RESUMO
Activating the O2 molecule is at the heart of a variety of technological applications, most prominently in energy conversion schemes including solid oxide fuel cells, electrolysis, and catalysis. Perovskite oxides, both traditionally-used and novel formulations, are the prime candidates in established and emerging energy devices. This work shows that the as-cleaved and unmodified CaO-terminated (001) surface of Ca3Ru2O7, a Ruddlesden-Popper perovskite, supports a full monolayer of superoxide ions, O2-, when exposed to molecular O2. The electrons for activating the molecule are transferred from the subsurface RuO2 layer. Theoretical calculations using both, density functional theory (DFT) and more accurate methods (RPA), predict the adsorption of O2- with Eads = 0.72 eV and provide a thorough analysis of the charge transfer. Non-contact atomic force microscopy (nc-AFM) and scanning tunnelling microscopy (STM) are used to resolve single molecules and confirm the predicted adsorption structures. Local contact potential difference (LCPD) and X-ray photoelectron spectroscopy (XPS) measurements on the full monolayer of O2- confirm the negative charge state of the molecules. The present study reports the rare case of an oxide surface without dopants, defects, or low-coordinated sites readily activating molecular O2.
RESUMO
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 (E ads = 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.As ternary perovskite-type oxides are increasingly used in fuel cells and catalysis, greater understanding of their surface chemical properties is required. Here the authors report a pronounced ordering of hydroxyls on the cleaved (001) surface of Ca3Ru2O7 perovskite induced by O-octahedral rotation and tilt.
RESUMO
The adsorption of CO2 on the Fe3O4(001)-(2 × 2)R45° surface was studied experimentally using temperature programmed desorption (TPD), photoelectron spectroscopies (UPS and XPS), and scanning tunneling microscopy. CO2 binds most strongly at defects related to Fe2+, including antiphase domain boundaries in the surface reconstruction and above incorporated Fe interstitials. At higher coverages,CO2 adsorbs at fivefold-coordinated Fe3+ sites with a binding energy of 0.4 eV. Above a coverage of 4 molecules per (2 × 2)R45° unit cell, further adsorption results in a compression of the first monolayer up to a density approaching that of a CO2 ice layer. Surprisingly, desorption of the second monolayer occurs at a lower temperature (≈84 K) than CO2 multilayers (≈88 K), suggestive of a metastable phase or diffusion-limited island growth. The paper also discusses design considerations for a vacuum system optimized to study the surface chemistry of metal oxide single crystals, including the calibration and characterisation of a molecular beam source for quantitative TPD measurements.
RESUMO
Although 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)O3(n+1) (n = 1, 2) using low-temperature scanning tunnelling microscopy, X-ray photoelectron spectroscopy, and density functional theory. These layered perovskites cleave between neighbouring 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.