Unusual synergistic effect in layered Ruddlesden-Popper oxide enables ultrafast hydrogen evolution

Efficient electrocatalysts for hydrogen evolution reaction are key to realize clean hydrogen production through water splitting. As an important family of functional materials, transition metal oxides are generally believed inactive towards hydrogen evolution reaction, although many of them show high activity for oxygen evolution reaction. Here we report the remarkable electrocatalytic activity for hydrogen evolution reaction of a layered metal oxide, Ruddlesden-Popper-type Sr2RuO4 with alternative perovskite layer and rock-salt SrO layer, in an alkaline solution, which is comparable to those of the best electrocatalysts ever reported. By theoretical calculations, such excellent activity is attributed mainly to an unusual synergistic effect in the layered structure, whereby the (001) SrO-terminated surface cleaved in rock-salt layer facilitates a barrier-free water dissociation while the active apical oxygen site in perovskite layer promotes favorable hydrogen adsorption and evolution. Moreover, the activity of such layered oxide can be further improved by electrochemistry-induced activation

Formaldehyde Adsorption on the Anatase TiO 2

Formaldehyde (CH2O) adsorption on the anatase TiO2(101) surface was studied with a combination of experimental and theoretical methods. Scanning tunneling microscopy, noncontact atomic force microscopy, temperature-programmed desorption, and X-ray photoelectron spectroscopy were employed on the experimental side. Density functional theory was used to calculate formaldehyde adsorption configurations and energy barriers for transitions between them. At low coverages (<0.25 monolayer), CH2O binds via its oxygen atom to the surface 5- coordinated Ti atoms Ti5c (monodentate configuration). At higher coverages, many adsorption configurations with comparable adsorption energies coexist, including a bidentate configuration and paraformaldehyde chains. The adsorption energies of all possible adsorption configurations lie in the range from 0.6 to 0.8 eV. Upon annealing, all formaldehyde molecules desorb below room temperature; no other reaction products were detected

Ordered hydroxyls on Ca3Ru2O7(001)

As complex ternary perovskite-type oxides are increasingly used in solid oxide fuel cells, electrolysis and catalysis, it is desirable to obtain a better understanding of their surface chemical properties. Here we report a pronounced ordering of hydroxyls on the cleaved (001) surface of the Ruddlesden-Popper perovskite Ca3Ru2O7 upon water adsorption at 105 K and subsequent annealing to room temperature. Density functional theory calculations predict the dissociative adsorption of a single water molecule (Eads = 1.64 eV), forming an (OH)ads group adsorbed in a Ca-Ca bridge site, with an H transferred to a neighboring surface oxygen atom, Osurf. Scanning tunneling microscopy images show a pronounced ordering of the hydroxyls with (2 × 1), c(2 × 6), (1 × 3), and (1 × 1) periodicity. The present work demonstrates the importance of octahedral rotation and tilt in perovskites, for influencing surface reactivity, which here induces the ordering of the observed OH overlayers

Adsorption of water at the SrO surface of ruthenates

While perovskite oxides hold promise in applications ranging from solid oxide fuel cells to catalysts, their surface chemistry is poorly understood at the molecular level. Here we follow the formation of the first monolayer of water at the (001) surfaces of Sr(n+1)Ru(n)O(3n+1) (n = 1, 2) using low-temperature scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory. These layered perovskites cleave between neighboring SrO planes, yielding almost ideal, rocksalt-like surfaces. An adsorbed monomer dissociates and forms a pair of hydroxide ions. The OH stemming from the original molecule stays trapped at Sr-Sr bridge positions, circling the surface OH with a measured activation energy of 187±10 meV. At higher coverage dimers of dissociated water assemble into one-dimensional chains and form a percolating network where water adsorbs molecularly in the gaps. Our work shows the limitations of applying surface chemistry concepts derived for binary rocksalt oxides to perovskites

chapter 10 Thin oxide films as model systems for heterogeneous catalysts

This chapter summarizes efforts to use thin oxide films as model supports for heterogeneous catalysts. We demonstrate that the oxide film route provides a useful platform to study oxide surfaces, per se its interaction with species from the gas phase, supported metal and oxide nanoparticles using the entire tool box of surfaces science under ultrahigh vacuum conditions. The extension to use thin oxide films as template also under ambient conditions or under water, is discussed and the potential to use oxide films as genuine two-dimensional materials is exemplified with vitreous and crystalline silica films