Promoting Effect of Bulk Cs⁺ over Cs0.68Ti1.83O₄ for Ammonia Synthesis

In addition to hydrogen poisoning, efficient electron transfer between promoters and Ru particles plays a crucial role in enhancing the performance of Ru-based heterogeneous catalysts for ammonia synthesis. Traditionally, Ru-based catalysts containing surface promoters prepared via an impregnation method pay significant attention to the contact position between promoter and Ru particles. Previous studies have shown that a minimal amount of surface promoters significantly enhances the activity, but excessive promoters can encapsulate transition metals, reducing the catalytic activity. In this work, we identify a mechanism of bulk Cs promoter that differs from traditional surface cocatalysts. The ammonia synthesis activity of Ru/Cs0.68Ti1.83O4 with bulk Cs is seven times higher than that of Cs–Ru/TiO2 synthesized by the impregnation method. Low hydrogen poisoning is revealed by kinetics studies while using a bulk Cs promoter. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images confirm no distinct differences of the positions between Cs and Ru for the above catalysts, indicating that the positioning factor is not a key determinant influencing activity. X-ray absorption near-edge structure (XANES) results demonstrate that bulk and surface Cs follow different electron transfer mechanisms in Ru-based catalysts. Density functional theory calculations support the XANES results by showing that the bulk Cs-containing support Cs0.68Ti1.83O4 gives the Ru particles additional electrons. This novel electron transfer mechanism provides new ideas for the design and development of cocatalysts in ammonia synthesis.