US20140170328A1

US20140170328A1 - Electroless plating of ruthenium and ruthenium-plated products

Info

Publication number: US20140170328A1
Application number: US14/105,866
Authority: US
Inventors: David Anthony Boyd; Anwu Li
Original assignee: Noram Engineering and Constructors Ltd
Current assignee: Noram Engineering and Constructors Ltd
Priority date: 2012-12-14
Filing date: 2013-12-13
Publication date: 2014-06-19

Abstract

An electroless plating Ru bath for the deposition of Ru on the surface of a substrate comprises a Ru stock solution and hydrazine as a reducing reagent. Ru layers may be applied, for example, for use in membranes for the separation of hydrogen gas from mixtures of gases or to protect materials from corrosion. An example Ru stock solution comprises Ru chloride, hydrochloric acid, ammonia, nitrite salt, alkali hydroxide, and deionized water. The electroless plating bath may be applied to deposit ruthenium layers onto palladium layers to prepare Pd—Ru composite or alloy membranes or multilayer Pd—Ru composite or alloy membranes. Such membranes have example application to the separation of hydrogen from mixtures of gases.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119 of U.S. Application No. 61/737664 filed 14 Dec. 2012 and entitled ELECTROLESS PLATING OF RUTHENIUM AND RUTHENIUM-PLATED PRODUCTS which is hereby incorporated herein by reference for all purposes.

FIELD OF THE INVENTION

This invention relates to electroless plating, in particular to plating baths and associated methods for the electroless plating of ruthenium and to novel Ru-plated products including but not limited palladium-Ru composite membranes useful for applications such as hydrogen separation.

BACKGROUND

Hydrogen may be used as an energy carrier. Hydrogen is also used extensively to make products such as ammonia, methanol, gasoline, heating oil, and rocket fuel. There is a need for inexpensive and reliable ways to obtain pure hydrogen.
Currently, hydrogen is mainly produced via steam methane reforming, followed by conventional separation techniques, such as high and low shift reactions followed by pressure swing adsorption. Membrane separation using hydrogen-selective membranes is a cost-effective method for separating hydrogen from mixtures of gases at elevated temperatures.
Pd-based membranes prepared by electroless plating have been used for hydrogen separation. Such membranes can have high resistance to hydrogen embrittlement and oxidation, good thermal stability, favorable catalytic activity for hydrogen dissociation and recombination, and appropriate hydrogen permeability. Because Pd is expensive and its hydrogen permeability is generally inversely proportional to its thickness, Pd-based membranes are usually prepared in composite form comprising thin Pd-based layers, to provide high hydrogen permselectivity.
The literature describes some Pd, Pd—Cu, and Pd—Ag membranes. Such membranes can be difficult to alloy into homogeneous layers due to the different melting points of the metals which make them up. Such membranes also tend to be unstable at elevated temperatures. In addition, Pd and Pd—Ag composite membranes suffer severe hydrogen embrittlement problems at temperatures below 300° C. and 200° C., respectively.
Pd—Ru alloy foils are more stable at both high and lower temperatures than other Pd-based membranes. However, Pd—Ru alloy is hard and is therefore difficult to form into foils. As a result, Pd—Ru alloy foils are typically thick (>75 μm), limiting their application on a large-scale due to the expense of Pd.
Electroless plating is an auto-catalytic process that uses a reducing agent to deposit a thin layer of material, such as copper, nickel, silver, gold, Pd, or Ru, on the surface of some solid substrate by means of electrochemical reactions. Electroless deposition is typically performed in an aqueous solution containing metal ions, a reducing agent, complexing and buffering agents, and stabilizers. Unlike electroplating methods, electroless plating does not require an externally applied electric current to drive the deposition reaction Electrons derived from the heterogeneous oxidation of the reducing agent at a catalytically active region of the substrate surface reduce metal ions to metal atoms, which deposit on the substrate surface. Under the right conditions, a continuous metal deposit can be obtained.
Electroless plating has several advantages over techniques, such as evaporation and sputtering. Compared to these other techniques, electroless plating uses materials and equipment that are relatively inexpensive. Although electroless plating appears to be a simple and inexpensive technique, the chemical reactions occurring at the substrate surface can be complex. Conditions such as temperature, ion concentration in the plating bath, and the duration of time the substrate is contacted with the plating bath can affect plating quality and thickness. Electroless plating can be sensitive to contaminants.
Ru can be difficult to deposit by electroless plating because the Ru cation typically exists in multiple oxidation states. This can result in disproportionation reactions that take place over desirable heterogeneous film growth on a substrate. Methods to deposit Ru by electroless plating using various reducing reagents, such as sodium hypophosphite and sodium borohydride, have been proposed in the literature however, the layers of Ru deposited by such methods tend to be of low quality (e.g. affected by severe hydrogen embrittlement and/or co-deposited impurities).

