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A simulation of the mass-transfer effects on the kinetics of solid-gas reactions

Ortega, A
International Journal of Chemical Kinetics, 40 (2008) 217-222


A theoretical analysis of the influence of mass-transfer effect on the kinetic of solid-gas reactions has been carried out by assuming that the partial pressures of the gases generated in the reaction are proportional to the reaction rate. The influence of mass-transfer phenomena on the apparent activation energy, calculated by the isoconversional methods of Friedman, and on the reaction model is discussed. In the present study, simulated nonisotherm, isotherm, and controlled rate thermal analysis (CRTA) data have been used. Master plots based on the differential forms of the kinetic equations describing solid-state reactions have been employed by using the concept of the generalized time (theta), introduced by Ozawa; this permits the application of these master plots to the kinetic analysis of reactions whatever the type of temperature program used for recording the experimental data. it has been shown that when the simulated mass-transfer effect is present the variable effective activation energy E remains nearly constant while the reaction model approaches zero order.

Abril, 2008 | DOI: 10.1002/kin.20308

Transport properties and microstructure changes of talc characterized by emanation thermal analysis

Perez-Maqueda, LA; Balek, V; Poyato, J; Subrt, J; Benes, M; Ramirez-Valle, V; Buntseva, IM; Beckman, IN; Perez-Rodriguez, JL
Journal of Thermal Analysis and Calorimetry, 92 (2008) 253-258
Reactividad de Sólidos


Thermal behavior of talc samples (from locality Puebla de Lillo, Spain) were characterized by emanation thermal analysis (ETA), DTA and TG. The ETA, based on the measurement of radon release rate from samples, revealed a closing up of surface micro-cracks and annealing of microstructure irregularities of the talc samples on heating in the range 200-500 degrees C.

For ground talc sample a crystallization of non-crystalline phase formed by grinding, into orthorhombic enstatite was characterized as a decrease of radon mobility in the range 785-825 degrees C and by a DTA exothermal effect with the maximum at 830 degrees C. ETA results characterized the microstructure development of the talc samples on heating and served to evaluate their radon mobility and transport properties on heating and cooling. Transport properties of the talc samples were evaluated by using ETA experimental data measured during heating to 600 and 1300 degrees C, respectively, and subsequent cooling to room temperature.

Abril, 2008 | DOI: 10.1007/s10973-007-8819-8

Critical study of the isoconversional methods of kinetic analysis

Criado, J. M.; Sanchez-Jimenez, P. E.; Perez-Maqueda, L. A.
Journal of Thermal Analysis and Calorimetry, 92 (2008) 199-203
Reactividad de Sólidos


A critical study of the use of isoconversional methods for the kinetic analysis of non-isothermal data corresponding to processes with either a real or an apparent variation of the activation energy, E, with the reacted fraction, alpha, has been carried out using for the first time simulated curves. It has been shown that the activation energies obtained from model-free methods are independent of the heating rate. However, the activation energy shows a very strong dependence of the range of heating rates used for simulating the curves if the apparent change of E with alpha is caused by overlapping processes with different individual activation energies. This criterion perhaps could be used for determining if a real dependence between E and alpha is really occurring.

Abril, 2008 | DOI: 10.1007/s10973-007-8763-7

A universal curve for the magnetocaloric effect: an analysis based on scaling relations

Colodrero, Silvia; Ocana, Manuel; Miguez, Hernan
Langmuir, 24, (2008) 4430–4434
Materiales Coloidales - Materiales Ópticos Multifuncionales


Herein we present a fast, reliable method for building nanoparticle-based 1D photonic crystals in which a periodic modulation of the refractive index is built by alternating different types of nanoparticles and by controlling the level of porosity of each layer. The versatility of the method is further confirmed by building up optically doped photonic crystals in which the opening of transmission windows due to the creation of defect states in the gap is demonstrated. The potential of this new type of structure as a sensing material is illustrated by analyzing the specific color changes induced by the infiltration of solvents of different refractive indexes.

