Langmuir-Hinshelwood or Eley-Rideal Mechanisms

In Langmuir-Hinshelwood kinetics is it assumed that all species are adsorbed and accommodated (in thermal equilibrium) with the surface before they take part in any reactions. Hence, species react in the chemisorbed state on the surface. This is the prevailing situation in heterogeneous catalysis. 

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. 

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When a reaction takes place by a redox mechanism, other reactions may take place at the same time by Langmuir-Hinshelwood or Eley-Rideal mechanisms or by homogeneous reactions. These other reactions could be eliminated or minimized by using two reactors, one fed with air or oxygen and the other fed with reactant A. The catalyst would be circulated between the two units using moving-bed or fluidized-bed technology. 

Rate equation can be expressed in power form or in terms of the Langmuir-Hinshelwood or Eley-Rideal mechanism. For example, the reaction of acetic acid and n-butanol in the liquid phase catalyzed by HZSM-5 proceeds according to a rate equation which is of the first order with respect to acetic acid and of the zeroth order with respect to n-butanol. It has been suggested that vapor-phase esterification of acetic acid with ethanol proceeded on decationized Y zeolites by a reaction between strongly adsorbed acetic acid and ethanol. The reaction rate is expressed by a Rideal-type rate equation, where the influence of the pressures of the acid dimer and products as well as the pressures of the reactants were taken into account. ... 

It needs to be mentioned that, within the Mars—van Krevelen-type catalytic cycle, the elementary steps of CO oxidation by lattice oxygen may stiU follow a Langmuir—Hinshelwood or Eley—Rideal mechanism. In the Langmuir—Hinshelwood mechanism, the CO molecule adsorbs first on the ceria surface before undergoing the oxidation, whereas in the Eley— Rideal mechanism, the CO molecule attacks directly the surface oxygen from the gas phase. As we have already mentioned when we discussed... 

In order to explain the mechanism of the Fisher-Tropsch reaction, some authors derived the Langmuir-Hinshelwood or Eley-Rideal types of rate expressions for the reactant consumption, where in the majority of cases the rate-determining step is supposed to be the formation of the building block or monomer, methylene [134],... 

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,... 

Similar equations were written by Eley [204] for the exchange of N2 with N2 catalyzed by Fe or W, and mechanisms such as Eq. XVIII-33 have come to be known as Eley-Rideal mechanisms. Mechanisms such as that of Eq. XVIII-32 are now most commonly called Langmuir-Hinshelwood mechanisms (see... 

Mechanistic kinetic expressions are often used to represent the rate data obtained in laboratory studies, and to explain quantitatively the effects observed in the field. Several types of mechanisms have been proposed. These differ primarily in complexity, and on whether the mechanism assumes that one compound that is adsorbed on the catalyst surface reacts with the other compound in the gas phase, eg, the Eley-Rideal mechanism (23) or that both compounds are adsorbed on the catalyst surface before they react, eg, the Langmuir-Hinshelwood mechanism (25). 

In the presence of chlorine atoms, the chlorine radical appears to be the active surface species. It is not possible from our limited data to establish whether most reaction occur via Langmuir-Hinshelwood or Rideal-Eley mechanisms. 

The central question in this discussion is again whether the reaction takes place between two adsorbed species (Langmuir-Hinshelwood mechanism) or between a gas phase CO molecule and an oxygen adatom (Eley-Rideal mechanism). All of the investigation cited indicated that under steady-state... 

If, on the other hand, surface reaction determined the overall chemical rate, equation 3.68 (or 3.69 if an Eley-Rideal mechanism operates) would represent the rate. If it is assumed that a pseudo-equilibrium state is reached for each of the adsorption-desorption processes then, by a similar method to that already discussed for reactions where adsorption is rate determining, it can be shown that the rate of chemical reaction is (for a Langmuir-Hinshelwood mechanism) ... 

Thus, assuming that one of the mechanisms (either the Langmuir -Hinshelwood or the Eley-Rideal) is irreversible, the second mechanism must also be assumed to be irreversible provided that K2 = 0. If the process is carried out at high temperatures and K2 is a minute value, the equality K4 = K2K3 can also be fulfilled in the case when the fourth step is reversible and the third is practically irreversible. It does not contradict the principle of detailed equilibrium. 