BRIEF DESCRIPTION OF THE DRAWINGS

An example embodiment illustrating an application of the invention is illustrated in the attached figure of the drawings. The figure disclosed herein is illustrative and non-limiting.

FIG. 1 is a schematic illustration showing a stainless steel tube or disk with a porous ceramic base plated with a Pd—Ru composite membrane.

FIG. 2 is a sample image of a Pd—Ru composite membrane distributed on a stainless steel disk.

FIG. 3 is a sample image of a Pd—Ru composite membrane distributed on a stainless steel disk, as captured by a scanning electron microscope (SEM).

FIG. 4 is an example of the permeation flux and the selective permeation flux of hydrogen gas across a Pd—Ru composite membrane as a function of time.

DETAILED DESCRIPTION

Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive sense.
One aspect of this invention relates to electroless plating of Ru. This aspect provides a bath for electroless plating of Ru comprising an Ru stock solution and a hydrazine reducing agent. The bath has a pH of at least 13. Such a bath may be applied, for example, to the production of Ru-based membranes useful for hydrogen separation or other purposes. The Ru-based membranes have many applications, including the separation of hydrogen from mixtures of gases and the protection of surfaces used in corrosive environments from corrosion. Other aspects of this invention provide methods for depositing Ru onto the surface of a substrate by electroless plating.
In an example embodiment, a Ru stock solution comprises a source of Ru ions and a water soluble nitrite salt. In some embodiments the Ru stock solution comprises a source of Ru ions, a water soluble nitrite salt, and any one or more of the following: hydrochloric acid, a complexing reagent, and an alkali hydroxide.
Ruthenium chloride is a preferred source of Ru ions, however, persons skilled in the art will recognize that other suitable sources of Ru ions such as, for example, ruthenium nitrate, K₂[Ru(NO)Cl₅], or [Ru(NO)(NH₃)₅]Cl₃may be used in the alternative.
When mixed with Ru ions, the complexing reagent forms a Ru complex that is stable but reducible under electroless plating bath conditions. NH₃H₂O is a preferred complexing reagent, however, persons skilled in the art will recognize that other suitable complexing reagents that form stable but reducible complexes with Ru may be used in the alternative.
The alkali hydroxide comprises an alkali metal cation (e.g. Na⁺ or K⁺) and hydroxide anions (OH⁻). The alkali hydroxide is provided in an amount suitable to control pH. Sodium nitrite is the preferred water soluble nitrite salt, however any suitable nitrite salt may be used.
An example Ru stock solution can be obtained by dispersing Ru chloride in deionized water and adding small amounts of HCl, ammonia, alkali hydroxide, and NaNO₂. In some cases, the molar ratio of ammonia/Ru is at least 150, and in some cases is in excess of 200. The molar ratio of NO₂ ⁻/Ru may be at least 1, and in some cases is in excess of 5. In some cases, the molar ratio of NO₂ ⁻/Ru is in the range of 3 to 5.
For example, the Ru stock solution can comprise 1.0×10⁻³M to 2.0×10⁻¹M RuCl₃×H₂O (Ru mass, 38%), 4.0×10⁻²M to 5.0×10⁻¹M HCl, 1.0 M to 21 M NH₃H₂O (28%), 1.0×10⁻¹M to 1.0 M NaOH, and 4.0×10⁻²M to 4.5×10⁻¹NaNO₂in deionized water. In some embodiments the concentration of Ru ions in the stock solution is about 1.0×10³M to about 2.0×10¹M. In some embodiments, the Ru stock solution comprises 0.10% weight to 0.51% weight Ru, 0.15% weight to 1.46% weight HCl, 1.7% weight to 35.7% weight NH₃, 0.4% weight to 4% weight NaOH, and 0.28% weight to 3.11% weight NaNO₂in deionized water. Further, in some embodiments, the Ru stock solution comprises 1.8×10⁻²M RuCl₃×H₂O (Ru mass, 38%), 8.8×10⁻²M HCl, 6.6 M NH₃H₂O (28%), 2.2×10⁻¹M NaOH, and 5.7×10⁻²M NaNO₂in deionized water.
