Platinum group metals: Why are platinum and palladium great catalysts and not the others?

Question

Platinum and palladium are great catalysts. At the same time, other metals of the so-called platinum group metals are not. What are the atomic level reasons for this?

Answer 1

I think some of the other transition elements have also catalytic properties. For example, rhodium is generally used in addition to platinum or palladium in catalytic converters to oxidize carbon monoxide and hydrocarbons on one hand, and reduce $\ce{NO_x}$ to nitrogen on the other hand.

So, the choice is often made on the basis of relative cost.

But of course, not all transition elements are adequate for these catalytic properties:

• Metals such as silver and copper have a high affinity for sulfur and form metal sulfides and sulfates. As this happens, there will be progressively less metal available to function as a catalyst. While platinum tends not to become totally or irreversibly poisoned, i.e It can be efficiently recycled.
• Another point is linked to "Tammann temperatures" of these metallic elements: Actually, in a catalytic converter, metal is used in the form of nanoparticles dispersed on the surface of a porous support material. When temperature increases the particles starts to becomes mobile and coalesce (i.e. the catalytic activity is lost). This process is well-known under the name "sintering" and it becomes tangible as the metal is close to its Tammann temperature. This temperature is often defined as the half of the metal's melting point on the absolute temperature scale. In fact, metals such as gold, silver and copper have a Tammann temperature that is well below the average temperature of exhaust-gas ($600-700^0\mathrm{C}$). Tammann Temperature is $750^0\mathrm{C}$ for platinum, and $640^0\mathrm{C}$ for palladium. While it is only $405^0\mathrm{C}$ for copper, $345^0\mathrm{C}$ for silver and $395^0\mathrm{C}$ for gold.

To sum up, the candidate transition element for use as a catalyst in a catalytic converter: 1) has a low affinity to poisons such as sulfur compounds; 2) can be efficiently recycled; 3) has a relatively high melting point.

Answer 2

For those like me who weren’t sure, the platinum group metals are made up of platinum, palladium, iridium, ruthenium, rhodium and osmium.

I don’t understand where you’re getting the idea from that only two of those be good catalysts. It all depends on the application you are looking into.

• Wilkinson’s catalyst, a very important reduction catalyst, is rhodium-based.
• The synthetically extremely important Sharpless asymmetric dihydroxylation uses osmium tetroxide as part of its catalytic system.
• The Noyori asymmetric hydrogenation is ruthenium-based.
• Both Crabtree’s catalyst and the Cativa process are iridium-based.

And these are just very random examples for the usage of each of those metals. There’s a million more. Just follow the ASAP articles of JACS or the Early View articles of the Angewandte to see novel catalytic uses for each of those metals pop up all the time. Catalysis journals will naturally feature even more (albeit maybe not quite as novel).

Conclusion: All of those are great catalysts. (But so are most other transition metals, too.)

Answer 3

Platinum, palladium, and rhodium have a D-Orbital electron structure on the surface of their atoms which encourages the temporary sticking of other molecules. When other molecules stick, the overall electron cloud is changed in shape allowing the stuck molecules to rearrange into new compounds. The catalyst does not change. The rearranged molecules eventually are pushed out by new input ones (temperature caused motion) and the cycle continues.

Please see The Science of Catalysts And Catalytic Converters Dr Emma Schofield, Johnson Matthey Technology Centre. She says it's a quantum effect.

Answer 4

A good catalyst facilitates a reaction at a lower temperature and pressure while not participating in the reaction itself. Platinum has 4 unpaired electrons in its outermost shell as are the other elements in this group, but this shell is a d orbital which is furthest from the nucleus and very weakly forms molecular bonds. In fact Platinum is at the bottom of the metal reactivity series.

Since its reactivity is so low while its outermost shell is accommodating for weak bonds, it lends itself well as a reaction platform for more strongly bonding reactions. The reactants do temporarily weakly bond to the platinum atom until the other reactant finds the reactant through normal thermal motion and grabs the matching reactant away in a stronger bond leaving the platinum un-bonded. Think of the lone platinum atom as a match-maker weakly holding reactants until the suitor reactant finds its partner and takes away from platinum matchmaker. Palladium is similar in this regard.