Michaela Kogler

In Situ FT-IR Spectroscopic Study of CO2 and CO Adsorption on Y2O3, ZrO2, and Yttria-Stabilized ZrO2

In situ FT-IR spectroscopy was exploited to study the adsorption of CO2 and CO on commercially available yttria-stabilized ZrO2 (8 mol % Y, YSZ-8), Y2O3, and ZrO2. All three oxides were pretreated at high temperatures (1173 K) in air, which leads to effective dehydroxylation of pure ZrO2. Both Y2O3 and YSZ-8 show a much higher reactivity toward CO and CO2 adsorption than ZrO2 because of more facile rehydroxylation of Y-containing phases. Several different carbonate species have been observed following CO2 adsorption on Y2O3 and YSZ-8, which are much more strongly bound on the former, due to formation of higher-coordinated polydentate carbonate species upon annealing. As the crucial factor governing the formation of carbonates, the presence of reactive (basic) surface hydroxyl groups on Y-centers was identified. Therefore, chemisorption of CO2 most likely includes insertion of the CO2 molecule into a reactive surface hydroxyl group and the subsequent formation of a bicarbonate species. Formate formation following CO adsorption has been observed on all three oxides but is less pronounced on ZrO2 due to effective dehydroxylation of the surface during high-temperature treatment. The latter generally causes suppression of the surface reactivity of ZrO2 samples regarding reactions involving CO or CO2 as reaction intermediates.

Hydrogen Surface Reactions and Adsorption Studied on Y2O3, YSZ, and ZrO2.

The surface reactivity of Y2O3, YSZ, and ZrO2 polycrystalline powder samples toward H2 has been comparatively studied by a pool of complementary experimental techniques, comprising volumetric methods (temperature-programmed volumetric adsorption/oxidation and thermal desorption spectrometry), spectroscopic techniques (in situ electric impedance and in situ Fourier-transform infrared spectroscopy), and eventually structural characterization methods (X-ray diffraction and scanning electron microscopy). Reduction has been observed on all three oxides to most likely follow a surface or near-surface-limited mechanism involving removal of surface OH-groups and associated formation of water without formation of a significant number of anionic oxygen vacancies. Partly reversible adsorption of H2 was proven on the basis of molecular H2 desorption. Dictated by the specific hydrophilicity of the oxide, readsorption of water eventually takes place. The inference of this surface-restricted mechanism is further corroborated by the fact that no bulk structural and/or morphological changes were observed upon reduction even at the highest reduction temperatures (1173 K). We anticipate relevant implications for the use of especially YSZ in fuel cell research, since in particular the chemical state and structure of the surface under typical reducing high-temperature conditions affects the operation of the entire cell.

Methane Decomposition and Carbon Growth on Y2O3, Yttria-Stabilized Zirconia, and ZrO2

Carbon deposition following thermal methane decomposition under dry and steam reforming conditions has been studied on yttria-stabilized zirconia (YSZ), Y2O3, and ZrO2 by a range of different chemical, structural, and spectroscopic characterization techniques, including aberration-corrected electron microscopy, Raman spectroscopy, electric impedance spectroscopy, and volumetric adsorption techniques. Concordantly, all experimental techniques reveal the formation of a conducting layer of disordered nanocrystalline graphite covering the individual grains of the respective pure oxides after treatment in dry methane at temperatures T ≥ 1000 K. In addition, treatment under moist methane conditions causes additional formation of carbon-nanotube-like architectures by partial detachment of the graphite layers. All experiments show that during carbon growth, no substantial reduction of any of the oxides takes place. Our results, therefore, indicate that these pure oxides can act as efficient nonmetallic substrates for methane-induced growth of different carbon species with potentially important implications regarding their use in solid oxide fuel cells. Moreover, by comparing the three oxides, we could elucidate differences in the methane reactivities of the respective SOFC-relevant purely oxidic surfaces under typical SOFC operation conditions without the presence of metallic constituents.

A high-temperature, ambient-pressure ultra-dry operando reactor cell for Fourier-transform infrared spectroscopy

The construction of a newly designed high-temperature, high-pressure FT-IR reaction cell for ultra-dry in situ and operando operation is reported. The reaction cell itself as well as the sample holder is fully made of quartz glass, with no hot metal or ceramic parts in the vicinity of the high-temperature zone. Special emphasis was put on chemically absolute water-free and inert experimental conditions, which includes reaction cell and gas-feeding lines. Operation and spectroscopy up to 1273 K is possible, as well as pressures up to ambient conditions. The reaction cell exhibits a very easy and variable construction and can be adjusted to any available FT-IR spectrometer. Its particular strength lies in its possibility to access and study samples under very demanding experimental conditions. This includes studies at very high temperatures, e.g., for solid-oxide fuel cell research or studies where the water content of the reaction mixtures must be exactly adjusted. The latter includes all adsorption studies on oxide surfaces, where the hydroxylation degree is of paramount importance. The capability of the reaction cell will be demonstrated for two selected examples where information and in due course a correlation to other methods can only be achieved using the presented setup.

