Catalyst surface arrangement

In polymerizing these compounds, a reaction between a-TiCls and triethylaluminum produces a five coordinate titanium (111) complex arranged octahedrally. The catalyst surface has four Cl anions, an ethyl group, and a vacant catalytic site ( ) with the Ti(lll) ion in the center of the octahedron. A polymerized ligand, such as ethylene, occupies the vacant site ... 

Heat-flow microcalorimetry may be used, therefore, not only to detect, by means of adsorption sequences, the different surface interactions between reactants which constitute, in favorable cases, the steps of probable reaction mechanisms, but also to determine the rates of these surface processes. The comparison of the adsorption or interaction rates, deduced from the thermograms recorded during an adsorption sequence, is particularly reliable, because the arrangement of the calorimetric cells remains unchanged during all the steps of the sequence. Moreover, it should be remembered that the curves on Fig. 28 represent the adsorption or interaction rates on a very small fraction of the catalyst surface which is, very probably, active during the catalytic reaction (Table VI). It is for these... 

For a specific reaction, the reaction rates, as defined in eqs. (3.2)-(3.4), depend on the nature of the catalytic active site, the surface arrangement of the catalyst, the temperanire, and the reactants concentration. Surface arrangement here denotes the macroscopic and measurable catalyst basic properties ... 

The principal difference between these catalyst level reaction rates and the turnover frequency is that the latter does not depend on the surface arrangement of the catalyst, or in more practical terms, does not depend on the specific physicochemical characteristics of the catalyst as a composite of the active catalytic reagent plus the support. 

The previously reported relationship (eq. (3.10)) manifests one more important characteristic of the catalyst level rate coefficients—for the same reaction, temperature, catalytic agent, and support but for different surface arrangement, i.e. active site concentration, these coefficients will be different and this is an advantage of the usage of kp and in general, of turnover frequency (active site level reaction description). 

In order to avoid any confusion, the surface structure used in sensitive and insensitive reaction analysis has nothing to do with the surface arrangement used in the catalyst level rates analysis—the first refers to the microscopic level of the active site, whereas the latter to the catalyst level. 

Fifl. 9. Arrangement for the investigation of the interaction between catalyst and foreign molecules by measurement of the secondary electron yield of the catalyst surface [according to (84)J. 

This somewhat simplified picture of possible transitions from one mechanism to another can be expanded and supplemented by a finer differentiation of the factors influencing bond strength and catalyst acid-base properties. Such structural parameters are the number and nature of substituents on Ca and Cp and the nature of the group X. The action of a catalyst depends on its cation charge and radius, on anion basicity and on lattice and surface arrangement (for some details see ref. 67). A temperature increase usually shifts the mechanism in the direction of El. 

It is frequently convenient to describe the chemical properties of a catalyst surface by reference to surface sites. A surface site may be composed of one or more surface atoms or ions and may act as an active site for a given catalytic reaction, or may be inactive. It is characterized by the chemical reactivity of these atoms or ions and by their spatial arrangement at the surface. The site density is the number of sites per unit surface area. However, it is expedient to extend the use of this term to cases where surface area has no clear significance. In these cases the site density is referred to unit mass or unit volume of the catalyst. 

The activity of a catalyst depends on the nature, the number, the strength and the spatial arrangement of the chemical bonds that are transiently created between the reactants and the surface. The objective of the chemical characterization of the surface is a detailed description of the adsorbate-adsorbent bonds that a given catalyst will develop when contacted with a given reaction mixture. Therefore, chemical characterization should be done in situ in the course of the reaction itself. However, because of experimental limitations, this is seldom possible and catalyst surfaces are usually characterized by means of separate experiments. It is important to characterize the catalyst surface both before and after its use in a reaction. 

These examples are two of many in the literature illustrating how XAFS spectroscopy has been used to obtain detailed structural information about the active site —the species present on the catalyst surface after some pretreatment but prior to catalytic reaction. This type of XAFS analysis is ideally suited to samples in which there is a well-defined bonding arrangement between the species of interest and the support and all of the species are the same. Often there is no other way to obtain this information. 

The nature and arrangement of the pores determine transport within the interior porous structure of the catalyst pellet. To evaluate pore size and pore size distributions providing the maximum activity per unit volume, simple reactions are considered for which the concept of the effectiveness factor is applicable. This means that reaction rates can be presented as a function of the key component. A only, hence RA(CA). Various systems belonging to this category have been discussed in Chapters 6 and 7. The focus is on gaseous systems, assuming the resistance for mass transfer from fluid to outer catalyst surface can be neglected and the effectiveness factor does not exceed unity. The mean reaction rate per unit particle volume can be rewritten as... 

In each of these studies the siterproduct relationship was determined on the basis of product distribution data obtained from standard, steady state catalytic reactions. While this approach can provide evidence for the type of site(s) responsible for the formation of certain products it cannot give any indication of the number of such sites that are present on the catalyst surface. Since the activities of the various types of sites are different, it is possible that a small number of very active sites could dominate product formation. In order to relate the extent of produet formation with the number of specific types of sites present an experimental arrangement is needed which obviates these site activity differences. One way of doing this is to use the catalyst surface as a stoichiometric reagent so that each site reacts only once. In this way there will always be a 1 1 site product molecule ratio regardless of the rates at which the different types of sites react. 

As mentioned in Chapter 3, the octahedral models used to describe the active sites on metal surfaces are not compatible with the presence of three different types of saturation sites on a catalyst surface so another model must be developed. On consideration of the fee crystal structure, which is that of most catalytically active metals, it can be seen that the bulk atoms in these metals are bound to twelve nearest neighbor atoms using the lobes of the t2g d orbitals. The octahedraily oriented eg orbitals are directed toward but not bonded to the next nearest neighbors in the crystal lattice as shown in Fig. 4.1.1 This atomic orientation precludes the presence of any octahedral arrangement involving M-M bonds. 

The present finding shows the critical importance of the atomic-scale design of the surface of a material. The arrangements of Ti4 + ions and O2- ions create new catalytic functions for desired chemical processes. The active sites can be produced in situ under the catalytic reaction conditions, even if there are no active sites at the surface before the catalysis. The surface is also modified by adsorption of a reactant to form a new surface with a different add-base character from the intrinsic property. The dynamic acid-base aspect at a catalyst surface is the key issue to regulate the acid-base catalysis, which may provide a new strategy for creation of acid-base catalysts. 

---