Estrategias de mejora de las propiedades funcionales del K(0,5)Na(0,5)NbO(3)

Los materiales piezoeléctricos más utilizados en una gran cantidad de aplicaciones son las cerámicas basadas en el titanato circonato de plomo (PZT). No obstante, el principal inconveniente de estos materiales recae en la elevada toxicidad del plomo y los problemas ambientales y de salud que provoca. Así, la comunidad científica se centra en la búsqueda de nuevos materiales piezoeléctricos libres de plomo. La familia de materiales libres de plomo basados en el niobiato de sodio y potasio (KNN) presenta unas propiedades piezoeléctricas y ferroeléctricas prometedoras. Este tipo de materiales son conocidos desde los años 1960, pero para poder utilizarlos industrialmente es necesario superar algunas dificultades, sobre todo, durante la síntesis y la sinterización. No obstante, se han logrado preparar cerámicas basadas en el KNN de alta calidad mediante la utilización de métodos de preparación complejos como el prensado en caliente o mediante Spark Plasma Sintering. Recientemente, el interés por los materiales libres de plomo aumentó notablemente a raíz de las excelentes propiedades piezoeléctricas de las cerámicas basadas en el KNN dopadas con Li, Ta y Sb. Este sistema complejo despertó un elevado interés en la comunidad científica, ya que podría abrir una nueva vía para substituir los materiales piezoeléctricos basados en plomo. No obstante, es importante destacar que sólo se han estudiado un número limitado de dopantes que pueden influir en las propiedades funcionales de los materiales y, frecuentemente, los resultados obtenidos por distintos grupos de investigación son contradictorios. Por lo tanto, es necesario seguir investigando el efecto de distintos dopantes en las cerámicas de KNN. Asimismo, se debería tener en cuenta la toxicidad de los elementos introducidos en las cerámicas de KNN, sobre todo, cuando se encuentran en elevadas concentraciones, como por ejemplo, el litio, tántalo, antimonio o cobre. Por lo tanto, el principal objetivo de esta tesis doctoral es investigar diferentes estrategias para desarrollar nuevos sistemas sencillos basados en el KNN. En este trabajo se han estudiado y optimizado la preparación de cerámicas basadas en el KNN mediante métodos de síntesis que facilitan la obtención de cerámicas densas, estos métodos son, además, reproducibles y escalables a nivel industrial. En concreto, tanto el tamaño de partícula como la distribución del tamaño de partícula se han controlado en cada etapa de la síntesis, así, se ha logrado disminuir la temperatura de síntesis entre 100 ºC y 200 ºC, respecto a las temperaturas empleadas habitualmente en la preparación de cerámicas basadas en el KNN. Las densidades relativas de las cerámicas obtenidas son superiores al 96 %. Se ha estudiado, también, el efecto de dopajes de tipo dador y de tipo aceptor, evaluando tanto el efecto de diferentes dopantes como su concentración. Mediante el dopaje se han mejorado tanto las propiedades dieléctricas como las propiedades piezoeléctricas de las cerámicas preparadas. Se ha observado un incremento del 45% del coeficiente piezoeléctrico d33 respecto al KNN puro para bajas concentraciones de dopante. Las propiedades piezoeléctricas del sistema dopado con Li, Ta y Sb son comparables a las propiedades que presentan las cerámicas basadas en el PZT. Sin embargo, sus pérdidas dieléctricas y mecánicas son demasiado elevadas para poder utilizar éstas cerámicas en dispositivos de potencia. En este sentido, se han logrado disminuir las pérdidas dieléctricas y mecánicas modificando las condiciones de sinterización, así, las cerámicas sinterizadas bajo atmósfera inerte son más estables que las sinterizadas bajo aire. Finalmente, mediante un control exhaustivo de la estequiometría y la estabilidad de las soluciones precursoras, entre otros parámetros, se han podido preparar capas delgadas basadas en el KNN. Se ha logrado desarrollar un método de preparación de capas delgadas altamente orientadas de una manera reproducible y escalable industrialmente. Además, se ha podido confirmar la relación entre el grado de orientación de la capa y sus propiedades.Presently, the family of lead-free ceramics showing the most promising piezoelectric and ferroelectric properties is based on potassium and sodium niobate (KNN). The use of this type of ceramic presents a number of challenges, especially during the synthesis, and sintering. The interest of lead-free ceramics increased markedly when Li, Ta and Sb dopants greatly improved the piezoelectric properties of KNN ceramics. Modified KNN compositions promise to be a new generation of environmentally safer piezoelectric materials.Thus, it is necessary to continue investigating the effect of dopants in KNN ceramics. Furthermore, the toxicity of the different elements used should be considered, mainly when high concentrations of new elements (e.g. lithium, tantalum, antimony, copper) are substituted in KNN ceramics. The principal goal of this thesis is to explore different strategies to develop new and simple systems based on KNN ceramics. The processing of pure and modified KNN ceramics has been studied and optimized so that the reproducibility and scalability of the synthesis methods can be controlled, which facilitates the creation of high density ceramics using normal processing methods. In particular, the particle grain size and particle size distribution have been controlled for all synthesis steps, as a consequence, the synthesis temperature has been reduced by 100 ºC - 200 ºC. The densities of the so-obtained ceramics are higher than 96% for all single phase ceramics. The effect of donor and acceptor dopants has been studied, focusing on the effect of different dopant and of their concentration. The dielectric and functional properties of the KNN based ceramics have been improved when doping. Even by low amounts of dopants the piezoelectric coefficient can be increased by 45% with respect to pure KNN. Although the piezoelectric properties of complex KNN based systems are comparable to those of PZT based ceramics, their dielectric and mechanical losses at room temperature are too high for them to be used in power devices. Consequently, the dielectric and mechanical losses have been improved by modifying the sintering conditions; ceramics sintered in an inert atmosphere are more stable than those sintered in air. Finally, the processing of sodium potassium niobate based thin films has been successfully achieved by controlling the stoichiometry, the stability of the precursor solutions, and so on. Consequently, a reproducible and scalable method has been developed to obtain highly oriented KNN based thin films. Moreover, the relationship between the orientation degree and the properties is confirmed in this thesis

