Laser Raman Spectroscopy of Precipitates: Applications in Catalysis

Precipitates are important precursors in the preparation of catalysts, pigments, fillers, and other metal oxides and sulfides. Despite the extensive investigations of these colloidal systems, the mechanism of precipitate formation and particle growth remains unclear. The processes involved during precipitation are complex and are sensitive to many environmental factors. The solutions of the metal salts used in the precipitation also may be complex, particularly for mutli-valent ions. Precipitate solutions are extremely difficult to characterize in situ with most spectroscopic techniques. The use of Raman spectroscopy is highly advantageous since precipitates may be examined in aqueous solution without interference from the presence of water. A series of bismuth molybdate selective oxidation catalysts were characterized during preparation using in situ Raman spectroscopy. Spectra were recorded during the precipitation and aging of stoichiometric γ-Bi2MoO6 and Bi2Mo3O12 catalysts. The formation of these phases was related to the slate of aggregation of molybdenum and bismuth species. Various techniques were used to obtain the Raman spectra of the precipitates, which are briefly described.