Method for recovering platinum group metals from catalytic structures

Abstract

A method for recovering platinum group metals from a catalytic structure, such as a fuel cell membrane electrode assembly, involving dissolution of the platinum group metal by treating the catalytic structure in an electrolytic cell with a suitable electrolyte containing a complexing agent and introducing an electric current into the electrolytic cell; and subsequently re-precipitating the platinum group metal by increasing the pH of the electrolyte system and adding a reducing agent.

Claims

1. A method for recycling platinum group metals from a catalytic structure, comprising the steps of: a) dissolving the platinum group metal by treating the catalytic structure in an electrolytic cell with a suitable electrolyte including a complexing agent and introducing an electric current into the electrolytic cell; and re-precipitating from said electrolyte including a complexing agent the platinum group metal by increasing the pH of the electrolyte system and adding a reducing agent; wherein the platinum group metal and the electrolyte are not separated prior to re-precipitation of platinum on the catalytic structure; wherein said current is a bias for dissolving platinum with a constant potential with intermittent pulses of lower potential for reestablishing the state of the surface of the catalytic structure; and wherein said current is not an alternating current.

2. A method according to claim 1, wherein the electrolyte further comprises a wetting agent.

3. A method according to claim 2 wherein said wetting agent, comprises an ionic surfactant or a nonionic surfactant.

4. A method according to claim 1, wherein the platinum group metal is re-precipitated as pure noble metal catalyst powder, or as part of an alloy or re-deposited directly on a support material.

5. A method according to claim 1, wherein the platinum group metal is re-precipitated on a suitable substrate by nucleating the platinum group metal directly on the desired substrate structure.

6. A method according to claim 5 wherein said structure comprises carbon particles.

7. A method according to claim 1, wherein the electrolyte system pH is adjusted according to the conditions of the re-precipitation method chosen.

8. A method according to claim 1, wherein the complexing agent is any ligand system with donor atoms belonging to either the carbon group or the groups of pnictogens.

9. A method according to claim 8 wherein said carbon group comprises one or more members selected from the group consisting of cyanides, alkynes, alkenes, and aromatics.

10. A method according to claim 8 wherein said pnictogen comprises one or more members selected from the group consisting of amines, phosphenes, arsenes, chalcogens, and halogens.

11. A method according to claim 1, wherein the reducing agent is selected from liquid organic compounds.

12. A method according to claim 11 wherein said liquid organic compound comprises one or more members selected from the group consisting of glycols, alcohols, antioxidants, and formic acid.

13. A method according to claim 1, wherein the electrolytic cell is subjected to a suitable potential profile for dissolving the noble metal in the chosen electrolyte system.

14. A method according to claim 1 wherein said current cycles between 0.55V and 1.3 V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the mass of platinum dissolved by potential cycling in 1 M HCl electrolyte versus time.

(2) FIG. 2 shows UV-visual absorption spectra of the aqueous phase a) at the end of the platinum dissolution experiment (solid line), b) containing solutions of K.sub.2PtCl.sub.4 (dashed line) and H.sub.2PtCl.sub.6 (dotted line) diluted in 1 M HCl.

(3) FIG. 3 shows a cyclic voltammogram of the re-deposited Pt/C catalyst.

(4) FIG. 4 shows thermogravimetric analysis of re-deposited Pt/C catalysts obtained with (solid line) and without (dashed line) increase of pH prior to platinum reduction.

(5) FIG. 5 shows XRD pattern of platinum re-deposited on carbon. Raw data (grey line) and smoothed curve (black line).

DETAILED DESCRIPTION OF THE INVENTION

(6) In the following a specific embodiment of the present invention is described in more detail.

(7) The concept of the present invention is to first bring platinum into solution electrochemically and then use the resulting solution directly for re-deposition of platinum nanoparticles on a carbon substrate is investigated. In this example pure platinum wire was dissolved by a potentio-dynamic treatment in dilute hydrochloric acid electrolyte. Pure platinum was selected in order not to pollute the solution with corroded contacts or substrate, and to enable a simple gravimetric way of monitoring the amount of platinum dissolved.

(8) The platinum containing solution was then used for re-deposition with no other pretreatment than a simple pH adjustment using sodium hydroxide. Ethanol was used as the solvent and reducing agent in order to reduce the dissolved platinum to platinum nanoparticles. A commercial carbon support material was added to the reaction mixture in order to nucleate platinum directly on carbon. This facilitated the collection and handling of the platinum nanoparticles and minimized the losses upon transfer.

