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A tellurate compound with CoII/CoIII mixed valence states and lithium ions within orthogonally oriented channels was realized in Li3Co1.06(1)TeO6. The single-crystal structure determination revealed two independent and interpenetrating Li/O and (Co,Te)/O substructures with octahedral oxygen coordination of the metal atoms. In contrast to other mixed oxides, a honeycomb-like ordering of CoO6 and TeO6 octahedra was not observed. Li3Co1.06(1)TeO6 crystallizes orthorhombically with the following unit cell parameters and refinement results: Fddd, a = 588.6(2), b = 856.7(2), c = 1781.5(4) pm, R1 = 0.0174, wR2 = 0.0462, 608 F2 values, and 33 variables. Additional electron density in tetrahedral voids in combination with neighboring face-linked and under-occupied octahedral lithium sites offers an excellent possible diffusion pathway for lithium ions. According to the symmetry of the crystal structure the diffusion pathways in Li3Co1.06(1)TeO6 were found in two orthogonal orientations. The CoII/CoIII mixed valence was investigated via X-ray photoelectron spectroscopy (XPS), revealing a composition comparable to that derived from single-crystal X-ray diffractometry. Magnetic susceptibility measurements underlined the coexistence of CoII and CoIII, the title compound, however, showed no magnetic ordering down to low temperatures. The ionic conductivity of Li3Co1.06(1)TeO6 was determined via alternating current (AC) electrochemical impedance spectroscopy and was found to be in the range of 1.6 × 10−6 S cm−1 at 573 K. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The surface chemical properties of undoped tetragonal ZrO2 and the gas-phase dependence of the tetragonal-to-monoclinic transformation are studied using a tetragonal ZrO2 polymorph synthesized via a sol–gel method from an alkoxide precursor. The obtained phase-pure tetragonal ZrO2 is defective and strongly hydroxylated with pronounced Lewis acidic and Brønsted basic surface sites. Combined in situ FT-infrared and electrochemical impedance measurements reveal effective blocking of coordinatively unsaturated sites by both CO and CO2, as well as low conductivity. The transformation into monoclinic ZrO2 is suppressed up to temperatures of ∼723 K independent of the gas phase composition, in contrast to at higher temperatures. In inert atmospheres, the persisting structural defectivity leads to a high stability of tetragonal ZrO2, even after a heating–cooling cycle up to 1273 K. Treatments in CO2 and H2 increase the amount of monoclinic ZrO2 upon cooling (＞85 wt%) and the associated formation of either Zr-surface-(oxy-)carbide or dissolved hydrogen. The transformation is strongly affected by the sintering/pressing history of the sample, due to significant agglomeration of small crystals on the surface of sintered pellets. Two factors dominate the properties of tetragonal ZrO2: defect chemistry and hydroxylation degree. In particular, moist conditions promote the phase transformation, although at significantly higher temperatures as previously reported for doped tetragonal ZrO2. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Structural and chemical degradation mechanisms of metal-free yttria stabilized zirconia (YSZ-8, 8 mol% Y2O3 in ZrO2) in comparison to its pure oxidic components ZrO2 and Y2O3 have been studied in carbon-rich fuel gases with respect to coking/graphitization and (oxy)carbide formation. By combining operando electrochemical impedance spectroscopy (EIS), operando Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS), the removal and suppression of CH4- and CO-induced carbon deposits and of those generated in more realistic fuel gas mixtures (syngas, mixtures of CH4 or CO with CO2 and H2O) was examined under SOFC-relevant conditions up to 1273 K and ambient pressures. Surface-near carbidization is a major problem already on the “isolated” (i.e. Nickel-free) cermet components, leading to irreversible changes of the conduction properties. Graphitic carbon deposition takes place already on the “isolated” oxides under sufficiently fuel-rich conditions, most pronounced in the pure gases CH4 and CO, but also significantly in fuel gas mixtures containing H2O and CO2. For YSZ, a comparative quantification of the total amount of deposited carbon in all gases and mixtures is provided and thus yields favorable and detrimental experimental approaches to suppress the carbon formation. In addition, the effectivity and reversibility of removal of the coke/graphite layers was comparably studied in the pure oxidants O2, CO2 and H2O and their effective contribution upon addition to the pure fuel gases CO and CH4 verified. [ABSTRACT FROM AUTHOR]

Abstract: A set of different Pd/Ga2O3 catalysts has been tested as suitable CO2-selective catalysts in methanol steam reforming, formaldehyde steam reforming and formic acid decomposition. Special focus was put on two different polymorphic forms of Ga2O3, namely α-Ga2O3 and γ-Ga2O3 and their comparison to the use of the more common β-Ga2O3 as oxide support of small Pd particles. The formation of eventually CO2-selective intermetallic Pd–Ga phases started at around 523K on both supports. Pd2Ga at low temperatures (between 523K and 673K) and generally PdGa at higher temperatures (at and above 673K) were the only intermetallic phases observed. Both thermodynamically metastable oxide supports transformed into the stable β-Ga2O3 structure at 873K reduction temperature. In contrast to previous studies, a highly CO2-selective state in methanol steam reforming (CO2 selectivity>70%) could be established on both catalysts after reduction at 673K (α-Ga2O3) and 523K (γ-Ga2O3), where the active state of the catalysts can be characterised as Pd2Ga/α-Ga2O3 and Pd2Ga/γ-Ga2O3, respectively. α-Ga2O3 itself is reasonably CO2-selective in methanol steam reforming (∼80%), although not very active. In contrast, the γ-Ga2O3 polymorph is neither CO2-selective, nor active and resembles the behaviour of β-Ga2O3. The high CO2-selectivity of α-Ga2O3 is explained in terms of a higher surface basicity alongside efficient decarboxylation of intermediary formed formic acid. As crucial parameters for the observed CO2-selectivity in methanol steam reforming, decarboxylation of formic acid and oxidation of formaldehyde proceeding efficiently via a low-temperature reaction channel over either oxide-supported Pd2Ga intermetallic catalysts were identified. [Copyright &y& Elsevier]