Low-temperature partial oxidation of methane over Pd–Ni bimetallic catalysts supported on CeO2

Monometallic Pd and Ni and bimetallic Pd–Ni catalysts supported on CeO2 are prepared via mechanochemical and conventional incipient wetness impregnation methods and tested for the production of syngas by the partial oxidation of methane. Compared with monometallic Ni/CeO2 and Pd/CeO2, bimetallic Pd–Ni/CeO2 catalysts show considerable higher methane conversion and syngas yield. Additionally, the bimetallic catalysts prepared by ball milling produce syngas at lower temperature. Different preparation parameters, such as metal loading, Pd/Ni ratio, milling energy, milling time and order of incorporation of the metals are examined. The best performance is obtained with a bimetallic catalyst prepared at 50 Hz for 20 min with only 0.12 wt% Pd and 1.38 wt% Ni. Stability tests demonstrate superior stability for bimetallic Pd–Ni/CeO2 catalysts prepared by a mechanochemical approach. From the characterization results, this is explained in terms of an impressive dispersion of metal species with a strong interaction with the surface of CeO2.Peer ReviewedPostprint (published version

Bimetallic Ru–Pd supported on CeO2 for the catalytic partial oxidation of methane into syngas

A series of monometallic Ru, Pd, and bimetallic Ru–Pd catalysts loaded on CeO2 support have been prepared via mechanochemical and conventional incipient wetness impregnation methods and used in the partial oxidation of methane (POM) to obtain synthesis gas (H2 and CO). The influence of the preparation method, the order of addition of the metals, the Ru:Pd metal ratio, and the milling energy and time for samples prepared by the mechanochemical method, have been evaluated between 300 and 600 °C. The results revealed that bimetallic Ru–Pd/CeO2 catalysts outperform monometallic Ru–CeO2 and Pd–CeO2 for POM, both in terms of catalytic activity and stability. Additionally, the bimetallic Ru–Pd/CeO2 catalysts prepared by ball milling produced syngas at a much lower temperature compared to the conventional catalysts prepared by incipient wetness impregnation. Raman spectroscopy, temperature programmed reduction (H2–TPR), X–ray photoelectron spectroscopy (XPS) and high–resolution transmission electron microscopy (HRTEM) have been used to characterize the catalysts before and after reaction.Peer ReviewedPostprint (published version

