Session: 05-03: NOx Control for Alternative Fuels

Paper Number: 172431

172431 - Challenges in Nh3-Scr Over Cu-Ssz-13 Under Hydrogen Engine Exhaust: Impact of H2 and H2o on Catalyst Performance and Stability 

Abstract: 

Hydrogen-powered internal combustion engines (H₂-ICEs) have garnered significant attention in recent years as a low-carbon alternative to conventional engines. However, emissions nitrogen oxides (NO_x: NO, NO₂, N₂O) remain a challenge. Copper-exchanged small-pore zeolites (Cu-SSZ-13) represent the state-of-the-art in diesel exhaust aftertreatment, where they effectively reduce NO_x via the selective catalytic reduction (SCR) with NH₃ (4NO + 4NH₃ → 4N₂ + 3H₂O). Extensive studies have elucidated the reaction mechanisms, performance, and stability of Cu-SSZ-13 under typical diesel exhaust conditions. The SCR process follows a redox mechanism in which isolated Cu sites cycle between Cu²⁺ and Cu⁺ through sequential reduction and oxidation half-cycles (RHC and OHC).

However, the stability and behavior of these active sites under H₂-ICE exhaust conditions, characterized by higher H₂O concentrations (up to 25 vol%) and the potential presence of unburned H₂, remain largely unexplored. In this work, we investigate the performance and durability of Cu-SSZ-13 under H₂-ICE-relevant exhaust conditions using a synthetic bench-flow reactor, supported by SCR kinetic modeling, temperature-programmed adsorption-desorption, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and density functional theory (DFT) calculations.

At low temperatures (<250 °C), NO_x conversion decreased under 20% H₂O, coinciding with a reduction in both activation energy and pre-exponential factor, suggesting increased rate limitation by the OHC due to hindered Cu mobility. At higher temperatures (>200 °C), NO_x conversion was further suppressed in the presence of H₂. Characterization with H₂-temperature-programmed reduction revealed Cu²⁺ is reduced to Cu⁺ above 200 °C, forming a non-redox-active Cu⁺ pool that diminishes SCR efficiency. During steady-state SCR at 250 °C, NO concentrations fluctuated dynamically with H₂ exposure, as NO_x species desorbed upon Cu²⁺ reduction to Cu⁺. DRIFTS analysis showed that H₂ decreased surface-adsorbed NO⁺ and NO₃^-, with reversible adsorption observed upon H₂ removal.

Hydrothermal aging under 20% H₂O led to loss of low-temperature activity via conversion of mobile Z₁CuOH to Z₂Cu species and loss of high-temperature activity due to CuO_x formation. The presence of 1000 ppm H₂ further accelerated degradation. EPR analysis of fresh and aged samples revealed that hydrothermally aged samples exhibited ~72% loss of isolated Cu sites and formation of CuAl₂O₄, linked to dealumination. NMR confirmed extensive framework dealumination under combined H₂O and H₂ exposure, indicating compromised catalyst stability under H₂-ICE exhaust.

These findings suggest that Cu-SSZ-13 may not offer sufficient efficiency or durability as an NH₃-SCR catalyst for H₂-ICE applications. Insights from this study are currently guiding the design of next generation aftertreatment catalysts optimized for hydrogen engine exhaust conditions.

Presenting Author: Dhruba Jyoti Deka Pacific Northwest National Laboratory

Presenting Author Biography: Dhruba Jyoti Deka is a Chemical Engineer at Pacific Northwest National Laboratory (PNNL), where he leads research in automotive emissions catalysis. His current work focuses on mechanistic understanding of NOx SCR on Cu-CHA catalysts, including under H2 and NH3 internal combustion engine exhaust conditions. Prior to joining PNNL, he was an R&D Associate Staff at Oak Ridge National Laboratory (ORNL), where he investigated spatiotemporal evolution of reactive species inside aftertreatment catalysts and co-invented a catalytic CO₂ solvent regeneration technology that received an R&D 100 Award in 2024.
Dhruba earned his Ph.D. in Chemical Engineering from The Ohio State University under the mentorship of Prof. Umit S. Ozkan, focusing on high-temperature electrocatalysis for CO₂ reduction and NH₃ synthesis. His research has resulted in 26 peer-reviewed publications, multiple patents, and talks at leading conferences including NAM, ACS, AIChE, ICEF and the DOE AMR. He actively mentors graduate students and postdocs and contributes to collaborative technology development efforts spanning fundamental science and applied energy solutions.

Authors:

Dhruba Jyoti Deka Pacific Northwest National Laboratory
Garam Lee Pacific Northwest National Laboratory
Eric Walter Pacific Northwest National Laboratory
Yong Wang Pacific Northwest National Laboratory
Kenneth Rappe Pacific Northwest National Laboratory