Water dissociation and KOH formation on potassium-covered MgO/Ru(001)

The sequential adsorption and reaction of H2O with K on MgO thin films have been studied using TDS and XPS. Upon H2O adsorption on K/MgO at 100 K, two hydrated species of K+(H2O)(n)(-) and K+(H2O)(n)(OH)(-) and formed, result ing in corresponding H2O-TDS peaks at 200 and 270 K, respectively. The ther mal conversion fi om hydrated K to hydrated KOH leads to H-2 desorption at 190-220 K. The formation of KOH is evidenced by the O Is bonding energies a t 530.6-530.9 eV. Both additional K deposition and thermal activation favor H2O dissociation. Annealing the coadsorbed surface to 500-600 K results in desorption and decomposition of KOH. The coincident desorption of K2O, H2O , and K at theta(K) > 0.3 ML strongly suggests the decomposition pathway 2K OH --> K2O + H2O, which further produces K desorbing into the gaseous phase and oxygen being retained on the surface. The KH species formed during H2O dissociation is also evidenced by the coincident desorption of H-2 and K a t 470-490 K for theta(K) greater than or equal to 1 ML.