REDUCTION STUDY WITH CO OF SOME SUPPORTED AND UNSUPPORTED HETEROPOLYACIDS WITH KEGGIN STRUCTURE

摘要

A detailed study on the oxidation states of the heteropolyacids H_3PMo_(12)O_(40)H_2O (HPM) and H_4PVMo_(11)O (HPVM) function of thermal treatmenl was performed. A slight degree of reduction was observed for HPVM with the thermal treatment temperature increasing, in contrast with HPM where the sample at room temperature underwent a slight reduction and at higher temperatures the same one was completely oxidized in the described work conditions. The UV-VIS studies of the reduction-reoxidstion cycles reveal different redox properties: HPM is completely reoxidated, while HP VM is incompletely reoxidated due to the presence of V~(5+) ion. The presence of V(Ⅴ) ion, with a higher electron affinity than Mo(Ⅵ), improve the electron transfer and consequently, decrease the apparent activation energy of the CO oxidation reaction. By the reoxldation, the diffusion of the oxygen species from the surface into the bulk is the rate limiting step. Supported heteropolyacids exhibit superior redox properties for the CO oxidation reaction as consequence of the higher specific surface area.