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)- are formed, resulting in corresponding H2O-TDS peaks at 200 and 270 K, respectively. The thermal conversion from hydrated K to hydrated KOH leads to H2 desorption at 190−220 K. The formation of KOH is evidenced by the O 1s bonding energies at 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 θK > 0.3 ML strongly suggests the decomposition pathway 2KOH → 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 H2 and K at 470−490 K for θK ≥ 1 ML.