why is palladium/platinum a good catalyst?
I'm guessing that the answer would be heterogeneous. Hydrogenation reactions use Pt and/or Pd metal as the catalyst. But these are always solids while the reaction is between H2 and either a gas or a liquid. Consequently the catalyst is in a different phase than the reactants, making them heterogeneous. I'd go with that one.
Sparky 22 is wrong. Metals like palladium and platinum and nickel do absorb hydrogen. For example many times when doing a catalytic hydrogenation with Raney-Nickel you do not need to add H2 because enough H2 is already absorbed on the metal.
Palladium and platinum can form partial bonds with other molecules. By forming these partial bonds, the bond in the actual molecule gets weaker and weaker and hence, the bond becomes easier to break. Let's say for example a hydrogen molecule. There is a single bond between the 2 hydrogen atoms. Platinum/palladium will form partial bonds with the 2 hydrogen atoms. By doing so, the single bond BETWEEN THE 2 HYDROGEN ATOMS gets weaker and weaker. Hence, a smaller amount of energy is needed to break the bond between the 2 hydrogen atoms (the hydrogen molecule). As the amount of energy needed to overcome the bond between the 2 hydrogen atoms gets smaller, we say the activation energy for the reaction has been reduced. Hence, a greater amount of bonds in hydrogen molecules can be broken in a smaller time, and therefore, we say palladium/platinum has catalysed the reaction.
Palladium and platinum are good catalysts because they _adsorb_ certain molecules, especially gases. Adsorption is NOT the same as _absorption_. Adsorption is a process by which a chemical (gases or liquid) physically becomes attached to the _surface_ of an adsorbate (a solid). Once gas molecules are adsorbed on the surface of a catalyst, they are much nearer each other and more likely to react. Absorption is a process by which a gas or liquid is incorporated throughout the volume of an absorbate (a solid), rather than just on the surface. The fact the palladium happens to _absorb_ hydrogen molecules well probably has little to do with its function as a catalyst and is probably just a coincidence. In fact, two of the main reactions in a catalytic converter don't involve molecules with hydrogen atoms at all. On the other hand, WHY platinum and palladium are good catalysts are a mystery. Presumably, they are good at enhancing the Van der Waals forces that cause adsorption with other molecules, but WHY the platinum family of metals do this better than other metals is not fully understood (I don't think).
