Reactions of gas-phase H atoms with atomically and molecularly adsorbed oxygen on Pt(111)

摘要

The interaction of gas-phase H atoms with ordered and disordered adlayers of atomic oxygen, hydroxyl, and molecular oxygen on Pt(111) surfaces was investigated by in situ mass spectrometry and post-reaction TPD (temperature programed desorption). Exposure of oxygen adlayers to gas-phase H atoms at 85 K leads to formation of H_2O via two consecutive hydrogenation reactions: H(g)+O(a)->OH(a) followed by H(g)+OH(a)->H_2O(g,a). Both reaction steps are highly exothermic, and nascent H_2O molecules partially escape into the gas phase before being thermally accommodated on the surface. Empty surface sites and hydrogen bonding promote thermalization of H_2O. Separate experiments performed with OH-covered Pt(111) surfaces reveal that the hydrogenation of hydroxyl is a slow reaction compared to the hydrogenation of atomic oxygen;additionally, the abstraction of H from OH by gas-phase D atoms, OH(a)+D(g)->O(a)+HD(g), was detected. Abstraction of H from adsorbed H_2O was not observed. Admission of gas-phase H atoms to O_2-covered Pt(111) surfaces at 85 K leads to the desorption of O_2 and H_2O. The thermodynamic stability of the HO_2 radical suggests that the reaction is initiated by hydrogenation of molecular oxygen, O_2(a)+H(g)->HO_2. The intermediate HO_2 either decomposes via dissociation of the HO-O bond, HO_2->OH(a)+O(a), finally leading to the formation of H_2O (approx85%), or via dissociation of the H_O_2 bond thus leading to desorption of O_2 (approx15%). The whole reaction sequence of formation and decomposition of HO_2 is fast compared to the formation of H_2O via hydrogenation of atomic oxygen and hydroxyl. The observed coverage dependence of the reaction kinetics indicates the dominance of hot-atom mediated reactions.