Determination of the electronic and geometric structure of Cu sites during methane conversion over Cu-MOR with X-ray absorption spectroscopy

Abstract

The endeavor to activate methane through low temperature partial oxidation to a partially oxidized species on Cu-exchanged zeolites relies on the understanding of the reaction mechanism. This necessitates knowing the electronic and geometric structure of the Cu sites under the different reaction steps that participate in the reaction. In situ high energy resolved fluorescence detected X-ray absorption spectroscopy (HERFD XAS) and extended X-ray absorption fine structure (EXAFS) spectroscopy at the Cu K-edge were explored to monitor the structure of the Cu atoms in Cu-MOR under the different gas feeds. After activation in oxygen, the XAS spectra showed distinct signatures of Cu2+ character. After cooling down in oxygen, and reaction with methane, the spectral signatures indicated a mixed environment of Cu2+/Cu+ states during methane interaction, where the majority of the Cu sites were reduced. Desorption of the formed intermediate by interaction with water showed a partially reduced character where a small reoxidation was attributed the loss of sorbed species and possibly reoxidation by oxygen in the feed.