Probing the Local Environment in Potassium Salts and Potassium-Promoted Catalysts by Potassium Valence-to-Core X-ray Emission Spectroscopy

Potassium plays an important role in biology as well as a promoter in heterogeneous catalysis. There are, however, limited characterization techniques for potassium available in the literature. This study elucidates the potential of element-selective X-ray emission spectroscopy (XES) for characterizing the coordination environment and the electronic properties of potassium. A series of XES measurements were conducted, primarily focusing on the VtC transition (Kβ_2,5) of potassium halides (KCl, KBr, and KI) and oxide-bound potassium salts, including potassium nitrate (KNO₃) and potassium carbonate (K₂CO₃). Across the series of potassium halides, the VtC transition energy is observed to increase, as accurately reproduced by TDDFT calculations. Molecular orbital analysis suggests that the Kβ_2,5 transition is primarily derived from halide <i>n</i>p contributions, with the primary factor influencing the energy shift being the metal-ligand distances. For oxide ligands, an additional Kβ″ transition appears alongside the Kβ_2,5, which is attributed to a low-energy ligand <i>n</i>s, as elucidated by theoretical calculations. Finally, the XES spectra of two potassium-promoted catalysts for ammonia decomposition/synthesis were measured. These spectra show that potassium within the catalyst is distinct from other K salts in the VtC region, which could be promising for understanding the role of potassium as an electronic promoter.