钼基金属氧化物催化剂在丙烷氧化脱氢反应中的活性位研究
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摘要
本文研究了铝基金属氧化物催化剂表面结构及其在丙烷氧化脱氢反应中的反应活性位和反应性能。主要研究了担载量和金属原子比对担载型氧化钼、钼酸镍催化剂结构的影响并采用原位紫外拉曼光谱技术对担载型钼酸镍催化剂在氧化脱氢反应中的活性位进行了探索。
实验中所使用的担载型催化剂均采用浸渍法合成,采用拉曼光谱,X射线光电子能谱,X射线衍射,程序升温还原等技术进行表征,并使用固定床反应器进行了反应性能测试。对于担载型氧化钼催化剂,在低担载量时,催化剂载体表面主要形成孤立结构钼物种,随着担载量的增加,逐渐达到单层,表面物种主要是二维多聚氧化钼,在高担载量时,晶相氧化钼大量形成。单层催化剂具有最佳的还原性能。对于担载型钼酸镍催化剂,担载量较低时催化剂表面形成的是尖晶石结构铝酸镍和高分散的氧化钼物种,随着担载量的增加氧化钼物种发生聚集,催化剂还原温度下降。高担载量时形成β-NiMoO_4晶体和晶相氧化钼,还原温度升高。本研究表明催化剂的结构和还原性可以通过担载量和钼镍原子比调变,提高担载量和钼镍原子比有利于γ-Al_2O_3表面β-NiMoO_4结构的形成。β-NiMoO_4被认为是许多反应的活性相或前体。我们的结果为文献中钼过量钼酸镍催化剂催化活性提高的原因提供了可能的解释,对制备以β-NiMoO_4为活性相的催化剂具有指导意义。
对40wt.%NiO-MoO_3/γ-Al_2O_3(Mo:Ni=2:1)催化剂的原位拉曼研究表明:各化学键的可还原性顺序为:Mo-O-Mo桥键>NiMoO_4的Mo=O>MoO_3的Mo=O。在丙烷氧化脱氢反应条件下,Mo-O-Mo桥键和NiMoO_4晶格中的Mo=O均参与反应,桥氧键的流通能力强于端基氧,桥氧参与的程度更大。MoO_3的端基氧在丙烷氧化脱氢反应过程中可能发生迁移,补充至桥氧空位等其它晶格氧空穴。
This work focuses on the structure and catalytic active sites of Mo-based metal oxide catalysts in propane oxidative dehydrogenation. The effects of loading and molar ratio on the structure and reducibility of MoO_3/γ-Al_2O_3 and NiO-MoO_3/γ-Al_2O_3 catalysts has been investigated. The active sites of NiO-MoO_3/γ-Al_2O_3 catalysts for the oxidative dehydrogenation of propane were studied using in situ UV Raman spectroscopic technique.
The catalysts used in this study were prepared by impregnation method, and characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Temperature-programmed reduction and Fixed-bed catalytic reactor techniques. For MoO_3/γ-Al_2O_3 samples the isolated MoO3 is the major structure at low loading. As the loading increases the polymerized molybdate is formed on the surface. At high loading, the formation of crystalline MoO_3 occurs on the surface. The two-dimentional polymerized MoO_3 shows the highest reducibility among the MoO_3/γ-Al_2O_3 catalysts with different loadinds.
For NiO-MoO_3/γ-Al_2O_3 catalysts, different surface species can be formed depending on the loading and molar ratio. At low loading spinel-like NiAl_2O_4 and dispersed MoO_3 are major species onγ-Al_2O_3, and the reducibility of Ni and Mo species is promoted by each other. The interaction between Ni and the support is stronger than that between Mo and the support. With increasing the loading the aggregation of the Mo species occurs, which leads to a decrease of the reduction temperature. At high loading crystallineβ-NiMoO_4 and MoO_3 can be formed on the surface, which results in a peak shift to high temperature side. In particular, excess Mo was found to promote the formation ofβ-NiMoO_4 onγ-Al_2O_3.
For 40wt.% NiO-MoO_3/γ-Al_2O_3 catalyst, the results from in situ UV Raman spectroscopic study shows that their reducibility decreases in the sequence: Mo-O-Mo>Mo=O in NiMoO_4 lattice>Mo=O in MoO_3 lattice. A correlation between the catalytic activity/selectivity and the structure proposes that Mo-O- Mo bond and Mo=O in NiMoO_4 are crucial active sites for the oxidative dehydrogenation of propane.
