Local structure determination in amorphous and disordered systems, which are crystallographically intractable, is critical for gaining an understanding of their properties. X-ray absorption spectroscopy (XAS) is an excellent tool for local structure determination in amorphous and disordered environmental materials. Amorphous manganese oxides produced by the freshwater bacterium Leptothrix discophora SP6 (SP6-MnOx) and disordered molybdenum vanadate catalysts are two materials with properties that make them active catalysts in the natural environment and in industry, respectively. L. discophora SP6 oxidizes Mn2+ aq) to form insoluble, amorphous Mn3+/4+ oxides (SP6-MnOx). The structure and the morphology of SP6-MnOx were characterized by transmission electron microscopy (TEM), XAS, including full multiple scattering analysis, and powder X-ray diffraction (XRD). The biogenic precipitate consists of nanoparticles that are approximately 10 nm by 100 nm in dimension with a fibrillar morphology that resembles twisted sheets. This biogenic MnOx is composed of sheets of edge-sharing of Mn4+O6 octahedra that form layers. The detailed analysis of the EXAFS spectra indicate that 12 ± 4% of the Mn4+ layer cation sites in SP6-MnOx are vacant, whereas the analysis of the XANES suggests that the average oxidation state of manganese is 3.8 ± 0.3. Therefore, the average chemical formula of SP6-MnOx is Mn+yMn3+0.12[□0.12Mn4+ 0.88]O2·zH2O, where Mnn+ represents hydrated interlayer cations, □0.12 represents Mn4+ layer cation vacancies, and Mn3+ 0.12 represents hydrated cations that occupy sites above/below layer cation vacancies. Molybdenum vanadates are active towards catalytic oxidative dehydrogenation of alkanes to alkenes. Single crystal x-ray and powder neutron diffraction studies of Mg2.5VMoO8 revealed that 1/16th of the Mg2+ sites are vacant, and Mo6+ and V5+ cations are crystallographically disordered within the structure. High frequency Raman vibrational bands (> 1000 cm-1) similar to the bands from Mo=O double bonds on oxide surfaces, suggest that higher order Mo-O bonds are associated with the cation vacancies. Quantum mechanical simulations of the Raman spectrum of Mg2.5VMoO8 confirm that the higher order bonds are associated with these vacancies. XAS, measured at the Mo and V K-edges, suggests that Mo6+ cations are angularly distorted and likely order on sites adjacent to Mg2+ cation vacancies.

Last modified

• 05/10/2018

Creator

• Ian Saratovsky

DOI

• https://doi.org/10.21985/N2510D

Subject

• Chemistry

Keyword

• Biogenic manganese oxides

Date created

• 2006-12-22

Resource type

• Dissertation

Rights statement

• In Copyright