The mechanism involved in the catalytic reaction of hydrogenation of ethylene over a platinum catalyst can be described in several steps:1. Adsorption: The hydrogen and ethylene molecules adsorb onto the surface of the platinum catalyst. The hydrogen molecules dissociate into individual hydrogen atoms upon adsorption, while the ethylene molecules remain intact.2. Formation of intermediate species: The adsorbed hydrogen atoms react with the adsorbed ethylene molecules to form an intermediate species, ethylidyne C2H3 , which is also adsorbed onto the platinum surface.3. Reaction: The ethylidyne species reacts with another adsorbed hydrogen atom to form ethane C2H6 , which is still adsorbed onto the platinum surface.4. Desorption: The ethane molecule desorbs from the platinum surface, leaving the catalyst surface free for further reactions.The morphology and crystallographic orientation of the platinum catalyst play crucial roles in determining its activity and selectivity. The activity of the catalyst is influenced by the number of available active sites on the surface, which depends on the surface area and the arrangement of atoms. The selectivity of the catalyst is determined by the specific crystallographic planes present on the surface, as different planes have different binding energies and reactivity towards the reactants.For example, platinum nanoparticles with a high proportion of 111 crystallographic planes have been shown to exhibit higher activity and selectivity for ethylene hydrogenation compared to those with a high proportion of 100 planes. This is because the 111 planes have a higher binding energy for hydrogen and ethylene, leading to a more efficient reaction.In summary, the mechanism of ethylene hydrogenation over a platinum catalyst involves adsorption, formation of intermediate species, reaction, and desorption. The morphology and crystallographic orientation of the platinum catalyst significantly affect its activity and selectivity, with certain crystallographic planes providing more favorable binding energies and reactivity for the reactants.