Electroless plating bath of iridium

Abstract: The present invention relates to a plating bath of the hydrazine type for electroless plating on the surface of a plated substance with iridium. The first plating bath is an electroless plating bath of iridium which contains a hydrazine complex of iridium and has pH of 1-7. The second plating bath is an electroless plating bath of iridium which contains hydrazine hydrate and/or hydrazinium salt, and iridium halide and/or halogenoiridate in the molar ratio of 1-10 and has pH of lower than 3. The plating bath of this invention is used, for example, for producing of a junction of a cation exchange membrane and iridium, which is used for a water electrolytic cell of the macromolecule solid electrolyte type.

Description: FIELD OF THE INVENTION The present invention relates to a plating bath of the hydrazine type for electroless plating on the surface of a plated substance with iridium. This plating bath can be used, for example, not only for producing of a junction of a cation exchange membrane and iridium, which is used for a water electrolytic cell of the macromolecule solid electrolyte type but also for electroless plating of general-purpose metals such as Cu, Fe, and Ni, valve metals such as Ti, Ta, and Nb, products made of macromolecules, glass, ceramics, etc. Furthermore, the present invention relates to processes for producing junctions for electrolysis by electroless plating with iridium using the above plating bath. BACKGROUND OF THE INVENTION Polyelectrolyte water electrolytic process is known as one of water electrolytic processes used for producing hydrogen and oxygen. This is an electrolytic process using perfluorocarbon sulfonic acid membrane as solid electrolyte and supplying pure water to an anode chamber. Platinum or a carbon membrane supporting platinum is used for cathode and a cation exchange membrane joined with platinum and iridium by electroless plating (JP-B-2-20709), or a cation exchange membrane joined with a membrane supporting mixed oxide of iridium and ruthenium by hot press method (JP-A-52-78788) is used for anode as a cation exchange membrane and an electrode incorporated into this electrolytic cell. A substance having low overvoltage is used as metal or metal oxide suited for a catalytic electrode. Namely, a platinum electrode is used for hydrogen side and a iridium electrode is used for oxygen side. A process for producing Pt/M/Pt and Pt/M/Pt-lr (M is a cation exchange membrane) using electroless plating process is described in Patent Abstracts of Japan, Vol. 10, No. 4 (C-322),and in JP-A-60-162 780 and JP-B-2-20709. The plating bath used for this process contains iridium halide and hydrazine, or iridium halide, hydrazine, and hydroxylamine and the pH is 3-10, preferably 7-9. When plating is performed by use of this conventional bath liquid, iridium is deposited on platinum layer certainly. However, this process has a defect that the utilization of iridium in bath liquid is lower than 50% and that the adhesion of deposited iridium is not good. SUMMARY OF THE INVENTION The object of this invention is to provide an improved electroless plating bath of iridium which can attain stabilisation of bath liquid, improvement of the utilization of iridium and the improvement of the adhesion of deposited iridium in order to be suited for producing a junction of a cation exchange membrane and iridium. It has been found that the above object can be achieved according to the present invention by an electroless plating bath of iridium comprising specific hydrazine complexes of iridium or specific iridium compounds and hydrazine compounds and having a pH of 3 or below. Subject-matter of the present invention according to a first aspect is an electroless plating bath of iridium which comprises a bath liquid having a pH of 1-3 and containing a hydrazine complex of iridium selected from the group consisting of H[lr(N 2 H 5 )Cl 5 ], K[lr(N 2 H 5 )Cl 5 ], and mixtures thereof. The pH of the first plating bath is 1-3, preferably 2-3, more preferably 2.4-2.8. The pH is preferably controlled by adding a pH adjustor selected from a group consisting of N 2 H 4 ·. H 2 O, a hydrazinium salt, an alkali hydroxide, and a mixture thereof. The hydrazinium salt preferably is N 2 H 5 CI, N 2 H 6 CI 2 , or a mixture thereof. The hydrazinium complex of iridium is H[lr(N 2 H 5 )Cl 5 ], K[lr(N 2 H 5 )Cl 5 ], or a mixture of them. The temperature of the first plating bath is preferably about 50-100 °C, more preferably 60-90 °C. The concentration of iridium is preferably 0.5mM-5mM, more preferably 2mM-3mM. According to a second aspect a further subject-matter of this invention is an electroless plating bath of iridium which comprises hydrazine hydrate and/or hydrazinium salt, and iridium halide and/or halogenoiridate in a molar ratio of 1-10 and has a pH of 2.8 or lower. The hydrazine hydrate and/or hydrazinium salt preferably is N 2 H 4 . H 2 O, N 2 H 4 . HCl, N 2 H 4 . H 2 SO 4 , or a mixture thereof. The preferred iridium halide and/or halogenoiridate is H 2 lrCl 6 . 