Unraveling the role of MgO in the Ru-Ba/MgO catalyst for boosting ammonia synthesis: Comparative study of MgO and MgAlOx supports

The growing emphasis on decarbonization and the use of renewable energy has sparked considerable interest in ammonia (NH3) synthesis under mild conditions, accompanied by the concurrent development of catalysts. Among these catalysts, BaO-promoted Ru has often been reported as a highly efficient catalyst for this reaction. Despite MgO being traditionally regarded as an optimal support for this catalytic system, its precise role has remained ambiguous. This study aims to elucidate the specific role of MgO as a support for Ru-BaO catalysts. Using pure MgO and MgO-Al2O3 mixed oxide (MgAlOx) with different MgO contents (MgO wt% = 5, 30, and 70) as supports, catalysts impregnated with equal amounts of Ba and Ru were prepared and analyzed to explore their NH3 synthesis activity and structural properties. Our observations reveal that while MgO does not directly promote Ru, it maintains robust basicity of BaO, enabling BaO to effectively function as an electronic promoter. In contrast, BaO supported on AlOx domain shows suppressed basicity, resulting in inertness as a promoter. Consequently, the Ru-Ba/MgO catalyst forms highly active Ru-BaO-MgO interfacial sites, leading to significantly higher activity, lower activation energy, and a distinct reaction mechanism compared to Ru-Ba/MgAlOx catalysts. This work sheds light on the importance of sequential interactions, i.e. support–promoter and promoter–active center interactions, in facilitating catalytic activity, thereby providing valuable insights essential for the design of advanced catalysts.