Search Results

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 (SCR) is a viable option for control of oxides of nitrogen (NOx) from diesel engines. Currently, copper zeolite (Cu-zeolite) SCR catalysts are favored for configurations where the exhaust gas temperature is below 450°C for the majority of operating conditions, while iron zeolite (Fe-zeolite) SCR catalysts are preferred where NOx conversion is needed at temperatures above 450°C. The selection of Cu-zeolite or Fe-zeolite SCR catalysts is based on the different performance characteristics of these two catalyst types. Cu-zeolite catalysts are generally known for having efficient NOx reduction at low temperatures with little or no NO2, and they tend to selectively oxidize ammonia (NH3) to N2 at temperatures above 400°C, leading to poor NOx conversion at elevated temperatures.

This paper discusses the poisoning of a selective catalytic reduction (SCR) catalyst by trace levels of platinum originating from an upstream diesel oxidation catalyst (DOC). A diesel aftertreatment system consisting of a DOC, urea based SCR Catalyst and a DPF was aged and evaluated on a 6.4 liter diesel engine dynamometer. The SCR catalyst system consisted of an Fe-zeolite catalyst followed by a Cu-zeolite catalyst. After approximately 400 hours of engine operation at varied exhaust flow rates and temperatures, deactivation of the SCR catalyst was observed. A subsequent detailed investigation revealed that the Cu catalyst was not deactivated and the front half of the Fe-based catalyst showed severe deactivation. The deactivated portion of the catalyst showed high activity of NH3 conversion to NOx and N2O formation. The cause of the deactivation was identified to be the presence of trace Pt contamination.

A laboratory study was performed to assess the potential capability of TWC+LNT/SCR systems to satisfy the Tier 2, Bin 2 emission standards for lean-burn gasoline applications. It was assumed that the exhaust system would need a close-coupled (CC) TWC, an underbody (U/B) TWC, and a third U/B LNT/SCR converter to satisfy the emission standards on the FTP and US06 tests while allowing lean operation for improved fuel economy during select driving conditions. Target levels for HC, CO, and NOx during lean/rich cycling were established. Sizing studies were performed to determine the minimum LNT/SCR volume needed to satisfy the NOx target. The ability of the TWC to oxidize the HC during rich operation through steam reforming was crucial for satisfying the HC target.

A laboratory study was performed to assess the potential capability of passive TWC+SCR systems to satisfy the Tier 2, Bin 2 emission standards for lean-burn gasoline applications. In this system, the TWC generates the NH3 for the SCR catalyst from the feedgas NOx during rich operation. Therefore, this approach benefits from high feedgas NOx during rich operation to generate high levels of NH3 quickly and low feedgas NOx during lean operation for a low rate of NH3 consumption. It was assumed that the exhaust system needed to include a close-coupled (CC) TWC, an underbody (U/B) TWC, and an U/B SCR converter to satisfy the emission standards during the FTP and US06 tests while allowing lean operation for improved fuel economy during select driving conditions. Target levels for HC, CO, and NOx during lean/rich cycling were established. With a 30 s lean/10 s rich cycle and 200 ppm NO lean, 1500 ppm NO rich and the equivalent of 3.3 L of SCR volume were required to satisfy the NOx target.

An investigation into the post-LNT NH3 generation and NOx slip over an LNT was completed for the purposes of determining the proper calibration for NOx reduction over a downstream SCR. Experiments were conducted in an engine dynamometer on the DW12 2.2L diesel engine that was calibrated for rich operation. The catalyst system was evaluated at 325°C for three different engine speed/load points. Engine speeds of 1750, 2000, and 2500 rpm were chosen, resulting in a LNT space velocities of 35,000 - 68,000 hr-1. Increased lean times were tested, which resulted in NOx loadings on the LNT that ranged from 0.10 - 0.50 grams NOx/L catalyst. In addition, varied degrees of richness were tested, corresponding to lambda sensor readings of 0.90, 0.92, and 0.94. LNT performance was evaluated based on it's ability to provide a stoichiometric ratio of reactants to the SCR for NOx conversion. Gross and net NOx conversion was evaluated for a LNT-SCR system.

Selective catalytic reduction (SCR) is a technology capable of meeting Tier 2 Bin 5 emissions levels of oxides of nitrogen (NOX) for diesel engines. Base metal zeolite catalysts show the best combination of thermal durability and NOX conversion activity. It is shown in this work that some base metal zeolite catalysts can store high levels of hydrocarbons (HCs). Also, base metal zeolite catalysts can catalyze oxidation of HCs under certain conditions. Oxidation of stored hydrocarbons can lead to permanent catalyst deactivation due to the exotherm generated in the SCR catalyst (over-temperature condition leading to SCR catalyst damage). This paper discusses a laboratory bench test to characterize hydrocarbon storage and burn-off characteristics of several SCR catalyst formulations, as well as engine dynamometer tests showing hydrocarbon storage and exotherm generation.

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.

Promising lean NOx trap (LNT) results on lean-burn gasoline engines have encouraged the development of LNTs for diesel applications. Although the fundamentals of LNT are common for both gasoline and diesel applications, there are major differences due to the character of engine operation and control strategies. The sulfur tolerance and thermal durability of current state-of-the-art diesel LNTs under the conditions that represent the thermal and chemical conditions in diesel exhaust were investigated in a laboratory flow reactor. Sulfur poisoning and thermal aging are unavoidable factors contributing to diesel LNT deactivation. The results show that sensitivity to sulfur poisoning varies with the catalyst formulations, and in some formulations the sulfur poisoning appears reversible. However, the thermal deactivation is permanent regardless of its cause, i.e., LNT de-sulfation (deSOx) or diesel particular filter (DPF) regeneration.

Alkali and alkaline earth metal impurities found in diesel fuels are potential poisons for diesel exhaust catalysts. Using an accelerated aging procedure, a set of production exhaust systems from a 2011 Ford F250 equipped with a 6.7L diesel engine have been aged to an equivalent of 150,000 miles of thermal aging and metal exposure. These exhaust systems included a diesel oxidation catalyst (DOC), selective catalytic reduction (SCR) catalyst, and diesel particulate filter (DPF). Four separate exhaust systems were aged, each with a different fuel: ULSD containing no measureable metals, B20 containing sodium, B20 containing potassium and B20 containing calcium. Metals levels were selected to simulate the maximum allowable levels in B100 according to the ASTM D6751 standard. Analysis of the aged catalysts included Federal Test Procedure emissions testing with the systems installed on a Ford F250 pickup, bench flow reactor testing of catalyst cores, and electron probe microanalysis (EPMA).