Understanding the cyclic behavior of passive NOx adsorbers under low and high concentrations of CO

Janos  Szanyi, Pacific Northwest National Laboratory

Passive NOx adsorber (PNA) technology has been considered to address the removal of NOx from the exhaust of lean burn engines at low temperatures (<180 °C) where the NH₃ SCR catalysts exhibit insufficient activity.  Pd-loaded small pore zeolites have shown promising NOx storage capacity as well as release temperatures in the range where SCR catalysts show optimal performance (200<T<450 °C).  Studies aimed at understanding the longterm durability of these PNA materials revealed that repeated NOx uptake and release cycles in simulated exhaust streams containing CO resulted in performance degradation.  Here we report on our studies aimed at understanding the 1. Effect of CO in the exhaust stream on the PNS performance during repeated cycles and 2. The regeneration of “deactivated” PNA materials due to the presence of high concentration CO in the exhaust stream.

We carried out cycles of NOx uptake and release experiments on a 1.7wt% Pd/FER PNA material using a simulated exhaust stream containing either 200 or 4000 ppm CO.  The key findings of this study can be summarized as follows: 1. In order to achieve high NOx adsorption capacity the Pd/FER PNA requires high CO concentrations.  The NOx adsorption capacity in the presence of 200 ppm CO is about 50% of that can be achieved at a CO level of 4000 ppm. 2. The NOx release temperature shifts to lower values as the CO concentration in the exhaust gas stream increased from 200 to 4000 ppm.  The newly developed NOx release feature at high CO concentration coincides with the onset of CO oxidation and can be attributed to local overheating of the adsorption sites.  3. Cycling to 420 °C only results in a gradual decrease in both the NOx release and CO oxidation onset temperatures.  4. High temperature calcination regenerates this “deactivated” PNA material, as both the NOx adsorption capacity and NOx release temperature regain their initial values.

In this presentation the PNA performance of the Pd/FER material under cyclic conditions, in the presence of CO in different concentrations will be compared and contrasted to that of Pd/SSZ-13.