For a long period, Horiuti-Polanyi mechanism was considered as the universal mechanism of metallic activation of dihydrogen. However, in 2013, Vile et al reported that the hydrogenation of propyne to propene with the catalysis of Ag follows an associative scheme instead of splitting the hydrogen molecule first. Yang et al examined the mechanism of hydrogenation of acrolein in some materials (e.g., gold-based system) using extensive first-principle calculations. Therefore, both researches suggested the preference for the non-Horiuti-Polanyi mechanism (Figure ) in some cases. In this part, we will take the hydrogenation of acrolein as an example, to introduce the trend of mechanism. The hydrogenation reaction can progress with both hydrogen atoms and H2 molecules on both terminal oxygen and carbon. However, the activation energy of hydrogenation on the terminal carbon significantly differs from the activation energy of reaction on terminal oxygen (Table , Table ) since there is an obvious structural difference between the two reacting regions. Generally speaking, when the activation energies of dihydrogen activation of dissociation decreases (i.e., the strength of chemisorption decreases), the activation energy of hydrogenation by the molecular H2 is more likely to be lower than the hydrogenation by the atomic hydrogen. In addition, the activation energy of hydrogenation by the molecular H2 is usually lower at the terminal oxygen than at the terminal