Active sites ideal crystals

In this exercise we shall estimate the influence of transport limitations when testing an ammonia catalyst such as that described in Exercise 5.1 by estimating the effectiveness factor e. We are aware that the radius of the catalyst particles is essential so the fused and reduced catalyst is crushed into small particles. A fraction with a narrow distribution of = 0.2 mm is used for the experiment. We shall assume that the particles are ideally spherical. The effective diffusion constant is not easily accessible but we assume that it is approximately a factor of 100 lower than the free diffusion, which is in the proximity of 0.4 cm s . A test is then made with a stoichiometric mixture of N2/H2 at 4 bar under the assumption that the process is far from equilibrium and first order in nitrogen. The reaction is planned to run at 600 K, and from fundamental studies on a single crystal the TOP is roughly 0.05 per iron atom in the surface. From Exercise 5.1 we utilize that 1 g of reduced catalyst has a volume of 0.2 cm g , that the pore volume constitutes 0.1 cm g and that the total surface area, which we will assume is the pore area, is 29 m g , and that of this is the 18 m g- is the pure iron Fe(lOO) surface. Note that there is some dispute as to which are the active sites on iron (a dispute that we disregard here). 

As the crystal surface exposed to the atmosphere is usually not ideal, specific sites exist with even much lower co-ordination numbers. This is shown schematically in Fig. 3.5, which gives a model comprising so-called step, kink and terrace sites (Morrison, 1982). This analysis suggests that even pure metal surfaces contain a wide variety of active sites, which indeed has been confirmed by surface science studies. Nevertheless, catalytic surfaces often behave rather homogeneously. Later it will be discussed why this is the case. In short, the most active sites deactivate easiest and the poorest active sites do not contribute much to the catalytic activity, leaving the average activity sites to play the major role. 

Several different test reactions have been suggested to evaluate the catalytic activity of an acid catalyst as a measure of the number and strength of the active sites. The ideal test reaction is experimentally easy, fast, reproducible, requires only a small amount of catalyst, has simple kinetics, and should show little deactivation. It should also not be diffusion limited and affected by the particle or crystal size. While no one reaction fits all these criteria perfectly, we and apparently others - find that hexane cracking comes closer to the ideal than most other reactions. 

X-ray crystallography and NMR spectroscopy are both powerful tools for the investigation of protein structure and conformation. Vital information may be obtained about the binding sites and catalytic sites of enzymes, particularly if the enzyme can be crystallized with the natural substrate or some smaller, analogue molecule in place at the active site. It occurred to us that our epoxyalkyl glycosides, suitably modified to contain either an iodine (heavy) or fluorine (magnetically active) atom, would be ideal molecules to assist in X-ray crystallographic and NMR analyses, respectively. 

Another argument in favor of XRD is that catalytic activity should be related to the defects (or "nanostructure") of a solid catalyst. Active sites are nonequilibrium structures and are therefore not part of the bulk structures that are most commonly determined by diffraction analysis. XRD is, however, in many ways sensitive to the deviations from perfect ordering of the unit cells in a perfect crystal hence, XRD can be used to detect the nanostructure representing deviations from an ideal crystal. 

An examination of the flavocytochrome 2 active site, as defined by the crystal structure (Xia and Mathews, 1990), clearly shows that if His373 formed a hydrogen bond to the substrate hydroxyl, then the hydrogen on the a-carbon would be ideally placed for hydride transfer to flavin N-5 as shown in Figure 4a. Tyr254 would then come into play, stabilising the transition state (in which the a-carbon would have less sp and more sp character). Thus a hydride transfer mechanism is certainly consistent with recent... 

The development of new catalytic materials needs to be complemented with detailed studies of the surface chemistry of catalysis at the molecular level in order to better define the requirements for the catalytic active sites. The wide array of modem spectroscopies available to surface scientists today is ideally suited for this task (see Surfaces). Surface science studies on catalysis typically probe reaction intermediates on model metal samples under well controlled conditions. This kind of study is traditionally carried out in ultrahigh vacuum (UHV) systems such as that shown in Figure 10. Single crystals or other well-defined metal surfaces are cleaned and characterized in situ by physical and chemical means, and then probed using a battery of surface sensitive techniqnes snch as photoelectron (XPS and UPS), electron energy loss (ELS... 

If the metal catalyst particles were present only in the form of these idealized crystals, then the number of active comer atoms present would be very low. However, STO evaluations of dispersed metal catalysts have shown that these active atoms are present in rather large amounts, at times as high as 30%-35% of the total metal atoms present. Such high surface concentrations of the highly unsaturated atoms can only be accounted for by the presence of the irregular particle shapes that were observed using dark field TEM imaging techniques. Additional active sites are probably present as adatoms on the 111 (M) and 100 (K) planes as shown in Fig. 4.4. 

If it is assumed that the activities of species in their regular sites are dose to unity, and that the ideal crystal without defects is initially denoted as nil, then Equation (3.4) can be simplified to the following form ... 

Ample experimental evidence indicates that the habit of crystallites of the catalyst has a profound influence on the activity and selectivity of the reaction, which results in the appearance of structure sensitivity of the selective oxidation reactions at oxide catalysts (39). However, little is known about the origin of this phenomenon and about the differences of the structure of active sites present at various crystal planes. Even less is known about the role of defects present at the surface of an oxide, in determining the catalytic properties. Only recently studies of the properties of (100) surface of a monocrystal of NiO revealed that an ideal surface is chemically inert and the reactivity of the system increases only if defects are introduced in the surface (40). At such a surface, dissociation of molecular water to form hydroxyl groups is observed in contrast to an ideal surface which is inactive in water dissociation. 

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