Birnessite Structure Research Articles

Hydrated layered manganese dioxide:: III. Role of bismuth oxide on the redox behaviour of hydrated layered manganese dioxides

The previous studies to the hydrated layered manganese dioxides were extended to show the role of bismuth oxide in the mechanism of redox reactions. The steps identified for the mechanism of the redox reactions were suggested to be: (1) a short reversible one-phase reaction of charge transfer n≅0.23 electron per formula unit; (2) a reversible two-phase reaction from the birnessite structure to a hydrotalcite-carbonate phase of an average Mn oxidation state of +2.6; and, finally, (3) another two-phase reaction from the hydrotalcite-like intermediate to manganese hydroxide. In recharging of the later phase, a two-phase reaction took place yielding β-MnOOH (Feitknechtite) which may be further oxidised to birnessite. The bismuth oxide was found to stabilise the intermediate phases (hydrotalcite during reduction or β-MnOOH during oxidation) through the adsorption of the dissolved Bi(III) species on the active sites found mainly on the sides of the layered oxide. This adsorption formed a protective layer against the aggressive species which cause the transformation to the inactive 3D structure (e.g. Mn 3O 4). The protection is still effective as long as the adsorption equilibrium of bismuth species predominates.

Structure of K-rich Birnessite synthesized by Thermal Decomposition of KMnO4

Structure of K-rich Birnessite synthesized by Thermal Decomposition of KMnO₄

Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH

The structural transformation of high pH Na-rich buserite (NaBu) to H-exchanged hexagonal birnessite (HBi) at low pH was studied by simulation of experimental X-ray diffraction patterns. Four HBi samples were prepared by equilibration of NaBu at constant pH in the range pH 5-2. The samples differ from each other by the presence of one (at pH 2 and 3) or two (at pH 4 and 5) phases, and by the structural heterogeneity of these phases which decreases with decreasing pH. The sample obtained at pH 5 is a 4:1 physical mixture of a 1H phase ( a = 4.940 A, b = a /√3 = 2.852 A, c = 7.235 A, β = 90°, γ = 90°) and of a 1M phase ( a = 4.940 A, b = a /√3 = 2.852 A, c = 7.235 A, β = 119.2°, γ = 90°) in which successive layers are shifted with respect to each other by + a /3 along the a axis as in chalcophanite. Both the 1H and 1M phases contain very few well-defined stacking faults at pH 5. At pH 4, the sample is a 8:5 physical mixture of a 1H phase containing 15% of monoclinic layer pairs and of a 1M phase containing 40% of orthogonal layer pairs. Any further decrease of the pH leads to the formation of a single defective 1H phase. This 1H phase contains 20% and 5% of monoclinic layer pairs at pH 3 and 2, respectively. Independent of pH, all phases contain 0.833 Mnlayer cations, 0.167 vacant layer sites, and 0.167 interlayer Mn cations located either above or below layer vacancies per octahedron. A structural formula is established at each pH. The origin of the observed phase and structural heterogeneities has been analyzed. 1H and 1M phases are assumed to inherit their specific structural and crystal chemical features from the two distinct NaBu modifications. NaBu type I, with a high proportion of Mn4+layer cations, is thought to be responsible for the monoclinic layer stacking because this configuration allows Mn cations from adjacent layers to be as far as possible from each other, thus minimizing the electrostatic repulsion between these high charge cations. In contrast, NaBu type II has a high interlayer charge induced by Mn3+layer for Mn4+layer substitutions. Consequently, the 1H phase has a high amount of interlayer protons and achieves compensation of the unfavorable overlap of layer and interlayer Mn cations, in projection on the ab -plane, by the presence of strong hydrogen bondings between layers. The higher proportion of defined stacking faults in both 1H and 1M phases at pH 4 compared to pH 5 can be attributed to the increase in reaction rate with decreasing pH. At lower pH (3 and 2) the formation of strong hydrogen bonds between adjacent layers controls the layer stacking mode and leads to the formation of a unique 1H phase. The proportion of well-defined stacking faults in this phase decreases from pH 3 to 2.

Effects of Water, Cations, and Structure on Energetics of Layer and Framework Phases, NaxMgyMnO2·nH2O

The energetics of 14 synthetic manganese dioxides, including framework phases with todorokite structure and layer phases with birnessite and buserite structures, were studied using high-temperature solution calorimetry in molten 3Na2O4·MoO3 at 977 K. The composition of these phases with a general formula of NaxMgyMnO2·nH2O was systematically varied. Using the measured enthalpies of drop solution, the enthalpies of formation from oxides based on 1 mol of NaxMgyMnO2·nH2O at 298 K, ΔHfo(oxides), were obtained. The ΔHfo(oxides) data show no obvious correlation with either content of charge-balancing cations or content of water for the layer and framework phases. Using a new parameter, the hydration number of the charge-balancing cations (N(hyd) = the number of water molecules per cation), all of the ΔHfo(oxides) data can be linearly correlated. The linear correlations are ΔHfo(oxides) = 16.5N(hyd) − 73.3 for the layer phases, and ΔHfo(oxides) = 11.7N(hyd) − 64.4 for the framework phases. The average N(hyd) values for the birnessite, buserite, and todorokite phases are 2, 4, and 2, respectively.

Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite; I, Results from X-ray diffraction and selected-area electron diffraction

Synthetic Na-rich birnessite (NaBi) and its low pH form, hexagonal birnessite (HBi), were studied by X-ray and selected-area electron diffraction (XRD, SAED). SAED patterns were also obtained for synthetic Sr-exchanged birnessite (SrBi) microcrystals in which Sr was substituted for Na. XRD confirmed the one-layer monoclinic structure of NaBi and the one-layer hexagonal structure of HBi with subcell parameters a = 5.172 Aa, b = 2.849 Aa, c = 7.34 Aa, beta = 103.3 degrees and a = 2.848 Aa, c = 7.19 Aa, gamma = 120 degrees , respectively. In addition to super-reflection networks, SAED patterns for NaBi and SrBi contain satellite reflections. On the basis of these experimental observations, structural models for NaBi and HBi are proposed. NaBi consists of almost vacancy-free Mn octahedral layers. The departure from the hexagonal symmetry of layers is caused by the Jahn-Teller distortion associated with the substitution of Mn (super 3+) for Mn (super 4+) . The supercell A = 3a parameter arises from the ordered distribution of Mn (super 3+) -rich rows parallel to [010] and separated from each other along [100] by two Mn (super 4+) rows. The superstructure in the b direction of NaBi type II (B = 3b) comes from the ordered distribution of Na cations in the interlayer space. The maximum value of the layer negative charge is equal to 0.333 v.u. per Mn atom and is obtained when Mn (super 3+) -rich rows are free of Mn (super 4+) . The idealized structural formula proposed for NaBi type II is Na (sub 0.333) (Mn (super 4+) (sub 0.722) Mn (super 3+) (sub 0.222) Mn (super 2+) (sub 0.055) )O 2 . NaBi type I has a lower amount of Mn (super 3+) and its ideal composition would vary from Na (sub 0.167) (Mn (super 4+) (sub 0.833) Mn (super 3+) (sub 0.167) )O 2 to Na (sub 0.25) (Mn (super 4+) (sub 0.75) Mn (super 3+) (sub 0.25) )O 2 . Satellites in SAED patterns of NaBi crystals result from the ordered distribution of Mn (super 4+) and Mn (super 2+) pairs in Mn (super 3+) -rich rows with a periodicity of 6b. The structure of HBi consists of hexagonal octahedral layers containing predominantly Mn (super 4+) with variable amounts of Mn (super 3+) and layer vacancies. The distribution of layer vacancies is inherited from the former Mn (super 3+) distribution in NaB. Interlayer Mn cations are located above or below vacant layer sites. The driving force of the NaBi to HBi transformation is probably the destabilization of Mn (super 3+) -rich rows at low pH.

Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite; II, Results from chemical studies and EXAFS spectroscopy

Solution chemical techniques were used to study the conversion of synthetic Na-rich buserite (NaBu) to hexagonal (H 1 -exchanged) birnessite (HBi) at low pH. The low-pH reaction is broadly characterized by the exchange of structural Na 1 with solution H 1 and the partial loss of Mn 21 to the aqueous phase. The desorption of Na 1 in two temporally distinct steps indicates the existence of two types of binding sites for this cation. Mn 21 EXAFS measurements on Na-rich birnessite (NaBi) show that this mineral is primarily a layered structure formed by edge-sharing MnO6 octahedra, with no evidence for triple- corner (TC) sharing Mn. HBi is significantly different with strong evidence for TC-sharing Mn and therefore layer vacancies. The relative numbers of edge (E)-sharing and TC-shar- ing neighbors determined from EXAFS measurements on HBi is consistent with SAED results (Drits et al. 1997), which suggest that the layer vacancies are restricted to every third row of Mn cations, with 50% of the Mn sites along these rows vacant. The density of vacancies in the entire layer is therefore one in six of layer Mn sites. Polarized EXAFS measurements on orientated films of NaBi and HBi confirm the absence of TC-sharing Mn in NaBi and indicate that Mn adsorbed at layer vacancy sites in HBi at pH 4 is dominantly Mn 31 . The intensity of the TC-sharing contribution to the Mn EXAFS spectra of HBi samples increases with increasing pH from pH 2 to 5, and supports a mechanism of formation involving both the direct migration of layer Mn 31 to interlayer TC-sharing positions and re-adsorption of Mn 21 from solution onto layer vacancy sites. The migration of Mn 31 cations into the interlayer releases the steric strain associated with the Jahn-Teller distortion of these octahedra. This model of the NaBu-to-HBi conversion explains the transformation from orthogonal to hexagonal layer symmetry, respectively, as reported by Drits et al. (1997). Analysis of the Zn EXAFS spectrum of Zn 21 -exchanged birnessite shows that Zn 21 also occupies TC-sharing positions at layer vacancy sites. The results of this study strongly suggest that lattice cation vacancies are of critical importance in adsorption and electron transfer processes occurring at the surface of this mineral.

