Engineering topics

Lee, Garam

Publications and source records attributed to Lee, Garam.

5 records

DOE OSTI2026

Insights into the mechanisms of NH3 inhibition on Cu-CHA SCR catalysts

This work elucidates the atomic-scale mechanism behind ammonia (NH3) inhibition during the selective catalytic reduction (SCR) of NO? on Cu-CHA catalysts, a key issue limiting low-temperature emission control. Using SCR kinetic analysis, operando electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT), we demonstrate that NH3 inhibition primarily slows the oxidation half-cycle (OHC), while the reduction half-cycle (RHC) remains unaffected. DFT simulations reveal that excess NH3 substantially increases the diffusion barrier for CuI ions, hindering formation of essential CuII-oxo dimer intermediates and thus suppressing OHC kinetics. Operando EPR studies confirm that this inhibition strongly depends on operating temperature and catalyst Cu loading. Our findings highlight strategies to counteract NH3 inhibition, including optimizing Cu loading, precisely managing NH3:NO feed ratios, and enhancing CuI ion mobility through tailored catalyst design and operational adjustments, thereby advancing the efficiency of emission control technologies in automotive applications.

Deka, Dhruba Jyoti↗

DOE OSTI2024

Nonthermal Plasma-Stimulated C–N Coupling from CH 4 and N 2 Depends on the Presence of Surface CH x and Plasma-Phase CN Species

Formation of C–N containing compounds from plasma-catalytic coupling of CH 4 and N 2 over various transition metals (Ni, Pd, Cu, Ag, and Au) is investigated using a multimodal spectroscopic approach, combining polarization-modulation infrared reflection–absorption spectroscopy (PM-IRAS) and optical emission spectroscopy (OES). Through sequential experiments utilizing CH 4 and N 2 nonthermal plasmas, we minimize plasma-phase reactions and identify key intermediates for C–N coupling on metal surfaces. Results show that simultaneous CH 4 and N 2 exposure with plasma stimulation produces surface C–N species. However, N 2 –CH 4 sequential exposure does not lead to C–N species formation, while CH 4 –N 2 sequential exposure reveals the presence of CH x surface species and CN radical species as key precursors to C–N species formation. From further analysis using X-ray photoelectron spectroscopy and liquid chromatography–mass spectrometry, the influence of exposure conditions on the degree of nitrogen incorporation and the nature of C–N species formed were revealed. The work highlights the importance of surface chemistry and exposure conditions in surface C–N coupling with plasma stimulation.

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DOE OSTI2024

Observation and Characterization of Vibrationally Active Surface Species Accessed with Nonthermal Nitrogen Plasmas

Polycrystalline Ni, Pd, Cu, Ag, and Au foils exposed to nonthermal plasma (NTP)-activated N 2 are found to exhibit a vibrational feature near 2200 cm –1 in polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS) observations that are not present in the same materials exposed to N 2 under nonplasma conditions. The feature is similar to that reported elsewhere and is typically assigned to chemisorbed N 2 . We employ a combination of temperature-dependent experiments, sequential dosing, X-ray photoelectron spectroscopy, isotopic labeling, and density functional theory calculations to characterize the feature. Results are most consistent with a triatomic species, likely NCO, with the C and O likely originating from ppm-level impurities in the ultrahigh-purity (UHP) Ar and/or N 2 gas cylinders. Here, this work highlights the potential for nonthermal plasmas to access adsorbates inaccessible thermally as well as the potential contributions of ppm-level impurities to corrupt the interpretation of plasma catalytic chemistry.

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DOE OSTI2022

Recent Advances in Plasma Catalysis

Plasma catalysis is the integration of plasmas and catalysts to achieve reactant conversions and product selectivities that are inaccessible with plasmas or catalysts alone. While chemical transformations via plasma and catalysis are individually well-developed and optimized in many cases, efficient and effective plasma catalysis coupling remains primitive. Molecular understanding of plasma catalysis is further challenged by the complicated natures of plasma and catalysis separately. In this Virtual Issue, we collect 35 examples highlighting recent advances in plasma catalysis that were published in ACS journals from 2019 to 2021. Here, we categorized these 35 into six classes, ranging from fundamental plasma/surface characterizations to applied research on chemical transformations and catalyst synthesis, as presented in Figure 1. We hope this collection is helpful especially to those new to this emerging area.

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DOE OSTI2021

Direct Observation of Plasma-Stimulated Activation of Surface Species Using Multimodal In Situ/Operando Spectroscopy Combining Polarization-Modulation Infrared Reflection-Absorption Spectroscopy, Optical Emission Spectroscopy, and Mass Spectrometry

Nonthermal plasmas (NTPs) produce reactive chemical environments, including electrons, ions, radicals, and vibrationally excited molecules, that can drive chemistry at temperatures at which such species are thermally inaccessible. There has been growing interest in the integration of conventional catalysis with reactive NTPs to promote novel chemical transformations. Unveiling the full potential of plasma-catalytic processes requires a comprehensive understanding of plasma-catalytic synergies, including characterization of plasma-catalytic surface interactions. In this work, we report on a newly designed multimodal spectroscopic instrument combining polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS), mass spectrometry, and optical emission spectroscopy (OES) for the investigation of plasma–surface interactions such as those found in plasma catalysis. In particular, this tool has been utilized to correlate plasma-phase chemistry with both surface chemistry and gas-phase products in situ (1) during the deposition of carbonaceous surface species via NTP-promoted nonoxidative coupling of methane and (2) during subsequent activation of surface deposits with an atmospheric pressure and temperature argon plasma jet on both nickel (Ni) and silicon dioxide (SiO 2 ) surfaces. For the first time, the activation of carbonaceous surface species by a NTP on Ni and SiO 2 surfaces to form hydrogen gas and C 2 hydrocarbons was directly observed, where both PM-IRAS and OES measurements suggest that they may form through different pathways. Furthermore, this unique tool for studying plasma–surface interactions could enable more rational design of plasma-stimulated catalytic processes.

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