Failing catalysts

["Storhok, Eric (E.M.)" <estorhok@domain.elided>]

Wow, to be considered the "Chief Technologists..." that is an honor!

The short answer is, yes, a catalyst can fail at idle and work elsewhere.  But I'm sure you are all interested in the long answer...

So let me remind you that there are 2 main ways that catalysts fail- 1) Chemical poison, and 2) Temperature.  There are quite a few others, but these are the 2 that still cause us the most headaches.

Chemical is usually some metal being deposited on the catalyst, which blocks active sites on the catalysts surface.  Turns out that certain metals are attracted to the sites just like NOx, CO, O2, HC's... are.  Lead is an obvious one, a related and not so obvious one is silicon- which is why you need to be careful which RTV you use on your car.  One other that is a minor poison, but over the long haul is a problem is phosphorus, commonly found in most oils.  This isn't a problem as long as your car doesn't burn oil.  The failure mode for chemical poisoning is that it degrades and stops working all the time.

Temperature is another issue.  If you get the catalyst hot enough, the metals that are on the surface of the cat will start to sinter together.  Those of you M.E.'s out there know that sintering is when a powder metal is heated up, so that the powder structure begins to become a solid structure.  So some of the precious metal on the cats starts to glob together and not work so well.  Depending on how hot it gets and for how long, the failure mode will shift.  For Rich's case, I'm betting that it was a short over heat, as the catalysts still works once it's lit off, but the activation temperature has risen enough that idle emissions are up.

So, that is my guess- an over temp has raised the activation temperature of the catalyst so that it doesn't work at idle anymore.  You do all know why they are so expensive, yes?  Platinum, Palladium ($700-900/oz each), and Rhodium ($2000/oz) are the main ingredients. 

Eric Storhok
Core Emissions and Fuel Economy Team
Advanced Powertrain Engineering
(313)33x75011
MD 35, FPC-B
Ford Motor Company

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