March 25, 2024
Introduction to Raney Nickel-Common Catalyst for Alkaline Electrolyzer
The most commonly used catalyst in alkaline hydrogen production electrolyzers is Raney nickel. It is a solid heterogeneous catalyst with a porous structure and a large specific surface area. Its main components are nickel and a small amount of aluminum. With the application of Raney nickel, nickel meshes perform better in their toughness and corrosion resistance. This article will give a detailed introduction to Raney nickel.
Table of Contents
Raney Nickel State
Dry Raney nickel appears in gray or gray-black powder. However, this state of Raney nickel is rare due to its poor safety. Commercially available Raney nickel is usually soaked in water, which can greatly reduce the risk of its use, and water has no effect on many hydrogenation reactions. Water-free Raney nickel is used only when hydrogenation under anhydrous conditions is clearly required.
Another type of Raney nickel adds some activated carbon as a stabilizer. The appearance of this type of Raney nickel is obviously black. It is safer than the common gray dry Raney nickel but still has potential risks.
History of Raney Nickel
In 1897, French chemist Paul Sabatier discovered in biochemical experiments established by American chemist James F. Boyce that substances containing trace amounts of nickel can accelerate the rate of addition of organic matter and hydrogen.
This discovery led to the application of nickel in the hydrogenation of organic matter. However, the catalytic effect of ordinary metal nickel powder cannot achieve the expected purpose, and this is true for nickel powder synthesized by various physical methods.
Since the 1920s, American engineer Murray Raney has been dedicated to finding efficient hydrogenation catalysts. In 1924, he used an alloy with a nickel/silicon ratio of 1:1, treated it with concentrated sodium hydroxide to dissolve the silicon, and obtained a porous structure. This structure is 5 times more effective in catalyzing the hydrogenation of cottonseed oil than ordinary nickel powder.
Inspired by this, Rainey used various nickel alloys to try to create such porous structures. Finally, it was found that nickel-aluminum alloy is the most suitable, especially the alloy with a nickel/aluminum ratio of 1:1. The porous nickel obtained after being fully corroded with sodium hydroxide solution has the highest catalytic activity. Raney officially applied for a patent in 1926. This discovery has been used till today. The nickel-aluminum alloy used in the production of Raney nickel is still optimal at 1:1.
Since “Raney” is a registered trademark of W. R. Grace and Company, strictly speaking, only products produced by the Grace Davison division of this company can be called “Raney Nickel”. Catalysts produced by other companies that have microporous structures and physical and chemical properties similar to Raney nickel can only be called “metal skeleton catalysts” or “sponge-metal catalysts.” But now “Raney” has been regarded as a name that can be used like aspirin.
Processing of Raney Nickel
Production of Raney Nickel
Commercially, the nickel-aluminum alloy required for Raney nickel is produced by fusing catalytically active metal nickel and aluminum in a furnace, the resulting melt is quenched and cooled, and then pulverized into uniform fine particles.
Design of Alloy Components
In the design of alloy components, two factors should be considered.
One is the composition ratio of nickel and aluminum in the alloy. As the nickel-aluminum ratio changes, different nickel/aluminum phases will be produced during the quenching process. They have different leaching properties, which may result in the final product having a completely different porous structure. Usually equal masses of nickel and aluminum are used for fusion.
The second is the proportion of adding the third metal. During the quenching process, a small amount of a third metal, such as zinc or chromium, is sometimes added. Their addition changes the composition and phase diagram of the alloy, resulting in different leaching properties and thus higher catalytic activity, so they are called “accelerators”.
In the preparation process of Raney nickel, the raw material nickel used is a carcinogen (Group 2B, EU Category 3) and teratogen considered by the International Agency for Research on Cancer, and inhaling fine oxidized Aluminum particles can cause aluminous pneumoconiosis, so care must be taken when preparing Raney Nickel.
During the activation process, because its surface area is gradually increasing and it continues to absorb the hydrogen produced by the leaching reaction, the Raney nickel formed after activation is moderately flammable. Therefore, the reactions in which Raney nickel participates should be in an inert gas environment for processing.
Activation of Raney Nickel
The high catalytic activity of Raney nickel comes from the catalytic properties of nickel itself and its porous structure. The porous structure comes from the removal of aluminum from nickel-aluminum alloys with concentrated sodium hydroxide solution. This process is called leaching. Simplified leaching reaction is as follows:
2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2
Since the leaching reaction brings the activity of the catalyst and the hydrogen produced is stored in the catalyst, it is also called activation.
