Catalytic Mechanisms Rideal

Using kinetic models of typical catalytic mechanisms (Eley-Rideal and Langmuir-Hinshelwood (LH) mechanisms) as examples, we found parametric domains, in which the hypergeometric representation is an excellent approximation... 

Steps 1 through 9 constitute a model for heterogeneous catalysis in a fixed-bed reactor. There are many variations, particularly for Steps 4 through 6. For example, the Eley-Rideal mechanism described in Problem 10.4 envisions an adsorbed molecule reacting directly with a molecule in the gas phase. Other models contemplate a mixture of surface sites that can have different catalytic activity. For example, the platinum and the alumina used for hydrocarbon reforming may catalyze different reactions. Alternative models lead to rate expressions that differ in the details, but the functional forms for the rate expressions are usually similar. 

Whether a catalytic reaction proceeds via a Langmuir-Hinshel vood or Eley-Rideal mechanism has significant implications for the kinetic description, as in the latter case one of the reactants does not require free sites to react. However, Eley-Rideal mechanisms are extremely rare, and we will assume Langmuir-Hinshelwood behavior throughout the remainder of this book. 

Perhaps the most extensive computational study of the kinetics of NO reactions on Rh and Pd surfaces has been provided by the group of Zgrablich. Their initial simulations of the NO + CO reaction on Rh(lll) corroborated the fact that the formation of N-NO intermediate is necessary for molecular nitrogen production [83], They also concluded that an Eley-Rideal mechanism is necessary to sustain a steady-state catalytic regime. Further simulations based on a lattice-gas model tested the role of the formation of... 

Detailed microkinetic models are available for CO, H2 and HC oxidation on noble metal(s) (NM)/y-Al203-based catalysts (cf., e.g. Chatterjee et al., 2001 Harmsen et al., 2000, 2001 Nibbelke et al., 1998). The model for CO oxidation on Pt sites includes both Langmuir-Hinshelwood and Eley-Rideal pathways (cf., e.g., Froment and Bischoff, 1990). Microkinetic description of the hydrocarbons oxidation is more complicated, particularly due to a large number of different reaction intermediates formed on the catalytic surface. Simplified mechanisms, using just one or two formal surface reaction steps,... 

Whilst, in principle, kinetic measurements should allow a differentiation between the two possible mechanisms, it must be noted that in catalytic hydrogenation reactions relatively few examples are sufficiently clear cut to allow this differentiation to be made. Thus, for example, it is quite commonly found that the experimentally observed orders of reaction are zero in the unsaturated substrate A and unity in hydrogen. Such results are readily interpreted by the adjacent-site mechanism by assuming A to be much more strongly adsorbed than hydrogen or by the Rideal— Eley type of mechanism. Clearly, kinetic measurements alone are insufficient for the establishment of mechanism. 

The observed effects of structure on rate and on orientation, confirmed by the Brown selectivity relationship, show that there is no basic difference between heterogeneous catalytic alkylation of aromatic compounds and homogeneous electrophilic aromatic substitution, cf. nitration, sul-phonation etc. This agreement allows the formulation of the alkylation mechanism as an electrophilic attack by carbonium ion-like species formed on the surface from the alkene on Br0nsted acidic sites. The state of the aromatic compound attacked is not clear it may react directly from the gas phase (Rideal mechanism ) [348] or be adsorbed weakly on the surface [359]. 

In every gas/solid catalytic cycle, at least one of the reactants must at some point be adsorbed on the catalyst surface. Let us consider the reaction A + B —> C. There are two options (Figure 4.2) In the first, both reactants A and B are first adsorbed on the catalyst, migrate to each other, and react on the surface, giving the product C, which is desorbed into the gas phase. This pathway, which we have already met in Chapter 2, is the Langmuir-Hinshelwood mechanism. The other option is that A is adsorbed on the catalyst surface, and B subsequently reacts with it from the gas phase to give C (the so-called Eley-Rideal mechanism [22]). The Langmuir-Hinshelwood mechanism is much more common, partly because many reactants are activated by the adsorption on the catalyst surface. 

Several examples of analysis of simple kinetic schemes that are linear in respect to catalytic intermediates but whose initial or final reaction groups include several "external" reactants are considered following. Obviously, according to such schemes, some of the catalytic intermediates are interacting directly with the external reactants. In this situation, the catalytic heterogeneous reaction is usually said to follow the Eley-Rideal mechanism. 

Example 6 Catalytic reaction with two initial reactants, one of which interacts with the catalytic intermediate by the Eley Rideal mechanism... 

The second step is the interaction of the external reactant R2 with intermediate Ki by the Eley-Rideal mechanism. The stationary rate of this catalytic stepwise process is... 

Rideal, E. K., A note on a simple molecular mechanism for heterogeneous catalytic reactions, Proc. Camb. Phil. Soc., 35, 130-132 (1939). 

Eley-Rideal (ER), and stepwise (SW) mechanisms. The LH model was tested by Mark et al. in the C02 reforming of methane.50 It assumed that both reactant species of CH4 and C02 are adsorbed onto the catalyst active sites separately. Adsorbed reactants then associatively react on the active sites and lead to H2 and CO product formation. The basic model is established on the basis that the reactant species of CH4 and C02 follow the first-order behavior. In the ER mechanism, one of the two reactants (either CH4 or C02) is adsorbed onto the catalyst surface in adsorption equilibrium. The adsorbed species then react with the other reactant from the gas phase, and H2 and CO are formed subsequently.51 The SW mechanism assumes that CH4 dissociatively adsorbed (active carbon and hydrogen species) on the catalytic surface. The active carbon reacts with C02 in the gas phase and produces two equivalents of CO. 

For the case of supported model catalyst under surface science conditions an Eley-Rideal mechanism has so far not been unambiguously identified. However, for gas-phase clusters, an Eley-Rideal type mechanism, in which the reactant has to collide at the right position with the cluster-complex to initiate the reaction, is more likely to proceed due to the reduced size of the catalytically active unsupported cluster. 

---