Mal-Soon Lee | Pacific Northwest National Laboratory (original) (raw)

Papers by Mal-Soon Lee

Bulletin of the American Physical Society, Mar 3, 2020

Bulletin of the American Physical Society, 2019

The Journal of Chemical Physics, 2021

In catalysis, MgO is often used to modify the acid–base properties of support oxides and to stabi... more In catalysis, MgO is often used to modify the acid–base properties of support oxides and to stabilize supported metal atoms and particles on oxides. In this study, we show how the sublimation of MgO powder can be used to deposit MgO monomers, hither on anatase TiO2(101). A combination of x-ray electron spectroscopy, high-resolution scanning tunneling microscopy, and density functional theory is employed to gain insight into the MgO monomer binding, electronic and vibrational properties, and thermal stability. In the most stable configuration, the Mg and O of the MgO monomer bind to two surface oxygens and one undercoordinated surface titanium, respectively. The additional binding weakens the Mg–O monomer bond and makes Mg more ionic. The monomers are thermally stable up to 600 K, where the onset of diffusion into the TiO2 bulk is observed. The monomeric MgO species on TiO2(101) represent an ideal atomically precise system with modified acid–base properties and will be employed in ou...

Proceedings of the National Academy of Sciences, 2021

Significance The design and synthesis of hierarchically ordered oxides remains a critical challen... more Significance The design and synthesis of hierarchically ordered oxides remains a critical challenge in material science and catalysis. Here, we demonstrate that well-ordered homotopic arrays of mono-oxo (MoO 3 ) 1 can be easily prepared on anatase TiO 2 (101) via the deposition of (MoO 3 ) n oligomers. As revealed by our combined experiential and theoretical studies, the oligomers spontaneously decompose and self-assemble into chemically identical and thermally stable monomers. The oligomer decomposition is permitted at room temperature due to the dynamic coupling of decomposition steps to the lattice phonons of TiO 2 . We identify transient mobility of the oligomers as key to the self-assembly of the complete overlayer. The ease of preparation and thermal stability of this atomically precise system makes it highly suitable for a broad range of applications.

ACS Symposium Series, 2020

In recent years, enormous efforts have been devoted to improving the atom efficiency of metal cat... more In recent years, enormous efforts have been devoted to improving the atom efficiency of metal catalysts by decreasing the size of metal particles to the limit of isolated atoms in heterogeneous catalysis, termed single-atom catalysts (SACs). In addition to being atom-efficient, these SACs are increasingly gaining attention because of the unique reactivity and selectivity they exhibit in industrially relevant reactions, including both oxidation and hydrogenation reactions. Clear parallels can be drawn between heterogeneous SACs and their analogous homogeneous catalysts. In this chapter, we highlight SACs that demonstrate this relationship and report on relevant, recent developments in the field of single-atom catalysis.

Journal of Catalysis, 2020

Ce document est protégé par la loi sur le droit d'auteur. L'utilisation des services d'Érudit (y ... more Ce document est protégé par la loi sur le droit d'auteur. L'utilisation des services d'Érudit (y compris la reproduction) est assujettie à sa politique d'utilisation que vous pouvez consulter en ligne. [https://apropos.erudit.org/fr/usagers/politiquedutilisation/] Cet article est diffusé et préservé par Érudit.

Angewandte Chemie International Edition, 2020

In this Communication, the information in the acknowledgements is incomplete. The updated acknowl... more In this Communication, the information in the acknowledgements is incomplete. The updated acknowledgement must read: "G.J.Y. is grateful for the support by the 111Project (B17020) of China and the postdoctoral international exchange program of China.

Angewandte Chemie, 2020

Angabe der unten stehenden Digitalobjekt-Identifizierungsnummer (DOI) zitiert werden. Die deutsch... more Angabe der unten stehenden Digitalobjekt-Identifizierungsnummer (DOI) zitiert werden. Die deutsche Übersetzung wird gemeinsam mit der endgültigen englischen Fassung erscheinen. Die endgültige englische Fassung (Version of Record) wird ehestmöglich nach dem Redigieren und einem Korrekturgang als Early-View-Beitrag erscheinen und kann sich naturgemäß von der AA-Fassung unterscheiden. Leser sollten daher die endgültige Fassung, sobald sie veröffentlicht ist, verwenden. Für die AA-Fassung trägt der Autor die alleinige Verantwortung.

