Aqueous sulfuric acid solution suitable for electroplating a platinum film

Abstract

A method for the manufacture of an aqueous sulfuric acid solution suitable for electroplating a platinum film, the method comprising heating Pt(NH3)2(NO2)2 with aqueous sulfuric acid at a temperature in the range of 80 to 90 °C.

Description

[0001] Aqueous sulfuric acid solution suitable for electroplating a platinum film

[0002] The invention relates to an aqueous sulfuric acid solution suitable for electroplating a platinum film, to a method for its manufacture and to its use for electroplating a platinum film.

[0003] US 3,206,382 discloses a method of electrodepositing platinum metal which comprises electrolyzing an electrolyte having a pH value below 2 and consisting essentially of an aqueous solution consisting essentially of H2Pt(NO2)2SO4 (Dihydrogen dinitrosulfatoplatinate(ll), so-called “Pt-DNS”), and remainder water. Example V of US 3,206,382 describes the preparation of H2Pt(NO2)2SO4by boiling a solution of H2Pt(NO2)4, prepared by passing a solution of K2Pt(NC>2)4through a cation exchange column, with H2SO4. According to US 3,206,382 the reaction is exemplified by: H2Pt(NC>2)4+ H2SO4— > H2Pt(NC>2)2SO4+ 2 HNO2. To summarize, US 3,206,382 teaches an aqueous sulfuric acid solution suitable for electroplating a platinum film.

[0004] It has now been found that such type of aqueous sulfuric acid solution is considerably well suited for electroplating a platinum film, when it meets certain limitations. Said considerable suitability finds its expression by at least a relatively high current efficiency in the Hull cell test. The Hull cell test is well-known to the skilled person and it can be carried out according to the methodology mentioned below in Example 1.

[0005] The aqueous sulfuric acid solution of the invention comprises H2[Pt(NC>2)2SO4] and has a pH in the range of 0.10 to 0.50 and a platinum content in the range of 2.0 to 5.0 wt.% (% by weight) with a SO4:Pt weight ratio in the range of 2.5:1 to 3.5:1. Nota bene, it is essential, that the aqueous sulfuric acid solution of the invention simultaneously meets a pH in the range of 0.10 to 0.50, a platinum content in the range of 2.0 to 5.0 wt.% and a SO4:Pt weight ratio in the range of 2.5:1 to 3.5:1.

[0006] Besides water as the main constituent, sulfuric acid, and ammonium sulfate, the aqueous sulfuric acid solution of the invention comprises the platinum species H2[Pt(NO2)2SO4] as well as other unidentified platinum species as can be derived from the solution’s1H-decoupled195Pt- NMR spectrum. The aqueous sulfuric acid solution of the invention does not comprise any other acid than sulfuric acid, i.e. no organic acid and no other inorganic acid; in particular and expressly, it does not comprise any other intentionally added acid or substance.

[0007] In an embodiment, a1H-decoupled195Pt-NMR spectrum of the aqueous sulfuric acid solution of the invention (for example, made according to the procedure of Example 1 below) recorded with H2PtCle as internal standard may exhibit or even exhibits a downfield broad resonance A with a chemical shift at 5= - 837 ppm and two broad resonances B and C upfield with chemical shifts at 5= - 1431 ppm and at 5= - 1466 ppm. The ratio of the area percentages of said three resonances may be or even is in the range of 12 to 15 area-% (resonance A, from which it may be assumed that with a certain degree of probability it may be assignable to H2[Pt(NO2)2SO4]), 27 to 31 area-% (resonance B, assignable to an unidentified platinum species B’) 56 to 60 area- % (resonance C, assignable to an unidentified platinum species C’), wherein the area-% of said three resonances total 100 area-%.

[0008] In another embodiment, a1H-decoupled195Pt-NMR spectrum of an aqueous sulfuric acid solution of the invention (for example, made according to the procedure of Example 2 below) recorded with H2PtCle as internal standard may exhibit or even exhibits a downfield broad resonance A1 with a chemical shift at 6 = - 834 ppm and three broad resonances B1 , C1 and D1 upfield with chemical shifts at 5 = -1348 ppm, 5= - 1431 ppm and at 5= - 1466 ppm. The ratio of the area percentages of said four resonances may be or even is in the range of 10 to 13 area-% (resonance A1, from which it may be assumed that with a certain degree of probability it may be assignable to H2[Pt(NO2)2SO4]), 21 to 25 area-% (resonance B1 , assignable to an unidentified platinum species B1’), 22 to 26 area-% (resonance C1 , assignable to an unidentified platinum species CT) and 41 to 45 area-% (resonance D1 , assignable to an unidentified platinum species D1’), wherein the area-% of said three resonances total 100 area- %.

[0009] The invention relates also to a method of manufacture by way of which the aqueous sulfuric acid solution of the invention can be obtained. This method for the manufacture of the aqueous sulfuric acid solution of the invention does not require any cation exchange process and it prevents the disadvantages thereof, for example, the disadvantage of some platinum being kept in a cation exchange resin which would typically require effort of recycling platinum. To be even more precise and expressly, the method of the invention does not comprise any cation exchange process.

