Abstract
Passive NOx adsorption (PNA) has been recently developed as a promising technology for controlling the NOx emissions during the cold start period. In this work, we illustrate a CO-assisted mechanism by combining experimental and kinetic modeling studies. Pd/SSZ-13 has been synthesized, characterized and evaluated as a PNA in low-temperature NOx adsorption and temperature program desorption cycles, to represent multiple cold start periods. The gas compositions were also systemically changed, where both the effect of varying NOx and CO feed was evaluated in the presence of high water and oxygen contents. A kinetic model was developed to simulate the profiles of NO and NO2, including three initial Pd sites (Z-Pd(II)Z-, Z-[Pd(II)OH]+ and PdO). It is concluded from XPS and in situ DRIFTS experiments, flow reactor measurements and modelling observations that CO reduces Pd(II) species to Pd(I)/Pd(0) species, which increases the stability of the stored NOx species, resulting in a release above the urea dosing temperature. The model could well describe the experimental features, including the effect of CO. In addition, the model was used for full-scale catalytic converter simulations.
Highlights
The market of vehicles has largely increased with the growth of the global economy, accompanied with rising concerns for the produced NOx [1]
With kinetic restrictions at low temperature for lean NOx traps and the issue of ineffective urea dosing at low temperature (
It has been reported that Pd/SSZ-13 is adequate for passive adsorp tion of NOx owing to its considerable NOx adsorption ability, H2O and sulfur resistance [10,13,44]
Summary
The market of vehicles has largely increased with the growth of the global economy, accompanied with rising concerns for the produced NOx [1]. Ppm NO2 / 400 ppm NO / 2% O2/ 7% H2O They further developed a kinetic model in their recent paper [23] using Pd/SSZ-13 including 2 initial Pd sites (Z-Pd2+Z-, Z-[PdOH]+), as the active sites during uptake of NO, CO and C2H4. We find an irregular effect by changing CO concentration, influencing the reaction network, which needs to be described our developed model Another objective in this work is to ascertain the precise role of CO addition in enhancing NOx stability based on experiments and kinetic modelling. Three initial Pd sites, Z-Pd(II)Z-, Z-[Pd(II)OH]+ and PdO were used as a base for the model It was concluded from both experiments and kinetic modelling that low concentrations of CO reduced Pd(II) sites to lower valence Pd(I)/Pd(0), subsequently enhancing the adsorption of NOx. The model could describe the exper imental findings well. A full-scale catalytic converter model was further developed to demonstrate the influence of spatial and temporal temperature gradients on PNA during cold-start simulations
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