Nana Zhu,Lu Zhu,Baojing Zhang,Peizhong Feng,Shiheng Li,Philipp V. Kiryukhantsev-Korneev,Evgeny A. Levashov,Xuanru Ren,Xiaohong Wang
An innovative two-step approach of self-propagating high-temperature synthesis (SHS) and spark plasma sintering (SPS) was developed to rapidly fabricate MoSi 2 and Mo(Al,Si) 2 ceramics for high-temperature anti-oxidation applications. The SHS process predominantly promoted the synthesis of high-purity and high-yield MoSi 2 and Mo(Si,Al) 2 phases in the alloyed powders. Subsequently, dense and crack-free MoSi 2 and Mo(Al,Si) 2 ceramics were produced using SPS. 1500 °C oxidation tests of the ceramics (100 h) revealed the formation of a protective SiO 2 oxide layer on the surface of MoSi 2 ceramics, while an Al-Si-O composite glassy oxide layer formed on Mo(Si,Al) 2 ceramics, which exhibited better thermal stability and lower oxygen permeability compared to the single SiO 2 oxide layer. However, an excessive Al content (>0.05 at.%) compromised the oxidation resistance due to the emergence of a Si-depleted Mo 5 (Si,Al) 3 layer with inferior oxidation resistance, which was caused by the high-temperature diffusion of Si. Therefore, via this novel two-step SHS-SPS technique compact and crack-free Mo(Si,Al) 2 ceramics can be rapidly synthesized at high temperatures. When trace amount of Al was added (0.05 at.%), Mo(Si 0.95 Al 0.05 ) 2 showed optimum high-temperature oxidation resistance.
Graphical Photocatalysts enhance the ability to absorb low-energy photons due to the reduction of the band gap. Through the photocatalytic mechanism, it is known that when there is irradiation with light (hv) energy entering Mg 2+ -Mo 6+ -Al 3+ -MoO 4 2− -Layered double hydroxides (LDHs), highly active hole-electron pairs will be generated, and a large amount of ⋅OH will be generated after the reaction, which can completely oxidize the phenol in water into CO 2 and H 2 O. Mg 2+ -Mo 6+ -Al 3+ -MoO 4 2− -Layered double hydroxides (LDHs) materials were successfully prepared via a hydrothermal method by doping Mo 6+ in hydroxide laminates and inserting MoO 4 2− to replace the interlayer anions of LDHs materials. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), Fourier transform infrared spectrometer (FTIRS) and ultraviolet-visible spectrophotometer (UV). The electronic density of states was calculated using the density functional theory (DFT) of Vienna Ab-initio Simulation Package (VASP). The effects of Mo 6+ and MoO 4 2− on the structure, morphology, electronic structure and photocatalytic performance of Mg 2+ -Al 3+ -LDHs were systematically studied. The conditions for phenol degradation by Mg 2+ -Mo 6+ -Al 3+ -MoO 4 2− -LDHs were optimized by single factor analysis and response surface methodology. The results show that the MoO 4 2− anion reduces the band gap width and thus enhances the absorption of low-energy photons. The interaction between cations and anions enhances the photocatalytic performance. When the dosage of Mg 2+ -Mo 6+ -Al 3+ -MoO 4 2− -LDHs was 0.03 g, the temperature was 40 °C and the visible light irradiation was 15 min, the removal rate of phenol reached 94.76 %.
Shuo Cao,Ying Ma,Wei Chu,Bingsen Zhang,Bo Peng,Yuefeng Liu
Molybdenum carbides have received attention for the reverse water gas shift (RWGS) reaction. Normally, MoCx-based catalysts are synthesized via a high-temperature carbonization of 1100 K, leading to the agglomeration of the active phase. In order to minimize the atomic motion leading to single growth, insertion of heteroatoms has been attempted with the usage of components in the catalyst substrate. The current work applied the self-assembling strategy to prepare a Mo–O–Al composite instead of traditional impregnation before carburization. The dynamic structure evolution and detailed coordination environment of the active phase are investigated by in situ X-ray diffraction, quasi in situ X-ray photoelectron spectroscopy, in situ transmission electron microscopy, and temperature-programmed surface reaction techniques. Our findings reveal that Al doping enables carburization at a lower temperature (200 °C) via the topotactic pathway, yielding α-MoCx (fcc); even at a loading as high as 40 wt %, the catalyst maintains an ultrasmall particle size (1 nm). The optimized catalyst exhibits a RWGS reaction rate of 52.1 μmolCO2·gMo–1·s–1 at 400 °C, representing a 2.5-fold enhancement over bulk β-Mo2C. This work provides a possible solution for maintaining high dispersion at high metal loading for catalyst preparation, specially requiring a high temperature during either synthesis or reaction.
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