The hydrazine reducing reagent is added to the Ru stock solution. The resulting electroless plating Ru bath has a pH of at least 13. In some cases, the pH is in the range of 13 to 14. In some cases, the pH is at least 13.3. In some cases the pH is not greater than 13.7. For example, the pH may be in the range of 13.3 to 13.7.
The molar ratio of hydrazine/Ru is at least 2 in some embodiments. The molar ratio is typically kept below 30 and is typically in the range of 5 to 20. However, this is not mandatory. In some cases the molar ratio of hydrazine/Ru is in excess of 30.
A stock solution as described above may be used to deposit a layer of Ru onto a substrate by electroless plating. Any of a wide variety of materials may be used as a substrate. For example, the substrate may comprise porous or nonporous stainless steel, porous ceramic, graphite, carbon, properly activated glass, Cr, Co, Au, Fe, Mo, Ni, Pd, Pt, Rh, Ru, Ag, Al, Sn, or W. In applications for which a Ru-containing layer is intended to be used for gas separation or similar applications the substrate may be gas-permeable (e.g. the substrate may be porous). The substrate may have any suitable form. For some applications, substrates in the form of sheets, tubes or disks are convenient. To achieve uniform surface coatings of Ru by electroless plating it is desirable that the surface of the substrate be smooth.
Prior to using a bath to perform electroless plating of Ru the substrate can be activated with a catalyst, such as Pd nuclei, in order to shorten the induction period at the beginning of Ru plating. For example, substrate activation may comprise one or more cycles of impregnation of the substrate in dilute Pd chloride solution and drying. This may be followed by hydrogen reduction at approximately 450° C. for a suitable time such as about 1 hour.
The electroless plating Ru bath may be used to deposit Ru onto a substrate by heating the bath and immersing the substrate in the heated bath. In some cases, the substrate is added to the Ru stock solution, heated, and the hydrazine reducing reagent is then added to the heated bath. The temperature of the electroless plating Ru bath may, for example be 40° C. or higher. In some cases the bath has a temperature of 70°C. or more. The temperature is typically kept below 70°C. In some cases, the temperature is in the range of 50°C. to 60° C.
When the desired amount of Ru plating is achieved, the substrate can be removed from the bath.
Example applications of the electroless plating Ru bath and the method for depositing Ru on a substrate surfacing using electroless plating include plating substrates to be used in hydrogen gas separation and plating substrates used in corrosive environments. Due to the thermal stability of Ru, stainless steel, and ceramic, these materials are ideally suited for use in high temperature hydrogen gas separation. For example, a hydrogen separation membrane may comprise a porous substrate (of e.g. stainless steel and/or ceramic) supporting a thin layer comprising Ru and Pd. The Ru may be deposited by electroless plating as described above. The Pd in the membrane may also be deposited by electroless plating, for example, Pd may be deposited using any known electroless plating technique for Pd, such as that described by Anwu Li, et al. in Preparation of thin Pd-based composite membrane on planar metallic substrate: Part II: Preparation of membranes by electroless plating and characterization, Journal of Membrane Science 306 (2007) 159-165. Pd may be deposited in other ways in the alternative.
