Heterogeneous surface-catalysed

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),... 

In this section we are going to consider the implementation of first-order heterogeneous chemical reactions coupled to the electron transfer. In these processes the electroactive species are transformed in a surface-catalysed chemical process that can be characterised through a first-order rate constant fchet (m s ) that is independent of the applied potential. Although a more detailed description of these systems may need to consider the possible adsorption/desorption of the species involved in the heterogeneous chemical transformation, the formalism presented in this section enables a first, simpler description with only one additional unknown variable fchet [13]. 

All heterogeneously catalysed processes must be preceded by gas adsorption on the surface of the solid catalyst before the reaction. There are two fundamental kinds of mechanistic situations that can arise in the surface-catalysed transformation (see Figure 3) [6]. 

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. 

Free radicals are atoms or groups of atoms possessing an odd (unpaired) electron. Radical recombination occurs when active flame propagating species (O , H and OH) recombine (heterogeneously) on particle surfaces or (homogeneously) as a result of gas phase reactions catalysed by alkali metal atoms in the flame, e.g. 

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 " . ... 

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 ... 

Bond [294] used comparisons between homogeneously and heterogeneously catalysed interconversions of unsaturated hydrocarbons to deduce that the reactive state of an adsorbed hydrocarbon may reasonably be assumed to be a jr-complex (see Sect. 3.2, p. 22). On this assumption, a molecular orbital model appropriate to a face-centred cubic metal was developed. By considering the direction of emergence and degree of occupation of the metal atomic orbitals at the (100), (110) and (111) faces, assuming that the atomic orbitals on the surface keep the same orientation as in the bulk metal, which may not be valid [295], he concluded that the (111) planes were least suited to the adsorption requirements of... 

One suggestion made was that all reactions subject to the influence of negative catalysts were in reality reactions already catalysed by a trace of some other substance present in small enough amount to be more or less completely removed even by the small quantity of negative catalyst added. Examples of this kind of action are known and indeed the poisoning of a catalytic surface in a heterogeneous reaction is something of the kind. Nevertheless examples of inhibition are known which do not seem to be explicable in this way. 

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. 

Figure 2.1 Physical (1,2,6,7) and chemical (3-5) steps involved in the following heterogeneously catalysed model reaction A + B —> C. For sake of simplification the surface reaction (4) is supposed to occur between chemisorbed A and nonadsorbed B molecules. 1, Diffusion of A (la) and B (lb) molecules from the homogeneous phase to the external surface of catalyst particle. 2, Diffusion of A (2a) and B (2b) molecules along the pores. 3, Chemisorption of A on the active site. 4, Reaction between chemisorbed A and nonadsorbed B with formation of C chemisorbed on the active site. 5, Desorption of C from the active site. 6, Diffusion of C (6c) out of the pore. 7, Diffusion of C (7c) from the pore mouth to the homogenous phase...

Recently two heterogeneous TPAP catalysts were developed which could be recycled successfully and displayed no leaching In the first example the tetraalkylammonium perruthenate was tethered to the internal surface of mesoporous silica (MCM-41) and was shown [ 101] to catalyse the selective aerobic oxidation of primary and secondary allylic and benzylic alcohols (Fig. 17). Surprisingly, both cyclohexanol and cyclohexenol were unreactive although these substrates can easily be accommodated in the pores of MCM-41. No mechanistic interpretation for this surprising observation was offered by the authors. 

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