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The adsorption of CO on two different mixed-terminated, single-crystal surfaces of zinc oxide, ZnO(10 1 − 0) and ZnO(11 2 − 0), was investigated by employing infrared reflection absorption spectroscopy (IRRAS) using p- and s-polarized light. For both surfaces, one negative CO band was observed only for p-polarized light, indicating that CO is bound to the surface Zn 2 + sites in a nearly upright geometry. In addition, we observed a substantial coverage-induced frequency shift for CO adsorption on both ZnO surfaces. For ZnO(10 1 − 0), the ν (C-O) band shows a red shift of 16 cm − 1 when increasing the CO coverage from 0.5 ML (2185 cm − 1 ) to 1 ML (2169 cm − 1 ). On the ZnO(11 2 − 0) surface, for isolated CO molecules, a frequency of 2192 cm − 1 was detected. The CO band shifts to 2170 cm − 1 at full monolayer. On both surfaces, the coverage-dependent frequency shift is attributed to a combination of dynamic and substrate-mediated static adsorbate–adsorbate interactions.

The adsorption of two carboxylic acids, benzoic acid (BA) and terephthalic acid (TPA), on a single crystal rutile TiO$_{2}$(110) substrate was studied using infrared reflection–absorption spectroscopy (IRRAS) in conjunction with DFT calculations. On the basis of the high-quality IR data (in particular for the OH bands), various adsorbate species with different geometries could be identified. The adsorption of both, BA and TPA, on TiO$_{2}$(110) leads to deprotonation of carboxylic acids and protonation of substrate O-atoms. At low coverage, the deprotonated BA molecule adsorbs on TiO$_{2}$(110) in an upright, bidentate configuration, while the TPA molecule adopts a flat-lying geometry with both carboxylates bound to the surface in a monodentate geometry. At higher coverages, a transition from flat-lying to upright-oriented TPA molecules occurs. At saturation coverage, both BA and TPA molecules undergo dimerization indicating the presence of pronounced attractive intermolecular interactions. We propose that the BA dimers are stabilized by the interaction between adjacent phenyl rings, while the TPA dimerization is attributed to the formation of double hydrogen bonds between adjacent apical carboxylic groups.

The interaction of NO with the O-rich RuO 2 (1 1 0) surface, exposing coordinatively unsaturated O-bridge, O-cus, and Ru-cus atoms, was studied at 300 K by thermal desorption spectroscopy (TDS) and high-resolution electron energy-loss spectroscopy (HREELS). The conclusions are validated by isotope substitution experiments with 18 O. During exposure to NO an O···N–O surface group (NO 2 -cus) is formed with O-cus. Additionally, a smaller number of empty Ru-cus sites are filled by NO-cus. If one warms the sample to 400 K, NO 2 -cus does not desorb but decomposes into O and NO again, the latter being either released into gas phase or adsorbed as NO-cus. With O-bridge such a surface group is not stable at 300 K. Our experiments further prove that O-cus is more reactive than O-bridge.

The most successful method to unravel the microscopic mechanisms governing reactions in heterogeneous catalysis is the “surface science” approach which is based on well-controlled studies on model catalysts (usually single crystal surfaces) under ultrahigh vacuum (UHV) conditions [G. Ertl, Angew. Chem. 47 (2008) 3524]. In this review our recent vibrational spectroscopic studies on selected model reactions at various single-crystalline metal oxide surfaces are summarized. Two vibrational spectroscopic methods, high resolution electron energy loss spectroscopy (HREELS) and Fourier-transform infrared spectroscopy (FTIRS), were applied to characterize the adsorbed species and to elucidate the elementary processes of chemical reactions at oxide surfaces ranging from well-defined single crystals to modified surfaces with deliberately introduced defects. The combination of both methods allows us to extend the vibrational spectroscopic studies from ideal to complex systems.

Ethylene adsorbed on the stoichiometric RuO 2 (110) surface at 85 K is investigated applying high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy in combination with isotope labeling experiments. Under the applied conditions (ultra-high vacuum, no continuous reactant supply) ethylene does not react with stoichiometric RuO2(110) and is desorbed molecularly at 320 K. Warming to 260 K leads to a rearrangement of C 2 H 4 from n- to σ-bonded.

