Method for precipitating platinum group metal particles

Abstract

The present invention relates to a method for recovering platinum group metals from a feed containing one or more precursor compounds of one or more platinum group metal ions, the method comprising the steps of: (i) supplying a feed containing one or more precursor compounds to a cathode compartment of an electrochemical cell having a cathode including a gas diffusion electrode having a porous electrochemically active material to form a liquid phase in the cathode compartment; (ii) supplying a CO2-containing gas to the cathode compartment; (iii) applying a potential to the cathode to cause electrochemical reduction of CO2 to CO; and (iv) recovering precipitated particles of the one or more platinum group metals in elemental form from the liquid phase.

Description

Technical Field

[0001] The present invention relates to a method for recovering a platinum group metal from a feed containing one or more precursor compounds of one or more platinum group metal ions according to the preamble of claim 1.

Background Art

[0002] Since metals often have quite different properties at the nanometer scale, metal-based nanoparticles have attracted much attention in recent years. Nanoparticles of platinum group metals such as Pt, Pd, Rh, etc. have gathered special interest due to their unique catalytic activity. Platinum group metals are expensive and have limited geological sources, but they are the best catalysts for proton exchange membrane fuel cells (PEMFC) or direct alcohol fuel cells (DAFC). Currently, the strategies for synthesizing platinum group metal nanoparticles aim to optimize the properties of the nanoparticles and reduce the required amount of the metal. In addition to this, there is a strong demand for strategies that can preferably achieve the secondary recovery of platinum group metals in the form of nanoparticles.

[0003] As a method often used for the synthesis of platinum group metal nanoparticles, for example, chemical reduction using hydrogen gas as a reducing agent for the shape-controlled synthesis of colloidal Pt nanoparticles can be mentioned. When H2 is used for the synthesis of carbon-supported Pt metal nanoparticles (Zhang et al. 2013 12 ), it has been found that in the absence of CO, large polydisperse particles are generated by the use of pure H2, and in the presence of CO, monodisperse Pt nanocubes are formed. In most of the disclosed methods, H2 and / or CO are introduced into the reaction of the system at different temperatures either by bubbling into a solution containing a platinum group metal compound or by contacting a platinum group metal feed with a gas flow in a furnace designed according to the purpose. The use of electrochemically generated H2 from a water reduction reaction (WRR) for synthesizing Pt nanoparticles with a specified crystal orientation has been disclosed in several reports 13 .

[0004] To date, no method has been disclosed that enables the effective recovery of platinum group metals in elemental form from a diluted liquid supply logistics of a precursor compound in which the corresponding platinum group metal ions are dissolved, dispersed, or suspended.

[0005] Known methods for synthesizing or recovering Pt group metal nanoparticles are disadvantageous in that they are neither selective nor sustainable. Some methods require the addition of hazardous chemical reagents that need to be removed after metal recovery, the use of support materials, or the supply of H2 using vigorous bubbling. Some known methods are energy-intensive because they must be carried out at relatively high temperatures, such as 200°C. Some methods are unsuitable for Pt recovery because they use expensive electrode materials such as Pt electrodes.

[0006] Patent Document 1 discloses the electrochemical synthesis of nanoparticles of metal oxides or metal hydroxides, such as iron oxide, copper hydroxychloride and zinc hydroxychloride (spin transition materials), scorodite and mixed metal (hydride) oxide libraries (i.e., barnesite, layered double hydroxide and spinel), by a process called gas-diffusion electrocrystallization (GDEx). According to the method of Patent Document 1, oxygen is supplied to a solution in which compounds of these metals are dissolved, and the solution is reduced on a gas-diffusion cathode, thereby generating hydroxyl ions (OH) - ) and strong oxidizing agents (H2O2 and HO2 - These generate ) which act as reaction intermediates for the chemical oxidation precipitation of stable nanoparticles of the metal compound. However, Patent Document 1 does not disclose the precipitation of platinum group metals in elemental form from a feed containing precursor compounds of the corresponding metal ions. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] European Patent No. 3242963 [Overview of the project] [Problems that the invention aims to solve]

[0008] Therefore, there is a need for a method that enables the isolation of platinum group metals in elemental form from complex supply chains containing one or more precursor compounds of one or more platinum group metal ions.

[0009] Therefore, the present invention aims to provide a method for recovering platinum group metal particles in elemental form from a feed containing one or more precursor compounds of one or more platinum group metals. [Means for solving the problem]

[0010] This is achieved according to the present invention by a method for demonstrating the technical features of the feature portion of claim 1.

[0011] According to the method of the present invention, a method for recovering platinum group metals from a feed containing one or more precursor compounds of one or more platinum group metal ions is, (i) A step of supplying a feed containing one or more precursor compounds to the cathode compartment of an electrochemical cell having a cathode including a gas diffusion electrode having a porous electrochemical active material, thereby forming a liquid phase in the cathode compartment, (ii) A process of supplying CO2-containing gas to the cathode compartment, (iii) A step of applying a potential to the cathode that causes the electrochemical reduction of CO2 to CO, (iv) A step of recovering one or more precipitated particles of platinum group metals from the liquid phase in elemental form, Includes.

[0012] The feedstock can originate from a wide range of sources, many of which are industrial in origin. In particular, the feedstock may originate from processes for recovering platinum group metals or precious metals from recycled used products, such as circuit boards, electronic equipment, catalysts, fuel cells, glass, ceramics, pigments, medical and dental supplies, pharmaceuticals, photocells, and superalloys. A problem with such industrial waste feedstock is that the concentration of platinum group metal ions can be quite low, sometimes only a few hundred grams per ton of waste. The feedstock is often aqueous, but may contain one or more organic solvents.

[0013] A feed containing one or more precursor compounds of one or more platinum metal group ions may be supplied to the cathode compartment as either a solid or liquid feed containing one or more precursor compounds. The liquid feed may be a solution in which one or more precursor compounds of one or more platinum metal ions are dissolved, or a dispersion or suspension of one or more precursor compounds in a liquid dispersant.

