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1. Oxygen-atom vacancy formation and reactivity in polyoxovanadate clusters

   https://doi.org/10.1039/D0CC05920J  

   Petel, Brittney E. ; Matson, Ellen M. ( November 2020 , Chemical Communications)

   null (Ed.)

   Reducible metal oxides (RMOs) are widely used materials in heterogeneous catalysis due to their ability to facilitate the conversion of energy-poor substrates to energy-rich chemical fuels and feedstocks. Theoretical investigations have modeled the role of RMOs in catalysis and found they traditionally follow a mechanism in which the generation of oxygen-atom vacancies is crucial for the high activity of these solid supports. However, limited spectroscopic techniques for in situ analysis renders the identification of the reactivity of individual oxygen-atom vacancies on RMOs challenging. These obstacles can be circumvented through the use of homogeneous complexes as molecular models for metal oxides, such as polyoxometalates. Summarized herein, a sub-class of polyoxometalates, polyoxovanadate–alkoxide clusters, ([V 6 O 7 (OR) 12 ] n ; R = CH 3 , C 2 H 5 ; n = 2−, 1−, 0), are explored as homogeneous molecular models for bulk vanadium oxide. A series of synthetic strategies have been employed to access oxygen-deficient vanadium oxide assemblies, including addition of V(Mes) 3 (thf), tertiary phosphanes, and organic acids to plenary Lindqvist motifs. We further detail investigations surrounding the ability of these oxygen-deficient sites to mediate reductive transformations such as O 2 and NO x 1− ( x = 2, 3) activation. 

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2. In Situ Studies of the Formation of Tungsten and Niobium Oxide Nanoparticles: Towards Automated Analysis of Reaction Pathways from PDF Analysis using the Pearson Correlation Coefficient

   https://doi.org/10.1002/cmtd.202200034  

   Kjær, Emil T. S. ; Aalling‐Frederiksen, Olivia ; Yang, Long ; Thomas, Nancy K. ; Juelsholt, Mikkel ; Billinge, Simon J. L. ; Jensen, Kirsten M. Ø. ( August 2022 , Chemistry–Methods)

   Using Pair Distribution Function (PDF) analysis of in situ total scattering data, we investigate the formation of tungsten and niobium oxides in a simple solvothermal synthesis. We use Pearson Correlation Coefficient (PCC) analysis of the time resolved PDFs to both map the structural changes taking place throughout the synthesis and identify structural models for precursor and product through PCC‐based structure mining. Our analysis first shows that ultra‐small tungsten and niobium oxide nanoparticles form instantaneously upon heating, with sizes between 1.5 and 2 nm. We show that the main structural motifs in the nanoparticles can be described with structures containing pentagonal columns, which is characteristic for many bulk tungsten and niobium oxides. We furthermore elucidate the structure of the precursor complex as clusters of octahedra with O‐ and Cl‐ligands. The PCC based methodology automates the structure characterization and proves useful for analysis of large datasets of for example, time resolved X‐ray scattering studies. The PCC is implemented in ‘PDF in the cloud’, a web platform for PDF analysis.

    

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3. Site-selective halogenation of mixed-valent vanadium oxide clusters

   https://doi.org/10.1039/D0DT01077D  

   Maiola, Michela L. ; Petel, Brittney E. ; Brennessel, William W. ; Matson, Ellen M. ( January 2020 , Dalton Transactions)

