Co-adsorption and reactions of Na and H 2O on graphite

Abstract

Experimental data for the co-adsorption of sodium and water on graphite (0001) at low temperature (95 K) and the subsequent thermal transformations has been obtained by thermal desorption spectroscopy (TDS), work function change (Δ φ) measurements, and high-resolution electron energy loss spectroscopy (HREELS). Water molecules co-adsorbed in the low Na coverage regime do not dissociate, but are strongly perturbed. The interaction results in the formation of hydrated ion shells. The estimated number of water molecules in the core (inner) shell is four. A series of thermal desorption peaks, which are related to the degree of stabilization by the H 2O–Na complexes formed, emerge with increasing water coverage. Up to 20 ± 3 water molecules can be affected per co-adsorbed Na-atom. The Na–H 2O interaction increases the water desorption energy by as much as 100 ± 20 meV/water molecule. Thermal interaction leads to the formation of sodium hydroxide species at ∼200 K, which subsequently decomposes to yield simultaneous desorption of Na and H 2O at ∼565 K.