Doctor of Philosophy
Israel E. Wachs
The selective catalytic reduction (SCR) of NOx with NH3 to environmentally innocuous N2 and H2O plays a crucial role in reducing undesirable NOx emissions from mobile applications and stationary installations. SCR catalysts are the core of the SCR system. Two general families of SCR catalysts have been commercialized today: Vanadia-based catalysts and Zeolite-based catalysts. This study examines the current fundamental understanding and recent advances of the supported V2O5‐WO3/TiO2 catalyst system and supported CuSSZ-13 catalyst system. The objectives of the dissertation were to resolve the fundamentals of catalytic active sites, surface acidity and surface reaction intermediates for the SCR reaction, and then apply the new fundamental insights to unravel the fundamental molecular structure-reactivity relationships.
Surface tungsten oxide (WO5) sites behave both as textural stabilizer and chemical and promoters for the SCR reaction by supported V2O5/TiO2 catalysts. As a textural stabilizer, the surface WO5 sites retard surface area loss of the TiO2 support and its transformation from the bulk anatase to rutile phase at elevated temperatures and hydrothermal conditions. It is critical to prevent formation of the TiO2(rutile) since it can form a solid solution with V4+O6, which causes catalyst deactivation by depleting the catalytic active surface V5+O4 sites. As a chemical promoter, the addition of surface WO5 sites increases the SCR reaction rate even though the surface WO5 sites do not directly participate in the SCR reaction because of their low redox activity. The chemical promotion by surface WO5 sites is proceeds by an indirect structural effect since the surface WO5 sites facilitate oligomerization of the surface VO4 sites. Given the need for two adjacent surface VO4 sites for the SCR bimolecular reaction, the presence of two adjacent surface VO4 sites in surface vanadia oligomers leads to more efficient performance of the SCR reaction. There is no electronic promotion effect between surface WO5 and VO4 sites on the TiO2 support. The hydrothermally treated catalysts did not possess any surface NH3* species on Lewis acid sites and only contained surface NH4+* species on Brønsted acid sites. Essentially the same SCR activity was found for the calcined catalysts, with both Lewis and Brønsted acid sites, and corresponding hydrothermally treated catalysts, with only Brønsted acid sites, demonstrating tha Brønsted acid sites, primarily associated with the surface VOx sites, represent the catalytic active sites for the SCR reaction. This unique situation allowed for resolving the long-standing argument about the role of Lewis and Brønsted acid sites in the SCR reaction.
In situ Raman and IR spectroscopy reveal that the SSZ-supported CuxOyHz contain isolated copper species : ZCu2+OH and Z2Cu2+, and dicopper species : trans-μ-1,2-peroxo dicopper(II) ([Cu2O2]2+) and mono-(μ-oxo) dicopper(II) ([Cu2O]2+) on the surface of SSZ-13 support. In situ Raman spectroscopy suggest that both surface dicopper species [Cu2O2]2+ and [Cu2O]2+ also participate in steady state SCR reaction. In situ IR spectroscopy shows that both surface NH3* species on Lewis acid sites and surface NH4+* species on Brønsted acid sites are present in the catalyst and participate in the standard SCR reaction. The introduction of Cu increases the concentration of surface NH3* Lewis species and decreases the concentration of Brønsted NH4+* species. The highly active new Lewis acid sites created by Cu species markedly increase when Cu loading increases. The excellent hydrothermal stability of the SSZ-13 support results in the stabilization of the surface acid sites for low temperature SCR activity.
These new unprecedented insights are able to resolve the long-standing debate that existed for decades in literature
Lai, Jun-Kun, "Nature of Active Sites and Surface Reaction Intermediates during SCR of NO with NH3 by Supported V2O5‐WO3/TiO2 Catalysts and Cu-SSZ-13 Catalysts" (2020). Theses and Dissertations. 5675.
Available for download on Sunday, January 30, 2022