6H 2 O, Na 2 lrCl 6 , K 2 lrCl 6 , K 3 lrCl 6 , lrCl 3 , lrCl 4 . H 2 O, or a mixture thereof. The molar ratio of hydrazine hydrate and/or hydrazinium salt to iridium halide and/or halogenoiridate is 1-10 (namely former/latter=1 -10/1), preferably 1.3-2. The temperature of the second plating bath preferably is about 50-100 °C, more preferably 60-90 °C. The concentration of iridium in the second plating bath is preferably 0.5mM-5mM, more preferably 2mM-3mM. The pH of the second plating bath is 2.8 or lower, preferably 2.4-2.8. The pH preferably is controlled by adding a pH adjustor selected from a group consisting of N 2 H 4 . H 2 O, a hydrazinium salt, an alkali hydroxide, and a mixture thereof. The preferred hydrazinium salt is N 2 H 5 Cl, N 2 H 6 Cl 2 , or a mixture thereof. According to a third aspect and a fourth aspect the present invention refers to a process used for producing a junction for electrolysis by electroless plating with iridium, and to a process for producing a junction for electrolysis by roughening the surface of an ion exchange membrane, making the membrane adsorb the solution of a metal salt, and then performing single-sided or double-sided plating on a platinum layer with iridium using the plating bath of this invention as defined above. The following effects are achieved by using the plating bath of this invention. 1) The effective electroless plating of iridium is possible by autocatalytic reaction. 2) The stabilization and prolonging life of the plating bath of iridium for producing the junction of the cation exchange membrane and iridium, which is used for the solid polyelectrolyte water electrolytic process, are permitted, a working process of production can be shortened, and a process control is simplified. As a result, a decrease in cost for producing the junction can be achieved. 3) A process is obtained by which adhesive and high-purity iridium can be plated in desired thickness. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows sectional views of electrolytic junctions (A), (B) and (C). Figure 2 is a graph showing plating yields. Figure 3 is a graph showing autocatalytic activities of iridium. DESCRIPTION OF PREFERRED EMBODIMENTS The inventors found that synthesizing hydrazine complexes H[lr(N 2 H 5 )Cl 5 ] and K[lr(N 2 H 5 )Cl 5 ], collecting the crystals, and then immersing an ion exchange membrane joining platinum catalytic layer in the aqueous solution of the crystals, the degree of conversion of plating deposition improves to more than 85% compared to 50% achieved by the conventional process. Continuing to analyze this reaction, the inventors found that supplying the above-mentioned complexes and immersing the membrane while controlling the bath liquid so as to keep pH 1-3 during reduction, the plating bath can be used continuously more than 10 turns and that an iridium layer with desired coating thickness can be obtained. Namely, the first bath liquid of this invention is an electroless plating bath of iridium which comprises the above specific hydrazine complexes of iridium and has a pH of 1-3. This first bath liquid also makes possible to join iridium singly with a cation exchange membrane. In this case, iridium cation or cationic colloid is chemically adsorbed on the cation exchange membrane, iridium cation is reduced with an aqueous solution of a reductant such as borohydride, alkylborane, hydrazine salt, and dithionite to form an iridium catalytic layer, and then the iridium layer can be grown using the bath of this invention. Provided that in the case of the aim at water electrolysis a junction obtained by joining the ion exchange membrane with iridium directly does not have satisfactory corrosion resistance against a sulfonic acid membrane and satisfactory catalytic ability for recombination of diffusing gases (O 2 in H 2 , H 2 in O 2 ) in the membrane and the purity of the gases is low. Namely, the corrosion resistance (resistance to dissolution in the membrane) of metals (namely, Pt and Ir) in contact with the sulfonic acid membrane is expressed by relationship Pt > Ir, and the catalytic ability for recombination of the diffusing gases (O 2 in H 2 , H 2 in O 2 ) in the membrane from the junction electrode is also expressed by relationship Pt > Ir. The kinds of junctions are shown in Figure 1(A), (B), and (C). (A) is a junction which consists of Pt/M/Pt and is excellent in corrosion resistance against the sulfonic acid membrane and in purity of the formed gases, but it has a defect