Particle sites, heterogeneous

General Observations About x. its Relationship to the Overall Partitioning Coefficient and to the Concept of Surface-Site Heterogeneity. One approach to metal/particle surface interactions which has been developed, historically, in a variety of forms, is a conceptual model that assumes only two conditions for surface sites occupied by an adsorbate or unoccupied. In applying this approach to the solid/aqueous solution interface, the adsorption... 

The Brunaurer Emmett Teller specific surface area (BET SSA) of carbon black is influenced by the size, the porosity, and the surface microstructure of the primary particles. Surface heterogeneities given by graphitic planes at the surface, amorphous carbon, crystallite edges, and slit-shaped cavities representing adsorption sites of different energies describe the surface microstructure. 

Fig. 15.1. The principal surface and particle sites for heterogeneous catalysts.

Figure 11.8 Reduced adsorption rate of particles at heterogeneous surfaces (fec/ c) versus spherical site coverage (6s). Points denote the experimental results. The solid line shows the theoretical results calculated from Equation 11.21 with K= and the dashed like shows the theoretical results calculated by neglecting the coupling, that is, ff=0.

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

Rate of polymerization. The rate of polymerization for homogeneous systems closely resembles anionic polymerization. For heterogeneous systems the concentration of alkylated transition metal sites on the surface appears in the rate law. The latter depends on the particle size of the solid catalyst and may be complicated by sites of various degrees of activity. There is sometimes an inverse relationship between the degree of stereoregularity produced by a catalyst and the rate at which polymerization occurs. 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

In catalysis active sites are operative that allow for an alternative reaction path. For a satisfactory catalyst this alternative pathway leads to higher rates and higher selectivity. In heterogeneous catalysis reactant molecules adsorb at active sites on the catalyst surface at the surface sites reactions occur and products are desorbed subsequently. After desorption, active sites are again available for reactant molecules and the cycle is closed. In homogeneous catalysis the situation is essentially identical. Here complexation and decomplexation occur. A complication in heterogeneous catalysis is the need for mass transfer into and out of the catalyst particle, which is usually porous with the major part of the active sites at the interior surface. 

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