[0014] It will be obvious to those skilled in the art that the liquid phase in the cathode compartment contains the feed.

[0015] Surprisingly, the method of the present invention has been found to enable the precipitation of particles or clusters of particles of the corresponding platinum group metal in elemental form from a feed containing one or more precursor compounds of one or more platinum group metal ions. These particles are suitable for recovery for further use. The precipitation of platinum group metal particles in elemental form has been found to occur mainly in the cathode solution, allowing for simplified recovery of the metal particles from the liquid phase.

[0016] The precipitated particles can take the form of nanoparticles having a particle size of up to 100 nm. The precipitated particles can also take the form of aggregates or clusters of metal particles, which have a particle size larger than 100 nm. Advantageously, the particle size can be controlled by manipulating one or more method parameters using the method of the present invention. For example, to control the particle size of the precipitated particles, the concentration of one or more precursor compounds, the flow rate when supplying the electrolyte to the cathode compartment, the gas flow rate, the conductivity of the electrolyte, etc., can be changed.

[0017] Depending on the composition of the feed, the particles may contain one type of platinum group metal or a mixture of two or more different platinum group metals.

[0018] In this way, platinum group metal particles can be recovered in a form particularly suitable for further processing. For example, the precipitated particles of elemental platinum group metals may be redissolved in a solvent suitable for a specific application, dispersed in a dispersant to obtain a suspension or slurry, or formed into dry pellets.

[0019] Advantageously, the method of the present invention is an electrochemical process, and the charge consumed is mainly related to the electrochemical reduction of CO2 to CO. When water is present in the liquid phase, the reduction of water to H2 contributes to charge consumption. CO2 can be supplied to the gas side of a gas diffusion electrode, where the electrochemical reduction of CO2 to CO occurs upon polarization of the gas diffusion electrode; therefore, the use of a gas diffusion electrode solves the problem of low solubility of CO2, particularly in aqueous feed systems. This reduction reaction occurs mainly on the working surface of the gas diffusion electrode. Under these conditions, electrochemical reduction of platinum group metal ions to elemental form may occur, but this is a feature that can be minimized by the present invention. This is an advantage because deposition at the electrode limits the range over which particle size can be controlled, complicates the recovery of metal particles, and often requires the disposal of the electrode.

[0020] While the inventors do not wish to be bound by this theory, under the conditions of the present invention, we surmise that CO is the main reaction product of CO2 reduction at the gas diffusion electrode. The CO formed by the reduction of CO2 can control the size of the resulting metal particles by interacting with the platinum group metal clusters formed thereby. CO can act, for example, as a capping agent capable of controlling the size of metal particles. This is surprising, given that, according to the teachings of Patent Document 1, CO is expected to act rather as a chemical oxidizing agent.

[0021] While the reduction of some platinum group metal ions can often occur at the surface of the gas diffusion electrode by electrochemical reduction, the reduction of platinum group metal ions primarily occurs within the cathode compartment, particularly in the cathode liquid. Although we do not wish to be bound by this assumption, we consider that the reduction of platinum group metals occurs mainly adjacent to the electrode, where the reaction products of CO2 reduction, mainly CO, extend to the electrolyte. It is assumed that the transport of CO to the electrolyte occurs at a faster rate than the transport of platinum group metal ions from the solution to the electrode. Reduction at the electrode surface is usually limited, and often nonexistent, due to the low concentration of platinum group metal ions in the liquid phase and the resulting restriction of material transport. This is advantageous because it facilitates the recovery of platinum group metal particles from the liquid phase and eliminates the need to separate the metal particles from the electrode.

[0022] The method of the present invention can be carried out as a continuous process or a batch process, and the liquid phase contained in the cathode compartment can be recirculated.

[0023] In a preferred embodiment, the liquid phase may contain one or more solvents selected from the group consisting of water, a protic solvent capable of generating H2, an organic solvent selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), acetone, an ionic liquid, a deep eutectic solvent, and mixtures of two or more thereof. Accordingly, the protic solvent is preferably selected from the group consisting of alcohols, ammonia, amines, amides, ionic liquids, acetic acid, and mixtures of two or more of the above compounds. In particular, the alcohol may be selected from the group consisting of methanol, ethanol, n-propanol and isopropyl alcohol, and mixtures of two or more thereof. Since industrial waste streams are usually aqueous, the feed preferably contains water.

[0024] When water derived from either the feed or the catholyte is present in the liquid phase in the cathode compartment, it is presumed that a water reduction reaction to generate H2 occurs simultaneously (Reaction 3) 22 Advantageously, the method of the present invention is an electrochemical process in which H2 is generated in the liquid phase. CO2 + 2H + + 2e - ⇔ CO + H2O E 0 = -0.106 V SHE (1) CO2 + 2H2O + 2e - ⇔ CO + 2OH - E 0 = -0.934 V SHE (2) 2H2O + 2e - ⇔ H2 + 2OH - E 0 = -0.828 V SHE (3)

[0025] By using a gas diffusion electrode, a sufficient amount of CO2 can be supplied and a degree of H2 formation sufficient to deposit platinum group metal particles from the feed in elemental form can be achieved. Although the inventors do not wish to be bound by this theory, it is presumed that H2 formed when the liquid phase contains a protic solvent chemically reduces platinum group metal ions supplied to the cathode compartment to elemental form.

[0026] Other reaction products produced by CO2 reduction include the following: CO2 + 2H + +2e - ⇔HCOOH E°=-0.250 V SHE CO2 + 2H2O + 2e - ⇔HCOO - +OH - E°=-1.078 V SHE CO2 + 6H + +6e - ⇔CH3OH+H2O E°=0.016 V SHE CO2 + 5H2O + 6e - ⇔CH3OH+6OH - E°=-0.812 V SHE CO2 + 8H + +8e - ⇔CH4+2H2O E°=0.169 V SHE CO2 + 6H2O + 8e - ⇔CH4+8OH - E°=-0.659 V SHE 2CO2 + 12H + +12e - ⇔CH2CH2+4H2O E°=0.064 V SHE 2CO2 + 8H2O + 12e - ⇔CH2CH2+12OH - E°=-0.764 V SHE

[0027] It is noteworthy that these reactions mainly occur in the presence of an electrolytic catalyst and can be ignored in the absence of the electrolytic catalyst.

