Heterogenous catalyst confusion

[234ff]

I encountered catalytic hydrogenation recation, where alkene is converted into alkane using Pt/C or Pd/C as catalyst, but I have a question:

Bond dissociation energy of H2 = 436kJ/mol. But when H2 is adsorbed on the metal surface, the bond is broken. How come this simple step can break this strong bond? Where does the 436kJ energy come from / what gives this activation energy?

[chenbeier]

First the bond is not broken. H2 will be adsorbed on platinum. The same alkene does. There will interaction and hydrogen will bond to alkene. On the surface Pt-H and Pt- C bonds will built and in further reaction new C-H bonds will developed and the meralbond will be broken again.