Insight from first principles into the nature of the bonding between water molecules and 4d metal surfaces

We address the nature of the bond between water molecules and metal surfaces through a systematic density-functional theory (DFT) study of H2O monomer adsorption on a series of close-packed transition metal surfaces: Ru(0001), Rh(111), Pd(111), and Ag(111). Aiming to understand the origin behind energetic and structural trends along the 4d series we employ a range of analysis tools such as the electron reactivity function, decomposition of densities of states, electron density differences, and inspection of individual Kohn–Sham orbitals. The results obtained from our DFT calculations allow us to rationalize the bonding between water and transition metal surfaces as a balance of covalent and electrostatic interactions. A frontier orbital scheme based on so-called two-center four-electron interactions between the molecular orbitals of H2O—mainly the 1b1— and d-band states of the surface proves incisive in understanding these systems.