SCR catalyst substrates are typically made from the same ceramic honeycomb material as standard automotive catalysts. They are generally coated with either oxides of base metals such as vanadium, or zeolites of copper or iron.
The advantage of zeolite SCR catalysts is that they have greater thermal durability and can operate at both lower temperatures during start up, and the higher temperatures that typically occur during regeneration. Vanadium can work well in automotive applications but is more commonly found in industrial settings.
Early SCR systems tended to place the urea/diesel exhaust fluid (DEF) injector and SCR catalyst downstream of other emissions control devices. It is now becoming increasingly common to see parts containing both SCR and DPF substrates, or even in some cases a single DPF substrate coated as an SCR catalyst.
It is also quite common to see a final catalyst installed after the SCR catalyst in the exhaust system to remove any remaining ammonia. This is called an ammonia slip catalyst. Ammonia slip can occur when:
- Excess urea/DEF is injected
- Temperatures are too low for the ammonia to react
- The SCR catalyst has degraded
Download a free copy of our SCR technical poster here.