Unimolecular surface reaction

A. Unimolecular Surface Reactions We suppose the type reaction to be... 

Prove that for a unimolecular surface reaction, the order of reaction with respect to reactant decreases from unity to zero as adsorption of the reactant increases from slight adsorption to strong adsorption. 

First-Order Unimolecular Surface Reaction For the reaction... 

We have thus far written unimolecular surface reaction rates as r" = kCAs assuming that rates are simply first order in the reactant concentration. This is the simplest form, and we used it to introduce the complexities of external mass transfer and pore diffusion on surface reactions. In fact there are many situations where surface reactions do not obey simple rate expressions, and they frequently give rate expressions that do not obey simple power-law dependences on concentrations or simple Arrhenius temperatures dependences. 

Data on alkyl radical oxidation between 300° and 800°K. have been studied to establish which of the many elementary reactions proposed for systems containing alkyl radicals and oxygen remain valid when considered in a broad framework, and the rate constants of the most likely major reactions have been estimated. It now seems that olefin formation in autocatalytic oxidations at about 600°K. occurs largely by decomposition of peroxy radicals rather than by direct abstraction of H from an alkyl radical by oxygen. This unimolecular decomposition apparently competes with H abstraction by peroxy radicals and mutual reaction of peroxy radicals. The position regarding other peroxy radical isomerization and decomposition reactions remains obscured by the uncertain effects of reaction vessel surface in oxidations of higher alkanes at 500°-600°K. 

The relation of homogeneous unimolecular changes to the corresponding catalytic reactions on surfaces will be discussed in a later section. 

In more detail, our approach can be briefly summarized as follows gas-phase reactions, surface structures, and gas-surface reactions are treated at an ab initio level, using either cluster or periodic (plane-wave) calculations for surface structures, when appropriate. The results of these calculations are used to calculate reaction rate constants within the transition state (TS) or Rice-Ramsperger-Kassel-Marcus (RRKM) theory for bimolecular gas-phase reactions or unimolecular and surface reactions, respectively. The structure and energy characteristics of various surface groups can also be extracted from the results of ab initio calculations. Based on these results, a chemical mechanism can be constructed for both gas-phase reactions and surface growth. The film growth process is modeled within the kinetic Monte Carlo (KMC) approach, which provides an effective separation of fast and slow processes on an atomistic scale. The results of Monte Carlo (MC) simulations can be used in kinetic modeling based on formal chemical kinetics. 

We now consider that the elementary reaction will be the unimolecular surface reaction expressed by the following equation... 

In unimolecular surface reactions, a single reactant species is adsorbed and reacts on the surface. A good example is given by catalysed nucleophilic solvolyses discussed further in Sect. 2.1. The reaction scheme may then be written... 

Table 5.2.2 Pre-exporer ia "actors for selected unimolecular surface reactions.

In certain cases, a quasi-unimolecular surface process occurs, for example, surface diffusion to a molecule-forming site or heterogeneous dissociation of an adsorbed intermediate (e.g., RCOO -> R -t- CO 2 in the Kolbe reaction). Then... 

A different model [11] that can be used to obtain the kinetics equation for a pyrolytic reaction is adapted from the theory developed for the kinetics of heterogeneous catalytic reactions. This theory is described in literature for various cases regarding the determining step of the reaction rate. The case that can be adapted for a pyrolytic process in solid state is that of a heterogeneous catalytic reaction with the ratedetermining step consisting of a first-order unimolecular surface reaction. For the catalytic reaction of a gas, this case can be written as follows ... 

Case I Irreversible Reaction (Unimolecular). Consider the following mechanistic equation for a reaction occurring on a catalyst surface ... 

Similar conventions are applied to reactions on surfaces. If individual molecules undergo decomposition or isomerization on a surface the reaction is usually described as unimolecular, even though the surface atoms are also involved. This, however, would not be done if the material of the surface entered into the final products. 

Reaction Models with a RDS - Unimolecular Surface Reactions... 

Finally, to summarize, a variety of the traditional rate expressions for reactions on an ideal surface has been examined, and many of their derivations have been discussed in detail. These include L-H and R-E models describing unimolecular and bimolecular reactions on surfaces with either one type of active site or two types of active sites. If a RDS other than that for a surface reaction is proposed, i.e., either an adsorption or a desorption step, then a H-W rate expression is derived. These standard rate laws, which assume a RDS exists, are frequently referred to and utilized, and they are summarized in Table 7.10. Many other forms of a rate expression, which do not assume a RDS and utilize the SSA, can be derived based on the reaction sequence proposed. 

The same equation applies for a unimolecular surface reaction (desorption), except that now the units of rate are number of species per cm per second since both the partition functions and the surface concentrations are in units of number per cm. Changing to molar units, the C and q quantities are divided by Avogadro s number, N, so that the rate of desorption is now given by... 

Unimolecular surface reaction controlling, with one component adsorbed more strongly than the other. It has been shown [6] that para hydrogen is more strongly adsorbed on silica gel than is ortho, so that it seems likely that para hydrogen is also held more tightly by the ferric oxide gel catalyst. 

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