Investigating the High-Temperature Water/MgCl₂ Interface through Ambient Pressure Soft X-ray Absorption Spectroscopy

Abstract

Magnesium chloride is a prototypical deliquescent material whose surface properties, although central for Ziegler− Natta cataysis, have so far remained elusive to experimental characterization. In this work, we use surface-selective X-ray absorption spectroscopy (XAS) at ambient pressure in combination with multivariate curve resolution, molecular dynamics, and XAS theoretical methods to track in real time and accurately describe the interaction between water vapor and the MgCl 2 surface. By exposing MgCl 2 to water vapor at temperatures between 595 and 391 K, we show that water is preferentially adsorbed on five-coordinated Mg 2+ sites in an octahedral configuration, confirming previous theoretical predictions, and find that MgCl 2 is capable of retaining a significant amount of adsorbed water even under prolonged heating to 595 K. As a consequence, our work provides first experimental insights into the unique surface affinity of MgCl 2 for atmospheric water. The developed technique is proven highly sensitive to the modifications induced by adsorbates on a given low-Z metal based surface and may be useful in the toolbox required to disentangle the mechanisms of interfacial chemical processes.

Exploring the Methane to Methanol Oxidation over Iron and Copper Sites in Metal–Organic Frameworks

“…Conversely, optical, Mössbauer, and XPS techniques furnish direct insights into surface composition and on the local structural and electronic properties of the adsorption sites. Notably, XAS measurements in both the soft (0.1-3 keV) [112][113][114][115] and hard (3-100 keV) [116][117][118][119][120][121][122] energy ranges may provide-at the same time-information on the nature, structure, and time evolution of the key intermediates with element-specificity and high sensitivity. It should be noted that the majority of works reported in the literature employed a combination of the above-mentioned spectroscopic techniques, supported by DFT calculations, to characterize the catalytic active sites and the intermediate species formed while performing the MTM conversion over iron-and copper-bearing MOFs.…”