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

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 surface. By exposing MgCl to water vapor at temperatures between 595 and 391 K, we show that water is preferentially adsorbed on five-coordinated Mg sites in an octahedral configuration, confirming previous theoretical predictions, and find that MgCl 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 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.