Surface phase composition of iron molybdate catalysts studied by UV Raman spectroscopy

Catalytic performance depends largely on the surface properties. Here, the surface phase composition of iron molybdate catalysts with different Mo/Fe atomic ratios calcined at different temperatures was studied by UV Raman spectroscopy. In this study, the surface enrichment of MoO3 that the Mo-rich catalysts undergo during calcination treatment is revealed by the increase in the Raman intensity of MoO3. X-ray photoelectron spectroscopy (XPS) analysis provides further evidence that the concentration of Mo increases evidently with increasing calcination temperature. The surface enrichment of MoO3 also results in a significant variation of the morphologic appearance as evidenced by the presence of a needle-like material assigned to MoO3 on the surface, while the surface phase composition remains almost unchangeable for the sample with a stoichiometric atomic ratio. These results are attributable to the formation of a(-) substitution structure of Fell ions in an octahedral coordination by Mo6+ ions in Mo-rich catalysts, and MoO3 is easier to segregate from such a structure as a second phase concentrated on the surface