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

Cold Tolerance of the Male Gametophyte during Germination and Tube Growth Depends on the Flowering Time

In temperate climates, most plants flower during the warmer season of the year to avoid negative effects of low temperatures on reproduction. Nevertheless, few species bloom in midwinter and early spring despite severe and frequent frosts at that time. This raises the question of adaption of sensible progamic processes such as pollen germination and pollen tube growth to low temperatures. The performance of the male gametophyte of 12 herbaceous lowland species flowering in different seasons was examined in vitro at different test temperatures using an easy to handle testing system. Additionally, the capacity to recover after the exposure to cold was checked. We found a clear relationship between cold tolerance of the activated male gametophyte and the flowering time. In most summer-flowering species, pollen germination stopped between 1 and 5 °C, whereas pollen of winter and early spring flowering species germinated even at temperatures below zero. Furthermore, germinating pollen was exceptionally frost tolerant in cold adapted plants, but suffered irreversible damage already from mild sub-zero temperatures in summer-flowering species. In conclusion, male gametophytes show a high adaptation potential to cold which might exceed that of female tissues. For an overall assessment of temperature limits for sexual reproduction it is therefore important to consider female functions as well

Not so biodegradable: Polylactic acid and cellulose/plastic blend textiles lack fast biodegradation in marine waters.

The resistance of plastic textiles to environmental degradation is of major concern as large portions of these materials reach the ocean. There, they persist for undefined amounts of time, possibly causing harm and toxicity to marine ecosystems. As a solution to this problem, many compostable and so-called biodegradable materials have been developed. However, to undergo rapid biodegradation, most compostable plastics require specific conditions that are achieved only in industrial settings. Thus, industrially compostable plastics might persist as pollutants under natural conditions. In this work, we tested the biodegradability in marine waters of textiles made of polylactic acid, a diffused industrially compostable plastic. The test was extended also to cellulose-based and conventional non-biodegradable oil-based plastic textiles. The analyses were complemented by bio-reactor tests for an innovative combined approach. Results show that polylactic acid, a so-called biodegradable plastic, does not degrade in the marine environment for over 428 days. This was also observed for the oil-based polypropylene and polyethylene terephthalate, including their portions in cellulose/oil-based plastic blend textiles. In contrast, natural and regenerated cellulose fibers undergo complete biodegradation within approximately 35 days. Our results indicate that polylactic acid resists marine degradation for at least a year, and suggest that oil-based plastic/cellulose blends are a poor solution to mitigate plastic pollution. The results on polylactic acid further stress that compostability does not imply environmental degradation and that appropriate disposal management is crucial also for compostable plastics. Referring to compostable plastics as biodegradable plastics is misleading as it may convey the perception of a material that degrades in the environment. Conclusively, advances in disposable textiles should consider the environmental impact during their full life cycle, and the existence of environmentally degradable disposal should not represent an alibi for perpetuating destructive throw-away behaviors

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

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

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

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

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