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  • directional emission  (1)
  • oxidative coupling of methane (OCM)  (1)
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  • 1
    ISSN: 1572-879X
    Keywords: oxidative coupling of methane (OCM) ; SrO-La2O3/CaO ; basicity ; CO2-TPD ; carbonates ; in situ FT-IR ; CO2 adspecies
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract 20%SrO-20%La2O3/CaO catalyst (SLC-2), prepared by impregnation, has shown 18% CH4 conversion and 80% C2-selectivity for the oxidative coupling of methane (OCM) at 1073–1103 K with CH4∶O2 molar ratio=9∶1 and total flow rate of 100 ml/min. Addition of SrO onto La2O3/CaO (LC) catalyst strengthens the surface basicity and leads to an increase in CH4 conversion and C2-selectivity. Meanwhile, the reaction temperature required to obtain the highest C2-yield increases with increasing SrO content. The formation of carbonate on the catalyst surface is the main reason for the deactivation of LC and SLC catalysts. If the amount of CO2 added into the feed is appropriate and the reaction temperature is high enough, there is no deactivation at all. In such case, the added CO2 will suppress the formation of CO2 produced via the OCM reaction, therefore, improves the C2-selectivity. The FT-IR spectra of CO2 adspecies recorded at different temperatures show that CO2 interacts easily with the catalyst surface to form different carbonate adspecies. Unidentate carbonate is the main CO2 adspecies formed on the catalyst surface. On the LC catalyst surface, the unidentate carbonate was first formed on Ca2+ cations at room temperature. If the temperature is higher than 473 K, it will form on La3+ cations. On the SLC catalyst surface, if the temperature is lower than 573 K, only the unidentate carbonate formed on Ca2+ cations could be observed. When the temperature is higher than 673 K, it will then form on Sr2+ cations. This suggests that the unidentate carbonate can migrate on the LC and SLC catalyst surface on one hand, and on the other hand, that the surface composition of SLC catalysts is dynamic in nature. On the basis of both the decomposition temperatures of the carbonate species, and the temperature dependence of the δΝ value which is the difference of symmetric and asymmetric stretching frequencies of surface carbonates, the in situ FT-IR technique offered two approaches to measure the surface basicity of the SLC catalyst. The results thus obtained are in good agreement with that of CO2-TPD. The role of the surface basicity of the SLC catalyst is also discussed.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2022-05-26
    Description: © The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Dong, E., Zhang, Y., Song, Z., Zhang, T., Cai, C., & Fang, N. X. Physical modeling and validation of porpoises' directional emission via hybrid metamaterials. National Science Review, 6(5), (2019): 921-928, doi:10.1093/nsr/nwz085.
    Description: In wave physics and engineering, directional emission sets a fundamental limitation on conventional simple sources as their sizes should be sufficiently larger than their wavelength. Artificial metamaterial and animal biosonar both show potential in overcoming this limitation. Existing metamaterials arranged in periodic microstructures face great challenges in realizing complex and multiphase biosonar structures. Here, we proposed a physical directional emission model to bridge the gap between porpoises’ biosonar and artificial metamaterial. Inspired by the anatomical and physical properties of the porpoise's biosonar transmission system, we fabricated a hybrid metamaterial system composed of multiple composite structures. We validated that the hybrid metamaterial significantly increased directivity and main lobe energy over a broad bandwidth both numerically and experimentally. The device displayed efficiency in detecting underwater target and suppressing false target jamming. The metamaterial-based physical model may be helpful to achieve the physical mechanisms of porpoise biosonar detection and has diverse applications in underwater acoustic sensing, ultrasound scanning, and medical ultrasonography.
    Description: E.D., Y.Z., Z.S., T.Z. and C.C. acknowledge the financial support in part by the National Key Research and Development Program of China (2018YFC1407504), the National Natural Science Foundation of China (41676023, 41276040 and 41422604). N.X.F. acknowledges the support from the MIT Energy Initiative grant. Z.S. thanks the China Scholarship Council for the financial support of his oversea study in Woods Hole Oceanographic Institution.
    Keywords: porpoise's physical model ; metamaterials ; biosonar ; directional emission
    Repository Name: Woods Hole Open Access Server
    Type: Article
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