Marzo, 2008 | DOI: 10.1021/la703987r

Nanostructured Ti-M mixed-metal oxides: Toward a visible light-driven photocatalyst

Kubacka, A.; Fernandez-Garcia, M.; Colon, G.
Journal of Catalysis, 254 (2008) 272-284
Materiales y Procesos Catalíticos de Interés Ambiental y Energético


In this report we investigate the structure-activity relationship in Ti-M (M = V, Mo, Nb, W) mixed-metal oxides with anatase structure used for the photoelimination of toluene under sunlight-type excitation. These systems were prepared by a microemulsion method, and their physicochemical properties were characterized by a multitechnique approach using X-ray diffraction-Rietveld, photoelectron spectroscopy, and Raman and UV-visible spectroscopy. The preparation method allowed the incorporation of up to around 20 at % of Mo, Nb, and W, whereas a significantly inferior solubility limit, below 5 at % was observed for V. The presence of nonpunctual defects, intimately related to the existence of MOx clustering, produced electronic mid-gap states involved in charge recombination and appeared to be the most negative factor influencing photoactivity. The maximization of photoactivity occurred for Ti-M samples with the highest doing level, together with a minimum structural disturbance of the anatase-type structure. The physicochemical bases for the photoactivity behavior of the Ti-M samples as function of the M content and, in particular, the presence of partial (for a specific Ti-M series) or global maxima are discussed. 

Marzo, 2008 | DOI: 10.1016/j.jcat.2008.01.005

Sorption properties of mesoporous multilayer thin films

Fuertes, MC; Colodrero, S; Lozano, G; Gonzalez-Elipe, AR; Grosso, D; Boissiere, C; Sanchez, C; Soler-Illia, GJDAA; Miguez, H
Journal of Physical Chemistry C, 112 (2008) 3157-3163
Nanotecnología en Superficies y Plasma - Materiales Ópticos Multifuncionales


The vapor sorption properties of multilayers made of ordered mesoporous thin films with tailored composition and mesostructure are herein investigated. Optical reflectance measurements versus partial pressure of several vapors are performed to analyze the interplay between the affinity to and the accessibility of the different types of layers in the structure. We find that the behavior of a mesoporous oxide layer within the multilayer largely differs from that of the isolated thin film, its sorption properties being determined by the interaction with neighboring films. An explanation of the phenomena observed in these complex systems is provided in terms of the pore size, the affinity of each type of layer to specific compounds, and the effect of neighboring layers in the sorption properties of bilayers by an independent environmental ellipsometric study.

Marzo, 2008 | DOI: 10.1021/jp710612y

Preparation of Au-CeO2 and Au-Al2O3/AISI 304 austenitic stainless steel monoliths and their performance in the catalytic oxidation of CO

Martinez, LM; Frias, DM; Centeno, MA; Paul, A; Montes, M; Odriozola, JA
Chemical Engineering Journal, 136 (2008) 390-397
Química de Superficies y Catálisis


The use of austenitic stainless steel (AISI 304) as metallic substrate for monolithic catalytic devices is explored in this work as well as an alternative procedure to the washcoating of Al2O3 and CeO2 supported Au catalysts. Thermal treatment of the metallic surface ensured a good adherence of the catalysts. Some preparation parameters, like the number of dipping and drying steps, were controlled in order to obtain an homogeneous and adherent layer of catalyst. The resulting surfaces were characterized by X-ray diffraction (XRD), N-2 Sorption and scanning electron microscopy (SEM), and were tested in the CO oxidation reaction. The highest conversion to CO2 was obtained with the monolith coated with 1% Au-CeO2 in the presence of water.

Marzo, 2008 | DOI: 10.1016/j.cej.2007.05.040

Self-assembled polyhydroxy fatty acids vesicles: a mechanism for plant cutin synthesis

Heredia-Guerrero, JA; Benitez, JJ; Heredia, A
Bioessays, 30 (2008) 273-277
Materiales de Diseño para la Energía y Medioambiente


Despite its biological importance, the mechanism of formation of cutin, the polymeric matrix of plant cuticles, has not yet been fully clarified. Here, for the first time, we show the participation in the process of lipid vesicles formed by the self-assembly of endogenous polyhydroxy fatty acids. The accumulation and fusion of these vesicles (cutinsomes) at the outer part of epidermal cell wall is proposed as the mechanism for early cuticle formation.

Marzo, 2008 | DOI: 10.1002/bies.20716

Relation between growth dynamics and the spatial distribution of intrinsic defects in self-assembled colloidal crystal films

Lozano, G; Miguez, H
Applied Physics Letters, 92 (2008) 091904
Materiales Ópticos Multifuncionales


Herein we establish a clear relation between the parameters that govern the growth dynamics and the structural quality of colloidal crystal films. We report an optical analysis of the spatial distribution of intrinsic defects in colloidal crystal films and correlate our results with a theoretical model describing the growth dynamics of such lattices. We find that the amount of defects fluctuates periodically and decreases along the growth direction of the lattice. We demonstrate that these spatial variations are a direct consequence of the temporal oscillations of the crystal film formation velocity, which are inherent to the colloidal particle deposition process.

