Hydrogen Evolution from Metal–Surface Hydroxyl Interaction

The redox interaction between hydroxyl groups on oxide surfaces and metal atoms and clusters deposited thereon, according to which metals get oxidized and hydrogen released, is an effective route to tune both the morphological (particle size and shape) and electronic (oxidation state) properties of oxide-supported metals. While the oxidation state of the metals can straightforwardly be probed by X-ray based methods (e.g., XPS), hydrogen is much more difficult to capture, in particular in highly reactive systems where the redox interaction takes place directly during the nucleation of the metals at room temperature. In the present study, the interaction of Pd with a hydroxylated MgO(001) surface was studied using a combination of vibrational spectroscopy, electronic structure studies including Auger parameter analysis, and thermal desorption experiments. The results provide clear experimental evidence for the redox nature of the interaction by showing a direct correlation between metal oxidation and hydrogen evolution at slightly elevated temperature (390 K). Moreover, a second hydrogen evolution pathway opens up at 500 K, which involves hydroxyl groups on the MgO support and carbon monoxide adsorbed on the Pd particles (water–gas shift reaction)

Interaction of Water with the CaO(001) Surface

The interaction of water with the CaO(001) surface has been studied from ultrahigh-vacuum to submillibar water vapor pressures and at temperatures of 100 and 300 K using well-structured CaO(001)/Mo(001) thin-film model systems. Infrared reflection absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM) in combination with density functional theory (DFT) calculations have been employed to reveal the correlation between the vibrational frequencies of the hydroxyl groups and the distinct hydroxylated surface phases that develop as a function of water exposure. In the low-coverage regime, water monomers, small water clusters, and one-dimensional water chains are formed on the CaO(001) surface. At increasing water coverages, water-induced structural disorder is observed, indicating partial solvation of Ca²⁺ ions. Partial transformation of CaO(001) into a Ca­(OH)₂-like phase occurs upon dosing of water at submillibar water pressure. In addition, it was found that interfacial hydroxyl groups formed on the CaO(001) surface during water exposure at temperatures as low as 100 K shift the temperatures for ice desorption and for the transition from amorphous solid water to crystalline ice by 10 K as compared to those for the nonhydroxylated MgO surface

Formation of Water Chains on CaO(001): What Drives the 1D Growth?

Formation of partly dissociated water chains is observed on CaO(001) films upon water exposure at 300 K. While morphology and orientation of the 1D assemblies are revealed from scanning tunneling microscopy, their atomic structure is identified with infrared absorption spectroscopy combined with density functional theory calculations. The latter exploit an ab initio genetic algorithm linked to atomistic thermodynamics to determine low-energy H₂O configurations on the oxide surface. The development of 1D structures on the <i>C</i>_4<i>v</i> symmetric CaO(001) is triggered by symmetry-broken water tetramers and a favorable balance between adsorbate–adsorbate versus adsorbate–surface interactions at the constraint of the CaO lattice parameter