François Rochet: Synchrotron Radiation Ambient Pressure XPS studies of smectite clays: hydration and heterogeneous radiolysis of inserted pyridine

Near ambient photoemission x-ray photoelectron spectroscopy (NAP- XPS) was used in the mbar range at TEMPO beamline (SOLEIL) to study the hydration of swelling smectites (hectorite, saponite). Smectites make an important class of clays consisting in negatively charged phyllosilicate 2D sheets, with cations (Na+, Cs+, Sr2+ etc.) inserted between them.

The core-level spectra (counterions, silicate layer) showed that the hydration process of the cations can be followed as a function of increasing humidity via sizeable shifts in their binding energies when water enters into the interstice. The local information obtained from XPS can be compared to results from more global probes like XRD and to MD simulations.

Water confined in the interlayer is sensitive to radiolysis induced by the synchrotron radiation. When a pyridine partial pressure is added to the water pressure of 1W hydrated Sr2+-hectorite, a pyridine/water/Sr2+ complex is detected from the Sr 3d and N 1s core-level binding energies. It likely presents a favorable situation for the radiolytic breaking of the O-H bond of water molecules adsorbed in the clay and the subsequent decomposition of the organic molecule, leading to the steady-state production of gaseous N2. The physical and chemical causes of the present observation are discussed, relevant to nuclear waste management and astrochemical studies.