WO2019211318A1

WO2019211318A1 - Method for dissolving precious metals

Info

Publication number: WO2019211318A1
Application number: PCT/EP2019/061137
Authority: WO
Inventors: Shuang Ma ANDERSEN; Raghunandan Sharma
Original assignee: Syddansk Universitet
Priority date: 2018-05-02
Filing date: 2019-05-01
Publication date: 2019-11-07
Also published as: EP3788177A1

Abstract

The present invention relates to a method for dissolving, recovering, extracting or separating precious metals. Efficient recovery of precious metals from industrial spent catalysts through electrochemical means using dilute acid/acid-free electrolytes is of significant importance from both the industrial and the environmental points of view. Owing to their large surface-to-volume ratio, fast dissolution through electrochemical potential cycling is possible. However, redeposition of the dissolved metal species on larger particles reduces the process efficiency significantly. In one aspect of the present invention, faster dissolution can be attained by inhibition of particle growth using surface switching species (SSS) functioning to selectively block and expose the metal surface to facilitate dissolution and inhibit redeposition, respectively. In another aspect of the present invention, the redeposition may be minimized by removing the dissolved Pt species redepositiong them (in metal form) electrochemically on a second working electrode.

Description

METHOD FOR DISSOLVING PRECIOUS METALS

FIELD OF THE INVENTION

The present invention relates to a method for dissolving metals, such as precious metals (PMs).

BACKGROUND OF THE INVENTION

PMs such as the platinum group metals (PGMs) are in general used as catalysts for fossil fuel powered vehicle or system exhaust gas purification, organic chemical reactions, as well as fuel cell and electrolyzer electrodes. PMs are also constituents of electronic components and microcircuits. Therefore, the recovery of PMs from used materials is of crucial importance, in particular, due to their rarity and costs.

The recovery of PMs from spent catalyst and other materials is currently mainly based on physical separation using melting temperature and density difference, and/or chemical dissolution processes in an acidic bath.

For example, dissolution processes generally occurs in strong acid or in strong oxidizing acid mixture such as aqua regia and / or toxic complexing agent such as cyanide.

Following the dissolution, the metals are then separated from the solution by addition of a reducing agent or cation resulting a salt of low solubility.

In general, the dissolution process requires a hazardous, highly oxidizing, corrosive and toxic chemicals that is difficult to handle, and a large amount of heat to maintain the acid temperature. Additionally, aqua regia, long been used for PMs dissolution, gives off NOx producing environmental pollution issues.

Cyanides, such as sodium cyanide, are effective for PMs dissolution, but need careful handling and adequate liquid waste treatment due to their high toxicity.

Therefore, the classic smelting and chemical dissolution is neither environmental friendly nor inexpensive.

Electrochemical dissolution may solve some of the disadvantage of the chemical dissolution. Electrochemical dissolution of PMs, such as through potential cycling, leads to the formation of oxides on the outer surface, followed by dissolution, is an effective method to dissolve PMs. However, redeposition of dissolved PM soluble species during potential cycling and hence growth of larger particles, due to Ostwald ripening on the surface, may reduce the efficiency of the

electrochemical dissolution process.

Furthermore, saturation with dissolved PM ions of the solution obtained via electrochemical dissolution causes the shift of the equilibrium of the reaction of dissolution, i.e. shift the reaction equilibrium towards the redeposition of the PM, thus reducing the efficiency of dissolution.

Therefore, there is a need for a more efficient method of dissolving PMs, which limits to the minimum the redeposition of the PM soluble species onto the catalytic structures or other structures from which the PM soluble species are originated.

Hence, an improved method for dissolving PMs would be advantageous, and in particular, a more efficient and/or reliable method or dissolving PMs would be advantageous.

OBJECT OF THE INVENTION

It is an object of the present invention to wholly or partly overcome the above disadvantages and drawbacks of the prior art. More specifically, it is an object to improve the efficiency of electrochemical dissolution of PMs.

In particular, it may be seen as a further object of the present invention to provide a method for dissolving PMs that solves the above-mentioned problems of the prior art and improves the efficiency of electrochemical dissolution.

An object of the present invention may also be seen as to provide an alternative to the prior art.

SUMMARY OF THE INVENTION

Thus, the above described object and several other objects are intended to be obtained in one aspect of the invention by providing a method of dissolving, recovering extracting or separating metals, by subjecting a source of metals, such as metals from catalytic structures, to potential cycling in an electrochemical or electrolytic cell.

