A PROCESS FOR PRECIPITATING PARTICLES OF PLATINUM GROUP METALS

Abstract

The present invention relates to a process for recovering platinum group metals from a feed containing one or more precursor compounds of one or more platinum group metal ions, wherein the process comprises the steps of (i) supplying to a cathode compartment of an electrochemical cell equipped with a cathode comprising a gas diffusion electrode with a porous electrochemically active material, the feed containing the one or more precursor compounds 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 which is such as to cause electrochemical reduction of the CO2 to CO, (iv) and recovering from the liquid phase precipitated particles of the one or more platinum group metals in clemental form.

Claims

1. A process for recovering platinum group metals from a feed containing one or more precursor compounds of one or more platinum group metal ions, wherein the process comprises the steps of (i) supplying to a cathode compartment of an electrochemical cell equipped with a cathode comprising a gas diffusion electrode with a porous electrochemically active material, the feed containing the one or more precursor compounds to form a liquid phase in the cathode compartment, (ii) supplying a CO.sub.2 containing gas to the cathode compartment, (iii) applying a potential to the cathode which is such as to cause electrochemical reduction of the CO.sub.2 to CO, (iv) and recovering from the liquid phase precipitated particles of the one or more platinum group metals in elemental form.

2. The process of claim 1, wherein the CO.sub.2 is supplied to the gas diffusion electrode.

3. The process of claim 1 or 2, wherein the liquid phase contains one or more solvents selected from the group comprising water, a protic solvent capable of generating H.sub.2, an organic solvent selected from the group of dimethylsulfoxide (DMSO), dimethylformamide (DMF), acetone, an ionic liquid, a deep eutectic solvent, and mixtures of two or more hereof.

4. The process of claim 3, wherein the protic solvent is selected from the group comprising an alcohol, ammonia, an amine, an amide, an ionic liquid, acetic acid, and mixtures of two or more of the afore mentioned compounds.

5. The process of claim 4, wherein the alcohol is selected from the group comprising methanol, ethanol, n-propanol and isopropylalcohol and mixtures of two or more hereof.

6. The process of any one of the previous claims, wherein the feed containing the one or more platinum group metal ions is a liquid feed containing an aqueous solution of the one or more precursor compounds.

7. The process according to any one of the previous claims, wherein the feed is liquid feed, in particular a solution, containing the one or more precursor compounds of the one or more platinum group metal ions dissolved therein, or a dispersion or a suspension of the one or more precursor compounds in a liquid dispersant.

8. The process of any one of the previous claims, wherein the liquid phase has a temperature which ranges from 10? C. to 100? C., preferably from 15-75? C., more preferably from 15-50? C., most preferably room temperature.

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

10. The process according to any one of the previous claims, wherein the feed of the platinum group metals contains one or more metals selected from the group of Pd, Pt, Rh, Ru, Os, Ir.

11. The process according to any one of the previous claims, wherein to the cathode a charge is applied of between ?10 and -1000 mAcm.sup.?2, preferably below 50 mA cm.sup.?2.

12. The process according to any one of the previous claims, wherein the precipitated particles are nanoparticles with a particle size of maximum 100 nm.

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

14. The process according to any one of the previous claims, wherein the anode is a Pt electrode.

15. The process according to any one of the previous claims, wherein the cathode is a carbon based gas diffusion electrode.

Description

LIST OF FIGURES

[0051] FIG. 1.1 shows the reactions that occur in the cathode compartment when (a) using O.sub.2, and (b) using CO.sub.2, as the gas feedstock to be electrochemically reduced. In (a), the products of the oxygen reduction reaction (ORR) (1) are oxidizing chemical species that turn metal ions into metal oxides (2) or hydroxides (3) nanoparticles. In (b), the H.sub.2 from the water reduction (WRR) (1), acts a reducing agent while CO from the CO.sub.2 reduction reaction (CRR) (2) acts as a capping agent for the synthesis of elemental nanoparticles from metal ions in solution (3). The equilibrium of unreacted CO.sub.2 to HCO3 and CO32 (4) is crucial to consume the OH generated in the WRR and CRR and interfere with the process.