that overvoltage to the oxygen evolution is high. (B) is a junction in which the sulfonic acid membrane is directly plated with iridium. In this case, the junction is effective for lowering oxygen overvoltage, but it has a defect that the corrosion resistance is low and that the purity of formed oxygen gas is also low. Compared with them, a junction of Pt/M/Pt-lr type shown in (C) can obtain excellent ability in all respects of corrosion resistance, purity of formed gases, and oxygen overvoltage. The pH of electroless plating bath of iridium is 3-10, preferably 7-9 in the known process. It had been thought that platinum acts as a catalyst under this condition, and that incipient reaction and reaction proceed on the platinum surface but the reaction stops when platinum is covered. The inventors reexamined reduction which had been thought that iridium has no autocatalytic ability to hydrazine in the conventional electroless plating of iridium, and found that inactivation of the Ir surface occurs when the surface is covered with a hydrated iridium oxide layer and that this is attributed to NH 3 formed by side reaction of hydrazine salt added excessively, and completed this invention. The complex used for the plating bath of this invention is H[lr(N 2 H 5 )Cl 5 ], K[lr(N 2 H 5 )Cl 5 ], or a mixture thereof. K[lr(N 2 H 5 )Cl 5 ] can be obtained as high-purity crystals. An initial make-up of electrolytic bath liquid can be also prepared using an intact reaction mixture liquid after complexing without collecting H[lr(N 2 H 5 )Cl 5 ] as crystals. In this case, the inclusion of a by-product salt has some influence, and the utilization of iridium slightly falls, but the fall is within 5%, and there is hardly trouble when the complex is used in a batch type bath. The control of bath liquid composition can be performed by adjusting pH, Ir concentration, temperature, etc., and the supply can be performed by use of an aqueous solution of hydrazine complex of iridium, hydrazinium salt, N 2 H 4 ·. H 2 O, KOH, NaOH, etc. The pH is 1-3, preferably 2-3, more preferably 2.4-2.8. When the pH exceeds 3, the Ir surface is subject to inactivation. Since the rate of reduction falls remarkably when the pH is lower than 1, the plating bath becomes unpractical. The temperature of the bath liquid is about 50-100 °C, preferably 60-90 °C. The growth rate of plating is slow below 50 °C, and the evaporation loss of the bath liquid is high above 100 °C, which is undesirable for operation. The preferred concentration of iridium is 0.5mM-5mM, more preferably 2mM-3mM. A continuous bath liquid can be also used controlling the concentration of iridium. The plating bath with the above-mentioned concentration of iridium is used for the initial make-up of electrolytic bath liquid in the case of the batch type bath. It is preferable to supply N 2 H 4 . H 2 O or hydrazinium salt in order to control pH lowered with the progress of plating. A pH adjustor selected from a group consisting of N 2 H 4 . H 2 O, a hydrazinium salt, an alkali hydroxide, and a mixture thereof can preferably be used for the control of pH. N 2 H 5 CI, N 2 H 6 Cl 2 , etc. are preferred examples of the hydrazinium salt. Iridium acts as an autocatalyst only under the above mentioned plating condition. Accordingly, it is possible to perform thick plating of iridium with metallic gloss on the iridium surface continuously. This makes the plating bath of this invention particularly preferable as a bath liquid for joining iridium on the membrane surface of a macromolecular membrane such as an ion exchange membrane. Such junction is used for water electrolysis, halogeno acid electrolysis, halide electrolysis as a solid polyelectrolyte electrolytic process. Industrial materials such as electronic parts or electrode materials of metals, e.g., copper, nickel, iron alloy thereof, titanium, tantalum, etc. are given as objects to which the plating bath of this invention can be applied other than the above-mentioned objects. The plating bath can be also applied to materials which can undergo ordinary electroless plating such as synthetic resin, e.g., ABS resin, polyamide resin, polycarbonate resin, etc., glass, ceramics, etc. It is preferable to perform surface active treatment of the plated substance in advance. In the case of metal, after cleaning the surface of the metal, the metal is immersed in a solution of a salt such as palladium, platinum, rhodium, ruthenium, gold, silver etc. If necessary, the metal is activated by immersion reduction treatment in a solution of borohydride, etc. continuously, and then the metal is immersed in the plating bath of this invention. In the case of macromolecular materials, glass, and ceramics, after ordinary surface hydrophiling, sensitization treatment, and the same