Rate-determining steps surface reactions

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... 

This may be explained by the bifunctional theory of electrocatalysis developed by Watanabe and Motoo [14], according to which Pt activates the dissociative chemisorption of methanol to CO, whereas Ru activates and dissociates water molecules, leading to adsorbed hydroxyl species, OH. A surface oxidation reaction between adsorbed CO and adsorbed OH becomes the rate-determining step. The reaction mechanism can be written as follows [15] ... 

Under most of the natural conditions, the rate of dissolution of carbonate minerals is far less than that expected for rate control by diffusion. The chemical reaction at the water-mineral interface is then assumed to be the rate-determining step. This reaction consists in the attachment or interactions of reactants with specific surface sites where the critical crystal bonds are weakened, which, in turn, allows the detachment of anions and cations of the surface into the solution. 

The best choice of the alloying or co-deposited elements depends on which step of the latter mechanism is the rate-determining step (normally reaction (VIII)). Different results have been obtained with ternary and quaternary combinations of these metals with rhodium, molybdenum, and iridium [104,144-156], so there is a lot left to be explored with regard to this interesting process. In the next figure, the current vs. potential profiles of a positive going scan for the different platinum modified surfaces in acidic methanol solution are shown. 

A kinetic model is based on an LH mechanism and is represented in Scheme 3-4. The rate-determining step is reaction 3 involving formation of TiN at the surface on a nitrogen site and the desorption of a dimethylamine radical. The deposition rate can be described by following formula... 

It is very difficult to connect the catalyst structure with reaction kinetics, however, and this remains a challenge for the XXI century. There are two main reactivity models the Langmuir-Hinselwood mechanism that involves reaction by interaction between two surface-bonded substrate atoms in the rate-determining step, and the Rideal-Eley mechanism that involves interaction between a surface-bonded substrate atom and a molecule in the gas phase in the rate-determining step. Most reactions have kinetics that are consistent with the first model. ... 

The measurement of a from the experimental slope of the Tafel equation may help to decide between rate-determining steps in an electrode process. Thus in the reduction water to evolve H2 gas, if the slow step is the reaction of with the metal M to form surface hydrogen atoms, M—H, a is expected to be about If, on the other hand, the slow step is the surface combination of two hydrogen atoms to form H2, a second-order process, then a should be 2 (see Ref. 150). 

The mechanism of the synthesis reaction remains unclear. Both a molecular mechanism and an atomic mechanism have been proposed. Strong support has been gathered for the atomic mechanism through measurements of adsorbed nitrogen atom concentrations on the surface of model working catalysts where dissociative N2 chemisorption is the rate-determining step (17). The likely mechanism, where (ad) indicates surface-adsorbed species, is as follows ... 

The quantitative solution of the problem, i.e. simultaneous determination of both the sequence of surface chemical steps and the ratios of the rate constants of adsorption-desorption processes to the rate constants of surface reactions from experimental kinetic data, is extraordinarily difficult. The attempt made by Smith and Prater 82) in a study of cyclohexane-cyclohexene-benzene interconversion, using elegant mathematic procedures based on the previous theoretical treatment 28), has met with only partial success. Nevertheless, their work is an example of how a sophisticated approach to the quantitative solution of a coupled heterogeneous catalytic system should be employed if the system is studied as a whole. 

If the rate-determining step were not a surface reaction but adsorption of competing reactants, we should obtain from the corresponding equations the expression which is formally identical with Eq. (15), in which, however, the relative reactivity is given by the expression S = fc dsA/ adsB. On the basis of data on competitive reactions only, these two cases cannot there-... 

Calderbank et al. (C6) studied the Fischer-Tropsch reaction in slurry reactors of 2- and 10-in. diameters, at pressures of 11 and 22 atm, and at a temperature of 265°C. It was assumed that the liquid-film diffusion of hydrogen from the gas-liquid interface is a rate-determining step, whereas the mass transfer of hydrogen from the bulk liquid to the catalyst was believed to be rapid because of the high ratio between catalyst exterior surface area and bubble surface area. The experimental data were not in complete agreement with a theoretical model based on these assumptions. 

In this approximation we assume that one elementary step determines the rate while all other steps are sufficiently fast that they can be considered as being in quasi-equilibrium. If we take the surface reaction to AB (step 3, Eq. 134) as the rate-determining step (RDS), we may write the rate equations for steps (1), (2) and (4) as ... 

It is important to realize that the assumption of a rate-determining step limits the scope of our description. As with the steady state approximation, it is not possible to describe transients in the quasi-equilibrium model. In addition, the rate-determining step in the mechanism might shift to a different step if the reaction conditions change, e.g. if the partial pressure of a gas changes markedly. For a surface science study of the reaction A -i- B in an ultrahigh vacuum chamber with a single crystal as the catalyst, the partial pressures of A and B may be so small that the rates of adsorption become smaller than the rate of the surface reaction. 

The evolution of methylchlorosilanes between 450 and 600 K is consistent with the 550 - 600 K typical for the catalytic Rochow Process [3]. It is also reasonably consistent with the evolution of methylchlorosilanes at 500 - 750 K reported by Frank and Falconer for a temperature programmed reaction study of the monolayer remaining on a CuaSi surface after catalytic formation of methylchlorosilanes from CHaCl at higher pressures [5]. Both of these observations suggest that the monolayer formed by methyl and chlorine adsorption on pure CuaSi is similar to that present on active catalysts. For reference, methylchlorosilanes bond quite weakly to tiie surface and desorb at 180 - 220 K. It can thus be concluded that the rate-determining step in the evolution of methylchlorosilanes at 450 - 600 K is a surface reaction rather an product desorption. 

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