[0028] In the presence of organic solvents, in addition to CO, other capping agents, such as citric acid and / or oxalic acid, may be formed, which may be useful in controlling the particle size of the precipitated elemental platinum metal particles.

[0029] In a preferred embodiment, CO2 is supplied to the gas diffusion electrode, particularly to the gas side of the gas diffusion electrode.

[0030] A liquid feed containing one or more platinum group metal ions may be a liquid feed containing water. In addition to water, the liquid feed may contain one or more of a variety of commonly used industrial solvents, examples of which are disclosed above. More preferably, a liquid feed containing one or more platinum group metal ions is a liquid feed of such metal compounds in water.

[0031] If water is present in the cathode solution, the reduction of CO to CO by CO2 is presumed to involve the following reaction, which additionally leads to the formation of H2: CO2 + 2H2O + 2e - ⇔CO+2OH - E 0 =-0.934 V SHE (2) 2H2O + 2e - ⇔H2+2OH - E 0 =-0.828 V SHE (3)

[0032] The main reduction product produced by CO2 reduction according to the present invention is CO, but the possibility of by-product formation cannot be ruled out, especially when an organic solvent is present in the liquid phase. Examples of such by-products of CO2 reduction include oxalates, formates, acetates, citrates, methanol, ethanol, methane, ethylene, ethylene oxide, and other compounds generally known to those skilled in the art, and it has been observed that their presence does not adversely affect the reduction of the desired metal ion or the precipitation of platinum group metals in metallic form. However, the presence of an electrocatalyst is required for the formation of at least some of these products to occur.

[0033] CO2 gas can be supplied to either the gas compartment of the gas diffusion electrode, the liquid phase, or both. It is preferable to supply CO2 gas to the gas compartment of the gas diffusion electrode when the formation of nanoparticles or smaller particles is anticipated. If the formation of larger particles, for example, those larger than 100 nm, is anticipated, CO2 gas can be supplied to the liquid phase. Supplying CO2 to the liquid phase offers the additional advantage of minimizing the risk of platinum group metal hydroxide formation and favoring the formation of metal particles.

[0034] A feed containing one or more platinum group metal ions may contain one type of platinum group metal ion or a mixture of two or more different platinum group metal ions. The method of the present invention makes it possible to precipitate platinum group metals in elemental form, regardless of the properties of the platinum group metals.

[0035] The method of the present invention is usually carried out at a temperature below the boiling point of the liquid phase. In a preferred embodiment in which the liquid phase contains water, the method of the present invention is preferably carried out at a temperature of up to 100°C, preferably 10°C to 100°C, more preferably 15°C to 75°C, most preferably 15°C to 50°C, and especially at room temperature. Conventional methods, on the other hand, require the use of high temperatures above 100°C, often up to 210°C, and therefore consume energy.

[0036] The method of the present invention can be advantageously carried out in a single step in a single reactor at atmospheric pressure and a suitable temperature.

[0037] The pH of the liquid phase at the start of the method of the present invention can vary over a wide pH range, but preferably the pH of the liquid phase at the start of the method is up to 5.0, preferably up to 4.0, and more preferably up to 3.0. At these pH values, the risk of platinum group metals precipitating as hydroxides can be minimized. The inventors have observed that the pH of the cathode solution can gradually progress toward an alkaline pH during the reaction. In one embodiment of the present invention, the pH of the liquid phase may be progressively changed as the precipitation of platinum group metals in metallic form progresses. In practice, the products produced by the electrochemical reduction of CO2 as described above act as buffers, counteracting an undesirable rise in the pH of the liquid phase. Typically, the pH rises to a certain maximum pH determined by the composition of the liquid phase. According to an alternative embodiment, a buffer may be added to the liquid phase to maintain the pH within the above limits. According to a preferred embodiment, the pH can be kept below 10, more preferably below 8.0, and most preferably below 7.0.

[0038] The pH of the liquid phase at the start of the method of the present invention may be at least 7.0, preferably at least 8.0, more preferably about 9.0, or even 11.0, for example, when the precipitation of element Pt is to be targeted. The pH may be gradually changed during the reaction, which often means a decrease to, for example, pH 8.0 or 9.0.

[0039] The pH of the liquid phase at the start of the method of the present invention is preferably adjusted considering the properties of the platinum group metal ions present in the liquid phase. Advantageously, if the liquid phase contains only Pt ions as platinum group metal ions, the pH may be gradually changed up to pH 11, because the formation of Pt hydroxide is expected to occur only when the pH exceeds this level.

[0040] Within the scope of the present invention, platinum group metals mean one or more metals selected from the group Pd, Pt, Rh, Ru, Os, and Ir, and include two or more mixtures of these metals. Individual metal ions may be present in the feed at similar concentrations, but the concentrations of individual platinum group metal ions may differ. Metal ions may be present in the feed in a single oxidation state, for example, Pd 2+ or Rh 3+ It can exist as such. However, metal ions may exist in two or more different oxidation states in the feed, for example, Pt 2+ and Pt 4+ It may also be included as a mixture with other materials. The supply may contain platinum group metals in elemental form, for example, Pt 0 , Pd 0 or Rh 0 It will become clear that it can also be contained in this form. Typically, metal particles in this elemental form can also act as crystallization seeds.

[0041] The method of the present invention is suitable for recovering platinum group metals from solutions of a wide variety of origins. The method of the present invention is suitable for reusing platinum group metals from spent applications, such as platinum group metal ions in feedstock derived from catalytic converters, printed circuit boards, electronic components, etc. The method of the present invention is also suitable for reusing platinum group metals from waste flows, such as slag, tailings, wastewater, leachates, etc.

[0042] When the method of the present invention is primarily used for the recovery of platinum group metals from industrial waste flows, the concentration of platinum group metal ions in these waste flows may be quite low (e.g., in the range of ppm to ppb). However, the method of the present invention is also suitable for the recovery of metals from higher concentration solutions.

