Deciphering Radiolytic Oxidation in Halide Aqueous Solutions: A Pathway Toward Improved Synchrotron NAP-XPS Analysis

Synchrotron radiation near ambient pressure X-ray photoemission spectroscopy (SR NAP-XPS) has been an invaluable tool for examining gas/liquid and liquid/solid interfaces. Despite its benefits, concerns have emerged regarding beam damage in NAP-XPS experiments, particularly involving condensed liquid water, because of the high dose rates, greater than 105 Gy·s–1. This study investigates the radiolytic effects on the chemistry of concentrated NaX sodium halide solutions (X = Cl, Br, I) and Mg–Cl solution formed over the layered double hydroxide [Mg2Al(OH)6]+[Cl–]. The formation of oxidized species XO– as the radiolytic end product under soft X-ray irradiation is discussed in detail. We examine the impact of known parameters (such as the dose rate) on the abundance of XO–. The observed scatter in the data likely arises from still unrecognized or insufficiently controlled parameters (such as solute concentration or solution hydrodynamics). Deciphering these radiolytic effects in halide solutions allows us to propose guidelines for their better identification, understanding and control, ultimately improving the reliability of synchrotron NAP-XPS analysis for interfaces relevant to environmental chemistry and electrochemistry.