(9) The platinum on carbon material obtained was characterized by cyclic voltammetry, thermogravimetric analysis and powder x-ray diffraction for the detection, quantification and particle size estimation of platinum. Furthermore, UV-vis spectrophotometry was carried out on the platinum containing 1 M HCl electrolyte used for the re-deposition, in order to gain insight on the form in which platinum was dissolved.

(10) Electrochemical Dissolution of Platinum

(11) Platinum wire (d=0.3 mm, Dansk delmetal A/S) was used as the working electrode in a three-electrode electrochemical setup along with a graphite rod as counter electrode and a Radiometer REF401 standard calomel reference electrode. 30 ml of 1 M HCl (Hydrochloric acid 37%, ACS reagent grade, Sigma-Aldrich) was used as the electrolyte. In this electrochemical cell, the platinum wire was subjected to potential cycles between 0.55 V and 1.3 V vs. SHE (standard hydrogen electrode) at a scan rate of 100 mV.Math.s.sup.1. The conditions were controlled by a custom build potentiostat. The amount of platinum dissolved was monitored by periodic weighing of the platinum wire.

(12) The dissolution was stopped when the mass loss from the platinum wire reached 11 mg.

(13) Re-Deposition of Platinum Nanoparticles on Carbon

(14) For the reduction of the dissolved platinum species, a modified version of a method reported by Teranishi et al. was used [27]. The electrolyte (30 ml), now containing dissolved platinum, was adjusted to pH>10 by addition of 5 M sodium hydroxide (NaOH), before being mixed with 50 ml of ethanol. Vulcan XC-72 carbon powder was added to the reaction mixture in order to enable nucleation of platinum directly on carbon. An amount of high surface area carbon (Vulcan XC-72R with 250 m.sup.2/g) was added in order to produce a 20 wt. % platinum on carbon. The solution was refluxed for 2.5 hours. Argon purge was utilized in order to eliminate oxygen.

(15) After cooling to room temperature, the mixture was centrifugated at 4500 rcf for 20 minutes. The precipitate was repeatedly washed with ultrapure water until pH of the wash water was neutral and no chloride ions could be detected by addition of 0.1 M silver nitrate (AgNO.sub.3). The washed precipitate was dried overnight at 95 C.

(16) Characterization

(17) Cyclic voltammetry for the Pt/C sample was carried out in a three necked electrochemical cell using a rotating disk electrode setup. A Zahner Elektrik IM6ex workstation controlled with Thales software version 2.0 was used as a potentiostat. The working electrode was a mirror polished glassy carbon rotating disc electrode with a surface area of 0.196 cm.sup.2 connected to a rotating shaft from Pine Instruments. The counter electrode was a platinum wire kept in a glass tube fitted with a ceramic frit, whereas a Gaskatel dynamic hydrogen electrode was used as the reference electrode. A total volume of 220 ml of 0.5 M perchloric acid electrolyte (Suprapur grade from Sigma-Aldrich) was used. All the water used during the electrochemical characterization was of an ultrapure grade with resistance larger than 18.2 M.

(18) 8 mg of platinum supported on carbon was ultrasonically dispersed in a total volume of 5 ml of ultrapure water for half an hour. A 20 l aliquot was drop coated onto the surface of the glassy carbon electrode in order to produce a platinum loading of 30 g/cm.sup.2. The electrode was dried for approximately one hour, while mounted on the rotating shaft in its inverted position, as previously reported by Garsany et al. [28]. The electrolyte was purged with argon at a flow of 30 ml min.sup.1 from one hour prior to and throughout the experiment in order to produce an oxygen free electrolyte. During all the measurements the rotation speed of the electrode was maintained at 400 rpm.

(19) The working electrode was electrochemically cleaned by potential sweeping for 20 cycles between 0 V and 1.3V vs. SHE at a scan speed of 200 mV/s. In order to measure the specific electrochemical activity, 10 cycles were recorded at 50 mV s.sup.1 between 50 mV and 1300 mV vs. SHE. All the measurements were compensated for 80% of the ohmic loss.

