Probing the\nLocal Environment in Potassium Salts and\nPotassium-Promoted Catalysts by Potassium Valence-to-Core X‑ray\nEmission Spectroscopy

Potassium plays an important role in biology as well\nas a promoter\nin heterogeneous catalysis. There are, however, limited characterization\ntechniques for potassium available in the literature. This study elucidates\nthe potential of element-selective X-ray emission spectroscopy (XES)\nfor characterizing the coordination environment and the electronic\nproperties of potassium. A series of XES measurements were conducted,\nprimarily focusing on the VtC transition (Kβ<sub>2,5</sub>)\nof potassium halides (KCl, KBr, and KI) and oxide-bound potassium\nsalts, including potassium nitrate (KNO<sub>3</sub>) and potassium\ncarbonate (K<sub>2</sub>CO<sub>3</sub>). Across the series of potassium\nhalides, the VtC transition energy is observed to increase, as accurately\nreproduced by TDDFT calculations. Molecular orbital analysis suggests\nthat the Kβ<sub>2,5</sub> transition is primarily derived from\nhalide <i>n</i>p contributions, with the primary factor\ninfluencing the energy shift being the metal–ligand distances.\nFor oxide ligands, an additional Kβ″ transition appears\nalongside the Kβ<sub>2,5</sub>, which is attributed to a low-energy\nligand <i>n</i>s, as elucidated by theoretical calculations.\nFinally, the XES spectra of two potassium-promoted catalysts for ammonia\ndecomposition/synthesis were measured. These spectra show that potassium\nwithin the catalyst is distinct from other K salts in the VtC region,\nwhich could be promising for understanding the role of potassium as\nan electronic promoter.