Studies of Platinum Dispersion and Catalyst Architecture for Lean NOx Reduction
Shakya, Bijesh Man
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The combination of NOx storage and reduction (NSR) and selective catalytic reduction (SCR) catalyst is a promising technology for the reduction of NOx emission from the exhaust of lean-burn or diesel engine vehicles. In the combined NSR/SCR system, NH3 generated in LNT during the rich phase is utilized in the SCR for additional NOx conversion. Therefore, the performance of the combined NSR/SCR depends strongly on the NH3 generating function of the NSR catalyst. Earlier studies show that lower Pt dispersion NSR catalysts give higher selectivity to NH3 making them ideal candidates for this particular application. In the first part of the work, we performed experiments on lower Pt dispersion catalysts to gain insights on the mechanistic effects of Pt dispersion on NOx conversion and selectivity. We also developed an improved crystallite-scale model of NSR that explicitly accounts for the crystallite scale gradients of the stored NOx. The calibrated model is able to capture the effects of Pt dispersion, rich phase duration and overall cycle time on cycle-averaged conversion and selectivity trends. In the second part, we carried out a simulation study of dual-layer NSR+SCR monolithic catalyst using (1+1)-D model of catalytic monolith with individually-calibrated global kinetic models. Simulations show that multiple combinations of catalyst loading can attain a given NOx conversion and N2 selectivity, and that there exists a loading of SCR washcoat for a given NSR catalyst for which the NOx conversion is maximum. Simulations of the dual-brick monolith are also performed to analyze the effects of catalyst architecture. Under identical conditions, the simulations show that dual-layer catalyst outperforms the dual-brick largely because of the better utilization of generated NH3. Finally, we performed an optimization study to identify optimal loading and configuration of combined Fe+Cu zeolite catalyst that gives overall high NOx removal efficiency over a broad range of temperature. Simulations suggest that the brick configuration in which Fe- brick is followed by Cu- catalyst is slightly better than dual-layer in which Fe- is coated on top of Cu- architecture. This is attributed to the diffusional limitations in the washcoat that is more pronounced in the dual-layer catalysts.