1,720,965 research outputs found
Probing the structure and reactivity of metal-organic frameworks in operando
No full textThis thesis utilizes synchrotron-based X-ray techniques, advanced data analysis methodologies, and theoretical calculations to comprehensively investigate the structural, electronic, and reactive properties of metal-organic frameworks (MOFs) under operando conditions. In the first study, the structural and catalytic properties of MIL-100(Fe) were explored. It was observed that MIL-100(Fe) forms open Fe(II) sites upon thermal activation, which can catalyze the methane-to-methanol (MTM) conversion using molecular oxygen. A combination of X-ray spectroscopy and diffraction techniques was employed to monitor multiple reaction cycles, providing insight into the possible reaction intermediates. DFT calculations were used to propose a reaction mechanism involving O2 and CH4 interactions with Fe(II) sites to form methanol, highlighting stages where methyl radical intermediates might escape, leading to catalyst deactivation. This study indicated that while MIL-100(Fe) shows some catalytic activity, it is limited for industrial MTM processes, prompting the need for more stable trimetallic iron-based MOFs. The second study focused on water adsorption/desorption mechanisms in Mg-MOF-74. A combination of theoretical MD calculations, AP-NEXAFS, and PXRD revealed how water adsorption causes significant structural changes in the MOF's unit cell, specifically expanding the a axis and contracting the c axis. A machine-learning analysis of NEXAFS data showed residual water adsorbed at high temperatures, even when a significant number of open Mg2+ sites are present. Further, a subsequent study highlighted that different metal ions in MOF-74 significantly influence water retention capabilities, with Mg-MOF-74 retaining more water at high temperatures compared to Co-MOF-74 and Ni-MOF-74. Finally, surface defect formation and reactivity in HKUST-1 were investigated using NEXAFS. It was shown that oxidative decarboxylation leads to defective Cu(I)/Cu(II) paddlewheels that can be restored by exposure to CO2. These findings demonstrate how integrating complementary X-ray techniques with theoretical models provides deep insights into MOF properties, enabling the optimization of MOF performance for industrial applications
In-Depth XANES and EXAFS Characterization of the Ag+ Ion Coordination in Dimethyl Sulfoxide Solution
No full textX-ray absorption near-edge structure (XANES) spectroscopy has been used, in conjunction with extended X-ray absorption fine structure (EXAFS), to determine the coordination structure of the Ag+ ion in a dimethyl sulfoxide (DMSO) solution. From the EXAFS data analysis, the Ag-O first shell distance in DMSO was found to be 2.31(3) & Aring;, with 4.1(5) oxygen atoms surrounding the Ag+ ion, in fair agreement with previous results. This technique did not allow us to determine the geometry of the 4-fold coordination complex and a quantitative analysis of the XANES region was carried out to shed light on this issue. The XANES data analysis confirmed the presence of a four-coordinated complex, unambiguously showing that a regular tetrahedral [Ag(DMSO)(4)](+) complex is formed when silver triflate is dissolved in DMSO solution
Hydrophobicity as the key to understanding the nanostructural behavior of eutectic mixtures upon apolar cosolvent addition
Full text linkThe influence of the addition of the apolar n-hexane (HEX) cosolvent on the structural arrangement of eutectic mixtures with different degrees of hydrophobicity, namely butylated hydroxytoluene (BHT), L-menthol (MEN), thymol (TYM), and choline chloride (ChCl), has been studied with a combined approach using small- and wide-angle X-ray scattering and molecular dynamics simulations. The cosolvent introduction has a similar impact on the molecular scale-length aggregation in BHT:MEN:HEX, TYM:MEN:HEX, and ChCl:TYM:HEX mixtures at different 1:3:H, 1:2:H, and 1:7:H molar ratios, specifically a dramatic perturbation of the main interactions present in the pure eutectics where hydrogen-bonds dominate. On larger scale-lengths, HEX addition results in a homogeneous electron-density distribution in the BHT:MEN:HEX and TYM:MEN:HEX mixtures due to a high affinity of the cosolvent for the BHT, MEN, and TYM components. Conversely, the presence of the more hydrophilic ChCl compound in the ChCl:TYM 1:7 system is the driving force for the segregation mechanism of this component which causes the formation of nano-scale inhomogeneities at high HEX concentrations, before macroscopic phase separation. The different degree of hydrophobicity is therefore key to understanding the nanostructural behavior of these eutectics in the presence of an apolar cosolvent. These findings have important implications for the employment of deep eutectic solvent mixtures, as the formation of pseudo-phase aggregates can help explaining the macroscopic behavior of these alternative media in applications like extraction procedures
