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Chevrel Phases
Published in David A. Cardwell, David C. Larbalestier, I. Braginski Aleksander, Handbook of Superconductivity, 2023
The Chevrel phase materials that transform fully from a rhombohedral structure at high temperatures to a triclinic structure at low temperatures have low electronic density of states (Lachal et al., 1983) and are non-superconducting. However, the superconductivity can be restored if pressure is applied to prevent the triclinic transition occurring. In BaMo6S8 for example, applying a pressure of 4 GPa changes the material from a triclinic semiconductor to a mixed triclinic–rhombohedral phase that is metallic with a TC of 12 K (Yao et al., 1988). The structural instabilities in the Chevrel phase superconductors may enhance the electron–phonon coupling. In Eu1.2Mo6S8 (Chu et al., 1981), (shown in Figure B3.12) superconductivity is also close to the metal–insulator transition and can be tuned by pressure. In (Sn1-xEuxM)1.2Mo6S8 (Harrison et al., 1981), the transition is tuned by Sn content (or carrier concentration). These properties are found in other Chevrel phase materials and is reminiscent of the HTS materials where high-TC values also occur in materials with relatively low carrier concentration that are in proximity to the metal–insulator transition. There has been intense research into alternative microscopic mechanisms for superconductivity following the discovery of the high-temperature superconductors in the late 1980's. The Uemura plot provides empirical evidence that the cuprate and bismuthate high-temperature superconductors, the organic, Chevrel phase and heavy Fermion systems all belong to a single class of superconductors where TC is proportional to the (small) ns/m* (carrier density/effective mass) (Uemura et al., 1991; Harshman and Mills Jr, 1992; Uemura, 2004) as shown in Figure B3.13. In the cuprates, the high values of TC and the lack of a clear isotope effect suggest a non-phononic mechanism (Batlogg et al., 1987). For Chevrel phase materials, the carrier density (ns ∼ 2 holes/unit cell) and the effective mass (m* ∼ me) are rather robust numbers from both experiment and theory. Their presence on the Uemura plot suggests that Chevrel phase materials, which have a well-known chemistry and electronic structure, may be model systems in which to investigate non-standard mechanisms for superconductivity because of the simplifications which follow from their (almost) cubic (isotropic) structure.
Insight into the reaction mechanism of ethanol steam reforming catalysed by Co–Mo6S8
Published in Molecular Physics, 2019
Qian Zhang, Ling Guo, Xiaoli Zheng, Minmin Xing, Zijun Hao
In SRE, the catalyst must assist the cleavage of C–H, C–C and O–H bonds, with the fragments recombining to produce CO, CO2 and H2. As the catalysts play a pivotal role in the efficiency of ethanol reforming, numerous experimental and theoretical investigations have been made in this field [2-6]. The noble metal catalysts (Pt [7-9], Pd [10,11], Rh [12-14], Ir [15-17] and Ru [18]) have shown catalytic activity toward the SRE reaction, due to their greater ability to break C–C bonds [19,20]. However, the high cost of noble metals has shifted the attention to earth-abundant metals such as Co catalysts [21], which are also effective in breaking C–C bonds and shows high activity in SRE. It has been reported that Co-based catalysts are able to catalyse SRE reactions at a relatively low temperature (∼350−400°C) [22-24]. Moreover, Co catalysts also possess superior selectivity for the overall SRE reaction, ranking the best among 14 transition metals supported on Al2O3, according to the study by Haga et al. [25]. And Karim et al. [26] comparing the catalytic behaviour of supported Rh and Co catalysts concluded that Co was a better option than Rh. On the other hand, for the SRE reaction, Co/Al2O3 shows the highest hydrogen selectivity among Co/Al2O3, Co/MgO and Co/SiO2 catalysts [27]. It is obvious that different support plays a vital role in the catalytic activity, selectivity. The Mo6S8 cluster is the structural building block of the Chevrel phase of molybdenum sulphide and has a highly symmetric cage-like structure with an octahedral Mo6 metallic core. Co–Mo6S8 has been found to be active catalysts for methanol synthesis, and the reaction intermediates and transition states (TSs) involved in methanol synthesis are more stable on Co–Mo6S8 than on Rh–Mo6S8 [28].