Jose Manuel Gallardo Amores

Characterization of manganese and iron oxides as combustion catalysts for propane and propene

Abstract Mn2O3–Fe2O3 powders have been prepared by a coprecipitation method. The pure compounds have been characterized as constituted of α-Fe2O3 (haematite) and α-Mn2O3 (bixbyite) while the mixed oxides are constituted of a mixture of haematite- and bixbyite-structure solid solutions. The observed reciprocal solubilities calculated from the XRD patterns approach the thermodynamic ones. α-Mn2O3 is more active than α-Fe2O3 as catalyst for both propane and propene oxidation. However, α-Mn2O3 is less active than Mn3O4 powder. Propene oxidation is in all cases very selective to CO2 while propane oxidation gives rise to significant amounts of propene on α-Fe2O3. Mn2O3–Fe2O3 powders are slightly more active than α-Mn2O3 as combustion catalysts. The selectivities to propene upon propane oxidation decrease with increasing Mn content.

FT Raman and FTIR studies of titanias and metatitanate powders

FT Raman and FTIR/FTFIR skeletal spectra of different TiO2 powders, of both synthetic and commercial origin, and of Sr, Ba, Co and Ni metatitanates are reported and discussed in relation to the predictions of factor group analyses. The ability of vibrational techniques to show the presence of brookite impurities in both anatase and rutile, and of rutile in anatase, as well as to give morphological and surface information is emphasized. The spectra of SrTiO3, BaTiO3, NiTiO3 and CoTiO3 are also discussed in relation to their different structures (pervoskite and ilmenite-type).

Anatase crystal growth and phase transformation to rutile in high-area TiO2, MoO3–TiO2 and other TiO2-supported oxide catalytic systems

Anatase-to-rutile phase transition has been studied on high-area TiO2-anatase and bicomponent systems containing molybdena and, for comparison, cobalt oxide, copper oxide, vanadia and corresponding TiO2-rutile based systems, using TG–DTA, XRD and surface-area measurements. These materials were prepared by impregnation and (co)precipitation methods. Conversion to rutile and crystal size growth strongly depend on the minority phase oxide. It is found that, while Cu and V oxides speed up both phase transition and particle sintering, Mo and Co oxides inhibit them. Moreover, the rutile particles obtained by phase transition are always much larger than both the starting and residual anatase particles. A mechanism of rutile formation by coalescence and phase transformation of anatase particles is proposed.

Solid-state and surface chemistry of CuO–TiO2(anatase) powders

CuO–TiO2 samples have been prepared by impregnation of two different preformed TiO2 supports with copper nitrate, and have been characterized by X-ray diffraction (XRD), thermogravimetry–differential thermal analysis (TG–DTA), Fourier-transform infrared (FTIR), Fourier-transform far-infrared (FT-FIR) and ultraviolet–visible (UV-VIS) spectroscopies. The surface properties have been investigated through surface area and porosity measurements, and FTIR spectra of surface OH groups and of adsorbed carbon monoxide at low temperature. The surface is thought to consist of complexes of Cu2+ and Cu+, involving surface Ti–O groups of the support as the ligands. These complexes can be reduced by CO under very mild conditions with the appearance of small Cu metal clusters. Ti4+ centres are also in part exposed on the surface, but their electron-withdrawing effect is weakened by the surface copper complexes. Surface copper ions act as electron-donor centres, giving rise to absorption over in the entire visible range, with a parallel weakening of the TiO2, absorption edge. Surface copper complexes have a pronounced accelerating effect on the anatase-to-rutile phase transition. A parallelism between the CuO–TiO2 and the V2O5–TiO2 systems is proposed.

Characterization of coprecipitated Fe2O3–Al2O3 powders

Powders belonging to the Fe2O3–Al2O3 system prepared by a coprecipitation method have been characterized from the point of view of their solid-state and surface structure by X-ray diffraction (XRD), Fourier-transform (far) infrared (FTIR/FT-FIR), diffuse reflectance ultraviolet–visible (DR-UV–VIS) Spectroscopy, BET surface area and porosity and adsorption of probe molecules. In samples calcined at 673 K a disordered defective spinel-type phase, γ-FeAlO3, has been found, containing Fe3+ in octahedral sites and Al3+ in both octahedral and tetrahedral sites, both in the bulk and on the surface. Samples calcined at 1173 K are composed of mixtures of two saturated solid solutions with corundum–haematite-type structures. The surface of these biphasic materials is dominated by the higher reactivity of α-Fe2O3 with respect to α-Al2O3, although tetrahedral Al3+ is also found at the surface of the saturated solid solution of Fe2O3 in α-Al2O3. Al3+ added to Fe2O3 tends to stabilize the spinel-type phase and to hinder sintering, while Fe3+ added to Al2O3 accelerates the γ→α phase transition and sintering.