(20) TGA/DSC analysis was carried out on a Netzsch STA449 F3 simultaneous thermal analyzer using 5 K/min heating rate with 50 ml/min of 4:1 nitrogen to oxygen gas mixture.

(21) For powder x-ray diffraction, the Pt/C sample was dispersed in ethanol and then applied drop wise to a flat plastic sample holder in order to produce a thin film. After evaporation of the ethanol, the X-ray pattern of the sample was collected on a Siemens D5000 powder diffractometer with a Cu K radiation source. The XRD pattern was recorded from 20 to 85 with a step size of 0.020 and 10 seconds per step.

(22) UV-visual absorption spectra of the platinum solutions were collected on a Shimadzu UV-1650PC UV-Visible spectrophotometer using quartz cells. The samples were referenced to 1 M HCl.

(23) Referring to FIG. 1 there is shown the mass of platinum dissolved by potential cycling in 1 M HCl electrolyte versus time. The dissolution of platinum by potential cycling in 1 M HCl electrolyte, as monitored by periodic weighing of the platinum wire working electrode, is illustrated in FIG. 1. The dissolution rate is constant throughout the experiment.

(24) Referring to FIG. 2 there is shown UV-visual spectrophotometry of the aqueous phase a) at the end of the platinum dissolution experiment, b) containing solutions of K.sub.2PtCl.sub.4 and H.sub.2PtCl.sub.6 diluted in 1 M HCl. In order to shed light on the form in which platinum is dissolved, UV-visual spectrophotometry of the platinum solution in 1 M HCl was carried out before and after platinum redeposition on carbon (see FIG. 2). UV-visual spectra of aqueous solutions of potassium tetrachloroplatinate and hexachloroplatinic acid diluted in 1 M HCl were also recorded. Whereas, the Pt(II) salt gives rise to an absorption peak at 217 nm with a shoulder at 228 nm, the Pt(IV) acid exhibits absorption peaks at 210 nm and 262 nm, which are coinciding with the peak positions found for the platinum electrochemically dissolved in 1 M HCl.

(25) Referring to FIG. 3 there is shown cyclic voltammogram of the re-deposited Pt/C catalyst. The purpose of the cyclic voltammetry on the fabricated Pt/C catalyst was to detect platinum by revealing its well-known features: hydrogen adsorption and desorption, platinum oxide formation and platinum oxide reduction. These features are clearly found in FIG. 3, confirming the presence of platinum on the carbon substrate. Moreover, the hydrogen desorption region can be used to estimate the specific electrochemical surface area (ECSA) of the platinum supported on carbon, which serves to evaluate the platinum particle size under the assumption of perfectly spherical particles. The specific electrochemical surface area (ECSA) is found as the ratio between the electrochemical surface area and the mass of platinum in the thin film electrode. The electrochemical surface area is in turn found by integration of the hydrogen desorption region followed by correction for the double layer charge contribution and use of the assumption of 210 C per cm.sup.2 of platinum surface area with full hydrogen monolayer coverage. In this case, the ECSA amounts to 43 m.sup.2/g.

(26) Referring to FIG. 4 there is shown thermogravimetric analysis on re-deposited Pt/C catalysts.

(27) The amount of platinum loaded on carbon was quantified by thermogravimetric analysis. The samples were heated in 1:4 oxygen to nitrogen atmosphere to combust the carbon and leave behind the platinum residue (see FIG. 4). Assuming that the residue after heating to 900 C. is pure platinum, the platinum loading is approximately 17.8 wt %, when the mass at 125 C. is taken as the dry weight of the Pt/C sample. FIG. 4 also shows data for a Pt/C sample obtained without increasing the pH prior to platinum re-deposition, in which case the platinum loading on carbon was much lower (5.6 wt %).

(28) Referring to FIG. 5 there is shown XRD pattern of platinum re-deposited on carbon. Raw data (grey line) and smoothed curve (black line). In order to estimate the size of the platinum particles, powder x-ray diffraction patterns were collected (FIG. 5). Peaks are found at 39.8, 46.0, 67.7, as well as 81.5 2 which respectively correspond to reflections from the 111, 200, 220 and 311 crystallographic planes of the platinum face-centered cubic lattice. Using the Scherrer equation, the average size of the crystalline domains is 4-5 nm, which can be considered a lower limit on the actual platinum particle size.

REFERENCES

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