Nanomaterials in Catalysis: Study of Model Reactions.
- Autores: Ricardo José Chimentao
- Directores de la Tesis: Jesús Eduardo Sueiras Romero (dir. tes.), Francesc Medina Cabello (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2007
- Idioma: inglés
- Tribunal Calificador de la Tesis: José Luis García Fierro (presid.), Francisco López Bonillo (secret.), Guido Busca (voc.), Sònia Abelló Cros (voc.), Jordi Llorca (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Metal nanoparticles catalysts considered in this work included systems consisting essentially of a single metal component (Ag) and bimetallic system. Bimetallic systems of miscible (Au-Cu and Au-Ag) and immiscible components (Ir-Au) have been investigated. The study of these materials with chemical probes including chemisorption and selected catalytic reaction, in conjunction with physical and chemical methods such as electron microscopy, X-ray diffraction (XRD), temperature programmed reduction (TPR), UV-vis, photoelectron spectroscopy (XPS) has been performed to contribute the knowledge of their structures. The fact that surface composition can differ substantially from bulk composition was also studied. High resolution electron microscopy (HRTEM) provided independent evidence of the highly dispersed nature of the metal clusters.
Concepts concerning the effect of additives on the structure and physicochemical and catalytic properties of oxide catalysts in selective oxidation reactions were considered.
This work was also carried out in an attempt to study the different oxygen species formed on silver. Of particular interest is to understand the role and nature of the elusive subsurface O species in the function of silver as an oxidant catalyst. The effect of additives on acid-base and redox properties and on their bearing on catalytic performance was discussed, with particular emphasis on alkaline metal additives such as cesium and sodium. The phenomena occurring in monophasic and supported mixed oxide containing the additives, including modification of structure, segregation, defect formation and spillover of the reactants was also described.
The nature of the surface of the bimetallic systems was the question of interest. Will the individual clusters be monometallic or will they contain atoms of both metals and therefore be bimetallic? It is interesting to consider how the state of dispersion the metal catalysts affects the relationship between chemisorption capacity or catalytic activity and catalyst composition. The bimetallic clusters discussed thus far have been combination of a Group VIII and a Group IB metal. In spite of the great importance in petrochemistry, skeletal hydrocarbon reactions, such as hydrogenolysis of methylcyclopentane, offers an interesting fingerprint of the dependence of specific rate on catalyst structure. The first reported case of a reaction with large dependence on catalyst structure was performed for neopentane on platinum. This result led Boudart to classify reactions on metals as: (i) facile or structure-insensitive reactions, for which the specific rate does not depend upon the size of the metal particle, and (ii) demanding or structure sensitive reactions, for which the specific rate is highly dependent on the metal dispersion. The rate per surface metal atom is the fundamental interest. Thus, the metal dispersion is extremely important parameter to be determined. An experimental scheme based on selective H2 chemisorption was developed to investigate the surface composition of bimetallic system. With this capability, the activity of such a catalyst was referred to the amount of metal in the surface rather than to the metal content of the catalyst as a whole.
The interaction between catalytically active metal oxide particles and oxide carriers greatly influences their structure and size. Vanadia catalysts constitute also a relevant example of the influence of this interaction. Accordingly, it was decided to study the effect of the support (TiO2 and MCM-41) with the aim of understanding the interrelations on the catalytic properties of V2O5 as a way to improve its performance for the selective oxidation of ethanol.
Finally, the catalytic role of the different basic sites in hydrotalcite interlayer was envisaged through the study of the styrene epoxidation in liquid phase. Hydrotalcite-like compounds are not only interesting for their industrial applications, but are also beautiful examples of the scientific preparation of catalysts. All the stages of the preparation of a catalyst based on hydrotalcite-like precursor need precise chemical foundations in order to avoid inhomogeneties and/or chemical segregations, which would be detrimental to the properties of the final compounds. Layered double hydroxides (LDHs), also known as hydrotalcites or anionic clays, are a class of ionic lamellar compounds made of positive-charged hydroxide layers with charge balancing anions and water molecules sandwiched between layers. Exfoliated Mg-Al layered double hydroxide in water was investigated in the styrene epoxidation. The change in the basic properties during the rehydration process of the calcined samples as well the influence on catalytic activity was studied. Hydrotalcites are very attractive for this type of oxidation reaction because their ability to give Lewis type acid-base bifunctional catalysts or basic catalysts with Brönsted type sites, proceeding from the mixed oxide and the meixnerite-like structures, respectively.
Resumen En este trabajo se han sintetizado y estudiado diferentes catalizadores a base de nanopartículas metálicas con diferentes tamaños y morfologías. Se han estudiado tanto sistemas monometálicos como bimetálicos con componentes de diferentes miscibilidad (por ejemplo sistemas Au-Cu y Au-Ag de total miscibilidad, así como Ir-Au de prácticamente total inmiscibilidad) con la finalidad de estudiar el cambio que producen tanto en el tamaño como en la morfología de las nanopartículas obtenidas.
Los nanomateriales obtenidos han sido estudiados mediante el empleo de diferentes técnicas de caracterización como pueden ser, fisisorción, quimisorción microscopia electrónica (SEM-TEM), difracción de rayos X (XRD), reducción a temperatura programada (TPR), espectroscopia ultravioleta-visible (UV-vis), espectroscopia fotoelectrónica (XPS), etc., así como mediante diferentes ensayos catalíticos modelo con la finalidad de correlacionar las propiedades físico-químicas de dichos materiales con los métodos de síntesis de éstos.
Se ha hecho especial énfasis en el estudio de las diferentes especies de oxígeno que presentan estos nanomateriales en superficie, así como su correlación con la actividad catalítica que presentan en diferentes reacciones de oxidación selectiva de olefinas. Por otra parte, la modificación de las propiedades ácido-base de dichos materiales y su carácter redox cuando se adiciona metales alcalinos del tipo sodio y cesio, también ha sido exhaustivamente estudiado.
La formación de sistemas bimetálicos y su homogeneidad, así como el efecto de dispersión y el cambio en sus propiedades físico-químicas que hace un metal sobre el otro, ha sido también objeto de estudio en este trabajo. Este hecho se ha puesto en evidencia en el estudio de reacciones de hidrogenolisis del metilciclopentano que permite una buena comprensión de la actividad catalítica, así como de la dependencia de la estructura en la actividad catalítica previamente ya descrita por Boudart.
La interacción entre las especies metálicas cataliticamente activas y los soportes empleados también pueden ejercer una notable influencia en la estructura y tamaño de la especie activa, y por consiguiente en el comportamiento catalítico. En este contexto, se han estudiado la interacción de óxido de vanadio en diferentes soportes como TiO2 y MCM-4, para comprender el comportamiento catalítico del V2O5 en otra reacción test como es la de oxidación selectiva del etanol.
Finalmente, el papel del los diferentes sitios básicos de materiales tipo hidrotalcita, el estudio de la exfoliación de dichos materiales y el cambio en sus propiedades básicas después de diferentes protocolos de rehidratación han sido estudiados en otra reacción test como es la reacción de epoxidación del estireno en fase liquida.
