IR AND ESR STUDIES OF THE REDUCED HETEROPOLYMOLYBDOVANADOPHOS PHORIC ACIDS H 3+ n PMo 12- n V n O 40 ( n =0,2,4)

Abstract

The reducibility and isotope exchange ability of different oxygen bonds (O b , O p , O t ) in the Keggin structure of heteropolymolybdatesH 3+ n PMo 12- n V n O 40 containing zero, two or four vanadium atoms as well as the changes of structure of heteropolyacids during reduction were investigated by IR, ESR and 18 O isotope exchange. The results obtained showed that the order in the reducibility of different oxygen bonds is O b >>O p ≈O t , the replacement of Mo by V affects obviously the reducibility of various oxygen bonds: O b increases while O p and O t decrease. 18 O exchange rate of O b , O p , O t is not different, comparatively speaking, the isotope exchange of oxygen of the heteropolyacids containing vanadium atoms is more difficult than PMo 12 . The ESR spectra indicate that two kinds of Mo 5+ ions with different coordinations are present in PMo 12 ( n =0) during reducing process, their g values are g ⊥ A =1.971, g ∥ A =1.912 and g ⊥ B =1.965, g ∥ B =1.861 belonging respectively to the distorted octahedron and square pyramidal coordinatlon. Only the ESR spectrum due to V atom could be observed after Mo is replaced by V. At the initial stage of reduction the VO 2+ or V 4+ (OH) coordination state is formed, g values of the distorted octahedron are: g ⊥ =1.983, g ∥ =1.932, which in turn, finally, converts to a distorted tetrahedron, and g value becomes g ∥ =1.932. Based on the experimental results it may be concluded that the terminal oxygen atoms (O t , M=O) play an important role in the localization of reduced electrons in gas phase catalysis while the bridging oxygen (O b ) may only cause electron transfer between different ions.