active treatment as that in the case of the above-mentioned metal, the pretreated substance is also immersed in the plating bath of this invention. It is preferable that the junction for solid polyelectrolyte water electrolysis consists of Pt/M/Pt-lr for the above-mentioned reason. First, for example, the Pt/M/Pt junction is prepared by the adsorption-reduction process according to the process described in JP-B-2-20709 to obtain this junction. Namely, the surface of the ion exchange membrane is roughened, the solution of the metal salt (for example, the salt of platinum, palladium, rhodium, iridium, ruthenium, etc.) was adsorbed by the membrane. Subsequently, if necessary, the membrane is reduced by a sodium borohydride solution, a hydrazine solution, etc. to form the first layer of about 0.1-1 µm, and then single-sided or double-sided plating is performed on the platinum layer with iridium using the plating bath of this invention. Iridium is an essential catalyst to lower oxygen overvoltage at the anode, but it is not particularly significant to join iridium with the cathode. Joining iridium with the cathode is performed for the purpose of labor-saving, automating of plating operation, and avoiding pollution in the membrane on plating. If single-sided plating is desired, plating can be performed putting two membranes upon each other or covering the other side with resist. The plating bath of this invention does not need hydroxylamine salt, which has been needed in order to stabilize the bath liquid in the above-mentioned conventional process. Filtering off fallen catalytic metal powder in the bath liquid carefully and circulating the bath liquid as performed in general electroless plating, the plating bath can be used for a long time without autodecomposition. In addition, the inventors found that the following conditions are important: 1) A supply of necessary and sufficient amount of hydrazine to inhibit side reaction of excess N 2 H 4 ; 2) Prevention of inactivation of the Ir surface by keeping the pH at 2.8 or lower. To promote the point 1) effectively, the inventors prepared an aqueous solution containing hydrazine hydrate and/or hydrazinium salt, and iridium halide and/or halogenoiridate in a proportion approximate to the component ratio of the hydrazine complex of iridium and examined the solution. As a result, it was found that it is preferable to keep the pH at 2.8 or lower in order to reduce the influence of the by-product NH 3 and to adjust the molar ratio of hydrazine/iridium 1-10 to satisfy the above-mentioned two conditions. Namely, the second bath liquid of this invention is an electroless plating bath of iridium which comprises hydrazine hydrate and/or hydrazinium salt, and iridium halide and/or halogenoiridate in a molar ratio of 1-10 and has pH of 2.8 or lower. N 2 H 4 . H 2 O, N 2 H 4 . HCI, N 2 H 4 . H 2 SO 4 , etc. are preferred examples of hydrazine hydrate and/or hydrazinium salt used for the second plating bath. These are used singly or in combination. H 2 lrCl 6 . 6H 2 O, Na 2 lrCl 6 , K 2 lrCl 6 , K 3 lrCl 6 , lrCl 3 , lrCl 4 . H 2 O, etc. are preferred examples of iridium halide and/or halogenoiridate. These are used singly or in combination. The molar ratio of hydrazine hydrate and/or hydrazinium salt to iridium halide and/or halogenoiridate is 1-10 (namely, former/latter=1-10/1), preferably 1.3-2. When this molar ratio is less than 1, a reducing agent is insufficient and an excess of iridium halide and/or halogenoiridate remains, which results in a lowering of plating yield. It is preferable to keep the upper limit of this molar ratio at about 10 mainly in terms of economy. The concentration of iridium in the bath liquid is preferably 0.5mM-5mM, more preferably 2mM-3mM. The liquid is used continuously controlling this concentration, or the plating bath with the above-mentioned concentration of iridium is used for the initial make-up of electrolytic bath liquid in the case of the batch type bath. The control of the bath liquid is performed adjusting pH, Ir concentration, temperature, and the supply is performed by use of the above-mentioned iridium compound, hydrazinium salt, and alkali hydroxide. The pH of the second plating bath is kept at 2.8 or lower, preferably at 2.4-2.8. When this pH is higher than 2.8, iridium tends to inactivate. It was found that the utilization of Ir is remarkably improved compared with the conventional process as shown in figure 2 as a result of the batch test. In addition, it was found that iridium acts as an autocatalyst and that the coating thickness increases as shown in figure 3 when the pH is 2.4-2.8. Other constitutions, plating process, etc. of the second plating bath are the same as those of the first plating bath. EXAMPLES The present invention is described