Synthesis, Characterization, and Photocatalytic Activity of Y‑Doped CeO₂ Nanorods

Yttrium-doped ceria (YDC) nanorods were prepared by hydrothermal synthesis and characterized using Raman, UV–vis, transmission electron microscopy, scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray powder diffraction. The ceria nanorods showed an increase in the amount of oxygen vacancies with an increase in the Y concentration. When the doping level is <30%, the optical band gap of the doped ceria is lower than that of pure ceria nanorods. At < 50% of Y doping, the composite nanorods exhibited a higher photocatalytic activity for the degradation of model organic dyes compared to the pure ceria at room temperature, and the catalyst with 10% loading showed the maximum photocatalytic efficiency. However, at 100 °C, the photocatalytic activity significantly improved for all the nanorods with different Y loadings, and the greatest improvement was obtained for the sample with the highest number of oxygen vacancies

Luminescent Metal–Organic Framework for the Selective Detection of Aldehydes

The detection of toxic, hazardous chemical species is an important task because they pose serious risks to either the environment or human health. Luminescent metal–organic frameworks (LMOFs) as alternative sensors offer rapid and sensitive detection of chemical species. Interactions between chemical species and LMOFs result in changes in the photoluminescence (PL) profile of the LMOFs which can be readily detected using a simple fluorometer. Herein, we report the use of a robust, Zn-based LMOF, [Zn5(μ3-OH)2(adtb)2(H2O)5·5 DMA] (Zn-adtb, LMOF-341), for the selective detection of benzaldehyde. Upon exposure to benzaldehyde, Zn-adtb experiences significant luminescent quenching, as characterized through PL experiments. Photoluminescent titration experiments reveal that LMOF-341 has a detection limit of 64 ppm and a Ksv value of 179 M–1 for benzaldehyde. Furthermore, we study the guest–host interactions that occur between LMOF-341 and benzaldehyde through in situ Fourier transform infrared and computational modeling employing density functional theory. The results show that benzaldehyde interacts more strongly with LMOF-341 compared to formaldehyde and propionaldehyde. Our combined studies also reveal that the mechanism of luminescence quenching originates from an electron-transfer process

Stability and Hydrolyzation of Metal Organic Frameworks with Paddle-Wheel SBUs upon Hydration

Instability of most prototypical metal organic frameworks (MOFs) in the presence of moisture is always a limitation for industrial scale development. In this work, we examine the dissociation mechanism of microporous paddle wheel frameworks M­(bdc)­(ted)_0.5 [M = Cu, Zn, Ni, Co; bdc = 1,4-benzenedicarboxylate; ted = triethylenediamine] in controlled humidity environments. Combined in situ IR spectroscopy, Raman, and Powder X-ray diffraction measurements show that the stability and modification of isostructual M­(bdc)­(ted)_0.5 compounds upon exposure to water vapor critically depend on the central metal ion. A hydrolysis reaction of water molecules with Cu–O–C is observed in the case of Cu­(bdc)­(ted)_0.5. Displacement reactions of ted linkers by water molecules are identified with Zn­(bdc)­(ted)_0.5 and Co­(bdc)­(ted)_0.5. In contrast,. Ni­(bdc)­(ted)_0.5 is less susceptible to reaction with water vapors than the other three compounds. In addition, the condensation of water vapors into the framework is necessary to initiate the dissociation reaction. These findings, supported by supported by first principles theoretical van der Waals density functional (vdW-DF) calculations of overall reaction enthalpies, provide the necessary information for determining operation conditions of this class of MOFs with paddle wheel secondary building units and guidance for developing more robust units

Selective, Sensitive, and Reversible Detection of Vapor-Phase High Explosives via Two-Dimensional Mapping: A New Strategy for MOF-Based Sensors

A new strategy has been developed for the effective detection of high explosives in vapor phase by fluorescent metal–organic framework (MOF) sensors. Two structurally related and dynamic MOFs, (Zn)₂(ndc)₂P·<i>x</i>G [ndc = 2,6-naphthalenedicarboxylate; P =1,2-bis­(4-pyridyl)­ethane (bpe) or 1,2-bis­(4-pyridyl)­ethylene (bpee); G = guest/solvent molecule], exhibit a two-dimensional signal response toward analytes of interest in the vapor phase, including aromatic and aliphatic high explosives (e.g., TNT and RDX). The interaction between analytes and the MOF has been studied using in situ infrared absorption spectroscopy and a DFT computational method

CO₂ capture by hybrid ultramicroporous TIFSIX-3-Ni under humid conditions using non-equilibrium cycling

