The adsorption of CO 2 on the Fe 3 O 4 (001)-( √ 2 × √ 2)R45 • surface was studied experimentally using temperature programmed desorption (TPD), photoelectron spectroscopies (UPS and XPS), and scanning tunneling microscopy. CO 2 binds most strongly at defects related to Fe 2+ , including antiphase domain boundaries in the surface reconstruction and above incorporated Fe interstitials. At higher coverages, CO 2 adsorbs at fivefold-coordinated Fe 3+ sites with a binding energy of 0.4 eV. Above a coverage of 4 molecules per (• unit cell, further adsorption results in a compression of the first monolayer up to a density approaching that of a CO 2 ice layer. Surprisingly, desorption of the second monolayer occurs at a lower temperature (≈84 K) than CO 2 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 mea-

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 Srn+1RunO3n+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.

Ca3Ru2O7(001), an oxide surface without dopants, defects, or low-coordinated sites, readily activates molecular oxygen to O2–.

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.

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