[0043] CO2 can be supplied as a pure gas or in the form of a gas mixture with other gases, such as one or more inert gases including He, Ar, and N2. If necessary, the CO2-containing gas may include air, synthesis gas, or any other CO2-containing gas. When using air, synthesis gas, or any other CO2-containing gas, competitive reactions may occur, but these may be negligible or acceptable.

[0044] When using a gas mixture, those skilled in the art can adjust the mole fraction of CO2 to be sufficiently high to allow precipitation. Therefore, preferably, the mole fraction of CO2 in the gas mixture is at least 0.15, more preferably at least 0.020, but most preferably, it may be as high as 1 to ensure that a favorable oxidation-reduction potential of the electrolyte for precipitation can be achieved in the cathode solution. On the other hand, an excessively low partial pressure of CO2 may slow the reaction rate.

[0045] Advantageously, the CO2 gas flow rate is maintained within a range that allows for optimization of mass transfer, thereby minimizing metal deposition at the gas diffusion electrode and promoting H2 diffusion into the liquid phase. Those skilled in the art can adapt the CO2 gas flow rate considering the dimensions of the electrochemical reactor, the concentration of the precursor compound in the cathode liquid, etc.

[0046] It will be apparent to those skilled in the art that the cathode compartment may contain a cathode liquid, and that in the course of the method of the present invention, an electrolyte may be supplied to the cathode compartment at a suitable flow rate, for example, a flow rate that supports the achievement of a desired mass flow rate.

[0047] In a preferred embodiment, the electrochemical potential acting on the gas diffusion cathode is the reduction potential relative to the reference electrode, preferably below the thermodynamic pH potential equilibrium region of CO2 stability in water, more preferably below the region of water thermodynamic stability, and preferably not within the region of hydrogen thermodynamic stability in order to ensure water electrolysis that forms hydrogen. The electrochemical potentials at which CO2 reduction and water electrolysis occur are well known to those skilled in the art.

[0048] Advantageously, depending on whether the formation of larger particles is anticipated or whether metal deposition is supported more than H2 generation, the current is -10 mA / cm². 2 ~-50 mA / cm 2 A charge of -50 A / cm is applied to the electrode. 2At values ​​exceeding this range, overproduction of H2 may occur, leading to an increase in cell resistance / cell potential, which could cause the gas diffusion electrode to malfunction. [Brief explanation of the drawing]

[0049] [Figure 1-1] Figure 1.1 shows the reactions that occur in the cathode compartment when (a) O2 is used and (b) CO2 is used as the gas supply material to be electrochemically reduced. In (a), the product of the oxygen reduction reaction (ORR) (1) is an oxidizing chemical species that converts metal ions into nanoparticles of metal oxides (2) or hydroxides (3). In (b), H2 from the water reduction (WRR) (1) acts as a reducing agent, and CO from the CO2 reduction reaction (CRR) (2) acts as a capping agent for synthesizing elemental nanoparticles from metal ions in solution (3). The equilibrium (4) from unreacted CO2 to HCO3- and CO32- consumes the OH- generated in WRR and CRR and is very important to hinder the process. [Figure 1-2] Figure 1.2: This is a schematic diagram of an electrochemical reactor suitable for use in the present invention. [Figure 2] Figure 2.1: This figure shows the gradual change in pH versus time during reduction of CO2 at -1.4 V in the Ag / AgCl Rh strip solution in Example 2. *Sampling point. Figure 2.2: This figure shows the gradual change in Rh concentration as a function of pH in the Rh strip during the GDEx process in Example 2. The data at the top of each column represents the metal removal rate (%). [Figure 3] Figure 3.1: This figure shows the gradual changes in pH and Rh concentration over time during the reduction of Ag / AgCl with CO2 at -1.4 V in Example 3. Figure 3.2: This figure shows the XRD pattern of the product obtained in Example 3. [Figure 4-1]Figure 4.1: This figure shows the gradual change in pH versus time during the reduction of CO2 to Ag / AgCl in the leaching solution of Example 4 at -1.4 V. Inset: Color change of the solution over time. *Sampling point. Figure 4.2: This figure shows the gradual change in PGM concentration versus time during the reduction of CO2 to Ag / AgCl by GDEx in the leaching solution of Example 4 at -1.4 V. [Figure 4-2] Figure 4.3: This figure shows the XRD pattern of the product obtained using CO2 as the gas feed for the GDEx process in Example 4. [Figure 5-1] Figure 5.1: This figure shows the gradual change in pH versus time during the reduction of CO2 to Ag / AgCl in the leachate solution at -1.4 V. Inset: Color change of the solution over time. *Sampling point. In Figure 5.1, Sample 1 refers to the leachate shown in Figure 4.1 of Example 4, and Sample 2 corresponds to the leachate used in this Example 5. Figure 5.2: This figure shows the gradual change in PGM concentration versus time during the reduction of CO2 to Ag / AgCl in the leachate solution of Example 5 by GDEx at -1.4 V. [Figure 5-2] Figure 5.3: This figure shows the XRD pattern of the product obtained using CO2 as the gas feed for the GDEx process in Example 5. [Figure 6] a) This figure shows the gradual change in pH as a function of charge consumed throughout the GDEx process when CO2 is replaced with Ar. b) This figure shows the X-ray diffraction pattern of the product obtained for the Pd solution. c) This figure shows the X-ray diffraction pattern of the product obtained for the Rh solution of Example 6. [Figure 7] Left: This figure shows the gradual change in pH as a function of charge consumed per unit volume throughout the entire GDEx process when elemental nanoparticles are generated by flowing Ar in the gas phase and bubbling CO2 in the cathode reservoir. Right: This figure shows the X-ray diffraction pattern (top) and SEM micrograph and distribution histogram (bottom) of the elemental nanoparticles. The white scale bars are 1 μm for the following metals: (a) Pt; (b) Pd; (c) Rh in Example 7. [Modes for carrying out the invention] [Examples]

[0050] The present invention is further illustrated in the following embodiments.

