one water soluble precursor compound comprising the metal
element or a metalloid element or two or more thereof, in the form of one or
more nano particles of at least one reaction product, according to the
preamble
of the first claim.
The present invention further relates to nano particles, in particular
nano crystals obtained with that process and 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, like controlling
particle growth, crystallinity, stability and reproducibility. A high quality
synthesis procedure should desirably produce nano particles with a controlled
size distribution, often a narrow size distribution is aimed at. 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
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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 around 100 nm. In a
few cases, formation of nanocrystals with an average particle size below 100
nm has been observed, however these exhibit highly polydisperse (non-
uniform) size and shape distributions.
U520060068026 discloses a thermal electrochemical method for
preparing a colloidal stable suspension of zero-valent naked metal or metal-
alloy 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
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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 present in a dissolved state in the solution that is to be treated,
for
example the chemical precursors are 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 and
probably
also due to their high economic value. A key measure to anticipate REE supply
vulnerabilities is recycling from end-of-life products; yet recycling
possibilities
are 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 Al discloses a method for the extraction and
separation of lanthanoid 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. To recover the
target metal, the adsorbent compound with the metal adsorbed to it is
contacted with a back-extraction solution.
Another method for removing ionic species from fluids, for example
impaired water supplies, which makes use of capacitive deionization is
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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 ¨ which they do not carry out
electron transfer reactions. 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.
US2015/0200082 Al discloses a method of manufacturing particles of
metal hydroxides, such as indium, tin, copper, gallium, zinc, aluminum, iron,
nickel, manganese, and lithium, or an alloy containing at least one type
selected from these metals, wherein the metal particles have a uniform
particle diameter of 100 nm. A gas diffusion electrode is submerged in an
electrolyte, for example ammonium nitrate, the anode is defined by the metal
or the conductive metal oxide. Oxygen supplied to the cathode is reduced at
the gas-liquid interface, thereby generating hydroxide ions (02+2H20+4e- ¨>
4011-), causing the pH to increase as the process continues. The standard
electrode potential (+0.40 V) of the reduction reaction of the oxygen being
higher than the standard electrode potential (+0.01 V) of the reduction
reaction of the nitrate ion, reduction of the nitrate ions rarely occurs.
Metal
ions eluted from the anode during electrolysis react with the hydroxide ions
in
the electrolytic solution to form metal hydroxide particles. The alkaline
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conditions in which the reaction is carried out cause particle diameters to
increase, and obtaining the metal hydroxides as particles with a uniform
desired particle diameter is hard to achieve. If so desired, pH and
temperature
of the electrolyte can be stabilized. When analysing the morphology of the
5 particles, it appeared that each particle consists of a plurality of
smaller
particles aggregated to form a larger particle.
US-A-3.073.763 discloses a method for the electrolytic production of iron
oxides or mixtures of metal oxides which predominantly contain iron oxide,
using an iron anode and one or more other metal anodes and a cathode
consisting of a conductive material, e.g. metal or carbon. An alkaline salt
solution, the anions of which do not form insoluble salts with the metals of
the
anodes and have a greater affinity to hydrogen than to iron is used as the
electrolyte. In the course of the process, the metals at the anode will
dissolve
and contact the alkaline medium which has formed adjacent the cathode. If
oxygen is supplied, the metal ions are converted in the electrolyte solution
into
the corresponding oxides, which precipitate in solution as particles having a
large average particle size in the order of 100 nm or more. In the process of
US-A-3.073.763 no reduction of oxygen takes place.
US-A-4.067.788 discloses a method for producing finely divided metal
powders, in particular powders of nickel, silver, gold and the platinum group
metals, having particles ranging in size from about 0.5 micron to about 10
micron. The anode is made of the metal to be finely divided as the oxide, the
hydroxide or the metal itself. The anode is immersed in an alkaline solution
of
a salt, having a typical pH of between 8 and 12, the anion of the salt being
soluble when combined with said metal. The cathode is a catalyzed air
electrode. Since the metal ions produced at the surface of anode do not
immediately react with the hydroxyl ions of the solution, and since the
insoluble oxides or hydroxides are formed in solution at a perceptible
distance
from the surface of the anode, the particle size may be controlled by
recirculation of the electrolyte. In order to further increase the particle
size of
the oxide or hydroxide, a portion of the solid material is returned to the
cell in
which the process is carried out to serve as nuclei on which additional oxide
or
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hydroxide is deposited. The final step in the process is the chemical
reduction
of the metal oxide or metal hydroxide to the finely divided metal, for example
using formaldehyde or inverted sugar.
UA-S-6.235.185 discloses a method and an apparatus for producing
particles of a metal compound such as a metal oxide or a metal hydroxide
having particle diameters of not greater than 1 micron, in particular 1 to
1000
nm, preferably 5 to 100 nm, The term "particle diameter" herein means the
volume average particle diameter of 200 or more particles observed by means
of a transmission electron microscope and a sharp particle size distribution,
starting from a less expensive starting material and using a simplified
production facility. Thereto an electrolytic solution which contains metal
ions
as a starting material for the ultrafine metal compound particles is provided
in
the anode chamber, an alkaline electrolytic solution is provided in the
cathode
chamber. A voltage is applied between the anode and the cathode, to induce
transfer of the metal ions from the anodic side chamber to the cathodic side
chamber through an ion exchange layer. Ultrafine metal compound particles
are precipitated in the alkaline electrolytic solution in the cathode chamber.
Examples of metals suitable for use with the method of US-A-6.235.185
include lithium, magnesium, aluminum, calcium, scandium, titanium,
vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium,
germanium, rubidium, strontium, yttrium, zirconium, niobium, molybdenum,
ruthenium, rhodium, palladium, cadmium, indium, tin, antimony, tellurium,
cesium, barium, tantalum, tungsten, osmium, platinum, gold, mercury, lead,
bismuth, cerium, neodymium, samarium and europium, or combinations
hereof.
US2004/0108220 Al discloses a process for the production of amorphous
and/or crystalline mixed oxides of metals with mean particle diameters
ranging from 1 to 500 nm, more particularly metals of the third to fifth main
group or the secondary groups of the periodic system. Ions dissolved in an
organic electrolyte of those metals of which the (mixed) oxide is to be
produced
in the electrolyte of the cathode compartment, are electrochemically reduced
at
the cathode in the presence of air as an oxidizing agent. The cathode
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compartment is separated from the anode compartment by a porous partition
diaphragm which is permeable to cations and anions, but prevents the passage
of elemental halogen from the anode to the cathode compartment. Preferred
organic electrolytes include alcohols, polyalcohols or mixtures and
derivatives
thereof, ketones, ethers, nitrites, organic carbonates and aromatic compounds,
tetrahydrofuran, acetone, acetonitrile, toluene. The organic electrolyte may
contain small quantities of water, for example from about 0.01 to about 2% by
weight. Air introduced in the form of small bubbles on the one hand, provides
for a fine distribution of the oxidizing agent in the cathode compartment and
on the other hand counteracts formation of solid metal oxide on the cathode by
providing a constant mixing of the electrolyte, and instead ensures that the
metal oxide particles are flushed from the cathode and dispersed in the
electrolyte.
Although US2004/0108220 Al is said to produce particles of amorphous
and/or crystalline mixed oxides of metals with mean particle diameters
ranging from 1 to 500 nm, the examples do not reveal the actual particle size
obtained. Moreover, the process involves the use of an organic electrolyte and
is therefore not suitable for example for recovering metal ions from aqueous
solutions.