in detail by giving examples as follows. Synthesis of iridium-hydrazine complexes Iridium-hydrazine complexes were synthesized according to the method in Gmelin, "Handbuch der Anorganishen Chemie Ir. (1978), p.188, (Berichte der Deutschen Chemischen Gesellshaft, 56 , 2067 (1923) cited there)). Synthesis 1 (Synthesis of H[Ir(N 2 H 5 )Cl 5 ]) Two hundred millilitre of 10% N 2 H 4 · HCl was added to 15g of K 2 [IrCl 6 ] and the mixed liquid was heated on a water bath. After the evolution of gas ceased, the reaction was stopped. A pinkish brown solution was obtained. Concentrating the solution under reduced pressure, the crystals were collected, and 0.8g of primary crystals were obtained. The purity of the composition H[Ir(N 2 H 5 )Cl 5 ] was not high. Synthesis 2 (Synthesis of K[Ir(N 2 H 5 )Cl 5 ]) [Pt(NH 3 ) 4 ]Cl 2 was added to the pinkish brown solution obtained by synthesis 1 to precipitate Pt(NH 3 ) 4 [Ir(N 2 H 5 )Cl 5 ] 2 . After this precipitation was taken out, K 2 [PtCl 4 ] was added to the aqueous solution of this precipitation to double-decompose Pt(NH 3 ) 4 [Ir(N 2 H 5 )Cl 5 ] 2 . Removing the precipitation of the Pt salt, the reaction liquid was concentrated under reduced pressure to give crystals of K[Ir(N 2 H 5 )Cl 5 ]. Synthesis 3 (Synthesis of K[Ir(N 2 H 5 )Cl 5 ]) Three grams of K 2 [IrCl 6 ] was dissolved in 300ml of boiling water and 140 ml of 10% N 2 H 4 · HCl was slowly added. After the addition, the evolution of gas soon ceased and a reddish brown solution was obtained. This solution was concentrated to 10ml, cooled to room temperature, and then a concentrated aqueous solution containing 2.3g of [Pt(NH 3 ) 4 ]Cl 2 was added thereto to precipitate [Pt(NH 3 ) 4 ] [Ir(N 2 H 5 )Cl 5 ] 2 immediately. This precipitation was filtered off and washed thoroughly with 3N hydrochloric acid. This precipitation was added to a concentrated aqueous solution containing 2.6g of K 2 [PtCl 4 ]. Stirring was continued for 2 hours at room temperature, K[Ir(N 2 H 5 )Cl 5 ] was extracted in the solution, and an insoluble matter was filtered out. Finally, the solution was evaporated to dryness to give crystals of K[Ir(N 2 H 5 )Cl 5 ]. Example 1 The surface of a perfluorocarbonsulfonic acid cation exchange membrane "Nafion 117" (du Pont Co.) was roughened by sand blast, followed by boiling the membrane with 10% hydrochloric acid, and washing it with hot water. This pretreated membrane was set in an acrylic plating cell, immersed in a 1mg/ml aqueous tetraammineplatinum solution, and allowed to stand for 3 hours. After water washing, the membrane was immersed in a 0.05% aqueous NaBH 4 solution at room temperature to 60 °C for 4 hours, and an about 1mg/cm 2 (membrane area) of platinum layer was deposited on the surface of the reaction membrane by reduction. This membrane is termed Pt junction membrane hereinafter. An iridium plating bath having the following composition was prepared using K[Ir(N 2 H 5 )Cl 5 ]. K[Ir(N 2 H 5 )Cl 5 ] 1.0g (Ir: 0.433g) water 750ml pH (initial) 2.8 The iridium plating bath having the above-mentioned composition was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N N 2 H 4 · H 2 O was added to the bath liquid with a micropump connected to a pH controller to keep the pH 2.2-2.8. After 4 hours, 4.08mg/cm 2 (membrane area) of an iridium layer was obtained. The plating yield was 94.2%. This result is shown in Figure 2. Example 2 An iridium plating bath having the following composition was prepared using H[Ir(N 2 H 5 )Cl 5 ]. H[Ir(N 2 H 5 )Cl 5 ] 1.0g (Ir: 0.401g) water 750ml pH (initial) 2.8 (adjusted with N 2 H 4 · H 2 O) The iridium plating bath having the above-mentioned composition was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N N 2 H 4 · H 2 O was added to the bath liquid with the micropump connected to the pH controller to keep the pH 2.3-2.8. After 4 hours, 3.75mg/cm 2 (membrane area) of an iridium layer was obtained. The plating yield was 93.5%. Example 3 The same iridium plating bath as that of Example 1 was prepared. The above-mentioned iridium plating bath was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N N 2 H 5 Cl was added to the bath liquid with the micropump connected to the pH controller to keep the pH about 2.8. After 4 hours, 4.33mg/cm 2 (membrane area) of an iridium layer was obtained. The plating yield was 99.9%. Example 4 The same iridium plating bath as that of Example 1 was prepared. The above-mentioned iridium plating bath was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N N 2 H 4 · H 2 O was added to the bath liquid with the micropump connected to the pH controller to keep the pH about 2.8. After 4 hours, 4.32mg/cm 2 (membrane area) of an iridium layer was obtained. Then the plating yield was 99.9%. Example 5 The same iridium plating bath as that