Catalytic converters and oxygen..

Sorry. The correct URL is

It will absorb the free oxygen as long as the cat is the type that allows for oxygen absorption. i.e., has a Cerium washcoat.

During engine running, yes. Under certain conditions, the cat will periodically burp off excess oxygen, this can be seen while monitoring the post-cat oxygen sensor(s) voltage signal.

Yes. Excess oxygen can and will effect the reduction beds capability of reducing Nox. That is why misfires and lean running conditions contribute/can cause NOx failures.

I had hoped that my buddy Mike and his staff of certified technicians would have answered your questions. I even waited more than an hour!

Guess not.

Good question none the less...

=IIRC=

the switching rate of the 1st O2 sensor can be 4 times/second at nominal engine speed. the downstream cat sensor should switch at about once every 127 toggles of the upstream sensor.

This frequency of switching is compared in the ecm, and if it goes

Lg

So then there are some cats that can *only* absorb oxygen atoms they crack from oxides of nitrogen?

If so, how do you know which type you've got?

Could some of that "burped off" oxygen be re-absorbed by the oxidation bed?

I'm guessing that excess exhaust oxygen (or shortage of CO) will eventually result in oxygen saturation of the reduction catalyst element. And when this happens, reduction catalytic reaction cannot occur because the element would be unable to absorb the resulting oxygen atom, so NO would pass through un-catalysed. Correct?

Oxygen to be absorbed comes from the reduction of NOx and from what is left over from combustion

AFAIK the Cerium washcoat wasn't used prior to OBD2

I don't think it is. AFAIK the oxygen storing capability exists only in the oxidation bed.

The reduction bed is usually (always?) ahead of the oxidation bed. Shortage of CO is not the same thing as excess O2.

You can not reduce (separate) O2 from NOx by adding O2 into it.

I'm getting my information from these two PDFs:

Page eleven of the second PDF states that reduced CO and increased O2 levels will result in an increase in NO. To me that suggests that those conditions would cause higher NO levels because the reduction bed would become oxygen-saturated and thus be unable to crack the NO apart.

Am I still missing something?

Here's what Ford Motor Company has to say on the subject;

The Catalyst Efficiency Monitor uses an oxygen sensor before and after the catalyst to infer the hydrocarbon efficiency based on oxygen storage capacity of the ceria and precious metals in the washcoat.

And GMs version;

To help aid in these processes, most modern TWCs also contain a base metal known as cerium. Cerium has the ability to attract and release oxygen in the exhaust stream.

So I've discovered as well.

It does, actually. That's what I've uncovered from copious sources so far.

It ADsorbs oxygen onto the surface of the noble metals for immediate supply, and ABsorbs oxygen into the underlying cerium oxide coating (as aarcuda said) for use during rich stages. The metals and the cerium are porous enough that there is oxygen transfer between the two coatings.

Yes, and it oxidises by cracking the HCs, then hooking each atom up with oxygen it's stored. It turns CO into CO2 by adding to it one stored oxygen atom. You cannot oxidise without oxygen to oxidise with.

The reduction side also turns CO into CO2 during the rich phase. It uses its stored oxygen from secondary air, and from cracked NO to do this.

I'm finding that high O2 is not desirable. CO2 is the primary indicator of an efficient engine and emissions control devices.

High oxygen content means something's not working correctly.

See above.

I've found a great many references to the use of cerium oxide as an absorptive medium for oxygen, as you say. The use of ceria appears to date almost from the earliest use of monolithic converters.

It appears further that the catalyst metals (Pt, Pd & Rh) do *adsorb* (surface storage only) oxygen, as well as other substances that may be present. This is why catalyst reactions are hampered by such things as TEL and sulfur: Adsorption of materials other than oxygen will take up surface space, displacing oxygen, thus interfering with catalyst reaction.

Three of the best references I've found so far:

The catalyst does briefly hold the released oxygen before re-use in the oxidation step, and the ceria helps do that. But it does NOT act as a big sponge to "absorb" oxygen. It would work OK without any ceria at all, but works better with a site to briefly bind the oxygen before re-using it. The net "oxygen content" of the catalyst is constant.