3 added 140 characters in body edited Jan 30 '13 at 4:30 ManishEarth 11k1212 gold badges6161 silver badges108108 bronze badges Pd$$\ce{Pd}$$ can dissociate H2$$\ce{H2}$$ because the resulting Pd-H$$\ce{Pd-H}$$ bonds are more stable than the starting H2$$\ce{H2}$$. But the reason why Pd$$\ce{Pd}$$ is so good at dissociating H2$$\ce{H2}$$ is related to the energy barrier to bond formation. The dissociation of H2$$\ce{H2}$$ on a Pd$$\ce{Pd}$$ surface (and on Pt$$\ce{Pt}$$ and maybe several other metals) has no barrier. So you don't need to put the H2$$\ce{H2}$$ in an excited state to go over a barrier and create Pd-H$$\ce{Pd-H}$$ bonds. On Cu$$\ce{Cu}$$ for example, the bonding is possible but there is a barrier, you need to excite H2$$\ce{H2}$$ to dissociate it. Here is a good theoretical articleHere is a good theoretical article (with a great title): http://web.mit.edu/andrew3/Public/Papers/1995/Hammer/1995_Nature_Why%20gold%20is%20the%20noblest_Hammer.pdf It shows the different barriers to dissociation of H2$$\ce{H2}$$ on Pt$$\ce{Pt}$$, Ni$$\ce{Ni}$$, Cu$$\ce{Cu}$$ and Au$$\ce{Au}$$. It also gives an explanation for such differences (more physics than chemistry). Pd can dissociate H2 because the resulting Pd-H bonds are more stable than the starting H2. But the reason why Pd is so good at dissociating H2 is related to the energy barrier to bond formation. The dissociation of H2 on a Pd surface (and on Pt and maybe several other metals) has no barrier. So you don't need to put the H2 in an excited state to go over a barrier and create Pd-H bonds. On Cu for example, the bonding is possible but there is a barrier, you need to excite H2 to dissociate it. Here is a good theoretical article (with a great title): http://web.mit.edu/andrew3/Public/Papers/1995/Hammer/1995_Nature_Why%20gold%20is%20the%20noblest_Hammer.pdf It shows the different barriers to dissociation of H2 on Pt, Ni, Cu and Au. It also gives an explanation for such differences (more physics than chemistry). $$\ce{Pd}$$ can dissociate $$\ce{H2}$$ because the resulting $$\ce{Pd-H}$$ bonds are more stable than the starting $$\ce{H2}$$. But the reason why $$\ce{Pd}$$ is so good at dissociating $$\ce{H2}$$ is related to the energy barrier to bond formation. The dissociation of $$\ce{H2}$$ on a $$\ce{Pd}$$ surface (and on $$\ce{Pt}$$ and maybe several other metals) has no barrier. So you don't need to put the $$\ce{H2}$$ in an excited state to go over a barrier and create $$\ce{Pd-H}$$ bonds. On $$\ce{Cu}$$ for example, the bonding is possible but there is a barrier, you need to excite $$\ce{H2}$$ to dissociate it. Here is a good theoretical article (with a great title): It shows the different barriers to dissociation of $$\ce{H2}$$ on $$\ce{Pt}$$, $$\ce{Ni}$$, $$\ce{Cu}$$ and $$\ce{Au}$$. It also gives an explanation for such differences (more physics than chemistry). 2 added 2 characters in body edited Jan 30 '13 at 3:29 Guillaume 31611 silver badge44 bronze badges Pd can dissociate H2 because the resulting Pd-H bonds are more stable than the starting H2. But the reason why Pd is so good at dissociating H2 is related to the energy barrier to bond formation. The dissociation of H2 on a Pd surface (and on Pt and maybe several other metals) has no barrier. So you don't need to put the H2 in an excited state to go over a barrier and create Pd-H bonds. On Cu for example, the bonding is possible but there is a barrier, you need to excite H2 to dissociate it. ThereHere is a good theoretical article (with a great title) here: http://web.mit.edu/andrew3/Public/Papers/1995/Hammer/1995_Nature_Why%20gold%20is%20the%20noblest_Hammer.pdf It shows the different barriers to dissociation of H2 on Pt, Ni, Cu and Au. It also gives an explanation for such differences (more physics than chemistry). Pd can dissociate H2 because the resulting Pd-H bonds are more stable than the starting H2. But the reason why Pd is so good at dissociating H2 is related to the energy barrier to bond formation. The dissociation of H2 on a Pd surface (and on Pt and maybe several other metals) has no barrier. So you don't need to put the H2 in an excited state to go over a barrier and create Pd-H bonds. On Cu for example, the bonding is possible but there is a barrier, you need to excite H2 to dissociate it. There a good theoretical article (with a great title) here: http://web.mit.edu/andrew3/Public/Papers/1995/Hammer/1995_Nature_Why%20gold%20is%20the%20noblest_Hammer.pdf It shows the different barriers to dissociation of H2 on Pt, Ni, Cu and Au. It also gives an explanation for such differences (more physics than chemistry). Pd can dissociate H2 because the resulting Pd-H bonds are more stable than the starting H2. But the reason why Pd is so good at dissociating H2 is related to the energy barrier to bond formation. The dissociation of H2 on a Pd surface (and on Pt and maybe several other metals) has no barrier. So you don't need to put the H2 in an excited state to go over a barrier and create Pd-H bonds. On Cu for example, the bonding is possible but there is a barrier, you need to excite H2 to dissociate it. Here is a good theoretical article (with a great title): http://web.mit.edu/andrew3/Public/Papers/1995/Hammer/1995_Nature_Why%20gold%20is%20the%20noblest_Hammer.pdf It shows the different barriers to dissociation of H2 on Pt, Ni, Cu and Au. It also gives an explanation for such differences (more physics than chemistry). 1 answered Jan 30 '13 at 3:24 Guillaume 31611 silver badge44 bronze badges Pd can dissociate H2 because the resulting Pd-H bonds are more stable than the starting H2. But the reason why Pd is so good at dissociating H2 is related to the energy barrier to bond formation. The dissociation of H2 on a Pd surface (and on Pt and maybe several other metals) has no barrier. So you don't need to put the H2 in an excited state to go over a barrier and create Pd-H bonds. On Cu for example, the bonding is possible but there is a barrier, you need to excite H2 to dissociate it. There a good theoretical article (with a great title) here: http://web.mit.edu/andrew3/Public/Papers/1995/Hammer/1995_Nature_Why%20gold%20is%20the%20noblest_Hammer.pdf It shows the different barriers to dissociation of H2 on Pt, Ni, Cu and Au. It also gives an explanation for such differences (more physics than chemistry).