Aqueous sulfuric acid solution suitable for plating platinum film

Abstract

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

Description

[0001] This invention relates to an aqueous sulfuric acid solution suitable for electroplating platinum films, its preparation method, and its use in electroplating platinum films.

[0002] US 3,206,382 discloses a method for platinum metal electrodeposition, comprising electrolyzing an electrolyte with a pH below 2 and consisting essentially of an aqueous solution, which is composed primarily of H₂Pt(NO₂)₂SO₄ (platinum(II) dihydrodinitrosulfate, so-called "Pt-DNS") and the balance water. Example V of US 3,206,382 describes the preparation of H₂Pt(NO₂)₄ by boiling a solution of H₂Pt(NO₂)₄ (prepared by passing a solution of K₂Pt(NO₂)₄ through a cation exchange column) together with H₂SO₄. According to US 3,206,382, the reaction is illustrated as follows: H₂Pt(NO₂)₄ + H₂SO₄ → H₂Pt(NO₂)₂SO₄ + 2HNO₂. In summary, US 3,206,382 teaches an aqueous sulfuric acid solution suitable for platinum film electroplating.

[0003] It has been found that this type of aqueous sulfuric acid solution is well-suited for platinum film electroplating when certain limiting conditions are met. This high suitability is manifested in the Hull cell test as at least relatively high current efficiency. The Hull cell test is well known to those skilled in the art and can be performed according to the method mentioned in Example 1 below.

[0004] The sulfuric acid aqueous solution of the present invention contains H2[Pt(NO2)2SO4] and has a pH in the range of 0.10 to 0.50 and a platinum content in the range of 2.0 wt% to 5.0 wt%, wherein the SO4:Pt weight ratio is in the range of 2.5:1 to 3.5:1. However, it is important that the sulfuric acid aqueous solution of the present invention simultaneously satisfies the requirements of a pH in the range of 0.10 to 0.50, a platinum content in the range of 2.0 wt% to 5.0 wt%, and an SO4:Pt weight ratio in the range of 2.5:1 to 3.5:1.

[0005] In addition to sulfuric acid and ammonium sulfate, and water as the main component, the sulfuric acid aqueous solution of the present invention also contains platinum species H2 [Pt(NO2)2SO4], and can be adjusted according to the solution's... 1 H-decoupling 195 Other unidentified platinum species deduced from Pt-NMR spectra. The sulfuric acid aqueous solution of the present invention does not contain any other acids besides sulfuric acid, that is, it does not contain organic acids or other inorganic acids; specifically and explicitly, it does not contain any other intentionally added acids or substances.

[0006] In one embodiment, the sulfuric acid aqueous solution of the present invention (e.g., prepared according to the procedure of Example 1 below) is recorded using H2PtCl6 as an internal standard. 1 H-decoupling 195 The Pt-NMR spectrum may exhibit, or even more so, a low-field-width resonance A at a chemical shift δ = -837 ppm, and two high-field-width resonances B and C at chemical shifts δ = -1431 ppm and δ = -1466 ppm, respectively. The ratio of the area percentages of the three resonances may be, or even more so, in the range of 12 area% to 15 area% (resonance A, from which it can be inferred that it has a certain probability of being attributed to H2[Pt(NO2)2SO4]), in the range of 27 area% to 31 area% (resonance B, which can be attributed to an unidentified platinum species B'), and in the range of 56 area% to 60 area% (resonance C, which can be attributed to an unidentified platinum species C'), wherein the sum of the area percentages of the three resonances is 100 area.

[0007] In another embodiment, the sulfuric acid aqueous solution of the present invention (e.g., prepared according to the procedure of Example 2 below) is recorded using H2PtCl6 as an internal standard. 1 H-decoupling 195 The Pt-NMR spectrum may exhibit, or even more so, a low-field-width resonance A1 at a chemical shift δ = -834 ppm, and three high-field-width resonances B1, C1, and D1 at chemical shifts δ = -1348 ppm, δ = -1431 ppm, and δ = -1466 ppm. The ratio of the area percentages of the four resonances may be, or even more so, in the range of 10 area% to 13 area% (resonance A1, from which it can be inferred that it has a certain probability of being attributed to H2[Pt(NO2)2SO4]), 21 area% to 25 area% (resonance B1, which can be attributed to an unidentified platinum species B1'), 22 area% to 26 area% (resonance C1, which can be attributed to an unidentified platinum species C1'), and 41 area% to 45 area% (resonance D1, which can be attributed to an unidentified platinum species D1'), wherein the sum of the area percentages of the three resonances is 100 area.