Thin Films of Octahedral Molecular Sieves of Manganese Oxide

Octahedral layered molecular sieve materials (OL-1) having the birnessite structure have been synthesized using sol-gel preparation methods. The sol obtained by redox reaction between sucrose and KMnO{sub 4} is then used to make films via (a) immersion, (b) spray coating, (c) spin coating, and (d) spatula coating. Crystalline OL-1 thin films have been prepared by methods a-c. Thin homogeneous films are most readily prepared by spray coating. Spin coating leads to noncrystalline and manganese-deficient films. The films have been characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, and scanning Auger microscopy. The microscopy and Auger experiments show that there was no charging, implying these samples are good electrical conductors. The films are free from large amounts of impurity ions and carbonaceous deposits formed during the syntheses. 25 refs., 5 figs., 3 tabs.

Synthesis and chimie douce reactions in lithium phyllomanganate

Lithium phyllomanganate, Li xMnO 2 · yH 2O (x = 0.25–0.29, y ≈ 0.6) with the birnessite structure cannot be obtained directly by oxidation of Mn(OH) 2 in Li-containing medium. It was obtained by a two-step topotactic reaction involving an acidic treatment of sodium phyllomanganate, followed by ion exchange in LiOH at room temperature. The lamellar lithium manganate is characterized by electron diffraction, magnetic measurements and thermogravimetry, showing that the structure is retained after water loss up to 500 °C. The material cannot be chemically oxidized by bromine, cerium(IV), or nitryl fluoborate. However, sodium and lithium phyllomanganates can be delithiated reversibly in electrochemical cells at ca. 4 V vs. Li Li + .

Transformations of Synthetic Birnessite as Affected by pH and Manganese Concentration

AbstractThe amount of Mn2+ adsorbed or removed from solution by birnessite is several times greater than its reported cation exchange capacity. Extractability of the sorbed Mn2+ decreases with aging. It is uncertain whether the sorbed Mn2+ is oxidized on the surface or incorporated into the structure of birnessite. Using X-ray powder diffractometry and transmission electron microscopy, a study was conducted to examine the mineralogical alteration of birnessite after treatment with various concentrations of MnSO4 and solution pH.The sorbed Mn2+ was not directly oxidized and remained on the birnessite surface. The sorption of Mn2+ was followed by alteration of birnessite with the formation of new Mn minerals. The specific Mn minerals formed were governed by the pH of the reaction, and the rate of the transformation was determined by Mn2+ concentration and pH. Nsutite and ramsdellite were identified at pH 2.4, crypto-melane at pH 4, groutite at pH 6, and manganite at pH 8. Other Mn minerals formed at these and other pH levels could not be identified. As the concentration of Mn in the solution decreased, the time required to form new minerals from the birnessite increased. The newly formed phases were the result of structural conversion since dissolution of birnessite and reprecipitation of new phases were not observed.

Reaction Scheme for the Oxidation of as(III) to as(V) by Birnessite

AbstractThe oxidation of As(III) to As(V) by K-birnessite was examined at different temperatures, pHs, and birnessite/As(III) ratios. Experiments ranged in duration from 5 to 64 hr, and solution and solid products were determined at several intervals. All experiments showed that the reaction produced large amounts of K+ to solution and very little Mn2+. As(V) was released to solution and incorporated into the K-birnessite. The oxidation was initially rapid and then slowed. The oxidation of As(III) was probably facilitated initially by autocatalytic Mn-As(V) reactions occurring mostly in the interlayer, in which large amounts of As(V) and K+ could be easily released to solution. The reaction also slowed when interlayer Mn was exhausted by forming Mn-As(V) complexes. Mn(IV) could only be acquired from the octahedral sheets of the birnessite. The two-stage reaction process proposed here depended on the layered structure of birnessite, the specific surface, and presence of exchangeable cations in K-birnessite.