Low Oxygen Mobility Research Articles

Fe-substituted Ba-hexaaluminate with enhanced oxygen mobility for CO2 capture by chemical looping combustion of methane

Abstract While Fe-based oxygen carriers (OC) are regarded to be promising for chemical looping combustion (CLC), the decrease of CO2 selectivity during deep reduction process and the severe agglomeration of Fe2O3 often occur after multiple redox cycles due to the low oxygen mobility. Herein, Fe-substituted Ba-hexaaluminates (BaFexAl12–xO19, denoted as BFxA-H, x = 1 and 2) prepared by a modified two-step method exhibited not only higher amount of converted oxygen (Ot) and CH4 conversion (77% and 81% vs. 17% and 75%) than those prepared by the traditional co-precipitation method (BFxA-C, x = 1 and 2) but also high CO2 selectivity above 92% during the nearly whole reduction from Fe3+ to Fe2+. Furthermore, the BFxA-H exhibited the excellent recyclability during 50 cycles. The better performance was ascribed to the markedly enhanced oxygen mobility which resulted from dominant occupancy of Fe cations in Al(5) sites (Fe5: 71% and 70% vs. 49% and 41%) in mirror planes of hexaaluminate leading to larger amount of lattice oxygen coordinated with Fe5 (O Fe5) (0.45 and 0.85 mmol/g vs. 0.31 and 0.50 mmol/g). The improvement of oxygen mobility also favored the preservation of chemical state of Fe cations in hexaaluminate structure in the re-oxidation step, resulting in the excellent recyclability of BFxA-H.

Electrochemical Performance and Stability of LSM-ESB Cathode

Among (La0.8Sr0.2)0.95MnO3-δ (LSM) based cathodes LSM-(Er0.2Bi0.8)2O3 (LSM-ESB) composite shows promising performance and is one of potential cathodes for low temperature solid oxide fuel cells (LT-SOFCs). However, the high conductivity ESB exhibits oxygen-ordering at temperatures ≤600°C, resulting in an ordered super-lattice with low oxygen mobility. Here the electrochemical performance of LSM-ESB was investigated using electrochemical impedance spectroscopy (EIS) as a function of temperature, oxygen partial pressure (pO2), bias, and time to determine possible electrochemical reaction steps and elucidate the oxygen reduction reaction (ORR) mechanism. LSM-ESB shows a strong pO2 dependence and above 650°C the non-stoichiometry of LSM plays an important role in oxygen transport between solid phases under biased conditions. In addition, effects of ion conductivity of electrolyte on the performance of LSM-ESB are determined. Also, the high temperature reactivity between LSM and ESB solid phases is studied and the possible degradation mechanism is proposed. Our results provide insights in the oxygen transport on LSM-ESB through the comprehensive study.

Electrochemical Performance and Stability of LSM-ESB Cathode

Among (La0.8Sr0.2)0.95MnO3-δ (LSM) based cathodes LSM-(Er0.2Bi0.8)2O3 (LSM-ESB) composite shows promising performance and is one of potential cathodes for low temperature solid oxide fuel cells (LT-SOFCs). The high conductivity ESB exhibits oxygen-ordering at temperatures ≤600°C, resulting in an ordered super-lattice with low oxygen mobility. Here the electrochemical performance of LSM-ESB was investigated using electrochemical impedance spectroscopy (EIS). The cathode area specific resistance (ASR) of LSM-ESB is a function of LSM/ESB ratio, suggesting that the importance of co-existence of electronic and ionic conducting phases. Under cathodic polarization, ASR of cell decreases as a function of bias while ASR remains almost the same under anodic polarization. Also, the high temperature reactivity between LSM and ESB solid phases is studied. Our finding shows that A site deficiency of LSM ABO3 perovskite structure is important to prevent the formation of strontium bismuth oxide.

Oxidative Dehydrogenation of n-Butane Over Vanadium Magnesium Oxide Catalysts Supported on Nano-Structured MgO and ZrO2: Effect of Oxygen Capacity of the Catalyst