of Example 1 was prepared. The above-mentioned iridium plating bath was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N NaOH was added to the bath liquid with the micropump connected to the pH controller to keep the pH about 2.8. After 4 hours, 4.05mg/cm 2 (membrane area) of an iridium layer was obtained. The plating yield was 94.0%. Example 6 The iridium plating bath was used continuously in this example. The iridium plating bath was prepared in the same manner as that of Example 1, the iridium plating bath was circulated in the plating cell, and an iridium layer was grown on the Pt junction membrane. Secondly, K[Ir(N 2 H 5 )Cl 5 ] was added to supply consumed iridium, the concentration of K[Ir(N 2 H 5 )Cl 5 ] was kept 2-3mM, and the iridium layer was grown on the Pt junction membrane prepared by the alternate process. This operation was repeated 10 times and the iridium plating bath was used continuously. In addition, the above-mentioned iridium plating bath was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N N 2 H 5 Cl was added to the bath liquid with the micropump connected to the pH controller to keep the pH 2.0-3.0. The 3mg/cm 2 (membrane area) of iridium layers were formed on each Pt junction membrane. The plating yields of this continuous layer plating were kept over 90%. This result is shown in Table 1. (Plating result by continuous use of iridium bath liquid) Number of growth Initial amount of Ir (g) Growth amount of Ir (mg/cm 2 ) Ir plating yield (%) 1 0.435 4.07 93.9 2 0.416 3.97 93.3 3 0.423 4.05 93.6 4 0.431 4.02 92.4 5 0.436 3.90 92.0 6 0.440 3.93 92.2 7 0.445 3.85 91.0 8 0.441 3.87 90.5 9 0.450 3.95 90.6 10 0.452 3.91 90.3 Example 7 The Pt junction membrane was formed on the cation exchange membrane in the same manner as that of Example 1. The iridium plating bath having the following composition was prepared using K 2 IrCl 6 . K 2 IrCl 6 1.1g (Ir: 0.435g) N 2 H 4 · HCl 0.31g water 750ml pH (initial) 2.8 The iridium plating bath having the above-mentioned composition was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N N 2 H 5 Cl was added to the bath liquid with the micropump connected to the pH controller to keep pH 2.3-2.8. After 4 hours, 3.85mg/cm 2 (membrane area) of an iridium layer was obtained. The plating yield was 88.6%. This result is shown in Figure 2. Example 8 The iridium plating bath having the following composition was prepared using K 2 IrCl 6 . K 2 IrCl 6 1.1g (Ir: 0.435g) N 2 H 4 · HCl 0.31g water 750ml pH (initial) 2.8 The iridium plating bath having the above-mentioned composition was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N N 2 H 5 Cl was added to the bath liquid with the micropump connected to the pH controller to keep the pH about 2.8. After 4 hours, 4.35mg/cm 2 (membrane area) of an iridium layer was obtained. The plating yield was 99.9%. Example 9 The iridium plating bath having the following composition was prepared using H 2 IrCl 6 . H 2 IrCl 6 0.91g (Ir: 0.430g) N 2 H 4 · HCl 0.31g water 750ml pH (initial) 2.8 (adjusted with 1/10N NaOH) The iridium plating bath having the above-mentioned composition was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N NaOH was added to the bath liquid with the micropump connected to the pH controller to keep the pH about 2.4-2.8. After 4 hours, 3.79mg/cm 2 (membrane area) of an iridium layer was obtained. The plating yield was 88.1%. Example 10 The iridium plating bath having the following composition was prepared using Na 2 IrCl 6 . Na 2 IrCl 6 1.0g (Ir: 0.430g) N 2 H 4 · HCl 0.28g water 750ml pH (initial) 2.8 (adjusted with 1/10N HCl) The iridium plating bath having the above-mentioned composition was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N N 2 H 4 · H 2 O was added to the bath liquid with the micropump connected to the pH controller to keep the pH 2.4-2.8. After 4 hours, 3.83mg/cm 2 (membrane area) of an iridium layer was obtained. The plating yield was 89.1%. Example 11 The iridium plating bath having the following composition was prepared using IrCl 3 . First, 6.7g of iridium (III) chloride was dissolved in 500ml of water. On the other hand, a solution in which 3.1g of hydrazinium chloride was dissolved in 300ml of water was prepared. The aqueous hydrazinium solution and 2ml of concentrated hydrochloric acid were added to the aqueous iridium chloride solution with stirring, the temperature was raised, and kept at 90 °C. Then, the aqueous iridium complex solution was concentrated to about 50ml, cooled to room temperature, and then the volume of the solution was adjusted to 100ml. Ten millilitre of the solution was collected, this solution was diluted to 750ml with water, and 1/10N NaOH was added thereto to adjust the pH 2.8. This solution was used as an iridium plating bath. The iridium