Nanomaterials in Catalysis: Study of Model Reactions.

Abstract

Metal nanoparticles catalysts considered in this work included systems consisting<br/>essentially of a single metal component (Ag) and bimetallic system. Bimetallic systems<br/>of miscible (Au-Cu and Au-Ag) and immiscible components (Ir-Au) have been<br/>investigated. The study of these materials with chemical probes including chemisorption<br/>and selected catalytic reaction, in conjunction with physical and chemical methods such<br/>as electron microscopy, X-ray diffraction (XRD), temperature programmed reduction<br/>(TPR), UV-vis, photoelectron spectroscopy (XPS) has been performed to contribute the<br/>knowledge of their structures. The fact that surface composition can differ substantially<br/>from bulk composition was also studied. High resolution electron microscopy (HRTEM)<br/>provided independent evidence of the highly dispersed nature of the metal clusters.<br/>Concepts concerning the effect of additives on the structure and physicochemical and<br/>catalytic properties of oxide catalysts in selective oxidation reactions were considered.<br/>This work was also carried out in an attempt to study the different oxygen species formed<br/>on silver. Of particular interest is to understand the role and nature of the elusive<br/>subsurface O species in the function of silver as an oxidant catalyst. The effect of<br/>additives on acid-base and redox properties and on their bearing on catalytic performance<br/>was discussed, with particular emphasis on alkaline metal additives such as cesium and<br/>sodium. The phenomena occurring in monophasic and supported mixed oxide containing<br/>the additives, including modification of structure, segregation, defect formation and spillover<br/>of the reactants was also described.<br/>The nature of the surface of the bimetallic systems was the question of interest. Will the<br/>individual clusters be monometallic or will they contain atoms of both metals and<br/>therefore be bimetallic? It is interesting to consider how the state of dispersion the metal<br/>catalysts affects the relationship between chemisorption capacity or catalytic activity and<br/>catalyst composition. The bimetallic clusters discussed thus far have been combination of<br/>a Group VIII and a Group IB metal. In spite of the great importance in petrochemistry,<br/>skeletal hydrocarbon reactions, such as hydrogenolysis of methylcyclopentane, offers an<br/>interesting fingerprint of the dependence of specific rate on catalyst structure. The first<br/>reported case of a reaction with large dependence on catalyst structure was performed for<br/>neopentane on platinum. This result led Boudart to classify reactions on metals as: (i)<br/>"facile" or "structure-insensitive" reactions, for which the specific rate does not depend<br/>upon the size of the metal particle, and (ii) "demanding" or "structure sensitive"<br/>reactions, for which the specific rate is highly dependent on the metal dispersion. The<br/>rate per surface metal atom is the fundamental interest. Thus, the metal dispersion is<br/>extremely important parameter to be determined. An experimental scheme based on<br/>selective H2 chemisorption was developed to investigate the surface composition of<br/>bimetallic system. With this capability, the activity of such a catalyst was referred to the<br/>amount of metal in the surface rather than to the metal content of the catalyst as a whole.<br/>The interaction between catalytically active metal oxide particles and oxide carriers<br/>greatly influences their structure and size. Vanadia catalysts constitute also a relevant<br/>example of the influence of this interaction. Accordingly, it was decided to study the<br/>effect of the support (TiO2 and MCM-41) with the aim of understanding the<br/>interrelations on the catalytic properties of V2O5 as a way to improve its performance for<br/>the selective oxidation of ethanol.<br/>Finally, the catalytic role of the different basic sites in hydrotalcite interlayer was<br/>envisaged through the study of the styrene epoxidation in liquid phase. Hydrotalcite-like<br/>compounds are not only interesting for their industrial applications, but are also beautiful<br/>examples of the scientific preparation of catalysts. All the stages of the preparation of a<br/>catalyst based on hydrotalcite-like precursor need precise chemical foundations in order<br/>to avoid inhomogeneties and/or chemical segregations, which would be detrimental to<br/>the properties of the final compounds. Layered double hydroxides (LDHs), also known<br/>as hydrotalcites or anionic clays, are a class of ionic lamellar compounds made of<br/>positive-charged hydroxide layers with charge balancing anions and water molecules<br/>sandwiched between layers. Exfoliated Mg-Al layered double hydroxide in water was<br/>investigated in the styrene epoxidation. The change in the basic properties during the<br/>rehydration process of the calcined samples as well the influence on catalytic activity<br/>was studied. Hydrotalcites are very attractive for this type of oxidation reaction because<br/>their ability to give Lewis type acid-base bifunctional catalysts or basic catalysts with<br/>Brönsted type sites, proceeding from the mixed oxide and the meixnerite-like structures,<br/>respectively.

En este trabajo se han sintetizado y estudiado diferentes catalizadores a base de<br/>nanopartículas metálicas con diferentes tamaños y morfologías. Se han estudiado tanto<br/>sistemas monometálicos como bimetálicos con componentes de diferentes miscibilidad (por<br/>ejemplo sistemas Au-Cu y Au-Ag de total miscibilidad, así como Ir-Au de prácticamente<br/>total inmiscibilidad) con la finalidad de estudiar el cambio que producen tanto en el tamaño<br/>como en la morfología de las nanopartículas obtenidas.<br/>Los nanomateriales obtenidos han sido estudiados mediante el empleo de diferentes<br/>técnicas de caracterización como pueden ser, fisisorción, quimisorción microscopia<br/>electrónica (SEM-TEM), difracción de rayos X (XRD), reducción a temperatura<br/>programada (TPR), espectroscopia ultravioleta-visible (UV-vis), espectroscopia<br/>fotoelectrónica (XPS), etc., así como mediante diferentes ensayos catalíticos modelo con la<br/>finalidad de correlacionar las propiedades físico-químicas de dichos materiales con los<br/>métodos de síntesis de éstos.<br/>Se ha hecho especial énfasis en el estudio de las diferentes especies de oxígeno que<br/>presentan estos nanomateriales en superficie, así como su correlación con la actividad<br/>catalítica que presentan en diferentes reacciones de oxidación selectiva de olefinas. Por otra<br/>parte, la modificación de las propiedades ácido-base de dichos materiales y su carácter<br/>redox cuando se adiciona metales alcalinos del tipo sodio y cesio, también ha sido<br/>exhaustivamente estudiado.<br/>La formación de sistemas bimetálicos y su homogeneidad, así como el efecto de dispersión<br/>y el cambio en sus propiedades físico-químicas que hace un metal sobre el otro, ha sido<br/>también objeto de estudio en este trabajo. Este hecho se ha puesto en evidencia en el<br/>estudio de reacciones de hidrogenolisis del metilciclopentano que permite una buena<br/>comprensión de la actividad catalítica, así como de la dependencia de la estructura en la<br/>actividad catalítica previamente ya descrita por Boudart.<br/>La interacción entre las especies metálicas cataliticamente activas y los soportes empleados<br/>también pueden ejercer una notable influencia en la estructura y tamaño de la especie<br/>activa, y por consiguiente en el comportamiento catalítico. En este contexto, se han<br/>estudiado la interacción de óxido de vanadio en diferentes soportes como TiO2 y MCM-4,<br/>para comprender el comportamiento catalítico del V2O5 en otra reacción test como es la de<br/>oxidación selectiva del etanol.<br/>Finalmente, el papel del los diferentes sitios básicos de materiales tipo hidrotalcita, el<br/>estudio de la exfoliación de dichos materiales y el cambio en sus propiedades básicas<br/>después de diferentes protocolos de rehidratación han sido estudiados en otra reacción test<br/>como es la reacción de epoxidación del estireno en fase liquida.