In general, the saturation of simple olefinic and acetylenic multiple bonds occurs readily in the presence of any typical hydrogenation catalyst. Platinum metals mainly rhodium are used as hydrogenation catalysts. These catalysts, when used in hydrogenations, are reduced to the metal form; thus, it is the metal that is the actual catalyst. Other catalysts, such as copper chromate, zinc oxide and metal sulfides, which are also active in hydrogenations, participate in the catalytic reactions in the oxide or sulfide form. The choice of catalyst applied in the hydrogenation of a certain unsaturated hydrocarbon depends on several factors, such as the reactivity of the substrate and the experimental conditions (pressure, temperature, solvent, liquid- or gas phase reaction).