Structure–Property Relationships in the Y₂O₃–ZrO₂ Phase Diagram: Influence of the Y‑Content on Reactivity in C1 Gases, Surface Conduction, and Surface Chemistry

The C1 chemistry of Y-doped ZrO₂ samples (3, 8, 20, and 40 mol % Y₂O₃; 3-YSZ, 8-YSZ, 20-YSZ, and 40-YSZ) was comparatively studied with respect to the correlation of electrochemical properties and surface chemistry in CH₄, CO, and CO₂ atmospheres by electrochemical impedance (EIS) and spectroscopic (FT-IR) methods up to 1273 K to unravel the influence of the Y-doping level. A consistent picture with respect to qualitative and quantitative surface modifications as a function of temperature and gas-phase composition evolves by performing highly correlated <i>operando/in situ</i> measurements. A detailed study of carbon deposition in CH₄ and CO and adsorption of CO and CO₂, but also proof of the strong influence of the surface chemistry, is included. Carbon deposition during treatment in CH₄ and CO at temperatures <i>T</i> ≥ 1023 K is a common feature on all materials, irrespective of the Y content. On the 40-YSZ sample, the thinnest, but at the same time fully percolated, carbon layer was generated, and hence, “metallic” conductivity was apparent. This goes along with the fact that 40-YSZ is most unreactive toward adsorption, suggesting a direct link between homogeneous deposition and suppressed reactivity. For all Y-doped samples, temperature regions with different charge carrier activation energies could be identified, perfectly corresponding to significant changes in surface chemistry. Due to the different degree of hydroxylation and the different ability to chemisorb CO and CO₂, the influence of the surface chemistry on the electrochemical properties is varying strongly as a function of Y-content

Surface Reactivity of YSZ, Y₂O₃, and ZrO₂ toward CO, CO₂, and CH₄: A Comparative Discussion

The C1-surface chemistry of catalytically and technologically relevant oxides (YSZ, ZrO₂, and Y₂O₃) toward CH₄, CO, and CO₂ was comparatively studied by electrochemical impedance (EIS) and spectroscopic (FT-IR) methods. Highly correlated <i>in situ</i> measurements yield a consistent picture with respect to qualitative and quantitative surface modifications as a function of temperature and gas phase composition. This includes not only a detailed study of carbon deposition in methane and adsorption of CO and CO₂ but also proof of the strong influence of surface chemistry. On all studied oxides, carbon deposited during methane treatment grows dynamically forming interconnected islands and eventually a continuous conducting carbon layer at <i>T</i> ≥ 1073 K. Before methane dissociation via gas phase radical reactions/H-abstraction and carbon growth, a complex redox interplay of total oxidation as well as formate and carbonate formation leads to associated surface and grain conductivity changes. For CO adsorption, these measurements yield data on the time and temperature dependence of the adsorbate- and carburization-induced conductivity processes. In that respect, an equivalent circuit model in dry CO allows to disentangle the different contributions of grain interiors, grain boundaries, and electrode contributions. For YSZ, temperature regions with different charge carrier activation energies could be identified, perfectly corresponding to significant changes in surface chemistry. Hydroxyl groups, carbonates, or formates strongly influence the impedance properties, suggesting that the conductivity properties of YSZ, e.g., in a realistic reforming gas mixture, cannot be reduced to exclusive bulk ion conduction. Because of the different degree of hydroxylation and the different ability to chemisorb CO and CO₂, the influence of the surface chemistry on the electrochemical properties is varying strongly: in contrast to ZrO₂, the impact of the studied C1-gases on YSZ and Y₂O₃ is substantial. This also includes the reoxidation/reactivation behavior of the surfaces

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

Structural and Electrochemical Properties of Physisorbed and Chemisorbed Water Layers on the Ceramic Oxides Y₂O₃, YSZ, and ZrO₂