The Journal of Physical Chemistry C, 2017

We have studied the mechanism of intercalation and methane adsorption from a H 2 O/CH 4 /CO 2 mix... more We have studied the mechanism of intercalation and methane adsorption from a H 2 O/CH 4 /CO 2 mixture on a prototypical swelling shale component, Ca−montmorillonite. We employed ab initio molecular dynamics simulations at 323 K and 90 bar to obtain molecular level information on adsorption energetics, speciation, and structural and thermodynamic properties. Interaction of CH 4 with surface Lewis acidic sites (Ca 2+ , surface OH) results in large induced dipoles (∼1 D) that lead to relatively strong adsorption energies compared to interactions of the normally apolar CH 4 that level off once a CH 4 layer is formed. Intercalated CH 4 , also exhibits large induced dipoles at lower hydration levels, when the interaction with Ca 2+ cations are less hindered. CO 2 displaces CH 4 in the coordination sphere of the cations (in the interlayer) or on the surface, thereby driving CH 4 extraction. Our simulations indicate that there is an optimal pressure range (∼70−90 bar) where scCO 2-facilitated CH 4 extraction will be maximized.

Catalysis Today, 2019

We present a multiscale modeling study on the electro-reduction of benzaldehyde (BZY) on Au(111) ... more We present a multiscale modeling study on the electro-reduction of benzaldehyde (BZY) on Au(111) under realistic electrochemical conditions. To model the electrochemical cell, we adopt a capacitor model in which complex solvents are confined between a Au cathode and a carbon anode. Classical molecular dynamics simulations reveal that electrode charge density and the presence of alcohol show strong effects on the density, adsorption geometry and dynamics of benzaldehyde on the Au electrode. Under charging conditions, the surface concentration of benzaldehyde on the Au electrode decreases, while the content of other species increases. Finally, we proposed a scheme that correlates the electric current running through the Au/solvent interface with the applied bias. This study provides a molecular level understanding of how solvent composition, in this case water/alcohol content, controls the activity of electrocatalytic hydrogenation

Journal of the American Chemical Society, 2019

The catalytic sites of acidic zeolite are profoundly altered by the presence of water changing th... more The catalytic sites of acidic zeolite are profoundly altered by the presence of water changing the nature of the Brønsted acid site. High resolution solid-state NMR spectroscopy shows water interacting with zeolite Brønsted acid sites, converting them to hydrated hydronium ions over a wide range of temperature and thermodynamic activity of water. A signal at 9 ppm was observed at loadings of 2-9 water molecules per Brønsted acid site, and is assigned to hydrated hydronium ions on the basis of the evolution of the signal with increasing water content, chemical shift calculations, and the direct comparison with HClO4 in water. The intensity of 1 H-29 Si crosspolarization signal first increased and then decreased with increasing water chemical potential. This indicates that hydrogen bonds between water molecules and the tetrahedrally coordinated aluminum in the zeolite lattice weaken with the formation of hydronium ion-water clusters and increase the mobility of protons. DFT-based ab initio molecular dynamics studies at multiple temperatures and water concentrations agree well with this interpretation. Above 140 °C, however, fast proton exchange between bridging hydroxyl groups and water occur even in the presence of only one water molecule per acid site.

