Development of a stoichiometric magnesium potassium phosphate cement (MKPC) for the immobilization of powdered minerals

International audienceOrdinary Portland Cement (OPC)-based materials are not systematically adapted for immobilizing industrial hazardous waste, e.g. for aluminium powder or plutonium waste sludge. In such case, Magnesium Potassium Phosphate Cements (MKPC) represent an interesting alternative. The originality of this research is to develop a formulation of a MKPC paste for hazardous waste immobilization, which incorporates a maximum amount of such waste, preferably in powdered form. To this purpose, a stoichiometric MKPC paste is selected, and its properties are improved by powdered waste addition. Firstly, the physico-chemical mechanisms generating expansion in stoichiometric MKPC paste are analyzed. Swelling is attributed to a pH gradient in the paste, due to the progressive sedimentation of MgO particles in the fresh mix. Secondly, over-stoichiometric MgO is replaced by varying amounts of minerals simulating the waste, of different mineralogy and granulometry, in order to achieve sufficient workability and no swelling. An optimal formulation is proposed, which incorporates powdered fly ash at a fine-to-cement mass ratio (F/C) of 1. Its mechanical performance and endogenous dimensional changes are comparable to typical over-stoichiometric pastes, and they stabilize between 7 and 28 days

Original Oxo-Centered Frameworks in Bi3(VO4)O3 and Bi3.5(AsO4)(OH)0.5O3.5 by Supercritical Steam

International audienceTwo new bismuth compounds, oxovanadate Bi 3 (VO 4)O 3 and oxoarsenate Bi 3.5 (AsO 4)(OH) 0.5 O 3.5 , were prepared using supercritical hydrothermal pressure. Dealing with the anionic sublattice, both crystal structures are built on anti-oxo-OBi 4 / OBi 3 or-OBi 4 /OBi 5 units connected together in infinite corrugated 2D layers surrounded by isolated XO 4 (X = V or As) tetrahedra. These edifices complete a series initiated by the recent Bi 3 (PO 4)O 3 prepared under similar conditions. With the latter being assigned to the "simplest" bioxophosphate in terms of structural complexity, this aspect was investigated among the other compounds in their chemical ternaries. These phases are suggested to be high-pressure polymorphs, not possible to tackle when working at ambient pressure and temperature conditions

Calcium and copper substitution in stoichiometric and La-deficient LaFeO3 compositions: A starting point in next generation of Three-Way-Catalysts for gasoline engines

International audienceCalcium and copper substitutions, in A-site and B-site respectively, of parent stoichiometric LaFeO3 and La-deficient La0.7FeO3 perovskites led to significant improvements in the kinetics of CO and propene oxidation in typical three-way operating conditions. La-deficient La0.7Fe1-yCuyO3 perovskites were found more prone to surface copper oxide segregation leading to more active extra-framework copper oxide species in CO oxidation. Optimal performances were obtained on La0.7Fe0.8Cu0.2O3 composition. At higher Cu content, strong copper agglomeration leads to deactivation. More stable systems were obtained on dual substituted samples thanks to calcium substitution stabilizing copper inside the perovskite lattice and slowing down subsequent surface agglomeration. Rate enhancements in propene oxidation is observed on A-site deficient La0.6CaxFe0.8Cu0.2O3 with x ≤ 0.2 but a sharp loss in rate is observed on stoichiometric La0.6Ca0.4Fe0.8Cu0.2O3 perovskite explained by a progressive shift from suprafacial to intrafacial mechanism involving in this latter case the redox Fe4+/Fe3+ couple and lattice oxygen species

Preparation, characterization and DFT+U study of the polar Fe 3+ -based phase Ba 5 Fe 2 ZnIn 4 S 15 containing S = 5/2 zigzag chains

International audienceThe polar magnetic chalcogenide phase Ba5Fe2ZnIn4S15 was synthesized and its structure was solved by single crystal XRD. It is the first member with a 3d magnetic metal (Fe3+) in the Pb5ZnGa6S15-type structure family of wide bandgap materials with non-linear optical properties. The three-dimensional framework possesses a low dimensional magnetic character through the presence of weakly interacting zig-zag chains made of corner-sharing FeS4 tetrahedra forming chain 1, [FeS2]−∞. The latter chains are separated by InS4 tetrahedra providing weak magnetic super-super exchanges between them. The framework is also constituted by chain 2, [In3Zn1S9]7−∞ (chain of T2-supertetrahedra) extended similarly to chain 1 along the direction c and connected through InS4 tetrahedra. Symmetry analysis shows that the intrinsic polarization observed in this class of materials is mostly due to the anionic framework. Preliminary magnetic measurements and density functional theory calculations suggest dominating antiferromagnetic interactions with strong super-exchange coupling within the Fe-chains

On the Use of Dynamical Diffraction Theory To Refine Crystal Structure from Electron Diffraction Data: Application to KLa 5 O 5 (VO 4 ) 2 , a Material with Promising Luminescent Properties

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Impact of Dual Calcium and Manganese Substitution of La-deficient perovskites on structural and related Catalytic Properties: Future opportunities in next Three-Way-Catalyst Generation?