Vanadium-magnesium oxide catalysts supported on nano-structured MgO and ZrO2 (Mg3(VO4)2/MgO/ZrO2) were prepared by a wet impregnation method with a variation of Mg:Zr ratio (8:1, 4:1, 2:1, and 1:1). For comparison, Mg3(VO4)2/MgO and Mg3(VO4)2/ZrO2 catalysts were also prepared by a wet impregnation method. The prepared catalysts were applied to the oxidative dehydrogenation of n-butane in a continuous flow fixed-bed reactor. Mg3(VO4)2/MgO/ZrO2 (Mg:Zr = 4:1, 2:1, and 1:1) and Mg3(VO4)2/ZrO2 catalysts showed a stable catalytic activity during the whole reaction time, while Mg3(VO4)2/MgO/ZrO2 (8:1) and Mg3(VO4)2/MgO catalysts experienced a severe catalyst deactivation. Deactivation of Mg3(VO4)2/MgO/ZrO2 (8:1) and Mg3(VO4)2/MgO catalysts was due to their low oxygen mobility. Effect of oxygen capacity (the amount of oxygen in the catalyst involved in the reaction) of the supported Mg3(V04)2 catalysts on the catalytic performance in the oxidative dehydrogenation of n-butane was investigated. Experimental results revealed that oxygen capacity of the catalyst was closely related to the catalytic activity in the oxidative dehydrogenation of n-butane. A large oxygen capacity of the catalyst was favorable for obtaining a high catalytic activity in this reaction. Among the catalysts tested, Mg3(VO4)2/MgO/ZrO2 (4:1) catalyst with the largest oxygen capacity showed the best catalytic performance.

Influence of the nature of the reducible support on CO oxidation kinetics of supported Rhδ+ catalysts: SnO2 versus Ce0.68Zr0.32O2

CO–O2 kinetics was studied on Rh(0.3 wt%)/SnO2 and compared with that reported previously on Rh/Ce0.68Zr0.32O2. The lower oxygen mobility on SnO2 and the stronger bonding of CO with the Rhδ+ sites of Rh/SnO2, are suggested to account for the lower CO oxidation activity of Rh/SnO2 compared with that reported on Rh/Ce0.68Zr0.32O2.

Unusual catalytic behavior of β-Bi 2Mo 2O 9 in the oxidative dehydrogenation of n-butene to 1,3-butadiene

A β-Bi 2Mo 2O 9 catalyst was prepared by a co-precipitation method and applied to the oxidative dehydrogenation of n-butene to 1,3-butadine. It was found that the β-Bi 2Mo 2O 9 catalyst retained lower oxygen mobility than γ-Bi 2MoO 6. It was also revealed that the β-Bi 2Mo 2O 9 was thermally unstable and decomposed into α-Bi 2Mo 3O 12 and γ-Bi 2MoO 6 at a reaction temperature of 420 °C. However, the β-Bi 2Mo 2O 9 showed a higher catalytic performance than the α-Bi 2Mo 3O 12 and γ-Bi 2MoO 6 catalysts in the oxidative dehydrogenation of n-butene, in spite of its thermal instability and low oxygen mobility. Moreover, the β-Bi 2Mo 2O 9 showed a stable catalytic performance with time on stream without catalyst deactivation. The high and stable catalytic performance of β-Bi 2Mo 2O 9 can be attributed to the synergy effect of α-Bi 2Mo 3O 12 and γ-Bi 2MoO 6 formed via the decomposition of β-Bi 2Mo 2O 9 during the catalytic reaction, to the high intrinsic catalytic activity of β-Bi 2Mo 2O 9, and to the well-crystallized parts of β-Bi 2Mo 2O 9.

Ceria-based fluorite-like oxide solid solutions as catalysts of methane selective oxidation into syngas by the lattice oxygen: synthesis, characterization and performance

Polymerized precursor route [M.P. Pechini, US Patent 3, 330 (1967) 697] was applied for synthesis of dispersed ceria-based solid solutions Ce 1− x Me x 3+O 3− y (Me 3+= Sm 3+ or Bi 3+, x up to 0.5). The surface properties of those samples were modified by supporting Pt. Their bulk and surface structural features were characterized by using XRD, TEM, EXAFS, X-ray wide angle scattering (RED), UV–vis, Raman, FTIRS of lattice modes and adsorbed CO. To elucidate factors controlling the surface/bulk oxygen mobility and reactivity, these results were compared with the data of‘ the oxygen isotope exchange and/or H 2/CH 4 temperature-programmed reduction (TPR). Single-phase fluorite-like solid solutions with a nanodomain microstructure are characterized by distortion of the coordination polyhedra caused by dopants followed by the lattice local rearrangement into that of a lower symmetry at a high dopant content. The highest performance in methane selective oxidation into syngas by the lattice oxygen is found for samples where isolated oxygen vacancies dominate. Earlier studied Pt/Ce–Zr–La–O system [Stud. Surf. Sci. Catal. 143 (2002) 659; T.G. Kuznetsova, V.A. Sadykov, S.A. Veniaminov, G.M. Alikina, E.A. Paukshtis, Ya.N. Kulikova, E.M. Moroz, E.B. Burgina, V.A. Rogov, V.N. Kolomiichuk, V.V. Kriventsov, D.I. Kochubei, O.N. Martyanov, V.F. Yudanov, I.S. Abornev, S. Neophytides, ISO22, Europacat VI, September 2003, Innsbruck, Austria] has a lower oxygen mobility, is less stable in redox cycles and less selective for syngas. Bi–Ce–O system mainly combusts methane and is irreversibly decomposed by deep reduction due to metallic Bi evaporation.

