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Base metal/zeolite catalysts, particularly containing copper and iron, are among the leading candidates for treatment of NOx emissions for diesel applications. Even with the use of ultra low sulfur fuel, sulfur poisoning is still a durability issue for base metal/zeolite SCR catalysts. In this study, the impact of sulfur poisoning on SCR activity and the stored sulfur removal effectiveness were investigated on several Cu and Fe/zeolite SCR catalysts after different thermal aging. The impact of sulfur was more significant on the Cu than on Fe/zeolite SCR catalysts for the NOx activity. It was found that the sensitivity of thermal aging status to the sulfur poisoning impact was different. The impact of sulfur on NOx activity changed with thermal aging on some catalysts, while it remained relatively the same for other catalysts. The most thermally durable SCR catalyst was not necessarily the most durable to sulfur poisoning.

Selective Catalytic Reduction of NOx is a promising technology to enable diesel engines to meet certification under Tier 2 Bin 5 emissions requirements. SCR catalysts for vehicle use are typically zeolitic materials known to store both hydrocarbons and ammonia. Ammonia storage on the zeolite has a beneficial effect on NOx conversion; hydrocarbons however, compete with ammonia for storage sites and may also block access to the interior of the zeolites where the bulk of the catalytic processes take place. This paper presents the results of laboratory studies utilizing surrogate hydrocarbon species to simulate engine-out exhaust over catalysts formulated to operate in both low (≈175-500°C) and high temperature (≈250-600°C) regimes. The effects of hydrocarbon exposure of these individual species on the SCR reaction are examined and observations are made as to necessary conditions for the recovery of SCR activity.

Copper/zeolite catalysts are the leading urea SCR catalysts for NOx emission treatment in diesel applications. Sulfur poisoning directly impacts the overall SCR performance and is still a durability issue for Cu/zeolite SCR catalysts. Most studies on sulfur poisoning of Cu/zeolite SCR catalysts have been based on SO2 as the poisoning agent. It is important to investigate the relative poisoning effects of SO3, especially for systems with DOCs in front of Cu/zeolite SCR catalysts. It was observed that SCR activity was significantly reduced for samples poisoned by SO3 vs. those poisoned by SO2. The sulfur was released mainly as SO2 for both samples poisoned by SO2 and SO3. The temperatures and the magnitudes of released SO2 peaks however, were very different between the samples poisoned by SO2 vs. SO3. The results indicate that sulfur poisoning by SO2 and SO3 are not equivalent, with different poisoning mechanisms and impacts.