Enhanced kinetic stability of pure and Y-doped tetragonal ZrO2.

The kinetic stability of pure and yttrium-doped tetragonal zirconia (ZrO2) polymorphs prepared via a pathway involving decomposition of pure zirconium and zirconium + yttrium isopropoxide is reported. Following this preparation routine, high surface area, pure, and structurally stable polymorphic modifications of pure and Y-doped tetragonal zirconia are obtained in a fast and reproducible way. Combined analytical high-resolution in situ transmission electron microscopy, high-temperature X-ray diffraction, and chemical and thermogravimetric analyses reveals that the thermal stability of the pure tetragonal ZrO2 structure is very much dominated by kinetic effects. Tetragonal ZrO2 crystallizes at 400 °C from an amorphous ZrO2 precursor state and persists in the further substantial transformation into the thermodynamically more stable monoclinic modification at higher temperatures at fast heating rates. Lower heating rates favor the formation of an increasing amount of monoclinic phase in the product mixture, especially in the temperature region near 600 °C and during/after recooling. If the heat treatment is restricted to 400 °C even under moist conditions, the tetragonal phase is permanently stable, regardless of the heating or cooling rate and, as such, can be used as pure catalyst support. In contrast, the corresponding Y-doped tetragonal ZrO2 phase retains its structure independent of the heating or cooling rate or reaction environment. Pure tetragonal ZrO2 can now be obtained in a structurally stable form, allowing its structural, chemical, or catalytic characterization without in-parallel triggering of unwanted phase transformations, at least if the annealing or reaction temperature is restricted to T ≤ 400 °C.

High-Temperature Carbon Deposition on Oxide Surfaces by CO Disproportionation

Carbon deposition due to the inverse Boudouard reaction (2CO → CO2 + C) has been studied on yttria-stabilized zirconia (YSZ), Y2O3, and ZrO2 in comparison to CH4 by a variety of different chemical, structural, and spectroscopic characterization techniques, including electrochemical impedance spectroscopy (EIS), Fourier-transform infrared (FT-IR) spectroscopy and imaging, Raman spectroscopy, and electron microscopy. Consentaneously, all experimental methods prove the formation of a more or less conducting carbon layer (depending on the used oxide) of disordered nanocrystalline graphite covering the individual grains of the respective pure oxides after treatment in flowing CO at temperatures above ∼1023 K. All measurements show that during carbon deposition, a more or less substantial surface reduction of the oxides takes place. These results, therefore, reveal that the studied pure oxides can act as efficient nonmetallic substrates for CO-induced growth of highly distorted graphitic carbon with possible important technological implications especially with respect to treatment in pure CO or CO-rich syngas mixtures. Compared to CH4, more carbon is generally deposited in CO under otherwise similar experimental conditions. Although Raman and electron microscopy measurements do not show substantial differences in the structure of the deposited carbon layers, in particular, electrochemical impedance measurements reveal major differences in the dynamic growth process of the carbon layer, eventually leading to less percolated islands and suppressed metallic conductivity in comparison to CH4-induced graphite.

Evidence for dissolved hydrogen in the mixed ionic–electronic conducting perovskites La0.6Sr0.4FeO3−δ and SrTi0.7Fe0.3O3−δ

Two mixed ionic-electronic conducting, Fe-containing perovskites were investigated regarding their reducibility in dry H2, namely lanthanum strontium ferrite (LSF4, La0.6Sr0.4FeO3-δ) and strontium titanium ferrite (STF3, SrTi0.7Fe0.3O3-δ). Upon treatment under comparable reduction conditions, LSF4 is by far more affected by reduction and is reduced more deeply than STF3. Thermal treatments of fully oxidized or slightly reduced LSF4/STF3 at decreased O2 partial pressure lead to spontaneous desorption of O2. Temperature-programmed desorption (TPD) spectra of H2 reveal distinct differences in H2 and H2O desorption. A simple mass balance of H2 reveals that oxygen vacancies formed on STF3 are more resilient towards O2 re-oxidation compared to those on LSF4. The results also imply that substantial amounts of hydrogen are dissolved in the bulk of LSF4 or STF3. 4.9 × 10-2 mol H2 per mol LSF4 and 1.6 × 10-2 mol H2 per mol STF3 are incorporated if the specimens are heated in flowing/dry H2 up to 550 °C and 612 °C, respectively. For LSF4 this equals about 13 hypothetical ML of H2 and for STF3 about 20 hypothetical ML of H2. This conclusion is also supported by Fourier-transform infrared spectroscopy (FT-IR). FT-IR reveals water formation during static H2 treatment of LSF4/STF3, which indicates perovskite reduction. Furthermore, both samples behave extraordinarily hydrophobic and no chemistry involving surface hydroxy groups was observed.