Another application uses the novel electroless plating Ru bath disclosed herein to prepare Pd—Ru composite and alloy membranes used for the separation of hydrogen from gaseous mixtures.
FIG. 1 shows a substrate 11 comprising a porous base 12 coated with a thin layer of Pd and Ru 13. Layer 13 may comprise one or more layers of Pd deposited on the porous base 12 using any suitable technique for electroless plating of Pd and one or more layers of Ru using an electroless plating bath as disclosed herein.
After deposition the one or more Pd layers and the one or more Ru layers may be treated, for example by a heat treatment to yield layer 13. For example, a Pd—Ru composite membrane or multilayer Pd—Ru composite membrane may be annealed in situ during hydrogen permeation at a temperature of approximately 600° C. to yield a Pd—Ru alloy membrane.
Pd—Ru composite and alloy membranes maybe prepared by plating a substrate with thin layers of Pd and Ru in alternation. In some cases, the substrate is first plated with Pd using any known electroless plating method, such as that described by Li, et al. cited above. The substrate is then plated with Ru using the electroless plating Ru bath and method disclosed herein. Ru is a more robust metal and is more chemically inert than Pd making it ideally suited to be plated such that the exposed layer is the Ru layer. However, in the alternative, the Pd—Ru composite membrane can be prepared by first plating the substrate with Ru and subsequently plating the substrate with Pd.
In some embodiments, the overall Ru composition of the Pd—Ru composite and alloy membrane is in the range of about 1% weight to about 12% weight. In some cases, the overall Ru composition is in the range of about 3% weight to about 8% weight In some embodiments, the overall Pd composition of the Pd—Ru composite and alloy membrane is in the range of about 88% weight to about 99% weight. In some cases, the overall Pd composition is in the range of about 92% weight to about 97% weight.
In some embodiments, the thickness of the Ru layers of the Pd—Ru composite and alloy membrane is in the range of about 0.1 μm to about 2.5 μm. In some embodiments, the thickness of the Pd layers of the Pd—Ru composite and alloy membrane is in the range of about 1 μm to about 3 μm. In some embodiments, there are one or more layers of each of Pd and Ru.
In some embodiments, a membrane layer of one or more metals other than Pd and Ru may be prepared. Such additional deposited metals may alter the composition of the membrane layer resulting after heat treatment of the deposited metals. For example, a Pd—Ag—Ru composite membrane or a Pd—Ag—Ru alloy membrane may be prepared by electroless deposition of one or more layers of each of Pd, Ru and Ag. The electroless plating bath described herein may be applied for deposition of the Ru layers. For example, a Pd—Ag—Ru membrane may be prepared by sequentially, and in any order, plating a substrate with layers of Pd, Ag, and Ru. The substrate may be plated with Pd and Ag using known electroless plating methods, such as that described by Li, et al. cited above and by Rajkumar Bhandark, et al. in Pd—Ag membrane synthesis: The electroless and electro-plating conditions and their effect on the deposits morphology, Journal of Membrane Science 334 (2009) 50-63. In some cases, the Pd—Ag—Ru composite membrane is then annealed in situ during hydrogen permeation to yield a Pd—Ag—Ru alloy membrane.

