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本文用IR,ESR和~(18)O同位素交换研究了磷钼钒杂多酸Keggin结构中不同氧键(O_b,O_p,O_t)的可还原度和可交换性以及还原过程中杂多酸结构的变化。结果表明,氧键的可还原度按O_b>>O_p≈O_T变化,V取代Mo对不同氧键的可还原度有明显影响:O_b增大,而o_P和O_T降低。O_b,O_p,O_t同~(18)O的交换没有区別,相对地说,含钒杂多酸中的氧不如PMo_(12)中的那样易于进行同位素交换。ESR谱指出,PMo_(12)(n=0)在还原过程中生成二种配位不同的Mo~(5+)离子,g值各为此g_⊥~A=1.971,g_∥~A=1.912和g_⊥~B=1.965,g_∥~B=1.861,分别属于畸变八面体和四方锥体。V取代Mo后只有V的ESR谱,还原起始阶段形成VO~(2+)或V~(4+)(OH)配位状态。畸变八面体的g值为g⊥=1.983,g∥=1.932;最后生成含V~(4+)的畸变四面体,g∥=1.932。根据实验结果认为在气相催化反应中端氧(O_t,M=O)对还原电子起定域作用,而O_b则具有传递电子的作用。
In this paper, the reducibility and exchangeability of different oxygen bonds (O_b, O_p, O_t) in the Keggin structure of phosphomolybdovanadophosphoric acid were studied by IR, ESR and ~ (18) O isotope exchange, and the structure of heteropolyacid The change. The results showed that the reducibility of oxygen bond was changed by O_b >> O_p≈O_T. The substitution of V by V had a significant effect on the reducibility of different oxygen bonds: O_b increased and o_P and O_T decreased. O_b, O_p, O_t and ~ (18) O exchange no difference, relatively speaking, vanadium-containing heteropolyacid in oxygen as PMo_ (12) as easy to isotope exchange. The results of ESR indicated that two kinds of Mo ~ (5+) ions with different coordination numbers were formed during the reduction of PMo_ (12) (n = 0). The value of g_⊥ was 1.971 and the value of g_∥ ~ A was 1.912 And g_⊥ ~ B = 1.965, g_∥ ~ B = 1.861, belonging to distorted octahedron and tetragonal pyramids, respectively. After V replaced Mo, only ESR spectra of V formed the coordination state of VO 2+ or V 4+ OH at the initial stage of reduction. The g value of the distorted octahedron is g⊥ = 1.983, g∥ = 1.932. Finally, the distorted tetrahedron with V 4+ is generated, g∥ = 1.932. According to the experimental results, it is concluded that the terminal oxygen (O_t, M = O) plays a certain role in the reduction of electrons in the gas phase catalytic reaction, while O_b has the function of transferring electrons.