Langmuir-Hinshelwood type reactions

The desire to understand catalytic chemistry was one of the motivating forces underlying the development of surface science. In a catalytic reaction, the reactants first adsorb onto the surface and then react with each other to fonn volatile product(s). The substrate itself is not affected by the reaction, but the reaction would not occur without its presence. Types of catalytic reactions include exchange, recombination, unimolecular decomposition, and bimolecular reactions. A reaction would be considered to be of the Langmuir-Hinshelwood type if both reactants first adsorbed onto the surface, and then reacted to fonn the products. If one reactant first adsorbs, and the other then reacts with it directly from the gas phase, the reaction is of the Eley-Ridel type. Catalytic reactions are discussed in more detail in section A3.10 and section C2.8. 

If the rate of both single reactions is expressed separately, e.g. by means of the equations of Langmuir-Hinshelwood type (when written in a more general way and if the mechanism of both reactions with common reagent X will be the same), we obtain... 

The kinetics of a coupled catalytic reaction can be well described by equations of the Langmuir-Hinshelwood type, since these are able to express mutual influencing of single reactions. Power-law equations are not suitable for this purpose. 

The form of equations of the Langmuir-Hinshelwood type and the values of their constants, obtained by the study of isolated reactions, are valid also in the case of the coupled system, where more substances are present and a greater number of interactions is taking place simultaneously on the surface of the catalyst. 

The model considers the noble-metal catalyzed oxidation reactions of CO, two hydrocarbons of differing reactivities and H2, and the reaction kinetics was described by the global rate expressions of the dual-site Langmuir-Hinshelwood type [2]. 

Ihe HVDg exchange reaction. This reaction vras studied vd.th both sur-faces in the wide tenperature range from 100 K to 600 K details are ocnrnunicated in a separate article (25). The H./D- exchange is known to be of the Langmuir-Hinshelwood type and proceed via the following mechanism ... 

Recently, Praharso et al also developed a Langmuir-Hinshelwood type of kinetic model for the SR kinetics of i-Cg over a Ni-based catalyst. In their model, it was assumed that both the hydrocarbon and steam dissociatively chemisorb on two different dual sites on the catalyst surface. The bimolecular surface reaction between dissociated adsorbed species was proposed as the ratedetermining step. The following generalized rate expression was proposed ... 

The electrooxidation of CO to C02 is, similar to its electroless counterpart in gas-phase catalysis, one of the most widely studied electrochemical reaction processes [131,152]. It is generally assumed that the electrooxidation of adsorbed CO proceeds primarily via a Langmuir-Hinshelwood type mechanism involving either adsorbed water molecules or, at higher electrode potentials, adsorbed surface hydroxides (see Figure 6.25) according to... 

When a simple, fast and robust model with global kinetics is the aim, the reaction kinetics able to predict correctly the rate of CO, H2 and hydrocarbons oxidation under most conditions met in the DOC consist of semi-empirical, pseudo-steady state kinetic expressions based on Langmuir-Hinshelwood surface reaction mechanism (cf., e.g., Froment and Bischoff, 1990). Such rate laws were proposed for CO and C3H6 oxidation in Pt/y-Al203 catalytic mufflers in the presence of NO already by Voltz et al. (1973) and since then this type of kinetics has been successfully employed in many models of oxidation and three-way catalytic monolith converters... 

A silica-supported Sn—V—P—O catalyst (Sn/V/P = 1/9/3) was investigated by Onsan and Trimm [244]. Working with a flow reactor at about 520°C, a maximum selectivity of 75% to acrylonitrile was reached at a contact time of ca. 230 g sec l-1 and an oxygen/propene/ammonia ratio of 2/1/1.75. The authors assume that the six principal products (acrylonitrile, acetonitrile, HCN, CO, C02, N2) are formed by six parallel reactions and in the first instance apply power rate equations. A more detailed analysis reveals that a Langmuir—Hinshelwood type rate equation, surface reaction being rate-determining, properly describes the production of acrolein plus acrylonitrile from propene, viz. 