plating bath having the above-mentioned composition was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N N 2 H 5 Cl was added to the bath liquid with the micropump connected to the pH controller to keep the pH 2.2-2.7. After 4 hours, 3.79mg/cm 2 (membrane area) of an iridium layer was obtained. The plating yield was 87.3%. Example 12 The Pt junction membrane was formed on the cation exchange membrane, and the iridium plating bath was prepared in the same manner as that of Example 7. The above-mentioned iridium plating bath was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N N 2 H 4 · H 2 O was added to the bath liquid with the micropump connected to the pH controller to keep the pH near 2.8. After 4 hours, 4.32mg/cm 2 (membrane area) of an iridium layer was obtained. The plating yield was 99.9%. Example 13 The same iridium plating bath as that of Example 7 was prepared. The iridium plating bath having the above-mentioned composition was circulated in the plating cell while the bath temperature was kept at 70 °C, and 1/10N NaOH was added to the bath liquid with the micropump connected to the pH controller to keep the pH near 2.8. After 4 hours, 3.89mg/cm 2 (membrane area) of an iridium layer was obtained. The plating yield was 90.3%. Example 14 The iridium plating bath was used continuously. The iridium plating bath was prepared in the same manner as that of Example 7, the iridium plating bath was circulated in the plating cell, and an iridium layer was grown on the Pt junction membrane. Secondly, K 2 IrCl 6 was added thereto to supply consumed iridium, the concentration of K 2 IrCl 6 was kept 2-3mM, and the iridium layer was grown on the Pt junction membrane prepared by the alternate process. This operation was repeated 10 times and the iridium plating bath was used continuously. The plating yields of this continuous layer plating were kept above 85%. (Plating result by continuous use of iridium bath liquid) Number of growth Initial amount of Ir (g) Growth amount of Ir (mg/cm 2 ) Ir plating yield (%) 1 0.433 3.88 89.5 2 0.426 3.79 89.0 3 0.433 3.86 89.2 4 0.435 3.88 89.1 5 0.424 3.77 88.8 6 0.426 3.79 89.0 7 0.423 3.74 88.3 8 0.428 3.77 88.1 9 0.436 3.83 87.8 10 0.433 3.77 87.1 Example 15 The presence of autocatalysis of iridium was confirmed in this example. The relationship between the initial amount of Ir and the amount of Ir deposited on the Pt junction membrane was examined under the condition of reaction temperature of 70 °C and reaction time of 4 hours about the iridium complex process (K 2 IrCl 6 /NH 2 NH 2 · HCl system, pH 2.3-2.8) according to the present invention and the conventional process (K 2 IrCl 6 /NH 2 NH 2 /NH 2 OH · HCl system, pH 7.0-7.2). In the case of the iridium complex process, the proportionality holds between the initial amount of Ir and the growth amount of Ir, and the growth rate of Ir to the initial amount of Ir was always over 90%. On the other hand, in the case of the conventional process, when the initial amount of Ir is less than 250mg, the proportionality is found between the initial amount of Ir and the growth amount of Ir. However, even if the initial amount of Ir exceeds 250mg, the growth amount of Ir is constant at about 200mg. This result shows that Ir is deposited on the Pt surface and that the Ir layer becomes thick, namely, that the Ir layer grows by the autocatalytic ability of Ir in the case of using the iridium complex. However, the iridium complex is inactivated by by-product NH 3 in the case of the conventional process. This shows that Ir grows while the Pt surface is exposed but the reaction stops when Ir covers the Pt surface. These results are shown in Figure 3. Comparative example 1 The iridium plating bath having the following composition was prepared using potassium chloroiridate (IV). K 2 IrCl 6 0.626g (Ir: 0.249g) 5%NH 2 OH · HCl 20ml 20%N 2 H 4 · H 2 O 8ml water 500ml pH (initial) 7.2 The Pt junction membrane was immersed in the plating bath having the above-mentioned composition, and the bath temperature was kept at 70 °C for 4 hours. Meanwhile, without adjusting the pH, the pH rises to 7.2-9.1. After 4 hours, 1.19mg/cm 2 of an iridium layer was obtained. The plating yield was 47.8%. This result is shown in Figure 2. Comparative example 2 The iridium plating bath was prepared in the same manner as that of Example 1. Secondly, 1/10N NaOH was added to the bath liquid to adjust the pH 7.0. The above-mentioned iridium plating bath was circulated in the Pt junction membrane, and 1/10N NaOH was added to the bath liquid at 70 °C to keep the pH near 5. However, since autodecomposition of the iridium complex proceeded in the course of the growth reaction and the Ir metal was deposited, it was impossible to plate the Pt junction membrane with iridium selectively.

