High-resolution models of solar granulation: the 2D case

Using grid refinement, we have simulated solar granulation in 2D. The refined region measures 1.97*2.58 Mm (vertical*horizontal). Grid spacing there is 1.82*2.84 km. The downflows exhibit strong Kelvin-Helmholtz instabilities. Below the photosphere, acoustic pulses are generated. They proceed laterally (in some cases distances of at least the size of our refined domain) and may be enhanced when transversing downflows) as well as upwards where, in the photosphere they contribute significantly to 'turbulence' (velocity gradients, etc.) The acoustic pulses are ubiquitous in that at any time several of them are seen in our high-resolution domain. Their possible contributions to p-mode excitation or heating of the chromosphere needs to be investigated

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

Determination of the bulk cobalt valence state of co-perovskites containing surface-adsorbed impurities

We used thermogravimetric hydrogen reduction and iodometric titration to determine the bulk valence state of cobalt in Co-perovskites containing surface carbonate hydroxide or hydroxyl groups. It could be shown that thermogravimetric hydrogen reduction experiments are very sensitive to volatile surface groups, but due to their volatility, they can be specified and the bulk valence state of cobalt can still be deduced from these experiments. The iodometric titration is less sensitive to small volatile surface impurities, but precaution has to be taken that oxygen or iodine does not escape from the solution during dissolution of the sample. Best results were obtained if the sample was titrated during dissolution in a closed argon floated titration apparatus. We tested the two methods using LaCoO3 perovskite as a sample with a known valence state. Both methods delivered satisfactory results, and the valence state could be determined with an accuracy of better than 1%

X-ray absorption investigation of the valence state and electronic structure of La 1-xCa xCoO 3−δ in comparison with La 1-xSr xCoO 3−δ and La 1-xSr xFeO 3−δ

3d metal K-shell X-ray absorption spectra of perovskites with the composition La1-xCaxCoO3-delta (x=0, 0.2, 0.4, 0.5, 0.6, 0.8), La1-xSrxCoO3-delta (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) and La1-xSrxFeO3-delta (x=0, 0.2, 0.4, 0.5, 0.6, 0.8) are compared on the basis of pre-edges, white line features and extended fine structures. The measurements were performed at 300 K and for La1-xCaxCoO3-delta also at temperatures as low as 10-20 K. Going to low-temperature the measurements indicate an increase in t(2g)* and a decrease in e(g)* orbital occupancy, which is most accentuated in the LaCoO3 sample. Virtually no Co K-edge shift was observed for the La1-xCaxCoO3-delta and La1-xSrxCoO3-delta compounds and the Co-O distances are also not significantly reduced when La3+ is partially substituted by Ca2+ or Sr2+. From the pre-edge features of these perovskites we are tended to conclude that the t(2g)* orbitals are less, and the e(g)* orbitals are more occupied with increasing x in the Ca and Sr substituted compounds, whereas the total d-electron density is not changing. These results indicate that cobalt prefers a valence state of 3(+) in these Co perovskites. This could also be confirmed with iodometric titrations. The Fe perovskites behave differently. In contrast to the Co perovskites, for La1-xSrxFeO3-delta perovskites the Fe K-edge is shifted, the pre-edge features intensity is increasing and the Fe-O bond length is decreasing with increasing x. The valence states of the iron in the La1-xSrxFeO3-delta perovskites in fact increase as much as x increases. (C) 2011 Elsevier Inc. All rights reserved