EXAMPLE 1

Ru Stock Solution Preparation

0.65 g of RuCl₃×H₂O (Ru mass, 38%) was dispersed in deionized water and 1.0 mL of HCl (36%) was then added to form a suspension solution. When the suspension solution became clear, 60 mL NH₃H₂O (28%), 1.2 g NaOH, and 0.53 g NaNO₂were added. The final volume of the Ru stock solution was brought to 135 mL by adding deionized water.

EXAMPLE 2

Ru Electroless Plating at 60° C.

A solid stainless steel sheet with an area of approximately 2.0 cm²and a thickness of 0.4 mm was ultrasonically cleaned with acetone and water to remove any organic contaminants. The sheet was then activated by impregnating it with a solution of PdCl₂followed by a reduction in hydrogen at 400° C. to 450° C. The sheet was then placed in a glass vial with the activated surface facing upwards. 4.0 mL Ru stock solution prepared as in Example 1 was then added to the vial. The vial was heated in a 60° C. water bath. When the solution in the vial reached 60° C., 1.0 mL of 0.5 M hydrazine solution was added to the vial and the solution was stirred. Gas bubbles quickly formed in the solution, indicating that Ru plating was occurring. After 30 minutes of plating, the sheet was removed from the vial, rinsed with deionized water, and dried in an oven at 120° C. for 2 hours. The thickness of the Ru film formed on the sheet was approximately 1.9 μm (estimated by comparing the weight of the sheet before and after plating). The sheet was observed under a SEM and the Ru film appeared to be continuously and uniformly distributed on the sheet.

EXAMPLE 3

Ru Electroless Plating at 50° C.

A solid stainless steel sheet, cleaned and activated in the same batch as the sheet in Example 2, was placed in a glass vial, in 4.0 mL of the Ru stock solution prepared in Example 1. The vial was heated in a 50° C. water bath. When the solution in the vial reached 50° C., 1.0 mL of 0.5 M hydrazine solution was added to the vial and the solution was stirred. After 30 minutes of plating, the sheet was removed from the vial, rinsed with deionized water, and dried in an oven at 120° C. for 2 hours. The thickness of the Ru film formed on the sheet was approximately 1.1 μm (estimated by comparing the weight of the sheet before and after plating). The sheet was observed under SEM and the morphology, microstructure, and quality of the Ru layer appeared to be similar to the sample prepared in Example 1.

EXAMPLE 4

Ru Electroless Plating at 85° C.
A solid stainless steel sheet, cleaned and activated in the same batch as the sheet in Example 2, was placed in a glass vial, in 4.0 mL of the Ru stock solution prepared in Example 1. The vial was heated in a 85° C. water bath. When the solution in the vial reached 85° C., 1.0 mL of 0.5 M hydrazine solution was added to the vial and the solution was stirred. After 300 minutes, the sheet was removed from the vial, rinsed with deionized water, and dried in an oven at 120° C. for 2 hours. No Ru deposition was observed.

EXAMPLE 5

Pd—Ru Alloy Membrane Preparation

A porous stainless steel disk was polished, etched, coated, and activated as described in Example 2. The disk was first plated with Pd using the known electroless plating Pd bath described in Li, et al. The thickness of the Pd plating was approximately 6.5 μm. Ru was subsequently plated on top of the Pd as described in Example 1. After 30 minutes of plating, the thickness of the Ru layer was approximately 0.4 μm. The Pd—Ru composite membrane was then annealed in situ during hydrogen permeation at 600° C. Hydrogen permeance increased with time and became steady after 24 hours, suggesting that a Pd—Ru alloy was achieved. The overall thickness of the resulting Pd—Ru alloy membrane was 6.9 μm. The Pd—Ru alloy membrane was approximately 5% weight Ru and 95% weight Pd. The hydrogen permeability of the Pd—Ru alloy membrane was approximately 20% higher than that of a Pd membrane annealed in situ during hydrogen permeation at a temperature of 550° C.

EXAMPLE 6

Ru-Plated Product

The Ru-plated stainless steel sheet prepared in Example 1 and a non-plated stainless steel sheet with the same area and thickness as the sample used in Example 1 were both placed into a 1000 mL solution of 70% weight H₂SO₄maintained at 100° C. After one week, the non-plated stainless steel sheet had completely disappeared due to acid corrosion. After one month, no evident corrosion of the Ru-plated stainless steel sheet was observed.

