Horacio Falcón

On the correlation of Ni oxidation states and electronic conductivity of (R,A)NiO3-δ (R=lanthanides, A=alkaline earths, Th) perovskites with catalytic activity for H2O2 decomposition

Several Ni-containing perovskite-related oxides of stoichiometries RNiO3 (R=La, Pr, Eu), La1-xAxNiO3 (A=Sr, Th), LaNiO2.5, BaNiO2 and BaNiO3 were studied as catalysts for hydrogen peroxide decomposition. In all cases the samples were single-phase materials characterized structurally by X-ray and neutron power diffraction. In these perovskites, Ni cations exhibited oxidation states between 2+ and 4+, as previously determined by thermogravimetric (TG) analysis under reducing conditions. In a set of samples prepared at 1000°C under 200 bar of O2 pressure, the catalytic activity at 30°C, measured by the gasometric method, showed a maximum value for La0.9Sr0.1NiO3, containing 90% of Ni3+ and 10% of Ni4+. However, the activity was very poor for BaNiO3, with 100% Ni4+, due to the insulating character of this oxide. The activity rate of PrNiO3 and EuNiO3, showing characteristic metal–insulator transitions as a function of temperature, was much higher for PrNiO3, which is in the metallic regime at the measuring temperature, whereas EuNiO3 remained in the semiconducting (insulating) regime. A linear relationship between the pre-exponential factor and the apparent activation energy, known as the compensation effect, was found for the metallic samples, which suggests that the catalytic surface is heterogeneous, showing active sites with different activation energies.

Ferromagnetic behavior in La(Cu3−xMnx)Mn4O12 (x=1,2) perovskites

High oxygen pressure techniques were used to prepare two ferromagnetic compounds of the series La(Cu3−xMnx)Mn4O12 (x=1 and x=2). In a previous work, the end member LaCu3Mn4O12 was reported to be cubic, space group Im3¯, showing a ferrimagnetic structure driven by antiferromagnetic coupling between Cu and Mn magnetic moments below TC=361 K. The partial replacement of Cu2+ by Mn3+ cations does not change the symmetry or space group of La(Cu3−xMnx)Mn4O12. However, the introduction of Mn3+ cations at the square-planar 6b site, occupied by Cu, increases the valence mixing of Mn3+ and Mn4+ ions at the octahedral 8c site. This drives to an increase in the bonding distances in the MnO6 octahedron and a decrease of the Cu–O distance in the CuO4 polyhedron. Susceptibility measurements indicate that La(Cu3−xMnx)Mn4O12 becomes magnetically ordered below TC=310 K for x=1 and below TC=150 K for x=2. Neutron powder diffraction (NPD) data reveal a parallel arrangement between the (Cu/Mn)6b and Mn8c moments, in contrast w...

Preparation under high pressures and neutron diffraction study of new ferromagnetic RCu3Mn4O12 (R = Pr, Sm, Eu, Gd, Dy, Ho, Tm, Yb) perovskites

We report on the study of some new derivatives of the complex perovskite CaCu 3 Mn 4 O 12 . Ca 2+ cations can be replaced by rare earths in the RCu 3 Mn 4 O 12 (R = rare earths) family, implying an electron doping effect that dramatically affects the magnetic properties. These compounds must be prepared under high pressure conditions. We have synthesized for the first time, at 2 GPa in a piston-cylinder press, some new members of the family, with R = Pr, Sm, Eu, Gd, Dy, Ho, Tm and Yb, for which we describe structural and magnetization data. The materials with non-absorbing rare earths have been studied by means of neutron powder diffraction, allowing us to determine the subtle structural features which are related to the observed physical properties. In fact, for most of the R cations, the ferromagnetic T C increases remarkably up to 400 K.

High-pressure synthesis and study of the crystal and magnetic structure of the distorted SeNiO3 and SeMnO3 perovskites

We describe the preparation of SeMO(3) (M = Ni, Mn) under high pressure conditions (3.5 GPa), starting from reactive H(2)SeO(3) and MO mixtures, contained in sealed gold capsules under the reaction conditions 850 degrees C for 1 h. The polycrystalline samples have been studied by neutron powder diffraction (NPD) data and magnetization measurements. SeMO(3) (M = Ni, Mn) are orthorhombically distorted perovskites (space group Pnma). Below T(N) approximately 104 K (M = Ni) and T(N) approximately 53.5 K (M = Mn) these oxides experience an antiferromagnetic ordering, as demonstrated by susceptibility and NPD measurements. The magnetic reflections observed in the neutron patterns can be indexed with a propagation vector k = 0. Both compounds present the same magnetic structure, which is given by the basis vector (0, 0, A(z)). It can be described as antiferromagnetic (010) layers of magnetic moments lying along the c direction, which are antiferromagnetically coupled along the b direction. For the Ni(2+) ions, the ordered magnetic moment at T = 2.3 K is 2.11(3) micro(B), whereas for Mn(2+) at T = 2.6 K, |m| = 4.64(2) micro(B), consistent with the electronic configurations te (Ni(2+)) and te (Mn(2+)).