Several authors have studied the kinetics in detail and have proposed kinetic models. According to Trimm and Gabbay [329], the kinetics of the reaction over a Sn/Sb = 1/4 catalyst is best described by a Langmuir— Hinshelwood type of model, which reflects that hydrogen abstraction from butene, involving dissociatively adsorbed oxygen, is rate-controlling viz. 

Regarding the kinetics, the oxidation of o-xylene and o-tolualdehyde were compared for catalysts with different V/Ti ratios (Table 36). The ratio between partial and complete oxidation (X for o-xylene and Y for o-tolualdehyde) are influenced similarly, indicating that a change in the catalyst structure influences all the reaction steps. The oxidation of o-tolualdehyde in mixtures with o-xylene revealed that o-tolualdehyde reduces the o-xylene oxidation rate by a factor of about 2. The authors conclude that a redox model is inadequate and that hydrocarbon adsorption cannot be rate-determining. Adsorption of various products should be included, and equations of the Langmuir—Hinshelwood type are proposed. It should be noted that the observed inhibition is not necessarily caused by adsorption competition, but may also stem from different... 

A Langmuir—Hinshelwood type of model is suggested by Robb and Harriott [266] who studied the reaction at 420°C. They find that the intrinsic kinetics can be represented by... 

For the reaction of diethylether giving either ethylene and ethanol or ethylene and water, the validity of the Langmuir—Hinshelwood type rate equations has again been confirmed [82],... 

Equation (24) is, in fact, a Langmuir—Hinshelwood-type equation. Similar models with a single site surface reaction as the rate-determining step were used for other liquid phase esterifications [448,451]. Experimental data for the l-butanol-x>leic acid system were best fitted by eqn. (24) [452] or eqn. (25) [451]... 

A Langmuir-Hinshelwood type of rate expression is often used to describe the effect of the competitive adsorption of components m in the reaction mixture on the rate of hydrotreating of compound i ... 

Bonn6 [1] studied reaction kinetics of tetraphenylporphyrin model compounds over several catalysts and concluded that hydrogenation and hydrogenolysis occur on different sites. Therefore the reaction mechanism of VO-TPP hydrodemetallisation is described by a two-site model. It was also found that a Langmuir-Hinshelwood type of kinetics applies with small inhibition by hydrogen. 

In reality however, situations also exist where a more complex form of the rate expression has to be applied. Among the numerous possible types of kinetic expressions two important cases will be discussed here in more detail, namely rate laws for reversible reactions and rate laws of the Langmuir-Hinshelwood type. Basically, the purpose of this is to point out additional effects concerning the dependence of the effectiveness factor upon the operating conditions which result from a more complex form of the rate expression. Moreover, without going too much into the details, it is intended at least to demonstrate to what extent the mathematical effort required for an analytical solution of the governing mass and enthalpy conservation equations is increased, and how much a clear presentation of the results is hindered whenever complex kinetic expressions are necessary. 

B Simple Irreversible Reactions with Langmuir-Hinshelwood-Type Kinetics... 

Figure 19. Effectiveness factor ij of an irreversible monomolecular reaction with Langmuir-Hinshelwood-type kinetics versus the Weisz modulus 4>. Influence of intraparticle diffusion on the effective reaction rate (isothermal reaction in a flat plate, X/>iiS as a parameter, adapted from Satterfield [91]).

Roberts and Satterfield [87, 88] also treated bimo-lecular reactions of the type Ai + 6A2 — products, described by Langmuir-Hinshelwood-type models. In this case, we have the rate expression... 

The same approach as for irreversible Langmuir-Hinshelwood-type models can be extended to reversible reactions. Kao and Satterfield [61] developed a graphical method for monomolecular reversible reactions of the type Ai Aj, which is presented here as our last example. The method is based upon the following formulation of the net reaction rate ... 

---