In a further aspect, the invention relates to a method of producing a solution comprising metal ions, the method comprising, subjecting a source of metals, such as metals from catalytic structures, to potential cycling in an electrochemical cell, thereby producing a solution containing a desired concentration of metal ions.

The method could be applied to the dissolution of several metals, such as transition metal elements, e.g. PMs or PGMs.

PMs are precious metallic elements comprising Platinum (Pt), Palladium (Pd), Ruthenium (Ru), Iridium (Ir), Rhodium (Rh), Osmium (Os), Gold (Au) and Silver (Ag).

In a first aspect, the invention relates to a method of dissolving PMs by subjecting a source of PMs, such as PMs from catalytic structures, to potential cycling in an electrochemical cell.

In some embodiments, the electrochemical cell comprises a first working electrode (WEI), and a counter electrode (CE) and an electrolyte.

In some further embodiments, the source of PMs may be the WEI. Upon operation, the WEI will thus dissolve leaving the desired metal ions within the electrolyte solution.

The electrochemical cell used may have a simple two electrodes configuration comprising the WEI as anode and the CE as cathode.

In some preferred embodiments, the electrochemical cell further comprise a reference electrode (RE), so as to ensure correct application of the desired potential values.

The RE employed may be the reversible hydrogen electrode (RHE). Alternatively, other reference electrodes may be used, such as saturated calomel electrode (SCE) or Silver chloride electrode (Ag/AgCI).

In some further embodiments, the PMs are or comprise nanoparticles.

The method and the electrochemical cell of the invention have, in particular, the advantage of improving dissolution of PMs that are in nano size, e.g. are in the form of nanoparticles, having a diameter of few nanometers, such as of diameter 2-3 nm.

However, the method and the electrochemical cell of the invention may be also applied to WEI comprising PMs in a bulk form.

In some embodiments, the step of subjecting the source of PMs to potential cycling comprises sweeping a potential between two predefined voltage values applied between the WEI and the CE.

During potential cycling the potential applied between the WEI and the CE is swept in time, i.e. changed, such as ramped vs time, cyclically, i.e. after reaching the second predefined voltage value, the potential of the WEI is ramped in the opposite direction so to return to the initial first predefined voltage value.

In some other embodiments, a first value of the two predefined voltage values is a PM deposition potential at which the PM soluble species deposit as PM atom, such as in a range between 0.2 V and 0.6 V, such as 0.4 V. A second value of the two predefined voltage values is a PM dissolution potential at which the PM dissolves as PM soluble species, such as in a range between 1.0 V and 1.8 V, such as 1.6 V.

All potentials and voltage values referred herein are to be considered as vs. RHE, i.e. measured against RHE.

Accordingly, preferred potential cycling may occurs between 0.4 V and 1.6 V vs RHE. In some further embodiments, the electrolyte comprises a dilute acid, such as 1 M or 0.1 M, or lower concentration of HCI or HNO3 or H2SO4 or HCIO4 or a mixture thereof. The use of strong and corrosive acid is not suitable for the invention.

The electrolyte may be a suitable liquid electrolyte in which the electrochemical cell can be operated in between the desired potential range and contains ions, molecules or compounds having the ability of forming complexes with the dissolved PM ions.

The electrolyte may comprise inorganic ions, such as cyanide, halogen, ammonium or organic molecules or mixture thereof.

The organic molecules may be molecules or compounds comprising organic groups.

The inorganic ions used may be generated preferably from salts, for example, ammonium halide salts, such as NH4CI,.

In general, the presence of low acidic complexing agents working as electrolyte, such as ammonium-halide groups, has shown to improve the industrial friendliness by replacing acid.

Indeed, due to the presence of complexing agent forming complexes with the PM ions in solution, the PMs is stabilized in solution and thus kept dissolved, improving the dissolution efficiency.

Indeed, in one aspect, the invention relates to a method of dissolving a source of PMs, the method comprising : providing an electrolytic cell comprising an anode, a cathode and an electrolyte comprising ammonium ions and a complexing agent, wherein the anode is the source of PMs; imposing an opportune potential cycling between said anode and said cathode to dissolve the source of PMs.

