Eley-Rideal processes

The models within each group differed by assumptions of surface processes involving vibrational excited molecules (Eley-Rideal processes, etc.). These effects have so far not been identified as significant. The relevance of vibrationally excited molecules on the divertor dynamics seems to be pre-... 

Fig. 12. Dependence of the photoassisted rates of carbon monoxide oxidation over Ti02 (upper plots) and of isopropanol oxidation over ZnO (lower plots) on reactant pressure. Observed rate photo-oxidation at each pressure is subdivided, in the manner of eqns. (39) in the text, into a rate Vx displaying a Langmuir—Hinshelwood pressure dependence and another rate, V2, increasing linearly with reactant pressure and consistent with an Eley—Rideal process. Reproduced with permission from refs. 183 and 257.

Molecular rotation has two competing influences on the dissociation of diatomics [56, 57 and 58]- A molecule will only be able to dissociate if its bond is oriented correctly with respect to the plane of the surface. If the bond is parallel to the plane, then dissociation will take place, whereas if the molecule is end-on to the surface, dissociation requires one atom to be ejected into the gas phase. In most cases, this reverse Eley-Rideal process is energetically very... 

Another possibility is that a molecule from the gas phase reacts with an adsorbed molecule without adsorbing itself on the surface. Such a case is denoted as an Eley-Rideal process. For the mechanism... 

The literature on dynamics at the gas-solid interface is vast (see, for instance (179-184)). The choice of topics considered here in some detail will clearly be highly subjective. We will focus on those studies which have most in common with the other topics in this chapter and in the entire volume. We will restrict ourselves to the following areas (a) physisorption dynamics, (b) dissociative chemisorption of H2 on metals, (c) Eley-Rideal processes, (d) photochemistry of adsorbed molecule, and (e) cluster-catalyzed chemisorption. 

The second class of reactions includes the direct interaction of a gas-phase species with an adsorbed species to form a product which may either remain adsorbed or desorb into the gas phase (Eley-Rideal processes). For such processes, the rate of reaction can be written as... 

This abstraction experiment is significant in its own right, since it is apparently one of the first direct observations of a surface reaction via an Eley-Rideal process. Abstraction appears to be temperature-independent down to 123 K, suggesting that abstraction is unactivated or that the incident H atoms do not equilibrate with the surface before abstraction occurs. Both suggestions are probably true. A low activation barrier for abstraction from a Si surface (4.5 kcal/mol) has been measured by Abrefah and Olander [37], and a low barrier for abstraction from disilane (2.4 kcal/mol) has been... 

Figure 3.1. Schematic of bond making/breaking process considered in this chapter (a) atomic adsorption/desorption/scattering, (b) molecular adsorption/desorption/scattering, (c) direct dissocia-tion/associative desorption, (d) precursor-mediated dissociation/associative desorption, (e) Langmuir-Hinschelwood chemistry, (f) Eley-Rideal chemistry, (g) photochemistry/femtochemistry, and (h) single molecule chemistry. Solid figures generally represent typical intial states of chemistry and dashed figures the final states of the chemistry.

Direct reaction between an adsorbed species A(s) and a gas-phase molecule B is sometimes proposed. This reaction pathway is called the Eley-Rideal mechanism. Although such a mechanism may seem as reasonable as the Langmuir-Hinshelwood model discussed above, very few heterogeneous reactions are still thought to occur by the Eley-Rideal mechanism. (An exception seems to be when species B is a very reactive radical species, e.g., a gas-phase H-atom reacting with an adsorbed species, as is discussed in Problem 11.10, in which an Eley-Rideal pathway initiates the growth process.)... 

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) ... 

Equations similar to eqns. (5), (6) and (8) were obtained by Zhdanov [104] to describe the monomolecular adsorption and associative desorption and Eley-Rideal s bimolecular reaction. He examined the dependence of the rate constants of these processes on the surface coverages and discussed various approximations applied previously to describe the effect of lateral interaction of adsorbed molecules on the desorption rate constant. He also considered the effect of the lateral interaction on the pre-exponential factor of the rate constants for various processes, and in terms of the "precursor state model, the effect of ordering the adsorbed molecules on the sticking coefficient and the rate constant of monomolecular desorption. 

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. 

Within the frameworks of the lattice gas model it is reasonable to classify the elementary processes by the number of sites m, which a given process occurs on, i.e., one- and two-site cases. In the first case the changing parameter is the occupancy state of one site. The processes such as these include isomerization associated with changes in the internal degrees of freedom of the adspecies (ZA- ZB, i.e., transition of the adspecies from state A to state B), adsorption-desorption of the atoms and nondissociating molecules (A + Z- ZA), reaction according to the collision mechanism (A + ZB ->ZD + C, Eley-Rideal s-type mechanism). It should be remembered that ZA, Z and A denote adspecies A, empty lattice site and species A in the gaseous phase, respectively. 

Eley-Rideal) mechanism, one of the reactants comes directly from the fluid phase to react with the other, which is already chemisorbed. This procedure was devised to explain the kinetics of the hydrogen-deuterium reaction on certain metals (see Section 9.2), but has also been suggested for other reactions. The Mars-van Krevelen mechanism applies to oxidations catalysed by oxides that are easily reducible, and are therefore able to release their lattice oxide ions for the purpose of oxidising the other reactant they are then replaced by the dissociation of molecular oxygen. With gold catalysts supported on such oxides, it is sometimes proposed that this mechanism plays a part in the total process. 

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