Catalyst, free valencies

The valence band structure of very small metal crystallites is expected to differ from that of an infinite crystal for a number of reasons (a) with a ratio of surface to bulk atoms approaching unity (ca. 2 nm diameter), the potential seen by the nearly free valence electrons will be very different from the periodic potential of an infinite crystal (b) surface states, if they exist, would be expected to dominate the electronic density of states (DOS) (c) the electronic DOS of very small metal crystallites on a support surface will be affected by the metal-support interactions. It is essential to determine at what crystallite size (or number of atoms per crystallite) the electronic density of sates begins to depart from that of the infinite crystal, as the material state of the catalyst particle can affect changes in the surface thermodynamics which may control the catalysis and electro-catalysis of heterogeneous reactions as well as the physical properties of the catalyst particle [26]. 

The treatment of free electrons and holes as free valencies is very convenient in describing chemical processes on the surface of a semiconductor. The following properties (10, 11) must be attributed to the free valencies of the catalyst in such treatment ... 

The catalyst thus acts as a sort of polyradical influencing the course of the reaction in the same manner in which the introduction of free radicals into a homogeneous medium affects the course of a homogeneous reaction. In both cases, the reaction is accelerated because of the participation of the free valencies. In the case of heterogeneous catalysis, these free valencies are introduced by the catalyst itself. In the final account, it is they who sustain and regulate the process. 

In the 1950s, Semenov and Voevodskii [148] made an attempt to apply the concepts of the branching-chain reaction theory to the kinetics of heterogeneous catalysts. They applied the concept of free valencies migrating over the catalyst surface and of "semi-chemisorbed radicals. But their attempt was criticized (see, for example, ref. 149 where Temkin, using hydrogenation of ethylene on palladium as an example, proved experimentally the inapplicability of the chain theory concepts). 

In this paper, we propose to present a theoretical study of the catalytic hydrogenation of ethylene in order to compare the catalytic power of various transition metals. As a basic hypothesis, we have taken the fact that the ethylene molecule attaches itself to the metallic surface and this increases its reactivity. This is the same hypothesis as that of Coulson and Longuet-Higgins82 and Daudel and Sandorfy.38 These workers calculated the variation of the index of free valence. We have considered it more interesting to endeavour to calculate the electronic potential barrier. We have used the system of reaction levels as shown in Fig. 1, where V is the potential barrier in the absence of catalyst, Q the heat of chemisorption, and U the potential barrier in passing... 

Metals in a state of fine sub-division or colloidal form are rich in free valence bonds and hence they are more efficient catalysts than the metal in lumps. 

The intensive search for the active cocatalyst responsible for this activation led in 1977 to the isolation of MAO, a component in which aluminum and oxygen atoms are positioned alternately and free valences are saturated by methyl groups (65, 66). When metallocenes, especially zirconocenes, are combined with MAO, the resulting catalyst can polymerize olefins 10-100 times faster than those used in the most active Ziegler-Natta systems (67). 

The valleys V between the atoms of a catalyst, dealt with in the multiplet theory, corresponds to what is designed in a number of studies by means of free valencies or by asterisks 49, 50)... 

A molecule with a double bond adsorbed on a semiconducting catalyst surface converts into a radical bound with the lattice and having a free valence. A molecule with a single bond emerging from the gas phase may react with the free valence of such a radical and dissociate. 

Thus chemisorption and the associated energetic aspects play a crucial role in understanding heterogeneous catalysis [10]. The active centers on the catalyst surface are probably the result of free valences or electron defects, which weaken the bonds in the adsorbed molecules to such an extent that a reaction can readily occur. The course of a heterogeneously catalyzed reaction is compared to that of an uncatalyzed reaction in Figure 5-9. 

Extension of the model then led to the concept of active centers on the catalyst surface, presumably attributable to free valences or electron defects (see Section 5.3.3). Therefore, methods for characterizing catalyst surfaces are of great importance, and they play a key role in imderstanding catalysis. 

Many catalysts have the fee structure. The arrangement of the atoms in the above-mentioned surfaces is depicted in Figure 5-17. Also shown is the munber of neighboring atoms and free valences of the surface atoms for tiie example of the nickel lattice [T33]. The highest number of free valences, namely five, occurs for the prismatic faces. 

The development of nanotechnology during the last decades has led scientists to fabricate and analyze catalysts at the nanoscale. These nanostructured materials are usually high-surface-area metals or semiconductors in the form of NPs with excellent catalytic properties due to the high ratio of surface atoms with free valences to the cluster of total atoms. The catalysis takes place on the active surface sites of metal clusters in a similar mechanism as the... 

