WOx/ZrO2 catalysts: Part 4. Redox properties as investigated by redox cycles, XPS and EPR Original Research Article

The redox properties of WOx/ZrO2 catalysts with W-content from 1.0 to 14.3 atoms nm−2, prepared by equilibrium adsorption from hydrous zirconium oxide and ammonium metatungstate solutions, were investigated by redox cycles with H2 and O2, volumetric oxygen adsorption at room temperature (RT), XPS and EPR. After catalysts were heated at 1073 K in air, the various tungsten species (small WxOy clusters anchored to zirconia surface, polyoxotungstates species and crystalline WO3) reacted differently with H2 in the range 473–773 K. The reduction degree of small WxOy clusters up to 723 K slightly increased and then sharply increased, yielding at 773 K an average oxidation state (nav) equal to 5.2. Conversely, the reduction degree of polyoxotungstates and WO3 steeply increased from 523 to 773 K, reaching nav equal to 4.8. XPS analysis disclosed that, after treatment in H2 at 773 K, whereas W5+ formed on all the samples, W4+ species formed only in samples containing WO3 and polyoxotungstates. Depending on W-content, EPR detected various W5+ species in reduced ZW samples: isolated ions incorporated into the monoclinic zirconia; two kind of surface species, one in small clusters and the other in polyoxotungstates, resembling a bronze-like structure. The formation of O2− by oxygen adsorption at RT gave an indirect evidence of isolated W5+ surface species, irrespective of W-content. Volumetric O2 adsorption indicated that W5+ surface species were largely re-oxidized at RT, whereas W4+ species, formed in polyoxotungstates and WO3, were less reactive, possibly because these species were less accessible to the gas phase.