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

*out of bounds,* that is the ratio of S1 to S2, you gen an MIL.

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:

It appears from the first PDF (see page 2) that the cat produces CO2 from both beds. I can't see that happening without the reduction bed being able to absorb oxygen as well as the oxidation bed.

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?

1) the "oxidation" portion of the catalyst doesn't care if the oxygen it needs comes from an upstream "reduction" catalyst cracking NO or if it comes from air injected into the exhaust stream. That's what air-pumps are on cars for in the first place- to provide extra oxygen to the oxidation stage of the catalyst (all of it, in cars that don't have reduction cats). 2) The cat doesn't "absorb" oxygen at all. It facilitates the burning (or oxidation) of unburned hydrocarbons and carbon monoxide, ideally resulting in CO2 and water. Free oxygen will begin to pass through the catalyst any time that there is not much HC or CO left to "burn," so free oxygen coming out of the catalyst is a good indicator that its doing its job and burning all the leftover CO and HC in the exhaust stream. Its easy to build a sensor to test for O2, much harder to test for HC and NO directly. So the OBD-III onboard computers use the indirect indication of oxygen in the exhaust stream to confirm that the cat is working. 3) Get over the idea of a catalyst "absorbing" anything. It doesn't. It enables chemical reactions. That particular reaction (NOx -> O2 +N2) is pretty easy to enable. Both O2 and N2 are more stable than NOx, which is why you want to get rid of NOx- to get a reactive oxidizer out of the exhaust stream.

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 US Patent and Trademark Office: from the University of Waterloo, Ontario: last is not written in the best English, but the author's nativelanguage may be something other than English.)

also has some good Toyota Technical Articles on the subject.

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.