[0052] FIG. 1.2 is a schematic representation of an electrochemical reactor suitable for use with this invention.

[0053] FIG. 2.1 Evolution of pH vs time in example 2 during reduction of CO.sub.2 at ?1.4 V vs Ag/AgCl Rh strip solution. * Sampling points

[0054] FIG. 2.2 Evolution of the Rh concentration as function of the pH of the Rh strip in example 2 during the GDEx process. Data above each column represents the % of metal removal.

[0055] FIG. 3.1 shows the evolution of pH and concentration of Rh vs time in example 3 during reduction of CO.sub.2 at ?1.4 V vs Ag/AgCl.

[0056] FIG. 3.2 shows the XRD pattern of the product obtained in example 3.

[0057] FIG. 4.1 shows the evolution of pH vs time during reduction of CO.sub.2 at ?1.4 V vs

[0058] Ag/AgCl of the leachate solution of example 4. Inset, color changes of the solution through time. * Sampling points

[0059] FIG. 4.2 shows the evolution of PGM concentration vs time during the reduction of CO.sub.2 with GDEx at ?1.4 V vs Ag/AgCl of the leachate solution of example 4.

[0060] FIG. 4.3 shows the XRD pattern of the product obtained using CO.sub.2 as the gas feed for the GDEx process of example 4.

[0061] FIG. 5.1 shows the evolution of pH vs time during reduction of CO.sub.2 at ?1.4 V vs Ag/AgCl of the leachate solution. Inset, color changes of the solution through time. *Sampling points. In this Figure, Sample 1 refers to the leachate shown in Example 4, FIG. 4.1, whereas Sample 2 corresponds to the leachate used in this example 5.

[0062] FIG. 5.2 Evolution of PGM concentration vs time during the reduction of CO.sub.2 with GDEx at ?1.4 V vs Ag/AgCl of the leachate solution of example 5.

[0063] FIG. 5.3 shows the XRD pattern of the product obtained using CO.sub.2 as the gas feed for the GDEx process in example 5.

[0064] FIG. 6. a) Evolution of pH as function of charge consumed throughout the GDEx process when CO.sub.2 is replaced with Ar. b) X-ray diffraction pattern of the product obtained for the Pd solution. c) X-ray diffraction pattern of the product obtained for the Rh solution of example 6.

[0065] FIG. 7. Left: Evolution of pH a function of the charge consumed per unit volume throughout the GDEx process when Ar is flowed at the gas phase and CO.sub.2 is bubbled at the catholyte reservoir yielding elemental nanoparticles. Right: X-Ray diffraction patterns (up) and SEM micrographs and distribution histograms (down) of the elemental nanoparticles. The withe scale bar is 1 ?m for the following metals: (a) Pt; (b) Pd; (c) Rh of example 7.

[0066] The invention is further illustrated in the examples below.

MATERIALS AND METHODS

Chemicals

[0067] Hexachloroplatinic (IV) acid (H2PtCl6, Pt 39.93 wt %) (Johnson Matthey), palladium (II) chloride (PdCl2, 99.9%) (Sigma-Aldrich), rhodium (III) chloride hydrate (RhCl3.Math.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 and used as received from different sources without any further purification. Demineralized water was used throughout the experiments to prepare aqueous solutions.

Electrochemical Reactor

[0068] The electrochemical reactor (FIG. 1.2) consisted in a three-compartment electrochemical cell. Through the first compartment, gas (i.e. CO.sub.2, Ar) flows at a fixed rate, with a set overpressure, at the hydrophobic layer of the gas diffusion electrode (GDE). The catholyte and anolyte flow from, and to, 3-necked bottles serving as reservoirs, through the respective cell compartment. Both, catholyte and anolyte compartment, are separated by a FUMASEPZ? FAP-4130-PK anion exchange membrane. Both liquid compartments have an exposed surface area of 10 cm2 and a volume of 21 cm3. The GDE (VITO CORE?)1 was composed of an outer active carbon-polytetrafluoroethylene (C-PTFE with a C to PTFE ratio of 80:20) layer pressed onto a stainless-steel mesh serving as current collector. The active carbon employed was Norit? SX 1G (Cabot, Europe). The projected surface area of such cathode was of 10 cm2, with a BET specific surface area of ?450 m2 g-1 for the PTFE-bound active layer. A Ag/AgCl (3M KCl) electrode was used as reference electrode, placed via Luggin capillary close to the GDE. A platinum-coated tantalum plate electrode (Pt 10 ?m thicknes) was used as anode, this anode generated O2 or Cl2, which had a negligible influence on the electrochemical (cathodic) process and products of interest.

