POLYOXOMETALLATES AS MODELS FOR OXIDE CATALYSTS .1. AN UV VISIBLE REFLECTANCE STUDY OF POLYOXOMOLYBDATES - INFLUENCE OF POLYHEDRA ARRANGEMENT ON THE ELECTRONIC-TRANSITIONS AND COMPARISON WITH SUPPORTED MOLYBDENUM CATALYSTS
POLYOXOMETALLATES AS MODELS FOR OXIDE CATALYSTS .1. AN UV VISIBLE REFLECTANCE STUDY OF POLYOXOMOLYBDATES - INFLUENCE OF POLYHEDRA ARRANGEMENT ON THE ELECTRONIC-TRANSITIONS AND COMPARISON WITH SUPPORTED MOLYBDENUM CATALYSTS
复制标题
DOI：
10.1016/0021-9517(89)90170-x
复制
发表时间：
1989-10-01
影响因子：
7.3
通讯作者：
MASURE, D
中科院分区：
文献类型：
--
作者：
FOURNIER, M;LOUIS, C;MASURE, D
漫反射光谱是一种通常用于通过与固态或液态的参比样品进行比较来确定负载型氧化物催化剂中过渡金属离子的对称性和环境的技术。然而，对具有确定结构的模型多钼酸盐如Na 2 MoO 4·2 H2O、TBA 2 Mo 2 O 7（TBA =四丁基铵）、α-TBA 4 Mo 8 O26、α-和β-TBA 4SiMo 12 O 40、TBA 2 Mo 6 O 19、H8 As 4 Mo 12 O 50、α-TBA 6P 2 Mo 18 O 62等的检测，表明，较早的归属的紫外可见光谱带的负载钼催化剂可以重新考虑。结果表明，钼分散（钼团簇大小和团簇之间的距离）或钼支持相互作用有更大的影响比本地钼对称性。事实上，对多钼酸盐的实验光谱的分析表明：·-与早期的观点相反，与金属的更远环境的影响相比，局部对称性对配体-金属电荷转移的最低能量跃迁带的影响可以忽略不计。当团簇尺寸增加时，或者当极化效应增加和/或抗衡阳离子的尺寸减小时，观察到该带的加宽和红移。相反，当负载多钼酸盐时观察到窄化和蓝移，即，当它的分散度增加时，不同参数的递增效应可分类如下：局部对称全对称凝聚度（聚阴离子尺寸）极化效应的抗衡阳离子的抗衡阳离子分散体上的支持·-紫外-可见光谱不区分Mo之间的强相互作用的效果（通过氧键合Mo O Mo）通过聚阴离子尺寸的影响表现出较弱的效果，如聚阴离子相互作用在一些实施方案中，当抗衡阳离子足够小时发生静电作用（例如，通过氢键），或者由于抗衡阳离子的极化而产生静电作用。
Diffuse reflectance spectroscopy is a technique commonly used to determine the symmetry and the environment of transition metal ions in supported oxide catalysts by comparison with reference samples either in solid or liquid state. However, the examination of model polyoxomolybdates with well-established structures such as Na2MoO4· 2H2O, TBA2Mo2O7(TBA = tetrabutylammonium), α-TBA4Mo8O26, α- and β-TBA4SiMo12O40, TBA2Mo6O19, H8As4Mo12O50, α-TBA6P2Mo18O62, etc., indicates that earlier attributions of the UV-visible bands of supported Mo catalysts may be reconsidered. It is shown that Mo dispersion (Mo cluster size and distance between clusters) or Mo-support interactions have more influence than local Mo symmetry. Indeed, analysis of experimental spectra of polyoxomolybdates indicates that:•—in contrast to an earlier belief, the effect of the local symmetry on the lowest energy transition band of ligand-metal charge transfer is negligible compared to the effect of the more distant environment of the metal.•—a broadening and a red shift of this band are observed when cluster size increases, or when the polarizing effect increases and/or the size of the counter-cation decreases. On the contrary, a narrowing and a blue shift are observed when a polyoxomolybdate is supported, i.e., when its dispersion increases.•—the increasing effects of the different parameters may be classified as follows: local symmetry overall symmetry condensation degree (polyanion size) polarizing effect of the counter-cation size of the counter-cation dispersion on a support•—UV-visible spectra do not distinguish the effect of strong interaction between Mo (via oxobonding MoOMo) exhibited by the influence of polyanion size from weaker effects such as polyanion interaction (for example, via hydrogen bonding), occurring when the counter-cation is small enough, or from electrostatic effects due to polarization by the counter-cation.