The interaction of water with ZnO nanoparticles has been studied by means of diffuse reflectance infrared spectroscopy (DRIFTS) and ultra-high vacuum FTIR spectroscopy (UHV-FTIRS). Exposing clean ZnO powder to water at 323 K leads to both molecular and dissociative adsorption of H2O forming a number of hydroxyl species. All the OH bands are clearly identified by the adsorption of D2O showing the expected isotopic shifts. According to the vibrational and thermal stability data obtained from single crystal surfaces, the OH species observed on ZnO nanoparticles are identified as follows: (1) OH group (3620 cm(-1)) on the polar O-ZnO(0001[combining macron]) surface formed via dissociation of water on oxygen vacancy sites; (2) partial dissociation of water on the mixed-terminated ZnO(101[combining macron]0) surface yielding coexistent H2O ( approximately 3150 and 3687 cm(-1)) and OH species (3672 cm(-1)), where the molecularly adsorbed H2O is further identified by the characteristic scissoring mode at 1617 cm(-1); (3) isolated OH species (3639 and 3656 cm(-1)) formed on the mixed-terminated ZnO(101[combining macron]0) surface; (4) interaction of water with defects forming hydroxyl (or O-HO) species (3564 and 3448 cm(-1)).

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The interaction of CH 2 O with perfect and defective TiO 2 (110) surfaces (produced by overannealing and Ar ion sputtering methods) was studied by thermal desorption spectroscopy, high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations. Exposing the perfect TiO 2 (110) surface to CH 2 O at 100 K leads to the formation of physisorbed CH 2 O and to polymerization of CH 2 O, yielding paraformaldehyde. The latter is bound to the 5-fold coordinated surface Ti atoms and is found to decompose and release CH 2 O at about 270 K. On the defective TiO 2 (110) surface, CH 2 O adsorbs more strongly on oxygen vacancy sites, ultimately forming a diolate (-OCH 2 CH 2 O-) species, as demonstrated by HREELS. The assignment of the vibrational frequencies was aided by theoretical calculations on the DFT-B3LYP level. Upon heating to higher temperatures, this species undergoes deoxygenation, resulting in ethylene formation.

The activation of CO 2 is one of the most important topics in catalysis.[1] For example, one of the simple Zn-enzymecatalyzed processes, the hydration of CO 2 by carbonic anhydrase, has led to extensive mechanistic and theoretical studies of the interaction of CO 2 with Zn-OH. [2][3][4][5] Also, in heterogeneous catalysis, a detailed understanding of the surface chemistry of CO 2 is an important issue; interest in this topic ranges from developing new processes for an emplacement of this greenhouse gas to the synthesis of methanol from syngas (CO/CO 2 /H 2 ) over Cu/ZnO catalysts. [6] Numerous studies have been reported on CO 2 adsorption on clean metal surfaces, where frequently activation is found to occur via the formation of a bent CO 2 dÀ species. [7][8][9] For oxide surfaces much less information is available. This deficit is in part due to the poor electric conductivity of many oxides which severely complicates the application of electron-based spectroscopic methods. In particular, there is a lack of information concerning molecular vibrations from highresolution electron energy loss spectroscopy (HREELS).The application of HREELS on oxide surfaces is-in addition to the electric conductivity problem-severely limited by the presence of intense substrate lattice excitations (FuchsKliewer phonons [10] ) which obscure the relatively weak vibrational modes of adsorbed species.Herein we present the results of a systematic multitechnique experimental and theoretical study on the interaction of CO 2 with the mixed-terminated ZnO(101 0) surface. In contrast to other oxides, ZnO is sufficiently conductive that electron-based methods can be applied without significant difficulties. The results from HREELS, thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), He-atom scattering (HAS), and X-ray photoelectron spectroscopy (XPS) reveal a complicated scenario, comprising the presence of two different ordered phases. By employing accurate periodic density-functional theory (DFT) and wave-function-based quantum-chemical cluster calculations it could be shown that the previously proposed bidentate bonding of CO 2 to this ZnO surface [11] has to be revised. Exposure to CO 2 leads-even at temperatures below 100 Kto the formation of an unusual tridentate carbonate species with the two O atoms of the CO 2 molecule being almost equivalently bound to two different Zn surface atoms.In a first set of experiments, the phase diagram of CO 2 adlayers on this ZnO substrate was determined using HAS. This technique uses neutral He atoms with thermal energy so charging problems are avoided. HAS is a highly sensitive surface-analysis method, [12][13][14] and has been successfully used to determine the phase diagram of H 2 O on the same surface.[15] The HAS data show that exposure of the sample to very small amounts of CO 2 in two steps (first with 4 L at 260 K and then 8 L at 120 K; exposures are given in units of langmuir (1 L = 1.33 10 À6 mbar s)) results in the formation of a well-ordered (2 1) phase (Figure 1 ...

The electronic effects of H atoms at interstitial sites in ZnO have been investigated by high resolution electron energy loss spectroscopy (HREELS). A reversible doping is achieved by exposing single crystalline (0001)-oriented ZnO substrates to atomic hydrogen. At low temperatures, interstitial H atoms form shallow donor states. At sufficiently high temperatures, the electrons are excited into the conduction band. We use EELS to demonstrate the presence of plasmons resulting from this finite density of charge carriers in the conduction band. Above temperatures of 100 K, a strong, plasmon-induced broadening of the quasielastic peak in the HREELS data is observed. The analysis of the temperature dependence yields a donor level ionization energy of 25+/-5 meV.