A need remains for an economically feasible electrochemical process
which permits to recover from a water soluble precursor compound containing
a metal or metalloid element or two or more thereof, a reaction product
containing the metal or metalloid element or two or more thereof. A particular
need remains for an economically feasible process which permits to recover
such metal or metalloid elements in the form of crystalline nano particles of
a
compound or a mixture of two or more compounds containing the at least one
metal or metalloid element, wherein the crystalline nano particles have an
average particle diameter of 50 nm or smaller, preferably 30 nm or smaller,
more preferably 20 nm or smaller, most preferably 10 nm or smaller or even 5
nm or smaller. More particularly, a need remains for an electrochemical
process which permits to produce such crystalline particles with a controlled
particle size distribution, preferably a narrow particle size distribution or
a so-
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called monodisperse particle size distribution. This process should be
suitable
for recovering the commonly used metal or metalloid elements as well as those
which are less commonly used.
The present invention therefore aims at providing an economically
feasible method for isolating from at least one water soluble precursor
compound comprising a metal or a metalloid element or two or more thereof,
nano particles, in particular crystalline nano particles, of at least one
reaction
product comprising a metal or a metalloid element or two or more thereof.
This is achieved by the present invention with a method which shows
the technical features of the characterizing portion of the first claim.
Thereto the electrochemical process of this invention comprises the
steps of
- supplying the water soluble precursor compound to a water-based
catholyte of a cathode compartment of an electrochemical cell, equipped
with a cathode comprising a gas diffusion electrode, wherein the gas
diffusion electrode comprises a porous electrochemically active material
having a BET surface area of at least 50 m2/g,
- adjusting the pH of the catholyte to a pH which is smaller than the
pKa of
the water soluble precursor compound,
- supplying at least one oxidant gas to the gas diffusion electrode,
- subjecting the cathode to an electrochemical potential to cause
reduction of
the at least one oxidant gas to one or more of the corresponding peroxide,
ionic and/or radical reactive species capable of reacting with a cation
comprising the metal element, or the metalloid element or a mixture of
two or more thereof, to form at least one nano particle, in particular at
least one nano crystal, of the at least one reaction product with an average
particle size, in particular an average crystallite size equal to or smaller
than 30.0 nm.
The inventors have observed that subjecting the cathode to an
electrochemical potential which is chosen such that it is capable of causing
reduction of an oxidant gas supplied to it, permits to induce at the
electrochemically active surface of the gas diffusion electrode of the
cathode, in
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other words at the electrochemically active surface of the cathode, a redox
transformation. This redox transformation involves on the one hand a
reduction of the oxidant gas, and on the other hand an in situ oxidation of
the
metal and/or metalloid element contained in the water soluble precursor
compound, to a higher oxidation state. This is surprising, as the skilled
person
would typically expect a reduction reaction of the metal and/or metalloid
element to occur at the cathode. The assumption that an in situ oxidation of
the at least one metal and/or metalloid element takes place is supported by
the
observation that the conductivity of the catholyte decreases with an
increasing
degree of recovery of metal or metalloid ion from the aqueous solution, i.e.
an
increasing formation of nano particles, in particular nano crystals of the
reaction product. The inventors have also observed that the oxidized at least
one metal and/or metalloid element having a positive valence adsorb at least
temporarily to the electrochemically active surface of the cathode, and
thereby
form an interface with the catholyte.
The electrochemical potential or range of electrochemical potentials at
which reduction of the oxidant gas may occur, is well known to the skilled
person and is represented by the Nernst equation. The Nernst equation is an
equation that relates the reduction potential of a half-cell or the total
voltage
at any point in time to the standard electrode potential, temperature,
activity
and reaction quotient of the underlying reactions and species used. The Nernst
equation may be written as follows :
E = Eo + RT in aox
or E = Eo + RT inn
zF aRed zF
where
- E is the half-cell reduction potential at the temperature of
interest
- E is the standard half-cell reduction potential
- R is the universal gas constant, R = 8.314 472(15) J K-1 mop'
- T is the absolute temperature
- a is the chemical activity for the relevant species, where aRed is the
reducing agent and aox is the oxidizing agent. ax = yxcx, where yx is the
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activity coefficient of species X. (Since activity coefficients tend to unity
at low concentrations, activities in the Nernst equation are frequently
replaced by simple concentrations.)
- F is the Faraday constant, the number of coulombs per mole of
5 electrons: F = 9.648 533 99(24)x104 C mop'
- z is the number of moles of electrons transferred in the cell
reaction or
half reaction
- (2, is the reaction quotient.
At room temperature (25 C), RTIF may be treated like a constant and replaced
by
10 25.693 mV for cells.
The Nernst equation is frequently expressed in terms of base 10 logarithms
(i.e. common logarithms) rather than natural logarithms, in which case it is
written
as follows for a cell at 25 C:
n 0.059 aox
E = E- + 109113¨,
u-Red
This equation can also be writien as a function of pH, as follows:
n 0.059 aox , m
E=E - + ¨logio-- u.059 -pH
Z aRed Z
Where m represents the stoichiometric coefficient as per the reaction below:
aA + mH+ ze- <=' bB + cH20
(see Pourbaix M (1976) Atlas of Electrochemical Equilibria in Aqueous
Solutions. NACE Cebelcor. Houston, Tx.
The inventors believe that the redox transformation takes place at the
active surface of the electrochemically active layer of the gas diffusion
electrode, i.e. the external surface of the porous material forming the
electrochemically active layer, as well as at the internal surface of the
electrochemically active material or in other words on the active surface
within
the interior of pores of the porous electrochemically active material. The
fact
that the process also occurs within the pores of the electrochemically active
material has been observed when the electric polarization is stopped, by the
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release of the majority of the nano particles, in particular nano crystals
into
the electrolyte when stopping the electric polarization.
Without wanting to be bound by this theory, the inventors assume that
the reduction of the oxidant gas at the cathode 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 surface of the
cathode and create there a local environment confined within the pores of the
electrochemically active material. Within the pores of the electrochemically
active material, a characteristic pH and redox potential may develop, which
are different from that of both the solution and the external electrode
surface.
Supersaturation of the local environment with such species may occur.
The inventors further believe that the at least one water soluble
precursor compound is dissolved in the electrolyte, in particular in the
catholyte, in an at least partly dissociated state:
MA <---> M+ + A-
with 1\4+ representing a cation of the metal element or metalloid element or a
compound having a positive valence comprising the at least one metal element
or metalloid element, or a mixture of two or more of the afore mentioned
species. It is remarked that the at least one metal or metalloid ion or
compound comprising the at least one metal element or metalloid element may
have a positive valence of +1, +2, +3 or any other valence that may form for
that species. The cations of the dissolve water soluble precursor compound
contained in the aqueous electrolyte may migrate or diffuse from the
electrolyte solution towards the cathode, adhere or be adsorbed to the
external
surface of the electrochemically active material and be adsorbed within the
pores of the electrochemically active material, in particular to the active
sites
thereof. Adhesion of the positively charged cations to the electrochemically
active surface of the gas diffusion electrode may take place through various
mechanisms, and may involve adsorption forces, mainly Van der Waals forces.
Besides that, an electric double layer may be formed at the active surface
involving the cations, thus allowing for electrostatic adsorption or
capacitive
adsorption at the double layer, but reversible ion exchange adsorption,
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complexating or chelation and any other forms of adhesion or adsorption 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 material will be
present
in an at least partially dissociated state (C*-R-), especially when an
electrochemical potential is applied to the electrode. These dissociated sites
C*-R- carrying an electric charge, 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), which may dissociate into C*-R- and El+, 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 on the
electrochemically active surface may be accelerated and a local environment
may be created with extreme pH and redox conditions, which are different
from those found in the bulk of the electrolyte solution and on the bare
electrode surface:
C*-R + 02 (0+ e ¨). C*-R02ad=
C¨R02ad= + H20 + e ¨). C¨R02ad=Had + OH
C*-R02ad=Had + e- ¨> C*-R- + H02-R
One or more positively charged ions, i.e. metal or metalloid ions or a
compound
comprising one or more of the metal or metalloid element, may be then
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 species. Thereby a (polymetal ion
polyoxy radical) or a (polymetalloid ion polyoxy radical) may be formed, which
once adsorbed to an electrochemically active site and supersaturating such
site
may on the surface of the electrochemically active material of the cathode,
act
as a nucleation site for the formation of an oxidized compound comprising the
at least one metal and/or at least one metalloid ion.
