Factors Controlling the Reactivity of Catalytically Active Monolayers on Metal Substrates

The focus of this work is on the Pt/MS structures (MS = Au, Ir, Ru, or Pt substrate), as promising electrocatalysts and a prototype for more general systems: (active element monolayer)/(metal substrate) (AE/MS). We evaluate from first principles the effects of AE monolayer strain and the interlayer AE–MS electronic state hybridization on surface reactivity and reveal rationale for the interlayer hybridization to dominate over the strain effect in determining the AE/MS surface reactivity. We find, however, that, if AE is weakly bound to MS, the surface electronic structure does not suffice to characterize the surface reactivity, because of involvement of other factors related to lattice response to adsorption of a reaction intermediate. Guided by our findings, we trace surface reactivity to a newly introduced hybridization parameter that reflects important features of the electronic structure of the AE/MS surface, which are not taken into account in the original <i>d-</i>band center model

Design of Optimally Stable Molecular Coatings for Fe-Based Nanoparticles in Aqueous Environments

Magnetic nanoparticles are widely used in biomedical and oil-well applications in aqueous, often harsh environments. The pursuit for high-saturation magnetization together with high stability of the molecular coating that prevents agglomeration and oxidation remains an active research area. Here, we report a detailed analysis of the criteria for the stability of molecular coatings in aqueous environments along with extensive first-principles calculations for magnetite, which has been widely used, and cementite, a promising emerging candidate. A key result is that the simple binding energies of molecules cannot be used as a definitive indicator of relative stability in a liquid environment. Instead, we find that H⁺ ions and water molecules facilitate the desorption of molecules from the surface. We further find that, because of differences in the geometry of crystal structures, molecules generally form stronger bonds on cementite surfaces than they do on magnetite surfaces. The net result is that molecular coatings of cementite nanoparticles are more stable. This feature, together with the better magnetic properties, makes cementite nanoparticles a promising candidate for biomedical and oil-well applications

Anisotropic Ordering in 1T′ Molybdenum and Tungsten Ditelluride Layers Alloyed with Sulfur and Selenium

Alloying is an effective way to engineer the band-gap structure of two-dimensional transition-metal dichalcogenide materials. Molybdenum and tungsten ditelluride alloyed with sulfur or selenium layers (MX_2<i>x</i>Te_{2(1–<i>x</i>)}, M = Mo, W and X = S, Se) have a large band-gap tunability from metallic to semiconducting due to the 2H-to-1T′ phase transition as controlled by the alloy concentrations, whereas the alloy atom distribution in these two phases remains elusive. Here, combining atomic resolution <i>Z</i>-contrast scanning transmission electron microscopy imaging and density functional theory (DFT), we discovered that anisotropic ordering occurs in the 1T′ phase, in sharp contrast to the isotropic alloy behavior in the 2H phase under similar alloy concentration. The anisotropic ordering is presumably due to the anisotropic bonding in the 1T′ phase, as further elaborated by DFT calculations. Our results reveal the atomic anisotropic alloyed behavior in 1T′ phase layered alloys regardless of their alloy concentration, shining light on fine-tuning their physical properties <i>via</i> engineering the alloyed atomic structure

Competitive Coadsorption of CO₂ with H₂O, NH₃, SO₂, NO, NO₂, N₂, O₂, and CH₄ in M‑MOF-74 (M = Mg, Co, Ni): The Role of Hydrogen Bonding

The importance of coadsorption for applications of porous materials in gas separation has motivated fundamental studies, which have initially focused on the comparison of the binding energies of different gas molecules in the pores (i.e., energetics) and their overall transport. By examining the competitive coadsorption of several small molecules in M-MOF-74 (M = Mg, Co, Ni) with in situ infrared spectroscopy and ab initio simulations, we find that the binding energy at the most favorable (metal) site is not a sufficient indicator for prediction of molecular adsorption and stability in MOFs. Instead, the occupation of the open metal sites is governed by kinetics, whereby the interaction of the guest molecules with the MOF organic linkers controls the reaction barrier for molecular exchange. Specifically, the displacement of CO₂ adsorbed at the metal center by other molecules such as H₂O, NH₃, SO₂, NO, NO₂, N₂, O₂, and CH₄ is mainly observed for H₂O and NH₃, even though SO₂, NO, and NO₂ have higher binding energies (∼70–90 kJ/mol) to metal sites than that of CO₂ (38 to 48 kJ/mol) and slightly higher than that of water (∼60–80 kJ/mol). DFT simulations evaluate the barriers for H₂O → CO₂ and SO₂ → CO₂ exchange to be ∼13 and 20 kJ/mol, respectively, explaining the slow exchange of CO₂ by SO₂, compared to water. Furthermore, the calculations reveal that the kinetic barrier for this exchange is determined by the specifics of the interaction of the second guest molecule (e.g., H₂O or SO₂) with the MOF ligands. Hydrogen bonding of H₂O molecules with the nearby oxygen of the organic linker is found to facilitate the positioning of the H₂O oxygen atom toward the metal center, thus reducing the exchange barrier. In contrast, SO₂ molecules interact with the distant benzene site, away from the metal center, hindering the exchange process. Similar considerations apply to the other molecules, accounting for much easier CO₂ exchange for NH₃ than for NO, NO₂, CH₄, O₂, and N₂ molecules. In this work, critical parameters such as kinetic barrier and exchange pathway are first unveiled and provide insight into the mechanism of competitive coadsorption, underscoring the need of combined studies, using spectroscopic methods and ab initio simulations to uncover the atomistic interactions of small molecules in MOFs that directly influence coadsorption