Review of the Decomposition of Ammonia to Generate Hydrogen

Because of the problems associated with the generation and storage of hydrogen in portable applications, the use of ammonia has been proposed for on-site production of hydrogen through ammonia decomposition. First, an analysis of the existing systems for ammonia decomposition and the challenges for this technology are presented. Then, the state of the art of the catalysts used to date for ammonia decomposition is described considering the catalysts composed of noble and non-noble metals and their combinations, as well as novel materials such as alkali metal amides and imides. The effect of the supports and promoters used is analyzed in detail, and the catalytic activity obtained is compared. An analysis of the kinetics of the reaction obtained with different catalysts is also presented and discussed, including the reaction mechanism, the determining step of the reaction, and the apparent activation energy. Finally, the structured reactors used to date for the decomposition reaction of ammonia are explored, as well as the possibilities offered by catalytic membrane reactors, which allow the on-site simultaneous production and separation of hydrogen.Peer ReviewedPostprint (author's final draft

Improving the functional properties of (K0.5Na0.5)NbO3 piezoceramics by acceptor doping

ZrO2 and TiO2 modified lead-free (K0.5Na0.5)NbO3 (KNN) piezoelectric ceramics are prepared by a conventional solid-state reaction. The effect of acceptor doping on structural and functional properties is investigated. A decrease in the Curie temperature and an increase in the dielectric constant values are observed when doping. More interestingly, an increase in the coercive field E-c and remanent polarization P-r is observed. The piezoelectric properties are greatly increased when doping with small concentrations dopants. ZrO2 doped ceramic exhibits good piezoelectric properties with piezoelectric coefficient d(33) = 134 pC/N and electromechanical coupling factor k(p) = 35%. It is verified that nonlinearity is significantly reduced. Thus, the creation of complex defects capable of pinning the domain wall motion is enhanced with doping, probably due to the formation of oxygen vacancies. These results strongly suggest that compositional engineering using low concentrations of acceptor doping is a good means of improving the functional properties of KNN lead-free piezoceramic system. (C) 2014 Elsevier Ltd. All rights reserved.Postprint (published version

The catalytic activity of the Pr2Zr2-xFexO7±d system for the CO oxidation reaction

One of the alternatives to decrease the concentration of CO is its oxidation reaction to CO2, which can be made more efficient using catalysts. In this work, it is shown that pyrochlore structures are a promising candidate to act as heterogeneous catalysts due to their chemical and physical properties. For use as a catalyst in this reaction, the Pr2Zr2-xFexO7±d (x = 0, 0.05, 0.10, and 0.15) system was synthesized by the solvothermal method, firing the powder obtained at temperatures of 1200 and 1400°C. The diffraction patterns confirmed the pyrochlore structure as the single phase in all the nominal compositions. The Brunauer–Emmett–Teller method and dynamic light-scattering analysis showed an increase in the particle size and a decrease in the specific surface area when increasing the iron concentration and increasing the calcination temperature. The compositions that presented the best catalytic activity were the samples with the highest iron concentration. Moreover, these samples were able to convert all the CO oxidation reactions in a narrower temperature range than a conventional CeO2 sample. The presence of vacancies and the redox behavior of the elements present are the key factors for the catalysis of this system in the CO oxidation reaction.Postprint (published version

Operando NAP-XPS studies of a ceria-supported Pd catalyst for CO oxidation

Supported Pd/CeO2 catalytic systems have been widely investigated in the low-temperature oxidation of CO (LTO CO) due to the unique oxygen storage capacity and redox properties of the ceria support, which highly influence the structural, chemical and electronic state of Pd species. Herein, operando near-ambient pressure XPS (NAP-XPS) technique has allowed the study of a conventional Pd/CeO2 catalyst surface during the CO oxidation reaction under experimental conditions closer to the actual catalytic reaction, unfeasible with other surface science techniques that demand UHV conditions. SEM, HRTEM and XRD analyses of the powder catalyst, prepared by conventional incipient wetness impregnation, reveal uniformly CeO2-loaded Pd NPs of less than 2 nm size, which generated an increase in oxygen vacancies with concomitant ceria reduction, as indicated by H2-TPR and Raman measurements. Adsorbed peroxide (O22-) species on the catalyst surface could also be detected by Raman spectra. Operando NAP-XPS results obtained at the ALBA Synchrotron Light Source revealed two kinds of Pd species under reaction conditions, namely PdOx and PdII ions in a PdxCe1-xO2-d solution, the latter one appearing to be crucial for the CO oxidation. By means of a non-destructive depth profile analysis using variable synchrotron excitation energies, the location and the role of these palladium species in the CO oxidation reaction could be clarified: PdOx was found to prevail on the upper surface layers of the metallic Pd supported NPs under CO, while under reaction mixture it was rapidly depleted from the surface, leaving a greater amount in the subsurface layers (7% vs. 12%, respectively). On the contrary, the PdxCe1-xO2-d phase, which was created at the Pd–CeO2 interface in contact with the gas environment, appeared to be predominant on the surface of the catalyst. Its presence was crucial for CO oxidation evolution, acting as a route through which active oxygen species could be transferred from ceria to Pd species for CO oxidation.Peer ReviewedPostprint (published version