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For example, in the case of the cerium ion (Ce3 ), this may lead to the
following reactions:
C*¨R02ad=-+ 2Ce3+ + 2e- ¨> C*¨R02ad2Cead
C*¨R02ad2Cead + 202 (g) + 2e ¨). C*¨R02ad2Cead202ad=
C*¨R02ad2Cead202ad= + H20 + 1/ + 2e- ¨> C¨R02ad2Cead202ad2Had+OH-
The electrochemically active surface of the gas diffusion electrode and
the cathode includes the external surface as well as the active surface
present
in the pores of the electrochemically active material. In conventional gas
diffusion electrodes which make use of a porous electrochemically active
material, in particular a carbon based porous electrochemically active
material, the active surface present within the pores provides by far the
larger
part of the active surface as the porous material will usually have a high BET
surface of at least 50 m2/g. Thus, the electrochemical process of this
invention,
i.e. the formation of nano particles, in particular nano crystals of the
reaction
product, will mainly take place in the pores of the electrochemically active
material, in which supersaturation conditions may exist of the polyatomic ions
or radicals which are formed during reduction of the oxidant gas. As a result,
formation and growth of particles of the reaction product in the pores of the
porous electrochemically active material will be limited by the dimensions of
those pores. Depending on the nature of the porous material, the average
diameter of the nano particles, in particular the nano crystals, will often be
limited to a maximum of 5 nm. A dedicated choice of the electrochemically
active material, in particular the pore size distribution of the
electrochemically
active material which is represented by the BET surface of the porous
material, thus permits controlling the size of the nano particles, in
particular
the nano crystals of the reaction product, and thus the particle size of the
reaction product.
The electrochemical potential to which the gas-diffusion cathode is
subjected, is a reducing potential 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
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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. However, in certain embodiments the
formation of hydrogen may be preferred, e.g. if the targeted application of
the
formed nano particles, in particular the nano crystals is related to in-situ
hydrogen capture or storage.
The inventors have observed that the oxidized metal and/or metalloid
cations formed in the process of this invention and outlined in the reaction
schemes above, accumulate at the interface in the pores of the
electrochemically active material, in a physical state which is different from
the physical state of the surrounding liquid electrolyte, so that they may be
separated therefrom. The reaction product may for example accumulate as
particles of a product forming a solid or colloidal phase. 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 process
proceeds and form a physical state which is different from the physical state
of
the electrolyte, so that the nano particles of the reaction product may be
isolated 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, gel-
like,
foam-like or a Pickering emulsion-like phase.
Since the nature of the forces with which the reaction product
comprising the at least one metal element and/or metalloid element in an
oxidized state adheres to or is adsorbed to the electrochemically active
surface
of the gas diffusion electrode may vary with the nature of that
electrochemically active material and the nature of the oxidized compound,
release of the reaction product in the form of nano particles, in particular
nano
crystals, into the electrolyte may occur as such, or may need to be forced.
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Metal and metalloid elements may take various oxidation states and
may form, with the species which result from the reduction of the oxidant gas,
reaction products or compounds which contain one or more monoatomic or
polyatomic ions, in an oxidation state which leads to a phase that may be
5 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 (or redox)
potential and pH for the catholyte which permits formation of a separable
phase. Pourbaix "Atlas of electrochemical equilibria in aqueous solutions",
10 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, at standard conditions. Theoretical diagrams for a wide
variety of species can be constructed without undue burden based on the
premises provided therein, included those for oxidant gases other than oxygen
15 The skilled person is capable of identifying the electrochemical
potential at
which electrochemical reduction of the oxidant gas, and the corresponding
oxidation of the metal cation or metalloid cation may occur. This is
illustrated
in figure 18, which shows the electrochemical series of a few oxidant gases.
It has been observed that by varying the electric or electrochemical
potential at the cathode, the chemical composition of the reaction product may
be controlled. Without wanting to be bound by this theory, the inventors
assume 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.
J 0- o -
ce c,
ce\
o
ad, ,
E 4:'
\to\o
OFkadl a410,$) 0 /
Ce N, Ce Ce
0
-4roxide
This clustering may lead to local super-saturation on the active surface of
the
electrochemically active material including the active surface within the
pores
of the electrochemically active material, and the growth of the ionic or
radical
species present on that electrochemically active surface (for example surface
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peroxide) into critical nuclei, as opposed to crystal formation in the
electrolyte
solution known from the prior art.
A dedicated selection of the pore size distribution of the
electrochemically active material permits to obtain the nano crystals of the
reaction product in a selected crystal form, and to affect their lattice
parameters. In particular when use is made of an electrochemically active
material having a quite uniform pore size distribution, nano crystals of the
reaction product may be obtained which are monodisperse, or at least show a
small particle size distribution curve. Within the scope of the present
invention
with monodisperse is meant, particles where 75 % of the particles have a
particle size which differs with maximum 5 %, preferably maximum 3 %, more
preferably maximum 1 %.
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
present in the electrolyte. Supply of an oxidant gas to the cathode, in
particular to the gas diffusion electrode, may therefore be preferred in order
to
ensure maximum recovery of the reaction product comprising the at least one
metal or metalloid element in an oxidized state, from the water soluble
precursor compound dissolved in the water based electrolyte and optimize the
reaction rate. In practice, an oxidant gas supplied to the gas diffusion
electrode
will migrate from the hydrophobic side of the gas diffusion electrode, through
the gas chamber of the gas diffusion electrode, towards, into and through the
electrochemically active material of the gas diffusion electrode and cathode.
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, N203, 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.
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Preferably however, use is made of air due to economic reasons. Other oxidant
gases particularly suitable for use with this invention include those capable
of
forming oxidant mono-atomic radicals and/or oxidant polyatomic radicals, for
example oxygen, ozone, carbon dioxide etc.
Particularly preferred oxidant gases are those which may be reduced so
as to generate polyatomic ions, polyatomic radicals or polyatomic peroxides,
for
example those summarized in the table below:
hydrogen hydrogen
perchlorate C104-1 HSO4-1 HPO4-2
sulfate phosphate
dihydrogen
chlorate C103-1 H2PO4-1 peroxide 02-2
phosphate
chlorite C102-1 permanganate Mn04-1 tetraborate B4072
hypochlorite CO' periodate I04-1 borate B03-3
hydrogen
Nitrate NO3-1- HCO3-1
carbonate
Nitrite NO2-1 sulfate SO4-2
bromate Br03-1- sulfite S03-2
Iodate 103-1 carbonate CO3-2
The at least one 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 (CH3C00-), acetylide
(C22-), carbonate (C032-), peroxide (022-), phosphate (P043-), sulfate (S042-
),
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. Ethylene oxide may for example be reduced at an electrochemical
potential of -1.2V vs. SHE. For other organic gases the electrochemical
potential may easily be determined by the skilled person.
