I worked on new zeolite catalysts for diesel deNOx for several years. The revelation that Volkswagen designed the catalytic converters on their TDI diesel automobiles to cheat on smog tests was absolutely shocking and depressing to me.
- NOx storage-reduction (NSR, also called Lean NOxTraps/LNT) uses alkali earth metals such as Ba to bind NOx as nitrates, which then get reduced when the fuel injection switches into a richer mode. Pt/Ba/Al2O3 is a common formulation. Periodically, the catalyst gets poisoned by sulfur and must be regenerated, just like the catalyst bed in a drybox.
- Ammonia-SCR: In ammonia selective catalytic reduction (NH3-SCR), urea is sprayed into the exhaust stream at high temperature. It decomposes to ammonia and helps reduce NOx to N2. A separate tank in needed to store the urea and, of course, it will need to be refilled on a regular basis.
Thanks to Old Biddy for her knowledgeable thoughts on the issue. Thoughts/questions that I have:
- I had no idea that urea was used in diesel vehicles to cut down on NOx - that’s fascinating.
- How many people had to be in on this? (i.e. were involved in making the software to turn off the SCR systems?) I guess that it could be a very small group of people?
- Still, how in the hell did no one at Volkswagen notice this?
- How did anyone not notice this in the general car-buying public, too?
*But I do know about getting drunk.