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The successful commercialization of lean burn gasoline engines is dependent upon development of an effective emission aftertreatment system which can provide HC, CO, and NOx control not only under lean operating conditions, but also when the engine operates at the stoichiometric point under conditions of high engine speed and/or load. NOx adsorber catalysts (NOx traps) are capable of storing NOx under lean condition, and subsequently releasing and catalyzing its reduction under conditions rich of the stoichiometric point. Aftertreatment systems based on these types of catalysts show great potential for reaching current and future emission standards. Key to the successful application of NOx adsorber catalysts is the development of engine control strategies which maximize NOx conversion while minimizing the fuel economy penalty associated with adsorber regeneration. In this paper limitations associated with NOx trap adsorption and regeneration strategies are discussed.

A new adsorber system for reducing cold start HC emissions has been developed that offers a passive and simplified alternative to previous HC adsorber technologies. The series flow in-line adsorber concept combines existing catalyst technology with a zeolite based HC adsorber by simply incorporating one additional adsorber catalyst substrate into conventional catalytic converters without any valving, purging lines or special substrates. The HC adsorber catalyst consist of a durable zeolite, a washcoat binder, precious group metals and rare earth promoters on standard monolithic substrates. For selected vehicle applications, a single converter containing a light off catalyst, a catalyzed HC adsorber and a standard three-way catalyst can be used in the underfloor position. Even after severe engine aging, the vehicle FTP results show that this new technology remains effective in reducing the cold start HC emissions while providing good CO and NOx conversions.

The understanding of deactivation mechanisms is critical to the development of NOx adsorber catalysts with improved durability. The thermal deactivation of a state-of-the-art Pt/Rh based NOx adsorber catalyst is evaluated following oven agings at 800 and 900°C. Sulfur poisoning during lean/rich cycling is studied as a function of catalyst inlet temperature and SO2 concentration. Complementing these studies utilizing synthetic exhaust gas compositions, deactivation resulting from three different engine aging schedules is examined. The performance of engine-aged catalysts is evaluated as received, and following desulfurization procedures differing in inlet temperature and air/fuel ratio. The impact of aging schedules on NOx adsorption and three-way catalyst function is discussed with respect to precious metal dispersion, washcoat sintering, as well as sulfur build-up and oil-derived poisonings.

A new single-brick Ba + alkali metals NOx-Trap catalyst has been developed to replace a two-brick NOx-Trap system containing a downstream three-way catalyst. Major development efforts include: 1) platinum group metals selection for higher HC oxidation with potassium-containing washcoat, 2) alumina and ceria selection, and Rh architecture design for more efficient NOx reduction and 3) NiO to suppress H2S odor. Mitsubishi Motors' 1.8L GDI™ with this Delphi new NOx-Trap catalyst with H2S control achieves J-LEV standard with less cost and lower backpressure compared to the previous model. It is further discovered that incorporation of NiO into the NOx-Trap washcoat is effective for H2S control during sulfur purge but has a negative impact on thermal durability and sulfur resistance. Further study to improve this trade-off has been made and preliminary results of an advanced washcoat design are presented in this paper. Details will be reported in a future publication.

Performance of two types of NOx adsorber catalysts, one based on Ba and the other based on Ba with alkali metals, was compared fresh and after thermal aging. Incorporation of sodium(Na), potassium(K) and cesium(Cs) into NOx adsorber washcoat containing barium significantly increases the NOx conversions in the temperature range of 350-600°C over that of the alkali metal free NOx adsorber catalysts. NOx performance benefit and HC performance penalty were observed on both engine dynamometer and vehicle tests for the “Ba+alkali metals” NOx adsorber catalysts. “Ba+alkali metals” NOx adsorber catalysts also demonstrate superior sulfur resistance with better NOx performance after repeated sulfur poisonings and desulfations over the “Ba based” NOx adsorber catalysts.