[0010] The method of the invention comprises heating Pt(NHa)2(NO2)2 (so-called “Pt P-salt”) with aqueous sulfuric acid at a temperature in the range of 80 to 90 °C, preferably >80 to <90 °C. The main reaction may be exemplified by: Pt(NHa)2(NO2)2 + 2 H2SO4 — > H2Pt(NO2)2SO4 + (NH4)2SC>4. Pt P-salt is commercially available, for example, from Heraeus or from other precious metal chemical suppliers, for example Umicore or Johnson Matthey.

[0011] The Pt P-salt may be added to the aqueous sulfuric acid or vice versa. It is expedient to take care and prevent the temperature of the reaction mixture to exceed 50 °C during the addition, either by controlled addition and / or by taking internal or external measures for active cooling. In the method of the invention it is preferred to provide the Pt P-salt in the form of an aqueous suspension (suspension of Pt P-salt in water). Preferred is the addition of the aqueous sulfuric acid to such aqueous suspension of Pt P-salt.

[0012] In an embodiment, the method of the invention is carried out at a SO^Pt weight ratio in the range of 2.5:1 to 3.5:1 or at a range slightly below, and at a platinum content in the range of, for example, 8 to 12 wt.%. After the reaction is finished, the resulting solution is diluted with water to obtain a desired platinum content in the afore mentioned range of 2.0 to 5.0 wt.%.

[0013] Quantitative analysis of SO4 content and Pt content is well-known to the skilled person, both can be measured or monitored by ICP-OES analysis of sulfur and by gravimetric analysis of platinum within the reaction mixture, i.e. within a sample thereof taken at any point of time during the reaction during or after the heating process.

[0014] In an expedient embodiment and as already indicated, the heating may be performed at a SO4:Pt weight ratio slightly below a desired target SO4:Pt weight ratio (a finally desired SO4:Pt weight ratio) in the range of 2.5:1 to 3.5:1 and to adjust the SO4:Pt weight ratio after the heating by adding a corresponding amount of aqueous diluted sulfuric acid. To this end, it may be expedient to perform the heating at a SO4:Pt weight ratio slightly below the desired target SO4:Pt weight ratio; in other words, the method of the invention is here carried out at a SO4:Pt weight ratio in the range of, for example, 2.2 to 2.4 :1 up to 3.3 to 3.4 :1 and at a platinum content of 8 to 12 wt.%, followed by adjusting the SO4:Pt weight ratio after the heating by adding the corresponding amount of aqueous diluted sulfuric acid and thereafter followed by dilution with water to obtain a desired platinum content in the afore mentioned range of 2.0 to 5.0 wt.%.

[0015] It is expedient to carry out the heating process until the initially colorless reaction mixture of the Pt(NH3)2(NO2)2 with the aqueous diluted sulfuric acid has finally turned into a brown solution, after having previously turned into a blue-green and then into a green solution. Such heating process may be controlled colorimetrically. The duration of such heating period may lie in the range of, for example, 60 to 90 hours. Once the brown color has been achieved, the heating is stopped and the reaction is finished. The resulting solution is then allowed to cool down and, as far as necessary, the afore disclosed adjustments of SO4:Pt weight ratio and platinum content can be made.

[0016] The aqueous sulfuric acid solution of the invention can be used in electroplating applications where thin and homogeneous platinum films can be cathodically deposited onto substrates like, for example, substrates having metallic surfaces such as surfaces of copper, brass, titanium, iron or nickel or even non-metallic surfaces such as Nation®. Examples of substrates include in particular substrates for use in PEM fuel cell applications or substrates that have utilization in the production of hydrogen via the water splitting reaction, e.g. bipolar electrodes. 1 (according to the invention):

[0017] 510 g of sulfuric acid (96%) were mixed with 510 g of deionized water and allowed to cool to room temperature.

[0018] 160 g of Pt P-salt were charged into a 4L beaker with 300 mL deionized water and stirred with a mechanical overhead stirrer on a hot plate equipped with a thermocouple thermometer. The aqueous sulfuric acid solution prepared before was added at a rate such that the solution temperature did not exceed 50 °C. After complete addition of the aqueous sulfuric acid solution, the solution was then heated at 85 °C for 4 hours. The reaction mixture turned from a cloudy white suspension to a blue green solution followed by a translucent green solution. Once a translucent green color was observed, the reaction mixture was then filtered through a Buchner funnel, weighed, and sampled for platinum and sulfur content. The result for the S content was used to calculate the amount of sulfuric acid needed to charge to achieve a 3.5:1 SO4: Pt weight ratio; here, 85 g of sulfuric acid was calculated and added. The solution was transferred to a 4- neck 5L round bottomed flask equipped with a reflux condenser, thermowell, and mechanical stirrer. The solution was then further diluted by adding 300 ml deionized water and it was then heated to 90°C until it turned to a dark brown color. This heat treatment took 72 hours. After cooling, the reaction mixture was diluted with deionized water so as to achieve a platinum content of 4.3 wt.%. The pH of the solution was 0.40.

