Mn K-edge XANES Research Articles

Structural and Optical Properties of Mn-Incorporated BiVO4 Nanoparticles Synthesized by Sonochemical Process

In this work, Mn-incorporated BiVO4 nanoparticles with different Mn contents (0–5%) were synthesized by one-step sonochemical process. Structural phase of Mn-incorporated BiVO4 nanoparticles was extensively carried out by X-ray diffraction technique. The XRD results show that the structural phase of BiVO4 belongs to the monoclinic principal structure while the secondary phases of MnO, Mn2O3, and Mn3O4 structures are noticed in Mn-loaded samples. Chemical bondings of Mn-incorporated BiVO4 nanoparticles were characterized with Raman spectroscopy. Relevant chemical bondings confirm the existence of VO4 3− tetrahedron and the V–O band by Raman spectra. Diffuse reflection spectroscopy and FE-SEM were employed to examine their optical and morphological properties, respectively. The results indicate that the incorporation of Mn can induce the red-shift of optical absorption edge in visible range, which can decrease the optical band gap of BiVO4. Morphology of all samples demonstrates the difference in shape of the compounds with the incorporation of Mn contents. Oxidation number and local structure of Mn-incorporated BiVO4 nanoparticles were investigated by X-ray absorption spectroscopy. XAS results demonstrate that oxidation number of all elements correspond to Mn2+/Mn3+, Bi3+, and V5+ ions. The normalized Mn K-edge XANES spectra suggest that Mn atoms would not locate at the local site of Bi or V in BiVO4 crystal because the specific feature of experimental spectra are inconsistent with simulated spectra, which is corresponded to XRD results indicating the formation of Mn-based oxide structures with various oxidation states.

Effects of promoters (Mn, Mg, Co and Ni) on the Fischer-Tropsch activity and selectivity of KCuFe/mesoporous-alumina catalyst

Fischer–Tropsch synthesis (FTS) is one of the promising technologies to produce liquid fuels and fine chemicals from coal, natural gas and biomass. Significant improvements in reactor technology and catalyst development are the focus of research in this area. In this work, the effects of 1.0 wt% promotor such as Mn, Mg, Co and Ni on the FTS activity and selectivity of mesoporous alumina supported KCuFe catalyst were studied. All synthesized promoted catalysts were characterized using BET, XRD, H2-TPR, XPS, TEM and XANES. The FTS reaction was carried out in a fixed bed reactor at 270 °C, 300 psi, 2000 h−1 and H2/CO = 1.25. In this work, the Ni-promoted catalyst showed the highest methane selectivity (∼29 %). The Co-promoted catalyst showed the least Olefins/Paraffins (O/P) ratio and a ∼5% increase in methane selectivity. Mg-promoted catalyst performed average but better than Ni and Co-promoted catalysts. However, Mn-supported catalyst outperformed Mg, Ni and Co-promoted catalysts and resulted in higher C5+ selectivity along with a marginal increase in CO conversion. The ∼5%, 2%, and 2% decline in CO2, CH4 and C2-C4 selectivities were also registered upon the addition of Mn promotor. The improvement in FTS activity and selectivity of Mn-promoted catalyst is related to the increase in iron dispersion and reducibility, as seen during H2-TPR, XPS and XANES analyses. The presence of Mn+3 and Mn+4, as confirmed by Mn 2p XPS and Mn K-edge XANES, indicates that the interaction at the electronic level between manganese and iron has contributed to improved performance. This study concludes that Mn is the most effective promotor for the synthesized mesoporous alumina supported KCuFe catalyst for the FT synthesis process.

Weathering and precipitation after meteorite impact of Ni, Cr, Fe, Ca and Mn in K-T boundary clays from Stevns Klint

Ni, Cr, Fe, Ca and Mn K-edge XANES and EXAFS spectra were measured on K-T boundary clays from Stevns Klint in Denmark. According to XANES spectra and EXAFS analyses, the local structures of Ni, Cr and Fe in K-T boundary clays is similar to Ni(OH)2, Cr2O3 and FeOOH, respectively. It is assumed that the Ni, Cr and Fe elements in impact related glasses is changing into stable hydrate and oxide by the weathering and diagenesis at the surface of the Earth. Ca in K-T boundary clays maintains the diopside-like structure. Local structure of Ca in K-T clays seems to keep information on the condition at meteorite impact. Mn has a local structure like MnCO3 with divalent state. It is assumed that the origin on low abundant of Mn in the Fe-group element in K-T clays was the consumption by life activity and the diffusion to other parts.

Structural Changes Correlated with Magnetic Spin State Isomorphism in the S2 State of the Mn4CaO5 Cluster in the Oxygen-Evolving Complex of Photosystem II.