Marzo, 2008 | DOI: 10.1063/1.2883943

Reaction-formation mechanisms and microstructure evolution of biomorphic SiC

Varela-Feria, FM; Ramirez-Rico, J; de Arellano-Lopez, AR; Martinez-Fernandez, J; Singh, M
Journal of Materials Science, 43 (2008) 933-941
Materiales de Diseño para la Energía y Medioambiente


Biomorphic SiC is fabricated by liquid Si infiltration of a carbon preform obtained from pyrolized wood that can be selected for tailored properties. The microstructure and reaction kinetics of biomorphic SiC have been investigated by means of TEM, SEM, EBSD, and partial infiltration experiments. The microstructure of the material consists of SiC and Si and a small fraction of unreacted C. The SiC follows a bimodal size distribution of grains in the micrometer and the nanometer range with no preferential orientation. The infiltration-reaction constant has been determined as 18 x 10(-3) s(-1). These observations suggest that the main mechanism for SiC formation is solution-precipitation in the first stage of growth. If the pores in the wood are small enough they can be choked by SiC grains that act as a diffusion barrier between Si and C. If that is the case, Si will diffuse through SiC forming SiC grains in the nanometer range.

Febrero, 2008 | DOI: 10.1007/s10853-007-2207-4

Interface effects in the Ni 2p x-ray photoelectron spectra of NiO thin films grown on oxide substrates

Preda, I; Gutierrez, A; Abbate, M; Yubero, F; Mendez, J; Alvarez, L; Soriano, L
Physical Review B, 77 (2008) 075411
Nanotecnología en Superficies y Plasma


We report the Ni 2p x-ray photoelectron spectra of NiO thin films grown on different oxide substrates, namely, SiO2, Al2O3, and MgO. The main line of the Ni 2p spectra is attributed to the bulk component, and the shoulder at 1.5 eV higher binding energies to the surface component. The spectra of the NiO thin films show strong differences with respect to that of bulk NiO. The energy separation between the main peak and the shoulder increases with the substrate covalence. This indicates the strong covalent interactions between the NiO thin films and the oxide substrates, and reflects changes in the bonding at the interface from a more ionic to a more covalent interaction. These conclusions are supported by cluster model calculations with a reduced O 2p-Ni 3d hybridization.

Febrero, 2008 | DOI: 10.1103/PhysRevB.77.075411

Getting more out of X2T2O7 compounds with thortveltite structure: The bond-valence model

Alba, MD; Becerro, AI; Chain, P; Escudero, A; Gonzalez-Carrascosa, T
Journal of Solid State Chemistry, 181 (2008) 340-344
Materiales de Diseño para la Energía y Medioambiente


C-M2Si2O7 (M = RE and In) and crystals of composition X2T2O7 (T = P, As, Ge) with ionic radius of X less than 0.97 angstrom (X = Ni, Cd, Mg, Zn, Cu, Ca) are isostructural with the natural-occurring mineral thortveitite. In those compounds, the T-O bridging distance values vary considerably and there is no explanation to this fact in the literature. This paper will bring their structural characteristics out by the bond-valence model. It has been concluded that T-O bridging distance and mean T-O distance are linearly correlated to the total atomic valence of the bridging oxygen and the T atom (T = Si, P, As, Ge), respectively, and they are a function of the principal quantum number (n) in the valence shell of the atom T. Finally, we have applied successfully the model for the prediction of Ge-O distances of (In,A)(2)Ge2O7 (A = Fe,Y,Gd) compounds. 

Febrero, 2008 | DOI: 10.1016/j.jssc.2007.11.027

Permanent magnetism in thiol capped nanoparticles, gold and ZnO

Crespo, P.; Garcia, M. A.; Fernandez-Pinel, E.; De la Venta, J.; Merino, J. M.; Quesada, A.; Hernando, A.; Fernandez, A.; Penades, S.
Acta Physica Polonica A, 113 (2008) 515-520
Materiales Nanoestructurados y Microestructura


In spite of the diamagnetic behavior exhibited by bulk ZnO and Au, a ferromagnetic-like behavior is induced in nanoparticles of both systems by appropriate surface functionalization. By capping with thiol derivatized molecules, magnetic hysteresis is observed even at room temperature, whereas the magnetization has a very little temperature dependence. Capping induces an alteration of their electronic configuration that depends on the capping molecule, as evidenced by X-ray absorption spectroscopy, that strongly affects their magnetic properties.