A tellurate compound with Co II /Co III mixed valence states and lithium ions within orthogonally oriented channels was realized in Li 3 Co 1.06(1) TeO 6 . The single-crystal structure determination revealed two independent and interpenetrating Li/O and (Co,Te)/O substructures with octahedral oxygen coordination of the metal atoms. In contrast to other mixed oxides, a honeycomb-like ordering of CoO 6 and TeO 6 octahedra was not observed. Li 3 Co 1.06(1) TeO 6 crystallizes orthorhombically with the following unit cell parameters and refinement results: Fddd, a = 588.6(2), b = 856.7(2), c = 1781.5(4) pm, R 1 = 0.0174, wR 2 = 0.0462, 608 F 2 values, and 33 variables. Additional electron density in tetrahedral voids in combination with neighboring face-linked and under-occupied octahedral lithium sites offers an excellent possible diffusion pathway for lithium ions. According to the symmetry of the crystal structure the diffusion pathways in Li 3 Co 1.06(1) TeO 6 were found in two orthogonal orientations. The Co II /Co III mixed valence was investigated via X-ray photoelectron spectroscopy (XPS), revealing a composition comparable to that derived from single-crystal X-ray diffractometry. Magnetic susceptibility measurements underlined the coexistence of Co II and Co III , the title compound, however, showed no magnetic ordering down to low temperatures. The ionic conductivity of Li 3 Co 1.06(1) TeO 6 was determined via alternating current (AC) electrochemical impedance spectroscopy and was found to be in the range of 1.6 × 10 -6 S cm -1 at 573 K.

The surface chemical properties of undoped tetragonal ZrO 2 and the gas-phase dependence of the tetragonal-to-monoclinic transformation are studied using a tetragonal ZrO 2 polymorph synthesized via a sol-gel method from an alkoxide precursor. The obtained phase-pure tetragonal ZrO 2 is defective and strongly hydroxylated with pronounced Lewis acidic and Brønsted basic surface sites. Combined in situ FT-infrared and electrochemical impedance measurements reveal effective blocking of coordinatively unsaturated sites by both CO and CO 2 , as well as low conductivity. The transformation into monoclinic ZrO 2 is suppressed up to temperatures of ∼723 K independent of the gas phase composition, in contrast to at higher temperatures. In inert atmospheres, the persisting structural defectivity leads to a high stability of tetragonal ZrO 2 , even after a heating-cooling cycle up to 1273 K. Treatments in CO 2 and H 2 increase the amount of monoclinic ZrO 2 upon cooling (>85 wt%) and the associated formation of either Zr-surface-(oxy-)carbide or dissolved hydrogen. The transformation is strongly affected by the sintering/pressing history of the sample, due to significant agglomeration of small crystals on the surface of sintered pellets. Two factors dominate the properties of tetragonal ZrO 2 : defect chemistry and hydroxylation degree. In particular, moist conditions promote the phase transformation, although at significantly higher temperatures as previously reported for doped tetragonal ZrO 2 .

Two mixed ionic-electronic conducting, Fe-containing perovskites were investigated regarding their reducibility in dry H 2 , namely lanthanum strontium ferrite (LSF4, La 0.6 Sr 0.4 FeO 3-δ ) and strontium titanium ferrite (STF3, SrTi 0.7 Fe 0.3 O 3-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 O 2 partial pressure lead to spontaneous desorption of O 2 . Temperature-programmed desorption (TPD) spectra of H 2 reveal distinct differences in H 2 and H 2 O desorption. A simple mass balance of H 2 reveals that oxygen vacancies formed on STF3 are more resilient towards O 2 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 H 2 per mol LSF4 and 1.6 × 10 -2 mol H 2 per mol STF3 are incorporated if the specimens are heated in flowing/dry H 2 up to 550 °C and 612 °C, respectively. For LSF4 this equals about 13 hypothetical ML of H 2 and for STF3 about 20 hypothetical ML of H 2 . This conclusion is also supported by Fourier-transform infrared spectroscopy (FT-IR). FT-IR reveals water formation during static H 2 treatment of LSF4/STF3, which indicates perovskite reduction. Furthermore, both samples behave extraordinarily hydrophobic and no chemistry involving surface hydroxy groups was observed.

The surface reactivity of Y 2 O 3 , YSZ, and ZrO 2 polycrystalline powder samples toward H 2 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 H 2 was proven on the basis of molecular H 2 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.

Carbon deposition following thermal methane decomposition under dry and steam reforming conditions has been studied on yttria-stabilized zirconia (YSZ), Y 2 O 3 , and ZrO 2 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.

In situ FT-IR spectroscopy was exploited to study the adsorption of CO 2 and CO on commercially available yttria-stabilized ZrO 2 (8 mol % Y, YSZ-8), Y 2 O 3 , and ZrO 2 . All three oxides were pretreated at high temperatures (1173 K) in air, which leads to effective dehydroxylation of pure ZrO 2 . Both Y 2 O 3 and YSZ-8 show a much higher reactivity toward CO and CO 2 adsorption than ZrO 2 because of more facile rehydroxylation of Y-containing phases. Several different carbonate species have been observed following CO 2 adsorption on Y 2 O 3 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 CO 2 most likely includes insertion of the CO 2 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 ZrO 2 due to effective dehydroxylation of the surface during high-temperature treatment. The latter generally causes suppression of the surface reactivity of ZrO 2 samples regarding reactions involving CO or CO 2 as reaction intermediates.