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The at least one 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 at least one oxidant gas within the gas
mixture is not critical to the invention and may vary within wide ranges. Yet,
varying the oxidant gas partial pressure or the oxidant gas supply rate will
permit to control the size of the crystals of the reaction product containing
the
at least one metal and/or metalloid element. Varying the oxidant gas partial
pressure, in particular increasing or decreasing the partial pressure of the
at
least one oxidant gas, will also permit to control, in particular to increase
or
reduce the average lattice parameter of the reaction product crystals as
measured by X-ray diffraction measured over a given crystallographic plane or
measured using transmission electron microscopy imaging. Varying the
oxidant gas partial pressure or the oxidant gas supply rate will namely affect
the amount of oxidant gas that may accumulate on the electrochemically
active surface, in particular by the amount of oxidant gas present on the
active
surface within the pores of the porous electrochemically active material. This
amount of gas may in turn be controlled by controlling the gas supply rate to
the gas chamber of the gas diffusion electrode, and by controlling the
concentration of the oxidant gas in the gas that is supplied to the gas
chamber
of the gas diffusion electrode. When air is used as the oxidant gas, the flow
rate will usually vary between 100 and 400 ml/min, preferably between 200
and 400 ml/min.
The concentration of the water soluble precursor compound in the
electrolyte is not critical to the invention. However, the concentration of
the
water soluble compound is preferably at least 100 ppm. In general the
concentration of the water soluble compound will not be higher than 10 g/liter
as the efficiency of the process can hardly be improved at higher
concentrations. Therefore, the concentration of the water soluble compound in
the catholyte is preferably between 100 and 500 ppm. The concentration of the
water soluble precursor in the catholyte may for example be varied by varying
the rate with which the water soluble precursor is supplied to the catholyte.
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Such variation will in particular be done when variation of the particle size,
in
particular the crystal size of the reaction product is envisaged, since
varying
the concentration of the water soluble precursor permits varying the particle
size, in particular the crystal size of the reaction product. The inventors
have
observed that the particle size, in particular the crystal size may increase
with
increasing precursor concentration or that the particle size, in particular
the
crystal size may decrease with decreasing precursor concentration.
The nature of the compound which may be recovered from the at least
one water soluble precursor compound may be varied by selecting the
appropriate oxidant gas. When 02 or an 02 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.
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In a preferred embodiment, the porous electrochemically active
material for the gas diffusion electrode and the cathode is chosen such that
it
has an active surface which comprises a plurality of active sites provided by
surface functional groups, wherein the functional groups preferably contain
5 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 in
the
10 cathode compartment 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 water
soluble
15 precursor compound, more preferably below 5Ø Usually the pH will be at
least 1.5 as below this value the process slows down too much. The inventors
have observed that the pH of the catholyte gradually progresses towards
alkalinity in the course of the reaction, which is often above the
dissociation
constant of the acid or salt of the ionic metal or metalloid compound. In
20 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
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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() # 1-1+ (84 A- 4,q)
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- [H+ } [A-
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 7Ø Examples of weak
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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 11Ø 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 in the cathode compartment 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, NaC1 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
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or only a minor decrease of the overall conductivity has been observed. This
is
probably due to adhesion or adsorption of cations containing the at least one
metal element and/or metalloid element of the water soluble precursor
compound, to the electrochemically active surface of the cathode, which in
general is expected to attain a quasi stable level when all of the positive
valenced ions 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 nano particles, in particular the nano
crystals, from the electrochemically active material 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 recover metal or metalloid ions or a
cation containing such metal or metalloid element contained in the water
soluble precursor compound. Thereby recovery rates may be substantially
complete, and amount to more than 99.0 wt. % thereof. Recovery rates will
usually be at least 20 wt. % of the initial concentration of the at least one
metal or metalloid element present in the precursor compound, preferably at
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least 50 wt. %, more preferably at least 80% and most preferably more than 90
wt. % or even more than 99% thereof.
The electrochemical process of the present invention is suitable for
recovering components of a wide variety of water soluble precursor compounds,
in a wide variety of concentrations.
The present invention is suitable for isolating a wide variety of
compounds from an aqueous solution of the corresponding water soluble
precursor compound. The precursor compound 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
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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 El+, Lit, Nat, K+,
Cs, Mg2+, Ca2+, Sr2+, Ba2+, APP, Ag+, Zn2+, Fe2+, Fe3+, Cu2+, Cu + and
mixtures of
5 two or more hereof. Although the majority of the afore mentioned metal
and/or
metalloid ions may be used as such within the process of this invention, for
some of them, in particular Na and K may need to be used in a mixture with a
further metal and/or metalloid ion.
The water soluble precursor compound may be supplied as a precursor
10 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
15 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
20 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
25 formed from the precursor compound may contain crystalline oxide nano
particles, for example, but not limited to Ce02, La203, Co203, A1203, Cs20,
Li20, CoFe204, FeAs04, or non-stoichiometric forms thereof or hydrated forms
thereof (e.g. Ce0175). 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
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mixed oxides. In a fourth embodiment of this invention additional ionic
compounds may be supplied to the catholyte such as ionic liquids. As a result
of the presence of such an ionic liquid the structure of the reaction product
may comprise the ionic liquid products of its decomposition, for example but
not limited to terbutylammonium chloride or terbutylammonium chloride.
Surprisingly it has been found, and this has been confirmed by IR spectra of
the reaction product formed ¨ although the structure is not fully understood,
that by supplying an ionic liquid a foamy reaction product is obtained.
The particles of the reaction product 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-like, foam-like or Pickering emulsion-like phase. Usually a stable
dispersion or gel will be obtained. In order to improve the stability, the
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 water soluble precursor compound, the electrolyte at the cathode,
the suspension or dispersion at any convenient point of time. The additives
may be selected from the group of dispersants, stabilizers, surfactants,
polymers, copolymers, emulsifiers, cross-linking agents, capping agents and
free flow agents or mixtures thereof. Such addition may done to stop growth of
the nano particles in particular the nano crystals, to reduce the risk to
agglomeration of the nano particles, to provide a stable dispersion.
In a preferred embodiment of this invention, the electrochemically
active material of the gas diffusion electrode which forms part of the cathode
preferably comprises an active surface having a plurality of active sites with
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
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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,
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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.
The process of the present invention is suitable for use in a wide
variety of applications. The process of the present invention may for example
be used to produce nano crystals of selected materials in a selected particle
size distribution, with a selected crystal form, with selected lattice
parameters.
The process of the present invention may also be used to recover metal ions or
metalloid ions from an aqueous solution, and provides a suitable method for
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recovering for example precious metals, in particular resulting from
destruction of electronic materials, from aqueous solutions.
The reaction product obtained according to the present invention, in
the form of nano crystals, may have a wide variety of uses. For example for
reaction products containing cerium, the Ce02 or sub-oxides such as Ce0i 75
may be formed which are particularly suitable for the scavenging of oxidizing
agents such as oxygen or hydrogen peroxide. Their use as oxygen scavenging
agents may be of particular interest to the electronics industry as additives
for
screen-sealing agents. Their use as hydrogen peroxide scavenging agents may
be of particular interest to the cosmetic industry, for example but not
limited
to scavenging naturally-produced peroxides in hair follicles to prevent or
revert white hair. Other compositions, for instance those issued with the
reaction with ionic liquids may have use as hydrogen or CO2 scavengers,
among many other numerous applications.
The present invention also relates to a composition comprising at least
one aggregate particle of nano crystals obtained by the above described
process
wherein, the nano particles in particular the nano crystals have a particle
size,
in particular a crystal size of between 0.2 and 30.0 nm. Where a crystalline
product is obtained, the nano crystals will usually have a lattice parameter
of
between 1.0 and 18.0 nm, and any aggregate particles formed will have an
average particle size of < 30 nm. The reaction product may take the form of a
dispersion of the reaction product in an aqueous medium, with a solids content
of between 1.0 and 30.0 wt. %, preferably between 5.0 and 10.0 wt. %. The
aqueous medium may comprise the electrolyte, water or an aqueous solution.
