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Technical Paper

Lean Breakthrough Phenomena Analysis for TWC OBD on a Natural Gas Engine using a Dual-Site Dynamic Oxygen Storage Capacity Model

Oxygen storage capacity (OSC) is one of the most critical characteristics of a three-way catalyst (TWC) and is closely related to the catalyst aging and performance. In this study, a dynamic OSC model involving two oxygen storage sites with distinct kinetics was developed. The dual-site OSC model was validated on a bench reactor and a natural gas engine. The model was capable of predicting temperature dependence on OSC with H2, CO and CH4 as reductants. Also, the effects of oxygen concentration and space velocity on the amount of OSC were captured by the model. The validated OSC model was applied to simulate lean breakthrough phenomena with varied space velocities and oxygen concentrations. It is found that OSC during lean breakthrough is not a constant for a particular TWC catalyst and is dependent on space velocity and oxygen concentration. Specifically, breakthrough time exhibits a non-linear, inverse correlation to oxygen flux.
Technical Paper

Experimental and Kinetic Modeling of Degreened and Aged Three-way Catalysts: Aging Impact on Oxygen Storage Capacity and Catalyst Performance

The aging impact on oxygen storage capacity (OSC) and catalyst performance was investigated on one degreened and one aged (hydrothermally aged at 955 °C for 50 h) commercial three-way catalyst (TWC) by experiments and modeling. The difference of OSC between the degreened and aged TWCs was dependent on catalyst temperature. The largest difference was found at 600 °C, at which the amount of OSC decreased by 45.5%. Catalyst performance was evaluated through lightoff tests at two simulated engine exhaust conditions (lean and rich) on a micro-reactor. The aging impact on the catalyst performance was different under lean and rich environments and investigated separately. At the lean condition, oxidation of CO and C3H6 was significantly suppressed while oxidation of C3H8 was relatively less degraded. At the rich condition, the inhibition effect was more pronounced on the aged TWC and inhibiting hydrocarbon species from C3H6 partial oxidation can survive at temperatures up to 450 °C.
Technical Paper

Diagnostics of field-aged Three-Way Catalyst (TWC) on stoichiometric natural gas engines

Natural gas is one of the most attractive alternate to gasoline and diesel fuels due to its competitive price resulting from its abundant availability and lower GHG emissions. In stoichiometric natural gas engines, three-way catalysts (TWCs) are widely employed to convert the exhaust pollutants CO, HC (including CH4) and NOx to CO2, N2 and H2O. TWCs contain Platinum Group Metals (PGM) as the active centers for various reactions and ceria-zirconia (CeZrOx) material to provide oxygen storage capacity (OSC), which acts as a buffer for maintaining stoichiometric conditions under a broad range of air to fuel ratios. The activities of the PGM and OSC components of TWC degrade upon prolonged real life operation. This degradation could arise due to various factors such as high temperature hydrothermal exposure of catalysts for prolonged periods and masking/poisoning of the catalytic sites by, for example, chemical contaminates originating from lube oil consumption and/or coolant leakage.
Technical Paper

Development of a Kinetic Model to Evaluate Water Storage on Commercial Cu-Zeolite SCR Catalysts during Cold Start

Commercial Cu-Zeolite SCR catalyst can store and subsequently release significant amount of H2O. The process is accompanied by large heat effects. It is critical to model this phenomenon to design aftertreatment systems and to provide robust tuning strategies to meet cold start emissions and low temperature operation. The complex reaction mechanism of water adsorption and desorption over a Cu-exchanged SAPO-34 catalyst at low temperature was studied through steady state and transient experiments. Steady state isotherms were generated using a gravimetric method and then utilized to predict water storage interactions with respect to feed concentration and catalyst temperature. Transient temperature programmed desorption (TPD) experiments provided the kinetic information required to develop a global kinetic model from the experimental data. The model captures fundamental characteristics of water adsorption and desorption accompanied by the heat effects.