Nitrogen Oxide Uptake and Desorption on Pd/ZSM-5 and Pd/Fe/ZSM-5

Meeting future NOx automotive emission standards necessitates a reduction in NOx emissions in low temperature exhaust encountered during vehicle startup and low load operation (< 200 °C). Pd-based zeolite catalysts are a potential solution to achieving these emission reductions through low temperature NO trapping. This Passive NOx Adsorbers (PNA) adsorbs NOx at low temperatures and release NOx at high temperatures enabling catalytic reduction with existing technologies such as selective catalytic reduction. In this thesis, NOx uptake and desorption characteristics was studied on several Pd-based ZSM-5 catalysts. The effect of Fe addition to the Pd ZSM-5 catalyst composition was also investigated.

A flow reactor system equipped with a Fourier transform infrared (FTIR) spectrometry was used to investigate the uptake and release performance features of ZSM5, Pd(1%)/ZSM-5, Pd(2%)/ZSM-5, and Pd(1%)Fe(1%)/ZSM-5 catalysts over a range of uptake temperatures and flow rates. Diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) experiments were conducted to identify key surface species over a range of conditions. NOx was found to adsorb on both Pd2+ complexes and zeolitic acid sites. The addition of Fe resulted in better Pd dispersion and better NOx storage. Catalysts stored over half of all NOx introduced, but NOx stored on zeolitic acid sites desorbed at lower-than-ideal temperatures. The presence of H2O severely reduced NO uptake. Potential mechanisms are described that are consistent with the flow reactor and DRIFTS experiments.