Why do hydrogenation reactions of ethylene and nitrogen need catalysts?

Hydrogenation reactions involving ethylene and nitrogen require catalysts because these processes involve overcoming significant chemical barriers to facilitate the reaction between molecular hydrogen and the target compounds.1

For the hydrogenation of ethylene, catalysts—such as nickel, platinum, or rhodium complexes—are used to lower the activation energy required to add hydrogen across the double bond. These catalysts often function by forming intermediate complexes with the reactants, allowing the hydrogen and ethylene to interact in a way that facilitates the reduction of the carbon skeleton.12

In the case of nitrogen, the element is chemically inert, making it difficult to combine with other elements. The Haber-Bosch process utilizes a catalyst, along with high pressure and temperature, to overcome this inertness and successfully synthesize ammonia from hydrogen and nitrogen. In both instances, the catalyst is essential for enabling the reaction to proceed at a practical rate without being consumed in the process.312