In this aspect, the invention addresses the issue of the recovery of PMs through the use of strong acids as being highly corrosive and thus not environmental friendly. The invention thus offers as a solution the use of electrochemical dissolution through potential cycling in diluted acid bath or ammonium salt solution that forms stable ammonium chloride complexes during the potential cycling.

Thus, the invention provides a process of dissolution PMs from, e.g. spent catalysts, through electrochemical dissolution at pH>4.5.

In general, the presence of a complexing agent in the electrolyte improve the dissolution of the PM by forming a stable PM/complex solution.

For example, in dissolving Pt, the advantage of ammonium chloride over any other halide salt is that both Pt-chloride (PtCU/PtCl₆) and Ammonium chloro platinum complexes ((NH₄)x.PtCl_y) can be formed in presence of Ammonium halide salts.

In some other embodiments, the electrolyte comprises one or several surface switch species (SSS).

A SSS is defined as a species that can be transformed between a soluble and aprecipitaion, such as insoluble solid form, in the correspondent suitable electrolyte by controlling the potential applied.

The SSS is a species having the function of blocking the surface of the PMs when the potential applied may cause redeposition of the PMs. At the same time, the SSS is a species having the function of rendering the surface of the PMs available for dissolution, when the opportune dissolution potential of the PMs is applied.

In that, the SSS is able to switch the nature and composition of the PM surface being exposed to the electrolyte depending on the potential applied.

The SSS is a species having the property of being dissolvable before the PM in the electrolyte solution while the potential applied is the one correspondent to the dissolution potential of the PM. The SSS is also a species having the property of being able to deposit, i.e. precipitate, before the PM while the potential applied is the deposition potential of the PM soluble species, such as ion or complexes, thus blocking the surface for redeposition of the PM soluble species. Thus, in some embodiments, the one or several SSS have a dissolution potential lower than the PM dissolution potential and a deposition potential higher than the PM deposition potential.

The dissolution potential of the one or several SSS is the potential at which the SSS transforms into a soluble form.

The deposition potential of the one or several SSS is the potential at which the SSS transform into a precipitate, i.e. insoluble solid form.

The specific properties of the SSS ensure that the one or several SSS are the first species to dissolve during potential increase, i.e. during anodic sweep, and the first species to deposit during potential decrease i.e. during cathodic sweep.

Accordingly, when the SSS is precipitated on the surface of the source of PM, it will dissolve first when the potential is raised, i.e. during the anodic sweep.

The SSS are compatible with the PMs in a way that the SSS form, when the SSS deposition potential is applied, a precipitate, i.e. a layer, such as a thin atomic layer, precipitated onto the top surface of the PMs.

Once precipitated onto the PMs surface, the SSS, in its insoluble form, is not compatible with the PM soluble species, or the potential of the PM soluble species for depositing onto the SSS in its insoluble form is lower than the deposition potential of the PM soluble species onto PM itself, therefore the PM soluble species cannot deposit onto the precipitated SSS at the deposition potential of the SSS.

Though SSS is compatible on top of PM, the PM is incompatible on top of SSS, most probably due to atomic lattice mismatch or increased surface energy requires over potential.

In general, deposition of PM onto SSS is not possible, i.e. the two layer are not compatible, as within the potential range used, PM will not deposit onto the layer of precipitated SSS. This property of the SSS in respect to the PM excludes the possibility of the formation of a layer structure, i.e. avoid the possibility that a new layer of PM would deposit onto the precipitated SSS structure during cathodic sweep within the applied predefined potential limits.

Conversion between the two forms of the SSS, i.e. in solution and precipitated form, may be caused by different means.

In some embodiments, conversion between the two forms, in solution or precipitated in its solid form, occurs by applying a suitable potential value.

This has the advantage that the potential variation necessary for PM dissolution can be used for SSS precipitation and dissolution.

In some other embodiments, precipitation of SSS can be controlled by light irradiation, such as UV/Vis/IR light irradiation or by temperature changes, acoustic waves or variation of the magnetic field.

The SSS may be an additive, such as an inorganic ion or an organic molecule, i.e. molecules comprising organic groups.

In some embodiments, the one or several SSS are or comprise metal ions, such as transition metal ions, such as group 10, 11 or 12 metal ions.

In some embodiments, the transition metal ions are Cu or Ag ions.

For example, Cu salts, such as Cu(NC>3)2, CuCb, CuCI or CuS0₄, i.e. providing Cu⁺ and Cu²⁺ ions, may be dissolved into the electrolyte solution so as to obtain Cu ions in solution.

