Single-site mechanisms surface reactions

Single site. The. surface reaction may be a single-site mechanism in which only the site on which the reactant is adsorbed is involved in the reaction. For example, an adsorbed molecule of A may isomerize (or perhaps decompose) directly on the site to which it is attached, such as... 

Hint. For the single-site mechanism assume (as Mathur and Thodos did) that the surface reaction step can be written as ... 

The authors prefer reaction (a) as the main route to ethylene oxide and reaction (d) to combustion products. Nevertheless, they find that selectivity is not a strong function of temperature, suggesting that there is a common rate-determining step. Recently, they studied the effect of dichloro-ethane as a moderator and concluded that such a compound decreases the activity because silver sites are occupied by chloride ions. But the selectivity is higher because the single site mechanism is first order in surface species (epoxidation) and the dual site mechanism second order (combustion). 

The authors may well be correct when concluding that the surprising weakness of inhibition by aromatics results from slow desorption of a product. However, their model and rate equation appear questionable. Like aromatics, hydrogen is also strongly adsorbed, and so is as likely a candidate as toluene for accumulation on the surface. Also, a single-site mechanism is quite improbable with two strongly adsorbed products. Moreover, the "initial" rates were measured at conversions that entailed a decrease of up to 13% in fluid density, an effect not corrected for. Lastly, the trial equation was derived only from rates at low conversion and cannot be relied upon to reflect the behavior of the reaction as it progresses. 

Diatomic gas A2 undergoes dissociative adsorption, gas B does not require an active site on the catalytic surface because it attacks adsorbed atomic A from the gas phase, and gas C experiences single-site adsorption. Chemical reaction on the catalytic surface is rate limiting in the three-step mechanism. Stoichiometric proportions of A2 and B are present initially (i.e., a 1 2 feed of A2 and B). 

For a catalyzed surface reaction like the exchange of H2 with D2 we cannot talk about a single mechanism for the reaction. We must specify the experimental conditions (pressure, surface coverage, temperature, and surface structure) as the reaction mechanism is likely to change with changing conditions of the experiments. Also, since there are several reaction paths available at the various surface sites, even under specified experimental conditions it is likely that the experimental technique utilized to monitor the reaction rate and product distribution may not detect products that form along the various reaction branches with equal probability. Thus, a combination of techniques that are employed over a wide range of experimental variables is necessary to reveal the nature of the complex catalytic process. 

Situation (I) corresponds to a fluid isotropic solution where a uniform time averaged environment should exist. Under such conditions single exponential decay would be expected for the guest excited states and the photoreactivity should be predictable on the basis of a single effective reaction cavity. In situation (II) there should be two kinetically distinct excited states in two noninterconverting sites resulting in nonexponential decay of the excited state of A. The quantum efficiency of product formation and the product distribution may depend upon the percent conversion. An example of mechanism (II) is provided in Sch. 22 [137]. The ratio of products A, B, and C has been shown to depend on the crystal size. With the size of the crystal the ratio of molecules present on the surface and in the interior changes which results in different extents of reactions from two the distinct sites namely, surface and interior. 

This retardation of reaction rate with increase in reactant partial pressure is characteristic of catalytic reactions controlled by a surface reaction mechanism. Langmuir-Hinshelwood surface reaction rate mechanisms for single and dual site mechanisms are respectively ... 

Selective oxidation of CO in hydrogen over different catalysts has been extensively examined. Most research to date has occurred with formulations that include a precious metal component supported on an alumina carrier. The catalyst-mediated oxidation of CO is a multistage process, commonly obeying Langmuir-Hinshelwood kinetics for a single-site competitive mechanism between CO and 02. Initially, CO is chemisorbed on a PGM surface site, while an 02 molecule undergoes dissociative chemisorption either on an adjacent site or on the support in order for surface reaction between chemisorbed CO and O atoms to produce C02. 

A kinetic model for n-butane isomerization over sulfated zirconia catalysts is proposed involving two types of active sites on the catalyst surface and a bi-molecular reaction mechanism. Deactivation kinetics are included in which the two different active sites deactivate at different rates. The proposed model more accurately captures the activity trends observed experimentally with respect to time on stream behavior compared to a single site model with deactivation. 

Reactants A and B reversibly produce C and D via gas-soUd kinetics. A and C experience single-site adsorption as characterized by Langmuir isotherms. Reactant B attacks adsorbed A from the gas phase or from a physisorbed layer. In other words, B does not occupy active sites on the catalyst, and neither does product D. Single-site chemical reaction on the surface is the slowest step in the mechanism. Hence,... 

Components B, C, D, and A2B experience single-site adsorption, whereas diatomic A2 undergoes dual-site adsorption. Triple-site chemical reaction on the catalytic surface equilibrates on the time scale of the adsorption of reactant A2. The modified Langmuir-Hinshelwood mechanism is described by the following sequence of six elementary steps ... 

The feed stream is stoichiometric in terms of the two reactants. Diatomic A2 undergoes dissociative adsorption. Components B, C, and D experience single-site adsorption, and triple-site chemical reaction on the catalytic surface is the rate-controlling feature of the overall irreversible process. This Langmuir-Hinshelwood mechanism produces the following Hougen-Watson kinetic model for the rate of reaction with units of moles per area per time ... 

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