Surface Langmuir-Hinshelwood mechanism

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 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). 

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 oxidation of CO on Pt is one of the best studied catalytic systems. It proceeds via the reaction of chemisorbed CO and O. Despite its complexities, which include island formation, surface reconstruction and self-sustained oscillations, the reaction is a textbook example of a Langmuir-Hinshelwood mechanism the kinetics of which can be described qualitatively by a LHHW rate expression. This is shown in Figure 2.39 for the unpromoted Pt( 111) surface.112 For low Pco/po2 ratios the rate is first order in CO and negative order in 02, for high pco/po2 ratios the rate becomes negative order in CO and positive order in 02. Thus for low Pcc/po2 ratios the Pt(l 11) surface is covered predominantly by O, at high pco/po2 ratios the Pt surface is predominantly covered by CO. 

Ertl and his colleagues in 1997 reported detailed STM data for the oxidation of CO at Pt(l 11) surfaces, with quantitative rates extracted from the atomically resolved surface events.27 The aim was to relate these to established macroscopic kinetic data, particularly since it had been shown that no surface reconstruction occurred and the reaction was considered to obey the Langmuir-Hinshelwood mechanism, where it is assumed that the product (C02) is formed by reaction between the two adsorbed reactants, in this case O(a) and CO(a). Nevertheless, it was well known that for many features of the CO oxidation reaction at Pt(lll) there is no mechanism that is consistent with all features of the kinetics the inherent problem is that in general a reaction mechanism cannot be uniquely established from kinetics because of the possible contribution of intermediates or complications for which there might be no direct experimental evidence. 

Combined with their kinetic measurements, the authors proposed CO from the gas phase could directly react with oxygen atoms in the surface oxides, accounting for relatively high reactivity of this phase for CO oxidation. This mechanism, termed as Mars-Van Krevelen mechanism, challenges the general concept that CO oxidation on Pt group metals is dominated by the Langmuir-Hinshelwood mechanism, which proceeds via (1) the adsorption of CO and the dissociative adsorption of 02 and (2) surface diffusion of COa(j and Oa(j atoms to ultimately form C02. 

Recent reviews by Ertl and Engel have summarized most of the chemisorption and low pressure catalytic findings . In general, the reaction proceeds through a Langmuir-Hinshelwood mechanism involving adsorbed CO and O atoms. Under reaction conditions typical in most high pressure, supported catalyst studies, and most low pressure UHV studies, the surface is almost entirely covered by CO, and the reaction rate is determined by the rate of... 

However, for the Langmuir-Hinshelwood mechanism the rate will go through a maximum and end up at zero, when the surface is completely covered by A. This happens because the step B + < B cannot proceed when A blocks all sites. 

The reaction mechanism is a Langmuir-Hinshelwood mechanisms comprising G gases, S surface species and R elementary steps, where G, S and R are arbitrary. 

Deviations from this simple expression have been attributed to mechanistic complexity For example, detailed kinetic studies have evaluated the relative importance of the Langmuir-Hinshelwood mechanism in which the reaction is proposed to occur entirely on the surface with adsorbed species and the Eley-Rideal route in which the reaction proceeds via collision of a dissolved reactant with surface-bound intermediates 5 . Such kinetic descriptions allow for the delineation of the nature of the adsorption sites. For example, trichloroethylene is thought to adsorb at Ti sites by a pi interaction, whereas dichloroacetaldehyde, an intermediate proposed in the photo-catalyzed decomposition of trichloroethylene, has been suggested to be dissociatively chemisorbed by attachment of the alpha-hydrogen to a surface site... 

Soma s excellent infrared and kinetic study of ethene hydrogenation catalyzed by Pt/Al203 (423) showed clearly the dominant role played by the 77-adsorbed ethene species and by the reversibly adsorbed hydrogen that occurs at higher pressures in the form of on-top PtH. It also pointed to a Langmuir-Hinshelwood mechanism as the 77-adsorbed ethene was shown to compete with adsorbed H atoms for surface sites. 

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

The kinetics of the ammoxidation of xylenes over a vanadium catalyst and mixed vanadium catalysts were studied. The reaction rate data obtained were correlated with the parallel consecutive reaction scheme by the rate equations based upon the Langmuir-Hinshelwood mechanism where the adsorption of xylenes was strong. The reaction rates of each path are remarkably affected by the kind of xylene and catalyst. The results of the physical measurement of catalysts indicated that the activity and the selectivity of reaction were affected by the nature and the distribution of metal ions and oxygen ion on catalyst surface. 

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) ... 

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. 

This also holds for the other mechanism—known as the Langmuir-Hinshelwood mechanism—where the two reactant molecules have to be chemisorbed side by side. Such pairs can be formed statistically by the molecules hitting the surface and being bound by chemisorption forces at the very spots where they hit (localized adsorption) or they may result from collisions of the molecules moving along the surface of the catalyst-(mobile adsorption). It may again be stated that the formation of pairs in the case of immobile localized adsorption is more favorable for reaction than in the case of mobile adsorption. 

Similar effects may be obtained with promotors producing such surface-potential effects that the heat of chemisorption is low enough for chemical action. Low heats of chemisorption are especially needed in the case of the Langmuir-Hinshelwood mechanism (Sec. VII,5). 

---