Under very rich conditions, unburnt fuel will combust in the catalytic converter, drastically increasing the temperature and greatly speeding up oxidation, which degrades its effectiveness with regard to all pollutants, including NOx, and oftentimes pretty dramatically.

High temps in the catalytic converter can sinter the precious metals, reducing the surface area of catalytically active surfaces, but I believe the effect is much more pronounced in CO and HC activation. The conversion of NOx is much more constant.

Ah, I was under the impression this wasn't the case, so you're probably right. Do you have a source I could refer to?

Internal Combustion Engine Handbook by van Basshuysen and Schafer. My edition is rather old (2004) so there may a newer one with different info as diesel and emission tech has changed a bit in the last 20 years.

Thank you! I'll take a look and try to find a more up to date source if possible.