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Characterization of oxide catalysts using time-resolved XRD and XANES: Properties of pure and Sulfided CoMoO4 and NiMoO4

Abstract:

The phase transitions in a series of CoMoO<inf>4</inf> and NiMoO<inf>4</inf> 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 H<inf>2</inf>S and SO<inf>2</inf>. For small exposures of H<inf>2</inf>S and moderate temperatures (50-150 °C), one sees the formation of Sulfides and sulfates on the oxide catalysts. Exposure to large amounts of H<inf>2</inf>S at elevated temperatures (350-400 °C) leads to formation of pure NiMoS<inf>x</inf> and CoMoS<inf>x</inf> compounds (no SO<inf>x</inf> species) that are active as HDS catalysts. CoMoO<inf>4</inf> and NiMoO<inf>4</inf> are good sorbents for sulfur dioxide. The adsorption of SO<inf>2</inf> on the molybdates at room temperature produces SO<inf>3</inf> and SO<inf>4</inf> species without dissociation of SO<inf>2</inf>. Most of the SO<inf>3</inf> and SO<inf>4</inf> species decompose after heating to 400 °C. The sulfided CoMoO<inf>4</inf> and NiMoO<inf>4</inf> catalysts can be "regenerated" by heating in O<inf>2</inf> at 500 °C. © 2000 Elsevier Science B.V. All rights reserved.