Experimental and Modeling Study of Passive NOx Adsorbers: Pd-H-ZSM-5 and Pd-SSZ-13

Mugdha  Ambast, University of Houston

Experimental and Modeling Study of Passive NOx Adsorbers: Pd-H-ZSM-5 and Pd-H-SSZ-13

Mugdha Ambast, Abhay Gupta and Michael P. Harold*
Dept. of Chemical and Biomolecular Engineering, University of Houston,
Houston, TX 77204-4004, USA

The passive NOx adsorber (PNA) is an emerging technology to mitigate NOx emissions through capture and conversion during vehicle cold-start and low temperature operations. A combined experimental and modeling study is presented that advances the understanding and prediction of NO and NO2 trapping and release on Pd-H-ZSM-5 and Pd-H-SSZ-13. The effects of various operating parameters including temperature, O2, CO, C2H4 and H2O concentrations, flowrate, and material properties including Pd-loading and silica-to-alumina ratio (SAR) on the NO uptake and release features are investigated. The NO uptake on Pd-H-ZSM-5 in presence of water did not exceed NO/Pd ~0.4 indicating that at least half of the Pd is not in cationic form. In contrast, the NO/Pd ~ 1 for Pd-H-SSZ-13 for comparable Pd loading and SAR. A predictive reactor model containing a mechanistic-based microkinetic scheme was developed. A systematic approach is followed using a combination of FTIR experiments, in-situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy), literature data, and density functional theory calculations. Water is shown to significantly lower the NOx uptake, but the extent of inhibition is a strong function of temperature. Flowrate experiments reveal the NOx uptake process to be kinetically limited and not impacted by washcoat diffusion. A one-dimensional two-phase transient monolith model is used to predict and validate NOx uptake and temperature-programmed desorption (TPD) concentrations for H-ZSM-5 and Pd-H-ZSM-5 with and without H2O in the feed. The model considers Z-[Pd(II)OH]+, Z-Pd2+Z-, Z-Pd+ as the active sites for NOx adsorption together with the Brønsted sites in Pd-H-ZSM-5, the latter of which are ineffective when water is present in the feed. The model was validated for different uptake temperatures, feed flow rates and Pd-loadings, affording its use to identify the optimal catalyst formulation as well as operating strategy. The model predictions provide insight about the NOx trapping and release and helps to identify improved materials and to develop effective trapping strategies.