A. Martínez-de la Cruz

Synthesis of β-MoO3 by vacuum drying and its structural and electrochemical characterisation

Abstract The β-MoO 3 was obtained successfully free of α-MoO 3 through soft chemistry methods. The formation of β-MoO 3 with high purity was determined by the formation of the precursor MoO 3 ·2H 2 O when a solution of Na 2 MoO 4 ·2H 2 O was passed through a cation-exchange resin. A structural, spectroscopic and thermal study of the polymorph synthesised was made by XRD, electron dispersion spectroscopy (EDS), FTIR and TGA/DTA techniques, respectively, in order to make a study about the possibilities of β-MoO 3 as active material in a lithium battery. Electrochemical experiments showed a high ability of the β-MoO 3 to form lithium molybdenum bronzes via a lithium insertion reaction.

Electrochemical lithium insertion in some niobates MNb2O6 (M=Mn, Co, Ni, Cu, Zn and Cd)

Abstract Several oxides with the general formula MNb 2 O 6 (M=Mn, Co, Ni, Cu, Zn and Cd) have been tested as cathodes for lithium batteries. When discharging electrochemical cells using these niobates as active materials down 0.5 V, the maximum observed lithium content ( x ) in Li x MNb 2 O 6 was x =0.9, 0.9, 1.15, 1.5, 2.3 and 3.0 for M=Mn, Co, Zn, Cd, Ni and Cu, respectively. In situ X-ray diffraction experiments revealed in all cases a certain loss of crystallinity as the insertion reaction proceeds. Only the copper niobate showed an almost complete amorphization after the initial discharge. The kinetics of lithium insertion in these compounds was studied by electrochemical spectroscopy in a stepping potential mode.

Electrochemical lithium insertion in some nickel, zinc and cadmium vanadates

Abstract In this work we present a study of the electrochemical characteristics of lithium insertion in eight crystalline Ni, Zn and Cd vanadates. During the electrochemical study carried out down to 0.5 V vs. Li + /Li, CdV 2 O 6 and Cd 2 V 2 O 7 were found to be the compounds tested accepting the highest number of lithium atoms per metal atom (Li/∑M=1.75 and 1.63, respectively). The zinc and nickel vanadates tested accepted a smaller number of lithium atoms per formula unit. In situ X-ray diffraction experiments showed an almost complete amorphization of the cadmium vanadates at the end of the first discharge while nickel and zinc vanadates were not amorphous. Therefore, these results are in agreement with previous reports of larger lithium atom intake in amorphous than in crystalline materials. In any case, on cycling none of these vanadates perform as well as previously described vanadium compounds.

Lithium and sodium insertion in W3Nb14O44, a block structure type phase

Abstract A study of lithium and sodium insertion in W 3 Nb 14 O 44 , a block structure type phase, has been carried out. During the electrochemical insertion, the formation of several solid solution regions of general formula Li x W 3 Nb 14 O 44 (0 ≤ x ≤ 28) and Na y W 3 Nb 14 O 44 (0 ≤ y ≤ 0.5), have been observed. Structural changes originated by lithium insertion in the parent oxide, have been evaluated by X-ray powder diffraction.

A study of lithium insertion in W4Nb26O77: Synthesis and characterization of new phases

Abstract A study of lithium insertion in W 4 Nb 26 O 77 has been carried out. When lithium is inserted following an electrochemical route, several single-phase regions are formed in a reversible reaction. Some lithiated phases included in these regions have been synthesized by chemical reaction and characterized by X-ray powder diffraction.

Lithium insertion in two tetragonal tungsten bronze type phases, M8W9O47 (M=Nb and Ta)

A study of lithium insertion in two tetragonal tungsten bronze (TTB) type phases of general formula M 8 W 9 O 47 (M=Nb and Ta), is presented. The electrochemical insertion of up to 20 lithium atoms per formula unit in Nb 8 W 9 O 47 (Li/ΣM=1.2) proceeds through a reversible reaction with several single phase and one two-phase domains, while in Ta8W9O47 the reversibility of lithium insertion is limited to 15 atoms (Li/Sigma;M=0.9). Structural changes on Nb 8 W 9 O 47 as a function of the number of lithium atoms inserted have been studied by X-ray powder diffraction.