Synthesis Procedure

[0069] Stock solutions of precursor compounds of three platinum group metal ions (PGMs) (H2PtCl6, PdCl2, RhCl3) 0.1 M were prepared in NaCl 0.5 M and used to prepare the corresponding working solutions. The background electrolyte for the GDEx process consisted in a NaCl 0.5 M solution pH 3 adjusted with concentrated HCl. The catholyte solutions were prepared by mixing the background electrolyte and the PGMs stock solutions in order to have a final conctration of Pt4+, Pd.sup.2+ and Rh3+of 3.0 mM, respectively. The background electrolyte alone (i.e., without PGMs) was used as anolyte. 250 ml or 100 ml of the catholyte and anolyte were collocated in the 3-necked glass bottles and connected to the electrochemical reactor using marprene tubbing (Watson-Marlow) and pumped to their respective chamber with a flow rate of 40 mL min.sup.?1 using a peristaltic pump (530, Watson-Marlow).

[0070] The gases (i.e., CO.sub.2, Ar) were flowed in the gas chamber at 200 ml min.sup.?1, with an overpressure of 20 mbar. The solutions and gases were flushed through the cell for 15 minutes before starting the experiments (without electrode polarization). Chronopotentiomemetric experiments were carried out in a batch mode at -10 mA cm.sup.?2 using a Bio-Logic (VMP3) multichannel potentiostat. pH, charge and potential were monitored throughout all the experiments.

[0071] The pH was measured every 5 seconds with a Metrohm 781 pH/ion meter equipped with a Metrohm Unitrode pH electrode. Aliquots of 1 ml were taken from the catholyte at different times, 100 ?L of HCl 0.1 M were added to quench the reaction and avoid precipitation of unreacted metal ions. The aliquots were centrifuged, and filtered with a 0.3 ?m pore filter.

[0072] The filtered solutions were analyzed with an inductive coupled plasma-optical emission spectrometer (ICP-OES) (Varian 750 ES) to monitor the metal concentration in the liquid phase. The turning from a clear solution to a dark turbid solution was an indicative of the formation of the nanoparticles. The experiments were stopped when, after centrifuging an aliquot of the catholytethe supernatant became transparent. The nanoparticles were left to settle down overnight. The supernatant was decanted, and the nanoparticles resuspended using demineralized water and centrifuged at 10000 rpm with a Hettich Rotina 35 centrifuge to remove the remaining NaCl. The washing procedure was repeated until the conductivity of the supernatant was similar to the demineralized water. The products were dried under Ar atmosphere at room temperature and kept under storage for further characterization.

Characterization

[0073] X-ray diffraction (XRD): The dried samples were analysed by powder X-ray diffraction in Seifert 3003 T/T diffractometer operated at a voltage of 40 kV and a current of 40 mA with Cu K? radiation (?=1.5406 ?). The data were collected in the 20?-120? (2?) range with a step size of 0.05?. The profile fitting of the powder difraction patterns was performed with Highscore Plus (Malvern Pannalytical) using the inorganic crystal structure database (ICSD). Crystallite sizes were calculated using the Scherrer equation:2

D=(??)/(? cos ?) (1)

[0074] where D is the average crystallite size of the crystalline domains, ? is the Scherrer constant, tipically considered 0.89 for spherical particles, ? is the X-ray wavelength, ? is the line broadening at half the maximum intensity (FWHM) in radians, and ? (?) is the Bragg angle at a reflecting plane.

