Integrated In Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation

Sodium‐based catalysts (such as Na2WO4) were proposed to selectively catalyze OH radical formation from H2O and O2 at high temperatures. This reaction may proceed on molten salt state surfaces owing to the lower melting point of the used Na salts compared to the reaction temperature. This study provides direct evidence of the molten salt state of Na2WO4, which can form OH radicals, using in situ techniques including X‐ray diffraction (XRD), scanning transmission electron microscopy (STEM), laser induced fluorescence (LIF) spectrometry, and ambient‐pressure X‐ray photoelectron spectroscopy (AP‐XPS). As a result, Na2O2 species, which were hypothesized to be responsible for the formation of OH radicals, have been identified on the outer surfaces at temperatures of ≥800 °C, and these species are useful for various gas‐phase hydrocarbon reactions, including the selective transformation of methane to ethane. In situ characterizations at high temperature revealed that the molten state of Na2WO4 on the oxide surface produces a Na2O2‐rich outer surface that catalyzes OH radical formation from an O2/H2O mixture. This process is useful for H ion from hydrocarbons in the gas phase.