[0051] Materials and methods chemicals Hexachloroplatinic acid (H2PtCl6, Pt 39.93 wt%) (Johnson Matthey), palladium(II) chloride (PdCl2, 99.9%) (Sigma-Aldrich), rhodium(III) chloride hydrate (RhCl3·xH2O, 99.98%) (Sigma-Aldrich), sodium chloride (NaCl, 99.5%) (Acros Organics), hydrochloric acid (HCl, 37 wt%) (Sigma-Aldrich), carbon dioxide (CO2, 99.998%) (Air Liquide), and argon (Ar, 99.99%) (Air Liquide) were purchased from various sources and used as received without further purification. Demineralized water was used to prepare aqueous solutions throughout the experiment.

[0052] electrochemical reactor The electrochemical reactor (Figure 1.2) was a three-compartment electrochemical cell. In the first compartment, a gas (i.e., CO2, Ar) was flowed at a constant rate by setting an overpressure on the hydrophobic layer of the gas diffusion electrode (GDE). The cathodeliane and anodeliane flowed through their respective cell compartments, from one three-necked bottle acting as a reservoir to another. Both the cathodeliane and anodeliane compartments were separated by a FUMASEPZ® FAP-4130-PK anion exchange membrane. Both liquid compartments were separated by a 10 cm² 2 Exposed surface area and 21 cm² 3It has a capacity of . The GDE (VITO CORE (trademark)) 1 consisted of an outer activated carbon-polytetrafluoroethylene (C-PTFE with a C to PTFE ratio of 80:20) layer pressed onto a stainless steel mesh that acted as a current collector. The activated carbon used was Norit (trademark) SX 1G (Cabot, Europe). The projected surface area of ​​such a cathode was 10 cm² 2 Therefore, the BET specific surface area of ​​the PTFE-bound active layer is approximately 450 m². 2 The value was / g. An Ag / AgCl (3 M KCl) electrode was used as a reference electrode and placed near the GDE via a Lugin tube. A platinum-clad tantalum plate electrode (Pt thickness 10 μm) was used as the anode, which produced O2 or Cl2, but had only a negligible effect on the electrochemical (cathode) process and the target product.

[0053] Synthesis procedure 0.1 M stock solutions of precursor compounds of three platinum group metal ions (PGMs) (H2PtCl6, PdCl2, RhCl3) were prepared in 0.5 M NaCl and used to prepare the corresponding working solutions. The background electrolyte for the GDEx process was a 0.5 M NaCl solution (pH 3) prepared with concentrated HCl. The cathode solution was Pt 4+ , Pd 2+ and Rh 3+ The solutions were prepared by mixing the background electrolyte and PGM stock solution so that the final concentrations of each were 3.0 mM. The background electrolyte alone (i.e., without PGM) was used as the anode solution. 250 mL or 100 mL of the cathode solution and anode solution were placed in three-necked glass bottles and connected to an electrochemical reactor using Marplen tubing (Watson-Marlow), and the solutions were delivered to each chamber at a flow rate of 40 mL / min using a peristaltic pump (530, Watson-Marlow).

[0054] The gases (i.e., CO2, Ar) were flowed through the gas chamber at 200 mL / min using an overpressure of 20 mbar. The solution and gases were flushed through the cell for 15 minutes before the start of the experiment (without polarization of the electrodes). The chronopotentiometry experiment was performed using a Bio-Logic (VMP3) multichannel potentiostat at -10 mA / cm 2 The experiment was conducted in batch mode. pH, charge, and potential were monitored throughout the entire experiment.

[0055] pH was measured every 5 seconds using a Metrohm 781 pH / ion meter equipped with a Metrohm Unitrode pH electrode. At different time points, 1 ml aliquots were taken from the cathode solution and the reaction was quenched by adding 100 μL of 0.1 M HCl to avoid the precipitation of unreacted metal ions. The aliquots were centrifuged and filtered through a 0.3 μm pore filter.

[0056] The filtered solution was analyzed using an inductively coupled plasma-atomography spectrometer (ICP-OES) (Varian 750 ES) to monitor the metal concentration in the liquid phase. A change from a clear solution to a dark, turbid solution indicated the formation of nanoparticles. The experiment was terminated when the supernatant became clear after centrifugation of aliquots of the cathode solution. The nanoparticles were allowed to settle overnight. The supernatant was decanted, the nanoparticles were resuspended in demineralized water, and the remaining NaCl was removed by centrifugation at 10,000 rpm using a Hettich Rotina 35 centrifuge. The washing procedure was repeated until the conductivity of the supernatant was the same as that of demineralized water. The product was dried at room temperature under an Ar atmosphere and stored for further characterization.

[0057] Characteristic evaluation X-ray diffraction (XRD): Dry samples were analyzed by powder X-ray diffraction using a Seifert 3003 T / T diffractometer operated at a voltage of 40 kV and a current of 40 mA with Cu Kα rays (λ=1.5406 Å). Data were collected in the range of 20° to 120° (2θ) using a step size of 0.05°. Profile fitting of the powder diffraction patterns was performed using Highscore Plus (Malvern Pannalytical) with the Inorganic Crystal Structure Database (ICSD). Crystallite size was calculated using Scherrer's formula as follows:2 D = (κλ) / (βcosθ) (1) Here, D is the average crystallite size of the crystalline domain, κ is Scherrer's constant, which is typically considered to be 0.89 for spherical particles, λ is the X-ray wavelength, β is the focal width-of-wavelength (FWHM) at half the maximum intensity in radians, and θ(°) is the Bragg angle at the reflecting surface.

[0058] Scanning electron microscopy (SEM): Microscopic images of the dried sample were taken using a Philips XL30 FEG scanning electron microscope. The images shown were acquired using secondary electrons at an accelerating voltage of 30 kV. Samples were prepared by dispersing the powder in ethanol and sonicating it for 30 minutes. Then, 10 μL of the sample was dropped onto aluminum foil attached to a sample holder. The average particle size and distribution were evaluated by counting at least 100 particles using ImageJ (NIH) software. Subsequently, the data were fitted to a log-normal distribution to obtain the average particle size and standard deviation.

