Maria Martinez-Lara

Interstitial oxygen conduction in lanthanum oxy-apatite electrolytes

The La10 − x(SiO4)6O3 − 1.5x (9.33 ≤ 10 − x ≤ 9.73) apatite series has been prepared and hexagonal single phases were obtained in a narrow compositional range (9.33 ≤ 10 − x ≤ 9.60). The room temperature crystal structure of La9.55(SiO4)6O2.32 has been determined from joint Rietveld refinement of neutron and laboratory X-ray powder diffraction data: a = 9.7257(1) A, c = 7.1864(1) A, V = 588.68(1) A3, Z = 1, RwpN = 3.2%, RwpX = 7.7%, RFN = 1.8%, RFX = 1.9%. An interstitial site for the extra-oxygen has been determined in the position very recently predicted in a theoretical study using atomistic simulations. The high temperature crystal structures have been obtained from neutron powder diffraction, NPD, collected at 773 and 1173 K showing the thermal evolution of this interstitial site. Previously reported neutron data for La9.60(GeO4)6O2.40 have also been re-analysed establishing the existence, and thermal evolution, of this interstitial site. The electrical results suggest that the samples are oxide ion conductors. The plots of the imaginary parts of the impedance, Z″, and the electric modulus, M″, vs. log (frequency), possess maxima for both curves separated by two decades in frequency. Bulk conductivities have been obtained from the fitting of the complex impedance spectra with the appropriate equivalent circuit. Bulk activation energies have been determined from two Arrhenius plots, one representing the bulk conductivities and the other representing the frequencies of the modulus peak maxima, fmax(M″). A comparative discussion of the two series, La10 − x(TO4)6O3 − 1.5x (T = Si, Ge), is given.

Interstitial oxygen in oxygen-stoichiometric apatites

Several oxy-apatite materials La10−xSrx(TO4)6O3−0.5x (T = Ge, Si; 10−x = 9.00, 8.80, 8.65 and 8.00) and La9.33(Si1−xGexO4)6O2 (x = 0, 0.5, 0.67) have been prepared as highly crystalline phases. The impedance study showed that all samples are oxide ion conductors. However, bulk conductivities changed by more than 2 orders of magnitude at a given temperature for some compositions. A thorough study on the oxygen sublattices for oxygen-stoichiometric oxy-apatites has been carried out. Constant-wavelength neutron powder diffraction data have been collected for La9.33(SiO4)6O2. Time-of-flight neutron data have been collected for La9.33(Si0.5Ge0.5O4)6O2, La8Sr2(SiO4)6O2 and La8Sr2(GeO4)6O2. The room-temperature structures have been derived from joint Rietveld refinements of neutron and laboratory X-ray powder diffraction data. High temperature structures have been obtained only from Rietveld refinements of neutron powder diffraction data. The refinements show that La9.33(SiO4)6O2 and La9.33(Si0.5Ge0.5O4)6O2 contain interstitial oxygen, associated to vacancies at the oxygen channels. The amount of interstitial oxygen is negligible in La8Sr2(SiO4)6O2 and La8Sr2(GeO4)6O2. Hence, the novelty of this work is to explain the high oxide conductivity of the lanthanum-deficient samples which it is due to the presence of interstitial oxygens. Lanthanum stoichiometric samples do not have interstitial oxygens and, so, their conductivities are much lower.

Intercalation of ferrocene and dimethylaminomethylferrocene into α-Sn(HPO4)2•H2O and α-VOPO4•2H2O

The intercalation of ferrocene and dimethylaminomethylferrocene into α-tin(IV) hydrogen phosphate (SnP) and α-vanadyl phosphate has been investigated. Successful intercalation of 0.81 mol of dimethylamino-methylferrocene into α-SnP by an acid-base reaction in aqueous medium to form a bilayer of protonated amines was achieved. However, ferrocene was not intercalated under the same conditions. Intercalation of α-vanadyl phosphate by 0.11 mol of ferrocene in acetonic medium at room temperature was effected by a redox topotactic reaction. The voluminous dimethylaminomethylferrocene was not intercalated into α-vanadyl phosphate.

Synthesis of mixed zirconium-titanium phosphates by sol-gel processing

Mixed crystalline alpha zirconium- titanium phosphates with variable zirconium to titanium molar ratios have been prepared by the sol-gel process, using as precursors Zr[O(CH2)2CH3]4 and Ti[OCH(CH3)2]4 mixed at differents ratios which are hydrolized with phosphoric acid. Chemical analysis, X-ray diffraction, TEM-AEM, XPS and thermal analysis were used to characterize these materials. A broad range of titanium-zirconium phosphates solid solutions were found to form.

Silica pillared metal(IV) phosphate materials as supports for nickel catalysts

Two silica pillared phosphate materials containing different amounts of silica have been used as supports to prepare, by impregnation, two series of metallic nickel catalysts (8, 10 and 15% loading). Thermal programmed reduction curves of nickel show two peaks at ca. 690 and 838 K denoting two different sites for Ni 2+ ions assigned to the external and inner surface, respectively. These high reduction temperatures indicate high dispersion and strong interactions with the supports. XPS analyses show that the reduction degree at 873 K (2 h) ranges between 22.5 and 30%. The metal dispersion and particle size were calculated from hydrogen adsorption isotherms at 298 K. These nickel catalysts are active in the hydrogenation of benzene (at 443 K). In both series of catalysts no correlation between activities and metal dispersion was found, and turnover frequencies indicate an apparent structure sensitivity of the reaction, likely to be due to the presence of superficial unreduced nickel ions. TEM micrographs reveal that deactivation of catalyst occurs by formation of coke and filaments of carbon separating the nickel particles from the pillared materials.

