Surface sites, heterogeneous catalysts

Surface site heterogeneity of Ru/Si02 for ammonia synthesis, as well as the effect of K promotion, has been studied using SSITKA.K is a well known activity promoter for ammonia synthesis on Ru and Fe catalysts. The Ru/Si02 catalyst was studied at 673 K, 204 kPa, H2/N2 ratio of 3 and GHSV between 5000-23000. SSITKA experiments were carried out by switching between N2 and N2. 

It is therefore obvious to expect research efforts to immobilize the active sites of enzymes at the surface of heterogeneous catalysts. More in particular, molecular sieve zeolites are preferred solids in this respect. 

Homogeneous catalysts are often very active in the polymerization of ethylene, since weight for weight they have a larger active surface than heterogeneous catalyst systems. In other cases, however, homogeneous catalysts are less active. This may depend on the fact that they represent a complicated mixture of a variety of compounds, of which only some are active. In addition, the rate constants for soluble and insoluble sites may be different. Insoluble polymers can occlude the catalyst, thus lessening its activity. 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

Much work has been undertaken to understand the steps and intermediates by which the reaction occurs on the heterogeneous catalyst surface. However, the exact mechanism is not fully established. One approach assumes a first-step adsorption of carbon monoxide on the catalyst surface followed by a transfer of an adsorbed hydrogen atom from an adjacent site to the metal carbonyl (M-CO) ... 

A kinetic description of a heterogeneous catalytic reaction will in most cases be different when the reaction proceeds simultaneously with other reactions in a complex system, compared with the case where its kinetics was studied separately. The most important is the effect in the case where the reactions concerned take place on the same sites of the surface of a catalyst. Let us take, for example, the system of competitive reactions... 

Microporous catalysts are heterogeneous catalysts used in catalytic converters and for many other specialized applications, because of their very large surface areas and reaction specificity. Zeolites, for example, are microporous aluminosilicates (see Section 14.19) with three-dimensional structures riddled with hexagonal channels connected by tunnels (Fig. 13.38). The enclosed nature of the active sites in zeolites gives them a special advantage over other heterogeneous catalysts, because an intermediate can be held in place inside the channels until the products form. Moreover, the channels allow products to grow only to a particular size. 

We can think of a heterogeneous catalyst as a collection of active sites (denoted by ) located at a surface. The total number of sites is constant and equal to N (if there is any chance of confusion with N atoms, we will use the symbol N ). The adsorption of the reactant is formally a reaction with an empty site to give an intermediate I (or more conveniently R if we explicitly want to express that it is the reactant R sitting on an adsorption site). All sites are equivalent and each can be occupied by a single species only. We will use the symbol 6r to indicate the fraction of occupied sites occupied by species R, making N6r the number of occupied sites. Hence, the fraction of unoccupied sites available for reaction will be 1 - 0r The following equations represent the catalytic cycle of Fig. 2.7 ... 

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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