[0059] Example 1 - Blank Experiment Figure 2 (gray line) shows the gradual change in pH associated with the progression of CRR as a function of charge consumed per unit volume in the absence of PGM (blank). Starting from acidic conditions (i.e., pH approximately 3), when only 500 C / L is consumed, the bulk pH rises from 3 to approximately 6, and then increases to approximately 6 at the end of the experiment (10 4By the time of C / L consumption, the pH remained buffered to approximately 7.5. Both the CO2 reduction reaction to CO (reactions 1 and 2) and the water reduction reaction to H2 (reaction 3) were OH - This caused a rapid increase in the pH of the electrolyte. 15、24 In such a weakly alkaline environment, unreacted CO2 becomes HCO3. - It dissolves in the cathode solution as (reaction 4). The latter, at an alkaline pH, is CO3 2- It can be further deprotonated (reaction 5). In both equilibrium reactions, the OH produced at the cathode - It consumes some of the anions and provides a buffer for the electrolyte bulk. 24 . CO2 + OH - ⇔HCO3 - (4) HCO3 - +OH - ⇔CO3 2- +H2O (5)

[0060] Example 2. A stripping solution in 1 M HCl containing 1.035 M chloride anions, 0.7 M sulfate anions, with a pH of -0.02 and the following elemental concentrations (mg / L) was used as the starting solution for the matrix: 0.01 Pt, 0.01 Pd, 2.17 Rh, 1830 Al, 15.2 Si, 315 Mg, 101 Ce, 476 Zr, 0.29 Ba, 15.2 La, 24.4 Fe, 16.1 Ti, 11.3 Sr, 10.6 Nd, 22519 S, 62.2 Ca, 144 K, 42752 Na, <0.01 P, 0.76 Zn, 3.31 Ni, <0.01 Cu, 1.48 Ag, 3.79 Cr, 0.78 V, <0.01 Co, 0.64 As, 3.16 Mn, 0.78 Pb, 0.90 Mo, 3.80 Sn, <0.01 Ta, <0.01 Bi, <0.01 Sb, 46.6 Sc, 0.32 Au, and <0.01 Cd. This stripping solution was derived from the pretreatment of a used catalytic converter. Rhodium is highlighted because, by applying the method of the present invention, it is intended that Rh can be selectively separated from the remaining components for further reuse.

[0061] The operating conditions for the method of the present invention were the same as those presented in Example 1.

[0062] After polarization at the cathode, where CO2 reduction occurs in GDEx, the pH of the system gradually changes over time, as shown in Figure 2.a. Simultaneously, the removal of Rh proceeds as shown in Figure 2.c, and the concentration of the most important components in the solution gradually changes as shown in Table 1. After 18 hours of treatment, the system reaches pH 2, and 95.5% of Rh is removed from the solution (Table 2.1). At this point, most impurities remain in the solution. Furthermore, these removed impurities, which cannot be reduced to elemental form under the given conditions, precipitate as hydroxides on the electrode surface. This yields pure elemental rhodium nanoparticles.

[0063] Table 2.1. [Table 1]

[0064] Example 3. A synthetic solution intended to partially mimic the stripping solution of Example 2 was prepared in 1 M HCl as a matrix, containing the following elements in mg / L units: 1000 Fe, 1000 Mg, 5000 Al, and Rh in the range of 600-100. The effect of its concentration was evaluated. RhCl3 was used as the Rh precursor. The objective was selective recovery of Rh by GDEx using CO2 as the gas feed. The operating conditions for the method of the present invention were the same as those presented in the previous examples. In this case, the pH only gradually changed down to 2 by the method, which is the range in which the highest Rh recovery rate was observed in Example 2.

[0065] Figure 3.1 shows the gradual change in pH and Rh concentration over time. As shown in Table 3.1, after 12 hours of treatment and reaching a pH of approximately 1.5, over 98% of the Rh was removed from the solution. By the end of the experiment, 98.1% of the Rh was removed from the 100 mg / L Rh solution, 99.3% from the 350 mg / L Rh solution, and 99.5% from the 600 mg / L Rh solution. The removal rate (%) of remaining metal ions in the solution was highest when using the 100 mg / L Rh solution. However, in all cases, the removal of these impurities was minimal. Based on quantitative evaluation of the obtained product, 70% of the Rh initially present in the solution was recovered into elemental Rh nanoparticles. The purity of the obtained product was 100%, as shown by its XRD pattern (Figure 3.b). The remaining Rh (30%) was lost by being deposited on the electrode surface along with impurities.

[0066] Table 3.1 Metal removal rate (%) in Rh-mimicking strips at the end of the experiment [Table 2]

[0067] Example 4. A leachate was obtained by pretreatment of a used catalytic converter. The leachate was supported in a 6 M HCl + 31% H2O2 (9:1 v / v) solution with a pH of -0.873 and an ionic conductivity of 555 mS / cm. The concentrations (mg / L) of various elements in the leachate were 84.25 Pt, 136.5 Pd, 21.5 Rh, 11650 Al, 3461 Mg, 1687 Ce, 1100 Fe, 0.1 Zr, 10.5 Ba, 220 La, 78.1 Ti, 147 Sr, 98.4 Nd, 182 Cr, 4 V, and <0.01 Cu. PGMs are highlighted because, by applying the methods of the present invention, they can be selectively separated from the remaining components for further reuse.

[0068] The operating conditions for the method of the present invention were the same as those presented in the previous examples.

[0069] Figure 4.1 shows the gradual change in pH throughout the experiment. The various color changes that occur when the leachate is treated with GDEx are highlighted. After approximately 22 hours of treatment, the pH reached 0.5 (Figure 5.a), and approximately 99% of the PGMs were removed from the leachate (Figure 4.2). Table 4.1 shows the gradual change in the concentrations of various components in the leachate, where the minimal removal of non-PGM elements from the solution up to 7 hours of treatment is noteworthy, and the highest PGM removal rate was observed. As shown in Figure 4.3, the product recovered by the treatment consisted of a mixture of pure PGMs.