The present invention also relates to a method for selectively isolating
at least one metal element or a metalloid element or two or more thereof from
an aqueous solution comprising a water soluble precursor compound thereof,
wherein use is made of the process as described above.
The present invention further relates to a device for recovering a metal
element or a metalloid element or two or more thereof from at least one water
soluble precursor compound comprising a metal element or a metalloid
element or two or more thereof, in the form of one or more nano particles, in
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particular nano crystals of at least one reaction product, wherein the device
comprises
- means for supplying the water soluble precursor compound at least
partly dissolved in a water-based catholyte to a cathode compartment of
5 an electrochemical cell, equipped with a cathode comprising a gas
diffusion electrode, wherein the gas diffusion electrode comprises a
porous electrochemically active material having a BET surface area of
at least 50 m2/g,
- wherein the catholyte has a pH which is smaller than the pKa of the
10 water soluble precursor compound,
- means for supplying at least one oxidant gas to the gas diffusion
electrode,
- wherein the cathode has an electrochemical potential which is such
that
it is capable of causing reduction of the at least one oxidant gas to one
15 or more corresponding peroxide, ionic and/or radical reactive
species
capable of reacting with the metal element, the metalloid element or a
cation comprising such a metal or metalloid element or two or more
thereof, to form at least one nano particle, in particular at least one
nano crystal of the at least one reaction product with an average
20 crystallite size equal to or smaller than 30.0 nm.
The invention is further illustrated in the appending figures.
Figure la shows a schematic representation of the experimental
electrochemical half-cell reactor suitable for use with the present invention.
25 Figure lb shows the process of the present invention including the
reactions
and nano crystal formation within the pores of the electrochemically active
material.
Figure 2 shows the removal efficiency in % of Ce3+ ions from the bulk solution
in the presence of N2 supplied through the gas-diffusion cathode, in the
30 absence of oxidant gas, of example 1.
Figure 3 shows the electrochemical response in case no oxidant gas is supplied
through the gas-diffusion electrode, but only N2.
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Fig. 3a: Frequency response obtained in example 1 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- in example 1.
Fig. 3c and d show 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), in
example 1.
Fig. 3e shows a typical capacitive and pseudo-capacitive CV responses, in
example 1.
Figure 4 shows the extent of removal of the Ce3+ ions from the bulk solution
in
the presence of 02 as the oxidant gas supplied to the gas diffusion electrode
in
example 1.
Figure 5 shows the recovery efficiency (%) of the Ce3+ ions transformed into a
solid product recovered as precipitate after being released from the electrode
and sedimented in solution, in the presence of 02 as the oxidant gas supplied
through the gas-diffusion cathode, on the basis of dry weight of the recovered
product, in example 1.
Figure 6 shows the electrochemical response obtained for the experiments
where air was supplied through the gas-diffusion electrode, in example 1:
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.
Figure 7 shows the crystallite size and lattice parameter found for the
different initial Ce3+ concentrations studied, in example 1.
Figure 7a shows the crystallite size (220) for Ce02 and NaCl.
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Figure 7b shows the lattice parameter Ce02 and NaCl. There was a limit in
detection for both parameters at Ce below 20 mg.
Figure 8 shows transmission electron micrographies evidencing the
characteristic morphology of Ce02 nano particles with crystallite sizes
matching those obtained by XRD, in example 1.
Figure 9 shows transmission electron micrographics evidencing the
aggregation of the small crystalline nano particles of Figure 8 into larger
size
nano particles, of example 1.
Figure 10 shows shows the removal efficiency (%) of the different metal ions
from the bulk solution in the presence of air supplied through the gas-
diffusion
cathode.
Figure 11 shows a device suitable for carrying out the process of the present
invention.
Figure 12 shows High-resolution transmission microsocopy (HRTEM) of
nanoparticles obtained with example 1.
Figure 13 shows IR spectra of the precipitates obtained under different
experimental conditions with TBAB (fig. 13a) and IR spectra of the initially
employed reagents for the TBAB experiment (fig. 13b). The IR spectra for the
TBAC cases are shown in fig. 13c.
Fig. 14 shows nanocrystals precipitated in the presence of excess IL in
Example 7 and under in-situ hydrogen electrosynthesis. Small bubbles in the
size range of 0.1 mm were visible, occupying more than 10% of the volume of
the material floating.
Figure 15 shows the XRD pattern for the nano crystalline product obtained with
TBAB 30.
Figure 16 shows the charge in Coulombs, consumed during the experiments
carried out at different concentrations (molar ratios) of TBAB.
Figure 17 shows TGA characteristic of the nanocrystaline product formed with
TBAB in a molar ratio of 20.
Fig. 18 shows electrochemical series of a few oxidant gases, the current
density
distribution over different gas-diffusion cathode materials (different types
of
carbon), registered at different steady state reduction potentials.
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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
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(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,
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.
10 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
15 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
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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
5 distributor.
The electrochemically active material 4 is preferably a material
which has a high electric conductivity. This permits the electrochemically
active material to take away or bring the electron from and to the current
density distributor.
10 The electrochemically active surface may be formed of any
conductive materials or composite materials with a high surface area.
Examples of such electrode materials include carbon, carbon nanotubes,
graphite, carbon fiber, carbon cloth, carbon aerogel, graphene, metallic
powders, for example nickel, metal oxides, for example ruthenium oxide,
15 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 (Tr), or alloys
of
20 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
25 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
30 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
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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.
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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 < 7.0, more
preferably < 5.0, most preferably < 3.0 but no lower than 1.5.
The electrolyte will usually have an ionic conductivity of at least 1.0 mS/cm,
but it may amount to 70-100 mS/cm in practice.
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 CeC13 99.9% (REO) ampouled under argon was
received from Alfa Aesar. K2HPO4 was procured from Merck. HC 1 at 35%,
CeN309 .6H20 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)6Mo7024=4H20 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/AgC1 3 M KC1 (+200 mV vs SHE) was used as a reference
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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/AgC1 3 M KC 1 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' (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 la.
Gas diffusion electrodes
A multilayered VITO CORETM 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. Otherwise, a catalyzed electrode may be
employed, e.g. it was 20 wt% PTFE with 76 wt% activated carbon and 4 wt%
Ce02, which may improve the efficiency of the process but is not essential to
carry out the targeted synthesis.
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Electrolyte composition
Independent electrodes were tested as gas-diffusion cathodes, in presence of
air or N2 respectively, at the cathodic gas compartment. CeN309=6H20 was
added to the cathodic electrolyte, composed of 30 g/L NaC1 and 10 mM sodium
acetate dissolved in demineralized water and adjusted at pH 2.7 with HC1.
Sodium acetate is not essential to carry out the targeted synthesis, however
it
may impact its rate. Different concentrations of CeN309=6H20 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 H202 after 02
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 mainly by diffusion,
with
possible subsequent adsorption in the porous electrode active sites.
Otherwise,
water electrolysis to form hydrogen is not expected at the conditions of this
example.
Electrochemical Impedance Spectroscopy (EIS) was recorded at the
steady state polarization potential (-0.350 V) a frequency range from 3 kHz to
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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 <
5 10 pA 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
10 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 vs 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 H202
15 A spectrophotometric method was employed to determine the
concentration of H202 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)6Mo7024=4H20 into 500 mL deionized water. This solution was
kept in dark conditions to inhibit oxidation of I. Reagent B was prepared with
20 10 g KHP dissolved into 500 mL deionized water. The standard calibration
curve (not shown) was prepared from known H202 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
25 mixture was allowed to react for 5 minutes, before reading the
absorbance of
the solution at 351 nm [GSI Scientific Report. (2009) Helmholtzzentrum fiir
Schwerionenforschung, 2010-1].
