Three metal-support interaction

Three independent systems were used by Nicole, Tsiplakides, Pliangos, Verykios, Comninellis and Vayenas22 to show the mechanistic equivalence of NEMCA and metal-support interactions (Fig. 11.3). 

In section 11.3 we saw how a classical reaction engineering approach45 can been used to model both electrochemical promotion and metal support interactions. The analysis shows that the magnitude of the effect depends on three dimensionless numbers, II, J and Op (Table 11.3) which dictate the actual value of the promotional effectiveness factor. 

Promotion, electrochemical promotion and metal-support interactions are three, at a first glance, independent phenomena which can affect catalyst activity and selectivity in a dramatic manner. In Chapter 5 we established the (functional) similarities and (operational) differences of promotion and electrochemical promotion. In this chapter we established again the functional similarities and only operational differences of electrochemical promotion and metal-support interactions on ionic and mixed conducting supports. It is therefore clear that promotion, electrochemical promotion and metal-support interactions on ion-conducting and mixed-conducting supports are three different facets of the same phenomenon. They are all three linked via the phenomenon of spillover-backspillover. And they are all three due to the same underlying cause The interaction of adsorbed reactants and intermediates with an effective double layer formed by promoting species at the metal/gas interface (Fig. 11.2). 

Consequently the proven functional identity of classical promotion, electrochemical promotion and metal-support interactions should not lead the reader to pessimistic conclusions regarding the practical usefulness of electrochemical promotion. Operational differences exist between the three phenomena and it is very difficult to imagine how one can use metal-support interactions with conventional supports to promote an electrophilic reaction or how one can use classical promotion to generate the strongest electronegative promoter, O2, on a catalyst surface. Furthermore there is no reason to expect that a metal-support-interaction-promoted catalyst is at its best electrochemically promoted state. Thus the experimental problem of inducing electrochemical promotion on fully-dispersed catalysts remains an important one, as discussed in the next Chapter. 

The structure of supported rhodium catalysts has been the subject of intensive research during the last decade. Rhodium is the component of the automotive exhaust catalyst (the three-way catalyst) responsible for the reduction of NO by CO [1], In addition, it exhibits a number of fundamentally interesting phenomena, such as strong metal-support interaction after high temperature treatment in hydrogen [21, and particle disintegration under carbon monoxide [3]. In this section we illustrate how techniques such as XPS, STMS, EXAFS, TEM and infrared spectroscopy have led to a fairly detailed understanding of supported rhodium catalysts. 

The studies discussed above deal with highly dispersed and therefore well-defined rhodium particles with which fundamental questions on particle shape, chemisorption and metal-support interactions can be addressed. Practical rhodium catalysts, for example those used in the three-way catalyst for reduction of NO by CO, have significantly larger particle sizes, however. In fact, large rhodium particles with diameters above 10 nm are much more active for the NO+CO reaction than the particles we discussed here, because of the large ensembles of Rh surface atoms needed for this reaction [28]. Such particles have also been extensively characterized with spectroscopic techniques and electron microscopy we mention in particular the work of Wong and McCabe [29] and Burkhardt and Schmidt [30], These studies deal with the materials science of rhodium catalysts that are closer to the ones used in practice, which is of great interest from an industrial point of view. 

The EXAFS results have implications for the metal support interaction. The data in Table 9.1 indicate that the main interaction between rhodium and alumina occurs between reduced metal atoms and two to three oxygen ions in the surface of the support at a distance of 0.27 nm. It appears logical, therefore, to attribute the metal support interaction to bonding between oxygen ions of the support and induced dipoles inside the rhodium particle [19]. Although such bonding is weak on a per atom basis, the cumulative bond for the whole particle may be significant. 

Other factors, by metal type, amount of metal used, the degree of metal dispersion, the location of metal on support or metal-support interaction. On the other hand the concentration of metal across the diameter of catalyst grains also important. There are three cases of concentration distribution eggshell (metals only on the inside of support), eggwhite (the maximum concentration of metal between one-half and one-quarter of grain diameter) and eggyolk (the maximum concentration of metal within one-half of grain diameter). At the same metal content considerable differences were observed in the concentration distribution in the catalysts. 

The use of CeOs-based materials in catalysis has attracted considerable attention in recent years, particularly in applications like environmental catalysis, where ceria has shown great potential. This book critically reviews the most recent advances in the field, with the focus on both fundamental and applied issues. The first few chapters cover structural and chemical properties of ceria and related materials, i.e. phase stability, reduction behaviour, synthesis, interaction with probe molecules (CO. O2, NO), and metal-support interaction — all presented from the viewpoint of catalytic applications. The use of computational techniques and ceria surfaces and films for model catalytic studies are also reviewed. The second part of the book provides a critical evaluation of the role of ceria in the most important catalytic processes three-way catalysis, catalytic wet oxidation and fluid catalytic cracking. Other topics include oxidation-combustion catalysts, electrocatalysis and the use of cerium catalysts/additives in diesel soot abatement technology. 

The goal of this chapter is to summarize and systematize the phenomenology of the three phenomena, i.e., classical promotion, electrochemical promotion, and metal-support interactions, present their striking similarities and some common rules that govern them, and demonstrate their intimate link and common molecular mechanism. 

These three aspects in catalysis by metals enter in the general frame of structure-activity relationships. They have been the subject of reviews dealing with the (1) particle-size and plane-structure sensitivities [10], (2) ensemble-size sensitivity [11], and (3) metal-support interaction [9]. Depending on whether or not the turnover frequency (TOF), or rate per unit surface area or per accessible metal atom, is affected by the structure of the particle surface, the reactions have been called structure-sensitive or structure-insensitive [12]. The structure-activity relationships... 

Three aspects of the performance of supported catalysts are also discussed in this Chapter. With the development of techniques, as outlined above, for the characterization of supported metal catalysts, it seems timely to survey studies of crystallite size effect/structure sensitivity with special reference to the possible intrusion of adventitious factors (Section 5). Recently there has been considerable interest in the existence of (chemical) metal-support interactions and their significance for chemisorption and catalytic activity/ selectivity (Section 6). Finally, supported bimetallic catalysts are discussed for various reactions not involving hydrocarbons (hydrocarbon reactions over alloys and bimetallic catalysts have already been reviewed in this Series with respect to both basic research and technical applications ). References to earlier reviews (including some on techniques) that complement material in this Chapter are given in the appropriate sections. It might be useful, however, to note here some topics not discussed that also form part of the vast subject of supported metal and bimetallic catalysts and for which recent reviews are available, viz, spillover, catalyst deactivation, the growth and... 

In this short summary, we have tried to show how combined strategies are the best when studying the reactivity of metals supported on oxides. However, the complexity of some of the techniques implies that most laboratories work mainly with two or three techniques. On the other hand, interpretation of the experiments is not always straightforward. In this sense, the combined use of theory and experiment becomes beneficial. The remainder of this chapter is devoted to the use of theoretical tools and approaches commonly used in the study of the metal-support interaction. 

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