Processes of luminescence quenching in study of molecular oxygen diffusion in glassy matrices

The methods have been developed for studying oxygen mobility by donor quenching in solutions with uniform concentration and in geminate pairs in a solid phase. It is shown that the kinetics of luminescence variations with time is described by a diffusion model. Oxygen mobility has been studied at low temperatures (77 K) in hard polymer matrices, squalane, glasses of low-molecular hydrocarbons and alcohols. The methods can be used to measure very low oxygen mobility with a diffusion coefficient of about 10 −23 cm 2 s −1.

Concomitance of magnetic ordering and superconductivity in low oxygen mobility La 2CuO 4+x single crystals

The magnetic ordering in superconducting La 2CuO 4+x single crystals with low oxygen mobility was studied via μSR spectroscopy and neutron diffraction. A macroscopic separation at T PS = 250K into the oxygen-poor and the oxygen-rich phases occurs only in crystals with x > 0.03. The crystals with x ≤ 0.03 remain in the single ( Bmab) phase. Regardless of the presence or absence of the macroscopic phase separation, an antiferromagnetically or spin-glass ordered state appears at low temperature concomitantly with the onset of superconductivity with the transition temperatures from 8K to 25K.

00/00815 Model-based control design of a TCSC-compensated power system

While Fe-based oxygen carriers (OC) are regarded to be promising for chemical looping combustion (CLC), the decrease of CO2 selectivity during deep reduction process and the severe agglomeration of Fe2O3 often occur after multiple redox cycles due to the low oxygen mobility. Herein, Fe-substituted Ba-hexaaluminates (BaFexAl12–xO19, denoted as BFxA-H, x = 1 and 2) prepared by a modified two-step method exhibited not only higher amount of converted oxygen (Ot) and CH4 conversion (77% and 81% vs. 17% and 75%) than those prepared by the traditional co-precipitation method (BFxA-C, x = 1 and 2) but also high CO2 selectivity above 92% during the nearly whole reduction from Fe3+ to Fe2+. Furthermore, the BFxA-H exhibited the excellent recyclability during 50 cycles. The better performance was ascribed to the markedly enhanced oxygen mobility which resulted from dominant occupancy of Fe cations in Al(5) sites (Fe5: 71% and 70% vs. 49% and 41%) in mirror planes of hexaaluminate leading to larger amount of lattice oxygen coordinated with Fe5 (OFe5) (0.45 and 0.85 mmol/g vs. 0.31 and 0.50 mmol/g). The improvement of oxygen mobility also favored the preservation of chemical state of Fe cations in hexaaluminate structure in the re-oxidation step, resulting in the excellent recyclability of BFxA-H.

Microscopic phase separation inLa2CuO4+xinduced by the superconducting transition

The phase separation (PS) effect in superconducting ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4+x} (x<~0.04)$ single crystals with low oxygen mobility was studied via $\ensuremath{\mu}\mathrm{SR}$ spectroscopy, high-resolution neutron diffraction, and magnetic susceptibility. Despite the fact that all crystals are inside the miscibility gap $(0.01<x<0.06),$ only crystals with a sufficiently large excess oxygen concentration $x=0.04$ show a macroscopic phase separation according to the neutron-diffraction data. However, in all samples a phase transition to an ordered magnetic state was observed by $\ensuremath{\mu}\mathrm{SR}$ spectroscopy concomitantly with the onset of superconductivity. This effect is treated as a microscopic phase separation which is possibly driven by superconductivity.

In-air preparation of optimally doped YBCO

The possibility to prepare directly in air superconducting YBCO samples showing Tc, 0 above 90 K is reported and discussed. The most critical parameter in determining the final Tc is represented by the cooling rate in the lower temperature region, characterised by a lower oxygen mobility. The results point out that, if a suitable cooling rate (and therefore an adequate control of the intercalation kinetics) is utilised, the partial pressure of oxygen in air is sufficient to assure the formation of the orthorhombic phase with an oxygen content as high as 6.96.

