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

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 NiMoS_x and CoMoS_x compounds (no SO_x 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.