Low Temperature NOx Trapping in Pd-exchanged Zeolites Passive NOx Adsorbers: Density Functional Theory Simulations, Transient Microkinetic Modeling and Bond Analysis

Lean-burn designs of higher fuel-efficient diesel engines have resulted in exhaust emissions at considerably lower temperatures. However, leading emission control technologies (three-way catalysis, selective catalytic reduction, lean NOx traps) available to abate NOx, CO and hydrocarbon emission levels function at temperatures typically above 200 oC. Passive NOx adsorbers (PNA), using 1% Pd-exchanged SSZ-13 with CHA framework, have shown great promise in trapping NOx at temperatures below 150 oC and releasing them above 200 oC. With the help of density functional theory (DFT) simulations, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements combined with microkinetic modeling of simulated temperature programmed desorption (TPD), we have investigated the nature of relevant active sites. A transient microkinetic model developed primarily under dry conditions identified the dominant reaction pathways responsible for the cycling between Pd(II) and Pd(I) stabilized by its high NO binding energy. A second monolithic kinetic model was developed under wet conditions as a predictive tool to simulate NOx uptake. NO uptake in the presence of the reductant CO is enhanced accompanied by CO2 formation. The increased NO uptake is consistent with the strongly exothermic nature of CO oxidation and lower activation barrier. Density functional theory (DFT) simulations are susceptible to the choice of exchange-correlation (XC) functional and improper selection of the functional can lead to fallacious conclusions. One such example where the choice of XC functional can potentially alter qualitative conclusions is NO binding on Pd-exchanged SSZ-13 zeolites, which is relevant for passive NOx adsorption. We have investigated the binding properties predicted by different XC functionals on various possible active sites on Pd-exchanged SSZ-13, spanning multiple levels of theory. The accuracy of XC functionals was assessed by comparison to reliable measurements of relevant properties, including the gas phase reaction enthalpy of CO oxidation by NO2, gas phase frequencies of CO and NO, ionization energy of Pd, and reported frequencies of Z2Pd(II)(NO) and ZH(CO) sites. To analyze bonding properties at the electronic structure level, we employed electron density difference and crystal orbital Hamilton population (COHP) analyses. The benchmark results indicate that the hybrid meta-GGA functional TPSSh most reliably predicts different properties investigated, and the GGA BEEF-vdW performs adequately in describing relevant frequencies.