Probing the Catalytic Activity of Sulfate-Derived Pristine and Post-Treated Porous TiO2(101) Anatase Mesocrystals by the Oxidative Desulfurization of Dibenzothiophenes

Mesocrystals (basically nanostructures showing alignment of nanocrystals well beyond crystal size) are attracting considerable attention for modeling and optimization of functionalities. However, for surface-driven applications (heterogeneous catalysis), only those mesocrystals with excellent textural properties are expected to fulfill their potential. This is especially true for oxidative desulfuration of dibenzothiophenes (hard to desulfurize organosulfur compounds found in fossil fuels). Here, we probe the catalytic activity of anatases for the oxidative desulfuration of dibenzothiophenes under atmospheric pressure and mild temperatures. Specifically, for this study, we have taken advantage of the high stability of the (101) anatase surface to obtain a variety of uniform colloidal mesocrystals (approximately 50 nm) with adequate orientational order and good textural properties (pores around 3–4 nm and surface areas around 200 m2/g). Ultimately, this stability has allowed us to compare the catalytic activity of anatases that expose a high number of aligned single crystal-like surfaces while differing in controllable surface characteristics. Thus, we have established that the type of tetrahedral coordination observed in these anatase mesocrystals is not essential for oxidative desulfuration and that both elimination of sulfates and good textural properties significantly improve the catalytic activity. Furthermore, the most active mesocrystals have been used to model the catalytic reaction in three-(oil–solvent–catalyst) and two-phase (solvent–catalyst) systems. Thus, we have been able to observe that the transfer of DBT from the oil to the solvent phase partially limits the oxidative process and to estimate an apparent activation energy for the oxidative desulfuration reaction of approximately 40 kJ/mol in the two-phase system to avoid mass transfer limitations. Our results clearly establish that (101) anatase mesocrystals with excellent textural properties show adequate stability to withstand several post-treatments without losing their initial mesocrystalline character and therefore could serve as models for catalytic processes different from the one studied here.

Microwave-Assisted Coprecipitation Synthesis of LaCoO3 Nanoparticles and Their Catalytic Activity for Syngas Production by Partial Oxidation of Methane

LaCoO3 perovskite-type oxides were prepared by microwave-assisted coprecipitation route and investigated in the catalytic partial oxidation of methane (CPOM) to syngas. This preparation method aims to achieve higher specific surface areas than soft-chemical methods commonly used in the preparation of engineered materials. In an attempt to accomplish the creation of mesostructured porous LaCoO3, an ionic template such as cetyl trimethyl ammonium bromide has been used as endotemplate in some samples. The influence of pH and the type of precipitating agent has been studied. The materials have been characterized at different levels: morphology has been studied by scanning electron microscopy, textural properties by nitrogen adsorption-desorption at -196oC, structural analysis by X-ray diffraction, surface composition by X-ray photoelectron spectroscopy (XPS), thermal stability by Thermogravimetric Analysis (TGA) and carbon formation in spent catalysts, by Raman spectroscopy. Structure-activity correlations point out that the precipitating agent has a key role on the morphology and porosity of the resultant oxide, as well as on the average crystalline domain of lanthanum perovskite (catalyst precursor). Thus, the use of ammonium hydroxide as precipitant leads to materials with a higher surface area and a greater specific surface area of cobalt (per unit mass), improving their catalytic performance for the CPOM reaction. The best catalytic performance was found for the catalyst prepared using ammonium hydroxide as precipitant (pH 9) and without adding CTAB as endotemplate.

Moderate-pressure Synthesis and Neutron Diffraction Study of New Metastable Oxides

We have synthesized two new series of metastable oxides, namely REMn2O5 and RECu3Mn4O12 (RE = rare earths) under moderate pressure conditions. A novel series of ferrimagnetic oxides has been obtained by replacing Mn3+ by Fe3+ in the parent REMn2O5 compounds (RE = Y, Dy, Ho, Er, Tm, Yb). The crystal structure has been studied by neutron powder diffraction (NPD); it contains chains of edge-linked Mn4+O6 octahedra connected via dimeric groups of Fe3+O5 square pyramids. The magnetic susceptibility and the thermal evolution of the NPD patterns reveal the onset of a ferrimagnetic structure below TC ≈ 165 K, characterized by the propagation vector k = 0. Immediately below TC, the Fe3+ and Mn4+ moments lie in an antiparallel arrangement along the c-axis direction. At lower temperatures, the magnetic moment of the rare-earth cations also participates in the magnetic structure, adopting a parallel arrangement with the Fe3+ spins. Some new derivatives of CaCu3Mn4O12 have been prepared at moderate pressures of 2 GPa by replacing Ca2+ by RE3+ cations in the series RECu3Mn4O12 (RE = Pr, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb); the concomitant electronic injection leads to a substantial contribution to TC. The crystal structures of the new materials were refined in the space group Im3̅ from NPD data for the non-absorbing RE cations. The unit cell parameters are considerably expanded with respect to CaCu3Mn4O12, as a result of the electronic injection. The r. t. magnetic structure displays a ferrimagnetic coupling between Mn3+/4+ and Cu2+ spins; at low temperatures there is an antiferromagnetic coupling of the RE3+ moments with the Mn substructure, which substantially reduces the susceptibility and the saturation magnetization.