Bimetallic NiFe nanoparticles supported on CeO2 as catalysts for methane steam reforming

Ni-Fe nanocatalysts supported on CeO2 have been prepared for the catalysis of methane steam reforming (MSR) aiming for coke-resistant noble metal-free catalysts. The catalysts have been synthesized by traditional incipient wetness impregnation as well as dry ball milling, a green and more sustainable preparation method. The impact of the synthesis method on the catalytic performance and the catalysts’ nanostructure has been investigated. The influence of Fe addition has been addressed as well. The reducibility and the electronic and crystalline structure of Ni and Ni-Fe mono- and bimetallic catalysts have been characterized by temperature programmed reduction (H2-TPR), in situ synchrotron X-ray diffraction (SXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Their catalytic activity was tested between 700 and 950 °C at 108 L gcat-1 h-1 and with the reactant flow varying between 54 and 415 L gcat-1 h-1 at 700 °C. Hydrogen production rates of 67 mol gmet-1 h-1 have been achieved. The performance of the ball-milled Fe0.1Ni0.9/CeO2 catalyst was similar to that of Ni/CeO2 at high temperatures, but Raman spectroscopy revealed a higher amount of highly defective carbon on the surface of Ni-Fe nanocatalysts. The reorganization of the surface under MSR of the ball-milled NiFe/CeO2 has been monitored by in situ near-ambient pressure XPS experiments, where a strong reorganization of the Ni-Fe nanoparticles with segregation of Fe toward the surface has been observed. Despite the catalytic activity being lower in the low-temperature regime, Fe addition for the milled nanocatalyst increased the coke resistance and could be an efficient alternative to industrial Ni/Al2O3 catalysts.Peer ReviewedPostprint (published version

Investigation of the evolution of Pd-Pt supported on ceria for dry and wet methane oxidation

Efficiently treating methane emissions in transportation remains a challenge. Here, we investigate palladium and platinum mono- and bimetallic ceria-supported catalysts synthesized by mechanical milling and by traditional impregnation for methane total oxidation under dry and wet conditions, reproducing those present in the exhaust of natural gas vehicles. By applying a toolkit of in situ synchrotron techniques (X-ray diffraction, X-ray absorption and ambient pressure photoelectron spectroscopies), together with transmission electron microscopy, we show that the synthesis method greatly influences the interaction and structure at the nanoscale. Our results reveal that the components of milled catalysts have a higher ability to transform metallic Pd into Pd oxide species strongly interacting with the support, and achieve a modulated PdO/Pd ratio than traditionally-synthesized catalysts. We demonstrate that the unique structures attained by milling are key for the catalytic activity and correlate with higher methane conversion and longer stability in the wet feed.Peer ReviewedPostprint (published version

Curs 0: preparació per als estudis a l’EEBE

Aquest article presenta el desenvolupament i primers resultats d'ús d'un conjunt de cursos virtuals que pretenen proporcionar uns coneixements inicials bàsics de Matemàtiques, Física i !ímica als estudiants que accedeixen a estudis de grau a l'Escola d'Enginyeria de Barcelona Est (EEBE). Els cursos han estat desenvolupats sobre la plataforma Atenea (Moodle). El seu nucli el constitueixen un conjunt de materials per a autoaprenentatge que inclouen documents escrits, vídeos i tests d'autoavaluació. Els documents escrits i els vídeos corresponen tant a explicacions de teoria com a la resolució detallada d'exercicis. En el marc d'una prova pilot, els cursos, de seguiment voluntari durant el període transcorregut entre la matricula (mitjans de juliol) i l'inici de les classes (mitjans de setembre), van ser publicitats a tots els estudiants de nou accés del curs 2021-2022. Encara que la participació va ser més limitada del que s'esperava (únicament el 22% dels estudiants de nou accés es van inscriure), cal destacar que els estudiants que sí que van seguir els cursos van expressar majoritàriament una bona valoració dels mateixos (al respondre un qüestionari de satisfacció). Del desenvolupament dels cursos i de la realització de la prova pilot s'han obtingut unes quantes conclusions que també queden reflectides al final de l'article