[0019] The1H-decoupled195Pt-NMR spectrum of the solution was recorded with H2PtCle as internal standard. It exhibited a downfield broad resonance A with a chemical shift at 5= -837 ppm and two broad resonances B and C upfield with chemical shifts at 5= -1431 ppm and at 5= -1466 ppm. The ratio of the area percentages of said three resonances was 13 area-% (resonance A), 30 area-% (resonance B) and 57 area-% (resonance C).

[0020] A Hull cell test was performed by placing a titanium cathode and a platinum anode in a bath containing the solution. Electrical connections were placed on the cathode and the anode and a current (0.5 Amperes) was passed through the system for 600 seconds. After this time, the cathode was removed from the bath, rinsed with water and briefly oven dried. The cathode was first evaluated for its appearance as an unstable platinum coating would lead to inaccurate calculations. A cathode with a smooth, shiny appearance and no imperfections in the platinum coating was chosen for current efficiency calculations. The current efficiency was calculated by the ratio of the weight gain on the cathode measured experimentally (Wexp) versus the theoretical weight gain (Wtheo). Wtheo was determined by the following calculation: Wtheo = 195.084 g / mol ■ 0.5 Amp ■ 600 sec / 2 ■ 96500 Coulomb / mol = 0.05 g. Wexpwas determined to be 0.008980148 g. Therefore, a current efficiency (Wexp / Wtheo) x 100% = 17.96% was calculated.

[0021] Example 2 (according to the invention):

[0022] Example 1 was repeated with the only difference that after dilution with water, instead heating the solution at 90 °C, it was heated at 85 °C for 72 hours.

[0023] The1H-decoupled195Pt-NMR spectrum of the solution was recorded with H2PtCle as internal standard. It exhibited a downfield broad resonance A1 with a chemical shift at 5= - 834 ppm and three broad resonances B1 , C1 and D1 upfield with chemical shifts at 5= -1348 ppm, 5= -1431 ppm and at 5= -1466 ppm. The ratio of the area percentages of said three resonances was 10 area-% (resonance A1), 23 area-% (resonance B1), 24 area-% (resonance C1) and 43 area-% (resonance D1).

[0024] The Hull cell test performed under the same conditions like in Example 1 showed a current efficiency of 18 %.

[0025] An aqueous sulfuric acid solution suitable for electroplating a platinum layer having the same platinum content and the same pH like that of the solution of Example 1 was made by following the procedure of Example V of US 3,206,382. The Hull cell test performed under the same conditions like in Example 1 showed a current efficiency of 13.1 %.

Claims

Claims1. A method for the manufacture of an aqueous sulfuric acid solution suitable for electroplating a platinum film, the method comprising heating Pt(NH3)2(NO2)2 with aqueous sulfuric acid at a temperature in the range of 80 to 90 °C.

2. The method of claim 1 , wherein the Pt(NH3)2(NO2)2 is added in the form of an aqueous suspension to aqueous sulfuric acid or vice versa.

3. The method of claim 1 or 2 being carried out at a SO4:Pt weight ratio in a range of 2.5:1 to 3.5:1 and at a platinum content in a range of 8 to 12 wt.%, wherein, after the reaction is finished and the heating is stopped, the resulting solution is diluted with water to obtain a desired platinum content in a range of 2.0 to 5.0 wt.%.

4. The method of claim 1 or 2 being carried out at a SO4:Pt weight ratio in a range of 2.2 to 3.3:1 up to 2.4 to 3.4:1 and at a platinum content in a range of 8 to 12 wt.%, wherein, after the reaction is finished and the heating is stopped, the SC Pt weight ratio of the resulting solution is adjusted by adding a corresponding amount of aqueous diluted sulfuric acid, followed by diluting with water to obtain a desired platinum content in a range of 2.0 to 5.0 wt.%.

5. The method of any one of the preceding claims, wherein the heating is stopped and the reaction is finished, once the initially colorless reaction mixture of the Pt(NH3)2(NO3)2 with the aqueous diluted sulfuric acid has finally turned into a brown solution, after having previously turned into a blue-green and then into a green solution.

6. An aqueous sulfuric acid solution suitable for electroplating a platinum film, said solution comprising H2[Pt(NO2)2SO4] and having a pH in the range of 0.10 to 0.50 and a platinum content in the range of 2.0 to 5.0 wt.% with a SO4: Pt weight ratio in the range of 2.5:1 to 3.5:1.

7. The aqueous sulfuric acid solution suitable for electroplating a platinum film of claim 6, the solution being obtained by a method of any one of claims 1 to 5.

8. The aqueous sulfuric acid solution of claim 6 or 7, comprising no other acid than sulfuric acid.

9. Use of an aqueous sulfuric acid solution of any one of claims 6 to 8 in electroplating applications.

10. Method for electroplating a platinum film on substrates utilizing an aqueous sulfuric acid solution of any one of claims 6 to 8.

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