Study explains the failure mode of Cu/SAPO-34 SCR catalysts

12 April 2019

Copper-exchanged zeolites have been commonly used as SCR catalysts for diesel engines. Most of commercial Cu/zeolite catalysts introduced over the last decade utilize one of two types of zeolites: SAPO-34 or SSZ-13. Both of these zeolites share the chabazite structure.

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After their commercial launch, it became apparent that Cu/SAPO-34 catalysts slowly began to fail. Initially, the reason of the failure was not well understood. A new study by a team from the US Pacific Northwest National Laboratory (PNNL) found that the catalyst deactivation can be explained by irreversible degradation of the SAPO-34 framework that occurs at low temperatures in the presence of water [4130].

Cu/SAPO-34’s problems are linked to temperature: It remains stable above 100°C, but shows signs of damage below that threshold. That explains why Cu/SAPO-34 performed poorly in refuse trucks compared to long-haul ones. The frequent start-stop operations for refuse trucks cause the SCR catalysts to run at lower temperatures than their long-haul counterparts, thus accelerating their rate of failure.

The researchers analyzed catalyst samples with 2-dimensional pulsed electron paramagnetic resonance, a technique that isn’t typically used on industrial catalysts. It was found that Cu/SAPO-34 catalysts deteriorate even when sitting on the shelf in the laboratory, due to exposure to moisture in the air. Unfortunately, high levels of moisture exist in all engine exhausts.

Recalls for SCR catalysts have been announced by a number of manufacturers, including Cummins and Volvo. While none of the recalls was explicit about the cause of the problem, the catalyst failures are presumably linked to the low temperature durability issues of the SAPO-34 zeolite.

Source: PNNL