Raney nickel: Encyclopedia II - Raney nickel - Preparation

Raney nickel - Preparation

Raney nickel - Alloy preparation

Alloys are prepared commercially by melting the active metal (nickel in this case, but iron and copper "Raney-type" catalysts can be prepared as well) and aluminium in a crucible and quenching the resultant melt, which is then crushed into a fine powder.^[2] This powder may be screened for a specific particle size range depending on the application the catalyst may be required for.

The initial alloy composition is important because the quenching process produces a number of different Ni/Al phases that have different leaching properties. This may result in markedly different porosities in the end product. The most common starting alloy used in industry contains an equal amount per weight of nickel and aluminium, incidentally, the same ratio Murray Raney used in his discovery of Raney nickel.

During the quenching procedure, small amounts of a third metal, such as zinc or chromium, may be added. This is done to enhance catalytic activity, and as such this third metal is called a "promoter".^[2] Note that the addition of a promoter changes the alloy and its resulting phase diagram to that of a ternary alloy, leading to different quenching and leaching properties during activation.

Raney nickel - Activation

The porous structure of the catalyst arises from the selective removal of aluminium from alloy particles using aqueous sodium hydroxide. The simplified leaching reaction is given by the following chemical equation:

The formation of sodium aluminate (Na[Al(OH)₄]) requires that solutions of high concentration of sodium hydroxide are used in order to avoid the formation of aluminium hydroxide, which precipitates as bayerite.^[2] Hence sodium hydroxide solutions with concentrations of up to 5 molar are used. Bayerite may cause blocking of the pores formed during leaching, and with the subsequent loss of surface area, it can reduce the efficiency and activity of the catalyst.

The temperature used to leach the alloy has a marked effect on the surface properties of the catalyst. Commonly used temperatures range from 70 to 100 °C. The surface area of Raney nickel (and skeletal catalysts in general) tends to decrease with increasing leaching temperature.^[3] This is due to structural rearrangements within the alloy that may be considered analogous to sintering, where alloy ligaments would start adhering to each other at higher temperatures leading to the loss of the porous structure.

Before storage, the catalyst can be washed with distilled water at ambient temperature in order to remove any remaining traces of sodium aluminate. Oxygen-free water is preferred for storage in order to prevent oxidation of the catalyst, which would accelerate its aging process and result in reduced catalytic activity.^[2]

Adapted from the Wikipedia article "Preparation", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki