Langmuir-Hinshelwood type

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. 

In the context of Langmuir-Hinshelwood type kinetics the latter can be... 

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 ... 

Two Langmuir-Hinshelwood type equations also were tried. 

Langmuir-Hinshelwood mechanism, 19 78 Langmuir-Hinshelwood type rate expressions, 25 192... 

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... 

A large number of authors describe the oxidation kinetics by Langmuir—Hinshelwood type models. Depending on the particular L—H model selected, the mathematical difference between L—H models and redox models can be very small, although the former always contains more... 

A Langmuir—Hinshelwood type of mechanism is also supported by the work of Klugherz and Harriott [178] and Spath and Handel [300], These authors, and several others, advocate 02 (and O2-) as the active oxygen species. Carberry et al. [74] and Forzatti et al. [118] contributed to the evidence for Oj with their results on the effect of promotors. 7-Irradiation of a calcium-promoted silver catalyst enhanced the yield of ethylene oxide. It could be shown that, during irradiation, calcium migrated to the surface, increasing the 02 concentration there. 

Kinetic redox models, as formulated by Mars and van Krevelen [204], have not been considered in any recent work. Although the combined dependence on both propene and oxygen pressures does arise in certain investigations, the authors seem to ignore redox mechanisms completely and correlate their data with Langmuir—Hinshelwood type models. 

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],... 

Langmuir—Hinshelwood-type equations were applied in some cases. The kinetics of the vapour phase condensation of acetaldehyde with formaldehyde to acrolein at 275—300°C over sodium-containing silica gel... 

For a formal kinetic description of vapour phase esterifications on inorganic catalysts (Table 21), Langmuir—Hinshelwood-type rate equations were applied in the majority of cases [405—408,410—412,414,415]. In some work, purely empirical equations [413] or second-order power law-type equations [401,409] were used. In the latter cases, the authors found that transport phenomena were important either pore diffusion [401] or diffusion of reactants through the gaseous film, as well as through the condensed liquid on the surface [409], were rate-controlling. 

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 rate equation was derived on the assumption that in the first step formaldehyde is formed on the catalyst surface from adsorbed carbon monoxide and hydrogen. The subsequent conversion of formaldehyde to methanol was assumed to be the rate-determining step. The experimental data were best expressed by a rate equation of the Langmuir-Hinshelwood type ... 

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