(126e) Hydrocarbon Storage and Its Impact On NOx Performance of Small Pore Cu-Zeolite SCR Catalysts

Zeolite-based catalysts have emerged as a technology of choice for converting NOx to N₂ in diesel engine exhaust gas using NH₃ as a reductant, via a process called Selective Catalytic Reduction (SCR). It is well known that zeolites can store hydrocarbons (HCs) and that such stored HCs can have a detrimental effect on the SCR catalyst performance and durability. In particular, uncontrolled HC oxidation can cause irreversible degradation due to heat generation across the SCR catalyst. This concern was partially addressed with the recent discovery of highly active transitional metal exchanged small-pore zeolites [1]. These catalysts were speculated to be immune to poisoning by larger HCs, unable to enter the catalyst pore.

In this study, we employed propene (C₃H₆), representing small HCs, and n-dodecane (n-C₁₂H₂₆), representing large HCs, to probe their impact on NOx conversion, as well as NH₃ storage and oxidation functions of a commercial small-pore Cu-zeolite SCR catalyst.

We have demonstrated that under an oxidizing environment, simulating gas conditions in diesel exhaust, interaction with both small and large HCs can lead to substantial HC deposit formation on a small-pore, Cu-zeolite SCR catalyst. By varying adsorption conditions, we determined that in the presence of oxygen, physisorption was the dominant mechanism for HC storage at low temperatures, while chemical interaction was involved in their deposition at high temperatures (200-400°C). The latter was further confirmed by the H/C ratio analysis of the deposits, which indicated them to be carbon-rich. Experimental observations suggest that HC storage is an activated process and requires elevated temperatures to transform the HC to oxygenated species that get accumulated on the catalyst surface. Above 400°C, the HC storage declines due to oxidation of the stored HCs to form COx (CO and CO₂) and H₂O.  Under challenging SCR reaction conditions, Cu-zeolite with high HC deposit accumulation (above 1.5 g/L) showed a decrease in NOx conversion. Catalyst functional evaluation indicated that the oxidation functionality of the catalyst decreased, however the NH₃ storage capacity remained unchanged. The stored HCs can be readily cleaned by an oxygen-rich mixture at elevated temperatures or by using NO₂ as an oxidant at lower temperatures.

The combination of the experimental observations indicate that the accumulation of carbonaceous deposits originating from larger hydrocarbons occurs solely on the outer surface of the zeolite crystallites, resulting in a less profound effect on the NOx reduction performance, ammonia storage, and other catalyst characteristics compared to short-chain hydrocarbons, which can enter the pore.

[1] Luo J.Y., Yezerets A., Henry C., Hess H., Kamasamudram K., Chen H.Y., Epling W.S., (2012), “Hydrocarbon Poisoning of Cu-Zeolite SCR Catalysts”, SAE Paper 2012-01-1096