XANES studies of modified and newly synthesized nanostructured manganese oxides (original) (raw)
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Catalysis Today, 2005
The characterization of a series of manganese oxides prepared by different methods were performed by X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) analysis. Low crystallized phases were found by means of XRD. The XANES results indicated that the mean oxidation state value of all the samples was between 3+ and 4+. XANES analysis of spectra and extended X-ray absorption fine structure (EXAFS) data revealed that Mn was present in three different octahedral environments; one of them corresponded to the environment of Mn in the pyrolusite phase while the other two can be associated to Mn in ramsdellite-like environment and Mn in a octahedral site with Mn-vacancies in the second shell of coordination. The different oxides were analyzed in the reaction of total oxidation of ethanol and we found that the catalytic activity was enhanced when the couple Mn 3+ -Mn 4+ was present in the structure of the oxides. #
International Journal of Quantum Chemistry, 2003
The absorption edge in Mn K-edge X-ray absorption spectra of manganese oxide compounds shows a shift of several electronvolts in going from MnO through LaMnO 3 to CaMnO 3. On the other hand, in X-ray photoelectron spectra much smaller shifts are observed. To identify the mechanisms that cause the observed chemical shifts, 1s ionization as well as 1s 3 "4p" transition energies have been determined by electronic structure calculations on embedded Mn ions and embedded MnO 6 clusters. Systematic variation of the cluster geometry and the cluster embedding showed that the chemical shifts are predominantly determined by two effects: the changes in the Mn 3d occupation and the changes in the Madelung potential. The large chemical shift in the 1s 3 4p transition energies between different materials occurs because the two effects do not compensate each other. The chemical shifts obtained for the embedded MnO 6 clusters agree reasonably with the experimental shifts. The small sensitivity to the material observed for the Mn 1s ionization energies is explained by the near cancellation of the effects of the Madelung potential and the 3d occupation of the Mn ion.
Structure Analysis of Exfoliated Unilamellar Crystallites of Manganese Oxide Nanosheets
The Journal of Physical Chemistry B, 2006
Structure analysis of unilamellar manganese oxide nanosheets obtained via exfoliation of layered manganese oxides was carried out utilizing synchrotron radiation (SR) X-ray in-plane diffraction and polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) analyses. A combination of SR excitation and the total reflection of incoming X-rays provides signals strong enough for both analyses even from a monolayer of the MnO 2 nanosheets having a concentration of 0.7 µg cm-2. In addition, the mean oxidation state of constituent manganese ions in the MnO 2 sheets was estimated on the basis of XANES spectra, and bond valence sum calculations with the bond length obtained from the present EXAFS analyses. The obtained structural data revealed that the two-dimensional lattice of the MnO 2 sheets underwent a slight elongation upon delamination. These changes correspond to approximately 1% expansion of sheet area and 1-2% expansion of thickness, which can be understood by reduction of the mean oxidation number of manganese ions in the sheet through the exfoliation process.
Geochimica Et Cosmochimica Acta, 2012
Manganese (Mn) oxides are among the most reactive natural minerals and play an important role in elemental cycling in oceanic and terrestrial environments. A large portion of naturally-occurring Mn oxides tend to be poorly-crystalline and/or nanocrystalline, with not fully resolved crystal structures. In this study, the crystal structures of their synthetic analogs including acid birnessite (AcidBir), d-MnO 2 , polymeric MnO 2 (PolyMnO 2 ) and a bacteriogenic Mn oxide (BioMnO x ), have been revealed using atomic pair distribution function (PDF) analysis. Results unambiguously verify that these Mn oxides are layered materials. The best models that accurately allow simulation of pair distribution functions (PDFs) belong to the monoclinic C12/m1 space group with a disk-like shape. The single MnO 6 layers in the average structures deviate significantly from hexagonal symmetry, in contrast to the results of previous studies based on X-ray diffraction analysis in reciprocal space. Manganese occupancies in MnO 6 layers are estimated to be 0.936, 0.847, 0.930 and 0.935, for AcidBir, BioMnO x , d-MnO 2 and PolyMnO 2 , respectively; however, occupancies of interlayer cations and water molecules cannot be accurately determined using the models in this study. The coherent scattering domains (CSDs) of PolyMnO 2 , d-MnO 2 and BioMnO x are at the nanometer scale, comprising one to three MnO 6 layers stacked with a high disorder in the crystallographic c-axis direction. Overall, the results of this study advance our understanding of the mineralogy of Mn oxide minerals in the environment.
Chemical Vapor Synthesis and Characterization of Manganese Oxides
Chemical Vapor Deposition, 2011
Manganese oxide nanoparticles are synthesized by a chemical vapor process using manganese(0) carbonyl as the precursor. The synthesis temperature of the materials is varied from 500 to 1500 °C at 200 °C intervals. These nanomaterials are characterized by X‐ray powder diffraction (XRD), transmission electron microscopy (TEM), Brunauer‐Emmett‐Teller (BET) surface area, and X‐ray photoelectron spectroscopy (XPS). XRD shows that the manganese oxide nanoparticles synthesized at 500, 700 – 1100, 1300, and 1500 °C are mainly MnO2, Mn2O3, Mn2O3/Mn3O4, and Mn3O4, respectively. The TEM and BET results also confirm that the particle size is on the nanometer scale with a large specific surface area (SSA) of 81 – 215 m2 g−1. XPS indicates the catalysts to have manganese oxidation states of (2+), (3+), or (4+) depending on the synthesis temperature.