The Mn4CaO5 cluster in Photosystem II catalyzes the four-electron redox reaction of water oxidation in natural photosynthesis. This catalytic reaction cycles through four intermediate states (Si, i = 0 to 4), involving changes in the redox state of the four Mn atoms in the cluster. Recent studies suggest the presence and importance of isomorphous structures within the same redox/intermediate S-state. It is highly likely that geometric and electronic structural flexibility play a role in the catalytic mechanism. Among the catalytic intermediates that have been identified experimentally thus far, there is clear evidence of such isomorphism in the S2 state, with a high-spin (5/2) (HS) and a low spin (1/2) (LS) form, identified and characterized by their distinct electron paramagnetic resonance (EPR spectroscopy) signals. We studied these two S2 isomers with Mn extended X-ray absorption fine structure (EXAFS) and absorption and emission spectroscopy (XANES/XES) to characterize the structural and electronic structural properties. The geometric and electronic structure of the HS and LS S2 states are different as determined using Mn EXAFS and XANES/XES, respectively. The Mn K-edge XANES and XES for the HS form are different from the LS and indicate a slightly lower positive charge on the Mn atoms compared to the LS form. Based on the EXAFS results which are clearly different, we propose possible structural differences between the two spin states. Such structural and magnetic redox-isomers if present at room temperature, will likely play a role in the mechanism for water-exchange/oxidation in photosynthesis.

Effects of Fe doping on the structures and properties of hexagonal birnessites – Comparison with Co and Ni doping

Fe-doped hexagonal birnessite was synthesized by adding Fe3+ to the initial reactants, and the effects of Fe doping on the structures and properties of birnessite were investigated and compared with the effects of Co and Ni doping. The underlying mechanisms controlling the incorporation of transition metals (TMs) into the birnessite structure were proposed. Compared to the un-doped control, Fe-doped birnessite has weaker crystallinity, i.e., less stacking of the phyllomanganate sheets in the c direction, and larger surface area. Combination of X-ray photoelectron spectroscopy (XPS) and Mn K-edge XANES and EXAFS spectra demonstrates that Fe doping decreases the Mn average oxidation state (AOS) but has little effect on the basic layer structure and local Mn environments. Fe(III) located in the birnessite layers exhibits high-spin (HS) configuration whereas layer Mn(III) and Co(III) plausibly adopt low-spin (LS) state. The TMs decrease the thickness of birnessite plate crystals along the c axis and affect the unit cell parameter b in the order Fe>Ni>Co. Co and Fe incorporate into the birnessite layers by substitution for Mn(IV) while Ni substitutes for Mn(III). The substitution of TMs into the birnessite layers is governed by the coordination radius (CR), crystal field stabilization energy (CFSE) and oxidation state of the TMs. The variations in potassium contents in doped birnessites together with TM K-edge EXAFS data indicate that most of the Fe (∼81–82%) or Ni (∼66–76%) incorporated into the birnessite structure exists in the interlayer regions, while most of the Co (∼71–80%) occurs in the manganese layers. The compatibility of these TM ions in the birnessite layers is in the order Co>Ni>Fe. The smaller the difference between the CR of Fe, Co or Ni and Mn(IV) or Mn(III), the more dopants are compatible within the Mn layers.

Mn-containing porous silicates as catalysts for the solvent-free selective oxidation of alkyl aromatics to aromatic ketones with molecular oxygen

Mn-containing redox molecular sieves, i.e., highly crystalline microporous MnS-1 (0.6 wt% Mn, n(Si)/ n(Mn) = 160) and ordered mesoporous silicates MnMCM-41 and MnMCM-48 (1.8 wt% Mn, n(Si)/ n(Mn) = 51), were synthesized. The porosity of the products was determined by X-ray powder diffraction (XRD) and high -resolution transmission electron microscopy (HRTEM). The evidence for the presence of isolated Mn sites incorporated into the silicate framework was obtained by Mn K-edge XANES and EXAFS analysis, which provided direct information on valence state and local environment of Mn in these products. The analysis revealed the presence of predominantly Mn 3+ cations located at a 3-fold coordination in the silicate framework indicating Lewis acid sites in microporous (MnS-1) and mesoporous (MnMCM-41, MnMCM-48) silicate structures. The micro- and mesoporous silicates containing framework Mn 3+-sites selectively catalyze the oxidation of alkyl aromatics in benzylic position to aromatic ketones in the absence of any initiator using molecular oxygen as the terminal oxidant under mild, solvent-free, liquid-phase conditions.

