Heterogeneous surface-catalysed reactions

For a molecule characterised by a AH value of 40 k.I mol 1 and undergoing facile surface diffusion, i.e. a A/ dir value close to zero, then each molecule will visit, during its surface lifetime (10 r s), approximately 107 surface sites. Since the surface concentration a is given by a = NtSUIf, then for a AH value of 40 kJ mol-1 and zsurf= 10-6 s at 295 K, the value of a is 109 molecules cm-2. These model calculations are illustrative but it is obvious that no conventional spectroscopic method is available that could monitor molecules present at a concentration 10-6 monolayers. These molecules may, however, contribute, if highly reactive, to the mechanism of a heterogeneously catalysed reaction we shall return to this important concept in discussing the role of transient states in catalytic reactions. 

As the Beckmann rearrangement is believed to be a typical acid-catalysed reaction, many researchers have reported the relationship between the vapour phase reaction catalysis and the acidity of the catalysts tested on non-zeolitic catalysts - i2s- i3i. 318-334 and on zeolitic catalysts Another interesting point for the heterogeneous gas-phase Beckmann rearrangement is the location of the reaction on the catalyst and different studies have been published ° . The outer surface of the catalyst particle seems to be the most probable place for the Beckmann rearrangement supported by the traces of reagents, and notable amounts of by-products found only in the outer layers of the zeolite crystal. Development of new and more efficient catalysts have also been reported " . ... 

Moreover, as neither the concept of surface initiated homogeneous-heterogeneous reaction (11) can be invoked to explain our results, it can be stated that the methane partial oxidation reaction proceeds via a surface catalysed process which likely involves specific catalyst requirements. However, by comparing the HCHO productivity of the different catalytic systems previously proposed (9) with that of our 5% V205/Si02 catalyst, it emerges that our findings constitute a relevant advancement in this area (23),... 

A heterogeneously catalysed reaction involves several steps (Mady et al, 1976) (i) mass transport of fluid reactants to the surface, (ii) chemisorption of reactants on the surface, (iii) diffusion and chemical reaction at the surface and (iv) desorption and diffusion of products from the surface. Step (iii), involving the formation of surface intermediates, is the key step. Formation of surface intermediates, which ultimately give rise to products, was first proposed by Sabatier (see Burwell, 1973) and strikingly demonstrated by Sachtler Fahrenfort (1960) in the decomposition of formic acid... 

We begin with the simplest model scheme for a heterogeneously catalysed reaction, with Langmuir-Hinshelwood kinetics. A reactant P is adsorbed, reversibly, onto a surface S. There it may react to give a product C. which is immediately and irreversibly desorbed ... 

The problem is also more complex when heterogeneous catalysed reactions are considered. With porous catalyst pellets, reaction occurs at gas- or liquid-solid interfaces at the outer or inner sphere. When the reactants diffuse only slowly from the bulk phase to the exterior surface of the catalyst, gas or liquid film resistance must be taken into account. Pore diffusion resistance may be involved when the reactants move through the pores into the pellet. 

Diacetyl (DA) is used as a flavour enhancer in the food industry and is currently manufactured from methyl ethyl ketone (MEK) in homogeneous systems via an oxime intermediate (ref.1). In principle, DA can also be manufactured by the selective oxidation of MEK and several reports have appeared in the literature which apply heterogeneous catalysts to this task (refs. 2-4). A number of reports have specified the importance of basic or weakly acidic sites on the catalyst surface for a selectively catalysed reaction and high selectivities to DA at moderate conversions of MEK have been reported for catalysts based on C03O4 as a pure oxide and with basic oxides added conversely scission reactions have been associated with acidic oxide additives (refs. 2-4). Other approaches to this problem have included the application of vanadium phosphorus oxide (VPO) catalysts. Ai (ref. 5) has shown that these catalysts also catalyse the selective oxidation of MEK to DA. Indeed this catalyst system, used commercially for the selective oxidation of n-butane to maleic anhydride (ref.6), possesses many of the desired functionalities for DA formation from MEK, namely the ability to selectively activate methylene C-H bonds without excessive C-C bond scission. 

