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

Kazuhiro Takanabe, Abdulaziz M. Khan, Yu Tang, Luan Nguyen, Ahmed Ziani, Benjamin W. Jacobs, Ayman M. Elbaz, S. Mani Sarathy, Franklin (Feng) Tao, 2017

Image courtesy of Angewandte Chemie

Abstract

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.

Impact Statement

The authors used multiple in situ techniquest to study Na-based catalysts at high termperatures (>900 deg C) in OCM relevant conditions. As such, they were able to uncover the formation of the species responsible for OH radiacal formation.