[0075] Electron Scanning Microscopy (SEM): Micrographs of the dry samples were taken with a Philips XL30 FEG scanning electron microscope, Images presented were taken with secondary electrones and an acceleration voltage of 30 kV. The samples were prepared by dispersing the powders in ethanol and sonicating for 30 minutes. Then 10 ?L were dropped in a aluminum foil mounted on a sample holder. The mean particle size and distribution were evaluated by counting at least 100 particles using the software ImageJ (NIH). After that, data were fitted to a lognormal distribution to obtain the mean particle size and standard deviation.

Example 1Blank Experiment

[0076] In FIG. 2 (gray lines) is shown the evolution of pH as the CRR progressed as function of the charge consumed per volume unit, in the absence of PGMs (blank). Starting from acidic conditions (i.e., pH ?3), an increase of the bulk pH from 3 to ?6 when only 500 C L.sup.?1 had been consumed, after that, the pH remained buffered until ?7.5 by the end of the experiment (10.sup.4 C L.sup.?1 consumed). Both, the CO.sub.2 reduction reaction to CO (reactions 1 and 2) and the water reduction reaction to H.sub.2 (reaction 3) yield OH.sup.?, resulting in a fast increase of the pH in the electrolyte..sup.15,24 In such a mildly alkaline environment, unreacted CO.sub.2 dissolves in the catholyte (reaction 4) as HCO.sub.3?. The latter can be further deprotonated in alkaline pH to CO.sub.3.sup.2?(reaction 5). Both equilibrium reactions consume part of the OH anions generated at the cathode, resulting in the buffering of the electrolyte bulk..sup.24

CO.sub.2+OH.sup.?HCO.sub.3.sup.?(4)

HCO.sub.3.sup.?+OH.sup.?CO.sub.3.sup.2?+H.sub.2O (5)

Example 2

[0077] A stripping solution in 1 M HCl as the matrix, containing 1.035 M of chloride anions, 0.7 M of sulfate anions, and a pH of ?0.02, and concentrations of the following elements in mg L.sup.?1, was used as the starting solution: 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 issued from the pre-treatment of spent catalytic converters. Rhodium is highlighted as it is intended that by applying the method object of this invention, it can be selectively separated from the rest of the components so that the Rh can be further reused.

[0078] The operational conditions of the method object of this invention were the same as those presented in Example 1.

[0079] After starting the polarization at the cathode upon which CO.sub.2 reduction takes place during GDEx, the pH of the system evolves through time, as shown in FIG. 2.a. Simultaneously, the removal of Rh progresses as shown in FIG. 2.c, whereas the concentration of the most relevant components in the solution evolves as presented in Table 1. By 18 h of processing, the system has reached a pH of 2 and 95.5% of Rh has been removed from the solution (Table 2.1). At that point, most impurities are kept in solution. Furthermore, those impurities which are removed, they precipitate as hydroxides on the surface of the electrode, as they cannot be reduced in the conditions provided to their elemental forms. Thus, pure elemental rhodium nanoparticles are obtained.

TABLE-US-00003 TABLE 2.1 % of metal removal % of metal removal after time indicated (hours) Metal 4 h 18 h 24 h 43 h 48 h 60 h 80 h Rh 65.3 95.5 94.8 95.0 95.6 98.8 97.4 Al 10.5 20.5 24.3 73.0 91.2 99.6 99.2 Ba 7.9 20.4 26.5 40.2 43.4 91.0 94.7 Ce 9.5 16.6 21.5 33.6 37.0 99.9 99.9 Cr ?18.7 ?2.6 3.3 86.4 97.5 98.8 95.7 Fe ?10.0 2.5 8.3 89.9 99.1 99.9 99.9 La 10.7 18.7 23.4 35.0 38.0 99.9 99.9 Mg 10.4 20.4 25.0 36.7 36.8 96.2 99.6 Nd 10.2 18.1 22.4 34.5 37.8 99.9 99.9 Sr 10.1 21.0 24.9 36.1 36.0 76.4 81.0 Ti 8.9 26.8 31.0 96.7 99.9 99.9 99.9 V 8.0 19.1 26.3 96.4 99.9 99.9 97.6 Zr 10.1 28.2 32.8 88.7 98.7 99.5 99.3

