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Characterization of oxide catalysts using time-resolved XRD and XANES: Properties of pure and Sulfided CoMoO<inf>4</inf> and NiMoO<inf>4</inf>

Abstract:

The phase transitions in a series of CoMoO4 and NiMoO4 catalysts have been studied using time-resolved x-ray diffraction and calculations based on density functional theory. Important differences are observed in the temperatures and energetics for these transitions. X-ray absorption spectroscopy was used to study the sulfidation of the molybdates with H2S and SO2. For small exposures of H2S and moderate temperatures (50-150 °C), one sees the formation of Sulfides and sulfates on the oxide catalysts. Exposure to large amounts of H2S at elevated temperatures (350-400 °C) leads to formation of pure NiMoSx and CoMoSx compounds (no SOx species) that are active as HDS catalysts. CoMoO4 and NiMoO4 are good sorbents for sulfur dioxide. The adsorption of SO2 on the molybdates at room temperature produces SO3 and SO4 species without dissociation of SO2. Most of the SO3 and SO4 species decompose after heating to 400 °C. The sulfided CoMoO4 and NiMoO4 catalysts can be "regenerated" by heating in O2 at 500 °C. © 2000 Elsevier Science B.V. All rights reserved.