I got curious as to how catalytic converters work but I couldn’t find a complete yet clear explanation. Most of the equations leave platinum out and I’ve read that platinum is non-reactive with oxygen and carbon. In one otherwise detailed explanation, the writer ultimately resorts to saying that the platinum “holds on to” the carbon monoxide molecule.” So what exactly happens? Does PtCO (or whatever) actually occur, or is this “holding on” achieved by some means other than what is typically called a chemical reaction? Thanks in advance!
I’m thinking more GQ.
It’s a seemingly simple question about a simple reaction, but people are still researching it. The answer depends a bit on where along the scale of “what is an electron” to “I am Bill Goddard and today my beret is blue” scale you are.
The “holding on” refers to adsorption (with a D), which can range from just a loose van der Waals interaction to strong covalent bonding. It’s not a discrete PtCO because that Pt is connected to other PT atoms.
I recommending checking out the Wikipedia articles on:
Adsorption
Heterogeneous catalysis
Reactions on surfaces
and letting us know how you’d like us to tailor our responses, or if you have other questions.
Whoops that’s where I meant to put it. Thanks.
Ruken - has given the chemistry answer and I will trt the layperson’s interpretation.
So think of Platinum molecules as matchmakers that especially love marrying off oxygen molecules. In normal car exhaust, there is oxygen (O) and carbon monoxide (CO) but each of them is going their own separate merry way.
Absorption : This is where platinum gets the CO and the O to sit together and “talk”. To do this, it has to stop them from going their way and sit them down together in close quarters. Now platinum has the right chairs to do this.
Reaction : Once seated in close quarters, platinum creates the right environment (music, drinks etc :)) so that the CO and O pair up.
Desorption : Once CO and O are joined at the hip, Platinum now knows the trick to send them on to the world and do this all over again for the next pair.
Now as you can imagine, you want more Platinum molecules to talk to CO and O in the exhaust. But Platinum is a solid and only the molecules sitting on the surface can do the talking. So you grind the platinum to a fine powder so that there is more of them at the surface that can contact the gas. But now the fine powder will blow away with the gas, so you stick them to something like the air filter you use in your home. Now the platinum powder can do its job effectively 
Happy matchmaking or good reaction rates
Thanks, I learned more from your 30 second reply than I did in 25 minutes of googling. So looking over the industrial applications I see they all employ metals in some form. Is this because the free electrons in metallic bonds render those atoms more likely to create a Van der Waals interactions? What is it about platinum specifically that it is so commonly used as a heterogeneous catalyst? Thanks!
In this case vdW aren’t the dominant driver, AFAIK. Oxygen (dioxygen in air) interacts with a platinum surface by completely dissociating into individual atoms on the surface. I don’t know the exact orbital interactions*. And this is dependent on temperature and pressure. CO adsorbs onto the surface but does not dissociate. The adsorbed species are mobile, and if an oxygen meets a CO, they’ll bond and dissociate. The density of species on the surface tends to be higher than in the gas, so you get some acceleration from proximity but also from activation and dissociation of the oxygen by the platinum. Likewise, in Haber-Bosch, dinitrogen is activated by iron (or any number of other metals) into individual surface nitrogen atoms.
Catalysis on surfaces tends to be dominated by either the rate of some activation or by the rate of some dissociation. If the surface interacts too strongly with reagents, intermediates, or products, it may activate something easily but get clogged up with an intermediate or product. So you’ll sometimes see these “volcano” charts with a happy medium where the forces balance out. That seems to be platinum in this case.
*I get the impression you’re curious about this but I’m not up to speed on it, being more of a homo guy when it comes to catalysis. The fundamental mechanisms of chemisorption are the subject of ongoing research that I don’t really follow.
Also important for real-world catalysts are resistance to sintering (your catalytic converter gets hot) and to poisons. The best catalyst is not always the fastest. E.g. ruthenium is more active for Haber-Bosch than iron, but iron is cheap.
[off-topic historical note]
Since platinum is a rare expensive metal, I was curious how its use as a catalyst was discovered. Googling shows that the use of metal catalysts, including platinum, was investigated in the early 1800s (or even a little earlier) by Louis Jacques Thenard and then, especially Humphrey Davy. (Davy was eventually assisted in his research by Michael Faraday.) I guess that as they became aware of the use of metals as catalyst, they tried every metal they could find, even expensive ones.
Davy mentions platinum (calling it ‘platina’) in his 1806 Bakerian Lecture, On Some Chemical Agencies of Electricity. For this important work he received a 3000 francs prize offered by the French Emperor, although “Napoleon seems to have been somewhat peeved that the prize went to an Englishman when a state of war existed between England and France.”