Nanomaterials in Catalysis: Study of Model Reactions.

by Chimentao, Ricardo José

Abstract 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.