Session: 08-01: Poster Session

Paper Number: 148621

148621 - Investigation of N2o Formation During Selective Catalytic Nox Reduction on Cu-Exchanged Zeolite Catalysts 

Abstract: 

Nitrous oxide (N₂O), a greenhouse gas with ~300 times more global warming potential than CO₂, forms as a byproduct during selective catalytic NO_x reduction (SCR) reaction. With increased scrutiny on N₂O emissions, it has become important to understand the mechanism behind its formation on various catalysts and under varied reaction conditions, and utilize such knowledge for designing future SCR catalysts. Herein, we focused our study on Cu-exchanged small pore zeolites such as Cu/SSZ-13 (Cu/CHA), Cu/SAPO-34, Cu/SSZ-39 (Cu/AEI) and Cu/LTA to understand the effect of Cu-loading, support acidity, topology, reaction temperature and gas environment on N₂O formation. SCR activity measurements between 100°C-550°C on these catalysts reveal that N₂O generation displays a bimodal nature suggesting at least two different mechanisms. The low-to-medium temperature N₂O formation (100°C-350°C) is of particular interest because of the high probability of such exhaust temperatures in diesel vehicles and the lack of N₂O decomposition catalysts active in this range. The conventional wisdom is that thermal decomposition of in-situ formed NH₄NO₃ facilitates N₂O formation at such temperatures. However, NH₄NO₃ decomposition was found to occur above 250°C, thereby posing questions around the origin of N₂O below 250°C. Reaction kinetics measurements, in-situ deposited NH₄NO₃ temperature-programmed desorption and DRIFTS studies on Cu-SSZ-13 with various Cu-loading and Si/Al ratios reveal that N₂O formation indeed occurs from NH₄NO₃-like species, however the variations in nature and stability of these species lead to N₂O formation across the entire low-to-medium temperature range. A pool of NO₂ created by feed gas or in-situ generated NO₂ facilitates NH₄NO₃ formation. DRIFTS results showed that such NO₂ dimerizes to form N₂O₄ which then dissociates into NO⁺ and NO₃^- ions; the reaction of NO₃^- with Bronsted acid site (BAS) bound NH₃ leads to N₂O formation (BAS-catalyzed NH₄NO₃ decomposition) below 250°C. Presence of Cu-sites stabilizes nitrates, forming [Cu^II(NO₃)(NH₃)]-type species, which hydrolyzes to form NH₄NO₃ that subsequently decomposes at >275°C. Thus, Cu-sites, BAS, reaction gas composition and temperature- all these parameters play vital roles in N₂O formation during SCR. These understandings helped in engineering a multi-component composite catalyst formulation with high SCR activity with low N₂O byproduct formation.       

Presenting Author: Dhruba Jyoti Deka Pacific Northwest National Laboratory

Presenting Author Biography: Dhruba Deka is currently a Chemical Engineer in the Applied Energy Systems group at Pacific Northwest National Laboratory (PNNL), where his research focus lies in automotive catalysis. He received his PhD in 2020 from the Ohio State University with research work on high-temperature electro-catalysis. Following graduate school, he joined Oak Ridge National Laboratory where he worked for three years before moving to PNNL.

Authors:

Dhruba Jyoti Deka Pacific Northwest National Laboratory
Kenneth Rappe Pacific Northwest National Laboratory
Yong Wang Pacific Northwest National Laboratory