   Here, we expand on the synthesis and characterization of chloride-functionalized polyoxovanadate-alkoxide (POV-alkoxide) clusters, to include the halogenation of mixed-valent vanadium oxide assemblies. These findings build on our previously disclosed results describing the preparation of a mono-anionic chloride-functionalized cluster, [V 6 O 6 Cl(OC 2 H 5 ) 12 ] 1− , by chlorination of [V 6 O 7 (OC 2 H 5 ) 12 ] 2− with AlCl 3 , aimed at understanding the electronic consequences of the introduction of halide-defects in bulk metal oxides ( e.g. VO 2 ). While chlorination of the mixed-valent POV-ethoxide clusters was not possible using AlCl 3 , we have found that the chloride-substituted oxidized derivatives of the Lindqvist vanadium-oxide clusters can be formed using TiCl 3 (thf) 3 with [V 6 O 7 (OC 2 H 5 ) 12 ] n ( n = 1−, 0) or WCl 6 with [V 6 O 7 (OC 2 H 5 ) 12 ] 0 . Characterization of the chloride-containing products, [V 6 O 6 Cl(OC 2 H 5 ) 12 ] n ( n = 0, 1+), was accomplished via 1 H NMR spectroscopy, X-ray crystallography, and elemental analysis. Electronic analysis of the redox series of Cl-doped POV-alkoxide clusters via infrared and electronic absorption spectroscopies revealed all redox events are localized to the vanadyl portion of the cluster, with the site differentiated V III –Cl moiety retaining its reduced oxidation state across a 1.9 V window. These results present new synthetic routes for accessing chloride-doped POV-alkoxide clusters from mixed-valent vanadium oxide precursors. 

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4. Microstructure and mechanical properties of reaction‐hot‐pressed zirconium diboride based ceramics

   https://doi.org/10.1111/ijac.13263  

   Neuman, Eric W. ; Fahrenholtz, William G. ; Hilmas, Gregory E. ( May 2019 , International Journal of Applied Ceramic Technology)

   ZrB₂ceramics were prepared by in‐situ reaction hot pressing of ZrH₂and B. Additions of carbon and excess boron were used to react with and remove the residual oxygen present in the starting powders. Additions of tungsten were utilized to make a ZrB₂‐4 mol%W ceramic, while a change in the B/C ratio was used to produce a ZrB₂‐10 vol% ZrC ceramic. All three compositions reached near full density. The baseline ZrB₂and ZrB₂–ZrC composition contained a residual oxide phase and ZrC inclusions, while the W‐doped composition contained residual carbon and a phase that contained tungsten and boron. All three compositions exhibited similar values for flexure strength (~520 MPa), Vickers hardness (~15 GPa), and elastic modulus (~500 to 540 GPa). Fracture toughness was about 2.6 MPa m^1/2for the W‐doped ZrB₂compared to about 3.8 MPa m^½for the ZrB₂and ZrB₂–ZrC ceramics. This decrease in fracture toughness was accompanied by an observed absence of crack deflection in the W‐doped ZrB₂compared with the other compositions. The study demonstrated that reaction‐hot‐pressing can be used to fabricate ZrB₂based ceramics containing solid solution additives or second phases with comparable mechanical properties.

    

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5. Strengthening boron carbide through lithium dopant

   https://doi.org/10.1111/jace.16889  

   He, Yi ; Shen, Yidi ; Tang, Bin ; An, Qi ( November 2019 , Journal of the American Ceramic Society)

   The high strength of boron carbide (B₄C) is essential in its engineering applications such as wear‐resistance and body armors. Here, by employing density functional theory simulations, we demonstrated that the strength of B₄C can be enhanced by doping lithium to boron‐rich boron carbide (B₁₃C₂) to form r‐LiB₁₃C₂. The bonding analysis on r‐LiB₁₃C₂indicates that the electron counting rule (or Wade's rule) is satisfied in r‐LiB₁₃C₂whose formula can be written as r‐Li⁺(B₁₂)^2‐(CB⁺C). The shear deformation on r‐LiB₁₃C₂indicates that its ideal shear strength is larger than that of B₄C because of the existing of Li dopant. The failure process of r‐LiB₁₃C₂under ideal shear deformation initiates from breaking the icosahedral‐icosahedral B‐B bonds. Then these B atoms react with the middle B in the C‐B‐C chain, resulting in the disintegration of icosahedral clusters and brittle failure. More interesting, the nanotwinned r‐LiB₁₃C₂is even stronger than r‐LiB₁₃C₂because of the directional nature of covalent bonding at the twin boundaries. This suggests that the nanotwinned r‐LiB₁₃C₂has a significant enhanced strength compared to B₄C. Our simulation results illustrate the deformation mechanism of Li‐doped boron carbide and its nanotwinned microstructure. We proposed to improve the strength of boron carbide by doping Li into B₁₃C₂and increasing its twin densities.

    

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