Structure and properties of oxidative dehydrogenation catalysts based on MoO3/Al2O3
Structure and properties of oxidative dehydrogenation catalysts based on MoO3/Al2O3
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DOI：
10.1006/jcat.2000.3125
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发表时间：
2001-03-10
影响因子：
7.3
通讯作者：
Iglesia, E
中科院分区：
文献类型：
--
作者：
Chen, K;Xie, S;Iglesia, E
考察了钼氧化物结构对丙烷氧化脱氢（ODH）反应速率和选择性的影响。X-射线衍射和拉曼光谱、紫外-可见光谱和X-射线吸收光谱表明，分散态的钼的结构强烈依赖于钼的表面密度。二维MoOx低聚物优先形成Mo表面密度低于4 Mo/nm（2）。在较高的表面密度，这些MoOx低聚物共存于Al 2 O3表面与三维MoO 3。UV-可见光边缘能量随着Mo表面密度的增加而降低，与MoOx结构的生长一致。X射线吸收光谱中的近边缘光谱特征的演变和径向结构函数中的Mo-Mo散射峰的逐渐出现证实了MoOx域随着表面密度的增加而生长。每个Mo原子的ODH速率随着Mo表面密度的增加而增加，并且在接近4.5 Mo/nm（2）的样品中达到最大值;这种行为反映了表面Mo物种的反应性增加，因为所有MoOx物种都暴露在该表面密度范围内的域表面。如还示出的基于VOx的催化剂，周转率在二维域上比在孤立的单体上更高，并且它们随着MoOx域尺寸的增加而增加。使用动力学和X-射线吸收方法探测在H-2或C3 H8中的MoOx物种的还原速率;这些还原速率随着MoOx表面密度的增加而与ODH速率平行增加，显然是由于较大域使伴随还原过程的较高电子密度离域的能力。当表面密度增加到4.5 Mo/nm（2）以上时，ODH速率（每个Mo原子）降低，这是由于MoO 3微晶的形成导致的可接近性损失的结果。对于这些后一种样品，随着表面密度的增加，每BET表面积的ODH速率接近恒定值，因为这些样品中的所有暴露表面都位于具有相似反应性的二维或三维MoO 3结构内。丙烷ODH和丙烷燃烧反应的速率常数的比率随着表面密度的增加而增加，然后在大于5 Mo/nm（2）的值时保持恒定。这些影响似乎反映了Al-O-Mo物种吸附醇盐中间体的趋势，并有利于其顺序氧化为考克斯。丙烯的燃烧速率常数也降低相对于那些丙烷ODH的二维结构形式与增加钼表面密度。(C)北京：科学出版社.
The effects of MoOx structure on propane oxidative dehydrogenation (ODH) rates and selectivity were examined on Al2O3- supported molybdenum oxide catalysts with a wide range of Mo surface density (0.4-12 Mo/nm(2)). X-ray diffraction and Raman, UV-visible, and X-ray absorption spectroscopies showed that the structure of dispersed molybdena depends strongly on the Mo surface density. Two-dimensional MoOx oligomers formed preferentially for Mo surface densities below 4 Mo/nm(2). At higher surface densities, these MoOx oligomers coexist on Al2O3 surfaces with three-dimensional MoO3. UV-visible edge energies decrease with increasing Mo surface density, consistent with the growth of MoOx structures. The evolution of near-edge spectral features in the X-ray absorption spectra and the gradual appearance of a Mo-Mo scattering peak in the radial structure function confirmed the growth of MoOx domains with increasing surface density. ODH rates per Mo atom increased with increasing Mo surface density and reached a maximum value for samples with similar to4.5 Mo/nm(2); this behavior reflects an increase in the reactivity of surface Mo species, because all MoOx species are exposed at domain surfaces in this surface density range. As also shown for VOx-based catalysts, turnover rates are higher on two-dimensional domains than on isolated monomers and they increase as the MoOx domain size increases. The rates of reduction of MoOx species in H-2 or C3H8 were probed using kinetic and X-ray absorption methods; these reduction rates increased in parallel with ODH rates as the MoOx surface density increased, apparently as a result of the ability of larger domains to delocalize the higher electron density that accompanies the reduction process. As the surface density increased above 4.5 Mo/nm(2), ODH rates (per Mo atom) decrease, as a result of the loss of accessibility caused by the formation of MoO3 crystallites. For these latter samples, the ODH rate per BET surface area approached a constant value as the surface density increased, because all exposed surfaces in these samples reside within two- or three-dimensional MoO3 structures with similar reactivity. The ratio of rate constants for propane ODH and propane combustion reactions increased with increasing surface density and then remained constant for values above 5 Mo/nm(2). These effects appear to reflect the tendency of Al-O-Mo species to adsorb alkoxide intermediates and favor their sequential oxidation to COx. Propene combustion rate constants also decreased relative to those for propane ODH as two-dimensional structures form with increasing Mo surface density. (C) 2001 Academic Press.