The concentrations calculated of H202 are the average of the
quantitative results obtained with 5 averaged calibration curves, described by
30 the following equation:
A351 = 0.3687 CH202 R2 = 0.9991
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Where CH202 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 CuKa (A = 1.5405A) 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):
BA
D = Eq. 8
p1/2cos,9
where B is the Scherrer constant (0.89), A is the wavelength of the X-ray beam
(1.5405A), fl1/2 is the full width at half maximum of the diffraction peak and
0
is the diffraction angle.
Independent aqueous solutions with fixed concentrations of NaC1 and
sodium acetate (CH3COONa) were supplemented with varying concentrations
of Ce(NO3)3.6H20 (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 HC1. A colourless solution was formed
in
all cases. A constant potential of -0.350 V vs Ag/AgC1 (3M KC1) 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.min4). Under such
operational conditions water electrolysis is avoided; however, when air is
supplied through the GDE, 02 electrochemically reduces to H202, upon
availability of protons and electrons.
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The overall solution was considered to be electroneutral before the
electrochemical polarization was applied. Given the high concentration of
NaC1, 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 (CeT,i / mg):
REMeff = CTf X 100% Eq. 9
CTj
CeT,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
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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:
0 t,, = 0 mg.L-1 Ce3 .
= tf, Cce3ti = 0 mg.L-1 Ce3 .
A to, Cce3ti = 10 g.L-1 Ce3 .
A tf, Cce3ti = 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.
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Although Fig. 3 only presents the EIS and CV data obtained for the
systems without Ce(NO3)3.6H20 or those supplemented with 10 g.L-1 of
Ce(NO3)3.6H20, 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)3.6H20.
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 NaC1+ 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 NaC1 could be achieved at such high NaC1 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.L4 NaC1 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 0 groups as characterized by
scanning electron microscopy and energy dispersive X-ray spectroscopy.
02 supplied as the oxidant gas through the gas-diffusion cathode
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
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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)
5 Figure 4 shows the extent of transport of the Ce3+ ions (removal
efficiency %) from the bulk solution in the presence of 02 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
10 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 02 was supplied through the gas-diffusion
15 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
20 being released from the electrode and sedimented in solution, in the
presence
of 02 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 Ce02 isotropic nanocrystals, as
identified by XRD and microscopic evidence described later, which precipitated
25 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
30 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
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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
02 (ads)) and the electrosynthesized H202 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/AgC1 were not anymore
capacitive ion-storage or electrosorption alone but an electrocatalytic
reduction, presumably 02 reduction to H202. This constitutes part of the proof
that the process for the formation of the nano crystals does not occur in
solution as a result of pH changes but at the electrode porosity as a
consequence of the electrocatalytic reactions, as observed by the
characteristic
rate-limiting EIS response for adsorption that is linked tot he formation of
superoxide intermediaries in carbon materials. The latter response is namely
not observed when non-porous metallic catalysts are incorporated in the gas
diffusion electrode, as in that case the mechanism does not involve the
adsorption of such superoxide radicals which trigger the supersaturation
conditions within the pores of the electrochemically active material of the
electrode.
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:
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0 ti, Cce3ti = 0 mg.L-1 Ce3 .
= tf, Cce3ti = 0 mg.L-1 Ce3 .
A to, Cce3ti = 10 g.L-1 Ce3 .
A tf, Cce3ti = 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 Ce02 and NaCl.
Figure 7h shows the lattice parameter Ce02 and NaCl. There was a
limit in detection for both parameters at Ce below 20 mg.
Fig. 8 shows the steady state current density as a function of time for
different oxygen-reducing gas-diffusion cathodes, with and without Ce02
catalyst, after 120 min of electrocatalytic production of hydrogen peroxide.
N:
Norit, V: Vulcan-Norit, AB: Acetylene Black-Norit. All electrodes were
composed of a combination of 80% carbon mixture and 20% polymer (PVDF).
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 H202 found in solution (Figure 7a). At lower Ce3+
concentrations the crystal size of Ce02 is smaller whereas as the
concentration
increases the crystal size is larger. The average crystal size for Ce02 was
3.5 0.337 nm, whereas for NaC1 it was 45.1275 0.337. This makes possible
further separation either by re-dissolution of NaC1 with a pH where Ce02 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 H202 found in
solution. It is possible that Ce3+ plays a co-catalytic role in the
electrosynthesis
of H202 itself.
Figure 8 shows transmission electron micrographies evidencing the
characteristic morphology of Ce02 nano particles with crystallite sizes
matching those obtained by XRD .
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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.
To release the nano crystals from the electrochemically active layer of
the gas diffusion electrode, several techniques may be used. Suitable
techniques include reversing the polarity of the electrode, accelerating the
electrolyte flow rate. The fact that these techniques cause release of the
nano
crystals into the catholyte, is an additional proof of the process taking
place in
the pores of the electrochemically active layer and not in the electrolyte
solution. In fact, when comparing the size and morphology of the product
obtained by just changing the pH of the electrolyte a difference is clear.
Nano
crystals are not obtained and instead an amorphous product is visible as
appreciated by XRD. In addition, the nanocrystal product obtained by this
invention shows a characteristic monodisperse foam-like morpohology (see
Figure 12 and 14 HRTEM) whereas the product obtained by simple addition of
a base until an alkaline pH is reached shows high non-uniformity.
EXAMPLE 2.
Independent electrodes were tested as gas-diffusion cathodes, in
presence of air at the gas compartment (to provide 02 for its reduction to
H202,
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 Quantity
formula (mg.L-1-)
1 Cerium nitrate hexahydrate Ce(NO3)3 = 350
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Chemical name Chemical Quantity
formula (mg.L4)
6H20
2 Dysprosium nitrate x hydrate Dy(NO3)3=xH20 80
3 Erbium nitrate pentahydrate Er(NO3)3=5H20 53
4 Europium nitrate pentahydate Eu(NO3)3 = 6
5H20
Gadolinium nitrate Gd(NO3)3 = 59
hexahydrate 6H20
6 Holmium nitrate Ho(NO3)3 = 18
pentahydrate 5H20
7 Lanthanum nitrate La(NO3) 3 = 159
hexahydrate 6H20
8 Lutetium nitrate hydrate Lu(NO3)3=xH20 6
9 Neodymium nitrate Nd(NO3)3 = 206
hexahydrate 6H20
Praseodymium nitrate Pr(NO3)3=6H20 51
hexahydrate
11 Samarium nitrate Sm(NO3)3 = 53
hexahydrate 6H20
12 Terbium nitrate hexahydrate Tb(NO3)3 = 12
6H20
13 Thulium nitrate pentahydrate Tm(NO3) 3 = 8
5H20
14 Yttrium nitrate hexahydrate Y(NO3)3 = 6H20 536
Ytterbium nitrate Yb(NO3)3 = 50
pentahydrate 5H20
Additionally, 30 g/L NaC1 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
5 by means of ICP-MS.
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The process was applied at constant polarization at -0.350 my 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
5 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.
10 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.
15 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)3.6H20 by the start of
experimentation.
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Concentration
0 100 500 1000 5000
(PPm)
Catholyte pH 2.54 2.15 2.70 2.78 2.76
Conductivity
51.0 51.6 50.4 50.3 52
(mS.cm-1)
Anolyte pH 2.8 2.8 2.74 2.74 2.74
Conductivity
49.7 49.7 49.8 49.8 49.8
(mS.cm-1)
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/AgC1 (3M KO), 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)3.6H20 at the end of the experiment.
