Redox dynamics of sulphur with Ni/GDC anode during SOFC operation at mid- and low-range temperatures: An operando S K-edge XANES study

Sulphur poisoning of nickel-based solid oxide fuel cell (SOFC) anode catalysts is a well-documented shortcoming, but not yet fully understood. Here, a novel experiment is demonstrated to obtain spectroscopic information at operando conditions, in particular the molecular structure of sulphur species in the sulphur K-shell X-ray absorption near edge structure (XANES) region for a SOFC anode under realistic operando conditions, thus, with the flux of O2- from cathode to anode. Cooling from T = 550 degrees C stepwise down to 250 degrees C, 5 ppm H2S/H-2 reacting with Ni-gadolinium doped ceria (GDC) anode resulted in several sulphur species in different oxidation states (6+, 4+, 0, -2) and in amounts being at a minimum at high temperature. According to sulphur speciation analysis, the species could either relate to -SO42- or SO3 (g), -SO32- or SO2 (g), S-2 (g) or surface-adsorbed S atoms, and, Ni or Ce sulphides, respectively. The coexistence of different sulphur oxidation states as a function of temperature was analysed in the context of thermodynamic equilibrium calculations. Deviations between experimental results and calculations are most likely due to limitations in the speed of some intermediate oxidation steps as well as due to differences between stoichiometric CeO2 used in calculations and partially reduced Ce0.9Gd0.1O2-delta. (C) 2013 Elsevier B.V. All rights reserved

Verification of the membrane reactor concept for the methanol synthesis

The experimentally observed rise in the single pass methanol production with CO2 and H-2 (T = 200 degreesC, P = 4.3 bar) by in situ product removal over a perm-selective Nafion (R) membrane is verified by model simulations. For this purpose, software was developed which addresses the two rivalling processes (synthesis and permeation) for given catalyst activity and permeation performance of the membrane. The program predicts the production rate of all the reactor gas components leaving the reactor by explicit integration of the gas component and (reactor and mantle) space specific differential rate equations along the symmetry axes of the tubular membrane reactor. The activity of the CO2-tolerant catalyst used was characterized independently by kinetic model analysis; the experimentally determined permeation performance of the membrane for the various reactant gas components by Arrhenius type temperature dependencies. The model confirmed the membrane reactor results satisfactorily well. It was estimated that with 10 mum thin membrane surfaces implemented in commercial methanol synthesis plants and operated under technically relevant conditions (T = 200 degreesC, P = 40 bar, GHSV = 5000 h(-1)), the single pass reactor yield improves by 40% and that the additional costs for the membrane material are two production months only. (C) 2001 Elsevier Science B.V. All rights reserved

Synchrotron X-ray absorption of LaCoO3 perovskite

LaCoO3 perovskite was prepared at 700degreesC using citrate precursors. The product was then characterized with X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The powder XRD pattern indicates rhomboliedral (R (3) over barc) or its monoclinic I2/a subgroup symmetry. The electronic configuration and the short-range atomic structure of the LaCoO3 perovskite at room temperature were investigated using synchrotron near-edge X-ray absorption spectroscopy (XANES) and extended X-ray absorption spectroscopy (EXAFS). From the XANES region of the XAS we conclude that Co(III) is at least partly in its intermediate- or high-spin state, which is in accordance with most of the published literature on LaCoO3 perovskite. The EXAFS region of the LaCoO3 perovskite spectrum, which up to now was almost not investigated, was simulated satisfactorily for the first two radial structure peaks in terms of the dominant scattering contributions generated with the FEFF8 code and the structural information available from crystallographic data. The best simulation results were obtained with I2/a symmetry. The obtained amplitude reduction factor, zero-energy shift and Debye-Waller factors are useful reference values for data analyses of similar compounds like partly substituted LaCoO3 perovskite, such as La1-xCaxCoO3 or La1-xSrxCoO3, which are materials of technical interest in catalyst and other applications. (C) 2003 Elsevier Inc. All rights reserved

Gasification reactivity of charcoal with CO2. Part I: Conversion and structural phenomena

