WATER-ADSORPTION AND COADSORPTION WITH POTASSIUM ON GRAPHITE(0001)

Water and water coadsorbed with potassium on the basal plane of graphi te were studied with thermal desorption spectroscopy (TDS) and high-re solution electron energy loss spectroscopy (HREELS) in the temperature range 85-900 K. Water alone adsorbs nondissociatively on the clean gr aphite surface at 85 K, forming hydrogen bonded aggregates. Its struct ure depends both on the coverage and on substrate temperature. With in creasing coverage at 85 K(0.5-1.0 monolayer (ML)) the libration mode a t similar to 86 meV shows a rapid upward shift, indicating a phase tra nsition from a 2D to a 3D structure. The transition can also be induce d by annealing the low coverage structure. Water coadsorption with pot assium is nonreactive or reactive, depending on temperature and potass ium coverage. The nonreactive coadsorption at T-s = 85 K occurs only b elow a critical potassium coverage of BK less than or equal to 0.3 ML. It is characterized by substantial symmetry changes of the adsorbed w ater molecules, compared to the pure water adsorption, and is attribut ed to formation of hydrated-ion species on the surface. The surface so lvation number at the lowest K coverage is three to four H2O molecules per potassium atom. K and H2O react at submonolayer coverages at 120- 160 K to form surface KOH, KH, KxOy, and volatile products. The surfac e species gradually transforms/decomposes at elevated temperatures (20 0-500 K) to first form potassium-oxygen complexes that then serve as p recursors to graphite oxidation to CO2 at similar to 750 K.