Multiple Scattering Calculations of Bonding and X-ray Absorption Spectroscopy of Manganese Oxides
The Journal of Physical Chemistry A, 2003
We present near edge X-ray absorption spectra of manganese oxides at the Mn L 2,3 , Mn K, and O K edges to investigate the relative sensitivity of the edges to bonding and structure. Collectively, the spectra probe local electronic structure and intermediate range crystal structure. Spin independent full multiple scattering calculations of the Mn K edge give good agreement with data above threshold and qualitatively reproduce the prepeak that is observed for each compound. We show that the apparent prepeak for MnO is not due to p-d hybridization at the Mn atom (in accordance with symmetry principles) or quadrupolar transitions but originates from multiple scattering within the fifth shell. We present spin dependent multiple scattering calculations of the O K edge and show that this edge allows for a more direct description of the 3d states than either the Mn L edge or K edge prepeak, which are complicated by multiplet effects.
ACS Catalysis
In situ X-ray absorption and emission spectroscopies (XAS and XES) are used to provide details regarding the role of the accessibility and extent of redox activity of the Mn ions in determining the oxygen reduction activity of LaMnO 3 and CaMnO 3 , with X-ray absorption near-edge structure (XANES) providing the average oxidation state, extended X-ray absorption fine structure (EXAFS) providing the local coordination environment, and XES providing the population ratios of the Mn 2+ , Mn 3+ , and Mn 4+ sites as a function of the applied potential. For LaMnO 3 , XANES and XES show that Mn 3+ is formed, but Mn 4+ ions are retained, which leads to the 4e − reduction between 0.85 and 0.6 V. At more negative potentials, down to 0.2 V, EXAFS confirms an increase in oxygen vacancies as evidenced by changes in the Mn−O coordination distance and number, while XES shows that the Mn 3+ to Mn 4+ ratio increases. For CaMnO 3 , XANES and XES show the formation of both Mn 3+ and Mn 2+ as the potential is made more negative, with little retention of Mn 4+ at 0.2 V. The EXAFS for CaMnO 3 also indicates the formation of oxygen vacancies, but in contrast to LaMnO 3 , this is accompanied by loss of the perovskite structure leading to structural collapse. The results presented have implications in terms of understanding of both the pseudocapacitive response of Mn oxide electrocatalysts and the processes behind degradation of the activity of the materials.
Valence measurement of Mn oxides using Mn Kβ emission spectroscopy
Journal of Physics and Chemistry of Solids, 2000
High resolution Mn K b emission spectra provide a direct method to probe the effective spin state and charge density on Mn sites. Direct comparison of MnF 2 and MnO reveals significant changes due to the degree of covalency. The detailed shape and energy shift of the spectra for the perovskite LaMnO 3 and CaMnO 3 compounds are found to be very similar to Mn 2 O 3 and MnO 2 , respectively. Detailed Mn K b X-ray emission results on La 1Ϫx Ca x MnO 3 can be well fit by linear superpositions of the end member spectra. However, for x Ͻ 0:3; a retarded response is found. No evidence for Mn 2ϩ is found.
Manganese oxide minerals: Crystal structures and economic and environmental significance
Proceedings of the National Academy of Sciences, 1999
Manganese oxide minerals have been used for thousands of years—by the ancients for pigments and to clarify glass, and today as ores of Mn metal, catalysts, and battery material. More than 30 Mn oxide minerals occur in a wide variety of geological settings. They are major components of Mn nodules that pave huge areas of the ocean floor and bottoms of many fresh-water lakes. Mn oxide minerals are ubiquitous in soils and sediments and participate in a variety of chemical reactions that affect groundwater and bulk soil composition. Their typical occurrence as fine-grained mixtures makes it difficult to study their atomic structures and crystal chemistries. In recent years, however, investigations using transmission electron microscopy and powder x-ray and neutron diffraction methods have provided important new insights into the structures and properties of these materials. The crystal structures for todorokite and birnessite, two of the more common Mn oxide minerals in terrestrial depos...
Synthesis and Vibration Spectroscopy of Nano-Sized Manganese Oxides
Acta Physica Polonica A, 2018
X-ray diffraction, micro-Raman and the Fourier transform infrared spectroscopies as well as magnetometry measurements were performed on nanosized manganese oxides to probe their phase composition and magnetic properties. It was shown that the XRD method is less sensitive to phase composition of manganese oxide samples than spectroscopic methods. While in some samples the XRD method recognised only the manganosite MnO phase, the Raman and FT-IR methods revealed additionally the presence of the hausmannite Mn3O4 phase.