实验中所使用的担载型催化剂均采用浸渍法合成,采用拉曼光谱,X射线光电子能谱,X射线衍射,程序升温还原等技术进行表征,并使用固定床反应器进行了反应性能测试。对于担载型氧化钼催化剂,在低担载量时,催化剂载体表面主要形成孤立结构钼物种,随着担载量的增加,逐渐达到单层,表面物种主要是二维多聚氧化钼,在高担载量时,晶相氧化钼大量形成。单层催化剂具有最佳的还原性能。对于担载型钼酸镍催化剂,担载量较低时催化剂表面形成的是尖晶石结构铝酸镍和高分散的氧化钼物种,随着担载量的增加氧化钼物种发生聚集,催化剂还原温度下降。高担载量时形成β-NiMoO_4晶体和晶相氧化钼,还原温度升高。本研究表明催化剂的结构和还原性可以通过担载量和钼镍原子比调变,提高担载量和钼镍原子比有利于γ-Al_2O_3表面β-NiMoO_4结构的形成。β-NiMoO_4被认为是许多反应的活性相或前体。我们的结果为文献中钼过量钼酸镍催化剂催化活性提高的原因提供了可能的解释,对制备以β-NiMoO_4为活性相的催化剂具有指导意义。
对40wt.%NiO-MoO_3/γ-Al_2O_3(Mo:Ni=2:1)催化剂的原位拉曼研究表明:各化学键的可还原性顺序为:Mo-O-Mo桥键>NiMoO_4的Mo=O>MoO_3的Mo=O。在丙烷氧化脱氢反应条件下,Mo-O-Mo桥键和NiMoO_4晶格中的Mo=O均参与反应,桥氧键的流通能力强于端基氧,桥氧参与的程度更大。MoO_3的端基氧在丙烷氧化脱氢反应过程中可能发生迁移,补充至桥氧空位等其它晶格氧空穴。
This work focuses on the structure and catalytic active sites of Mo-based metal oxide catalysts in propane oxidative dehydrogenation. The effects of loading and molar ratio on the structure and reducibility of MoO_3/γ-Al_2O_3 and NiO-MoO_3/γ-Al_2O_3 catalysts has been investigated. The active sites of NiO-MoO_3/γ-Al_2O_3 catalysts for the oxidative dehydrogenation of propane were studied using in situ UV Raman spectroscopic technique.
The catalysts used in this study were prepared by impregnation method, and characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Temperature-programmed reduction and Fixed-bed catalytic reactor techniques. For MoO_3/γ-Al_2O_3 samples the isolated MoO3 is the major structure at low loading. As the loading increases the polymerized molybdate is formed on the surface. At high loading, the formation of crystalline MoO_3 occurs on the surface. The two-dimentional polymerized MoO_3 shows the highest reducibility among the MoO_3/γ-Al_2O_3 catalysts with different loadinds.
For NiO-MoO_3/γ-Al_2O_3 catalysts, different surface species can be formed depending on the loading and molar ratio. At low loading spinel-like NiAl_2O_4 and dispersed MoO_3 are major species onγ-Al_2O_3, and the reducibility of Ni and Mo species is promoted by each other. The interaction between Ni and the support is stronger than that between Mo and the support. With increasing the loading the aggregation of the Mo species occurs, which leads to a decrease of the reduction temperature. At high loading crystallineβ-NiMoO_4 and MoO_3 can be formed on the surface, which results in a peak shift to high temperature side. In particular, excess Mo was found to promote the formation ofβ-NiMoO_4 onγ-Al_2O_3.
For 40wt.% NiO-MoO_3/γ-Al_2O_3 catalyst, the results from in situ UV Raman spectroscopic study shows that their reducibility decreases in the sequence: Mo-O-Mo>Mo=O in NiMoO_4 lattice>Mo=O in MoO_3 lattice. A correlation between the catalytic activity/selectivity and the structure proposes that Mo-O- Mo bond and Mo=O in NiMoO_4 are crucial active sites for the oxidative dehydrogenation of propane.
引文
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