[0008] This invention also relates to a preparation method for obtaining the sulfuric acid aqueous solution of this invention. This method for preparing the sulfuric acid aqueous solution of this invention eliminates the need for any cation exchange treatment, thereby avoiding the disadvantages of such treatment, such as the residue of platinum in the cation exchange resin, which typically requires arduous recovery of the platinum. For the sake of greater precision and clarity, the method of this invention does not include any cation exchange treatment.

[0009] The method of the present invention involves heating Pt(NH3)2(NO2)2 (the so-called "Pt P salt") with an aqueous sulfuric acid solution at a temperature of 80°C to 90°C, preferably >80°C to <90°C. The main reaction can be illustrated as follows: Pt(NH3)2(NO2)2 + 2H2SO4 → H2Pt(NO2)2SO4 + (NH4)2SO4. The Pt P salt is commercially available from, for example, Heraeus or other precious metal chemicals suppliers (e.g., Umicore or Johnson Matthey).

[0010] PtP salts can be added to an aqueous sulfuric acid solution, or vice versa. Advantageously, during addition, the temperature of the reaction mixture is carefully controlled and prevented from exceeding 50°C by internal or external measures of active cooling. In the method of the invention, it is preferred that the PtP salts be provided in the form of an aqueous suspension (a suspension of PtP salts in water). Preferably, an aqueous sulfuric acid solution is added to this aqueous suspension of PtP salts.

[0011] In one embodiment, the method of the present invention is carried out with an SO4:Pt weight ratio in the range of 2.5:1 to 3.5:1 or slightly below this range and a platinum content in the range of, for example, 8% to 12% by weight. After the reaction is complete, the resulting solution is diluted with water to obtain the desired platinum content in the range of 2.0% to 5.0% by weight mentioned above.

[0012] Quantitative analysis of SO4 and Pt content is well known to those skilled in the art, and both can be measured or monitored by ICP-OES analysis of sulfur in the reaction mixture (i.e., in samples taken at any time during or after the heat treatment) and by gravimetric analysis of platinum.

[0013] In an advantageous embodiment, and as already noted, heating can be carried out at an SO4:Pt weight ratio in the range of 2.5:1 to 3.5:1, slightly lower than the desired target SO4:Pt weight ratio (the final desired SO4:Pt weight ratio), and the SO4:Pt weight ratio is adjusted after heating by adding a corresponding amount of dilute sulfuric acid aqueous solution. For this purpose, it may be advantageous to carry out heating at an SO4:Pt weight ratio slightly lower than the desired target SO4:Pt weight ratio; in other words, the method of the invention is carried out at an SO4:Pt weight ratio in, for example, the range of 2.2 to 2.4:1 to 3.3 to 3.4:1 and a platinum content of 8% to 12% by weight, followed by adjusting the SO4:Pt weight ratio after heating by adding a corresponding amount of dilute sulfuric acid aqueous solution, and then diluting with water to obtain the desired platinum content in the aforementioned range of 2.0% to 5.0% by weight.

[0014] Advantageously, the reaction mixture is heated until the initially colorless reaction mixture of Pt(NH3)2(NO2)2 and dilute sulfuric acid aqueous solution eventually turns into a brown solution, after previously turning blue-green and then green. This heating treatment can be controlled by colorimetric determination. The duration of this heating period can be, for example, in the range of 60 to 90 hours. Once brown color appears, heating is stopped and the reaction is terminated. The resulting solution is then cooled, and if necessary, the SO4:Pt weight ratio and platinum content can be adjusted as described above.

[0015] The sulfuric acid aqueous solution of the present invention can be used in electroplating applications, wherein a thin and uniform platinum film can be deposited onto a substrate, such as a metallic surface (e.g., a surface of copper, brass, titanium, iron, or nickel) or even a non-metallic surface (e.g., a Nafion platinum film), by cathodic deposition. ® The substrate of the substrate. Examples of substrates specifically include substrates for PEM fuel cell applications, or substrates for producing hydrogen through water splitting reactions, such as bipolar electrodes. Example

[0016] Example 1 (according to the present invention) :

[0017] Mix 510g of sulfuric acid (96%) with 510g of deionized water and allow it to cool to room temperature.

