Photo-induced transformation of soluble U(VI) to insoluble U(IV) is an effective way for uranium extraction and removal. However, the photoreduction process needs to be conducted under the protection of inert...
The ignition mechanism of the explosive particles under impact has been a hot topic, but the research progress is slow. With the rapid development of computer science, the three-dimensional discrete element technique (DM3) is regarded as an efficient and intuitive method to study the explosive ignition under impact. As is well known, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is one of the most effective explosive particles in performance, which has high density and energy and thus possesses a significant application. In this paper, the deformation and ignition of HMX particles under impact of drop hammer are investigated based on the three-dimensional discrete element technique. Specifically, the computational process for shock loading as well as chemical reaction is employed in DM3 model through using the state equation of Hugoniot, the reactive model of Arrhenius, the state equation of JWL. The results show that the size, degree of accumulation, defect and the force of drop hammer can definitely influence the ignition and propagation of HMX particles. Under the same shock loading, the particles on a small scale would produce less power. On the same scale of particle, the less the number of particles, the shorter the deformation time is, so the temperature increases more easily. As for the different shapes of single particles, the deformation and ignition first appear from the ‘top’ for the spire particles, and then the deformation and ignition of flat particles happens from ‘shear’. Specifically, there are two results of the internal defect HMX particles under impact: the particles with bigger size (discrete elements 256 × 34 = 8704) have a temperature advantage near the ‘hole’, while the temperature advantage of the particles with the smaller size (discrete elements 93 × 35 = 3814) appears on the ‘top’.
Ultrasmall metal nanoparticles (MNPs) and atomically dispersed metal sites (ADMSs) exhibit excellent catalytic activity and selectivity. However, they tend to aggregate into large particles during synthesis and catalysis due to high surface free energy. Aiming to break out of this dilemma, zeolite imidazolate frameworks (ZIFs) have proved to be ideal substrates for stabilizing the ultrafine metal nanostructures. The stabilizing mode includes confining MNPs inside their pore structures and converting intrinsic metal nodes into coordinately unsaturated metal sites (i.e., ZIF-confined metal catalysts). Furthermore, various MNPs and ADMSs can also be immobilized into ZIF-derived porous carbon through high-temperature pyrolysis (i.e., ZIF-derived carbon-supported metal catalysts). In brief, ZIFs and their derivatives have gradually emerged as good platforms for designing metal catalysts at nanoscale and even atomic level.In this review, recent progress in the structural analysis and synthetic strategies of metal catalysts supported by ZIFs and their derivatives is presented. Besides, current challenges and future directions of atomically dispersed metal catalysts are discussed. This article will provide valuable insights into rational design of metal catalysts with exquisite nanostructure and high catalytic performance to meet further material requirements in the catalytic field.
K E Y W O R D Ssingle-atom catalysts, synthetic strategy, ultrasmall metal nanoparticles, zeolite imidazolate frameworks, ZIF-derived carbon
INTRODUCTIONCatalysts have been broadly used in modern society, such as energy conversion, chemical production, and environmental protection. [1][2][3][4][5][6][7] Rational selection and fabrication of high-performance catalysts have always been theThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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