Catalytic reactions consist of a reaction cycle formed by a series of elementary reaction steps. Hence the rate expression is in general a function of many parameters. In heterogeneously catalysed reactions reactant molecules are adsorbed on the catalyst surface (characterized by equilibrium constants Kj), undergo chemical modifications on the surface to give adsorbed products with rate constants fc, and these products finally desorb. The overall catalyst activity and selectivity is determined by the composition and structure of its surface. Hence it is important to relate constants, such as fc and K with the chemical reactivity of the catalyst surface. 

The second type of heterogeneously catalysed reaction subject to external diffusion control behaves quite differently. Where the catalysis is strong enough for the reaction to be almost at equilibrium on the surface, the rate constant will contain both diffusion and thermodynamic terms. Equation (60) for unimolecular reactions is one example and another is eqn. (62) which applies to the initial stages of a general reaction. In the latter case [79]... 

Catalysed substitution reactions of an unusual kind are collected together in this section. In each case, the catalysis of the reaction by a homogeneous entity is assisted by the surface of a solid. The resulting reinforcement of catalytic effects is frequently described as synergistic. The homogeneous and heterogeneous catalysts quite often possess a species in common, for example Ag+ ions and solid Agl, and many of the homogeneously catalysed reactions exhibit autocatalysis as a result. 

The most efficacious catalysts to date have been the noble metals, carbons, and some insoluble oxides and salts. As was emphasized in Sect. 1.8, tests should always be carried out to confirm that heterogeneous catalysis is the true reason for any observed rate increase. One of these tests requires the catalytic rate to rise proportionately with the mass or area of the catalyst. While most reaction systems have satisfied this criterion, the rates of several carbon-catalysed reactions in the literature have been reported as increasing either much more or much less than expected when larger amounts of the solid were added. Pore diffusion could account for only some of these results. Since carbons are cheap catalysts with large surface areas, their aberrant behaviour in this respect would be worth serious investigation. 

The mesoporous solids developed by Mobil group in 1991 were found to be catalytically inactive and have attracted a considerable interest from researchers throughout the world to introduce catalytically active sites within these materials. For example, doping of aluminium into the silica generates Bronsted acid sites and the resulting materials can be used as solid acids in acid-catalysed reactions. It is also possible to deposit metal particles within the pores and to use these materials as redox catalysts in many chemical reactions. Another avenue for catalytic functionalisation is to tether metal complexes within pores in order to prepare heterogeneous catalysts. It has been observed in the process of functionalisation that MCM materials can lose mesoporosity, surface area and pore volume as shown by nitrogen... 

Textural properties are important in the field of catalyst design for heterogeneous catalysis Surface area and pore size determine the accessibility to active sites and this is often related to catalytic activity and selectivity in catalysed reactions Therefore textural properties are often a target of catalyst design. 

To complete this chapter, we would like to mention that recent monographs have reviewed the use of in-situ spectroscopies for monitoring heterogeneously catalysed reaction under supercritical conditions, although very few studies in this field has been devoted to the study of the fluid-solid interface.182 The use of a multi-technique approach in order to maximise information under real, in-situ conditions has also been reviewed recently.183 The combined use of powerful spectroscopies with simultaneous on-line analysis of the catalytic activity of the sample will become more widespread in application allowing an interpretation of catalytic behaviour in terms of the physico-chemical properties of the solid. The next frontier in spectroscopic characterisation of metal catalysts will consist of time-dependent analysis of the gas/liquid-solid interface, particularly with a view to analyse short-lived intermediates during catalysed reactions and with the aim to determine the response of the catalyst surface and relate these responses to the physico-chemical properties of the solid. 

D25.6 Heterogeneous catalysis on a solid surface requires the reacting molecules or fragments to encounter each other by adsorption on the surface. Therefore, the rate of the catalysed reaction is determined by the sticking probabilities of the species on the surface as described by Figure 25.28 of the text. 

SO2, oxidation with molecules, heterogeneous reactions in the gas phase, catalysed and non-catalysed reactions in liquid phase, and reactions on the surface of particles. These processes will now be discussed in turn. 

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