The Co-catalyzed formal [2+2+2] cyclization of cis allene 2.294 in the presence of T) -cyclopentadienyldicarbonylcobalt(I) [CpCo(CO)2] in boiling xylene at 300 W irradiation for 15 minutes converts it into a bicyclic yne-triene compound 2.297 in 66% yield (Scheme 2.97) [146]. This is a result of a formal ene-Diels—Alder reaction between the triple bond, an allene double bond bearing a methyl group, and two free valences of the Co-complex (structure 2.295), affording compound 2.296 with a five-membered ring. After the oxidative coupling, the final product 2.297 is formed by p-elimination accompanied by reductive elimination of the catalyst. This route has a potential for the synthesis of complex polycyclic molecules. 

Compounds of transition metals (Mn, Cu, Fe, Co, Ce) are well known as catalysts for the oxidation of hydrocarbons and aldehydes (see Chapter 10). They accelerate oxidation by destroying hydroperoxides and initiating the formation of free radicals. Salts and complexes containing transition metals in a lower-valence state react rapidly with peroxyl radicals and so when these compounds are added to a hydrocarbon prior to its oxidation an induction period arises [48]. Chain termination occurs stoichiometrically (f 1) and stops when the metal passes to a higher-valence state due to oxidation. On the addition of an initiator or hydroperoxide, the induction period disappears. 

The second attribute of the catalyst concerns its electronic structure, or more simply the valence electron count. Effective catalysts must, it seems, have < 18 VE, such that coordination of a substrate or the departure of a product does not itself pose a major kinetic barrier. Furthermore, it happens that the most stable valence states of the metal will differ by two units. Thus not only will the stoichiometry of atom transfer be supported, but also the mechanism. In the case of rhenium, the oxidation states are Re(V) and Re(VII) indeed scant indication of Re(VI) has been found in this chemistry, especially in a mononuclear species. Likewise, there is no indication of the involvement of free radical chemistry. 

Although valence band spectra probe those electrons that are involved in chemical bond formation, they are rarely used in studying catalysts. One reason is that all elements have valence electrons, which makes valence band spectra of multi-component systems difficult to sort out. A second reason is that the mean free path of photoelectrons from the valence band is at its maximum, implying that the chemical effects of for example chemisorption, which are limited to the outer surface layer, can hardly be distinguished from the dominating substrate signal. In this respect UPS, discussed later in this chapter, is much more surface sensitive and therefore better suited for adsorption studies. 

UPS studies of supported catalysts are rare. Griinert and coworkers [45] recently explored the feasibility of characterizing polycrystalline oxides by He-II UPS. A nice touch of their work is that they employed the difference in mean free path of photoelectrons in UPS, V 2p XPS and valence band XPS (below 1 nm, around 1.5 nm, and above 2 nm, respectively) to obtain depth profiles of the different states of vanadium ions in reduced V205 particles [45]. However, the vast majority of UPS studies concern single crystals, for probing the band structure and investigating the molecular orbitals of chemisorbed gases. We discuss examples of each of these applications. 

Inifiation. The trick here is to get the reaction started. Usually a catalyst is used, typically an organic peroxide such as ditertiary butyl hydroperoxide. Peroxide molecules are somewhat unstable, and when they re heated, they decompose and turn into highly reactive free radicals. As you ll recall, a radical is an almost-complete molecule, but all the valence requirements are not satisfied. So it is very anxious to meet up with some other molecule to satisfy its valence needs. The free radical, in the presence of an abundance of monomers, say a million to one ratio, will react with a monomer molecule. It becomes part of the molecule. In doing so, the unsatisfied valence condition now transfers to the end of the monomer. A new radical is formed. That s the start of the initiation step. 

A favorable combination of valence forces of both components seems to be the basic principle of the nickel-molybdenum ammonia catalyst. It has been found (50) that an effective catalyst of this type requires the presence of two solid phases consisting of molybdenum and nickel on the one hand and an excess of metallic molybdenum on the other. Similar conditions prevail for molybdenum-cobalt and for molybdenum-iron catalysts their effectiveness depends on an excess of free metal, molybdenum for the molybdenum-cobalt combination and iron for the molybdenum-iron combination, beyond the amounts of the two components which combine with each other. A simple explanation for the working mechanism of such catalysts is that at the boundary lines between the two phases, an activation takes place. In the case of the nickel-molybdenum catalyst, the nickel-molybdenum phase will probably act preferentially on the hydrogen and the molybdenum phase on the nitrogen. 

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