International audienceThe impact of La-deficiency and substitution by calcium and manganese in LaFeO3 has been investigated in Three-Way-Catalysis. Despite significant structural distortions, predominantly induced by calcium substitution, no significant phase segregation occurs. On the other hand, bulk and surface characterization revealed the stabilization of Mn4+ and Fe4+ in different extent. Kinetic analysis of CO and propene oxidation reactions and the NO reduction reaction showed that La0.7Fe0.8Mn0.2O3 exhibits the best performance. By contrast, a detrimental effect of calcium more pronounced for propene oxidation is observed. It was found that catalytic properties are neither related to bulk properties nor to the extent of specific surface area but are mostly governed by the redox couple Mn4+/Mn3+. The formation of Fe4+ improving with partial substitution of La by Ca in A-site was not found as a determining criterion

Undulated oxo-centered layers in PbLn 3 O 4 (VO 4 ) (Ln= La and Nd) and relationship with Nd 4 O 4 (GeO 4 )

International audienceSingle crystals of PbLa3O4(VO4) have been synthesized using the flux growth technique and characterized by X-ray diffraction. The crystal structure of the tittle phase was solved by charge flipping and refined to R1 = 0.024 (wR2 = 0.031) for 2777 reflections [I>3σ(I)]. The compound is orthorhombic and crystallized in the space group Cmcm: a = 5.8686(6)Å, b = 17.898(2)Å, c = 7.9190(7)Å, V = 831.8(1)Å3, Z = 4. The structure is built on [PbLa3O4]3+ layers with zig-zag cross-sections, surrounded by isolated (VO4)3- tetrahedra. Its crystal structure shows direct relationship with the isoformular Nd4O4(GeO4) compound which crystallized in the primitive non centrosymmetric Pb21m sub-group. Its stability in temperature and under air was checked as well as optical properties. In a second part, lanthanum was substituted by neodymium giving rise to a paramagnet and f→ f electronic excitations superposed to the broad absorption front below 3.05 eV related to the presence of VO4 groups

Undulated oxo-centered layers in PbLn 3 O 4 (VO 4 ) (Ln= La and Nd) and relationship with Nd 4 O 4 (GeO 4 )

International audienceSingle crystals of PbLa3O4(VO4) have been synthesized using the flux growth technique and characterized by X-ray diffraction. The crystal structure of the tittle phase was solved by charge flipping and refined to R1 = 0.024 (wR2 = 0.031) for 2777 reflections [I>3σ(I)]. The compound is orthorhombic and crystallized in the space group Cmcm: a = 5.8686(6)Å, b = 17.898(2)Å, c = 7.9190(7)Å, V = 831.8(1)Å3, Z = 4. The structure is built on [PbLa3O4]3+ layers with zig-zag cross-sections, surrounded by isolated (VO4)3- tetrahedra. Its crystal structure shows direct relationship with the isoformular Nd4O4(GeO4) compound which crystallized in the primitive non centrosymmetric Pb21m sub-group. Its stability in temperature and under air was checked as well as optical properties. In a second part, lanthanum was substituted by neodymium giving rise to a paramagnet and f→ f electronic excitations superposed to the broad absorption front below 3.05 eV related to the presence of VO4 groups

From useless humins by-product to Nb@graphite-like carbon catalysts highly efficient in HMF synthesis

International audienceHighly dispersed supported NbOx species were prepared via a deposition precipitation-carbonization (DPC)-like method. As precursors for niobium species and carrier ammonium niobate(V) oxalate hydrate and humins were used. Characterization of the resulted catalysts indicated bi-functional acid-base niobium species anchored onto a highly hydrophobic graphite-like carbon structure. These catalysts were investigated in the one-pot conversion of glucose to 5-hydroxymethylfurfural (HMF) in a biphasic system consisting of a mixture of a 20 wt% NaCl aqueous solution phase and an organic extracting phase (methyl-isobutyl-ketone (MIBK), 2-tert-butylphenol (TBP) or 2-sec-butylphenol (SBP)). The optimization conditions afforded the highest yield to HMF (96 %) for the GHNb1.2 catalyst (with 2.5 wt%Nb), the base/acid sites ratio of 1.76, in biphasic TBP/water system, at 180 °C after 8 h