Preparation, characterization and surface structure of coprecipitated high-area Srx TiO2 +x(0 ⩽x⩽ 1) powders

Powders with composition SrxTiO2 +x(x= 0, 0.02, 0.11, 0.42 and 1) have been prepared by coprecipitation from strontium hydroxide and titanium isopropoxide and have been characterized after drying at 393 K, and after calcination at 773 and 973 K, using XRD, TG–DTA, BET surface area and porosity measurements, FTIR–FTFIR and FT-Raman skeletal vibrational characterization, diffuse reflectance UV–VIS spectroscopy and FTIR spectra of the hydroxy groups and of adsorbed pyridine nad CO2. Strontium was found to inhibit anatase crystallization, sintering and transformation to rutile. Strontium tended to be deposited at the surface of anatase and gave rise to a decrease of the surface acidity and an increase of surface basicity. Moreover, it shifted the absorption edge of anatase to higher energies. For x= 1, the truly cubic SrTiO3 pervoskite phase was produced at room temperature with high surface area. The surface of SrTiO3 was definitely basic, with only strontium and oxygen ions exposed at the surface, and hydrogen impurities in the bulk.

Characterization of α-(Fe,Al)2O3 solid-solution powders

Monophasic powders constituting α-(Fe,Al)2O3 solid solutions have been prepared by a coprecipitation method and have been characterized, from the point of view of their solid-state and surface structures, by X-ray diffraction (XRD), Fourier-transform (far) infrared (FTIR/FT-FIR) and diffuse reflectance ultraviolet–visible (DR-UV–VIS) spectroscopy. BET surface area and porosity and adsorption of probe molecules. Their bulk and surface properties have been compared with those observed for pure α-Al2O3 and pure α-Fe2O3 as well as with biphasic materials constituting mixtures of two α-(Fe,Al)2O3 saturated solid solutions, with overall compositions external to the solubility limits of these phases. The maximum solubility of Fe2O3 in α-Al2O3 is near 11%, while the maximum solubility of Al2O3 in α-Fe2O3 does not exceed 7%, for samples calcined at 1173 K. Metastable solid-solution phases with higher solute concentrations can be obtained if the calcination temperature is limited to 673 K. Al3+ added to α-Fe2O3 tends to hinder sintering and stabilize the pore structure, while Fe3+ added to α-Al2O3 accelerates the γ→α phase transition and causes a reduction in the surface area. In spite of their structures, which forecast only octahedral cation coordination, tetrahedral Al3+ and Fe3+ rations are found, mainly at the surfaces of the solid solutions.

Synthesis and characterization of Fe-Ga mixed hydroxide powders

Several samples of iron gallium mixed oxyhydroxides with stoichiometry Fe 1–x Ga x (with x=0, 0.10, 0.25, 0.50, 0.75, 0.90) have been synthesized by a coprecipitation method. They have been characterised from the point of view of their solid state structure by X-ray diffraction (XRD), thermogravimetry and differential thermal analyses (TG-DTA), FTIR and diffuse reflectance (DR) UV-VIS spectroscopy. After drying at 393 K, only one crystalline phase denoted as α-(Fe,Ga)OOH, is found in the samples, i.e. a solid solution in the whole composition range in agreement with the continuous lowering of the unit cell parameter. When Ga is added in small amounts to α-FeOOH, it tends to notably hinder α-(Fe,Ga)OOH crystallisation. The electronic spectra suggest that clustering of Fe 3+ in dilute samples may occur. Low-temperature thermal decomposition of such mixed oxyhydroxides gives rise to corundum-hematite type solid solutions in the whole compositional range.

VO2F: a new transition metal oxyfluoride with high specific capacity for Li ion batteries

Hitherto unreported vanadium oxyfluoride VO2F has been synthesized using a solid state reaction at a pressure of 4 GPa and 800 °C. This long awaited vanadium oxyfluoride fills the existing gap of ReO3-type MO2F compounds of Group 5 elements, from which only NbO2F and TaO2F have been known to exist to date. VO2F crystallizes with the VF3-type structure, space group Rc, with a = 5.1226(1) A and c = 13.0686(3) A as determined by powder X-ray diffraction. Highly structured diffuse streaking observed in electron diffraction patterns evidences local O/F ordering. VO2F exhibits two regions upon discharge in a lithium cell, an upper sloped region in the range of 3.9–2.2 V and a lower plateau at 2.15 V. Discharge of VO2F to 1 V provides a gravimetric capacity of 450 mA h g−1. VO2F can reversibly insert up to 1 Li+ per vanadium above 2.15 V without destruction of the host structure, delivering a gravimetric capacity as high as 250 mA h g−1 and pointing to VO2F as a promising intercalation electrode.