The gasification reaction of fir charcoal with CO2 was studied by isothermal thermogravimetric analysis under kinetic control. The derived reaction rate (r = dX/dt) as a function of the converted carbon mass (X) was compared with random pore model predictions and found to be much higher at elevated conversion levels than predicted by theory. Similar enhanced reaction rate behaviour was evidenced after removing the natural alkali catalyst from the charcoal by acid washing, suggesting that with untreated charcoal the late reaction rate contribution stems from both, catalytic and additional structure effects. Literature attributes the unpredicted late reaction rate behaviour to the disintegration of the porous char particle into small fragments, which, in line with percolation theory predictions, seems to occur only after a critical conversion level has been reached. However, our gasification data reveal a gradual rise in the charcoal reactivity thereafter, suggesting a breaking up (embrittlement) of the solid phase accompanied by the exposure of fresh surface area from fracturing. The original random pore model derivation given by Bhatia and Perlmutter is extended to account also for these peculiarities and the resulting kinetic relation described our reaction rate data well over the entire conversion range. (C) 2002 Elsevier Science Ltd. All rights reserved

Oxygen transfer and catalytic properties of nickel iron oxides for steam reforming of methane

Nickel iron oxide (NiFe2O4) has been tested as a combined catalyst precursor and oxygen transfer material for improved conversion of methane into syngas. Thermogravimetric measurements with simultaneous gas analysis have demonstrated the superior behavior of nickel iron oxide compared to iron oxide. The enhanced catalytic activity is attributed to the generation of fresh nickel surfaces in the course of the reduction of the metal oxide. This allows for a periodic in situ generation of fresh, catalytically active nickel. (C) 2006 Elsevier Ltd. All rights reserved

Gasification reactivity of charcoal with CO2. Part II: Metal catalysis as a function of conversion

The catalytic influence of major metal species found with waste wood (Na, K, Ca, Mg, Zn, Pb, Cu) was studied during the gasification of nitrate salt impregnated charcoal with CO2 at 800degreesC in the kinetically controlled regime. In contrast with literature, the respective reaction rate data were analysed over the entire carbon conversion (X) range by using extended kinetic relations to quantify catalyst metal-specific reaction rate contributions not accounted for by the original random pore model of Bhatia and Perlmutter. The kinetic analysis provided valuable insights in the underlying mechanisms. With alkali nitrate impregnated charcoal, it is demonstrated that the often found reaction rate maximum around X similar to 0.7 may merely reflect increasing catalytic activity resulting from alkali accumulation in the charcoal, superimposed on structural changes in the charcoal micropore domain. Impregnated earth-alkali nitrates revealed substantial activity as well, but only during the early gasification stage (X < 0.2), hereby, underlining their sintering propensity in combination with the localised deposition of the earth-alkaline nitrate salt in the former wood cells by the impregnation procedure. Added heavy metal nitrates revealed no activity, apart from lowering the charcoal reactivity over the entire conversion range by ca. 15% compared with the untreated charcoal, suggesting inhibition by covering, hence, blocking of otherwise accessible active charcoal surface sites and/or by deactivation of neighbouring indigenous alkali due to immobilisation at the heavy metal oxide surfaces formed during pyrolysis. The extended kinetic relation reproduced all of our reaction rate data well over the entire gasification stage, hereby, supporting the superposition of micropore domain and catalyst specific effects. (C) 2002 Elsevier Science Ltd. All rights reserved

Speciation of zinc in municipal solid waste incineration fly ash after heat treatment: An X-ray absorption spectroscopy study

Fly ash is commonly deposited in special landfills as it contains toxic concentrations of heavy metals, such as Zn, Pb, Cd, and Cu. This study was inspired by our efforts to detoxify fly ash from municipal solid waste incineration by thermal treatment to produce secondary raw materials suited for reprocessing. The potential of the thermal treatment was studied by monitoring the evaporation rate of zinc from a certified fly ash (BCR176) during heating between 300 and 950 degreesC under different carrier gas compositions. Samples were quenched at different temperatures for subsequent investigation with X-ray absorption spectroscopy (XAS). The XAS spectra were analyzed using principal component analysis (PCA), target transformation (TT), and linear combination fitting (LCF) to analyze the major Zn compounds in the fly ash as a function of the temperature. The original fly ash comprised about 60% zinc oxides mainly in the form of hydrozincite (Zn-5(OH)(6)(CO3)(2)) and 40% inerts like willemite (Zn2SiO4) and gahnite (ZnAl2O4) in a weight ratio of about 3:1. At intermediate temperatures (550750 degreesC) the speciation underlines the competition between indigenous S and Cl with solid zinc oxides to form either volatile ZnCl2 or solid ZnS. ZnS then transformed into volatile species at about 200 degreesC higher temperatures. The inhibiting influence of S was found absent when oxygen was introduced to the inert carrier gas stream or chloride-donating alkali salt was added to the fly ash