If I understand correctly, H2 in the presence of Pd readily dissociates as it dissolves into the metal. With the dissociation energy for the H—H bond being so large, how is this possible?
At first I thought that the H atoms were falling to a lower energy level in the Pd that was somehow only available if they were monatomic, but if that were true wouldn't some heat be generated by the absorption?
------------------Update------------------
I just read that saturated palladium hydride, when exposed to oxygen, will generate heat and water on it's surface, $\ce{4 H}$(Pd) $\ce{ + O2}$(g) $\ce{\to 2 H2O}$(g, l). What is going on?
I'm guessing that when the $\ce{H}$ atoms leave the palladium are they still monatomic for an instant, but without the $\Delta H^0_f = 436 kJ/mol$ extra energy that free $\ce{H}$ atoms would normally have, as bypassed by the $\ce Pd$ somehow. Most of the $\ce{H}$ atoms simply fall in step with one another, reforming $\ce{H2}$ without a peep. But some latch onto nearby $\ce{O2}$ molecules, forming $\ce{H2O}$.
Am I hilariously wrong here?