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Koh, Katherine

Publications and source records attributed to Koh, Katherine.

8 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Formation of (Rh–Fe)–FeO x Complex Sites Enables Methanol Synthesis from CO 2

Here, we addressed the challenges of designing catalysts for selective CO 2 hydrogenation by incorporating oxide Fe species onto Rh nanoparticles. Nanoscopic FeO x domains created a “reverse catalyst” structure (i.e., a metal oxide supported on a metal) that increased the density of interfacial sites compared to traditional supported catalysts. The contact between the metal nanoparticle and the oxide overlayer induced the formation of a surface Rh-Fe alloy that stabilize methoxy groups while suppressing hydrogenolysis to methane. Sites at FeO x -metal interfaces interact with CO 2 sevenfold stronger than sites on metal surfaces, show larger energy barriers to cleave the C-O bonds, and offer a barrierless pathway for hydrogenation of methoxy species to methanol. Consequently, the multifunctional sites over FeO x /Rh-Fe catalysts highlight and meet the requirements of a selective methanol catalyst: strong interaction with CO 2 to ensure high density of transition states; metal sites to activate and make hydrogen available to surface intermediates; and high energy barriers for C-O bond cleavage to form carbides. These synthesis and catalytic chemistries, demonstrated for Rh-Fe-FeO x interfaces, enable us to overcome the limitations to the design of methanol production catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Influence of the Molecular Structure on the Electrocatalytic Hydrogenation of Carbonyl Groups and H 2 Evolution on Pd

We investigated the electrocatalytic hydrogenation (ECH) of model aldehydes and ketones over carbon supported Pd in aqueous phase. We propose reaction mechanisms based on kinetic measurements and on spectroscopic and electrochemical characterization of the working catalyst. The reaction rates of ECH and of the H 2 evolution reaction (HER) vary with the applied electric potential following trends that strongly depend on the organic substrate. The intrinsic rates of hydrogenation and H 2 evolution are influenced, in opposing ways, by the sorption of the reacting organic substrate. Strong interactions, i.e., higher standard free energies of adsorption of the organic compound, induce high hydrogenation rates but decrease the overall currents. The fast hydrogenation kinetics produces a hydrogen-depleted environment that kinetically hinders the HER and the bulk phase transition of Pd to a H-rich bulk Pd hydride, which is triggered by the applied potential in the absence of reacting organic compounds. As consequence of strong organic-metal interactions, hydrogenation dominates at low overpotential. However, the coverages of organic substrates on the metal surface decrease and the rates of H 2 evolution surpass those of hydrogenation with increasingly negative electric potential. We determined the range of electric potential favoring hydrogenation on Pd and quantitatively deconvoluted the effects of the sorption of the organic compound, and of the rates of proton coupled electron transfers, on the kinetics of both ECH and HER. The results indicate that electrocatalysis offers hydrogenation pathways for polar molecules that are different and, in some cases, faster than those dominating in the absence of an external electric potential.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Experimental Studies of the Dynamics of Adsorbed and Condensed Phase Iodine Species Formed in Severe Reactor Accidents (Final Project Report)

We have studied the chemical transformations of semi-volatile organic iodide species (RI) in the condensed phase as well as adsorbed onto the surfaces of environmentally relevant particles, e.g., soot (carbon) and dust (silica) under real-world conditions. The combination of chosen experimental methods provide a molecular-level insight into the chemical dynamics of organic iodides. Throughout this project, we have developed new experimental, quantum chemistry, and data analysis capabilities. All experimental work was performed with 127 I.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2021

The role of defects and solid/liquid interfacial interactions on controlling anisotropic growth of novel two-dimensional materials

Atomic-scale understanding about how interfacial properties are modified in the presence of defects and how they are correlated to the properties of nucleation and growth is critical for controlled materials growth on solid/liquid interfaces. The goal of this proposal is to establish preliminary data on the critical role of surface defects and chemical compositions of solvent on the structure, dynamics, reactivity, and free energetics at graphene/ionic liquid (IL) interfaces and their impact on two-dimensional (2D) growth of transition metal oxides on the surface. This study aims to develop generalized physical/chemical principles for directional growth at solid/liquid interfaces to achieve atomic-level control of two-dimensional materials synthesis. The proposed study of the effects of surface defects on the interfacial structure and reactivity will improve our understanding of the critical roles of heterogeneity, disorder, and solvents on 2D materials synthesis and lead to better control of directional growth of materials at solid/liquid interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Single-Use Destructive Assay for Uranium Hexafluoride Sampling

Sampling uranium hexafluoride (UF6) for the determination of enrichments by destructive analysis (DA) is a critical component in the International Atomic Energy Agency’s layered safeguards approach for uranium processing facilities. Typically, gram-quantity UF6 samples are collected during inspections and stored under tag-and-seal until transportation to an off-site analytical laboratory. The shipping times can be long, and evolving restrictions on radioactive/corrosive materials shipments may increasingly limit the IAEA’s ability to transport UF6 samples easily. Pacific Northwest National Laboratory has developed a low-cost UF6 sampling technology called Single-Use Destructive Assay (SUDA) that addresses these challenges, as well as provides DA sample geometries that can be tailored for different analytical methods, including potential on-site analyses. The SUDA samplers, along with a unique holder, are designed for direct attachment to existing taps at uranium processing facilities, allowing gaseous UF6 to come into direct contact with a zeolite film. The SUDA technology features the ability to capture uranium in a more easily shipped and handled form as the solid, more stable, and relatively less hazardous hydrated uranyl fluoride (UO2F2•nH2O), which is formed through the controlled hydrolysis of UF6. We have recently simulated uranium collection under enrichment plant sampling conditions to further improve our understanding of SUDA sampling. Presented here is our recent work on measuring the relationship between sampling conditions and uranium collection, which includes control of the uranium-mass-to-zeolite ratio and assessing variable UF6 gas and sampling parameters that can affect collection using the SUDA sampler.

Pope, Timothy R.↗
DOE OSTI2021

Aspects of Crystal Growth and Atomic-Scale Characterization of U/Th Age-Dating Particles

Trace analysis of particulate material collected on environmental swipe samples continues to be one of the cornerstones of the IAEA’s process of verifying member state compliance as part of the Non-Proliferation Treaty (NPT). While complementary to traditional bulk analysis, the ability to measure individual particles can be far more impactful from a radiochronometry perspective. The 234U/230Th radiochronometer is being evaluated as a means of establishing model ages for discrete particles with sufficient confidence. To support the extension of current capabilities, reference materials with a suitable particle form factor and well-documented purification date are actively sought for development with large geometry secondary ion mass spectroscopy.

Nuclear Safeguards, IAEA, Particle Standards↗