Hinshelwood, 455

Irving Langmuir was awarded the 1932 Nobel Prize in Chemistiy for his work in surface chemistry. He contributed to atomic theory and the understanding of atomic stracture by defining the concepts of valence shells and isotopes. 

Sir Cyril Norman Hinshelwood s early studies of molecular kinetics led to the publication of Thermodynamics for Students of Chemistry and The Kinetics of Chemical Change in 1926. He studied the explosive reaction of hydrogen and oxygen and described the phenomenon of chain reactions. Hinshelwood was jointly awarded the Nobel Prize in Chemistry in 1956 for his research into the mechanisms of chemical reactions. 

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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... 

Ref. 205). The two mechanisms may sometimes be distinguished on the basis of the expected rate law (see Section XVni-8) one or the other may be ruled out if unreasonable adsorption entropies are implied (see Ref. 206). Molecular beam studies, which can determine the residence time of an adsorbed species, have permitted an experimental decision as to which type of mechanism applies (Langmuir-Hinshelwood in the case of CO + O2 on Pt(lll)—note Problem XVIII-26) [207,208]. 

The Langmuir-Hinshelwood picture is essentially that of Fig. XVIII-14. If the process is unimolecular, the species meanders around on the surface until it receives the activation energy to go over to product(s), which then desorb. If the process is bimolecular, two species diffuse around until a reactive encounter occurs. The reaction will be diffusion controlled if it occurs on every encounter (see Ref. 211) the theory of surface diffusional encounters has been treated (see Ref. 212) the subject may also be approached by means of Monte Carlo/molecular dynamics techniques [213]. In the case of activated bimolecular reactions, however, there will in general be many encounters before the reactive one, and the rate law for the surface reaction is generally written by analogy to the mass action law for solutions. That is, for a bimolecular process, the rate is taken to be proportional to the product of the two surface concentrations. It is interesting, however, that essentially the same rate law is obtained if the adsorption is strictly localized and species react only if they happen to adsorb on adjacent sites (note Ref. 214). (The apparent rate law, that is, the rate law in terms of gas pressures, depends on the form of the adsorption isotherm, as discussed in the next section.)... 

If reaction XVIII-42 is the slow step, the Langmuir-Hinshelwood rate law is... 

Rate laws have also been observed that correspond to there being two kinds of surface, one adsorbing reactant A and the other reactant B and with the rate proportional to 5a x 5b- For traditional discussions of Langmuir-Hinshelwood rate laws, see Refs. 240-242. Many catalytic systems involve a series of intermediates, and the simplifying assumption of steady-state equilibrium is usually made. See Boudart and co-workers [243-245] for a contemporary discussion of such complexities. 

Derive the probable rate law for the reaction CO + j02 = CO2 as catalyzed by a metal surface assuming (a) an Eley-Rideal mechanism and (b) a Langmuir-Hinshelwood mechanism. 

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. 

The correct treatment of the mechanism (equation (A3.4.25), equation (A3.4.26) and equation (A3.4.27), which goes back to Lindemann [18] and Hinshelwood [19], also describes the pressure dependence of the effective rate constant in the low-pressure limit ([M] < [CHoNC], see section A3.4.8.2). 

Hinshelwood C N 1933 The Kinetios of Chemioal Change in Gaseous Systems 3rd edn (Oxford Clarendon)... 

Bernstein R B (ed) 1982 Chemioal Dynamios via Moleoular Beam and Laser Teohniques (The Hinshelwood Leotures, Oxford, 1980) (Oxford Oxford University Press)... 

Figure A3.9.1. Schematic illustrations of (a) the Langmuir-Hinshelwood and (b) Eley-Rideal mechanisms in gas-surface dynamics.

Rettner C T 1994 Reaction of an H-atom beam with Cl/Au(111)—dynamics of concurrent Eley-Rideal and Langmuir-Hinshelwood mechanisms J. Chem. Phys. 101 1529... 

The first step consists of the molecular adsorption of CO. The second step is the dissociation of O2 to yield two adsorbed oxygen atoms. The third step is the reaction of an adsorbed CO molecule with an adsorbed oxygen atom to fonn a CO2 molecule that, at room temperature and higher, desorbs upon fomiation. To simplify matters, this desorption step is not included. This sequence of steps depicts a Langmuir-Hinshelwood mechanism, whereby reaction occurs between two adsorbed species (as opposed to an Eley-Rideal mechanism, whereby reaction occurs between one adsorbed species and one gas phase species). The role of surface science studies in fomuilating the CO oxidation mechanism was prominent. 

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). 

Solid-catalyzed 7eactioixs. Some Langmuir-Hinshelwood mechanisms for the reference reaction A -I- B R -I- S (see also Tables 7.2, 7.3) ... 

Solid Catalyzed Reaction The pioneers were Langmuir (J. Am. Chem. Soc., 40, 1361 [1918]) and Hinshelwood Kinetics of Chemical Change, Oxford, 1940). For a gas phase reaction A + B Products, catalyzed by a solid, the postulated mechanism consists of the following ... 

Over the usual hmited range of conditions, a power law rate equation often appears to be as satisfactory a fit of the data as a more complex Langmuir-Hinshelwood equation. The example of the hydrogenation of oc tenes is shown in Fig. l-2d and l-2e, and another case follows. 

The first is ruled out because the constants physically cannot be negative. Although the other correlations are equally valid statistically, the Langmuir-Hinshelwood may be preferred to the power law form because it is more likely to be amenable to extrapolation. 

Various Langmiiir-Hinshelwood mechanisms were assumed. GO and GO2 were assumed to adsorb on one kind of active site, si, and H2 and H2O on another kind, s2. The H2 adsorbed with dissociation and all participants were assumed to be in adsorptive equilibrium. Some 48 possible controlling mechanisms were examined, each with 7 empirical constants. Variance analysis of the experimental data reduced the number to three possibilities. The rate equations of the three reactions are stated for the mechanisms finally adopted, with the constants correlated by the Arrhenius equation. 

Boudart (1956) and Weller (1956) discussed the applicability and need of Langmuir-Hinshelwood kinetics to describe the rate of industrially... 

The model is intrinsically irreversible. It is assumed that both dissociation of the dimer and reaction between a pair of adjacent species of different type are instantaneous. The ZGB model basically retains the adsorption-desorption selectivity rules of the Langmuir-Hinshelwood mechanism, it has no energy parameters, and the only independent parameter is Fa. Obviously, these crude assumptions imply that, for example, diffusion of adsorbed species is neglected, desorption of the reactants is not considered, lateral interactions are ignored, adsorbate-induced reconstructions of the surface are not considered, etc. Efforts to overcome these shortcomings will be briefly discussed below. 

The catalytic reaction of NO and CO on single crystal substrates, under ultra-high vacuum conditions, has been extensively studied. Neglecting N2O formation and CO desorption, the Langmuir-Hinshelwood mechanism of the NO + CO reaction can be described by the following sequence of steps [16,17] ... 

A dimer-dimer (DD) surface reaction scheme of the type (1/2)A2 + B2 B2A has been proposed in order to mimic the catalytic oxidation of hydrogen A2 is O2, B2 is H2, AB is OH and B2A is H2O. The model reaction proceeds according to the Langmuir-Hinshelwood... 

The monomer-monomer (MM) model, for the reaction A -H B —> AB, assumes the following Langmuir-Hinshelwood reaction schema ... 

The reaction scheme of the ZGB-DD model is based upon the Langmuir-Hinshelwood mechanism. Thus, it is assumed that the reaction occurs according to the following steps ... 

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