From simple measurements of the rate of a photocatalytic reaction as a function of the concentration of a given reactant or product, valuable information can be derived. For example, these measurements should allow one to know whether the active species of an adsorbed reactant are dissociated or not (22), whether the various reactants are adsorbed on the same surface sites or on different sites (23), and whether a given product inhibits the reaction by adsorbing on the same sites as those of the reactants. Referring to kinetic models is therefore necessary. The Langmuir-Hinshelwood model, which indicates that the reaction takes place between both reactants at their equilibrium of adsorption, has often been used to interpret kinetic results of photocatalytic reactions in gaseous or liquid phase. A contribution of the Eley-Rideal mechanism (the reaction between one nonadsorbed reactant and one adsorbed reactant) has sometimes been proposed. 

Besides volume processes wall collisions of hydrogen particles can contribute to the vibrational population. A direct process is the interaction of already vi-brationally excited molecules with the surface (v) +wall —> ff2(w) mostly depopulating the vibrational levels. Further fundamental mechanisms are the Langmuir-Hinshelwood and the Eley-Rideal mechanism. They are based on recombining hydrogen atoms or ions Hads/gas + Hads —> H2(v). In the first case an adsorbed particle at the surface recombines with another adsorbed particle (Langmuir-Hinshelwood mechanism). In the second case one particle from the gas phase recombines with an adsorbed particle (Eley-Rideal mechanism). For these processes the data base is scarce and often not determined from plasma material interaction experiments. A dependence on particle densities, surface material and surface treatment as well as surface temperature can be expected. 

A following stage of chemisorption process on the solid surface is a chemical reaction of the reactant immediately from the gas phase (Eley-Rideal mechanism) or between the intrinsic precursor and active sites (Langmuir-Hinshelwood mechajiism). Possible mechanisms of these reactions and formal kinetic equations have been discussed previously. 

The reaction of NO, NO2, and NH3 over a CuO/NiO on (X-Al203 catalyst was studied by Blanco et al. [114]. Ammonia chemisorbs on the catalyst surface forming NH2 species, whereas copper is reduced to Cu+. Evidence for the presence of Cu+ was obtained by means of XPS and FTIR. Then NO2 oxidizes Cu into Cu + to form NO. This adsorbed NO reacts with the NH2 species through a Langmuir-Hinshelwood mechanism. NO present in the gas phase can react with the catalyst through an Eley-Rideal mechanism forming a reduced copper site. The Cu+ sites are promptly reoxidized by O2 or NO2 O2 plays a minor role in the reoxidation. 

Other DFT calculations have shown that co-adsorption of H2O and O2 on Aug clusters, free or supported on MgO(lOO), leads to the formation of an 02- -H20 complex involving partial proton sharing or proton transfer, and leading to a hydroperoxy-like complex (HO2) [178]. This favors the activation of the 0—0 bond, i.e. the bond extension to values characteristic of a peroxo- or superoxo-Uke state. Consequently, the reaction with CO can occur with a small activation barrier of -0.5 eV, either through an Eley-Rideal mechanism if O2 is adsorbed on the top face of Aug clusters or through a Langmuir-Hinshelwood mechanism if O2 is adsorbed on the periphery of the cluster. 

Noble metals may follow either a Langmuir-Hinshelwood type of mechanism (reaction between adsorbed oxygen and an adsorbed reactant) or an Eley-Rideal mechanism (reaction between adsorbed oxygen and a gas phase reactant molecule). In the case of nucleophilic organic reactants (e.g., CO or alkenes), both the oxygen and reactant are adsorbed and react on the surface. 

A more complicated example is the oxidation of CO over Pt. Two typical mechanisms exist for this reaction (i) the Eley-Rideal or impact mechanism and (ii) the Langmuir-Hinshelwood or adsorption mechanism. The Eley-Rideal mechanism does not involve any interaction between catalytic intermediates one component from the gas phase, in this case oxygen, adsorbs on the catalyst surface forming a surface intermediate, and another component from the gas phase, in this case carbon monoxide, reacts with this surface intermediate ... 

Fig. 1. Variation of rate with reactant pressure for a bimolecular surface reaction proceeding by a Rideal—-Eley or a Langmuir—Hinshelwood mechanism.

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