Enero, 2008 | DOI: 10.12693/APhysPolA.113.515

"in situ" XPS studies of laser induced surface cleaning and nitridation of Ti

Lahoz, R; Espinos, JP; de la Fuente, GF; Gonzalez-Elipe, AR
Surface & Coatings Technology, 202 (2008) 1486-1492
Nanotecnología en Superficies y Plasma


A titanium foil has been subjected to laser irradiation "in situ" in a pre-chamber of an X-ray photoemission spectrometer under different atmospheres (vacuum, Ar, 02, air, N-2 and H-2). As a result of these treatments, a high amount of the carbon contamination layer was removed and other changes in composition were induced. Nitridation was achieved by laser irradiation under nitrogen. The most effective treatment protocol included an initial cleaning procedure induced by irradiation in vacuum, followed by a second irradiation process performed under nitrogen. Partial nitridation is also observed when irradiating under synthetic air. Lateral and depth analysis of the nitrogen concentration around the laser spot has been also carried out. It is found that the outermost layers present a similar concentration of nitrogen. In addition, the measured nitrogen profile indicates that the amount of nitrogen within the laser spot region is relatively lower than within the immediately surrounding area. Almost no nitrogen remains in the spot area after sputtering for 30 min. A model is proposed to account for the observed titanium surface nitridation processes.

Enero, 2008 | DOI: 10.1016/j.surfcoat.2007.06.061

Apatite and Portland/apatite composite cements obtained using a hydrothermal method for retaining heavy metals

Dominguez, MI; Carpena, J; Borschnek, D; Centeno, MA; Odriozola, JA; Rose, J
Journal of Hazardous Materials, 150 (2008) 99-108
Química de Superficies y Catálisis


Apatite and Portland/apatite composite cements containing steelwork dusts have been prepared using a low temperature hydrothermal method (200° C, 48 h). The produced solids were characterized by means of XRD, IR, and SEM-EDX, and the remaining liquid was analyzed by ICP. The results clearly show the capability of these cements to inertise the heavy metals contained in steelwork dusts, that is Fe, Pb, Mo, Cr, Mn, Ni, and Zn. In the case of apatitic cements, Fe, Mg, Cr, Mn, and Pb coming from steel dust replaced Ca in the divalent cation position of the apatite structure, while Si and Mo replaced P in tetrahedral position. The average crystal size of the apatite-containing dust is smaller than in pure apatite synthesized using the same procedure, which is related to the magnesium content of the dust, since magnesium seems to inhibit the crystal growth. XRD diagrams of composite cements show only peaks corresponding to phases observed in the single cements, and in that no new phases are found. However, EDX analysis reveals the introduction of cations coming from Portland cement into the apatite structure. From the results of water analysis it could be concluded that the capability of retention is higher in composite matrices than in the pure apatite one. In conclusion, the obtained data allow stating that the proposed method, the hydrothermal synthesis of steelwork dust containing cement, is a reliable one for immobilization of toxic residues containing heavy leachable cations.

Enero, 2008 | DOI: 10.1016/j.jhazmat.2007.04.091

Carbothermal synthesis of silicon nitride(Si3N4): Kinetics and diffusion mechanism

Ortega, A.; Alcala, M. D.; Real, C.
Journmal of Materials Processing Technology, 195 (2008) 224-231
Reactividad de Sólidos


Sample controlled reaction temperature (SCRT) has been used for the first time to study the kinetics of the carbothermal reduction of silica in nitrogen to obtain silicon nitride (Si3N4). It is noteworthy to point out that SCRT method allows both a good control of pressure in the sample surroundings and the use of reaction rates low enough to keep temperature gradients at a negligible level to avoid any heat or mass transfer phenomena. It has been shown that the carbothermal synthesis of silicon nitride is best described by a two-dimensional diffusion kinetic model with an activation energy of about 306 kJ/mol. Effects of experimental parameters on the activation energy has been investigated. By using SCRT it has been possible to obtain ceramic powder with a determined phase composition and a controlled microstructure. 

