The process of catalytic hydrogenation typically involves three steps: adsorption of the reactants onto the catalyst surface, the reaction at the catalyst surface, and desorption of the products from the catalyst. Initially, hydrogen gas and the unsaturated substrate adsorb onto the surface of the metal catalyst. The hydrogen molecules dissociate into atoms, which then interact with the substrate to form the hydrogenated product. Finally, the product desorbs from the catalyst, leaving the active sites available for further reactions.