The invention, in this aspect, thus relates to a process of improving dissolution of PMs, such as Pt or Au, based on the electrochemical dissolution of the PMs by potential cycling in electrolyte using acid or ammonium salt and in presence of other SSS, such as metal ions, inhibiting the redeposition of the PM during cathodic sweep. The SSS in solutions form a thin layer, such as a monolayer, onto the PMs, during the cathodic sweep and thus inhibiting the redeposition of the PMs. Complexing agents may also be present so as to further increase the dissolution rate by stabilizing the PM soluble species in solution.

The invention, in this aspect, has the advantage of proving a faster dissolution, such as three time faster, and can be used efficiently at low acid concentrations. The SSS may be present in very low concentration, such as in trace, i.e. in amount of few ppm.

The SSS may be easily recovered from the electrolyte solution and may be reused.

In a second aspect, the invention relates to a method of dissolving PMs, such as PMs from catalytic structures, comprising : dissolving the PMs by subjecting a source of PMs to potential cycling in an electrochemical cell in an electrolyte, the dissolving according to the first aspect of the invention; depositing the dissolved PMs.

In some embodiments, the electrochemical cell further comprises a second working (WE2) and the depositing occurs on the WE2 located within the electrochemical cell.

In some further embodiments, the method further comprises, alternating between the dissolving and the depositing by switching, such as changing, between applying a predefined potential between the WEI and the CE and applying a predefined potential between the WE2 and the CE.

Alternating between dissolving and depositing occur by swopping the predefined or appropriate potential when the potential reaches a value below the deposition potential of the PM soluble species.

In some further embodiments, the swopping occurs from dissolving, occurring at the WEI, to depositing, occurring at WE2, when a potential applied reaches a value below the PM deposition potential, i.e. the potential at which the PM soluble species deposit as PM, such as when the potential applied reaches a value below 0.4 V.

The WE2 may be a Pt electrode. In some other embodiments, the swopping occurs from depositing, occurring at the WE2, to dissolving, occurring at the WEI, when a potential applied reaches a value above said PM deposition potential, at which the PM soluble species deposit as PM, such as 0.4 V.

In the second aspect, the invention relates to the recovery of PMs, such as Pt, from different structures, through a two-step electrochemical process.

The first step comprises an electrochemical dissolution of the PM contained in a structure, such as a spent catalytic structure, used as a first working electrode in a three-electrode electrochemical cell, through potential cycling between given potential values.

The second step relates to an electrochemical deposition of the PMs on a second working electrode by reduction at a suitable potential value.

Dissolution and deposition occur within the same container and are obtained by alternating the use of the first and the second working electrode respectively.

By swopping between dissolution and deposition, it is thus possible to constantly remove PM soluble species from the solution thus improving the efficiency of the dissolution by shifting the equilibrium of the dissolution reaction towards the formation of PM soluble species.

The invention has particularly shown an increase in dissolution in electrolytic solution of ammonium chloride, nitric acid and ammonium nitrate.

In a third aspect, the invention relates to a method of recovering PMs, such as PMs from catalytic structures, comprising : dissolving the PMs according to the first aspect of the invention; depositing, such as depositing on a substrate, the PMs by chemical or electrochemical means.

The first, second and third and other aspects and embodiments of the present invention may each be combined with any of the other aspects and embodiments. These and other aspects and embodiments of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE FIGURES

The method for dissolving metals according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 is a schematic representation of the mechanism behind the use of SSS according to some embodiments of the invention.

Figure 2 is a bar chart comparing the dissolution of Pt during potential cycling in an electrolyte having Cu ion vs an electrolyte in which Cu ions are absent.

Figure 3 is a bar chart comparing the dissolution of Pt during potential cycling in an electrolyte having Cu ion vs an electrolyte in which Cu ions are absent with similar concentration of Cl ions.

Figure 4A and 4B are bar charts showing the influence in the dissolution of Pt using different sources of Cu ions.

Figure 5 is a graph comparing the dissolution of Pt during potential cycling in an electrolyte having Cu ion vs an electrolyte in which Cu ions are absent vs the number of cycles.

Figure 6 is a bar chart showing the influence of different type of SSS, such as different metal ions, on the dissolution of Pt.

Figure 7 is a bar chart comparing the dissolution of Au during potential cycling in an electrolyte having Cu ion vs an electrolyte in which Cu ions are absent.