Example 3

[0080] Synthetic solutions aiming to partly simulate the stripping solution of example 2 were prepared in 1 M HCl as the matrix, containing the following elements in mg L.sup.?1, 1000 Fe, 1000 Mg, 5000 Al, and Rh in the range of 600-100, to assess the effect of its concentration. RhCl.sub.3 was used as the Rh precursor. The selective recovery of Rh by GDEx using CO.sub.2 as the gas-feed was aimed. The operational conditions of the method object of this invention were the same as those presented in the previous Examples. In this case, the process was only let to evolve up to pH 2, which his the range in which the highest Rh recovery rate took place in Example 2.

[0081] FIG. 3.1 shows the evolution of pH and the concentration of Rh through time. After 12 h of processing and reaching PH ?1.5, >98% of Rh had been removed from the solution and by the end of the experiment 98.1% of Rh had been removed from the solution containing 100 mg L.sup.?1 Rh, 99.3% of Rh had been removed from the solution containing 350 mg L.sup.?1 Rh, and 99.5% of Rh had been removed from the solution containing 600 mg L.sup.?1 Rh, as shown in Table 3.1. The % of removal from the rest of the metal ions in solution was highest when using the solution containing 100 mg L.sup.?1 Rh. However, in all instances the removal of these impurities was minimal. Based on a quantitative evaluation of the resulting product, 70% of the Rh initially present in the solution was recovered into elemental Rh nanoparticles. The purity of the product obtained was 100%, as shown by its XRD pattern (FIG. 3.b). The rest of the Rh (30%) was lost by deposition in the electrode surface together with the impurities.

TABLE-US-00004 TABLE 3.1 % of metal removal of the metals in the Rh simulated strip at the end of the experiment % of metal removal Mg Al Fe Rh 600 mg/L 4.8 4.1 7.6 99.5 350 mg/L 9.9 8.8 11.0 99.3 100 mg/L 17.4 15.8 5.3 98.1

Example 4

[0082] A leachate solution was obtained from the pre-treatment of spent catalytic converters. The leachate was supported in a solution of HCl 6 M+H.sub.2O.sub.2 31% (9:1v/v), with a pH of ?0.873 and an ionic conductivity of 555 mS/cm. The concentration of the different elements in the leachate, in mg L.sup.?1, was: 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, <0.01 Cu. The PGMs are highlighted as it is intended that by applying the method object of this invention, they can be selectively separated from the rest of the components, for further reuse.

[0083] The operational conditions of the method object of this invention were the same as those presented in the previous examples.

[0084] FIG. 4.1 shows the evolution of pH through the experiment. Diffierent changes of color occurring during processing the leachate with GDEx are highlighted. At about 22 h of processing, pH 0.5 was reached (FIG. 5.a) and ?99% of the PGMs had been removed from the leachate (FIG. 4.2). Table 4.1 shoes the evolution on the concentration of the different components of the leachate, from which the miminal removal from solution from non-PGM elements is noteworthy, especially up to 7 h of processing, while the highest PGM removal rate has taken place. As shown in FIG. 4.3, the product recovered from the processing consisted of a mixture of pure PGMs.

TABLE-US-00005 TABLE 4.1 % of metal removal from the leachate at different sampling times. % of metal removal from the leachate at different sampling times. Metal 3 h 7 h 22 h 30 h 35 h 48 h Pt 73.5 97.4 99.4 99.9 99.8 99.5 Pd 79.2 97.7 98.9 99.3 99.2 98.6 Rh 53.8 89.6 99.2 99.8 99.7 99.3 Al 2.9 5.1 5.9 12.0 13.4 16.9 Ba 3.1 1.0 10.3 15.6 51.4 67.3 Ce 2.1 3.9 5.3 11.4 11.5 16.9 Cr 1.1 5.7 6.7 10.6 13.7 76.3 Fe 3.2 9.3 9.0 12.3 15.5 52.5 La 2.4 2.7 5.7 10.8 10.4 16.8 Mg 3.6 5.7 7.1 12.4 15.1 16.3 Nd 2.2 3.5 7.3 21.1 51.2 69.0 Sr 1.3 ?0.2 9.3 18.7 50.4 67.1 Ti 0.1 3.0 7.9 45.0 61.2 62.8 V 6.3 6.5 14.6 17.7 50.7 74.1 Zr ?0.3 11.9 58.2 85.7 87.2 89.6

