Thesis
Atomic-level insights into the hydrothermal stability of CU/SSZ-13 SCR catalysts for the abatement of diesel vehicle NOx emissions
Washington State University
Master of Science (MS), Washington State University
2017
Handle:
https://hdl.handle.net/2376/103353
Abstract
Previous NOx emissions abatement technologies lacked the following characteristics that are required in order to be utilized in modern lean-combustion diesel vehicles: effective NOx removal (1) under highly oxidizing conditions, (2) at relatively low exhaust temperatures, and (3) while maintaining integrity under extreme hydrothermal conditions. One of the most promising recent innovations which seems to possess these three favorable traits is copper ion-exchanged SSZ-13 small-pore zeolite catalyst (Cu/SSZ-13) for the selective catalytic reduction of NOx using ammonia as the reductant (NH3-SCR). Despite its success in commercial catalytic platforms, exploration into the reasons why Cu/SSZ-13 performs so well compared to its predecessors, chiefly in terms of hydrothermal stability, remains rather shallow. In previous studies, the two main types of isolated Cu sites for the SCR reaction have been identified (Cu2+-2Z and [Cu(OH)]+-Z, with Z standing for a zeolite framework coordination). Also, dealumination of the zeolite framework, simultaneous loss of isolated Cu sites, and subsequent copper oxide agglomeration (CuOx) have been purported to be contributors to the loss in SCR activity observed after severe hydrothermal aging. These assertions regarding the effects of hydrothermal aging on Cu/SSZ-13 have been professed largely without much basis due to the challenges of intertwining various atomic-level characterizations with reaction kinetics testing. In the work presented in this thesis, these challenges were tackled and the hydrothermal stability of Cu/SSZ-13 has been revisited with particular focus on differentiating the two isolated Cu species. Of notable significance, it was found through SCR kinetics of fresh and hydrothermally aged samples that [Cu(OH)]+-Z is ~1.5 times more active than Cu2+-2Z. Also, quantification studies of the two Cu species via in-situ dehydration EPR experiments concludes that Cu2+-2Z is the more hydrothermally stable species. The long-standing question of how CuOx interacts with detached Al species is also further revealed by MAS-NMR, STEM, etc. Finally, the detrimental effect that zeolite structural damage, dealumination, and Cu species compositional changes have on overall SCR activity was scrutinized. With these results, rational design principles regarding ideal Cu loading levels and zeolite Si/Al ratios were gained which can guide the future development of this catalyst formulation's hydrothermal durability.
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Details
- Title
- Atomic-level insights into the hydrothermal stability of CU/SSZ-13 SCR catalysts for the abatement of diesel vehicle NOx emissions
- Creators
- James Song
- Contributors
- Yong Wong (Degree Supervisor)
- Awarding Institution
- Washington State University
- Academic Unit
- Chemical Engineering and Bioengineering, School of
- Theses and Dissertations
- Master of Science (MS), Washington State University
- Publisher
- Washington State University; [Pullman, Washington] :
- Identifiers
- 99900525092401842
- Language
- English
- Resource Type
- Thesis