The Adsorption of Water on Metal Surfaces

The adsorption of water on most metal surfaces is typically rather weak and controlled by a balance between the strength of the metal-water bond and the water waterl interactions. Molecular water adsorbs on metal and metal oxide substrates through the donation and back-donation of electrons between the frontier molecular orbitals of water and the states of the metal near the Fermi level. 

On metal oxides weak hydrogen bond interactions can also evolve. The molecular orbital energy diagram for the gas-phase water molecule alone is shown in Fig. 6.2. 

The frontier orbitals are the Ibi, 3ai and the n states. We recognize these orbitals as two Px- and pj,-type oxygen atomic orbitals interacting with symmetric and antisymmetric hydrogen atomic orbital combinations (lb2 and 3ai, resprectively). The Ibi molecular orbital is the non-bonded 2p atomic orbital on oxygen. All three of these states are 

Michaelides and co-workersi have shown theoretically that for monolayers and bilayers of water on Ru, the predominant overlap occurs between the 2pa type 3ai orbital of water with a d 2 state on Ru. In addition there is overlap between the lone-pair Ibi orbital on water and the Ru dz2 state. The overlap and mixing of the metal surface state with the Ibi orbital is significantly stronger. An electronic analysis shows that there is a charge depletion from the Ibi and Ru d 2 states with a charge accumulation on the lower lying dxz and d states. There is also a small charge increase between the O and the Rul l. 

The adsorption of water onto the metal in the vapor phase is typically quite weak with adsorption energies on the order of 30-50 kJ/mol, with some notable exceptionsP . DFT slab calculations predict the adsorption of a single water molecule to be 50 kJ/mol on Ag(lll), 30 kJ/mol on Pd(lll), 30 kJ/mol on Pt(lll), 37 kJ/mol on Rh(lll), and 100 kJ/mol on Cu(llO). Noteworthy is the increased hydrophilicity of the Group IB noble metals compared with that of the group VIII metals. 

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