EXAMPLE 7

Pd—Ru Composite Membrane Hydrogen Gas Permeability and Selectivity

A porous stainless steel disk with a diameter of approximately 5.0 cm and a thickness of 1.37 mm was cleaned and activated as described in Example 2. The disk was first plated with Pd using the known electroless plating Pd bath described in Li, et al. The disk was then cleaned with deionized water, dried in an oven at 50° C., and placed in a glass vial. 4.5 mL of a Ru stock solution comprising 1.8×10⁻²M RuCl₃×H₂O (Ru mass, 38%), 8.8×10⁻²M HCl, 6.6 M NH₃H₂O (28%), 2.2×10⁻¹M NaOH, and 5.7×10⁻²M NaNO₂was added to the vial containing the Pd—plated disk. 9.0 mL of deionized water and 1.0 mL of 2.0 M aNaOH solution were added to the vial. The vial was heated in a 55° C. water bath. When the solution in the vial reached 55° C., 0.5 mL of 1.0 M hydrazine solution was added to the vial and the solution was stirred. After 30 minutes of plating, the disk was removed from the vial, rinsed with deionized water, and dried in an oven at 120° C. for 2 hours. The thickness of the Pd—Ru film formed on the disk was approximately 6.4 μm (estimated by comparing the weight of the sheet before and after plating). FIG. 2 shows the Pd—Ru composite membrane after preparation. The disk was observed under a SEM and the Pd—Ru composite film appeared to be continuously and uniformly distributed on the disk, as shown in FIG. 3.
FIG. 4 shows the permeation flux of hydrogen gas across the Pd—Ru composite membrane at a temperature between 350° C. to 400° C. for over 1000 hours. The difference in pressure across the membrane was 1 bar. After approximately 50 hours, the permeation flux of hydrogen gas across the membrane was approximately 13 m³/(m²h). The membrane was stable over 1000 hours.
Also shown in FIG. 4 is the selective permeation flux of hydrogen gas across the Pd—Ru composite membrane. A mixture of 75% hydrogen gas and 25% nitrogen gas was applied across the membrane at temperatures between 350° C. to 400° C. for over 1000 hours. The difference in pressure across the membrane was 1 bar. After approximately 50 hours, the permeation of hydrogen gas was a factor of approximately 15,000 greater than the permeation of nitrogen gas across the membrane. The membrane was stable over 1000 hours.

Interpretation of Terms

Unless the context clearly requires otherwise, throughout the description and the claims:

- “comprise”, “comprising”, and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.
- “connected”, “coupled”, or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof.
- “herein”, “above”, “below”, and words of similar import, when used to describe this specification shall refer to this specification as a whole and not to any particular portions of this specification.
- “or”, in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.
- the singular forms “a”, “an”, and “the” also include the meaning of any appropriate plural forms.

Words that indicate directions such as “vertical”, “transverse”, “horizontal”, “upward”, “downward”, “forward”, “backward”, “inward”, “outward”, “vertical”, “transverse”, “left”, “right”, “front”, “back”, “top”, “bottom”, “below”, “above”, “under”, and the like, used in this description and any accompanying claims (where present) depend on the specific orientation of the apparatus described and illustrated. The subject matter described herein may assume various alternative orientations. Accordingly, these directional terms are not strictly defined and should not be interpreted narrowly.
While processes or blocks are presented in a given order, alternative examples may have steps or blocks presented in a different order. Processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified to provide alternative embodiments (including subcombinations of described methods). Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, such processes or blocks may instead be performed in parallel, or may be performed at different times where this is technically practical.
Where an element or component (e.g. a solvent, solute, assembly, device, substrate, catalyst, activator, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention.
Specific examples of systems, methods and apparatus have been described herein for purposes of illustration. These are only examples. The technology provided herein can be applied to systems other than the example systems described above. Many alterations, modifications, additions, omissions and permutations are possible within the practice of this invention. This invention includes variations on described embodiments that would be apparent to the skilled addressee, including variations obtained by: replacing features, elements and/or acts with equivalent features, elements and/or acts; mixing and matching of features, elements and/or acts from different embodiments; combining features, elements and/or acts from embodiments as described herein with features, elements and/or acts of other technology; and/or omitting combining features, elements and/or acts from described embodiments.
It is therefore intended that the following appended aspects and claims hereafter introduced are interpreted to include all such modifications, permutations, additions, omissions and sub-combinations as may reasonably be inferred. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

Claims

What is claimed is:

1. A method for depositing Ru onto a surface of a substrate, the method comprising:

preparing an electroless plating bath having a pH of at least 13 comprising:

a Ru stock solution; and

a hydrazine reducing reagent; and

contacting the substrate with the electroless plating bath.