Although pyrazine-linked hybrid ultramicroporous materials (HUMs, pore size <7 Å) are benchmark physisorbents for trace carbon dioxide (CO2) capture under dry conditions, their affinity for water (H2O) mitigates their carbon capture performance in humid conditions. Herein, we report on the co-adsorption of H2O and CO2 by TIFSIX-3- Ni—a high CO2 affinity HUM—and find that slow H2O sorption kinetics can enable CO2 uptake and release using shortened adsorption cycles with retention of ca. 90% of dry CO2 uptake. Insight into co-adsorption is provided by in situ infrared spectroscopy and ab initio calculations. The binding sites and sorption mechanisms reveal that both CO2 and H2O molecules occupy the same ultramicropore through favorable interactions between CO2 and H2O at low water loading. An energetically favored water network displaces CO2 molecules at higher loading. Our results offer bottom-up design principles and insight into co-adsorption of CO2 and H2O that is likely to be relevant across the full spectrum of carbon capture sorbents to better understand and address the challenge posed by humidity to gas capture.</p

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

Modulation of water vapor sorption by a 4th generation metal-organic material with a rigid framework and self-switching pores

Hydrolytically stable adsorbents are needed for water vapor sorption related applications; however, design principles for porous materials with tunable water sorption behavior are not yet established. Here, we report that a platform of fourth-generation metal–organic materials (MOMs) with rigid frameworks and self-switching pores can adapt their pores to modulate water sorption. This platform is based upon the hydrolytically stable material CMOM-3S, which exhibits bnn topology and is composed of rod building blocks based upon S-mandelate ligands, 4,4-bipyridine ligands, and extraframework triflate anions. Isostructural variants of CMOM-3S were prepared using substituted R-mandelate ligands and exhibit diverse water vapor uptakes (20–67 cm3/g) and pore filling pressures (P/P0, 0.55–0.75). [Co2(R-4-Cl-man)2(bpy)3](OTf) (33R) is of particular interest because of its unusual isotherm. Insight into the different water sorption properties of the materials studied was gained from analysis of in situ vibrational spectra, which indicate self-switching pores via perturbation of extraframework triflate anions and mandelate linker ligands to generate distinctive water binding sites. Water vapor adsorption was studied using in situ differential spectra that reveal gradual singlet water occupancy followed by aggregation of water clusters in the channels upon increasing pressure. First-principles calculations identified the water binding sites and provide structural insight on how adsorbed water molecules affect the structures and the binding sites. Stronger triflate hydrogen bonding to the framework along with significant charge redistribution were determined for water binding in 33R. This study provides insight into a new class of fourth-generation (self-switching pores) MOM and the resulting effect upon water vapor sorption propertie

Mechanism of Preferential Adsorption of SO₂ into Two Microporous Paddle Wheel Frameworks M(bdc)(ted)_0.5

The selective adsorption of a corrosive gas, SO₂, into two microporous pillared paddle-wheel frameworks M­(bdc)­(ted)_0.5 [<i>M</i> = Ni, Zn; bdc =1,4-benzenedicarboxylate; ted = triethylenediamine] is studied by volumetric adsorption measurements and a combination of <i>in situ</i> infrared spectroscopy and <i>ab initio</i> density functional theory (DFT) calculations. The uptake of SO₂ in M­(bdc)­(ted)_0.5 at room temperature is quite significant, 9.97 mol/kg at 1.13 bar. The major adsorbed SO₂ molecules contributing to the isotherm measurements are characterized by stretching bands at 1326 and 1144 cm^–1. Theoretical calculations including van der Waals interactions (based on vdW-DF) suggest that two adsorption configurations are possible for these SO₂ molecules. One geometry involves an SO₂ molecule bonded through its sulfur atom to the oxygen atom of the paddle-wheel building unit and its two oxygen atoms to the C–H groups of the organic linkers by formation of hydrogen bonds. Such a configuration results in a distortion of the benzene rings, which is consistent with the experimentally observed shift of the ring deformation mode. In the other geometry, SO₂ establishes hydrogen bonding with −CH₂ group of the ted linker through its two oxygen atoms simultaneously. The vdW-DF-simulated frequency shifts of the SO₂ stretching bands in these two configurations are similar and in good agreement with spectroscopically measured values of physisorbed SO₂. In addition, the IR spectra reveal the presence of another minor species, characterized by stretching modes at 1242 and 1105 cm^–1 and causing significant perturbations of MOFs vibrational modes (CH_<i>x</i> and carboxylate groups). This species is more strongly bound, requiring a higher temperature (∼150 °C) to remove it than for the main physisorbed species. The adsorption configurations of SO₂ into M­(bdc)­(ted)_0.5 derived by infrared spectroscopy and vdW-DF calculations provide the initial understanding to develop microporous metal organic frameworks materials based on paddlewheel secondary-building units for SO₂ removal in industrial processes