[0070] Table 4.1. Percentage of metal removal from leachate at different sampling times. [Table 3]

[0071] Example 5. A leachate was obtained by pretreatment of a used catalytic converter. The leachate was supported in a 6 M HCl + 31% H2O2 (9:1 v / v) solution with a pH of -0.873 and an ionic conductivity of 555 mS / cm. The concentrations (mg / L) of various elements in the leachate were 90.25 Pt, 154.5 Pd, 25.1 Rh, 11750 Al, 3570 Mg, 1640 Ce, 498 Fe, 0.11 Zr, 4.36 Ba, 289 La, 88.1 Ti, 153 Sr, 104 Nd, 21.5 Cr, 4.4 V, and 9.5 Cu. PGMs are highlighted because, by applying the methods of the present invention, they can be selectively separated from the remaining components for further reuse.

[0072] The operating conditions for the method of the present invention were the same as those presented in the previous examples.

[0073] Figure 5.1 shows the gradual change in pH throughout the experiment. The various color changes that occur when the leachate is treated with GDEx are highlighted. Similar to Example 1, it is shown that most of the PGM was removed by 22 hours of treatment, and the treatment time of this leachate with GDEx was completed at 22 hours (i.e., see the comparison of the gradual change in pH for both experiments in Figure 5.1). The maximum pH achieved was approximately 0.75, at which point 99.9% of the PGM had been removed from the leachate (Figure 5.2). Nevertheless, Table 5.1 shows that the maximum PGM recovery rate was observed in the first 7 hours of treatment. As shown in Figure 5.3, the product recovered by the treatment consisted of a mixture of pure PGM.

[0074] Example 6. Replacement of CO2 with Ar in a gas compartment To demonstrate the role of CO2 equilibrium in buffering pH during the GDEx process, CO2 was replaced with Ar and the cathode was polarized at the same current density. As expected, if only the water reduction reaction occurs, OH - Due to ion generation, the pH rises from 3 to 11.5 without a significant buffering layer. The same experiment was repeated with metal-containing solutions. For Pd and Rh, OH -[PdCl4] 2- and [RhCl6] 3- Cl - It replaces and forms a hydroxide complex. Rh(OH)3 precipitates as is, but Pd(OH)2 is extremely unstable and is readily converted to hydrated PdO, as shown in the X-ray diffraction patterns (Figures S4b and S4c). 13 In experiments using Pd and Rh, OH - Consumption of Pd is observed at the pH plateau before reaching pH 11. In the case of Pd, a peak corresponding to metallic Pd is identified in the X-ray diffraction pattern (Figure S3b), indicating that a small amount of Pd 2+ Pd 0 This suggests that it was reduced to [a certain value]. These results highlight the importance of CO2 equilibrium in the GDEx process.

[0075] Example 7: Inflow of Ar in the gas phase of a GDEx reactor and bubbling of CO2 in the cathode liquid reservoir To demonstrate the role of CO generated during CO2 reduction in GDE during the GDEx process, Ar was flowed into the reactor's gas compartment, and CO2 was bubbled into the cathode reservoir. The cathode was polarized at the same current density. Under these conditions, water reduction to H2 and CO2 equilibrium in the bulk electrolyte occurred, but CO2 reduction to CO did not. As expected, the pH of the cathode was buffered to approximately 6 (in both the blank and metal-containing experiments). Metal nanoparticles were obtained, similar to the GDEx process (where CO2 is reduced by GDE). However, the average diameter of these products was larger, and their distribution was broader. These results highlight the importance of CO formation and presence in the GDEx process for controlling the size and distribution of the resulting products. [Explanation of Symbols]