Close-coupled or manifold catalysts have been extensively employed to reduce emissions during cold start by achieving quick catalyst light-off. These catalysts must have good thermal durability, high intrinsic light-off activity and high HC/CO/NOx conversions at high temperature and flow conditions. A number of studies have been dedicated to engine control, manifold design and converter optimization to reduce cold start emissions. The current paper focuses on the effect of catalyst design parameters and their performance response to different engine operating conditions. Key design parameters such as catalyst formulation (CeO2 vs. non CeO2), precious metal loading and composition (Pd vs. Pd/Rh), washcoat loading, catalyst thermal mass, substrate properties and key application (in use) parameters such as catalyst aging, exhaust A/F ratio, A/F ratio modulation, exhaust temperature, temperature rise rate and exhaust flow rate were studied on engine dynamometers in a systematic manner.

Although improvements in NOx adsorber formulations are increasing the sulfur resistance of these materials, and legislation continues to further restrict sulfur levels in fuels, sulfur poisoning remains as one of the key issues associated with successful commercialization of NOx adsorber technology throughout the world. Because of the stability of the sulfate poisons, high temperatures which stress the thermal stability of some of the most efficient NOx adsorbents are required for desulfation. Additionally, the rich condition which favors sulfur release simultaneously increases the H2S content of the emission. Sulfur traps offer the potential for reducing the formation of poisoning sulfates on downstream NOx adsorbents. Results characterizing the sulfur scavenging efficiency of these materials, as well as the conditions required for their regeneration will be presented. Strategies for their successful application on motor vehicles will be discussed.

Advances in extrusion die technology allow ceramic substrate suppliers to provide new monolithic automotive substrates with considerably higher cell densities and thinner wall thicknesses. These new substrates offer both faster light off and better steady state efficiencies providing new flexibility in the design of automotive catalytic converters. The effectiveness-NTU methodology is used to evaluate various design parameters of the HCD substrates. Various theoretical derivations are supported with experimental results on substrates with cell densities ranging from 400 to 1200 cells per square inch with varying wall thicknesses. Performance effects such as steady state conversion, transient response both thermal and emission, flow restriction and FTP emissions results are evaluated. Poison deposition is studied and the effects on emissions performance evaluated.

Dual-brick catalyst systems containing Pd-only catalysts followed by Pt/Rh three-way catalysts (TWCs) provide an effective strategy for managing Pt, Pd and Rh precious metal inventories while achieving LEV/ULEV emission standards. Engine aged dual-brick converters containing front Pd catalysts followed by rear Pd/Rh or Pt/Rh TWCs demonstrated LEV emission levels in an underfloor location on a TLEV calibrated 3.8L vehicle, and achieved ULEV emissions with air addition. Using identical advanced washcoat formulations stabilized with ceria-zirconia promoters, single-brick Pt/Rh TWCs demonstrated equivalent performance to Pd/Rh TWCs after thermally severe aging, and dual-brick [Pd + Pt/Rh] systems also had equivalent performance to [Pd + Pd/Rh] catalyst systems. While a Pd-only system also achieved 100K mi equivalent LEV emissions, both dual-brick options lowered emissions further using substantially lower loadings and more balanced precious metal usage.

SCR on filter, also known as SCRoF, SCRF, SDPF, has been utilized to meet the stringent light duty Euro 6 emission regulations. Close-coupled DOC-DEF-SCR on filter with underfloor SCR architectures, offer a balance of NOx performance at cold start and highway driving conditions. In contrast, the DOC-DPF-DEF-SCR architecture has been most commonly selected to meet the on-road and non-road heavy duty emission regulations worldwide. Diesel engines applied to off road vehicles can operate under higher loads for extended times, producing higher exhaust temperatures and engine out NOx emissions. New European Stage V emission regulations will mandate diesel particulate filter (DPF) adoption because of particulate number and more stringent particulate mass requirements. Three aftertreatment architecture choices with diesel particulate filters (DPF) were evaluated as candidates to fulfill the Stage V emission regulations.