Water Reaction Mechanism in Metal Organic Frameworks with Coordinatively Unsaturated Metal Ions: MOF-74

Water dissociation represents one of the most important reactions in catalysis, essential to the surface and nano sciences [e.g., Hass et al., Science, 1998 282, 265–268; Brown et al., Science, 2001, 294, 67–69; Bikondoa et al., Nature, 2005, 5, 189–192]. However, the dissociation mechanism on most oxide surfaces is not well understood due to the experimental challenges of preparing surface structures and characterizing reaction pathways. To remedy this problem, we propose the metal organic framework MOF-74 as an ideal model system to study water reactions. Its crystalline structure is well characterized; the metal oxide node mimics surfaces with exposed cations; and it degrades in water. Combining <i>in situ</i> IR spectroscopy and first-principles calculations, we explored the MOF-74/water interaction as a function of vapor pressure and temperature. Here, we show that, while adsorption is reversible below the water condensation pressure (∼19.7 Torr) at room temperature, a reaction takes place at ∼150 °C even at low water vapor pressures. This important finding is unambiguously demonstrated by a clear spectroscopic signature of the direct reaction using D₂O, which is not present using H₂O due to strong phonon coupling. Specifically, a sharp absorption band appears at 970 cm^–1 when D₂O is introduced at above 150 °C, which we attribute to an O–D bending vibration on the phenolate linker. Although H₂O undergoes a similar dissociation reaction, the corresponding O–H mode is too strongly coupled to MOF vibrations to detect. In contrast, the O–D mode falls in the phonon gap of the MOF and remains localized. First-principles calculations not only positively identify the O–D mode at 970 cm^–1 but derive a pathway and kinetic barrier for the reaction and the final configuration: the D (H) atom is transferred to the oxygen of the linker phenolate group, producing the notable O–D absorption band at 970 cm^–1, while the OD (or OH) binds to the open metal sites. This finding explains water dissociation in this case and provides insight into the long-lasting question of MOF-74 degradation. Overall, it adds to the understanding of molecular water interaction with cation-exposed surfaces to enable development of more efficient catalysts for water dissociation

Interaction of Acid Gases SO₂ and NO₂ with Coordinatively Unsaturated Metal Organic Frameworks: M‑MOF-74 (M = Zn, Mg, Ni, Co)

<i>In situ</i> infrared spectroscopy and <i>ab initio</i> density functional theory (DFT) calculations are combined to study the interaction of the corrosive gases SO₂ and NO₂ with metal organic frameworks M-MOF-74 (M = Zn, Mg, Ni, Co). We find that NO₂ dissociatively adsorbs into MOF-74 compounds, forming NO and NO₃^–. The mechanism is unraveled by considering the Zn-MOF-74 system, for which DFT calculations show that a strong NO₂–Zn bonding interaction induces a significant weakening of the N–O bond, facilitating the decomposition of the NO₂ molecules. In contrast, SO₂ is only molecularly adsorbed into MOF-74 with high binding energy (>90 kJ/mol for Mg-MOF-74 and >70 for Zn-MOF-74). This work gives insight into poisoning issues by minor components of flue gases in metal organic frameworks materials

Role of Hydrogen Bonding on Transport of Coadsorbed Gases in Metal–Organic Frameworks Materials

Coadsorption of multicomponents in metal–organic framework (MOF) materials can lead to a number of cooperative effects, such as modification of adsorption sites or during transport. In this work, we explore the incorporation of NH₃ and H₂O into MOFs preloaded with small molecules such as CO, CO₂, and SO₂. We find that NH₃ (or H₂O) first displaces a certain amount of preadsorbed molecules in the outer portion of MOF crystallites, and then substantially hinders diffusion. Combining <i>in situ</i> spectroscopy with first-principles calculations, we show that hydrogen bonding between NH₃ (or H₂O) is responsible for an increase of a factor of 7 and 8 in diffusion barrier of CO and CO₂ through the MOF channels. Understanding such cooperative effects is important for designing new strategies to enhance adsorption in nanoporous materials