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Noh, Hyunho

Publications and source records attributed to Noh, Hyunho.

8 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Electrochemically Determined and Structurally Justified Thermochemistry of H atom Transfer on Ti-Oxo Nodes of the Colloidal Metal–Organic Framework Ti-MIL-125

Titanium dioxide (TiO 2 ) has long been employed as (photo)electrodes for reactions relevant to energy storage and renewable energy synthesis. Proton-coupled electron transfer (PCET) reactions with equimolar amounts of protons and electrons at the TiO 2 surface or within the bulk structure lie at the center of these reactions. Because a proton and an electron are thermochemically equivalent to an H-atom, these reactions are essentially H-atom transfer reactions. Thermodynamics of H-atom transfer has a complex dependence on the synthetic protocol and chemical history of the electrode, the reaction medium, and many others; together, these complications preclude the understanding of the H-atom transfer thermochemistry with atomic-level structural knowledge. Herein, we report our success in employing open-circuit potential (E OCP ) measurements to quantitatively determine the H-atom transfer thermochemistry at structurally well-defined Ti-oxo clusters within a colloidally stabilized metal–organic framework (MOFs), Ti-MIL-125. The free energy to transfer H-atom, Ti 3+ O–H bond dissociation free energy (BDFE), was measured to be 68(2) kcal mol -1 . To the best of our understanding, this is the first report on using E OCP measurements to quantify thermochemistry on any MOFs. The proton topology, the structural change upon the redox reaction, and BDFE values were further quantitatively corroborated using computational simulations. Furthermore, comparisons of the E OCP -derived BDFEs of Ti-MIL-125 to similar parameters in the literature suggest that E OCP should be the preferred method for quantitatively accurate BDFE calculations. Here, the reported success in employing E OCP for nanosized Ti- MIL-125 should lay the ground for thermochemical measurements of other colloidal systems, which are otherwise challenging. Implications of these measurements on Ti-MIL-125 as an H-atom acceptor in chemical reactions and comparisons with other MOFs/metal oxides are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Photoelectrochemical Proton-Coupled Electron Transfer of TiO 2 Thin Films on Silicon

TiO 2 thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule–semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO 2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO 2 reduction, N 2 reduction, and H 2 evolution. On both n-type Si and irradiated p-type Si, TiO 2 reduction is proton-coupled with a 1e – :1H + stoichiometry, as demonstrated by the Nernstian dependence of the Ti 4+/3+ E 1/2 on the buffer pK a . Experiments were conducted with and without illumination, and a photovoltage of ∼0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO 2 /Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO 2 films after reduction reveals restructuring with the formation of islands, rendering TiO 2 films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Altogether, this work demonstrates that atomic layer deposition TiO 2 films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on “protective” metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators.

Electrodes↗
DOE OSTI2022

Medium-independent hydrogen atom binding isotherms of nickel oxide electrodes

Adsorption and transfer of hydrogen atoms at solid/solution interfaces are fundamental to heterogeneous catalysis for chemical energy transformations and other processes. Reported here are electrochemical and spectroelectrochemical measurements of the thermodynamics of H-atom binding to nickel oxide electrodes, both the average and the distribution of NiO–H bond dissociation free energies (BDFEs). These are perhaps the first measurements of binding isotherms at non-metal electrodes. Remarkably, both the BDFEs and the non-Langmuirian isotherms are the same in water, acetonitrile, and dimethylformamide, and with different buffers and proton activities. Such medium independence of the BDFEs and isotherms has not been previously reported for any binary material. The medium independence supports the common use of computed hydrogen binding energies as intrinsic descriptors of surface reactivity, while the broadened isotherms add a level of complexity to such analyses. Furthermore, this work demonstrates the capability to derive key thermodynamic parameters at chemically reactive solid-liquid interfaces.

