Water adsorption on metals is extremely important in a large number of interfacial phenomena related to astrochemical ice nucleation, atmospheric chemistry, heterogeneous catalysis, electrochemical and corrosion processes. For instance, the interaction of water and its constituents determine the kinetics of the oxygen reduction reaction (ORR) in fuel cell catalysis. Nonetheless, little is known about structures of water interaction with well-defined single crystal metal surfaces. Questions regarding how water, at various degrees of coverage, interfaces with the metal surfaces and what factors govern the water-metal bonding have not been satisfactorily addressed.
In this thesis, ab initio density functional theory has been used to study the adsorption of water on low-Miller-index Pd surfaces by varying the amount of protons involved in H-bonding of water adlayers, ranging from supported H2O monomers with no hydrogen bonding, to intact H2O monolayers in which a number of protons contribute to H-bonding, to H2O-OH mixed phases where all protons are involved in H-bonding.
On the basis of orbital analysis, we have provided a complete understanding of geometrical and electronic effects on water-palladium bonding as well as molecular mechanisms of elementary surface diffusion of the water monomers on Pd(111), Pd(100), Pd(110) and Al(100). A new bonding mechanism for a water monomer on Ni(111) and unique molecular mechanisms for atop-to-atop motions have been identified. Several structural models for intact H2O monolayers on M(110) surfaces (M=Pd, Cu, Ni) are proposed, and the rotated H-down structures have been found to be the most promising ground-state structures of the water adlayers, as compared to the H-down monolayer model. The coplanar atomic structures of mixed H2O/OH phases on Pd(111) and Pd(110) have also been determined. We observed that the competition between H-bonding and water-metal bonding actually govern the structures of the intact water monolayers on Pd(110), while in mixed H2O/OH phases, a synergistic effect of the interplay of water-palladium interaction and hydrogen bonding accounts for coplanar adlayer structures and the enhanced electron density which gives rise to the hydrophobic nature of the supported mixed phases on Pd(111) and Pd(110).
修改评论