The structural changes of the title compound during heating and under conditions of catalytic conversion of isobutyric acid to methacrylic acid were followed in situ by powder X-ray diffraction under ...continuous control of its activity. The results were verified by a postmortem phase analysis of practical supported catalyst samples used in kinetic reactors. The activity of the catalyst is correlated with its dehydrated form. A new cubic phase of a water-free vanadyl salt of the heteropolyacid (HPA) was found to be connected to a maximum conversion. This phase is isostructural to the unsubstituted anhydrous alkali-3-HPA salts and is metastable at ambient conditions with respect to rehydration. The catalyst material as a whole is metastable at any temperature above the onset of conversion with respect to a partially reversible decomposition into MoO3 and amorphous other components, Restructuring into crystalline forms of HPA is possible from the deactivated material upon dissolution and recrystallization at 323 K. In situ UV-VIS data and X-ray diffraction show the complete self-reorganization of the MoO3 phase and the amorphous V and P compounds into new Keggin anions indicating the possible living nature of the catalyst under reaction conditions which enable extended lifetimes beyond the stability limits found in the present in situ X-ray diffraction experiments.
13C NMR measurements were performed on different fullerene black materials in order to gain knowledge about the fullerene black structure, the incorporation of C
60 molecules in the fullerene black, ...the generation process of the fullerenes in the arc reactor, the influence of solvents and the amount of fullerenes not removable by solvents. Our results are consistent with the model that fullerene black is an amorphous carbon material containing a high amount of bent and spheroidal carbon fragments. The C
60 molecules are dispersed on the large fullerene black surface and rotate rapidly. A weak signal in the fullerene black at about 70 ppm could arise from polyynic carbon chains, one of the discussed intermediate compounds in the fullerene generation process. X-ray diffractometry shows that the treatment of the fullerene black raw material with tetrahydrofuran (THF) leads to structural relaxation and a rearrangement of carbon sheets.
13C NMR is rather insensitive to this modified texture of the fullerene black matrix indicating that the local carbon structure remains largely unchanged by solvent treatments. In the extracted fullerene black materials we did not find fullerene signals proving that the amount of possibly entrapped fullerenes is very small and that the conventional fullerene extraction with toluene can be regarded as complete.
A polycrystalline Rb
2CsC
60 sample with
T
c = 32.5 K (onset value) is investigated in the superconducting state by scanning tunneling spectroscopy (STS) using a low temperature scanning tunneling ...microscope (STM). At 4.5 K the superconducting energy gap Δ is evaluated from the measured quasiparticle density of states (DOS). Most gap values range from 3.3 to 5.7 meV, with an average of4.2 ± 0.6meV (2Δ/k
BT
c= 3.2 ± 0.5). The scattering is attributed to local variations in stoichiometry on the sample surface. However, a few spectra exhibit much larger energy gaps (≈ 6.2 to 9.5 meV). These large values cannot be understood within standard BCS theory (1.76 × k
BT
c = 4.9 meV). We therefore suggest an alternative explanation by assuming a superconductor-insulator-superconductor (SIS) tunneling junction. In this case a gap width of 4Δ (peak-to-peak distance in the DOS) is expected, which is in good agreement with our measurements.
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