2. A method according to claim 1, wherein the Ru stock solution comprises:

a source of Ru ions; and

a water soluble nitrite salt.

3. A method according to claim 2, wherein the electroless plating bath further comprises one or more of:

hydrochloric acid;

a complexing reagent; and

an alkali hydroxide.

4. A method according to claim 3, wherein the source of Ru ions comprises a chloride or nitrate of Ru.

5. A method according to claim 3, wherein the complexing reagent comprises ammonia.

6. A method according to claim 5, wherein the molar ratio of ammonia/Ru is at least 150.

7. A method according to claim 6, wherein the molar ratio of ammonia/Ru is at least 200.

8. A method according to claim 7, wherein the alkali hydroxide comprises sodium hydroxide or potassium hydroxide.

9. A method according to claim 2, wherein the water soluble nitrite salt comprises sodium nitrite or potassium nitrite.

10. A method according to claim 2, wherein the molar ratio of nitrite/Ru is at least 1.

11. A method according to claim 2, wherein the molar ratio of nitrite/Ru is in the range of 3 to 5.

12. A method according to claim 1, wherein the molar ratio of hydrazine/Ru is at least 2.

13. A method according to claim 12, wherein the molar ratio of hydrazine/Ru is in the range of 5 to 20.

14. A method according to claim 1, wherein the pH of the electroless plating of the bath is in the range of 13 to 14.

15. A method according to claim 14, wherein the pH is in the range of 13.3 to 13.7.

16. A method according to claim 1, comprising maintaining a temperature of the electroless plating bath at a temperature of at least 50° C.

17. A method according to claim 16, comprising maintaining a temperature of the electroless plating bath at a temperature of at least 60° C.

18. A method according to claim 1, wherein the Ru stock solution comprises an aqueous solution of:

1.0×10⁻³M to 2.0×10⁻¹M RuCl₃.×H₂O (Ru mass, 38%);

4.0×10⁻²M to 4.0×10⁻¹M HCl;

1.0 M to 21 M NH₃.H₂O (28%);

1.0×10⁻¹M to 1.0 M NaOH; and

4.0×10⁻²M to 4.5×10⁻¹M NaNO₂.

19. A method according to claim 1, wherein the Ru stock solution comprises an aqueous solution of:

0.10% weight to 0.51% weight Ru;

0.15% weight to 1.46% weight HCl;

1.7% weight to 35.7% weight NH₃;

0.4% weight to 4% weight NaOH;

0.28% weight to 3.11% weight NaNO₂.

20. A method according to claim 19, wherein the substrate comprises stainless steel.

21. A method according to claim 19, wherein the substrate comprises a ceramic.

22. A method according to claim 21, wherein the substrate is porous.

23. A method according to claim 19, comprising activating the substrate with a metal catalyst before contacting the substrate with the electroless plating bath.

24. A method according to claim 23, wherein the metal catalyst comprises Pd.

25. A method according to claim 1 applied for preparing a Pd—Ru composite membrane, the method comprising:

prior to contacting the substrate with the electroless plating bath, plating a surface of a substrate with Pd using electroless plating;

wherein the electroless plating bath has a pH of at least 13.

26. A method according to claim 25, wherein the electroless plating bath comprises:

a source of Ru ions; and

a water soluble nitrite salt.

27. A method according to claim 26, wherein the electroless plating bath comprises:

hydrochloric acid;

a complexing reagent; and

an alkali hydroxide.