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

Magnesium chlorideis a prototypical deliquescent materialwhosesurface properties, although central for Ziegler-Natta cataysis,have so far remained elusive to experimental characterization. Inthis 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 timeand accurately describe the interaction between water vapor and theMgCl(2) surface. By exposing MgCl2 to water vaporat temperatures between 595 and 391 K, we show that water is preferentiallyadsorbed on five-coordinated Mg2+ sites in an octahedralconfiguration, confirming previous theoretical predictions, and findthat MgCl2 is capable of retaining a significant amountof adsorbed water even under prolonged heating to 595 K. As a consequence,our work provides first experimental insights into the unique surfaceaffinity of MgCl2 for atmospheric water. The developedtechnique is proven highly sensitive to the modifications inducedby adsorbates on a given low-Z metal based surface and may be usefulin the toolbox required to disentangle the mechanisms of interfacialchemical processes.