I've seen many mentions of metal catalysts why metal? What do they do?

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I know a catalyst is something that speeds up a reaction, usually by lowering the activation energy, by adding intermediate steps, which are not so "high energy".

Why metal? What are they doing in the reaction? For example, I've seen platinum mentioned a lot, or also nickel. Do these two have something special? Could any metals be used?

Here's an example:

http://phys.org/news/2012-09-scientists-mechanism-water-splitting-catalysts.html
 

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here's what I think happens:

they provide a reactive surface for reactions to take place on because they have a relatively high surface energy - the atoms at the surface of a metal have dangling bonds, so to speak (are not bonded in the typical unit cell for the metal) and are less energetically favorable in that configuration than atoms on the interior.

When something like, say, a gas molecule, adheres to the surface, the high surface energy means that it might be more favorable for the gas, especially at high temperature, to break up into atoms that bond (temporarily) to the surface. But since diffusion is sped up at high temperatures, these atoms can also move around on the surface and if they meet another atom that they can form a covalent bond with, they'll form that bond and then diffuse away from the metal. Doesn't happen for all atoms, of course, otherwise there'd be a 100% yield. However, it does happen for a large fraction of them.

Different metals have different surface energies, so it might depend on the specific reaction.

Ceramics are also used for catalysis. Alumina, for example. Same principle; the ions at the surface of a ceramic are not part of a complete unit cell and thus are "left dangling". However some ceramics are totally inert, so I think it might have to do with the identity of the ions.

Why not say... polymers? Polymers are molecular solids; their surfaces have no dangling bonds because they are molecular solids, have lower surface energy and thus can't break things apart.
 

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