[CCoE Notice] MS Thesis Defense - Di Wang, Chemical Engineering

[Khator, Suresh]

TPD and Modeling study of the NH3 Uptake on Synthesized Fe- and Cu-Exchanged ZSM-5

Di Wang

Wednesday, November 9, 2011, 1:00 PM

Chemical Engineering Conference Room
Co-advisors: Dr. M. Harold and Dr. J. Rimer
Abstract
The influence of metal ion, adsorption temperature, exchange method and exchange degree on the NH3-uptake capacity and acidity features over a family of synthesized Fe- and Cu-exchanged ZSM-5 catalysts were investigated by temperature programmed desorption using ammonia (NH3-TPD) and a numerical model. Results reveal that both Brønsted and Lewis acidities were present in Fe- and Cu-exchanged ZSM-5. As the metal exchange degree increased, the number of Lewis acid sites increased but the Lewis acidity strength decreased. Our studies suggested that the Brønsted acidity was affected by the metal binding structure instead of exchange degree. Moreover, Cu ion exchange created a stronger acidity that stored NH3 in ZSM-5 catalysts at temperature above 500 oC, which may be caused by Cu Lewis acid sites or the enhanced Brønsted acidity. Cu-exchanged ZSM-5 samples also exhibited superior NH3 uptake capacity than Fe-exchanged samples, indicating that Cu may absorb NH3. The optimal metal exchange degrees that resulted in the best NH3 uptake performance were 0.74 (Fe/Al) for Fe-exchanged ZSM-5 and 0.94 (Cu/Al) for Cu-exchanged ZSM-5. The largest NH3 uptake capacity was also achieved at lowest adsorption temperature. In addition, a numerical model was developed to estimate the kinetic and thermodynamic parameters of the NH3 adsorption and desorption. The heat of adsorption for NH3 on H-ZSM-5 was determined to be 123kJ/mol, while Fe-exchanged samples had a range of 107 to 120kJ/mol, decreasing with increasing the Fe loading. We also derived the activation energies at zero coverage for two desorption states in Cu-ZSM-5 samples, 91-99kJ/mol and 196-201kJ/mol. Since Fe and Cu were reported to exhibit superior activity over different temperature range, this may be related to their different NH3 uptake properties, which is discovered in this study. This work also provides rational insights that can help guide future investigations into the relationship between NH3-SCR activity and zeolite acidity.
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