Claims: An electroless plating bath of iridium which comprises a bath liquid having a pH of 1-3 and containing a hydrazine complex of iridium selected from the group consisting of H[lr(N 2 H 5 )Cl 5 ], K[lr(N 2 H 5 )Cl 5 ], and mixtures thereof. The plating bath according to claim 1, wherein said pH is 2.4-2.8. The plating bath according to claim 1, wherein said bath liquid has a temperature of 50-100°C. The plating bath according to claim 1, wherein said bath liquid has an iridium concentration of 0.5mM-5mM. The plating bath according to claim 4, wherein said bath liquid has an iridium concentration of 2mM-3mM. The plating bath according to claim 1, wherein said pH is controlled by adding a pH adjustor, selected from the group consisting of N 2 H 4 · H 2 O, a hydrazinium salt, alkali hydroxide, and a mixture thereof. The plating bath according to claim 6, wherein said hydrazinium salt is selected from N 2 H 5 Cl, N 2 H 6 Cl 2 , or a mixture thereof. An electroless plating bath of iridium which comprises hydrazine hydrate and/or hydrazinium salt, and iridium halide and / or halogenoiridate in a molar ratio of 1 - 10 and has a pH of 2.8 or lower. The plating bath according to claim 8, wherein said hydrazine hydrate and/or hydrazinium salt is N 2 H 4 . H 2 O, N 2 H 4 . HCI, N 2 H 4 . H 2 SO 4 , or a mixture thereof. The plating bath according to claim 8, wherein said iridium halide and/or halogenoiridate is H 2 lrCl 6 . 6H 2 O, Na 2 lrCl 6 , K 2 lrCl 6 , K 3 lrCl 6 , IrCl 3 , IrCl 4 .H 2 O, or a mixture thereof. The plating bath according to claim 8, wherein said molar ratio of hydrazine hydrate and/ or hydrazinium salt to iridium halide and/or halogenoiridate is 1 - 10 ( namely former/latter= 1 - 10/1). The plating bath according to claim 11, wherein said molar ratio is 1.3-2. The plating bath according to claim 8, wherein the temperature of the bath liquid is 50-100°C. The plating bath according to claim 8, wherein said iridium concentration in the bath liquid is 0.5mM-5mM. The plating bath according to claim 14, wherein said iridium concentration in the bath liquid is 2mM-3mM. The plating bath according to claim 8, whose pH is 2.4-2.8. The plating bath according to claim 8, wherein said pH is controlled by adding a pH adjustor selected from the group consisting of N 2 H 4 · H 2 O, a hydrazinium salt, alkali hydroxide, and a mixture thereof. The plating bath according to claim 17, wherein said hydrazinium salt is selected from N 2 H 5 CI, N 2 H 6 Cl 2 , or a mixture of them. A process for producing a junction for electrolysis by electroless plating with iridium using the plating bath according to any of claims 1 to 18. A process for producing a junction for electrolysis by roughening the surface of an ion exchange membrane making the membrane adsorb a solution of a metal salt, and then performing single-sided or double-sided plating on a platinum layer with iridium using the plating bath according to any of claims 1 to 18.

Inventors: MAEZAWA SHOJI, MORI HIROAKI, OGURO KEISUKE, TORIKAI EIICHI, MAEZAWA, SHOJI, MORI, HIROAKI, OGURO, KEISUKE, TORIKAI, EIICHI
Applicants: AGENCY IND SCIENCE TECHN, HITACHI SHIPBUILDING ENG CO, NIPPON KOKAN KK, RES INST INNOVATIVE TECH EARTH, AGENCY OF INDUSTRIAL SCIENCE AND TECHNOLOGY, HITACHI ZOSEN CORPORATION, NKK CORPORATION, RESEARCH INSTITUTE OF INNOVATIVE TECHNOLOGY FOR THE EARTH
Application year: 1995
Main classification: C23C18
IPC codes: C25B9, C23C18