Effects of the Potassium Promoter upon the Structures of Mo/SiO₂ Catalysts

The interaction of the potassium promoter with the molybdenum components supported on silica was investigated and correlated to the sulfidation properties of the surface Mo（Ⅵ） species by using Laser Raman Spectroscopy （LRS）, Electron Spin Resonance （ESR） and X-Ray Diffraction （XRD） techniques. It was found that the interacting species composed of the potassium and molybdenum components could be formed as polymolybdeate with the anions corresponding to Mo₈O₂₆^4- and Mo₇O₂₄^6-, the formation of the bulk MoO₃ being greatly retarded. As shown by XRD results, large amount of the Mo（Ⅵ） species on the support could be transferred to the Mo（Ⅳ） ones during sulfidation. However, due to the effects of the potassium promoter, there were still a small amount of the Mo（Ⅵ） species, detected by LRS, which could not be completely sulfided and the Mo（Ⅴ） specise, detected by ESR, which evolved during sulfidation existed on the silica support. The effects of the promoter became significant with the increase of K / Mo ratios in the catalysts.