Figure 8, 9 and 10 are schematic representations of different potential

configurations of the electrochemical cell according to some different aspects of the invention.

Figure 11 shows a schematic representation according to one embodiment of the invention.

Figure 12 is a bar chart comparing the dissolution of Pt during potential cycling in an electrochemical cell according to figure 11 in which a second working electrode is used and operates as described below.

Figure 13 is a bar chart comparing the effect of lower potential during

electrochemical potential cycling on the %dissolution with and without presence of Cu⁺² ions. Figure 14 is a flow-chart of a method according to one aspect of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

Figure 1 is a schematic representation of the mechanism behind the use of SSS in the method of dissolving PMs according to some aspect of the invention.

Redeposition of dissolved PM soluble species, such as Pt soluble species, on PM particles, due to Ostwald ripening, during subsequent cathodic sweeps reduces the dissolution efficiency of the PMs.

The invention in one of its aspects inhibits the particle growth and enhance dissolution efficiency by using a SSS, such as Cu ions, that precipitate onto the PMs surface.

The deposition of SSS monolayer during cathodic scans blocks the PM from redeposition of Pt soluble species.

Figure 2 is a bar chart showing the dissolution percentage of a Pt working electrode having platinum nanoparticles (2-3 nm) supported on high surface area carbon, when potential cycling between 0.4V and 1.6 V is applied in a three electrode chemical cell configuration having a carbon rod as CE and a RHE reference electrode in HCI 0.1M, room temperature (20°C) and pressure (1 atmosphere).

After 25 cycles, it can be clearly seen that due to the presence of the CuCh dissolved in the electrolyte the increase in dissolution is in the area of 40%.

Figure 3 shows that the increase of dissolution cannot be only due to an increased concentration of Cl ions as even at similar concentration of Cl ions, the presence of Cu ions shows an increase of dissolution in the area of 20%.

Figure 4A shows the effect of Cu on Pt dissolution efficiency (potential cycling between 0.4 and 1.6 V at a rate of 100 mV/s for 25 cycles) in presence of (CUNC>3)2 as a source of Cu²⁺. While keeping the H⁺ concentration constant, presence of NO3 increases the dissolution efficiency slightly. Again, effect of Cu⁺⁺ towards increasing the dissolution of PM was clear even at Cu²⁺ concentration of 0.01 M. Fig. 4B shows the effect of presence of Cu²⁺ on Pt dissolution rate in non- complexing electrolytes (1 M H2SO4). Potential cycling between 0.4 and 1.6 V at a rate of 100 mV/s for 10k cycles in 1 M H2SO4 shows a dissolution of ~30% while in presence of 0.1 M CuSC , same electrochemical treatment attains a dissolution of ~75%. Hence, the effect of cu is not limited to only the electrolytes capable of complexing the Pt soluble species.

Figure 5 shows the effect of Cu on Pt dissolution efficiency in terms of the number of potential cycles (between 0.4 and 1.6 V at a rate of 100 mV/s) required to achieve >95% dissolution. In presence of Cu²⁺ (20), required number of potential cycles is reduced significantly (from 350 to 150), as compared to that in absence of Cu (10).

Figure 6 shows the effect of different metal ions on the dissolution efficiency.

While presence of Au and Fe shows down dissolution, Ni and Zn improve it slightly. However, the mechanism affecting the dissolution process in presence of metals not appropriate for the aforesaid surface switching mechanism, may be active participation in complex formation, alloying with Pt, etc.

Figure 7 shows the effect of Cu on dissolution of gold (bulk gold in form of 0.1 mm wire) through potential cycling (between 0.4 and 1.6 V at a rate of 100 mV/s in 1 M HCI). Similar to platinum, gold also shows enhanced dissolution in presence of Cu²⁺ (0.1 M CuCI₂).

Figure 8 is schematic representation of an electrochemical cell 3 comprising a cathode or counter electrode 6 and an anode or working electrode 11 according to some embodiments of the invention.

Figure 9 is a schematic representation of an electrochemical cell 4 comprising a cathode or counter electrode 7 and an anode or working electrode 12 and a reference electrode 9 according to some embodiments of the invention.

Figure 10 is a schematic representation of an electrochemical cell 5 comprising a cathode or counter electrode 8, a reference electrode 16 and two anodes or working electrodes 13 and 14 according to some aspects of the invention. The electrochemical cell 5 comprises a controlling means, such as a switch 15, that can change the application of potential to working electrode 13 and working electrode 14.

Figure 11 shows a schematic of a typical experimental setup used in one aspect of the present invention. During potential cycling, the working electrode is switched between WEI (for potentials higher than, for example 0.4 V) and WE2 (for potentials lower than, for example 0.4 V).

Figure 12 shows the effect of presence of WE2 for reduction and redeposition of dissolved PM soluble species on the percentage of dissolution. The lower potential limit for redeposition was set to 0.1 V while the dissolution was performed by potential cycling between 0.4 and 1.6 V at a scan rate of 100 mv/s for 100 cycles in 0.5 M NH4CI electrolyte kept at normal temperature and pressure.

Figure 13 shows the effect different lower potential limits (0.2, 0.4 and 0.6 V; with a fixed upper potential limit of 1.6 V) on dissolution of platinum through potential cycling. In 0.1 M HCI (1) electrolyte, the percentage of dissolution decreases when decreasing the lower potential from 0.4 to 0.2 V due to redeposition of dissolved Pt species on the platinum nanoparticles of the WE. In 0.1 M HCI+0.01M CuCb (2) electrolyte, the % dissolution remains almost constant when decreasing the lower potential from 0.4 to 0.2 V due to inhibition of redeposition of dissolved Pt species through formation of Cu layer on the platinum nanoparticles of the WE.

Figure 14 is a flow-chart of a method according to one aspect of the invention.

The method 19 of dissolving PMs, such as PMs from catalytic structures, comprises the steps of: (17) dissolving the PMs by subjecting a source of PMs to potential cycling in an electrochemical cell in an electrolyte according to figures 10 or 11; (18) depositing the dissolved PMs onto a substrate, such as the second WE.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. A method of dissolving precious metals (PMs) by subjecting a source of PMs, such as PMs from catalytic structures, to potential cycling in an electrochemical cell, wherein said electrochemical cell comprises a first working electrode (WEI), a counter electrode (CE), a reference electrode (RE) and an electrolyte, and wherein said source of PMs is said WEI, wherein

- a first value of said two predefined voltage values is a PM deposition potential at which the PM soluble species deposit as PM atom, and a second value of said two predefined voltage values is a PM dissolution potential at which the PMs dissolve as PM soluble species;

- said electrolyte comprises a dilute acid or salt solution, such as 1 M or 0.1 M, or lower concentration of HCI or HNO3 or H2SO4 or HCIO4 or NH3CI or a mixture thereof;

wherein said electrolyte comprises one or several surface switch species (SSS) having a dissolution potential lower than the PM dissolution potential and a deposition potential higher than said PM deposition potential.

2. A method according to claim 1, wherein said precious metals are or comprise nanoparticles.

3. A method according to any of the preceding claims, wherein said subjecting said source of PMs to potential cycling comprises sweeping a potential between two predefined voltage values applied between said WEI and said CE.

4. A method according to any of the preceding claims, wherein said one or several SSS are or comprise metal ions, such as transition metal ions, such as group 10, 11 or 12 metal ions.

5. A method according to claim 4, wherein said transition metal ions are Cu ions.

6. A method of dissolving PMs, such as PMs from catalytic structures, comprising : dissolving said PMs by subjecting a source of PMs to potential cycling in an electrochemical cell in an electrolyte according to any of the preceding claims 1-3; depositing said dissolved PM soluble species.

7. A method of dissolving PMs according to claim 6, wherein said electrochemical cell further comprises a second working (WE2) and wherein said depositing occurs on said WE2 located within said electrochemical cell.

8. A method according to any of the preceding claims 6-7, said method further comprising, alternating between said dissolving and said depositing by switching between applying a predefined potential between said WEI and said CE and applying a predefined potential between said WE2 and said CE.

9. A method according to claim 8, wherein said switching occurs from dissolving to depositing, when a potential applied reaches a value below said PM deposition potential, such as when said potential applied reaches a value below 0.4 V.

10. A method according to any of the preceding claims 8-9, wherein said switching occurs from depositing to dissolving, when a potential applied reaches a value above said PM deposition potential, such as 0.4 V.

11. A method of recovering PMs, such as PMs from catalytic structures,

comprising :

dissolving said PMs according to any of the preceding claims 1-3;

depositing said PMs by chemical or electrochemical means.