Structure of iron and manganese ions substituted in the framework of nanoporous AlPO-5 material

The structure of iron and managanese ions substituted in the framework of nanoporous AlPO-5 is determined by ex situ and in situ X-ray absorption spectroscopy. Fe K-edge XANES and EXAFS studies clearly indicate that iron ions are present as Fe(III) in octahedral coordination in the assynthesised material and tetrahedral coordination in the calcined material in both pure FeAlPO-5 and FeMnalPO-5. XANES and EXAFS results also indicate that reaction with hydrogen peroxide causes the removal of Fe(III) ions from the framework. Mn K-edge XANES and EXAFS of FeMnAlPO-5 samples indicate that Mn(II) ions are present in the framework, tetrahedrally coordinated, in the as-synthesised material but upon calcination it is found that the Mn(II) ions are removed from the framework, suggesting a different synthesis strategy is necessary to stabilise the Mn(II) ions in the framework simultaneously with Fe(III) ions.

Characterization of Cu 1.4Mn 1.6O 4/PPy composite electrodes

In this work, we have studied the multilayered polypyrrole(PPy)/oxide composite electrode on glassy carbon (GC) having the structure GC/PPy/PPy(Cu 1.4Mn 1.6O 4)/PPy using X-ray Photoelectron Spectroscopy and Mn K-edge and Cu K-edge XANES and EXAFS. The mixed oxide particles have been incorporated into the PPy matrix simultaneously to the electropolymerization of Py from a solution containing 0.1 M Py + 0.15 M KCl + Cu 1.4Mn 1.6O 4. The XPS data have shown that, prior to the incorporation of the oxide into the PPy matrix, it contains Cu +, Cu 2+, Mn 3+ and Mn 4+. The XPS, XANES and EXAFS results have shown that when the oxide is incorporated into the PPy matrix, the Cu + present in the original oxide suffers dismutation to give Cu 2+ and metallic Cu. The metallic Cu is segregated out of the spinel structure. The Mn K-edge XANES and EXAFS data show that, after the incorporation into the PPy matrix, Mn is present as Mn 3+ and Mn 4+ occupying octahedral sites in a spinel-related structure while the Cu K-edge XANES and EXAFS data indicate that copper occupies tetrahedral sites predominantly in that structure but having a large degree of disorder in the second and higher coordination shells.

Characterization of the Mn–Li ferrite system Li1–0.5xFe1.5x+1Mn1–xO4 (0.2 ≤ x ≤ 1)

The Mn–Li ferrite system Li1−0.5xFe1.5x+1Mn1−xO4 (x = 1, 0.8, 0.6, 0.4, 0.2) has been studied by means of X-ray diffraction, Mossbauer spectroscopy and Mn K- and Fe K-edge XANES/EXAFS. All the samples show the spinel-related structure with the lattice constant a gradually decreasing between a = 0.8310 nm (x = 1) and a = 0.8301 nm (x = 0.2). Mossbauer and Fe K-edge XANES/EXAFS results have shown that the oxidation state of Fe is +3 and that the fractions of Fe3+ ions occupying the octahedral and tetrahedral sublattices do not change significantly along the series. Mossbauer data recorded under applied magnetic field indicate the existence in the Mn-containing samples of canted spin structures for the Fe3+ ions on both the tetrahedral and octahedral sites, the canting angle being larger for the octahedral sublattice. Mn K-edge XANES and EXAFS have shown Mn to be present as Mn3+ and Mn4+ and to occupy only octahedral sites. The XANES/EXAFS results have also shown that the Mn4+/Mn3+ ratio increases with increasing Li/Mn content. The results indicate that as the extent of Li/Mn insertion increases the new inserted Li+ ions occupy tetrahedral sites and that some of the Li+ ions initially occupying octahedral sites are also driven into tetrahedral positions.

Structure and physical property changes of de-lithiated spinels for Li 1.02− xMn 1.98O 4 after high-temperature storage

High-temperature storage of the lithium manganese spinel cathode was examined for Li 1.02− x Mn 1.98O 4 using 1 M LiPF 6 in EC:DEC (1:1) electrolyte solution. The structure, magnetic properties, and valence state of Mn were determined before and after the storage using Mn K-edge XANES, X-ray diffraction, SQUID, ICP spectroscopy, and electrochemical measurements. After the storage at 80 °C for 6 days, single-phase property was observed at x=0 and 0.96 in Li 1.02− x Mn 1.98O 4, while multi-phase property was observed between the compositions, x=0.18 and 0.63. Shallow de-lithiated region near x=0.18 was easily affected by the storage; the Li/Li 1.02− x Mn 1.98O 4 cell showed large capacity failure from 111 to 40 mAh/g after the storage which corresponded to the phase transition with the increase in the valence state of Mn after the storage. Magnetic measurement was found to be quite sensitive and effective to detect the subtle structural changes caused by the high-temperature storage.