Strong metal-support interactions mechanism

The concept of mechanical fixation of metal on carbon makes catalytic applications at high temperatures possible. These applications require medium-sized active particles because particles below 2nm in size are not sufficiently stabilised by mechanical fixation and do not survive the high temperature treatment required by the selective etching. Typical reactions which have been studied in detail are ammonia synthesis [195, 201-203] and CO hydrogenation [204-207]. The idea that the inert carbon support could remove all problems associated with the reactivity of products with acid sites on oxides was tested, with the hope that a thermally wellconducting catalyst lacking strong-metal support interactions, as on oxide supports, would result. 

Recently, Pt/Nb20s catalysts have been investigated on dehydrogenation of alkanes. These systems have presented advantages on selectivity towards olefins when compared to Pt/ALOs or even Pt-Sn/A]20 , catalysts [4-6]. The promoting mechanism is related to both the SMSI (Strong Metal Support Interaction) effect and the low acidity of the support, which produce a sharp decrease in hydrogenolysis and aromatization, respectively [5]. 

A number of investigations have indicated that so-called "strong metal-support interactions (SMSI)" are caused by the migration of partially reduced oxide species onto the surface of titania supported metal particles (1 8). While this is an attractive theory in that it can account for the observed modifications in chemical properties of the metal particles, there is no agreed mechanism by which the postulated transport processes occur. 

The work of Tauster and coworkers (1,2) showed that hydrogen chemisorption is suppressed on group VIII metals supported on a series of oxides after these samples have been reduced at high temperatures. The term strong metal-support interactions (SMSI) was introduced to describe this behavior. A similar suppression in hydrogen chemisorption has since been reported for many other supported metal systems 0-5). However, the use of other chemical probes (4, 5) demonstrated that different mechanisms of metal-support interactions could exist for different types of oxides. Furthermore, even for a so-called SMSI oxide, the degree of interaction could be influenced by many parameters such as crystallite size and reduction temperature. It would thus be desirable to find an approach to systematically compare catalytic behavior of different systems. 

Before proceeding to consider kinetic equations and implied reaction mechanisms, we may note some other pertinent features of these reactions. (A) Benzene hydrogenation was subject to the influence of the Strong Metal-Support Interaction (Section 3.35) when titania and vanadium sesquioxide were used as supports for rhodium, platinum and iridium - even Pt/Si02 and Ni/Si02 when heated... 

Bifunctional and Spillover Catalysis. - It has been suggested above that the active involvement of the support in a catalytic reaction could appear as a metal-support interaction, and several recent papers demonstrate in some detail how this can come about. Catalysis of the hydrogenation of CO2 by Rh on various supports shows a strong dependence of turnover number on the kind of support used (see Table 2). Dispersions, D, are similar, as are activation energies, so the considerable differences in turnover number reside principally in the pre-exponential factor. A further study of this system by i.r. spectroscopy has identified CO on the metal and the formate ion on the support (although not in the case of Si02), and the favoured mechanism for CH4 formation involves a reaction of spillover H with CO2 chemisorbed on the support by insertion into surface OH groups. 

A massive electron transfer between the metal particles and the supports (or promoters) and the penetration of an electric field into the metal are thus not realistic ideas on the through-the-metal interaction. However, there is one mechanism for such an interaction which is well supported by the quantum theory of chemisorption when a covalent chemisorption bond is formed, it causes periodic variation (with the distance) in the chemisorption bond strength in its environment. At the nearest site a repulsion is felt, on the next-nearest an attraction, etc. [46a]. However, it is important to realize how strong this interaction is. A realistic estimate, based on observations of the field ion emission images, shows that these interactions are comparable in their strength to the physical (condensation) van der Waals forces [46b]. 

As the deposited oxide layer is well mixed, strong interaction between the oxides is expected, leading often to mechanically strong materials, but pretreatment procedures can be hindered. For instance, in the preparation of a metal-based catalyst, a reduced reducibility of the precursor is often encountered, and, as a result, a reduced availability of the catalytically active phase is encountered. Moreover, under strongly acidic or basic conditions, some support materials, e.g., alumina, may be dissolved, as mentioned before. Furthermore, the adhesion of the precipitated layer with the monolith substrate is often a point of concern, especially during drying and heat treatment. 

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