[CCoE Notice] MS Defense: NOx Storage and Release Activity on Pd/BEA Passive NOx Adsorber

[Grayson, Audrey A]

MS DEFENSE STUDENT: Anh Vu
DATE: Friday, April 21, 2017
TIME: 2:30 PM
PLACE:  Room E132 (Engineering Building 2)
DISSERTATION CHAIR: Dr. Lars Grabow
________________________________
TITLE:
NOx Storage and Release Activity on Pd/BEA Passive NOx Adsorber
Increasingly stringent vehicular emission regulation on NOx demands a better strategy to further mitigate tailpipe NOx exhaust. In terms of NOx reduction strategy, both current state-of-the-art technologies, selective catalytic reduction (SCR) and NOx storage and reduction (NSR), have shown optimal performance in warm conditions but are not fully functional at temperature below 200°C, commonly regarded cold-start condition. Thus, in order to lower NOx cold-start emission, the passive NOx adsorber (PNA) has been introduced based on the concept of low-temperature NOx storage developed during the research of NSR, with so far a very promising result thanks in part to high through-put comprehensive catalyst screening. Among the PNAs, Pd/BEA stands out due to its promising NOx storage capacity and well-controlled thermal desorption performance. In this Masters thesis, the NOx activity on Pd/BEA passive NOx adsorber was investigated.
NOx adsorption and desorption on Pd/BEA was studied using temperature-programmed desorption method. It was shown that CO improved NOx storage and induced NOx release to higher temperature on Pd/BEA. In-situ diffuse reflectance IR spectroscopy revealed the critical role of Pd chemical states, where lower oxidation state Pd would bind NOx more strongly. In addition to CO, the influence of SO2 was also addressed briefly and was shown to slightly decrease NOx performance on Pd/BEA, especially when CO was included in the feed.
Apart from the effect of gases, the desorption kinetics of NOx on Pd/BEA was also studied in an attempt to resolve the effect of heating rate on the controlled release of NOx. As the ideal PNA demands surface NOx to be retained until approximately 200°C before its desorption for downstream reduction (by SCR or NSR, which reaches maximum performance by that temperature), the heating rate becomes critical. Thermal desorption spectra were collected at different heating rate and were subsequently analyzed using a number of methods, including the well-known Polanyi-Wigner model, Redhead’s method, and peak width analysis. The kinetic set of activation energy of desorption, [cid:image002.png at 01D2B781.6AD76190] , and frequency factor, [cid:image003.png at 01D2B781.6AD76190] , produced from the first two methods differed from that obtained from the last one. However, via a better fit in the full simulation of experimental TPD data, the latter desorption kinetics, computed from peak width analysis, became more reliable. The discrepancy in ([cid:image002.png at 01D2B781.6AD76190], [cid:image003.png at 01D2B781.6AD76190] ) obtained across the TPD data at different heating rate was reconciled by the compensation effect, which eventually established an empirical relationship between [cid:image002.png at 01D2B781.6AD76190]  and [cid:image003.png at 01D2B781.6AD76190]  and contributed to the predictability of the model developed from the peak-width analysis.
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