[0018] 160 g of Pt salt was placed in a 4 L beaker with 300 mL of deionized water and stirred with a mechanical top stirrer on a hot plate equipped with a thermocouple thermometer. The previously prepared sulfuric acid aqueous solution was added at a rate that kept the solution temperature below 50 °C. After the sulfuric acid aqueous solution was added, the solution was heated at 85 °C for 4 hours. The reaction mixture changed from a turbid white suspension to a blue-green solution, and then to a translucent green solution. Once the translucent green color was observed, the reaction mixture was filtered through a Buchner funnel, weighed, and sampled to determine the platinum and sulfur content. The S content result was used to calculate the amount of sulfuric acid required to achieve an SO4:Pt weight ratio of 3.5:1; at this point, 85 g of sulfuric acid was calculated and added. The solution was transferred to a 4-necked 5 L round-bottom flask equipped with a reflux condenser, thermocouple sheath, and mechanical stirrer. The solution was then further diluted by adding 300 mL of deionized water and then heated to 90 °C until it turned dark brown. This heat treatment took 72 hours. After cooling, the reaction mixture was diluted with deionized water until the platinum content reached 4.3% by weight. The pH of the solution was 0.40.

[0019] H₂PtCl₆ was used as an internal standard to record the solution. 1 H-decoupling 195Pt-NMR spectra. The spectra show a low-field-width resonance A at a chemical shift δ = -837 ppm, and two high-field-width resonances B and C at chemical shifts δ = -1431 ppm and δ = -1466 ppm, respectively. The area percentage ratios of the three resonances are 13 area% (resonance A), 30 area% (resonance B), and 57 area% (resonance C), respectively.

[0020] Hull cell tests were conducted by placing a titanium cathode and a platinum anode in a bath containing the solution. Electrical connections were placed on the cathode and anode, and a current (0.5 amperes) was applied through the system for 600 seconds. Afterward, the cathode was removed from the bath, rinsed with water, and briefly dried. The appearance of the cathode was evaluated first, as an unstable platinum coating could lead to inaccurate calculations. Cathodes with a smooth, glossy appearance and flawless platinum coating were selected for current efficiency calculations. The experimentally measured weight increase on the cathode (W) was used to determine the current efficiency. exp ) and theoretical weight increase (W theo The current efficiency is calculated using the ratio of W to W. The following calculation determines W... theo :W theo =195.084 g / mol·0.5 amperes·600 seconds / 2·96500 coulombs / mol = 0.05 g. The measured W exp The value is 0.008980148g. Therefore, the current efficiency is calculated as follows: (W) exp / W theo )×100% = 17.96%.

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

[0022] Repeat Example 1, except that after dilution with water, the solution is heated at 85°C for 72 hours instead of at 90°C.

[0023] H₂PtCl₆ was used as an internal standard to record the solution. 1 H-decoupling 195 Pt-NMR spectra. The spectra show a low-field-width resonance A1 at a chemical shift δ = -834 ppm, and three high-field-width resonances B1, C1, and D1 at chemical shifts δ = -1348 ppm, δ = -1431 ppm, and δ = -1466 ppm. The area percentage ratios of the three resonances are 10 area% (resonance A1), 23 area% (resonance B1), 24 area% (resonance C1), and 43 area% (resonance D1), respectively.

[0024] The Hull tank test, conducted under the same conditions as in Example 1, showed a current efficiency of 18%.

[0025] Comparative Example 3

[0026] An aqueous sulfuric acid solution suitable for platinum plating was prepared following the procedure of Example V of US 3206382, having the same platinum content and pH as the solution in Example 1. Hull cell tests conducted under the same conditions as in Example 1 showed a current efficiency of 13.1%.

Claims

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

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

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

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

5. The method according to any one of the preceding claims, wherein the heating is stopped and the reaction is terminated once the initial colorless reaction mixture of Pt(NH3)2(NO2)2 and the dilute sulfuric acid aqueous solution eventually turns into a brown solution, the initial colorless reaction mixture having previously turned blue-green and then into a green solution.

6. An aqueous sulfuric acid solution suitable for electroplating platinum films, the solution containing 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 wt% to 5.0 wt%, wherein the SO4:Pt weight ratio is in the range of 2.5:1 to 3.5:

1.

7. The sulfuric acid aqueous solution suitable for electroplating platinum films according to claim 6, wherein the solution is obtained by the method according to any one of claims 1 to 5.

8. The sulfuric acid aqueous solution according to claim 6 or 7, wherein the sulfuric acid aqueous solution does not contain any acid other than sulfuric acid.

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

10. A method for electroplating a platinum film on a substrate, the method utilizing an aqueous sulfuric acid solution according to any one of claims 6 to 8.