The surface area of the finished product is usually measured by adsorption experiments on gases such as hydrogen. Experiments found that nickel was present in almost all contact areas. The average nickel contact area of commercial Raney nickel is 100 m²/g.
Methods to Attach Raney Nickel to Nickel Meshes
Commonly used methods include thermal spraying, plasma spraying, gas power spraying (cold spraying), roller coating and calcining methods.
Thermal Spraying
Thermal spraying uses high-temperature heat sources (heat sources such as arcs, plasma sprays, combustion flames, etc.) to heat materials until they are molten. The semi-molten material is atomized by high-speed airflow and sprayed on the surface of the purified and roughened parts to form a surface processing technology for spray coating.
In the process of spraying alloy particles being accelerated by heat and impacting on the surface of the substrate to form a coating, powder particles in a molten state or with a high plastic shape impact on the substrate at a certain flying speed and interact with the substrate, which is an important stage in the formation of the coating .
The general process flow is: pre-spraying treatment (cleaning, roughening, stress relief, etc.) → spray primer (some do not need it) → spray the working layer.
Plasma Spraying
The plasma arc driven by DC is used as the heat source to heat the material to a molten or semi-molten state and spray it at high speed onto the surface of the pretreated workpiece to form a firmly adherent surface layer.
The general process flow is: workpiece pretreatment → select appropriate plasma → adjust arc power → supply powder → adjust spraying distance and spraying angle → adjust relative movement speed of spray gun and workpiece → substrate temperature control.
Gas Power Spraying (Cold Spraying)
When the speed of the particle flow exceeds a certain value, the so-called critical speed, a hard and thick metal powder coating is deposited on the obstacle, which is called cold air power spraying.
Gas power spraying method is mainly determined by the pressure, temperature, gas consumption, powder supply, spraying distance, required power, particle size and hardness (microns) of the propelling gas.
The outstanding characteristics of cold spraying are that the chemical properties of the original powder material remain unchanged, it will not oxidize, and the porosity is low (because the particles will be severely deformed at high speed, but they will always be in a solid state).
Roller Coating and Calcining Method
The nickel-based alloy coating is applied to the electrode material to form an alloy coating with a uniform thickness and is then calcined in an inert gas (to avoid oxidation) environment to obtain the coating.
Raney Nickel Application to Nickel Meshes for Alkaline Electrolyzer
Traditional hydrogen generators use nickel mesh as the hydrogen evolution cathode. However, the nickel mesh currently used in hydrogen production electrolyzers on the market is directly woven from nickel wire.
As we all know, the surface of nickel wire is smooth with low roughness. To improve the specific surface area of the nickel mesh, technicians need to spray nickel-aluminum alloy on the surface of the nickel mesh.
It is this porous structure that greatly increases the specific surface area of the nickel mesh, thereby bringing higher catalytic performance and improving hydrogen production efficiency.
You may also be interested in the Surface Treatment of Nickel Mesh for more details.
Find A Professional Nickel Mesh Manufacturer
As a professional nickel wire mesh factory, NHGT is capable of providing high-quality Raney nickel meshes to meet the needs of different industries. If you are looking for a reliable nickel mesh supplier, just contact our team at info@nickelgreen.com or call us directly at +86 132 31116517 for urgent demand. We will reach out to you within 24 hours.
2 Comments
MohammadMarch 25, 2024
hi there,
What are the test required to measure the porosity of the nickel after plasma coating and leaching of aluminum.
CatherineMarch 25, 2024
Hi Mohammad,
For a reference, you may take the following method to measure the porosity of nickel after plasma coating and aluminum leaching :
1. Mercury Intrusion Porosimetry (MIP)
Principle: Measures pore size distribution by forcing mercury into pores under pressure.
Data Obtained:
Total porosity (%)
Pore size distribution (macro/meso/micropores)
Pore volume
Limitations:
High pressure may distort soft structures.
Not suitable for closed pores.
2. Gas Adsorption (BET & BJH Analysis)
Principle: Uses nitrogen (N₂) or argon (Ar) adsorption-desorption isotherms.
Data Obtained:
BET Surface Area (m²/g) – Indicates microporosity.
BJH Pore Size Distribution – For mesopores (2–50 nm).
Limitations:
Less effective for macropores (>50 nm).
Hope they are helpful.
