Pd-exchanged zeolites show great promise as passive NOx adsorbers (PNA) for the reduction of cold-start emissions from vehicle exhaust. However, the precise nature of the adsorption sites and the mechanisms underlying NOx storage in these materials remain unclear. Using quantum chemical simulations, we have investigated the structure and relative stability of Pd species exchanged into in H-CHA and assessed their potential for NO adsorption under various conditions. After a full configurational search was performed, phase diagrams were constructed to show which Pd speciation is favored at isolated Al atoms and proximate Al pairs for different operating conditions. While the most stable Pd2+ sites at NNNN Al pairs were found to be poor NO binding sites, calculated adsorption free energies for NO on Pd2+ and Pd+ sites at NNN Al pairs suggest that these are responsible for respectively the low-temperature and high-temperature desorption peaks observed in TPD experiments. These insights provide an important first step towards understanding the storage mechanism for NOx in Pd-exchanged zeolites, and the selection and design of more efficient PNA materials.

Theoretical study of Pd-exchanged zeolites for passive NOx adsorption

ACS Fall 2020 Virtual Meeting

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