Kinetic modeling of CO assisted passive NOx adsorption on Pd/SSZ-13

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.

Automated summary

The study illustrates a CO-assisted mechanism for passive NOx adsorption (PNA) by combining experimental and kinetic modeling studies. Pd/SSZ-13 was used as a PNA to control NOx emissions during cold start periods, with the effect of varying gas compositions evaluated. It was found that CO reduces Pd(II) species to Pd(I)/Pd(0) species, increasing the stability of stored NOx species.