Surface chemistry and stability of metastable corundum-type In2O3

To account for the explanation of an eventual sensing and catalytic behavior of rhombohedral In2O3 (rh-In2O3) and the dependence of the metastability of the latter on gas atmospheres, in situ electrochemical impedance spectroscopic (EIS), Fourier-transform infrared spectroscopic (FT-IR), in situ X-ray diffraction and in situ thermogravimetric analyses in inert (helium) and reactive gases (hydrogen, carbon monoxide and carbon dioxide) have been conducted to link the gas-dependent electrical conductivity features and the surface chemical properties to its metastability towards cubic In2O3. In particular, for highly reducible oxides such as In2O3, for which not only the formation of oxygen vacancies, but deep reduction to the metallic state (i.e. metallic indium) also has to be taken into account, this approach is imperative. Temperature-dependent impedance features are strongly dependent on the respective gas composition and are assigned to distinct changes in either surface adsorbates or free charge carrier absorbance, allowing for differentiating and distinguishing between bulk reduction-related features from those directly arising from surface chemical alterations. For the measurements in an inert gas atmosphere, this analysis specifically also included monitoring the fate of differently bonded, and hence, differently reactive, hydroxyl groups. Reduction of rh-In2O3 proceeds to a large extent indirectly via rh-In2O3 → c-In2O3 → In metal. As deduced from the CO and CO2 adsorption experiments, rhombohedral In2O3 exhibits predominantly Lewis acidic surface sites. The basic character is less pronounced, directly explaining the previously observed high (inverse) water–gas shift activity and the low CO2 selectivity in methanol steam reforming.DFG, SPP 1415, Kristalline Nichtgleichgewichtsphasen - Präparation, Charakterisierung und in situ-Untersuchung der Bildungsmechanisme

Near-infrared Spectroscopy for Remote Sensing of Porosity, Density, and Cubicity of Crystalline and Amorphous H2O Ices in Astrophysical Environments

We present laboratory spectra of pure amorphous and crystalline H _2 O ices in the near-infrared (NIR, 1–2.5 μ m/10,000–4000 cm ^−1 ) at 80–180 K. The aim of this study is to provide spectroscopic reference data that allow remotely accessing ice properties for icy objects such as icy moons, cometary ice, or Saturn rings. Specifically, we identify new spectral markers for assessing three important properties of ices in space: (i) porosity/fluffiness, (ii) bulk density of amorphous ice, and (iii) cubicity in crystalline ice. The analysis is based on the first OH-stretching overtone (2 ν _OH ) and the combinational band at 5000 cm ^−1 /2 μ m, which are potent spectral markers for these properties. By comparison of vapor-deposited, microporous amorphous solid water, pore-free low-, high-, and very-high-density amorphous ice, we are able to separate the effect of (bulk) density from the effect of porosity on NIR-spectra of amorphous ices. This allows for clarifying a longstanding inconsistency about the density of amorphous ice vapor-deposited at low temperatures, first brought up by Jenniskens & Blake. Direct comparison of NIR spectra with powder X-ray diffractograms allows us to correlate spectral features with the number of cubic stacking layers in stacking-disordered ice I _sd , ranging from fully cubic ice I _c to fully hexagonal ice I _h . We show that exposure times for instruments on the James Webb Space Telescope are in the hour range to distinguish these properties, demonstrating the usefulness of the neglected NIR spectral range for identifying ices in space