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.
To understand what they did, and why they did it, here’s a highly simplified short primer on automotive emissions control. In the United States, gas-powered cars have what is known as a three-way catalytic converter and use a combination of Rh, Pt, and/or Pd loaded onto a porous ceramic monolith through which the exhaust passes. Other additives, such as cerium oxide, may also be present to maximize catalytic performance. Three-way refers to the three things the converter does – oxidize CO to CO2, reduce NOx to nitrogen, and oxidize unburnt fuel and soot to CO2. Oxygen sensors and fuel injectors cycle the system back and forth between “lean” (excess O2) and “rich” (excess fuel) to make sure that NOx gets reduced and CO, soot and unburnt fuel get oxidized. In addition to the catalyst chemistry and fuel injector systems, there is a great deal of engineering technology present to maximize the performance under sub par conditions such as cold starts, etc.
However, the three-way catalytic converter technology is not suitable for application on so-called ‘‘lean-burn’’ engines that operate at high air/fuel ratios, including diesel-powered cars. The high air-fuel ratio makes it harder to reduce the NOx emissions. Prompted in part by the shift to cleaner diesel feeds, which are less likely to poison catalysts, and more stringent environmental standards, there’s been lot of research done in the last 15 years on developing better catalysts for diesel deNOx. So, what are the options for diesel/lean DeNOx? It would be advantageous to develop a diesel analog of the three-way converter, in which fuel injection strategies and a bit of extra fuel can be used to reduce NOx. This is called hydrocarbon-selective catalytic reduction (HC-SCR). We’re not really there yet. Several workaround technologies are in use.
1. 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.
2. 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.
Both NSR and NH3-SCR also require the use of additional catalysts to actually do the DeNOx. Currently, the state of the art for NH3-SCR is copper on zeolites,* but Pt has also been widely used, particularly in the NSR systems. In addition to the catalytic converter, diesel engines contain a diesel particulate filter (DPF) to collect soot. The soot can be burned off at high temperature.
Volkswagen is sort of vague on exactly what types of deNOx technologies they use, but from what I’ve read, it sounds like the smaller cars had NSR and the larger ones had a combination of NSR and NH3-SCR using Cu-zeolites (Cu-CHA). It is unclear whether the Cu-zeolites were used in all cars, or just the ones with SCR. When the cars are being driven, the steering column vibrates and the deNOx parts of the catalytic converter were not turned on. Urea was not sprayed into the exhaust pathway, and the feedback loop necessary for proper NSR was probably not turned on either. When the car was being smog tested, the steering column was stationary and the catalytic converter was turned on. I suspect the DPF technology was still being run properly, because soot is a much easier problem for a layperson to notice.
I’m still puzzled why they’d take the risk and cheat the system this way. Fine of $37,500 per car are nothing to scoff at. I’m also sort of dumbfounded that TDI owners didn’t notice that they never needed to fill their SCR tank, but perhaps VW just told them that it would not need to be filled very often. Did the catalytic converter technology not hold up to everyday driving, or did it just decrease the performance and mileage and they thought they’d sell more cars and happier owners if they bypassed it without telling people? At first, I cynically assumed the latter explanation, especially since only a few states do routine smog testing. But after thinking about it some more, I’m starting to wonder if the technology itself was not quite ready for prime time. Some people have speculated that the temperatures required might be higher and cause more engine wear and tear. I think it may depend on what catalyst is being used. The Cu-zeolite catalysts work best above 300C, but many NSR Pt catalysts perform quite well at 200-300C. But it may be that the Cu-zeolite technology just isn’t ready for prime time. Back when I worked on HC-SCR, the performance of a fresh catalyst was very good, but it would get deactivated over time. The commercialized system and the use of NH3-SCR were supposed to solve a lot of the problems that we observed, but perhaps that is not the case. If you only turn on the catalytic converted once in a while, this won’t be a problem. I’ve been working in another area of catalysis since 2010, so I haven’t been keeping up to date on the literature, so it could be that VW is just getting greedy and the catalytic converters work just fine.
Anyway, at the moment there are a lot of questions and not a lot of answers. Hopefully more details will come to light soon.