Year of Publication

2016

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department

Chemistry

First Advisor

Dr. Jack Selegue

Abstract

Model ceria catalysts were evaluated for NOx storage and desorption performance under lean conditions. Three different storage temperatures (80 °C, 120 °C, and 160 °C) were utilized to evaluate NOx storage. Higher temperatures resulted in higher NOx storage. It was observed that storage of platinum promoted ceria resulted in higher NOx storage compared to promotion with palladium. NOx desorption behavior of platinum promoted ceria indicated that the majority of NOx is released at high temperatures (> 350 °C), comparatively palladium promotion released more of the stored NOx at lower temperatures. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) indicated that platinum promotion results in NOx storage as thermally stabile nitrates, while palladium promotion results in NOx storage as thermally labile nitrites.

Doping ceria with trivalent rare earth oxides has been shown to improve NOx storage by generating lattice oxygen vacancies. Ceria doped with Pr, Y, La, Sm, and Nd at two different concentrations (5 and 20 mol%) and promoted with Pt were evaluated. Doping ceria with 5% Sm, Nd, and Pr improved the amount of NOx stored while the addition of Sm and La did not improve storage. Upon increasing dopant concentration, NOx storage decreased in all cases but Pr. However, increasing Pr concentration was found to increase NOx storage as well as low temperature NOx release. Ceria doped with Pr promoted with Pd increased the amount of NOx released at lower temperatures compared to Pt promotion, although palladium promotion resulted in lower storage. Similar DRIFTS spectra were obtained with Ce-Pr when promoted with Pt or Pd compared to model catalysts. Platinum promotion results in the storage of NOx at nitrates, which require high temperatures for removal. Comparatively, Pd promotion results in NOx stored at nitrites requiring lower temperatures for removal.

Ceria doped with Pr proved to be promising, although not thermally stable when exposed to high temperatures as may be seen during a DPF clean up. Therefor, stabilizing Ce-Pr catalysts with Zr were evaluated. It was found that stabilizing Ce-Pr with Zr was not found to be beneficial to the catalyst performance.

Digital Object Identifier (DOI)

http://dx.doi.org/10.13023/ETD.2016.379

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