Exploring the Methane to Methanol Oxidation over Iron and Copper Sites in Metal–Organic Frameworks
Full text linkThe direct oxidation of methane to methanol (MTM) is a significant challenge in catalysis and holds profound economic implications for the modern chemical industry. Bioinspired metal–organic frameworks (MOFs) with active iron and copper sites have emerged as innovative catalytic platforms capable of facilitating MTM conversion under mild conditions. This review discusses the current state of the art in applying MOFs with iron and copper catalytic centers to effectuate the MTM reaction, with a focus on the diverse spectroscopic techniques employed to uncover the electronic and structural properties of MOF catalysts at a microscopic level. We explore the synthetic strategies employed to incorporate iron and copper sites into various MOF topologies and explore the efficiency and selectivity of the MOFs embedded with iron and copper in acting as catalysts, as well as the ensuing MTM reaction mechanisms based on spectroscopic characterizations supported by theory. In particular, we show how integrating complementary spectroscopic tools that probe varying regions of the electromagnetic spectrum can be exceptionally conducive to achieving a comprehensive understanding of the crucial reaction pathways and intermediates. Finally, we provide a critical perspective on future directions to advance the use of MOFs to accomplish the MTM reaction
Structural and mechanistic insights into low-temperature CO oxidation over a prototypical high entropy oxide by Cu L-edge operando soft X-ray absorption spectroscopy
No full textHigh entropy oxides (HEOs) are an emerging class of materials constituted by multicomponent systems that are receiving special interest as candidates for obtaining novel and desirable properties. In this study we present a detailed investigation of the relevant intermediates arising at the surface of the prototypical HEO Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O during low-temperature CO oxidation. By combining Cu L-2,L-3-edge operando soft X-ray absorption spectroscopy (soft-XAS) with density functional theory simulations and in situ FT-IR spectroscopy, we propose that upon HEO exposure to CO at 235 degrees C reduced Cu(i) sites arise mostly coordinated to activated CO molecules and partly to bidentate carbonate species. When the HEO surface is then exposed to a stoichiometric mixture of CO + 1/2O(2) at 250 degrees C, CO2 is produced while bidentate carbonate moieties remain interacting with the Cu(i) sites. We structurally characterize the carbonate and CO preferential adsorption geometries on the Cu(i) surface metal centers, and find that CO adopts a bent conformation that may energetically favor its subsequent oxidation. The unique surface, structural and electronic sensitivity of soft-XAS coupled with the developed data analysis work-flow and supported by FT-IR spectroscopy may be beneficial to characterize often elusive surface properties of systems of catalytic interest
On the Role of Water in the Formation of a Deep Eutectic Solvent Based on NiCl(2)·6H(2)O and Urea
Full text link[Image: see text] The metal-based deep eutectic solvent (MDES) formed by NiCl(2)·6H(2)O and urea in 1:3.5 molar ratio has been prepared for the first time and characterized from a structural point of view. Particular accent has been put on the role of water in the MDES formation, since the eutectic could not be obtained with the anhydrous form of the metal salt. To this end, mixtures at different water/MDES molar ratios (W) have been studied with a combined approach exploiting molecular dynamics and ab initio simulations, UV–vis and near-infra-red spectroscopies, small- and wide-angle X-ray scattering, and X-ray absorption spectroscopy measurements. In the pure MDES, a close packing of Ni(2+) ion clusters forming oligomeric agglomerates is present thanks to the mediation of bridging chloride anions and water molecules. Conversely, urea poorly coordinates the metal ion and is mostly found in the interstitial regions among the Ni(2+) ion oligomers. This nanostructure is disrupted upon the introduction of additional water, which enlarges the Ni–Ni distances and dilutes the system up to an aqueous solution of the MDES constituents. In the NiCl(2)·6H(2)O 1:3.5 MDES, the Ni(2+) ion is coordinated on average by one chloride anion and five water molecules, while water easily saturates the metal solvation sphere to provide a hexa-aquo coordination for increasing W values. This multidisciplinary study allowed us to reconstruct the structural arrangement of the MDES and its aqueous mixtures on both short- and intermediate-scale levels, clarifying the fundamental role of water in the eutectic formation and challenging the definition at the base of these complex systems
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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