Effect of pretreatment on ceria-supported cobalt catalyst for ammonia synthesis

Abstract

Cerium in a catalyst may affect the catalytic reaction by providing surface hydroxyl groups. In this work, Co/CeO₂ catalysts with different properties of hydroxyl groups were prepared using different pretreatment atmospheres during the heat treatment process of the hydroxide gel with metal and cerium. The heat treatment process of Co/CeO₂ precipitate in different pretreatment atmospheres was monitored by mass spectrometry. Pulse chemisorption measurements in combination with XRD, TPR, XPS, TEM were used to reveal the relationship between the properties of the as-obtained samples and their catalytic performances in ammonia synthesis. It was found that the pretreatment atmosphere of Co/CeO₂ affected the dehydroxylation pathways. Most hydroxyls in CeO₂ can release H₂O by reacting with atomic hydrogen during the hydrogen reduction process. On the other hand, oxygen treatment resulted in the conversion of hydroxyls to oxides, and further hydrogen reduction promoted the reduction of Ce⁴⁺ to Ce³⁺ and the formation of the stable surface hydroxyl radicals. The active sites available for nitrogen dissociation and ammonia synthesis may decrease because some redundant hydrogen cannot be removed in time by combination with the stable hydroxyls, and thus the catalyst with oxygen treatment showed lower ammonia synthesis activity.