ACS nano, Jan 31, 2017

The structure, composition, and atomic distribution of nanoalloys under operating conditions are ... more The structure, composition, and atomic distribution of nanoalloys under operating conditions are of significant importance for their catalytic activity. In the present work, we use ab initio molecular dynamics simulations to understand the structural behavior of Au-Pd nanoalloys supported on rutile TiO2 under different conditions. We find that the Au-Pd structure is strongly dependent on the redox properties of the support, originating from strong metal-support interactions. Under reducing conditions, Pd atoms are inclined to move toward the metal/oxide interface, as indicated by a significant increase of Pd-Ti bonds. This could be attributed to the charge localization at the interface that leads to Coulomb attractions to positively charged Pd atoms. In contrast, under oxidizing conditions, Pd atoms would rather stay inside or on the exterior of the nanoparticle. Moreover, Pd atoms on the alloy surface can be stabilized by hydrogen adsorption, forming Pd-H bonds, which are stronger ...

Chemistry of Materials, 2020

Metal–organic framework (MOF) heterostructures exhibit unique properties beyond those of individu... more Metal–organic framework (MOF) heterostructures exhibit unique properties beyond those of individual components, but their design requires an understanding of energetic and kinetic controls at MOF–s...

Single-atom catalysts are often reported to have catalytic properties that surpass those of nanop... more Single-atom catalysts are often reported to have catalytic properties that surpass those of nanoparticles, while a direct comparison of sites common and different for both is lacking. Here we show that single atoms of Pt-group metals embedded into the surface of Fe3O4 have a greatly enhanced interaction strength with CO2 compared with the Fe3O4 surface. The strong CO2 adsorption on single Rh atoms and corresponding low activation energies lead to 2 orders of magnitude higher conversion rates of CO2 compared to Rh nanoparticles. This high activity of single atoms stems from the partially oxidic state imposed by their coordination to the support. Fe3O4-supported Rh nanoparticles follow the behavior of single atoms for CO2 interaction and reduction, which is attributed to the dominating role of partially oxidic sites at the Fe3O4-Rh interface. Thus, we show a likely common catalytic chemistry for two kinds of materials thought to be different, and we show that single atoms of Pt-group metals on Fe3O4 are especially successful materials for catalyzed reactions that depend primarily upon sites with the metal-O-Fe environment.

Ethanol is an important C2 platform molecule for producing value-added chemicals and distillate h... more Ethanol is an important C2 platform molecule for producing value-added chemicals and distillate hydrocarbon fuels (e.g., jet and diesel). Among these, catalytic upgrading of ethanol to butenes can ...

tions and experiments carried out separately on methane and water, the main components of the mid... more tions and experiments carried out separately on methane and water, the main components of the middle layers of Neptune and Uranus, show that at those conditions methane disproportionates into carbon-rich species and water dissociates to form an ionic fluid. Water becomes electronically conducting only at the conditions found in the deepest layers of the planets. More recent simulations on water/methane mixtures suggest a pressure-induced softening of the methane-water intermolecular repulsion that points to an enhancement of mixing under extreme conditions. In the mixtures, ionized water causes the progressive ionization of methane and the mixture becomes electronically conductive at milder conditions than pure water. Calculations on the crystalline counterparts, methane hydrate clathrates, suggest however a different picture: mixtures at low temperature become increasingly unstable, with increasing pressure, towards phase separation, despite the prediction of a solid-solid phase transition between MH-III, the known high-pressure form of methane hydrate, and a new hypothetical phase.

The dynamics of reactive intermediates are important in catalysis for understanding transient spe... more The dynamics of reactive intermediates are important in catalysis for understanding transient species, which can drive reactivity and the transport of species to reaction centers. In particular, the interplay between surface-bound carboxylic acids and carboxylates is important for numerous chemical transformations, including CO2 hydrogenation and ketonization. Here, we investigate the dynamics of adsorbed, dissociated acetic acid to provide insight into the formation of reaction intermediates using scanning tunneling microscopy experiments and density functional theory calculations. We demonstrate the concomitant diffusion of bidentate acetate and a bridging hydroxyl on anatase TiO2(101) and provide evidence for the transient formation of molecular, monodentate acetic acid. The diffusion rate is strongly dependent on the position of the bridging hydroxyl and the presence of adjacent bidentate acetate(s). A facile three-step diffusion process is proposed consisting of a bidentate ace...