Bimolecular reactions surface-catalyzed

Bimolecular Reactions. Models of surface-catalyzed reactions involving two gas-phase reactants can be derived using either the equal rates method or the method of rate-controlling steps. The latter technique is algebraically simpler and serves to illustrate general principles. 

For a solid-catalyzed reaction between two different molecules all the possibilities for the rate-determining step of the one-molecule reaction still exist. Thus, adsorption of either molecule can be rate determining a surface reaction involving only one of the molecules can be the rate-determining step and so forth. The two-molecule case introduces some new possibilities for the rate-determining step. In both the one-molecule and the two-molecule case the rate-determining step can be a surface bimolecular reaction. With the surface reaction in the two-molecule case, however, the reaction depends upon the relative abilities of the molecules to adsorb on the active sites. There are several possible cases. Both can adsorb weakly one can adsorb moderately well as the other adsorbs weakly they can both adsorb moderately well, competing effectively with each other for sites and one can adsorb very well as the other adsorbs weakly. Another problem arises when the two... 

The chemical reaction step is normally composed of various steps, and a broad diversity of rate laws and reaction mechanisms are relevant for surface-catalyzed reactions. However, if the simple assumption that the chemical reaction consists of a sole unimolecular or bimolecular elementary reaction or a rate determining simple reaction followed by one or more fast steps, is made, then the reaction kinetics can be mathematically treated [92],... 

In the Langmuir-Hinshelwood (L-H) mechanism for surface-catalyzed reactions, the reaction takes place between two surface-adsorbed species [4,5], As a substitute for concentration, we use surface coverage, and the rate is expressed in this term. We consider that the elementary reaction in the L-H mechanism is the bimolecular surface reaction expressed by the following equations ... 

Notice that the denominator is squared for a bimolecular surface reaction. In general, the exponent on the denominator is equal to the number of sites participating in a rate-determining surface-catalyzed reaction. Since trimolecular surface events are uncommon, the exponent of the denominator rarely exceeds 2. 

The exchange reaction does not have an integral order, but is a typical example of a complex bimolecular surface-catalyzed reaction 12). It is assumed that the hydroxyl hydrogen is the only exchangeable hydrogen in methanol under these conditions. In view of the nonexchangeability of... 

Many catalyzed surface reactions can be treated as a two-step process with an adsorption equilibrium followed by one rate-determining step (diffusion, surface reaction, or desorption). The surface reaction kinetics are usually discussed in terms of two limiting mechanisms, the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. In the LH mechanism, reaction takes place directly between species which are chemically bonded (chemisorbed) on the surface. For a bimolecular LH surface reaction. Aawith competitive chemisorption of the reactants, the rate of reaction is given by the following expression ... 

Kinetic studies indicate whether a surface reaction-controlled bimolecular reaction proceeds by an L-H or R-E mechanism. Equation 2.24 indicates that in the L-H mechanism -Ra) passes through a maximum when either Pa or Pb is increased while the other is fixed. The decrease in the rate at high Pa or Pb is rationalized by supposing that the more strongly adsorbed reactant displaces other species from the surface as its partial pressure is increased. This type of behavior was observed in the transition metal-catalyzed reaction of cyclopropane with hydrogen, where the strongly adsorbed hydrogen displaced cyclopropane from the surface [20]. In the R-E mechanism, on the other hand, -Ra) tends to become independent of reactant partial pressure when Pa is steadily increased (Eq. 2.25). 

Rebek and his co-workers have shown that replication - autocatalysis based on molecular recognition - best accommodates the facts observed in the reaction of 42 with 43, and that under the published conditions 44 is responsible for the autocatalysis. The results indicated template-catalyzed replication as the source of autocatalysis, where recognition surfaces and functional groups interact to form a productive termolecular complex. The mechanism demands that catalysis would be absent with esters that lack hydrogen-bonding sites. One complication of this system is that the initial product of this bimolecular preassociative mechanism is postulated to be a cw-amide, which isomerized to the frani-amide, the active form of template. This appears to be one major background reaction for product formation (Scheme 14). 

Zeolite H-T catalyzed the ketonization of short-chain carboxylic acids. The formation of anhydrides is a side reaction, occurring on the outer surface of the zeolite ctystals. The propionic and butyric acid molecules seem to have the optimum size for a bimolecular ketonization reaction inside an erionite cavity. The ketonization of carboxylic acids is an example of zeolite specificity in catalysis, illustrating the necessity of strict adaptation of the transition state of the reaction to the intracrystalline porosity of the zeolite. 

Metal oxides that have adjacent cations with coordination vacancies in certain common surface planes, like some Ti02 and ZnO surfaces, can catalyze ketoniz-ation. This suggests that a bimolecular surface reaction is rate determining. However, not all such catalysts are very selective for ketonization, especially at high conversion. Kuriacose and co-workers proposed a mechanism based on the condensation of two adsorbed carboxylates, or a carboxylate and a less ionic species, following an aldol condensation-type pathway (Scheme The aldol-... 

Kinetic treatments are relatively simple for spontaneous reactions, both unimolecular and bimolecular water-catalyzed reactions, because only the transfer equilibrium of the substrate between solvent and the association colloid has to be considered, Eq. (3). For example, the rate of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion (3) is a useful indicator of medium polarity, and reaction is inhibited by solvents that hydrogen bond to the carboxylate moiety [96]. The reaction is accelerated by a variety of colloidal species that incorporate 3. including ionic and zwitterionic micelles [97,98] and O/W microemulsions [99]. Reactions are slightly slower in microemulsions derived from cetyltrimethylammonium bromide (CTABr) than in the corresponding aqueous micelles, but changes in the alcohol cosurfactant or the hydrocarbon, or in their relative concentrations, do not have major rate effects, and it appears that these microemulsion droplets are similar to aqueous micelles as submicroscopic reaction media. These observations are consistent with estimates of surface polarities [99-101] determined with bound fluorescent probes [102]. 

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