h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

A 3way catalytic converter can as well reduce NOx according to

CO + NO -> CO2 + 1/2 N2 or
H2 + NO -> H2O + 1/2 N2

the carbonmonoxide is from the exhaust gas, the Hydrogen is builded at the converter as well from carbonmonoxide and water.

Yes, they can reduce some NOx (with the help of A/F mixture swinging), but that's not the main function of a catalytic converter. The emission system relies more on the EGR to accomplish that. Thank you for your interest in the article.

Previous answer: "Yes, they can reduce some NOx (with the help of A/F mixture swinging)..."

Actually, if the A/F mixture wasn't "swinging", the vehicle would rapidly descend into open loop operation, and would run rich. NOx production, of course, would then be very low, as it always is in overly rich mixtures, although CO and HC would be very high. It was only when we started to develop leaner running cars that NOx became a problem at all!

"...but that's not the main function of a catalytic converter."

NOT TRUE! A three-way catalytic converter's main function is to reduce all three major pollutants: HC, CO, and NOx. That's why it's called a three-way catalytic converter...

Previous answer: "The emission system relies more on the EGR to accomplish that. Thank you for your interest in the article."

ALSO NOT TRUE in almost all cases. This is a misconception that just won't go away easily, I'm afraid. Catlytic converters can be amazingly efficient when they and the oxygen sensor are both working well (and no, five crosscounts in ten seconds is not acceptable for an oxygen sensor, even at idle, but that's another story).

I have tested hundreds of vehicles under load on a dynamometer, testing them with the EGR systems enabled and then disabled. The amount of NOx reduction effected by the EGR system, measured in parts-per-million, is usually a fraction of the NOx reduction effected by the catalytic converter. Additionally, a vacuum-operated EGR system does not open up as much under heavy load as at cruise. Many vehicles which have completely inoperative EGR systems pass the stringent BAR-97 California emissions test anyway. Why? Because the cat is cleaning up the NOx!

Perhaps the previous post had in mind earlier vehicles with 2-way catalytic converters and EGR systems. Early (1975-1980) automotive catalytic converters could only reduce HC and CO, but you're obviously not talking about those. And if the EGR system is doing most of the work, why is it less and less common on new cars sold in California?