Example 5

[0085] A leachate solution was obtained from the pre-treatment of spent catalytic converters. The leachate was supported in a solution of HCl 6 M+H.sub.2O.sub.2 31% (9:1v/v), with a pH of -0.873 and an ionic conductivity of 555 mS/cm. The concentration of the different elements in the leachate, in mg L.sup.?1, was: 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, 9.5 Cu. The PGMs are highlighted as it is intended that by applying the method object of this invention, they can be selectively separated from the rest of the components, for further reuse.

[0086] The operational conditions of the method object of this invention were the same as those presented in the previous examples.

[0087] FIG. 5.1 shows the evolution of pH through the experiment. Different changes of color occurring during processing the leachate with GDEx are highlighted. As in Example 1 it was shown that by 22 h of processing most of the PGMs were removed, the processing time of this leachate by GDEx stopped by 22 h (i.e., see comparison between the pH evolution of both experiments in FIG. 5.1). The maximum pH attained was ?0.75, at which 99.9% of the PGMs had been removed from the leachate (FIG. 5.2). Nevertheless, in Table 5.1 it is shown that the maximum PGM recovery rate took place within the first 7 h of processing. As shown in FIG. 5.3, the product recovered from the processing consisted of a mixture of pure PGMs.

Example 6. Replacing the CO.SUB.2 .by Ar at the Gas Compartment

[0088] To demonstrate the role of the CO.sub.2 equilibrium to buffer the pH during the GDEx process, CO.sub.2 was replaced by Ar and the cathode was polarized at the same current density. As expected, when only the water reduction reaction takes place, due to the generation of OH.sup.? ions, the pH increases from 3 to 11.5 without any noticeable buffered zone. The same experiment was repeated with solutions containing the metals. For the case of Pd and Rh, the OH.sup.? displaces the Cl.sup.? of the [PdCl.sub.4].sup.2? and [RhCl.sub.6].sup.3? to form hydroxide complexes. Rh(OH)3 precipitate as such, while Pd(OH)2 is highly unstable and is easily transformed into hydrated PdO,.sup.13 as is shown in the X-ray diffraction patterns (FIG. S4b and FIG. S4c). The consumption of OH is noted in the pH plateau in the experiments with Pd and Rh, before reaching pH 11. For the case of Pd, in the X-ray diffraction patterns (FIG. S3b) peaks corresponding to the metallic Pd were identified suggesting that a small amount to Pd.sup.2+ was reduced to Pd.sup.0. These results highlight the importance of the CO.sub.2 equilibrium in the GDEx process.

Example 7 Flowing Ar at the Gas Phase of the GDEx Reactor and Bubblinc CO.SUB.2 .in the Catholyte Reservoir

[0089] To demonstrate the role of the CO generated during the CO.sub.2 reduction at the GDE during the GDEx process, Ar was flowed in the gas compartment of the reactor and CO.sub.2 was bubbled in the catholyte reservoir. The cathode was polarized at the same current density. At this conditions water reduction to H.sub.2 and CO.sub.2 equilibrium at the bulk electrolyte take place but not the CO.sub.2 reduction to CO. As expected, the pH of the catholyte is buffered (in both, blank and containing metals experiments) at ?6. As in the GDEx process (in which CO.sub.2 is reduced at the GDE) metallic nanoparticles were obtained. However the average diameter of these products was bigger and with a broader distribution. These results highlight the importance of the formation and presence of CO in the GDEx process to control the size and distribution of the products obtained.

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