Enero, 2008 | DOI: 10.1016/j.jmatprotec.2007.05.004

Influence of amine template on the photoactivity of TiO2 nanoparticles obtained by hydrothermal treatment

Colon, G; Hidalgo, MC; Navio, JA; Melian, EP; Diaz, OG; Dona, JM
Applied Catalysis B-Environmental, 78 (2008) 176-182
Fotocatálisis Heterogénea: Aplicaciones - Materiales y Procesos Catalíticos de Interés Ambiental y Energético


TiO2 nanoparticles have been prepared by amine template-assisted sol-gel precipitation and further hydrothermal treatment. We have investigated the effect of different amines (hydrazine and triethylamine) in the final surface and structural properties. It has been stated that the different amounts of amine could act as an interesting template upon hydrothermal treatment. Further thermal treatment also leads to a significant improvement in the photocatalytic properties of the studied systems. Surface and morphological features notably differ from TiO2 prepared using different synthetic routes. Wide surface and structural characterization of the samples have been carried out, and correlations with precipitation pH are pointed out from this characterization. In all cases, amine template TiO2 obtained exhibit high conversion values for phenol photo-oxidation reaction. Further calcination treatment of all the studied systems clearly leads to photocatalytic conversions higher than that exhibited by TiO2 Degussa P25. 

Enero, 2008 | DOI: 10.1016/j.apcatb.2007.09.019

Thermal Behavior of Ground Talc Mineral

Balek, V; Subrt, J; Perez-Maqueda, LA; Benes, M; Bountseva, IM; Beckman, IN; Perez-Rodriguez, JL
Journal of Mining and Metallurgy Section B-Metallurgy, 44 (2008) 7-17
Reactividad de Sólidos


Microstructure development and thermal behavior of ground talc mineral samples (from locality Puebla de Lillo,Spain) was characterized by X-ray diffraction, scanning electron microscopy(SEM),surface area measurements differential thermal analysis, thermogravimetry and emanation thermal analysis (ETA). A vibratory mill and grinding time 5 min. was used to prepare the ground talc sample. It was found that grinding caused an increase in the surface area of the natural talc from 3 m(2)g(-1) to 110 m(2) g(-1). A decrease of particle size after sample grinding was observed by SEM. The increase of structure disorder of the ground sample and the crystallite size reduction from 40 to 10 nm were determined from the XRD results. ETA results revealed a closing up of surface micro-cracks and healing of microstructure irregularities on heating in the range 200 - 500 degrees C of both un-ground and ground talc samples. For the ground talc sample a crystallization of non-crystalline phase into orthorhombic enstatite was characterized as by a decrease of radon mobility in the range 785-825 degrees C and by a DTA exothermal effect with the maximum at 830 degrees C. ETA results were used for the assessment of transport properties of the talc samples on heating.

Enero, 2008 | DOI: 10.2298/JMMB0801007B

Syntesis by reactive grinding of molibdenum iron bimetallic nitride

Roldan, MA; Ortega, A; Palencia, I; Real, C
Revista de Metalurgia, 44 (2008) 13-18
Reactividad de Sólidos


The transition metal nitride ternary show similar properties to the binary nitride and some times this behaviour are improved. In the present work, the molybdenum-iron nitride has been prepared by reactive grinding from the two metals under nitrogen atmosphere at a pressure of 11 bar. The characterization of the compound is presented and it is also shown a study of the stability of the nitride under several atmospheres.

Enero, 2008 | DOI:

Spectral Response of Opal-Based Dye-Sensitized Solar Cells

Mihi, A.; Calvo, M. E.; Anta, J. A.; Miguez, H.
Journal of Physical Chemistry C, 112 (2008) 13-17
Materiales Ópticos Multifuncionales


Herein we present an experimental study of the spectral dependence of the photogenerated current of opal-based solar cells. We analyze the incident photon-to-current conversion efficiency (IPCE) for dye-sensitized solar cells in which colloidal crystals are introduced in different configurations. We prove that a dye-sensitized nanocrystalline titanium oxide electrode moulded in the shape of an inverse opal shows a decrease of efficiency for the spectral region in which a photonic stop band opens up. Contrarily, when a standard thin film of disordered titania nanocrystallites is coupled to an inverse opal, the mirror effect of the photonic crystal at band gap frequencies increases the light harvesting efficiency of the cell and thus the IPCE. This effect is further demonstrated by coupling an inverse opal multilayer to a homogeneous electrode, with two well-defined spectral ranges of increased photogenerated current being detected.

Enero, 2008 | DOI: 10.1021/jp7105633