Studies on Surface Structures of Supported Molybdenum Catalysts by LRS and XPS

Laser Raman spectra of oxidized and sulfided molybdenum catalysts supported on γ-Al₂O₃, ZrO₂, CeO₂, and La₂O₃ were measured. According to MoO₃ monolayer model and when MoO₃ loading was equal to the monolayer saturated capacity on γ-Al₂O₃ catalyst, tetrahedral and octahedral coordinated Mo, bulk MoO₃ and molybdate were formed. On ZrO₂, two kinds of coordinated Mo were determined. Only octahedral Mo on CeO₂ and only tetrahedral Mo on La₂O₃ were observed. A strong support effect appeared. In the sulfided samples, surface oxysulfide molybdnum and little MoS₂ microcrystal on the catalysts were formed. The sulfidation process of Mo/ZrO₂ and Mo/CeO₂ was studied by XPS. Under the weaker sulfidation condition (300°C, 20min), the most of Mo⁺⁶ were reduced to Mo⁺⁴. The structure of highly dispersed Mo was destroied gradually and sulfur loading was increased step by step with sulfiding. The surface structures of oxidized and sulfided state were discussed according to the results of LRS and XPS.