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Low-sensitivity and high-energy explosives (LSHEs) are highly desired for their comprehensive superiority of safety and energy. Crystal packing is crucial to both the safety and energy, and therefore becomes of interest in energetic crystal engineering. This work carries out systemic analyses on the crystal packing of 11 existing LSHEs with both energy and safety close or superior to TNT. As a result, we find that the LSHE crystals wholly feature π−π stacking with the aid of intermolecular hydrogen bonding. Each LSHE molecule is πbonded with a big conjugated structure composed of all nonhydrogen atoms in the entire molecule. Intramolecular hydrogen bonding exists in most LSHE molecules with strongly active hydrogen bond (HB) donors of amino and hydroxyl groups, and various strength. These big π-conjugated structures and intramolecular HBs lead to planar molecules with high stability, settling a base of π−π stacking in crystals. With the help of intermolecular HBs, the π−π stacking holding the LSHE crystals appears in four modes. Among them, the face-to-face stacking (always offset) gives rationally the smallest steric hindrance when interlayer slide occurs in crystal, which is the reason for very low impact sensitivity. This work suggests that the planar conjugated molecular structure and intermolecular hydrogen bonding supporting the π−π stacking are necessary to the crystal engineering of LSHEs.

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The influence of different iron carbides on the activity and selectivity of iron-based Fischer−Tropsch catalysts has been studied. Different iron carbide phases are obtained by the pretreatment of a binary Fe/SiO 2 model catalyst (prepared by coprecipitation method) to different gas atmospheres (syngas, CO, or H 2 ). The phase structures, compositions, and particle sizes of the catalysts are characterized systematically by XRD, XAFS, MES, and TEM. It is found that in the syngas-treated catalyst only χ-Fe 5 C 2 carbide is formed. In the CO-treated catalyst, Fe 7 C 3 and χ-Fe 5 C 2 with a bimodal particle size distribution are formed, while the H 2 -treated catalyst exhibits the bimodal size distributed ε-Fe 2 C and χ-Fe 5 C 2 after a Fischer−Tropsch synthesis (FTS) reaction. The intrinsic FTS activity is calculated and assigned to each corresponding iron carbide based on the phase composition and the particle size. It is identified that Fe 7 C 3 has the highest intrinsic activity (TOF = 4.59 × 10 −2 s −1 ) among the three candidate carbides (ε-Fe 2 C, Fe 7 C 3 , and χ-Fe 5 C 2 ) in typical medium-temperature Fischer−Tropsch (MTFT) conditions (260−300 °C, 2−3 MPa, and H 2 /CO = 2). Moreover, FTS over ε-Fe 2 C leads to the lowest methane selectivity.