Porous silica pillared α-ZrTi phosphate-phosphonates materials

Abstract By the sol-gel method, there have been prepared different phosphate-phosphonates of zirconium-titanium with formula Zr 0.75 Ti 0.25 (HPO 4 ) 2− x (C 6 H 5 PO 3 ) x , X varies from 0.65 to 1.8. The intercalation of hydrolysed derivatives from aminopropyl-triethoxysilane, NH 2 (CH 2 ) 3 Si(OC 2 H 5 ) 3 , into these compounds gives rise, after heating at 773 K, to the formation of silic pillars between the layers, with basal spacing approximately 14.5 A, surface area up to 149m 2 /g and high acidity (1.1-1.6 mmol NH 3 g −1 between 373–673 K).

Epitactic ion-exchange reactions into vanadyl(IV) arsenate

Abstract The synthesis, structural characterization, thermal stability, and spectroscopic (IR, UV-vis-diffuse reflectance) properties of three vanadyl arsenates are described. Vanadyl(IV) bis(dihydrogenarsenate), [VO(H2AsO4)2] (I), lithium vanadyl arsenate, [(Li4VO(AsO4)2 · 0.5H2O] (II), and nickel(II) and lithium vanadyl arsenate, [(Li2.4Ni0.8VO(AsO4)2 · 4H2O] (III), have been prepared. (I) Tetragonal (a = 9.128 A; c = 8.128 A) is prepared by reduction with isobutanol or ethanol from vanadyl(V) arsenate. (II) Cubic (a = 9.024 A) is obtained from (I) by lithium ion-exchange, and (III) tetragonal (a = 9.106 A; c = 8.454 A) is made from (II) by Ni2+ ion-exchange. These exchange reactions are epitactic and the overall result is a topotactic transformation.

Interaction of aniline and benzidine with layered solids Mn+(UO2XO4) n − ·z H2O[M = H3O+, Cu2+, VO2+, Fe2+; X = P, As]

The layered acid solids of formula H3OUO2XO4· 3 H2O (X = As, P) intercalate aniline and benzidine arylamines, by protonation of the guest molecules. The intercalates maintain the original laminar structure.The insertion of aniline and benzidinein the metal derivativesM(UO2XO4)2·n H2O (M = Cu, VO, Fe) requires rather drastic conditions. Near and medium infrared spectra of intercalates in which M = VO2+ and Cu2+, indicate that the polymerization and/or oxidation of sorbed amines occurs; however, the guest-host reactions for Fe2+-derivatives are of the acid-base type.

Reaction of niobium phosphate with hydrazine: structural modification

The reaction between hydrazine and NbOPO4·nH2O (n<3) gives an amorphous solid with a high thermal and hydrolitic stability. The presence of hydrazine in the solid has been detected by several bands in the IR spectrum between 1500–1550 cm−1 and 3300–3400 cm−1. The solid maintains its amorphous nature after the elimination of hydrazine at 600°C, and below the crystallization temperature (800°C) we have an amorphous niobium phosphate free of hydrazine possibly with catalytic properties.

Structural and optical properties of hydrated lanthanon uranyl arsenates

Abstract An investigation of the structural and optical properties of the lamellar lanthanon derivatives of hydrogen uranyl arsenate (HUO 2 AsO 4 · 4H 2 O (HUAs)) is reported. The hydrated lanthanon uranyl arsenate compounds have the approximate composition Ln ( UO 2 AsO 4 ) 3 · xH 2 O (LnUAs) ( Ln ≡ La , Ce , Pr , Nd , Sm , Eu , Gd , Tb , Dy , Ho , Er , Tm , Yb and Lu , x ≈ 13–16) and are prepared by intercalation of the lanthanon ions into either HUAs or n -butylammonium uranyl arsenate. Upon intercalation of the tripositive guest ions, the lattice symmetry and crystallinity are reduced, as observed by X-ray powder diffraction. The LnUAs samples exhibit emission characteristic of the UO 2 2+ moiety. Those samples having Ln ≡ Ce , Pr , Nd , Sm , Ho and Er are very weakly emissive, whereas the remaining solids (with Ln ≡ La , Eu , Gd , Tb , Dy , Tm , Yb and Lu ) emit strongly at 298 K when excited with near-UV light. The emissive LnUAs samples have photoluminescence (PL) decay curves that are exponential and yield lifetimes for the strongly emissive samples in the approximate range of 8–100 μs with quantum yields of about 0.02–0.09. From these data, unimolecular radiative and non-radiative rate constants have been calculated for uranyl PL, yielding values of about (5–30) × 10 2 s −1 and about (1–10) × 10 4 s −1 respectively. For EuUAs there is extensive host-to-guest energy transfer, resulting in long-lived emission from the europium guest species. Electronic absorption and PL properties of the LnUAs samples are similar to those of their Ln ( UO 2 PO 4 ) 3 · xH 2 O counterparts