Mg_3(VO_4)_2的环己烷氧化脱氢性能与X射线光电子能谱分析

X-ray Photoelectron Spectroscopy and Catalytic Performance of Mg_3(VO_4)_2 Catalyst in Oxidative Dehydrogenation of Cyclohexane

采用X射线光电子能谱对柠檬酸法制备的Mg3(VO4)2催化剂表面物种进行了研究,并结合X射线衍射(XRD)、扫描电镜(SEM)、傅立叶红外光谱(FT-IR)和氢气程序升温还原(H2-TPR)对催化剂的表征结果及其在环己烷氧化脱氢反应中的催化性能,进一步探讨了Mg3(VO4)2催化剂表面物种与其催化性能之间的关系。研究结果表明,采用柠檬酸法制备的Mg3(VO4)2催化剂晶粒大小均匀,平均晶粒度大小为110nm且组分单一;催化剂表面上的氧物种为未完全还原氧物种O-、O 2-和晶格氧物种O2-,其中晶格氧为主要氧物种;催化剂表面上的钒物种为V4+和V5+物种,且V5+物种含量略高于V4+物种,但由于制备过程中Mg的偏析作用,催化剂表面含有较多孤立存在的VO4物种。在环己烷氧化脱氢反应中,吸附在催化剂表面的气相氧通过V4+V5+氧化还原偶的氧化还原转化为晶格氧参与反应。Mg3(VO4)2催化剂表面上的晶格氧物种和孤立存在的VO4物种有利于环己烯选择性的提高。

The surface species on Mg3(VO4)2 catalyst prepared by citric acid complexation method,were investigated by X-ray photoelectron spectroscopy(XPS) techniques.In the oxidative dehydrogenation of cyclohexane to cyclohexene,the relationship between the catalytic performance over Mg3(VO4)2 catalyst and the surface species on the catalyst was studied.With the characterization of X-ray diffraction(XRD),scanning electron microscopy(SEM),FT-IR spectroscopy(FT-IR) and H2-temperature programmed reduction(H2-TPR) techniques,the results indicated that well crystalline structures of Mg3(VO4)2 with a pure phase and a uniform granularity of particles of 110 nm mean size were obtained by the citric acid complexation.XPS spectra also indicated that the lattice oxygen was the main oxygen species among the three oxygen species(e.g.O-,O-2 and O2-) on the catalyst surface.Meanwhile,V4+ and V5+ species were coexisted on the catalyst surface.Although the molar amount of V5+ was a little more than that of V4+,the isolated VO4 species was the main vanadium species on the catalyst surface owing to the segregation of Mg.In the oxidative dehydrogenation of cyclohexane,gaseous oxygen participated in the reaction only after adsorption in other parts of the catalyst and then migrated through the lattice to the active sites by the reoxidation between V4+ and V5+.The characterization and the catalytic results also demonstrated that the catalytic performance was correlated to the lattice oxygen O2-and isolated VO4 species,which was beneficial to the improvement of the selectivity to cyclohexene.