Transient hydroxyl formation from water on oxygen-covered Au(111).

We present evidence for the formation of transient hydroxyls from the reaction of water with atomic oxygen on Au(111) and investigate the effect of adsorbed oxygen on the hydrogen bonding of water. Water is evolved in peaks at 175 and 195 K in temperature programed reaction experiments following adsorption of water on oxygen-covered Au(111). The peak at 175 K is ascribed to sublimation of multilayers of water, whereas the peak at 195 K is associated with oxygen-stabilized water or a water-hydroxyl surface complex. Infrared reflection absorption spectra are consistent with the presence of molecular water over the entire range of coverages studied, indicating that isolated stable hydroxyls are not formed. Isotopic exchange of adsorbed (16)O with H(2)(18)O following adsorption and subsequent temperature programed reaction, however, indicates that transient OH species are formed. The extent of oxygen exchange was considerable--up to 70%. The degree of oxygen exchange depends on the initial coverage of oxygen, the surface temperature when preparing oxygen adatoms, and the H(2)(18)O coverage. The hydroxyls are short-lived, forming and disproportionating multiple times before water desorption during temperature programed reaction. It was also found that chemisorbed oxygen is critical in the formation of hydroxyls and stabilizing water, whereas gold oxide does not contribute to these effects. These results identify transient hydroxyls as species that could play a critical role in oxidative chemical reactions on gold, especially in ambient water vapor. The crystallinity of adsorbed water also depended on the degree of surface ordering and chemical modification based on scanning tunneling microscopy and infrared spectra. These results demonstrate that oxidation of interfaces has a major impact on their interaction with water.

Water dissociation associated with NO2 coadsorption on Mo(110)-(1 x 6)-O: effect of coverage and electronic properties of oxygen.

Water dissociation on an oxygen-covered Mo(110) surface was investigated using temperature-programmed reaction spectroscopy (TPRS) and infrared reflectance absorbance spectroscopy (IRAS). Adsorbed hydroxyl formation is enhanced by increasing the coverage of chemisorbed oxygen prior to exposure to water up to saturation (0.66 ML). Additional oxidation of the surface using NO(2) suppresses the formation of hydroxyl species (OH). There is no detectable change in the reaction of NO(2) on Mo(110)-(1 x 6)-O when either the water or hydroxyl is adsorbed on the Mo(110)-(1 x 6)-O surface prior to NO(2) adsorption. In contrast, NO(2) induces the displacement of water into the gas phase and the conversion of hydroxyl species to molecular water. Infrared spectra show that the dissociation of NO(2) populates three types of terminal oxygen sites on Mo(110)-(1 x 6)-O, and the population of the terminal oxygen at step sites increases with respect to the amount of NO(2) deposited. Overall, these results suggest that the oxidic property of oxygen results in a lack of activity for the water dissociation.