Enhancement of photocatalytic properties of TiO2 for NO photo-oxidation by optimized sol–gel synthesis

TiO2 samples were prepared by the sol–gel method applying a factorial design in order to improve the photocatalytic properties of the semiconductor oxide for the nitric oxide (NO) photo-oxidation reaction. The temperature of calcination and the amount of alcohol and acid used in the course of the sol–gel reaction were selected as critical experimental variables. As the products of the factorial design, 27 TiO2 samples were obtained and characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy, and adsorption–desorption N2 isotherms. The photocatalytic activity of the TiO2 samples was evaluated in the NO photo-oxidation reaction under UV irradiation. The conversion degree of NO reached by each sample was associated with its physicochemical properties, finding a stronger dependence with the temperature of calcination and morphology of the samples. Different photocatalytic reaction parameters were modified such as mass of photocatalyst, irradiance, flow rate of gas, and relative humidity in order to evaluate their effect in the constants of velocity and adsorption. The TiO2 sample with the highest photocatalytic activity was exposed under different experimental conditions to evaluate its selectivity for the formation of innocuous nitrate ions as final product of the NO photo-oxidation reaction.

Electrochemical study of the reaction of lithium with Aurivillius and related phases

Abstract Some Aurivillius and Sillen phases have been tested as positive electrode materials for secondary lithium batteries through an electrochemical study. We have carried out an electrochemical and structural study of the lithium reaction with some of the most representative phases of the systems WO 3 -Bi 2 O 3 , MoO 3 -Bi 2 O 3 , and Nb 2 O 5 -Bi 2 O 3 . Although the amount of lithium atoms incorporated in all cases was large, due to irreversible structural transformations in the matrix of the host, specific capacity of the cells were dramatically lost after the first cycle. On the basis of in situ X-ray diffraction data of lithiated phases, we propose a mechanism of reaction.

Photocatalytic Activity of Doped NaTaO3 and NaNbO3 : (Y, La, Nd, Sm) Sol Gel on Degradation of Rhodamine B by UV Irradiation

The catalytic photodegradation of Rhodamine B with UV irradiation over NaMO3 (M= Ta and Nb) doped with Y2O3, La2O3, Nd2O3 and Sm2O3 has been studied in our laboratories. All catalysts were characterized by different techniques such as X-ray powder diffraction (XRD) and thermal analysis (DTA/TGA). Surface characterization of the solids was carried out from adsorption isotherms and infrared spectroscopy (FTIR). The band gap energy (Eg) of materials ranged from 4.1 to 2.9 eV. For comparative purpose the materials studied as catalysts were also synthesized by classical solid state reaction.

Performance of ZnO synthesized by sol-gel as photocatalyst in the photooxidation reaction of NO

ZnO samples were prepared by sol-gel method applying a factorial design in order to improve the photocatalytic properties of the semiconductor oxide in the NO photooxidation reaction. The concentrations of zinc acetate and ammonium hydroxide were selected as critical variables in the synthesis of ZnO. Nine samples of ZnO were obtained as product of the factorial design and were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy, and N2 adsorption-desorption isotherms. The photocatalytic activity of ZnO samples was associated with the physical properties developed by each sample according to its respective conditions of synthesis. Some photocatalytic reaction parameters, such as mass of photocatalyst, irradiance, and relative humidity, were modified in order to evaluate its effect in the photocatalytic conversion of NO. As a relevant point, the relative humidity played an important role in the increase of the selectivity of the NO photooxidation reaction to innocuous nitrate ions when ZnO was used as photocatalyst.