How is nickel used as a catalyst to harden oils?

In the process of hardening oils, nickel serves as a catalyst to facilitate the hydrogenation reaction, a method used to convert liquid oils into edible fats.12

To achieve this, finely divided nickel is typically suspended on a support material, such as kieselguhr (diatomaceous earth), at a concentration of approximately 0.05 to 0.10 percent. This mixture is introduced into a steel pressure-reaction vessel along with the oil and hydrogen. Under controlled conditions—typically ranging from 100 to 200 °C and varying pressures—the nickel catalyst enables hydrogen to add to the double bonds of the fatty acid radicals within the oil molecules.1

This reaction effectively reduces the degree of unsaturation in the oil, resulting in a harder consistency. Once the desired level of hardening is reached, the hot oil is filtered to remove the metallic nickel catalyst, which can then be recovered or reused. This industrial application of nickel was pioneered by the chemist Paul Sabatier in 1897.12