[0076] Drawing translation Figure 1.1 diffusion layer Oxide Hydroxide Figure 1.2 Anolyte Anolyte Pt plate anode Pt plate anode FUMASE (商標) Anion exchange membrane FUMASEP (trademark) anion exchange membrane Liquid phase Liquid phase VITO CORE (商標) cathode VITO CORE (trademark) cathode Gas phase Gas phase Catholyte (Metals) Catholyte (Metals) Product Product Figure 2.1 Time (h) Time (h) Rh Removal zone Rh Removal zone Removal of impurities from solution Removal of impurities from solution Figure 2.2 Rh concentration Rh concentration time (h) time (h) Rh is removed before solution reached pH 2 Rh is removed before solution reached pH 2 Figure 3.1 Time (h) Time (h) Concentration Concentration Figure 3.2 Position Position Cobalt Cobalt Rhodium Rhodium Figure 4.1 time (h) time (h) Figure 4.2 Concentration Concentration Time (h) Time (h) ~99% of removal from the solution Figure 4.3 Position Cobalt Leachate leachate Palladium Platinum Sodium Chloride Figure 5.1 Tin time Sample Figure 5.2 Concentration Time (h) 99.9% of removal from solution Figure 5.3 Position Cobalt 2nd Experiment Pt, Rh, Pd mixture Palladium Platinum Figure 6 Volume charge density Volume charge density Blank Blank Figure 7 Volume charge density Volume charge density Blank Blank References 1. P. Herves, M. Perez-Lorenzo, L. M. Liz-Marzan, J. Dzubiella, Y. Lu and M. Ballauff, Chem. Soc. Rev., 2012, 41, 5577-5587. 2. D. Banham and S. Ye, ACS Energy Lett., 2017, 2, 629-638. 3. E. Antolini, J. Power Sources, 2007, 170, 1-12. 4. S. Sui, X. Wang, X. Zhou, Y. Su, S. Riffat and C. Liu, J. Mater. Chem. A, 2017, 5, 1808-1825. 5. A. P. Reverberi, N. T. Kuznetsov, V. P. Meshalkin, M. Salerno and B. Fabiano, Theor. Found. Chem. Eng., 2016, 50, 59-66. 6. L. D. Rampino and F. F. Nord, J. Am. Chem. Soc., 1941, 63, 2745-2749. 7. T. S. Ahmadi, Z. L. Wang, T. C. Green, A. Henglein and M. A. El-Sayed, Science (80-. )., 1996, 272, 1924 LP - 1925. 8. J. M. Petroski, Z. L. Wang, T. C. Green and M. A. El-Sayed, J. Phys. Chem. B, 1998, 102, 3316-3320. 9. B. Wu, N. Zheng and G. Fu, Chem. Commun., 2011, 47, 1039-1041. 10. G. Chen, Y. Tan, B. Wu, G. Fu and N. Zheng, Chem. Commun., 2012, 48, 2758-2760. 11. S.-H. Chang, M.-H. Yeh, J.-J. Pan, K.-J. Chen, H. Ishii, D.-G. Liu, J.-F. Lee, C.-C. Liu, J. Rick, M.-Y. Cheng and B.-J. Hwang , Chem. Commun., 2011, 47, 3864-3866. 12. C. Zhang, SY Hwang and Z. Peng, J. Mater. Chem. A, 2013, 1, 14402-14408. 13. CF Zinola, J. Electrochem. Soc., 2017, 164, H170-H182. 14. C. Delacourt, PL Ridgway, JB Kerr and J. Newman, J. Electrochem. Soc., 2008, 155, B42. 15. T. Burdyny and WA Smith, Energy Environ. Sci., 2019, 12, 1442-1453. 16. RA Prato, V. Van Vught, S. Eggermont, G. Pozo, P. Marin, J. Fransaer and X. Dominguez-Benetton, Sci. Rep., 2019, 9, 15370. 17. G. Pozo, P. de la Presa, R. Prato, I. Morales, P. Marin, J. Fransaer and X. Dominguez-Benetton, Nanoscale, 2020, 12, 5412-5421. 18. G. Pozo, D. van Houtven, J. Fransaer and X. Dominguez-Benetton, React. Chem. Eng., 2020, 5, 1118-1128. 19. RA Prato M., V. Van Vught, K. Chayambuka, G. Pozo, S. Eggermont, J. Fransaer and X. Dominguez-Benetton, J. Mater. Chem. A, 2020, 8, 11674-11686. 20. N. Yang, SR Waldvogel and X. Jiang, ACS Appl. Mater. Interfaces, 2016, 8, 28357-28371. 21. J. Albo, M. Alvarez-Guerra, P. Castano and A. Irabien, Green Chem., 2015, 17, 2304-2324. 22. H. Ooka, MC Figueiredo and MTM Koper, Langmuir, 2017, 33, 9307-9313. 23. W. Zhou, J. Wu and H. Yang, Nano Lett., 2013, 13, 2870-2874. 24. N. Gupta, M. Gattrell and B. MacDougall, J. Appl. Electrochem., 2006, 36, 161-172. 25. RE Cameron and AB Bocarsly, Inorg. Chem., 1986, 25, 2910-2913. 26. CF Holder and RE Schaak, ACS Nano, 2019, 13, 7359-7365. 27. H. Einaga and M. Harada, Langmuir, 2005, 21, 2578-2584. 28. A. Henglein and M. Giersig, J. Phys. Chem. B, 2000, 104, 6767-6772. 29. P. Jeanty, C. Scherer, E. Magori, K. Wiesner-Fleischer, O. Hinrichsen and M. Fleischer, J. CO2 Util., 2018, 24, 454-462.

Claims

1. A method for recovering platinum group metals from a feed containing one or more precursor compounds of one or more platinum group metal ions, (i) A step of supplying a feed containing one or more precursor compounds to the cathode compartment of an electrochemical cell having a cathode including a gas diffusion electrode having a porous electrochemical active material, thereby forming a liquid phase in the cathode compartment, wherein the liquid phase is electrolyzed to produce H₂ and reduce metal ions, (ii) CO 2 A step of supplying the contained gas to the cathode compartment, (iii) CO to CO 2 A step of applying a potential to the cathode that causes electrochemical reduction, (iv) A step of recovering one or more precipitated particles of platinum group metals from the liquid phase in elemental form, Methods that include...

2. CO 2 The method according to claim 1, wherein the gas is supplied to the gas diffusion electrode.

3. The aforementioned liquid phase is water, H 2 The method according to claim 1 or 2, comprising one or more solvents selected from the group consisting of a protic solvent capable of producing, an organic solvent selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and acetone, an ionic liquid, a deep eutectic solvent, and mixtures of two or more thereof.

4. The method according to claim 3, wherein the protic solvent is selected from the group comprising alcohols, ammonia, amines, amides, ionic liquids, acetic acid, and mixtures of two or more of the above-mentioned compounds.

5. The method according to claim 4, wherein the alcohol is selected from the group comprising methanol, ethanol, n-propanol, and isopropyl alcohol, and mixtures of two or more thereof.

6. The method according to any one of claims 1 to 5, wherein the feed containing one or more platinum group metal ions is a liquid feed containing an aqueous solution of one or more precursor compounds.

7. The method according to any one of claims 1 to 6, wherein the feed is a liquid feed or solution in which one or more precursor compounds of one or more platinum group metal ions are dissolved, or a dispersion or suspension of the one or more precursor compounds in a liquid dispersant.

8. The method according to any one of claims 1 to 7, wherein the liquid phase has a temperature in the range of 10°C to 100°C.

9. CO 2 The method according to any one of claims 1 to 8, wherein the gas is supplied as a pure gas or in a mixture with one or more further gases.

10. The method according to any one of claims 1 to 9, wherein the platinum group metal supply contains one or more metals selected from the group consisting of Pd, Pt, Rh, Ru, Os, and Ir.

11. -10 mA / cm² at the cathode 2 The method according to any one of claims 1 to 10, wherein a charge of ~-1000 mA / cm² is applied.

12. The method according to any one of claims 1 to 11, wherein the precipitated particles are nanoparticles having a maximum particle size of 100 nm.

13. The method according to any one of claims 1 to 12, wherein the precipitated particles have a particle size of at least 100 nm.

14. The method according to any one of claims 1 to 13, wherein the anode is a Pt electrode.

15. The method according to any one of claims 1 to 14, wherein the cathode is a carbon-based gas diffusion electrode.

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