Efficient CeO2 and CeO2–Al2O3 supports for Ru as 3rd generation ammonia synthesis catalysts: enhanced kinetic mechanism over commercial Ru/CeO2

Abstract

Ceria (CeO₂) has been previously reported as a functional support for ruthenium (Ru) as an ammonia synthesis catalyst. However, lab-synthesized ceria materials usually present low surface areas, thereby limiting the generation of oxygen vacancies and the ammonia synthesis activity as a result of weak metal–support interactions. With the aim of overcoming this issue, we prepared, by a simple impregnation method, high surface area ceria and ceria–alumina supported Ru catalysts with improved ammonia synthesis performance at moderate temperatures. In this sense, lab-synthesized Ru/CeO₂ (with higher specific surface area and lower crystallinity than commercial ceria) showed stronger metal–support interactions than the commercial sample, which resulted in a superior global ammonia synthesis kinetic mechanism with more positive hydrogen reaction orders (i.e., more resistant to hydrogen inhibition) and significantly lower activation energies (46 vs. 61 kJ mol⁻¹). We found that the use of alumina as a structural support increased the surface area of ceria, thereby promoting the Ru–CeO₂ interaction and the catalytic performance. We analyzed the effect of the surface chemistry of two different commercial aluminas (acidic and basic) with similar surface areas. Basic alumina was found to increase the specific surface area of the catalyst to a larger extent as compared to acidic alumina. Thus, the Ru/CeO₂–Al₂O₃ catalyst with 50 wt% of basic alumina showed an ammonia synthesis activity of 1.9 mmol g⁻¹ h ⁻¹ at 400 °C and ambient pressure and an activation energy as low as 44.8 kJ mol⁻¹.