Dual-Site Dissociative Adsorption

Both mobile and immobile eoverages can again be considered but now, because the desorption step involves the recombination of two neighboring adsorbed species, the functions of f(9) and f (9) are different than before because of the difference in reaction (recombination) probabilities. 

Thus these two systems give identical mathematical forms for the Langmuir equation and they cannot be differentiated experimentally by uptake measurements alone. However, for immobile adsorption requiring site pairs, it has been shown statistically that complete saturation cannot be achieved and the highest coverage is about 90% of a full monolayer [6]. 

One straightforward way to test these isotherms is to linearize them, evaluate the linearity and, if acceptable, obtain values for the monolayer coverage, nm, and the adsorption equilibrium constant, K, from the slope and the intercept. For example, equation 5.17 for the single-site model can be rearranged to  

In either of these arrangements, the slope gives the monolayer uptake. 

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The basic problem can be stated as follows minimize an objective function fjx), where x is a vector variable comprising the parameters to be estimated, andXx) is the sum of squares to be minimized. For instance, in the oxidation of carbon monoxide, already mentioned in Chapter 4 and discussed in more detail in Chapter 11 (and in [Asprey, 1997]), the following Langmuir-Hinshelwood Dual Site Dissociative Adsorption Model is used to fit experimental data ... 

Single-site surface coverage by CO and methanol is described by a classic Langmuir isotherm, whereas H2 requires modification for dual-site dissociative adsorption. Hence,... 

The equation reflects dual site reversible adsorption. Methanol and formaldehyde compete for sites, while oxygen is dissociatively adsorbed on different sites. At a not-too-low oxygen pressure (>0.01 atm) the coverage of the oxygen sites is complete and the equation reduces to... 

Gomez-Sainero et al. (11) reported X-ray photoelectron spectroscopy results on their Pd/C catalysts prepared by an incipient wetness method. XPS showed that Pd° (metallic) and Pdn+ (electron-deficient) species are present on the catalyst surface and the properties depend on the reduction temperature and nature of the palladium precursor. With this understanding of the dual sites nature of Pd, it is believed that organic species S and A are chemisorbed on to Pdn+ (SI) and H2 is chemisorbed dissociatively on to Pd°(S2) in a noncompetitive manner. In the catalytic cycle, quasi-equilibrium ( ) was assumed for adsorption of reactants, SM and hydrogen in liquid phase and the product A (12). Applying Horiuti s concept of rate determining step (13,14), the surface reaction between the adsorbed SM on site SI and adsorbed hydrogen on S2 is the key step in the rate equation. 

Sachtler [195] proposed a dual-site mechanism in which the hydrogen is dissociated on the Ni surface and then migrates to the substrate that is coordinated to the adsorbed dimeric nickel tartrate species. In their model, adsorption of modifier and reactants takes place on different surface atoms in contrast to Klabunovskii s proposal. Adsorbed modifier and reactant are presumed to interact through hydrogen bonding (Scheme 14.5). The unique orientation of adsorbed modifier molecules leads to a sterically favored adsorbed reactant configuration to achieve this bonding. 

Various types of neutron scattering can be utilized to extract data on structure and dynamics for novel catalytic materials. By selectively deuterating an SSZ-13 zeolite, Cheetham and others" used ND performed on the Dual Beam Neutron Spectrometer (DUALSPEC) diffractometer at the Chalk River Laboratories and found that two acid sites are present in the unit cell of the zeohte. INS can be used to probe the mechanism of the catalytic reaction by looking at the change in the vibrational modes of the adsorbed molecules on the surface. Lennon et alP found that the interaction of HCl with a ]-alumina catalyst results in the dissociative adsorption of HCl, in which the hydroxyl groups terminally bound to A1 are replaced by chlorine. INS spectra reveal an in-plane deformation mode, 5 (OH), that can be resolved into two bands located at 990 and 1050 cm. ... 

Pignet and Schmidt have fitted a Langmuir-Hinshelwood model for non-competitive dual-site adsorption and with dissociative oxygen adsorption ... 

Epoxidation of butadiene occurs by addition of dissociatively-adsorbed oxygen to one of the localized C=C bonds to form epoxybutene. The addition of oxygen across the terminal carbon atoms does not occur to any measurable extent. The direct participation of molecular oxygen can be ruled out based both on selectivity arguments as well as the kinetic model for the reaction. The kinetics imply a dual site mechanism. One site, which is unpromoted, serves as the site for butadiene adsorption, while the second site, which is promoted, functions as the site for dissociative oxygen adsorption and epoxybutene formation. 

For a dual site mechanism with dissociative adsorption of hydrogen. 

Dual site formalism can be easily extended to other cases, e.g. for dissociative adsorption. 

Closure. After reading this chapter, the reader should be able to discuss the steps in a heterogeneous reaction (adsorption, surface reaction, and desorption) and describe what is meant by a rate-limiting step. The differences between molecular adsorption and dissociated adsorption should be explained by the reader, as should the different types of surface reactions (single site, dual site, and Eley-Rideal). Given heterogeneous reaction rate data, the reader should be able to analyze the data and to develop a rale law for Langmuir-Hinshelwood kinetics. The reader should be able to discrimi-... 

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