Concentration
0 100 500 1000 5000
(PPm)
Catholyte pH 11.5 12.47 11.62 11.8 5.37
Conductivity
49.1 49.9 51.6 50.7 47.4
(mS.cm-1)
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Anolyte pH 2.37 2.78 2.23 2.20 2.68
Conductivity
49.7 49.9 50.7 49.5 47.5
(mS.cm-1)
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/AgC1 3 M -0.150 -0.350 -0.550 -0.750 -0.950
KC1)
Catholyte pH 2.67 2.63 2.51 2.51 2.76
Conductivity 47.1
47.4 47.2 47.4 46.9
(mS.cm-1)
Anolyte pH 2.74 2.74 2.8 2.8 2.8
Conductivity 49.8
49.8 49.7 49.7 49.7
(mS.cm-1)
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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/AgC1 3 M -0.150 -0.350 -0.550 -0.750 -0.950
KC1)
Catholyte pH 5.37 6.55 6.92 8.45 8.5
Conductivity
45.2 46.6 46 44.1 45
(mS.cm-1)
Anolyte pH 2.68 2.13 2.15 2.13 2.15
Conductivity
47.5 51.0 50.3 50.5 50.8
(mS.cm-1)
After centrifugation and drying, a crystalline product could be recuperated
matching the characteristics of sassolite, except for that formed at -0.950 V
which had characteristics of nanocrysaline borax Na2B407.10H20. The latter
product may have application as dopant in flexible and fluorescent
electronics.
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The example below show the electrochemical series showing the reduction
potential of different oxidizing gases to peroxides or their polyatomic ions
or
radicals. The potentials reported in this table refer to the normal hydrogen
electrode (NHE). These potentials are calculated by Nernst equation.
TABLE F
Redox pair (half-cell reaction) Thermodynamic reduction potential of
the half-cell reaction at pH 1 (V vs
NH E)
NO/NO3- +0.958
02/H202 +0.547
02/1102* -0.13
CO2/HOOCCOO- -0.43
CO2/HCO2- -0.49
CO2/HC00- -0.61
EXAMPLE 5.
Eight different electrolyte compositions were tested, as described in example
1
but instead of cerium nitrate, the following independent compounds were
supplemented the catholyte in a concentration of 500 ppm, respectively:
1. Co(NO3)2 = 6 H20
2. Al(NO3)3 = 9 H20
3. CsC1
4. LiC1
5. KNO 3
6. Sm(NO3)3 = 6 H20
7. Er(NO3)3 = 5 H20
8. Lu(NO3)3 = x H20
9. Fe(NO3)3
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The process was carried out at -0.350 V. The operational volume of the
catholyte in each experiment was 125 mL. In all cases, a nano crystalline
product could be recovered except for KNO3. On each case where a precipitate
was found, the latter had characteristics (color, morphology) associated to
the
5 specific precursor employed, proving that nano crystalline materials can
be
formed with a wide diversity of metals from different groups in the periodic
table. In all cases the crystallite size was < 30 nm.
EXAMPLE 6.
10 A catholyte was prepared, identical to that explained for example 1 but
instead
of cerium nitrate, arsenic acid was employed in a solution containing 72 g/L
of
arsenic and iron chloride considering a Fe/As ratio of 1.5 and sulfuric acid
considering 5.61 g/L of S. The process was carried out in the same
experimental conditions as example 1 but at an operational temperature > 80
15 C. After processing, stable nanocrystalline scorodite was obtained,
with a
crystallite size <30 nm. This embodiment may be applicable to arsenic
immobilization.
EXAMPLE 7.
20 Experimental conditions
In the present example, the process claimed was studied under the presence of
ammonium-based ionic liquids (IL). More specifically, Tetrabutylammonium
bromide (TBAB) and tetrabutylammonium chloride (TBAC) were employed.
There are few literatures showing the use of ILs for the synthesis of metal
25 organic frameworks (M0Fs) by ionothermal synthesis (Lei Liu, David S.
Wragg, Hongyan Zhang, Ying Kong, Peter J. Byrne, Timothy J. Prior, John E.
Warren, Zhuojia Lin, Jinxiang Dong and Russell E. Morris. Ionothermal
synthesis, structure and characterization of three-dimensional zinc
phosphates. Dalton Trans., 2009, 6715-6718.). Few literatures also report that
30 nanoparticle-stabilized ILs may behave as Pickering emulsions in water
(Huan
Ma and Lenore L. Dai. Particle Self-Assembly in Ionic Liquid-in-Water
Pickering Emulsions. Langmuir, 2011, 27 (2), pp 508-512) i.e. they assemble
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themselves to fom cage-like structures wherein some gas may be trapped in.
These emulsions are typically stable as long as the medium is undisturbed.
Otherwise ILs can be used as electrolytes due to their conducting properties
(Chiappe and Rajamani 2012; Chiappe et al 2011 and Andrzej
Lewandowski, Agnieszka widerska-Mocek. Ionic liquids as electrolytes for
Li-ion batteries¨An overview of electrochemical studies. Journal of Power
Sources.Volume 194, Issue 2, 1 December 2009, Pages 601-609.).
All reagents were used as purchased without further purification.
Demineralized water was used throughout the study. Activated carbon
employed was Norit SX1G from Norit Americas Inc. Fluorinated ethylene
propylene resin (Teflon FEP 8000) was obtained from Dupont. Two ILs based
on ammonium base i.e. TBAB (Sigma-Aldrich 98.0%) and TBAC (Aldrich
97.0% (NI)) were purchased. Lanthanum nitrate hexahydrate
(La(NO3)9.6H20) was also purchased from AHrich (99.99% trace metals
basis). All reagents were stored as per recommendations of the corresponding
supplier. Handling of all reagents was carried out as per recommendations in
the corresponding Material Safety Data Sheets (MSDS). Experiments were
performed in a half-cell electrochemical reactors equivalent to those
described
in example 1, unless otherwise specified. The principal functions of Zirfon
here were to prevent oxygen eventually evolved at the counter-electrode from
reaching the working-electrode, to serve as ion permeable separator, and also
as a fluid transport barrier to maintain the desired pH at the catholyte
compartment. The electrodes and separator had a projected electrode surface
area of 10 cm2. The reacting (oxidant) gas flows (air or CO2, respectively)
were
fed through the cathode gas compartment on each individual experiment. Gas
flow rate was set at 200 mL/min (excess) in all cases and an overpressure of
30
mbarg 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 20 rpm (Watson-Marlow).
An acidic basal electrolyte was employed in all experiments related to this
example, consisting of demineralized water supplemented with HC 1 to achieve
a starting pH of 2.7. The effect of two independent oxidant gases was
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investigated, aka air (composition is Nitrogen: 78%; Oxygen: 20%; CO2: 0.03%,
inert gases mainly argon: 0.97%; water vapor: 1%) and CO2/N2 80%/20%,
respectively. La(NO3)3.6H20 was added to the cathodic electrolyte, at a
concentration of 500 ppm. Different molar ratios between the IL and
La(NO3)3.6H20 were investigated, as follows: 0, 0.1, 1, 10, 20, 30 and 86.5.
The
last molar ratio corresponds to a concentration of 0.1 M for the ILs. The
electrolyte at the counter-electrode compartment (anodic) was identical to
that
of the catholyte (acidic basal electrolyte), but without the addition of
lanthanum nitrate or IL.
