Production and preparation of Nickel catalysts
Alloy Preparation:
Commercially, the nickel-aluminum alloy required to produce Raney nickel is prepared by fusing catalytically active metals (nickel, iron or copper) with aluminum in a furnace, the resulting melt is quenched and cooled, and then crushed into uniform fine particles. In the design of the alloy components, two factors are considered. One is the ratio of nickel to aluminum composition in the alloy. As the ratio of nickel to aluminum varies, different nickel/aluminum phases are produced during quenching and they have different leaching properties, which may result in a very different porous structure of the final product. Usually equal masses of nickel and aluminum are used for fusion. The second is the proportion of the third metal added. During the quenching process, small amounts of third metals, such as zinc and chromium, are sometimes added. Their addition changes the composition and phase diagram of the alloy, resulting in different leaching properties and thus a higher catalytic activity, hence the term "accelerator".
Activation:
The high catalytic activity of Raney nickel comes from the catalytic nature of the nickel itself and its porous structure, which is derived from the removal of aluminum from nickel-aluminum alloys with concentrated sodium hydroxide solutions, a process known as leaching, which is simplified by the following leaching reaction.
2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2
Since the leaching reaction brings about the activity of the catalyst and the hydrogen gas produced is stored into the catalyst, it is also called activation. The surface area of the finished product is usually measured by adsorption experiments of gases (e.g. hydrogen). Nickel was found to be present in almost all the contact area. The average nickel contact area of commercial Raney nickel is 100 m² /g. Three main factors influence the outcome of the leaching reaction, they are the composition of the alloy, the concentration of sodium hydroxide used and the temperature of the leaching reaction.
As mentioned before, the alloy contains several nickel-aluminum phases. During the leaching process, the aluminum contained in the NiAl3 and Ni2Al3 phases is reacted first, while the aluminum contained in the NiAl phase reacts more slowly and can be retained by adjusting the leaching time, which is why it is called "selective leaching". A typical activated Raney nickel has 85% nickel by mass, which means that 2/3 of the atoms are nickel. The remaining aluminum in the NiAl phase helps to maintain this porous structure, providing structural and thermal stability to the catalyst.
The concentration of sodium hydroxide used for the leaching reaction needs to be relatively high, typically up to 5 mol/L, in order to rapidly convert the aluminum to water-soluble sodium aluminate (Na[Al(OH)4]) and avoid the formation of aluminum hydroxide precipitates. Once the precipitation of aluminum hydroxide is produced, the precipitation will block the formed pores and prevent the rest of the sodium hydroxide solution from entering the path of the alloy, making it harder for the remaining aluminum to react away. This results in a smaller surface area of the porous structure of the product and a reduction in catalytic activity.
The porous structure formed gradually during the leaching process has a strong tendency to reduce its surface area, and structural rearrangement occurs, with the pore walls bonding to each other, causing the porous structure to be destroyed. Therefore, the surface area and catalytic activity of Raney nickel decrease with the increase of leaching reaction temperature, while if the leaching temperature is very low, it will make the leaching reaction too slow, so the common leaching reaction temperature is between 70 and 100℃.
