Applying in situ transmission electron microscopy, the phase instability in potassium tungsten bronze (K x WO 3 , 0.18 < x < 0.57) induced by heating was investigated. The atomistic phase transition pathway of monoclinic K 0.20 WO 3 → hexagonal K m WO 3 (0.18 < m < 0.20) → cubic WO 3 induced by cationic defects (K and W vacancies) was directly revealed. Unexpectedly, a K + -rich tetragonal K n WO 3 (0.40 < n < 0.57) phase would nucleate as well, which may result from the blockage of K + diffusion at the grain boundaries. Our results point out the critical role of the cationic defects in mediating the crystal structures in K x WO 3 , which provide reference to rational structural design for extensive hightemperature applications.
Omphalane
diterpenoids usually contain a cyclohexane-fused bicyclo[3.2.1]octane
scaffold embedded with two continuous quaternary carbon centers, which
pose considerable challenges to synthetic chemists. Herein, we reported
the first total synthesis of omphalic acid with high stereochemical
control, featuring an intermolecular Diels–Alder cycloaddition,
ring reorganization through Criegee oxidative cleavage and programmed
aldol condensations, conformationally controlled hydrogenation, and
Pd-catalyzed carboxylation. The absolute configuration of omphalic
acid was defined for the first time via the asymmetric total synthesis
facilitated by a derivatization resolution protocol.
Knowledge regarding the phase and valence state evolution of molybdenum (Mo) and its oxides during the redox reaction is essential for advancing their energy applications (e.g., electrocatalysis), which unfortunately remains largely unexplored. Herein, the effects of atomic and electronic structures on the electrocatalytic performance of Mo/oxides core−shell structures are investigated on the basis of the combination of ex situ and in situ experiments. First, a two-step reaction pathway is revealed during the oxidation of nanoscale Mo: the formation of amorphous MoO 3 (A-MoO 3 ) shells followed by the nucleation of crystalline α-MoO 3 . It is shown that the electrocatalytic performance of A-MoO 3 is superior to that of α-MoO 3 , mainly due to more catalytically active sites in the former material. Furthermore, in situ transmission electron microscopy observations show that the A-MoO 3 shell can be rapidly reduced into metallic MoO 2 under reductive environment, which is likely to occur during the hydrogen evolution reaction measurement. Our in-depth characterization may contribute to the thorough and comprehensive understanding of the structural transition in Mo and its oxides during oxidative and reductive environments and thus serves as a reference for understanding the structure−property interplay for real energy applications.
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