The experiments were carried out at room temperature (18 2 C). The
parameters like H202 concentration, conductivity and pH were measured
before and after each experiments in both the sets. Three more cases were
carried out where catholyte was acidic basal electrolyte only, La(NO3)3.6H20
in
acidic basal, TBAC (0.1 M). At higher concentrations of TBAB (specially 0.1 M)
gave yellow precipitates or by-products in the anolyte chamber which may be
some compounds of bromide or nitrate. Using only TBAB (0.1 M) without the
metal nitrate in acidic basal electrolyte did not show positive results as the
potential did not reach to ¨0.95 V vs Ag/AgCl. At the highest concentration
for
both TBAB and TBAC the experiments were carried out at ¨0.35 V vs Ag/AgCl.
A Bio-Logic VMP3 potentiostat/galvanostat and frequency response analyzer
was used in order to perform the electrochemical measurements. EC-Lab
v.10.40 software was used for data acquisition. One linear scan voltammetry
(LSV) from 0 to -0.950 V followed by chronoamperometry (CA) experiments
were carried out at ¨0.950 V vs Ag/AgC1 until pH 11 was reached.
The concentration of La was quantitatively analyzed in samples from the
catholyte and anolyte solutions, respectively, by means of ICP-MS. First, the
sample solutions were pipetted from the supernatans. Each sample was
diluted by a factor x and acidified prior to analysis. For the 10x dilution
case, 1
ml of sample solution was mixed with 1 ml HNO3 and the result diluted to 10
ml with Milli-Q water. For the 100x dilution case, a dilution to a final
volume
of 100 ml was used. Calibration of the observed ion intensities was performed
against a series of calibration standards prepared on the basis of a 1000 mg/1
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Merck, Certipur, Cat No 1.70311.0100, Ce(NO3)3 solution in 2-3% HNO3.
Calibration standards of 0, 1, 2, 5, 10, 20 ppb, 100, 200, 500, 1000 and 2000
ppb were prepared. All measurements were performed with a Varian 820 ICP-
MS. On a daily basis, a mass calibration was performed; ion lenses and
gasflows were optimized to obtain a maximal signal-to-noise ratio with a 10
ppb multi-element standard provided by Varian.
The catholyte conductivities measured before the individual different
experiments carried out are presented in Table 1. By the end of the all
experiments the conductivity decreased sifnificantly, which was linked to a
decrease in both the amount of IL present in the electrolyte as well as on the
La concentration in the electrolyte (Table G2). The IL present in the
electrolyte was removed from the electrolyte 60-75% in all cases and partly
found in the precipitates. However, after the precipitates were washed with
distilled water, most of the IL there present was was washed out.
Table G. Initial and final conductivities of the catholytes employed for the
different experiments.
Experimental Initial conductivity Final conductivity
condition (mS/cm) (mS/cm)
No IL added 0.853 No change
TBAB 10 1.885 0.095
TBAB 20 2.72 0.190
TBAB 30 3.57 0.280
TBAB 86.5 7.57 0.300
TBAC 10 1.873 0.116
TBAC 20 2.78 0.160
TBAC 30 3.38 0.375
TBAC 86.5 7.56 0.378
Besides the IL, mg/L quantities of formic and acetic acid were found, without
having a particular correlation with the amount of IL added. The inventors
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believe that the formic and acetic acid quantified are issue from the
reduction
of CO2 or from the partial degradation of the IL. The inventors also believe
that these products may serve as linkers between the IL or its degradation
products and the nanocrystal products formed within the process, in a similar
way to what takes place in the more classical electrochemical formation of
MOFs. The product formed at -0.350 V has the appereance as classical
precipitates. The formation of the product at -0.350 V was compared to that
formed at at -0.950 V. As shown by the IR spectra in the Figures 13a, b and c,
the characteristic response and thus the characteristic bonds of the product
are identical and only differ in intensity in correlation with the IL
concentration employed and not by the type of IL. It is also shown that only
part of the IL characteristic bonds remain in the precipitate, suggesting that
at least part of the IL remains forming part of the structure of the
precipitate
product. The IR of the precipitate samples were compared before (P2) and after
(P3) washing, showing the removal of an excess of IL but still after washing
the structural effect of part of the IL remains. Thus, the product formed is
believed to be a metal-organic compound with characteristics of both the IL
and the nanocrystals.
From the above IR spectra of figure 13a, b and c respectively the
inventors could interpret that peak a is related to the bonding of lanthanum
and nitrogen in case of original La-nitrate. This peak becomes broader when
lanthanum is bonded to nitrogen of TBA. Peak b relates to N-CH2 bonding.
This peak however appears in the first Figure (unwashed part) but disappears
in the second Figure indicating that some TBAB molecules that lied as
surfactants on these compounds. The peak c (many peaks) are seen in case of
original TBAB (lower blue curve in figure 13b). These refer to the C-N bonds
in
aliphatic amines. In the Figure corresponding to the washed samples (P3) it
can be observed that some of these peaks are lost however not all indicating
that there are some changes in the bonding between the nitrogen and
methylene groups.
It should be noted that under the polarization condition applied at -
0.950 V, hydrogen formation is also possible. Under this condition, the
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precipitates still looked initially as discrete precipitates, however, it was
noticeable that they allow the sequestration of the hydrogen formed in situ
and ultimately form a foam-like product that floats over a column of the
catholyte due to the hydrogen it carries (see Figure 14). It is believed by
the
5 inventors that this product behaves similarly to a Pickering emulsion
where
the nanocrystals formed stabilize the remaining IL by self-assembly in
solution, forming the characteristic cage-like structures where the gas can be
sequestrated. The product also showed the characteristic of Pickering
emulsions of stability until the medium is perturbed by simple agitation.
10 Under such condition the hydrogen bubbles are released back into the
air. The
amount of hydrogen sequestrated in the nanocrystaline product formed was
estimated to be above 10% in volume with respect to the solid precipitate.
Figure 14 shows nanocrystals precipitated in the presence of excess IL
and under in-situ hydrogen electrosynthesis. Small bubbles in the size range
of
15 0.1 mm were visible, occupying more than 10% of the volume of the
material
floating.
The XRD patterns of the precipitate product washed were registered. Although
the XRD pattern did not correspond to any correspondence in the databases
available, the product showed characteristic features of high crystallinity
and
20 of a nanomaterial with small crystallite size (e.g. <10-5 nm). A
characteristic
XRD spectrum is shown for TBAB 30 in figure 15 below.
The characterization by scanning electron microscopy (SEM) showed N,
C, La and 0 and in the precipitate elemental analysis, with a strong
correlation in position, being a second indicator of the structural
incorporation
25 of at least part of the IL in the nanocrystals. Br and Cl from the IL
were also
found but lost when the precipitate was washed.
The elemental analysis of the products formed showed an approximate
proportion of 1:1 of hydrogen and carbon.
Based on the electrochemical response obtained, the charge consumed
30 for each experiment was quantified, the results are shown in figure 12.
An
exponential correlation between the charge consumed and the IL concentration
CA 02973289 2017-07-07
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61
employed was found, indicating that the IL indeed takes part on the
electrochemical process.
The TGA characterization (see figure 12) of the precipitates obtained
indicate that the nanocrystaline product contains not the full ionic liquid
but
only fractions of it, which may be incorporated after partial degradation of
the
IL during the process. It is believed that the polyatomic ion radicals issued
from the reduction of the oxidizing gas contribute to this IL degradation
process and ultimately on the linkage of the IL fractions to the final
precipitate
structure. Although the structure of the nanocrystaline product cannot be
fully
established at this moment, the authors support the rationale that at least
part of IL is found in the final nanocrystals, these being a metal-organic
product.
More than 83% of the initial La precursor could be removed when CO2
was used solely as the oxidizing gas, whereas more than 98% was removed
then the oxidant gas comprised air. The use of pure CO2 led to the formation
of
lanthanum carbonates, whereas the use of air led to the formation of the above
described metal-organic nanocrystaline product.