The O-bridge atoms on a stoichiometric RuO2(1 1 0) surface were removed by reaction with CO. The resulting reduced surface was then further exposed to CO. By means of thermal desorption spectroscopy and high-resolution electron energy-loss spectroscopy three adsorbed CO states were identified on bridge sites and assigned to double-bonded, single-bonded, and single-bonded species in the vicinity of O-bridge residues, respectively.

NH and NH2 reaction intermediates were prepared on the Ru(1 1 2 0) surface by thermal dehydrogenation of NH3 and studied using high-resolution electron energy-loss spectroscopy. NH2 and NH were isolated to a high degree, which allows to unambiguously identify their vibrational modes. From isotope experiments with 15ND3, frustrated translational and rotational (bending, rocking) modes were differentiated. NH2 exhibits a scissoring mode at 189.7 meV, a rocking mode at 165.4 meV and translational modes at 62.2 and 44.3 meV. NH2 is tilted relative to the surface normal within an adsorbate complex of Cs symmetry. For NH two rather low-lying bending modes at 83.2 and 88.2 meV were identified due to NH species presumably adsorbed in the two different threefold-hollow sites within the Ru(1 1 2 0)(1×1) unit cell, also in a tilted geometry. A long-standing wrong assignment of the NH bending mode could be corrected.

The thermal dehydrogenation of NH3 was studied on Ru(11(2) over bar0) using high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy. Following NH3 exposure at 85 K, the expected three dehydrogenation steps could be separated, allowing to isolate successively NH2, NH and N on the surface. The R u(11(2) over bar 0) surface is highly reactive as indicated by the complete dehydrogenation of NH3 for exposures up to 0.2 L. During annealing to 300 K both the channels for desorption and dehydrogenation are opened. Following adsorption at 85 K, the obtained nitrogen coverage of 0.11 ML is low, while for dissociative adsorption of NH3 at 300 K, a maximum coverage of 0.38 ML is achieved, and the stability of NH2 is increased.

The dissociation of CO on the Ru(1 1 2 0) surface was investigated by thermal desorption spectroscopy and high resolution electron energy loss spectroscopy (HREELS). Up to about 20% of a monolayer CO dissociates on the Ru(1 1 2 0) surface at room temperature. Oxygen (68 meV) and carbon (42 meV) are identified as dissociation products by HREELS. At temperatures below 240 K the precursor of dissociation can be stabilized. In this state, CO is tilted and presumably bonded at a kind of a fourfold hollow site. During annealing, CO2 formation is observed from CO reacting with oxygen from the dissociation.

The molecular adsorption of CO was studied on the Ru (1 1 2 0) surface using low energy electron diffraction, high resolution electron energy loss spectroscopy and thermal desorption spectroscopy at room temperature. Two ordered CO phases were observed, a p(1×2) phase formed at θCO=0.25 and a (1×2)p2mg phase formed at θCO=0.5 (saturation). In both phases, the CO molecules occupy the on-top sites. Two structural models are suggested.

H 2 O adsorption on Pt(1 1 1) was studied by high-resolution electron energy loss spectroscopy over a wide exposure range at 85 K. We conclude that the known loss of the ice multilayer at about 30 meV is due to T ⊥4 , the Pt–OH 2 translational mode perpendicular to the surface. This new assignment gives rise to a very consistent understanding of the vibrational spectra for thicker ice layers, for the bilayer, and for sub-monolayer coverages. For the bilayer two perpendicular modes are observed at 33 meV for the top H 2 O molecule and at 16.5 meV for the bottom H 2 O bonding to the surface Pt atom. For a new low-coverage phase only one T ⊥ is found at 15 meV. Following a recent scanning tunneling microscopy result of Morgenstern et al. [1] , the latter is assigned to H 2 O monomers adsorbed in chains at the Pt(1 1 1) step edges. The newly observed monomer translation mode T ⊥1 at 15 meV can be used as an indicator for small amounts of H 2 O co-adsorbed within O or OH layers.