Structure–Activity Relationships in RuCs/MgO Catalysts During Ammonia Synthesis

In the industrial synthesis of ammonia, a combination of high temperature and pressure is required to achieve a reasonable educt conversion. Efforts have been undertaken to lower these requirements by utilizing ruthenium-based catalysts promoted with alkali metals, which have the potential to lower the energy barrier associated with the dissociative adsorption of nitrogen. In this work, the structure of Ru and Cs species is probed in impregnated RuCs/MgO and Ru/MgO catalysts by operando X-ray absorption spectroscopy during reduction and ammonia synthesis at pressures up to 19 bar(a) in pure gas feed as well as the deactivation behavior with unpure feed containing 25 ppm oxygen. Interconversion of three types of Ru species, including RuO₂, highly dispersed RuO_x, and metallic Ru, occurs for both studied catalysts. Promotion by Cs leads to higher content of metallic Ru at the expense of dispersed RuO_x and results in higher NH₃ concentration at the reactor outlet. Exposure of the catalysts to traces of oxygen enables a gradual transformation of bare Cs⁺ cations to hydrated species [Cs(H₂O)_x]⁺. The irreversible deactivation of the catalyst is traced back to the leaching of cesium species, which has a disproportionate effect on the catalytic activity.