Mixed metal oxides are important industrial catalysts for the selective oxidation and ammoxidation of aromatics and alkenes and often contain Sb oxides as a component. For the preparation of a catalytically relevant system on the basis of monolayer-type catalysts, an alternative route as compared to the conventional impregnation was chosen by milling the dry compounds in a planetary mill. To get a closer insight into the spreading and oxidation properties of antimony oxide on titania, only the binary oxidic compounds Sb oxide and TiO2 as support were investigated in the present study. Photoelectron spectroscopy (XPS) investigations for surface analysis and X-ray absorption spectroscopy (XANES) for bulk phase analysis were applied. The various Sb oxides (Sb2O3, Sb2O4, and Sb2O5) show totally different spreading behavior. Only with the Sb(III) oxide on titania a significant increase of dispersion was detectable by means of XPS and temperature programmed reduction (TPR). The temperature of oxidation of the supported Sb(III) oxide in air was 100 °C lower as compared to the bulk phase oxidation. The final formula after oxidation of Sb(III) oxide can be calculated from XANES results as Sb6O13 and does not end up at a stoichiometry of Sb2O4.
The synthesis, local structure, and magnetism of lamellar iron(III) oxyhydroxide−surfactant composites prepared by two different methods have been investigated in detail. In the first method, Fe(II) solutions are oxidized by H2O2 in the presence of C n H2 n +1OSO3 −Na+ surfactants (n = 10, 12, 14, 16, 18), leading to lamellar composites with an inorganic wall thickness of around 28 Å. When a second method is used, namely, aging an Fe(III) solution for selected times after slightly increasing the pH with NH3 and subsequent addition of the surfactant, the inorganic wall thickness can be tuned between 19 and 26 Å, employing the same surfactants. EXAFS analysis of the Fe K edge X-ray absorption spectra reveals that the local structure of the inorganic part is a reminder of those found for the bulk iron oxyhydroxides goethite and akaganéite; that is, [Fe(O,OH)6] octahedra are predominantly connected by common edges and corners, the ratio of edge to corner sharing being similar to the mentioned bulk oxyhydroxides. Whereas coordination numbers for the first oxygen coordination shell are around 6, confirming an octahedral (or distorted octahedral) coordination around the Fe ions, coordination numbers found for the second and third Fe···Fe neighbors are low (around 2), indicating the presence of a considerable amount of vacancies around the central absorber ion or, as an alternative description, a low degree of condensation of the oxyhydroxide. Complementary to the local structural picture given by EXAFS, Mössbauer spectra elucidate the inorganic iron oxyhydroxide walls to be built up by domains of different crystallinity. The crystallinity is sensitive to the synthesis conditions used in the preparation. For example, under aging in the presence of NH3, longer aging times and higher temperatures result in a larger overall crystallinity of the inorganic part. By carefully controlling the reaction parameters, the thickness of the inorganic layers can be varied from around 19 Å to around 30 Å; also, the blocking temperatures of these superparamagnetic compounds observed by zero-field-cooled magnetization measurements can be controlled in the range between 4 and 30 K.
Antimony-modified vanadia-on-titania catalysts were prepared for the selective oxidation of o-xylene to phthalic anhydride by ball milling of powder mixtures followed by calcination. A binary Sb 2 O 3 -V 2 O 5 system was also prepared for comparison purposes. The resulting materials were physically characterized by surface area measurements, X-ray diffraction analysis (XRD), laser Raman spectroscopy, X-ray absorption fine structure (XAFS) spectroscopy, electron spin resonance (ESR), magnetic susceptibility determination, and 15 V solidstate NMR. The catalytic performance of the TiO 2 -supported materials was tested for o-xylene oxidation. After calcination of the Sb 2 O 3 -V 2 O 5 binary mixture at 673 K, Sb 3+ is almost quantitatively oxidized to Sb 5+ , while both V 3+ and V 4+ are detected. V 3+ and some V 4+ are most likely located in a nonstoichiometric VSbO 4 -like structure, while the majority of V 4+ preferentially concentrates within shear domains in oxygendeficient V 2 O 5-x particles. In the titania-supported catalyst system, both Sb 2 O 3 and V 2 O 5 spread on the anatase surface. Sb 3+ is oxidized to Sb 5+ , and V 3+ , V 4+ , and V 5+ are detected. VSbO 4 -like structures are not observed. The presence of antimony leads to the formation of presumably V 3+ -O-V 5+ redox couples. The paramagnetic centerssin contrast to the binary mixturesare largely isolated. Antimony preferentially migrates to the surface and appears to exhibit a dual function catalytically. It is inferred from the experimental data that the addition of antimony leads to site isolation and to a reduction of surface acidity. We suggest that V-O-V-O-V domains or clusters are interrupted by incorporation of Sb to form V-O-Sb-O-V species. As a consequence of this site isolation and a reduction of surface acidity, overoxidation of o-xylene is reduced. These two effects are therefore most probably responsible for the improved selectivity of the ternary catalyst system over the binary one toward phthalic anhydride.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.