A combination of operando Fourier transform infrared spectroscopy, operando electrochemical-impedance spectroscopy, and moisture-sorption measurements has been exploited to study the adsorption and conduction behavior of H₂O and D₂O on the technologically important ceramic oxides YSZ (8 mol % Y₂O₃), ZrO₂, and Y₂O₃. Because the characterization of the chemisorbed and physisorbed water layers is imperative to a full understanding of (electro-)­catalytically active doped oxide surfaces and their application in technology, the presented data provide the specific reactivity of these oxides toward water over a pressure-and-temperature parameter range extending up to, e.g., solid-oxide fuel cell (SOFC)-relevant conditions. The characteristic changes of the related infrared bands could directly be linked to the associated conductivity and moisture-sorption data. For YSZ, a sequential dissociative water (“ice-like” layer) and polymeric chained water (“liquid-like”) water-adsorption model for isothermal and isobaric conditions over a pressure range of 10^–5 to 24 mbar and a temperature range from room temperature up to 1173 K could be experimentally verified. On pure monoclinic ZrO₂, in contrast to highly hydroxylated YSZ and Y₂O₃, a high surface concentration of OH groups from water chemisorption is absent at any temperature and pressure. Thus, the ice-like and following molecular water layers exhibit no measurable protonic conduction. We show that the water layers, even under these rather extreme experimental conditions, play a key role in understanding the function of these materials. Furthermore, the reported data are supposed to provide an extended basis for the further investigation of close-to-real gas adsorption or catalyzed heterogeneous reactions

Metastable Corundum-Type In₂O₃: Phase Stability, Reduction Properties, and Catalytic Characterization

The phase stability, reduction, and catalytic properties of corundum-type rhombohedral In₂O₃ have been comparatively studied with respect to its thermodynamically more stable cubic In₂O₃ counterpart. Phase stability and transformation were observed to be strongly dependent on the gas environment and the reduction potential of the gas phase. As such, reduction in hydrogen caused both the efficient transformation into the cubic polymorph as well as the formation of metallic In especially at high reduction temperatures between 573 and 673 K. In contrast, reduction in CO suppresses the transformation into cubic In₂O₃ but leads to a larger quantity of In metal at comparable reduction temperatures. This difference is also directly reflected in temperature-dependent conductivity measurements. Catalytic characterization of rh-In₂O₃ reveals activity in both routes of the water–gas shift equilibrium, which gives rise to a diminished CO₂-selectivity of ∼60% in methanol steam reforming. This is in strong contrast to its cubic counterpart where CO₂ selectivities of close to 100% due to the suppressed inverse water–gas shift reaction, have been obtained. Most importantly, rh-In₂O₃ in fact is structurally stable during catalytic characterization and no unwanted phase transformations are triggered. Thus, the results directly reveal the application-relevant physicochemical properties of rh-In₂O₃ that might encourage subsequent studies on other less-common In₂O₃ polymorphs

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 <i>T</i> ≤ 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

Synthetic Access to Cubic Rare Earth Molybdenum Oxides RE₆MoO_12−δ (RE = Tm–Lu) Representing a New Class of Ion Conductors

Materials crystallizing in highly symmetric structures are of particular interest as they display superior physical properties in many relevant technological areas such as solid oxide fuels cells (SOFCs), catalysis, or photoluminescent materials. While the rare earth molybdenum oxides RE₆MoO₁₂ with the large rare earth cations RE = La to Dy crystallize in a cubic defect fluorite structure type (<i>Fm</i>3̅<i>m</i>, no. 225), the compounds with the smaller cations RE = Tm–Lu could hitherto only be synthesized in the rhombohedral defect fluorite structure type (<i>R</i>3̅, no. 148). In the following, new low temperature access to the rare earth molybdenum oxides RE₆MoO_12−δ (RE = Tm–Lu) crystallizing in the highly symmetric cubic bixbyite structure type (<i>Ia</i>3̅, no. 206) will be discussed. The three-step method comprises preparation of the rhombohedral phases by solution combustion (SC) reactions, their reduction including simultaneous structural transitions from the rhombohedral to the cubic phases, and subsequent reoxidations while preserving their cubic structures. Detailed studies on this process were performed on the compound Yb₆MoO_12−δ using TG-DTA, XPS, EDX, and X-ray powder diffraction (XRPD) measurements. In contrast to the rhombohedral phase Yb₆MoO₁₂, which does not show any ionic conductivity, the cubic bixbyite structured compound can be classified as a promising ionic conductor. Electrochemical impedance spectroscopy (EIS) revealed that bulk and grain boundary activation energy determined to be 144.6 kJ mol^–1 and 150.4 kJ mol^–1, respectively, range in the same regime as the conventional ionic conductor 8-YSZ. Furthermore, the new cubic phase Yb₆MoO_12−δ displays improved coloristic properties (UV–Vis spectroscopy) with a yellow hue value (CIE-Lab) being enhanced from <i>b</i>* = 26.0 of the rhombohedral to <i>b</i>* = 46.1 for the cubic phase, which is relevant for the field of inorganic pigments

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 <i>T</i> ≥ 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