Evgeny Fomin

Metal- and hydrogen-bonding competition during water adsorption on Pd(111) and Ru(0001).

The initial stages of water adsorption on the Pd(111) and Ru(0001) surfaces have been investigated experimentally by scanning tunneling microscopy in the temperature range between 40 and 130 K, and theoretically with density functional theory (DFT) total energy calculations and scanning tunneling microscopy (STM) image simulations. Below 125 K, water dissociation does not occur at any appreciable rate, and only molecular films are formed. Film growth starts by the formation of flat hexamer clusters where the molecules bind to the metal substrate through the O-lone pair while making H-bonds with neighboring molecules. As coverage increases, larger networks of linked hexagons are formed with a honeycomb structure, which requires a fraction of the water molecules to have their molecular plane perpendicular to the metal surface with reduced water-metal interaction. Energy minimization favors the growth of networks with limited width. As additional water molecules adsorb on the surface, they attach to the periphery of existing islands, where they interact only weakly with the metal substrate. These molecules hop along the periphery of the clusters at intermediate temperatures. At higher temperatures, they bind to the metal to continue the honeycomb growth. The water-Ru interaction is significantly stronger than the water-Pd interaction, which is consistent with the greater degree of hydrogen-bonded network formation and reduced water-metal bonding observed on Pd relative to Ru.

Coadsorption and interactions of O and H on Pd(111)

The interactions between oxygen and hydrogen coadsorbed on Pd(1 1 1) have been studied by scanning tunneling microscopy (STM) in the temperature range from 25 to 230 K. Atomic oxygen without coadsorbed hydrogen forms a (2×2) structure in a continuous layer or in the form of islands. Individual oxygen atoms are also observed between the islands at low temperature. Coadsorbed hydrogen modifies the STM image contrast and enhances the diffusion of the isolated oxygen atoms. Above 120 K hydrogen modifies the structure of the oxygen islands, transforming them into rows of oxygen atom pairs at nearest neighbor distances. Above 150 K, hydrogen causes all (2×2)-O islands to convert into a (√3×√3) structure, which is stable up to 210 K. Above 210 K the (√3×√3) structure transforms back to (2×2) due to dissolution of the surface hydrogen into the bulk. During these transformations the number of oxygen atoms on the surface remains unchanged. Above 220 K the oxygen population decreases by reaction with dissolved hydrogen to form H2O which desorbs from surface.

A scanning tunneling microscopy study of the reaction between hydrogen and oxygen to form water on Pd(111)

The reaction between hydrogen and oxygen to form water on Pd(111) was investigated by scanning tunneling microscopy. The reaction was followed by observing the decreasing size of the oxygen islands in the presence of coadsorbed hydrogen. The observations indicate that the reaction sites are the Pd step edges and that the reaction rate is limited by formation of OH at these sites.