13 HYDRO ENERGY↗
DOE OSTI2021

Free Energies of Proton-Coupled Electron Transfer Reagents and Their Applications

Here, we present an update and revision to our 2010 review on the topic of proton- coupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XHn in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H 2 ), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H 2 ) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Unexpected “Spontaneous” Evolution of Catalytic, MOF-Supported Single Cu(II) Cations to Catalytic, MOF-Supported Cu(0) Nanoparticles

A desirable feature of metal–organic frameworks (MOFs) is their well-defined structural periodicity and the presence of well-defined catalyst grafting sites (e.g., reactive –OH and –OH 2 groups) that can support single-site heterogeneous catalysts. However, one should not overlook the potential role of residual organic moieties, specifically formate ions that can occupy the catalyst anchoring sites during MOF synthesis. Here we show how these residual formate species in a Zr-based MOF, NU-1000, critically alter the structure, redox capability, and catalytic activity of postsynthetically incorporated Cu(II) ions. Single-crystal X-ray diffraction measurements established that there are two structurally distinct types of Cu(II) ions in NU-1000: one type with residual formate and one without. In NU-1000 with formate, Cu(II) solely binds to the node via the formate-unoccupied, bridging μ 3 –OH, whereas in the formate-free case, it displaces protons from two node hydroxo ligands and resides close to the terminal –OH 2 . Under an inert atmosphere, node-bound formate facilitates the unanticipated reduction of isolated Cu(II) to nanoparticulate Cu(0)—a behavior which is essentially absent in the formate-free analogue because no other sacrificial reductant is present. When the two MOFs were tested as benzyl alcohol oxidation catalysts, we observed that residual formate boosts the catalytic turnover frequency. Density functional calculations showed that node-bound formate acts as a sacrificial two-electron donor and assists in reducing Cu(II) to Cu(0) by a nonradical pathway. The negative Gibbs free energy of reaction (ΔG) and enthalpy of reaction (ΔH) indicate that the reduction is thermodynamically favorable. Here, the work presented here highlights how the often-neglected residual formate prevalent in nearly all zirconium-based MOFs can significantly modulate the properties of supported catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Stabilization of Low Valent Zirconium Nitrides in Titanium Nitride via Plasma-Enhanced Atomic Layer Deposition and Assessment of Electrochemical Properties

The increasing awareness of metal nitrides as feasible electrocatalysts in numerous energy-relevant transformations motivates greater synthetic control of mixed metal nitride compositions and their valence states. We report plasma-enhanced atomic layer deposition of titanium zirconium nitride, with an emphasis on the experimental parameters that influence the valence states of the two metal ions. We further examined the consequence of various ratios of Ti and Zr on the electrochemically active surface areas (ECSAs). Films with a Zr-to-Ti ratio of 1:2 or 1:1 yielded the highest ECSA; deviation from these values led to a large fraction of electrically insulating or semiconducting titanium or zirconium oxynitrides/oxides. The systematic examination establishes a fundamental structure-property relationship critical to assessing the electrochemical/electrocatalytic properties of metal nitrides.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Single‐Site, Single‐Metal‐Atom, Heterogeneous Electrocatalyst: Metal–Organic‐Framework Supported Molybdenum Sulfide for Redox Mediator‐Assisted Hydrogen Evolution Reaction

Abstract Synthesis of single‐site catalysts, whereby the local structure and surrounding chemical environments are identical, has been challenging, particularly in heterogeneous catalysis, as the support often presents spectrum of chemically distinct binding sites. Yet, the above criteria are crucial in attributing the apparent catalytic performance to the structural motif. The presented work augments on our previous work using monometallic molybdenum sulfide tethered within a zirconium‐based metal‐organic framework (MOF), NU‐1000; the monometallic nature enables all presented sites to be catalytically addressable. As the molybdenum sulfide species resided within two distinct pores (micro‐ and mesopores) of the MOF support, we have imparted uniformity in the local chemical environment by reducing the pore heterogeneity down to a single mesopore. Single‐site and single‐atom nature of the candidate catalyst was established via X‐ray diffraction measurements. Redox mediators were implemented, which, under reductive potentials, provide reduced species; they can effectively deliver the necessary reducing equivalences to the catalytic units that can otherwise not be addressed electrochemically due to the low electron mobility within the framework. Our results indicate the micropore‐allocated molybdenum sulfide is approximately four times more active as that in mesopores, whereas its catalytic mechanism is identical, underscoring the importance of controlling chemical environment beyond the active site.

Noh, Hyunho↗