Catalytic CO Oxidation over La1‐xSrxMnO3 at High CO Partial Pressures

AbstractThe catalytic CO oxidation over polycrystalline LaMnO3 exhibits a bistability at high CO concentration. Under atmospheric pressure, the regime of bistability extends from 200 to 220°C and widens with increasing CO concentration. A systematic study of the solid solution system La1‐xSrxMnO3 which crystallizes in the perovskite structure, shows that the width of the bistable regime decreases with an increasing amount of strontium substitution and no bifurcation was observed for x > 0.2. A model is proposed to explain the bistability, based on a CO‐driven reconstruction of the reduced surface, leading to pairs of Mn2 ions with a Mn‐Mn distance comparable to the spacing in the metal. These pairs provide reactive sites for CO oxidation and oxygen chemisorption. Such metal‐metal pairs are not found in the perovskite lattice but are a structural feature of the closely related hexagonal 4‐layered packing which is the normal crystal structure of SrMnO3. The rate of change back to the “oxidized” surface is found to correlate with the lattice oxygen mobility. With low oxygen mobility, the transition from the active into the less active state becomes slow and hysteresis may result. Strontium substitution increases the number of defects in the oxygen lattice and leads ultimately to an oxygen deficiency (reductive nonstoichiometry). The catalytic activity of the system peaks at x = 0.2, indicating the influence of both the oxygen mobility and the oxygen nonstoichiometry on the catalytic oxidation reaction.

Les paragenèses réduites ou oxydées de concentrations manganésifères des «schistes lustrés» de Haute-Maurienne (Alpes françaises)

A phengite quartzite interbedded in calcschists overlying an ultramafic body near Bonneval-sur-Arc (Savoie) contains Mn-deposits having undergone HP-LT metamorphism and displaying following assemblages : — Rhodochrosite - pyroxmangite - rhodonite - tirodite - spessartine - tephroite - sonolite - alleghanyite - friedelite. — Kutnahorite - alleghanyite - sonolite - galaxite - jacobsite - Mn-thuringite. Apatite, pyrophanite, siegenite, pentlandite, millerite, chalcocite, bornite are there accessory minerals ; neotocite, kellyite (?) and digenite secondary ones. — Quartz - piemontite - ardennite - spessartine - braunite - hematite - phyllosilicates - richterite. The juxtaposition of these oxidized and reduced assemblages dates back to the sedimentary origin of the rocks. This implies very low oxygen mobility during metamorphism, which is evidenced through the whole metamorphic pile. Pyroxmangite in equilibrium with rhodonite is abnormally Ca-rich. The composition of Mn-humites is shown to be stoichiometric and Mn(OH, F)2 ⇄ TiO2 to be the most important substitution involved in them. Maximum Al-substitution in piemontite is shown not to be dependent from the physical conditions but essentially from the chemical parameters. An attempt is made to delineate the galaxite-jacobsite solvus. The noticeable Ni and Co content of the phyllosilicates and the highly magnesian chemistry of the most oxidized assemblages are shown to be mere consequences of the high fO2. We discuss the chemistry of the enclosing rocks and the possible «oceanic » origin of these deposits which are equivalent to those described in Piémont (e. g. Saint-Marcel).

The Isotopic Exchange of Oxygen-18 with the Oxygen of Niobium (V) Oxide

The kinetics of the exchange of gas-phase oxygen-18 with the oxygen of niobium(V) oxide has been studied in a staticcirculation'' apparatus. Both the surface and bulk oxygen in the oxide possess low mobility and exhibit energetically uniform behaviour. From the experimental data for the specific rates of exchange, at 620 to 700 deg C, an activation energy of 61 kcal x mole/ sup -1/ was calculated. The reaction order was 0.6 in the oxygen pressure range 5 to 30 torr. Kinetic data are interpreted. Relationships between the low oxygen mobility in a transition metal oxide and the degree of covalency of the metal/oxygen bond are discussed. (GE)

The fundamental laws of the catalytic oxidation of hydrocarbons Communication 2. Oxygen adsorption on oxidation catalyst

1) The adsorption of oxygen on some typical oxidation catalysts: V2O5, MgCr2O4, and Ag was investigated. 2) We investigated the isotopic exchange of oxygen on metallic oxidation catalysts, silver and platinum, and demonstrated the low oxygen mobility on the surface of these metals. 3) We investigated the homolytic exchange of oxygep (O218 + O216→-2O16O18), which characterizes the oxygen dissociation step during adsorption on various catalysts. 4) We have shown that at low temperatures oxygen is partially dissociated on platinum, silver, and oxide catalysts.